Eric Pop is the Pease-Ye Professor of Electrical Engineering (EE) and, by courtesy, of Materials Science & Engineering and Applied Physics at Stanford and SLAC. He is also Senior Fellow at the Precourt Institute for Energy and he leads the Heterogeneous Integration focus area of the SystemX Alliance. Before Stanford, he spent several years on the faculty of UIUC, and in industry at Intel and IBM. His research interests include semiconductors, nanoelectronics, data storage, and energy. He received his PhD in EE from Stanford (2005) and three degrees from MIT (MEng and BS in EE, BS in Physics). His honors include the Intel Outstanding Researcher Award, the PECASE from the White House (highest honor given by the US government to early-career scientists & engineers), Young Investigator Awards from the Navy, Air Force, NSF CAREER, DARPA, and several best-paper awards with his students. He is an APS and IEEE Fellow, a Clarivate Highly Cited Researcher, he was Chair of the IEEE Device Research Conference (DRC) and IEEE Non-Volatile Memory Technology Symposium (NVMTS), and he has also served on committees of the APS, MRS, IEDM, and VLSI conferences. In his spare time he tries to avoid injuries while snowboarding, and in a past life he was a college radio DJ at KZSU 90.1. More information about the Pop Lab is at and on Twitter/X at @profericpop.

Academic Appointments

Administrative Appointments

  • Chair of EE faculty search committee, Electrical Engineering (2023 - Present)
  • Chair of EE Culture, Equity, and Inclusion (CEI) Committee, Electrical Engineering (2019 - 2022)
  • Co-Lead of Heterogeneous Integration Focus Area, SystemX Alliance (2015 - Present)

Honors & Awards

  • Pease-Ye Professorship, Stanford University (2023)
  • Viskanta Fellowship, Purdue University (2023)
  • APS Fellow, American Physical Society (2022)
  • Intel 2021 Outstanding Researcher Award, Intel (2021)
  • IEEE Fellow, IEEE (2021)
  • Highly Cited Researcher, Clarivate (2018)
  • Golden Reviewers List, IEEE Electron Device Letters (2017, 2013-09)
  • Most Cited Researchers List in EE, Elsevier (2016)
  • Okawa Foundation Grant, Okawa Foundation (2014)
  • Engineering Council Award for Excellence in Advising, UIUC (2013)
  • Award for Faculty Research, Xerox/UIUC (2011)
  • Center for Advanced Study (CAS) Fellowship, UIUC (2011)
  • Outstanding Presentation Award, E\PCOS Symposium (2011)
  • Senior Member, IEEE (2011)
  • AFOSR Young Investigator Program (YIP) Award, AFOSR (2010)
  • CAREER Award, NSF (2010)
  • ONR Young Investigator Program (YIP) Award, ONR (2010)
  • PECASE (Presidential) Award from the White House, ARO (2010)
  • List of Teachers Ranked as Excellent, UIUC (2009)
  • DARPA Young Faculty Award (YFA), DARPA (2008)
  • Arnold O. Beckman Research Award, UIUC (2007)

Boards, Advisory Committees, Professional Organizations

  • Fellow, APS (2022 - Present)
  • Fellow, IEEE (2021 - Present)
  • Member, AAAS (2012 - Present)
  • Member, APS (2011 - Present)
  • Senior Member, IEEE (2011 - Present)
  • Member, MRS (2007 - Present)

Program Affiliations

  • Stanford SystemX Alliance

Professional Education

  • Ph.D., Stanford University, Electrical Engineering (2005)
  • M.Eng., MIT, EECS (1999)
  • B.S., MIT, EECS (1999)
  • B.S., MIT, Physics (1999)

Current Research and Scholarly Interests

Research in the Pop Lab is at the intersection of nanoelectronics and nanoscale energy conversion. Most projects include both fundamental and applied, experimental and computational components. Some recent topics (as of 2013) include:

* Energy-efficient transistors, memory and integrated circuits
* Novel nanomaterials, e.g. graphene, BN, MoS2, carbon nanotubes, GeSbTe, etc.
* Fundamental physical limits of current and heat flow, e.g. ballistic electrons and phonons
* Applications of nanoscale energy transport, conversion and harvesting, e.g. thermoelectrics

For more details see the Pop Lab research website:

Stanford Advisees

All Publications

  • Nonvolatile Phase-Only Transmissive Spatial Light Modulator with Electrical Addressability of Individual Pixels. ACS nano Fang, Z., Chen, R., Froch, J. E., Tanguy, Q. A., Khan, A. I., Wu, X., Tara, V., Manna, A., Sharp, D., Munley, C., Miller, F., Zhao, Y., Geiger, S., Bohringer, K. F., Reynolds, M. S., Pop, E., Majumdar, A. 2024


    Active metasurfaces with tunable subwavelength-scale nanoscatterers are promising platforms for high-performance spatial light modulators (SLMs). Among the tuning methods, phase-change materials (PCMs) are attractive because of their nonvolatile, threshold-driven, and drastic optical modulation, rendering zero-static power, crosstalk immunity, and compact pixels. However, current electrically controlled PCM-based metasurfaces are limited to global amplitude modulation, which is insufficient for SLMs. Here, an individual-pixel addressable, transmissive metasurface is experimentally demonstrated using the low-loss PCM Sb2Se3 and doped silicon nanowire heaters. The nanowires simultaneously form a diatomic metasurface, supporting a high-quality-factor (406) quasi-bound-state-in-the-continuum mode. A global phase-only modulation of 0.25pi (0.2pi) in simulation (experiment) is achieved, showing ten times enhancement. A 2pi phase shift is further obtained using a guided-mode resonance with enhanced light-Sb2Se3 interaction. Finally, individual-pixel addressability and SLM functionality are demonstrated through deterministic multilevel switching (ten levels) and tunable far-field beam shaping. Our work presents zero-static power transmissive phase-only SLMs, enabled by electrically controlled low-loss PCMs and individual meta-molecule addressable metasurfaces.

    View details for DOI 10.1021/acsnano.4c00340

    View details for PubMedID 38639708

  • Design Guidelines for Oxide Semiconductor Gain Cell Memory on a Logic Platform IEEE TRANSACTIONS ON ELECTRON DEVICES Liu, S., Jana, K., Toprasertpong, K., Chen, J., Liang, Z., Jiang, Q., Wahid, S., Qin, S., Chen, W., Pop, E., Wong, H. 2024
  • Novel nanocomposite-superlattices for low energy and high stability nanoscale phase-change memory. Nature communications Wu, X., Khan, A. I., Lee, H., Hsu, C. F., Zhang, H., Yu, H., Roy, N., Davydov, A. V., Takeuchi, I., Bao, X., Wong, H. P., Pop, E. 2024; 15 (1): 13


    Data-centric applications are pushing the limits of energy-efficiency in today's computing systems, including those based on phase-change memory (PCM). This technology must achieve low-power and stable operation at nanoscale dimensions to succeed in high-density memory arrays. Here we use a novel combination of phase-change material superlattices and nanocomposites (based on Ge4Sb6Te7), to achieve record-low power density ≈ 5 MW/cm2 and ≈ 0.7 V switching voltage (compatible with modern logic processors) in PCM devices with the smallest dimensions to date (≈ 40 nm) for a superlattice technology on a CMOS-compatible substrate. These devices also simultaneously exhibit low resistance drift with 8 resistance states, good endurance (≈ 2 × 108 cycles), and fast switching (≈ 40 ns). The efficient switching is enabled by strong heat confinement within the superlattice materials and the nanoscale device dimensions. The microstructural properties of the Ge4Sb6Te7 nanocomposite and its high crystallization temperature ensure the fast-switching speed and stability in our superlattice PCM devices. These results re-establish PCM technology as one of the frontrunners for energy-efficient data storage and computing.

    View details for DOI 10.1038/s41467-023-42792-4

    View details for PubMedID 38253559

    View details for PubMedCentralID PMC10803317

  • Efficiency limit of transition metal dichalcogenide solar cells COMMUNICATIONS PHYSICS Nazif, K., Nitta, F. U., Daus, A., Saraswat, K. C., Pop, E. 2023; 6 (1)
  • Effect of Back-Gate Dielectric on Indium Tin Oxide (ITO) Transistor Performance and Stability IEEE TRANSACTIONS ON ELECTRON DEVICES Daus, A., Hoang, L., Gilardi, C., Wahid, S., Kwon, J., Qin, S., Ko, J., Islam, M., Kumar, A., Neilson, K. M., Saraswat, K. C., Mitra, S., Wong, H., Pop, E. 2023; 70 (11): 5685-5689
  • High Thermal Conductivity of Submicrometer Aluminum Nitride Thin Films Sputter-Deposited at Low Temperature. ACS nano Perez, C., McLeod, A. J., Chen, M. E., Yi, S. I., Vaziri, S., Hood, R., Ueda, S. T., Bao, X., Asheghi, M., Park, W., Talin, A. A., Kumar, S., Pop, E., Kummel, A. C., Goodson, K. E. 2023


    Aluminum nitride (AlN) is one of the few electrically insulating materials with excellent thermal conductivity, but high-quality films typically require exceedingly hot deposition temperatures (>1000 °C). For thermal management applications in dense or high-power integrated circuits, it is important to deposit heat spreaders at low temperatures (<500 °C), without affecting the underlying electronics. Here we demonstrate 100 nm to 1.7 μm thick AlN films achieved by low-temperature (<100 °C) sputtering, correlating their thermal properties with their grain size and interfacial quality, which we analyze by X-ray diffraction, transmission X-ray microscopy, as well as Raman and Auger spectroscopy. Controlling the deposition conditions through the partial pressure of reactive N2, we achieve an ∼3× variation in thermal conductivity (∼36-104 W m-1 K-1) of ∼600 nm films, with the upper range representing one of the highest values for such film thicknesses at room temperature, especially at deposition temperatures below 100 °C. Defect densities are also estimated from the thermal conductivity measurements, providing insight into the thermal engineering of AlN that can be optimized for application-specific heat spreading or thermal confinement.

    View details for DOI 10.1021/acsnano.3c05485

    View details for PubMedID 37796248

  • Spatially-Resolved Thermometry of Filamentary Nanoscale Hot Spots in TiO2 Resistive Random Access Memories to Address Device Variability. ACS applied electronic materials Swoboda, T., Gao, X., Rosário, C. M., Hui, F., Zhu, K., Yuan, Y., Deshmukh, S., Köroǧlu, Ç., Pop, E., Lanza, M., Hilgenkamp, H., Rojo, M. M. 2023; 5 (9): 5025-5031


    Resistive random access memories (RRAM), based on the formation and rupture of conductive nanoscale filaments, have attracted increased attention for application in neuromorphic and in-memory computing. However, this technology is, in part, limited by its variability, which originates from the stochastic formation and extreme heating of its nanoscale filaments. In this study, we used scanning thermal microscopy (SThM) to assess the effect of filament-induced heat spreading on the surface of metal oxide RRAMs with different device designs. We evaluate the variability of TiO2 RRAM devices with area sizes of 2 × 2 and 5 × 5 μm2. Electrical characterization shows that the variability indicated by the standard deviation of the forming voltage is ∼2 times larger for 5 × 5 μm2 devices than for the 2 × 2 μm2 ones. Further knowledge on the reason for this variability is gained through the SThM thermal maps. These maps show that for 2 × 2 μm2 devices the formation of one filament, i.e., hot spot at the device surface, happens reliably at the same location, while the filament location varies for the 5 × 5 μm2 devices. The thermal information, combined with the electrical, interfacial, and geometric characteristics of the device, provides additional insights into the operation and variability of RRAMs. This work suggests thermal engineering and characterization routes to optimize the efficiency and reliability of these devices.

    View details for DOI 10.1021/acsaelm.3c00782

    View details for PubMedID 37779889

    View details for PubMedCentralID PMC10537448

  • Spatially-Resolved Thermometry of Filamentary Nanoscale Hot Spots in TiO2 Resistive Random Access Memories to Address Device Variability ACS APPLIED ELECTRONIC MATERIALS Swoboda, T., Gao, X., Rosario, C. M., Hui, F., Zhu, K., Yuan, Y., Deshmukh, S., Koroglu, C., Pop, E., Lanza, M., Hilgenkamp, H., Rojo, M. 2023
  • Area-Selective Atomic Layer Deposition for Resistive Random-Access Memory Devices. ACS applied materials & interfaces Oh, I. K., Khan, A. I., Qin, S., Lee, Y., Wong, H. P., Pop, E., Bent, S. F. 2023


    Resistive random-access memory (RRAM) is a promising technology for data storage and neuromorphic computing; however, cycle-to-cycle and device-to-device variability limits its widespread adoption and high-volume manufacturability. Improving the structural accuracy of RRAM devices during fabrication can reduce these variabilities by minimizing the filamentary randomness within a device. Here, we studied area-selective atomic layer deposition (AS-ALD) of the HfO2 dielectric for the fabrication of RRAM devices with higher reliability and accuracy. Without requiring photolithography, first we demonstrated ALD of HfO2 patterns uniformly and selectively on Pt bottom electrodes for RRAM but not on the underlying SiO2/Si substrate. RRAM devices fabricated using AS-ALD showed significantly narrower operating voltage range (2.6 × improvement) and resistance states than control devices without AS-ALD, improving the overall reliability of RRAM. Irrespective of device size (1 × 1, 2 × 2, and 5 × 5 μm2), we observed similar improvement, which is an inherent outcome of the AS-ALD technique. Our demonstration of AS-ALD for improved RRAM devices could further encourage the adoption of such techniques for other data storage technologies, including phase-change, magnetic, and ferroelectric RAM.

    View details for DOI 10.1021/acsami.3c05822

    View details for PubMedID 37656599

  • Thiol-based defect healing of WSe2 and WS2 NPJ 2D MATERIALS AND APPLICATIONS Schwarz, A., Alon-Yehezkel, H., Levi, A., Yadav, R., Majhi, K., Tzuriel, Y., Hoang, L., Bailey, C. S., Brumme, T., Mannix, A. J., Cohen, H., Yalon, E., Heine, T., Pop, E., Cheshnovsky, O., Naveh, D. 2023; 7 (1)
  • Imaging the electron charge density in monolayer MoS2 at the Ångstrom scale. Nature communications Martis, J., Susarla, S., Rayabharam, A., Su, C., Paule, T., Pelz, P., Huff, C., Xu, X., Li, H. K., Jaikissoon, M., Chen, V., Pop, E., Saraswat, K., Zettl, A., Aluru, N. R., Ramesh, R., Ercius, P., Majumdar, A. 2023; 14 (1): 4363


    Four-dimensional scanning transmission electron microscopy (4D-STEM) has recently gained widespread attention for its ability to image atomic electric fields with sub-Ångstrom spatial resolution. These electric field maps represent the integrated effect of the nucleus, core electrons and valence electrons, and separating their contributions is non-trivial. In this paper, we utilized simultaneously acquired 4D-STEM center of mass (CoM) images and annular dark field (ADF) images to determine the projected electron charge density in monolayer MoS2. We evaluate the contributions of both the core electrons and the valence electrons to the derived electron charge density; however, due to blurring by the probe shape, the valence electron contribution forms a nearly featureless background while most of the spatial modulation comes from the core electrons. Our findings highlight the importance of probe shape in interpreting charge densities derived from 4D-STEM and the need for smaller electron probes.

    View details for DOI 10.1038/s41467-023-39304-9

    View details for PubMedID 37474521

  • A Purcell-enabled monolayer semiconductor free-space optical modulator NATURE PHOTONICS Li, Q., Song, J., Xu, F., van de Groep, J., Hong, J., Daus, A., Lee, Y., Johnson, A. C., Pop, E., Liu, F., Brongersma, M. L. 2023
  • Effect of Top-Gate Dielectric Deposition on the Performance of Indium Tin Oxide Transistors IEEE ELECTRON DEVICE LETTERS Wahid, S., Daus, A., Kwon, J., Qin, S., Ko, J., Wong, H., Pop, E. 2023; 44 (6): 951-954
  • Probing the Melting Transitions in Phase-Change Superlattices via Thin Film Nanocalorimetry. Nano letters Zhao, J., Khan, A. I., Efremov, M. Y., Ye, Z., Wu, X., Kim, K., Lee, Z., Wong, H. P., Pop, E., Allen, L. H. 2023


    Phase-change superlattices with nanometer thin sublayers are promising for low-power phase-change memory (PCM) on rigid and flexible platforms. However, the thermodynamics of the phase transition in such nanoscale superlattices remain unexplored, especially at ultrafast scanning rates, which is crucial for our fundamental understanding of superlattice-based PCM. Here, we probe the phase transition of Sb2Te3 (ST)/Ge2Sb2Te5 (GST) superlattices using nanocalorimetry with a monolayer sensitivity (∼1 Å) and a fast scanning rate (105 K/s). For a 2/1.8 nm/nm Sb2Te3/GST superlattice, we observe an endothermic melting transition with an ∼240 °C decrease in temperature and an ∼8-fold decrease in enthalpy compared to those for the melting of GST, providing key thermodynamic insights into the low-power switching of superlattice-based PCM. Nanocalorimetry measurements for Sb2Te3 alone demonstrate an intrinsic premelting similar to the unique phase transition of superlattices, thus revealing a critical role of the Sb2Te3 sublayer within our superlattices. These results advance our understanding of superlattices for energy-efficient data storage and computing.

    View details for DOI 10.1021/acs.nanolett.3c01049

    View details for PubMedID 37171275

  • Ambipolar Thickness-Dependent Thermoelectric Measurements of WSe2. Nano letters Chen, V., Lee, H. R., Köroğlu, Ç., McClellan, C. J., Daus, A., Pop, E. 2023


    Thermoelectric materials can harvest electrical energy from temperature gradients, and could play a role as power supplies for sensors and other devices. Here, we characterize fundamental in-plane electrical and thermoelectric properties of layered WSe2 over a range of thicknesses, from 10 to 96 nm, between 300 and 400 K. The devices are electrostatically gated with an ion gel, enabling us to probe both electron and hole regimes over a large range of carrier densities. We extract the highest n- and p-type Seebeck coefficients for thin-film WSe2, -500 and 950 μV/K respectively, reported to date at room temperature. We also emphasize the importance of low substrate thermal conductivity on such lateral thermoelectric measurements, improving this platform for future studies on other nanomaterials.

    View details for DOI 10.1021/acs.nanolett.2c03468

    View details for PubMedID 37141159

  • Uncovering the Different Components of Contact Resistance to Atomically Thin Semiconductors ADVANCED ELECTRONIC MATERIALS Ber, E., Grady, R. W., Pop, E., Yalon, E. 2023
  • Intrinsic and Extrinsic Factors Influencing the Dynamics of VO2 Mott Oscillators PHYSICAL REVIEW APPLIED Bohaichuk, S. M., Kumar, S., Islam, M., Rojo, M., Williams, R., Pitner, G., Jeong, J., Samant, M. G., Parkin, S. P., Pop, E. 2023; 19 (4)
  • Nanoscale temperature sensing of electronic devices with calibrated scanning thermal microscopy. Nanoscale Swoboda, T., Wainstein, N., Deshmukh, S., Koroglu, C., Gao, X., Lanza, M., Hilgenkamp, H., Pop, E., Yalon, E., Munoz Rojo, M. 2023


    Heat dissipation threatens the performance and lifetime of many electronic devices. As the size of devices shrinks to the nanoscale, we require spatially and thermally resolved thermometry to observe their fine thermal features. Scanning thermal microscopy (SThM) has proven to be a versatile measurement tool for characterizing the temperature at the surface of devices with nanoscale resolution. SThM can obtain qualitative thermal maps of a device using an operating principle based on a heat exchange process between a thermo-sensitive probe and the sample surface. However, the quantification of these thermal features is one of the most challenging parts of this technique. Developing reliable calibration approaches for SThM is therefore an essential aspect to accurately determine the temperature at the surface of a sample or device. In this work, we calibrate a thermo-resistive SThM probe using heater-thermometer metal lines with different widths (50 nm to 750 nm), which mimic variable probe-sample thermal exchange processes. The sensitivity of the SThM probe when scanning the metal lines is also evaluated under different probe and line temperatures. Our results reveal that the calibration factor depends on the probe measuring conditions and on the size of the surface heating features. This approach is validated by mapping the temperature profile of a phase change electronic device. Our analysis provides new insights on how to convert the thermo-resistive SThM probe signal to the scanned device temperature more accurately.

    View details for DOI 10.1039/d3nr00343d

    View details for PubMedID 37006192

  • Semimetal-Monolayer Transition Metal Dichalcogenides Photodetectors for Wafer-Scale Broadband Photonics ADVANCED PHOTONICS RESEARCH Sinn, H., Kumar, A., Pop, E., Newaz, A. 2023
  • High Thermal Conductivity Insulators for Thermal Management in 3D Integrated Circuits IEEE ELECTRON DEVICE LETTERS Koroglu, C., Pop, E. 2023; 44 (3): 496-499
  • High Number of Transport Modes: A Requirement for Contact Resistance Reduction to Atomically Thin Semiconductors IEEE TRANSACTIONS ON ELECTRON DEVICES Ber, E., Grady, R. W., Pop, E., Yalon, E. 2023
  • How Do Quantum Effects Influence the Capacitance and Carrier Density of Monolayer MoS2 Transistors? Nano letters Bennett, R. K., Pop, E. 2023


    When transistor gate insulators have nanometer-scale equivalent oxide thickness (EOT), the gate capacitance (CG) becomes smaller than the oxide capacitance (Cox) due to the quantum capacitance and charge centroid capacitance of the channel. Here, we study the capacitance of monolayer MoS2 as a prototypical two-dimensional (2D) channel while considering spatial variations in the potential, charge density, and density of states. At 0.5 nm EOT, the monolayer MoS2 capacitance is smaller than its quantum capacitance, limiting the single-gated CG of an n-type channel to between 63% and 78% of Cox, for gate overdrive voltages between 0.5 and 1 V. Despite these limitations, for dual-gated devices, the on-state CG of monolayer MoS2 is 50% greater than that of silicon at 0.5 nm EOT and more than three times that of InGaAs at 1 nm EOT, indicating that such 2D semiconductors are promising for improved gate control of nanoscale transistors at future technology nodes.

    View details for DOI 10.1021/acs.nanolett.2c03913

    View details for PubMedID 36786518

  • Energy Efficient Neuro-inspired Phase Change Memory Based on Ge4 Sb6 Te7 as a Novel Epitaxial Nanocomposite. Advanced materials (Deerfield Beach, Fla.) Khan, A. I., Yu, H., Zhang, H., Goggin, J. R., Kwon, H., Wu, X., Perez, C., Neilson, K. M., Asheghi, M., Goodson, K. E., Vora, P. M., Davydov, A., Takeuchi, I., Pop, E. 2023: e2300107


    Phase change memory (PCM) is a promising candidate for neuro-inspired, data-intensive artificial intelligence applications, which relies on the physical attributes of PCM materials including gradual change of resistance states and multilevel operation with low resistance drift. However, achieving these attributes simultaneously remains a fundamental challenge for PCM materials such as Ge2 Sb2 Te5 , the most commonly used material. Here we demonstrate bi-directional gradual resistance changes with ∼10x resistance window using low energy pulses in nanoscale PCM devices based on Ge4 Sb6 Te7 , a new phase change nanocomposite material. These devices show 13 resistance levels with low resistance drift for the first 8 levels, resistance on/off ratio of ∼1000, and low variability. These attributes are enabled by the unique microstructural and electrothermal properties of Ge4 Sb6 Te7 , a nanocomposite consisting of epitaxial SbTe nanoclusters within the Ge-Sb-Te matrix, and a higher crystallization but lower melting temperature than Ge2 Sb2 Te5 . These results advance the pathway towards energy-efficient analog computing using PCM. This article is protected by copyright. All rights reserved.

    View details for DOI 10.1002/adma.202300107

    View details for PubMedID 36720651

  • Efficiency Limit of Transition Metal Dichalcogenide Solar Cells Nazif, K., Nitta, F. U., Daus, A., Saraswat, K. C., Pop, E., IEEE IEEE. 2023
  • Approaching the quantum limit in two-dimensional semiconductor contacts. Nature Li, W., Gong, X., Yu, Z., Ma, L., Sun, W., Gao, S., Köroğlu, Ç., Wang, W., Liu, L., Li, T., Ning, H., Fan, D., Xu, Y., Tu, X., Xu, T., Sun, L., Wang, W., Lu, J., Ni, Z., Li, J., Duan, X., Wang, P., Nie, Y., Qiu, H., Shi, Y., Pop, E., Wang, J., Wang, X. 2023; 613 (7943): 274-279


    The development of next-generation electronics requires scaling of channel material thickness down to the two-dimensional limit while maintaining ultralow contact resistance1,2. Transition-metal dichalcogenides can sustain transistor scaling to the end of roadmap, but despite a myriad of efforts, the device performance remains contact-limited3-12. In particular, the contact resistance has not surpassed that of covalently bonded metal-semiconductor junctions owing to the intrinsic van der Waals gap, and the best contact technologies are facing stability issues3,7. Here we push the electrical contact of monolayer molybdenum disulfide close to the quantum limit by hybridization of energy bands with semi-metallic antimony ([Formula: see text]) through strong van der Waals interactions. The contacts exhibit a low contact resistance of 42 ohm micrometres and excellent stability at 125 degrees Celsius. Owing to improved contacts, short-channel molybdenum disulfide transistors show current saturation under one-volt drain bias with an on-state current of 1.23 milliamperes per micrometre, an on/off ratio over 108 and an intrinsic delay of 74 femtoseconds. These performances outperformed equivalent silicon complementary metal-oxide-semiconductor technologies and satisfied the 2028 roadmap target. We further fabricate large-area device arrays and demonstrate low variability in contact resistance, threshold voltage, subthreshold swing, on/off ratio, on-state current and transconductance13. The excellent electrical performance, stability and variability make antimony ([Formula: see text]) a promising contact technology for transition-metal-dichalcogenide-based electronics beyond silicon.

    View details for DOI 10.1038/s41586-022-05431-4

    View details for PubMedID 36631650

  • Stateful Logic Using Phase Change Memory IEEE JOURNAL ON EXPLORATORY SOLID-STATE COMPUTATIONAL DEVICES AND CIRCUITS Hoffer, B., Wainstein, N., Neumann, C. M., Pop, E., Yalon, E., Kvatinsky, S. 2022; 8 (2): 77-83
  • Strain-Enhanced Mobility of Monolayer MoS2. Nano letters Datye, I. M., Daus, A., Grady, R. W., Brenner, K., Vaziri, S., Pop, E. 2022


    Strain engineering is an important method for tuning the properties of semiconductors and has been used to improve the mobility of silicon transistors for several decades. Recently, theoretical studies have predicted that strain can also improve the mobility of two-dimensional (2D) semiconductors, e.g., by reducing intervalley scattering or lowering effective masses. Here, we experimentally show strain-enhanced electron mobility in monolayer MoS2 transistors with uniaxial tensile strain, on flexible substrates. The on-state current and mobility are nearly doubled with tensile strain up to 0.7%, and devices return to their initial state after release of the strain. We also show a gate-voltage-dependent gauge factor up to 200 for monolayer MoS2, which is higher than previous values reported for sub-1 nm thin piezoresistive films. These results demonstrate the importance of strain engineering 2D semiconductors for performance enhancements in integrated circuits, or for applications such as flexible strain sensors.

    View details for DOI 10.1021/acs.nanolett.2c01707

    View details for PubMedID 36198070

  • Understanding Interface-Controlled Resistance Drift in Superlattice Phase Change Memory IEEE ELECTRON DEVICE LETTERS Wu, X., Khan, A., Ramesh, P., Perez, C., Kim, K., Lee, Z., Saraswat, K., Goodson, K. E., Wong, H., Pop, E. 2022; 43 (10): 1669-1672
  • Electro-thermal Characterization of Dynamical VO2 Memristors via Local Activity Modeling. Advanced materials (Deerfield Beach, Fla.) Brown, T. D., Bohaichuk, S. M., Islam, M., Kumar, S., Pop, E., Williams, R. S. 2022: e2205451


    Translating the surging interest in neuromorphic electronic components, such as those based on nonlinearities near Mott transitions, into large-scale commercial deployment faces steep challenges in the current lack of means to identify and design key material parameters. These issues are exemplified by the difficulties in connecting measurable material properties to device behavior via circuit element models. Here we use the principle of Local Activity to build a model of VO2 / SiN Mott threshold switches by sequentially accounting for constraints from a minimal set of quasi-static and dynamic electrical and high spatial resolution thermal data obtained via in-situ thermoreflectance mapping. By combining independent data sets for devices with varying dimensions, we distill the model to measurable material properties and established device scaling laws. The model can accurately predict electrical and thermal conductivities and capacitances and locally active dynamics (especially persistent spiking self-oscillations). The systematic procedure by which we develop this model has been a missing link in predictively connecting neuromorphic device behavior with their underlying material properties, and should enable rapid screening of material candidates before employing expensive manufacturing processes and testing procedures. This article is protected by copyright. All rights reserved.

    View details for DOI 10.1002/adma.202205451

    View details for PubMedID 36165218

  • How to report and benchmark emerging field-effect transistors (July, 10.1038/s41928-022-00798-8, 2022) NATURE ELECTRONICS Cheng, Z., Pang, C., Wang, P., Le, S. T., Wu, Y., Shahrjerdi, D., Radu, I., Lemme, M. C., Peng, L., Duan, X., Chen, Z., Appenzeller, J., Koester, S. J., Pop, E., Franklin, A. D., Richter, C. A. 2022
  • Improved gradual resistive switching range and 1000x on/off ratio in HfOx PRAM achieved with a Ge2Sb2Te5 thermal barrier APPLIED PHYSICS LETTERS Islam, R., Qin, S., Deshmukh, S., Yu, Z., Koroglu, C., Khan, A. I., Schauble, K., Saraswat, K. C., Pop, E., Wong, H. P. 2022; 121 (8)

    View details for DOI 10.1063/5.0101417

    View details for Web of Science ID 000892460800016

  • Fast-Response Flexible Temperature Sensors with Atomically Thin Molybdenum Disulfide. Nano letters Daus, A., Jaikissoon, M., Khan, A. I., Kumar, A., Grady, R. W., Saraswat, K. C., Pop, E. 2022


    Real-time thermal sensing on flexible substrates could enable a plethora of new applications. However, achieving fast, sub-millisecond response times even in a single sensor is difficult, due to the thermal mass of the sensor and encapsulation. Here, we fabricate flexible monolayer molybdenum disulfide (MoS2) temperature sensors and arrays, which can detect temperature changes within a few microseconds, over 100× faster than flexible thin-film metal sensors. Thermal simulations indicate the sensors' response time is only limited by the MoS2 interfaces and encapsulation. The sensors also have high temperature coefficient of resistance, ∼1-2%/K and stable operation upon cycling and long-term measurement when they are encapsulated with alumina. These results, together with their biocompatibility, make these devices excellent candidates for biomedical sensor arrays and many other Internet of Things applications.

    View details for DOI 10.1021/acs.nanolett.2c01344

    View details for PubMedID 35899996

  • Unveiling the Effect of Superlattice Interfaces and Intermixing on Phase Change Memory Performance. Nano letters Khan, A. I., Wu, X., Perez, C., Won, B., Kim, K., Ramesh, P., Kwon, H., Tung, M. C., Lee, Z., Oh, I., Saraswat, K., Asheghi, M., Goodson, K. E., Wong, H. P., Pop, E. 2022


    Superlattice (SL) phase change materials have shown promise to reduce the switching current and resistance drift of phase change memory (PCM). However, the effects of internal SL interfaces and intermixing on PCM performance remain unexplored, although these are essential to understand and ensure reliable memory operation. Here, using nanometer-thin layers of Ge2Sb2Te5 and Sb2Te3 in SL-PCM, we uncover that both switching current density (Jreset) and resistance drift coefficient (v) decrease as the SL period thickness is reduced (i.e., higher interface density); however, interface intermixing within the SL increases both. The signatures of distinct versus intermixed interfaces also show up in transmission electron microscopy, X-ray diffraction, and thermal conductivity measurements of our SL films. Combining the lessons learned, we simultaneously achieve low Jreset 3-4 MA/cm2 and ultralow v 0.002 in mushroom-cell SL-PCM with 110 nm bottom contact diameter, thus advancing SL-PCM technology for high-density storage and neuromorphic applications.

    View details for DOI 10.1021/acs.nanolett.2c01869

    View details for PubMedID 35876819

  • Extended Scale Length Theory for Low-Dimensional Field-Effect Transistors IEEE TRANSACTIONS ON ELECTRON DEVICES Gilardi, C., Bennett, R. A., Yoon, Y., Pop, E., Wong, H., Mitra, S. 2022
  • Ultra-low-energy programmable non-volatile silicon photonics based on phase-change materials with graphene heaters NATURE NANOTECHNOLOGY Fang, Z., Chen, R., Zheng, J., Khan, A., Neilson, K. M., Geiger, S. J., Callahan, D. M., Moebius, M. G., Saxena, A., Chen, M. E., Rios, C., Hu, J., Pop, E., Majumdar, A. 2022


    Silicon photonics is evolving from laboratory research to real-world applications with the potential to transform many technologies, including optical neural networks and quantum information processing. A key element for these applications is a reconfigurable switch operating at ultra-low programming energy-a challenging proposition for traditional thermo-optic or free carrier switches. Recent advances in non-volatile programmable silicon photonics based on phase-change materials (PCMs) provide an attractive solution to energy-efficient photonic switches with zero static power, but the programming energy density remains high (hundreds of attojoules per cubic nanometre). Here we demonstrate a non-volatile electrically reconfigurable silicon photonic platform leveraging a monolayer graphene heater with high energy efficiency and endurance. In particular, we show a broadband switch based on the technologically mature PCM Ge2Sb2Te5 and a phase shifter employing the emerging low-loss PCM Sb2Se3. The graphene-assisted photonic switches exhibited an endurance of over 1,000 cycles and a programming energy density of 8.7 ± 1.4 aJ nm-3, that is, within an order of magnitude of the PCM thermodynamic switching energy limit (~1.2 aJ nm-3) and at least a 20-fold reduction in switching energy compared with the state of the art. Our work shows that graphene is a reliable and energy-efficient heater compatible with dielectric platforms, including Si3N4, for technologically relevant non-volatile programmable silicon photonics.

    View details for DOI 10.1038/s41565-022-01153-w

    View details for Web of Science ID 000820548600001

    View details for PubMedID 35788188

  • How to report and benchmark emerging field-effect transistors NATURE ELECTRONICS Cheng, Z., Pang, C., Wang, P., Le, S. T., Wu, Y., Shahrjerdi, D., Radu, I., Lemme, M. C., Peng, L., Duan, X., Chen, Z., Appenzeller, J., Koester, S. J., Pop, E., Franklin, A. D., Richter, C. A. 2022; 5 (7): 416-423
  • Substrate-dependence of monolayer MoS2 thermal conductivity and thermal boundary conductance JOURNAL OF APPLIED PHYSICS Gabourie, A. J., Koroglu, C., Pop, E. 2022; 131 (19)

    View details for DOI 10.1063/5.0089247

    View details for Web of Science ID 000827597400006

  • Nonequilibrium Phonon Thermal Resistance at MoS2/Oxide and Graphene/Oxide Interfaces. ACS applied materials & interfaces Zheng, W., McClellan, C. J., Pop, E., Koh, Y. K. 2022


    Accurate measurements and physical understanding of thermal boundary resistance (R) of two-dimensional (2D) materials are imperative for effective thermal management of 2D electronics and photonics. In previous studies, heat dissipation from 2D material devices was presumed to be dominated by phonon transport across the interfaces. In this study, we find that, in addition to phonon transport, thermal resistance between nonequilibrium phonons in the 2D materials could play a critical role too when the 2D material devices are internally self-heated, either optically or electrically. We accurately measure the R of oxide/MoS2/oxide and oxide/graphene/oxide interfaces for three oxides (SiO2, HfO2, and Al2O3) by differential time-domain thermoreflectance (TDTR). Our measurements of R across these interfaces with external heating are 2-4 times lower than the previously reported R of the similar interfaces measured by Raman thermometry with internal self-heating. Using a simple model, we show that the observed discrepancy can be explained by an additional internal thermal resistance (Rint) between nonequilibrium phonons present during Raman measurements. We subsequently estimate that, for MoS2 and graphene, Rint 31 and 22 m2 K GW-1, respectively. The values are comparable to the thermal resistance due to finite phonon transmission across interfaces of 2D materials and thus cannot be ignored in the design of 2D material devices. Moreover, the nonequilibrium phonons also lead to a different temperature dependence than that by phonon transport. As such, our work provides important insights into physical understanding of heat dissipation in 2D material devices.

    View details for DOI 10.1021/acsami.2c02062

    View details for PubMedID 35506655

  • Direct measurement of nanoscale filamentary hot spots in resistive memory devices. Science advances Deshmukh, S., Rojo, M. M., Yalon, E., Vaziri, S., Koroglu, C., Islam, R., Iglesias, R. A., Saraswat, K., Pop, E. 2022; 8 (13): eabk1514


    Resistive random access memory (RRAM) is an important candidate for both digital, high-density data storage and for analog, neuromorphic computing. RRAM operation relies on the formation and rupture of nanoscale conductive filaments that carry enormous current densities and whose behavior lies at the heart of this technology. Here, we directly measure the temperature of these filaments in realistic RRAM with nanoscale resolution using scanning thermal microscopy. We use both conventional metal and ultrathin graphene electrodes, which enable the most thermally intimate measurement to date. Filaments can reach 1300°C during steady-state operation, but electrode temperatures seldom exceed 350°C because of thermal interface resistance. These results reveal the importance of thermal engineering for nanoscale RRAM toward ultradense data storage or neuromorphic operation.

    View details for DOI 10.1126/sciadv.abk1514

    View details for PubMedID 35353574

  • Electrically driven reprogrammable phase-change metasurface reaching 80% efficiency. Nature communications Abdollahramezani, S., Hemmatyar, O., Taghinejad, M., Taghinejad, H., Krasnok, A., Eftekhar, A. A., Teichrib, C., Deshmukh, S., El-Sayed, M. A., Pop, E., Wuttig, M., Alo, A., Cai, W., Adibi, A. 2022; 13 (1): 1696


    Phase-change materials (PCMs) offer a compelling platform for active metaoptics, owing to their large index contrast and fast yet stable phase transition attributes. Despite recent advances in phase-change metasurfaces, a fully integrable solution that combines pronounced tuning measures, i.e., efficiency, dynamic range, speed, and power consumption, is still elusive. Here, we demonstrate an in situ electrically driven tunable metasurface by harnessing the full potential of a PCM alloy, Ge2Sb2Te5 (GST), to realize non-volatile, reversible, multilevel, fast, and remarkable optical modulation in the near-infrared spectral range. Such a reprogrammable platform presents a record eleven-fold change in the reflectance (absolute reflectance contrast reaching 80%), unprecedented quasi-continuous spectral tuning over 250nm, and switching speed that can potentially reach a few kHz. Our scalable heterostructure architecture capitalizes on the integration of a robust resistive microheater decoupled from an optically smart metasurface enabling good modal overlap with an ultrathin layer of the largest index contrast PCM to sustain high scattering efficiency even after several reversible phase transitions. We further experimentally demonstrate an electrically reconfigurable phase-change gradient metasurface capable of steering an incident light beam into different diffraction orders. This work represents a critical advance towards the development of fully integrable dynamic metasurfaces and their potential for beamforming applications.

    View details for DOI 10.1038/s41467-022-29374-6

    View details for PubMedID 35354813

  • <p>Temperature-dependent thermal resistance of phase change memory</p> APPLIED PHYSICS LETTERS Stern, K., Keller, Y., Neumann, C. M., Pop, E., Yalon, E. 2022; 120 (11)

    View details for DOI 10.1063/5.0081016

    View details for Web of Science ID 000790984800001

  • Electro-Thermal Confinement Enables Improved Superlattice Phase Change Memory IEEE ELECTRON DEVICE LETTERS Khan, A., Kwon, H., Chen, M. E., Asheghi, M., Wong, H., Goodson, K. E., Pop, E. 2022; 43 (2): 204-207
  • First Demonstration of Dual-Gated Indium Tin Oxide Transistors with Record Drive Current similar to 2.3 mA/mu m at L approximate to 60 nm and V-DS=1 V Wahid, S., Daus, A., Kumar, A., Wong, H., Pop, E., IEEE IEEE. 2022
  • Ultra-low energy programmable non-volatile silicon photonics based on phase-change materials with graphene heaters Fang, Z., Chen, R., Zheng, J., Khan, A., Neilson, K. M., Geiger, S. J., Callahan, D. M., Moebius, M. G., Saxena, A., Chen, M. E., Rios, C., Hu, J., Pop, E., Majumdar, A., Subramania, G. S., Foteinopoulou, S. SPIE-INT SOC OPTICAL ENGINEERING. 2022

    View details for DOI 10.1117/12.2632208

    View details for Web of Science ID 000870730200001

  • Nanoscale Phase Change Memory Arrays Patterned by Block Copolymer Directed Self-Assembly Tung, M. C., Khan, A., Kwon, H., Asheghi, M., Goodson, K. E., Pop, E., Wong, H., Panning, E. M., Liddle, J. A. SPIE-INT SOC OPTICAL ENGINEERING. 2022

    View details for DOI 10.1117/12.2611737

    View details for Web of Science ID 000839339400005

  • High-specific-power flexible transition metal dichalcogenide solar cells. Nature communications Nassiri Nazif, K., Daus, A., Hong, J., Lee, N., Vaziri, S., Kumar, A., Nitta, F., Chen, M. E., Kananian, S., Islam, R., Kim, K., Park, J., Poon, A. S., Brongersma, M. L., Pop, E., Saraswat, K. C. 2021; 12 (1): 7034


    Semiconducting transition metal dichalcogenides (TMDs) are promising for flexible high-specific-power photovoltaics due to their ultrahigh optical absorption coefficients, desirable band gaps and self-passivated surfaces. However, challenges such as Fermi-level pinning at the metal contact-TMD interface and the inapplicability of traditional doping schemes have prevented most TMD solar cells from exceeding 2% power conversion efficiency (PCE). In addition, fabrication on flexible substrates tends to contaminate or damage TMD interfaces, further reducing performance. Here, we address these fundamental issues by employing: (1) transparent graphene contacts to mitigate Fermi-level pinning, (2) MoOx capping for doping, passivation and anti-reflection, and (3) a clean, non-damaging direct transfer method to realize devices on lightweight flexible polyimide substrates. These lead to record PCE of 5.1% and record specific power of 4.4Wg-1 for flexible TMD (WSe2) solar cells, the latter on par with prevailing thin-film solar technologies cadmium telluride, copper indium gallium selenide, amorphous silicon and III-Vs. We further project that TMD solar cells could achieve specific power up to 46Wg-1, creating unprecedented opportunities in a broad range of industries from aerospace to wearable and implantable electronics.

    View details for DOI 10.1038/s41467-021-27195-7

    View details for PubMedID 34887383

  • Lateral electrical transport and field-effect characteristics of sputtered p-type chalcogenide thin films APPLIED PHYSICS LETTERS Wahid, S., Daus, A., Khan, A., Chen, V., Neilson, K. M., Islam, M., Chen, M. E., Pop, E. 2021; 119 (23)

    View details for DOI 10.1063/5.0063759

    View details for Web of Science ID 000729364800005

  • Transistors based on two-dimensional materials for future integrated circuits NATURE ELECTRONICS Das, S., Sebastian, A., Pop, E., McClellan, C. J., Franklin, A. D., Grasser, T., Knobloch, T., Illarionov, Y., Penumatcha, A. V., Appenzeller, J., Chen, Z., Zhu, W., Asselberghs, I., Li, L., Avci, U. E., Bhat, N., Anthopoulos, T. D., Singh, R. 2021; 4 (11): 786-799
  • Vibrational Properties of a Naturally Occurring Semiconducting van der Waals Heterostructure JOURNAL OF PHYSICAL CHEMISTRY C Costa, V., Liang, L., Vaziri, S., Miller, A., Pop, E., Newaz, A. M. 2021; 125 (39): 21607-21613
  • Application-driven synthesis and characterization of hexagonal boron nitride deposited on metals and carbon nanotubes 2D MATERIALS Chen, V., Shin, Y., Mikheev, E., Lin, Q., Martis, J., Zhang, Z., Chatterjee, S., Majumdar, A., Wong, H., Goldhaber-Gordon, D., Pop, E. 2021; 8 (4)
  • Field-effect at electrical contacts to two-dimensional materials (Jul, 10.1007/s12274-021-3670-y, 2021) NANO RESEARCH Guo, Y., Sun, Y., Tang, A., Wang, C., Zhao, Y., Bai, M., Xu, S., Xu, Z., Tang, T., Wang, S., Qiu, C., Xu, K., Peng, X., Han, J., Pop, E., Chai, Y. 2021
  • Sub-Nanosecond Pulses Enable Partial Reset for Analog Phase Change Memory IEEE ELECTRON DEVICE LETTERS Stern, K., Wainstein, N., Keller, Y., Neumann, C. M., Pop, E., Kvatinsky, S., Yalon, E. 2021; 42 (9): 1291-1294
  • Toward Low-Temperature Solid-Source Synthesis of Monolayer MoS2. ACS applied materials & interfaces Tang, A., Kumar, A., Jaikissoon, M., Saraswat, K., Wong, H. P., Pop, E. 2021


    Two-dimensional (2D) semiconductors have been proposed for heterogeneous integration with existing silicon technology; however, their chemical vapor deposition (CVD) growth temperatures are often too high. Here, we demonstrate direct CVD solid-source precursor synthesis of continuous monolayer (1L) MoS2 films at 560 °C in 50 min, within the 450-to-600 °C, 2 h thermal budget window required for back-end-of-the-line compatibility with modern silicon technology. Transistor measurements reveal on-state current up to 140 muA/mum at 1 V drain-to-source voltage for 100 nm channel lengths, the highest reported to date for 1L MoS2 grown below 600 °C using solid-source precursors. The effective mobility from transfer length method test structures is 29 ± 5 cm2 V-1 s-1 at 6.1 * 1012 cm-2 electron density, which is comparable to mobilities reported from films grown at higher temperatures. The results of this work provide a path toward the realization of high-quality, thermal-budget-compatible 2D semiconductors for heterogeneous integration with silicon manufacturing.

    View details for DOI 10.1021/acsami.1c06812

    View details for PubMedID 34427445

  • Field-effect at electrical contacts to two-dimensional materials. Nano research Guo, Y., Sun, Y., Tang, A., Wang, C., Zhao, Y., Bai, M., Xu, S., Xu, Z., Tang, T., Wang, S., Qiu, C., Xu, K., Peng, X., Han, J., Pop, E., Chai, Y. 2021: 1-7


    The inferior electrical contact to two-dimensional (2D) materials is a critical challenge for their application in post-silicon very large-scale integrated circuits. Electrical contacts were generally related to their resistive effect, quantified as contact resistance. With a systematic investigation, this work demonstrates a capacitive metal-insulator-semiconductor (MIS) field-effect at the electrical contacts to 2D materials: The field-effect depletes or accumulates charge carriers, redistributes the voltage potential, and gives rise to abnormal current saturation and nonlinearity. On one hand, the current saturation hinders the devices' driving ability, which can be eliminated with carefully engineered contact configurations. On the other hand, by introducing the nonlinearity to monolithic analog artificial neural network circuits, the circuits' perception ability can be significantly enhanced, as evidenced using a coronavirus disease 2019 (COVID-19) critical illness prediction model. This work provides a comprehension of the field-effect at the electrical contacts to 2D materials, which is fundamental to the design, simulation, and fabrication of electronics based on 2D materials.Electronic Supplementary Material: Supplementary material (results of the simulation and SEM) is available in the online version of this article at 10.1007/s12274-021-3670-y.

    View details for DOI 10.1007/s12274-021-3670-y

    View details for PubMedID 34336143

  • A Comprehensive Study of WSe2 Crystals Using Correlated Raman, Photoluminescence (PL), Second Harmonic Generation (SHG), and Atomic Force Microscopy (AFM) Imaging SPECTROSCOPY Schmidt, U., Bailey, C. S., Englert, J., Yalon, E., Ankonina, G., Pop, E., Hollricher, O., Dieing, T. 2021; 36 (7): 23-30
  • Graphene-based electromechanical thermal switches 2D MATERIALS Chen, M. E., Rojo, M., Lian, F., Koeln, J., Sood, A., Bohaichuk, S. M., Neumann, C. M., Garrow, S. G., Goodson, K. E., Alleyne, A. G., Pop, E. 2021; 8 (3)
  • High-performance flexible nanoscale transistors based on transition metal dichalcogenides NATURE ELECTRONICS Daus, A., Vaziri, S., Chen, V., Koroglu, C., Grady, R. W., Bailey, C. S., Lee, H., Schauble, K., Brenner, K., Pop, E. 2021
  • Uncovering Phase Change Memory Energy Limits by Sub-Nanosecond Probing of Power Dissipation Dynamics ADVANCED ELECTRONIC MATERIALS Stern, K., Wainstein, N., Keller, Y., Neumann, C. M., Pop, E., Kvatinsky, S., Yalon, E. 2021
  • Spectral decomposition of thermal conductivity: Comparing velocity decomposition methods in homogeneous molecular dynamics simulations PHYSICAL REVIEW B Gabourie, A. J., Fan, Z., Ala-Nissila, T., Pop, E. 2021; 103 (20)
  • Ultrathin Three-Monolayer Tunneling Memory Selectors. ACS nano Wang, C., Chen, V., McClellan, C. J., Tang, A., Vaziri, S., Li, L., Chen, M. E., Pop, E., Wong, H. P. 2021


    High-density memory arrays require selector devices, which enable selection of a specific memory cell within a memory array by suppressing leakage current through unselected cells. Such selector devices must have highly nonlinear current-voltage characteristics and excellent endurance; thus selectors based on a tunneling mechanism present advantages over those based on the physical motion of atoms or ions. Here, we use two-dimensional (2D) materials to build an ultrathin (three-monolayer-thick) tunneling-based memory selector. Using a sandwich of h-BN, MoS2, and h-BN monolayers leads to an "H-shaped" energy barrier in the middle of the heterojunction, which nonlinearly modulates the tunneling current when the external voltage is varied. We experimentally demonstrate that tuning the MoS2 Fermi level can improve the device nonlinearity from 10 to 25. These results provide a fundamental understanding of the tunneling process through atomically thin 2D heterojunctions and lay the foundation for developing high endurance selectors with 2D heterojunctions, potentially enabling high-density non-volatile memory systems.

    View details for DOI 10.1021/acsnano.1c00002

    View details for PubMedID 33944559

  • Carbon nanotube thermoelectric devices by direct printing: Toward wearable energy converters APPLIED PHYSICS LETTERS Lee, H., Furukawa, N., Ricco, A. J., Pop, E., Cui, Y., Nishi, Y. 2021; 118 (17)

    View details for DOI 10.1063/5.0042349

    View details for Web of Science ID 000677695700001

  • High-Performance p-n Junction Transition Metal Dichalcogenide Photovoltaic Cells Enabled by MoOx Doping and Passivation. Nano letters Nassiri Nazif, K., Kumar, A., Hong, J., Lee, N., Islam, R., McClellan, C. J., Karni, O., van de Groep, J., Heinz, T. F., Pop, E., Brongersma, M. L., Saraswat, K. C. 2021


    Layered semiconducting transition metal dichalcogenides (TMDs) are promising materials for high-specific-power photovoltaics due to their excellent optoelectronic properties. However, in practice, contacts to TMDs have poor charge carrier selectivity, while imperfect surfaces cause recombination, leading to a low open-circuit voltage (VOC) and therefore limited power conversion efficiency (PCE) in TMD photovoltaics. Here, we simultaneously address these fundamental issues with a simple MoOx (x 3) surface charge-transfer doping and passivation method, applying it to multilayer tungsten disulfide (WS2) Schottky-junction solar cells with initially near-zero VOC. Doping and passivation turn these into lateral p-n junction photovoltaic cells with a record VOC of 681 mV under AM 1.5G illumination, the highest among all p-n junction TMD solar cells with a practical design. The enhanced VOC also leads to record PCE in ultrathin (<90 nm) WS2 photovoltaics. This easily scalable doping and passivation scheme is expected to enable further advances in TMD electronics and optoelectronics.

    View details for DOI 10.1021/acs.nanolett.1c00015

    View details for PubMedID 33852295

  • High Current Density in Monolayer MoS2 Doped by AlOx. ACS nano McClellan, C. J., Yalon, E., Smithe, K. K., Suryavanshi, S. V., Pop, E. 2021


    Semiconductors require stable doping for applications in transistors, optoelectronics, and thermoelectrics. However, this has been challenging for two-dimensional (2D) materials, where existing approaches are either incompatible with conventional semiconductor processing or introduce time-dependent, hysteretic behavior. Here we show that low-temperature (<200 °C) substoichiometric AlOx provides a stable n-doping layer for monolayer MoS2, compatible with circuit integration. This approach achieves carrier densities >2 * 1013 cm-2, sheet resistance as low as 7 kOmega/□, and good contact resistance 480 Omega·mum in transistors from monolayer MoS2 grown by chemical vapor deposition. We also reach record current density of nearly 700 muA/mum (>110 MA/cm2) along this three-atom-thick semiconductor while preserving transistor on/off current ratio >106. The maximum current is ultimately limited by self-heating (SH) and could exceed 1 mA/mum with better device heat sinking. With their 0.1 nA/mum off-current, such doped MoS2 devices approach several low-power transistor metrics required by the international technology roadmap.

    View details for DOI 10.1021/acsnano.0c09078

    View details for PubMedID 33405894

  • Dynamic Hybrid Metasurfaces. Nano letters Abdollahramezani, S. n., Hemmatyar, O. n., Taghinejad, M. n., Taghinejad, H. n., Kiarashinejad, Y. n., Zandehshahvar, M. n., Fan, T. n., Deshmukh, S. n., Eftekhar, A. A., Cai, W. n., Pop, E. n., El-Sayed, M. A., Adibi, A. n. 2021


    Efficient hybrid plasmonic-photonic metasurfaces that simultaneously take advantage of the potential of both pure metallic and all-dielectric nanoantennas are identified as an emerging technology in flat optics. Nevertheless, postfabrication tunable hybrid metasurfaces are still elusive. Here, we present a reconfigurable hybrid metasurface platform by incorporating the phase-change material Ge2Sb2Te5 (GST) into metal-dielectric meta-atoms for active and nonvolatile tuning of properties of light. We systematically design a reduced-dimension meta-atom, which selectively controls the hybrid plasmonic-photonic resonances of the metasurface via the dynamic change of optical constants of GST without compromising the scattering efficiency. As a proof-of-concept, we experimentally demonstrate two tunable metasurfaces that control the amplitude (with relative modulation depth as high as ≈80%) or phase (with tunability >230°) of incident light promising for high-contrast optical switching and efficient anomalous to specular beam deflection, respectively. Our findings further substantiate dynamic hybrid metasurfaces as compelling candidates for next-generation reprogrammable meta-optics.

    View details for DOI 10.1021/acs.nanolett.0c03625

    View details for PubMedID 33481600

  • Advanced Data Encryption ​using 2D Materials. Advanced materials (Deerfield Beach, Fla.) Wen, C., Li, X., Zanotti, T., Puglisi, F. M., Shi, Y., Saiz, F., Antidormi, A., Roche, S., Zheng, W., Liang, X., Hu, J., Duhm, S., Roldan, J. B., Wu, T., Chen, V., Pop, E., Garrido, B., Zhu, K., Hui, F., Lanza, M. 2021: e2100185


    Advanced data encryption requires the use of true random number generators (TRNGs) to produce unpredictable sequences of bits. TRNG circuits with high degree of randomness and low power consumption may be fabricated by using the random telegraph noise (RTN) current signals produced by polarized metal/insulator/metal (MIM) devices as entropy source. However, the RTN signals produced by MIM devices made of traditional insulators, i.e., transition metal oxides like HfO2 and Al2 O3 , are not stable enough due to the formation and lateral expansion of defect clusters, resulting in undesired current fluctuations and the disappearance of the RTN effect. Here, the fabrication of highly stable TRNG circuits with low power consumption, high degree of randomness (even for a long string of 224  - 1 bits), and high throughput of 1 Mbit s-1 by using MIM devices made of multilayer hexagonal boron nitride (h-BN) is shown. Their application is also demonstrated to produce one-time passwords, which is ideal for the internet-of-everything. The superior stability of the h-BN-based TRNG is related to the presence of few-atoms-wide defects embedded within the layered and crystalline structure of the h-BN stack, which produces a confinement effect that avoids their lateral expansion and results in stable operation.

    View details for DOI 10.1002/adma.202100185

    View details for PubMedID 34046938

  • Engineering Thermal Transport across Layered Graphene-MoS2 Superlattices. ACS nano Sood, A., Sievers, C., Shin, Y. C., Chen, V., Chen, S., Smithe, K. K., Chatterjee, S., Donadio, D., Goodson, K. E., Pop, E. 2021


    Layering two-dimensional van der Waals materials provides a high degree of control over atomic placement, which could enable tailoring of vibrational spectra and heat flow at the sub-nanometer scale. Here, using spatially resolved ultrafast thermoreflectance and spectroscopy, we uncover the design rules governing cross-plane heat transport in superlattices assembled from monolayers of graphene (G) and MoS2 (M). Using a combinatorial experimental approach, we probe nine different stacking sequences, G, GG, MG, GGG, GMG, GGMG, GMGG, GMMG, and GMGMG, and identify the effects of vibrational mismatch, interlayer adhesion, and junction asymmetry on thermal transport. Pure G sequences display evidence of quasi-ballistic transport, whereas adding even a single M layer strongly disrupts heat conduction. The experimental data are described well by molecular dynamics simulations, which include thermal expansion, accounting for the effect of finite temperature on the interlayer spacing. The simulations show that an increase of ∼2.4% in the layer separation of GMGMG, relative to its value at 300 K, can lead to a doubling of the thermal resistance. Using these design rules, we experimentally demonstrate a five-layer GMGMG superlattice "thermal metamaterial" with an ultralow effective cross-plane thermal conductivity comparable to that of air.

    View details for DOI 10.1021/acsnano.1c06299

    View details for PubMedID 34813267

  • Diamond Integration on GaN for Channel Temperature Reduction Malakoutian, M., Xu, R., Ren, C., Pasayat, S., Sayed, I., Pop, E., Chowdhury, S., IEEE IEEE. 2021: 70-74
  • Sub-200 m Alloyed Contacts to Synthetic Monolayer MoS2 Kumar, A., Schauble, K., Neilson, K. M., Tang, A., Ramesh, P., Wong, H., Pop, E., Saraswat, K., IEEE IEEE. 2021
  • Ultralow-switching current density multilevel phase-change memory on a flexible substrate. Science (New York, N.Y.) Khan, A. I., Daus, A., Islam, R., Neilson, K. M., Lee, H. R., Wong, H. P., Pop, E. 2021; 373 (6560): 1243-1247


    [Figure: see text].

    View details for DOI 10.1126/science.abj1261

    View details for PubMedID 34516795

  • Uncovering Thermal and Electrical Properties of Sb2Te3/GeTe Superlattice Films. Nano letters Kwon, H., Khan, A. I., Perez, C., Asheghi, M., Pop, E., Goodson, K. E. 2021


    Superlattice-like phase change memory (SL-PCM) promises lower switching current than conventional PCM based on Ge2Sb2Te5 (GST); however, a fundamental understanding of SL-PCM requires detailed characterization of the interfaces within such an SL. Here we explore the electrical and thermal transport of SLs with deposited Sb2Te3 and GeTe alternating layers of various thicknesses. We find up to an approximately four-fold reduction of the effective cross-plane thermal conductivity of the SL stack (as-deposited polycrystalline) compared with polycrystalline GST (as-deposited amorphous and later annealed) due to the thermal interface resistances within the SL. Thermal measurements with varying periods of our SLs show a signature of phonon coherence with a transition from wave-like to particle-like phonon transport, further described by our modeling. Electrical resistivity measurements of such SLs reveal strong anisotropy (∼2000×) between the in-plane and cross-plane directions due to the weakly interacting van der Waals-like gaps. This work uncovers electrothermal transport in SLs based on Sb2Te3 and GeTe for the improved design of low-power PCM.

    View details for DOI 10.1021/acs.nanolett.1c00947

    View details for PubMedID 34270270

  • Reduced thermal conductivity of supported and encased monolayer and bilayer MoS2 2D MATERIALS Gabourie, A. J., Suryavanshi, S., Farimani, A., Pop, E. 2021; 8 (1)
  • Tuning electrical and interfacial thermal properties of bilayer MoS2 via electrochemical intercalation. Nanotechnology Xiong, F. n., Yalon, E. n., McClellan, C. n., Zhang, J. n., Aslan, O. B., Sood, A. n., Sun, J. n., Andolina, C. M., Al-Saidi, W. A., Goodson, K. E., Heinz, T. n., Cui, Y. n., Pop, E. n. 2021


    Layered two-dimensional (2D) materials such as MoS2 have attracted much attention for nano- and opto-electronics. Recently, intercalation (e.g. of ions, atoms, or molecules) has emerged as an effective technique to reversibly modulate material properties of such layered 2D films. Here we probe both electrical and thermal properties of Li-intercalated bilayer MoS2 nanosheets by combined electrical measurements and Raman spectroscopy. We demonstrate reversible modulation of carrier density over more than two orders of magnitude (from 0.8×1012 cm 2 to 1.5×1014 cm-2), and we simultaneously obtain the thermal boundary conduct-ance (TBC) between the bilayer and its supporting SiO2 substrate for an intercalated system for the first time. This thermal coupling can be reversibly modulated by nearly a factor of eight, from 14 ± 4.0 MWm-2K-1 before intercalation to 1.8 ± 0.9 MWm 2K-1 when the MoS2 is fully lithiated. These results reveal electrochemical intercalation as a reversible tool to modulate and control both electrical and thermal properties of 2D layers.

    View details for DOI 10.1088/1361-6528/abe78a

    View details for PubMedID 33601363

  • Two-Fold Reduction of Switching Current Density in Phase Change Memory Using Bi2Te3 Thermoelectric Interfacial Layer IEEE ELECTRON DEVICE LETTERS Khan, A., Kwon, H., Islam, R., Perez, C., Chen, M. E., Asheghi, M., Goodson, K. E., Wong, H., Pop, E. 2020; 41 (11): 1657–60
  • Ultrahigh Doping of Graphene Using Flame-Deposited MoO3 IEEE ELECTRON DEVICE LETTERS Vaziri, S., Chen, V., Cai, L., Jiang, Y., Chen, M. E., Grady, R. W., Zheng, X., Pop, E. 2020; 41 (10): 1592–95
  • Visualizing Energy Transfer at Buried Interfaces in Layered Materials Using Picosecond X-Rays ADVANCED FUNCTIONAL MATERIALS Nyby, C., Sood, A., Zalden, P., Gabourie, A. J., Muscher, P., Rhodes, D., Mannebach, E., Corbett, J., Mehta, A., Pop, E., Heinz, T. F., Lindenberg, A. M. 2020
  • Nonvolatile Electrically Reconfigurable Integrated Photonic Switch Enabled by a Silicon PIN Diode Heater. Advanced materials (Deerfield Beach, Fla.) Zheng, J., Fang, Z., Wu, C., Zhu, S., Xu, P., Doylend, J. K., Deshmukh, S., Pop, E., Dunham, S., Li, M., Majumdar, A. 2020: e2001218


    Reconfigurability of photonic integrated circuits (PICs) has become increasingly important due to the growing demands for electronic-photonic systems on a chip driven by emerging applications, including neuromorphic computing, quantum information, and microwave photonics. Success in these fields usually requires highly scalable photonic switching units as essential building blocks. Current photonic switches, however, mainly rely on materials with weak, volatile thermo-optic or electro-optic modulation effects, resulting in large footprints and high energy consumption. As a promising alternative, chalcogenide phase-change materials (PCMs) exhibit strong optical modulation in a static, self-holding fashion, but the scalability of present PCM-integrated photonic applications is still limited by the poor optical or electrical actuation approaches. Here, with phase transitions actuated by in situ silicon PIN diode heaters, scalable nonvolatile electrically reconfigurable photonic switches using PCM-clad silicon waveguides and microring resonators are demonstrated. As a result, intrinsically compact and energy-efficient switching units operated with low driving voltages, near-zero additional loss, and reversible switching with high endurance are obtained in a complementary metal-oxide-semiconductor (CMOS)-compatible process. This work can potentially enable very large-scale CMOS-integrated programmable electronic-photonic systems such as optical neural networks and general-purpose integrated photonic processors.

    View details for DOI 10.1002/adma.202001218

    View details for PubMedID 32588481

  • VO2 Switch for Electrostatic Discharge Protection IEEE ELECTRON DEVICE LETTERS Bohaichuk, S. M., Pelella, M. M., Sun, Y., Zhang, Z., Ramanathan, S., Pop, E. 2020; 41 (2): 292–95
  • Stacking Independence and Resonant Interlayer Excitation of Monolayer WSe2/MoSe2 Heterostructures for Photocatalytic Energy Conversion ACS APPLIED NANO MATERIALS Chen, J., Bailey, C., Cui, D., Wang, Y., Wang, B., Shi, H., Cai, Z., Pop, E., Zhou, C., Cronin, S. B. 2020; 3 (2): 1175–81
  • Flexible Low-Power Superlattice-Like Phase Change Memory Khan, A., Daus, A., Pop, E., IEEE IEEE. 2020
  • Improved Current Density and Contact Resistance in Bilayer MoSe2 Field Effect Transistors by AlO x Capping. ACS applied materials & interfaces Somvanshi, D. n., Ber, E. n., Bailey, C. S., Pop, E. n., Yalon, E. n. 2020; 12 (32): 36355–61


    Atomically thin semiconductors are of interest for future electronics applications, and much attention has been given to monolayer (1L) sulfides, such as MoS2, grown by chemical vapor deposition (CVD). However, reports on the electrical properties of CVD-grown selenides, and MoSe2 in particular, are scarce. Here, we compare the electrical properties of 1L and bilayer (2L) MoSe2 grown by CVD and capped by sub-stoichiometric AlO x . The 2L channels exhibit ∼20× lower contact resistance (RC) and ∼30× larger current density compared with 1L channels. RC is further reduced by >5× with AlO x capping, which enables improved transistor current density. Overall, 2L AlO x -capped MoSe2 transistors (with ∼500 nm channel length) achieve improved current density (∼65 μA/μm at VDS = 4 V), a good Ion/Ioff ratio of >106, and an RC of ∼60 kΩ·μm. The weaker performance of 1L devices is due to their sensitivity to processing and ambient. Our results suggest that 2L (or few layers) is preferable to 1L for improved electronic properties in applications that do not require a direct band gap, which is a key finding for future two-dimensional electronics.

    View details for DOI 10.1021/acsami.0c09541

    View details for PubMedID 32678569

  • Large temperature coefficient of resistance in atomically thin two-dimensional semiconductors Applied Physics Letters Khan, A., Khakbaz, P., Brenner, K. A., Smithe, K., Mleczko, M. J., Esseni, D., Pop, E. 2020; 116 (20)

    View details for DOI 10.1063/5.0003312

  • Ultra-scaled MoS2 transistors and circuits fabricated without nanolithography 2D MATERIALS Patel, K., Grady, R. W., Smithe, K. H., Pop, E., Sordan, R. 2020; 7 (1)
  • Nonvolatile Electrically Reconfigurable Integrated Photonic Switches Using Phase-Change Materials Zheng, J., Fang, Z., Wu, C., Zhu, S., Xu, P., Doylend, J. K., Deshmukh, S., Pop, E., Dunham, S., Li, M., Majumdar, A., IEEE IEEE. 2020
  • Highly confined plasmons in individual single-walled carbon nanotube nanoantennas Yu, S., Roberts, J., Lin, Q., Bohaichuk, S., Luo, Y., Choi, Y., Ho, P., Lee, K., Falk, A. L., Wilson, W. L., Pop, E., Wong, H., Fan, J. A., IEEE IEEE. 2020
  • Phase Change Material Integrated Silicon Photonics: GST and Beyond Fang, Z., Zheng, J., Xu, P., Deshmukh, S., Pop, E., Majumdar, A., Jiang, S., Digonnet, M. J. SPIE-INT SOC OPTICAL ENGINEERING. 2020

    View details for DOI 10.1117/12.2548309

    View details for Web of Science ID 000568488700001

  • Uncovering the Effects of Metal Contacts on Monolayer MoS2. ACS nano Schauble, K. n., Zakhidov, D. n., Yalon, E. n., Deshmukh, S. n., Grady, R. W., Cooley, K. A., McClellan, C. J., Vaziri, S. n., Passarello, D. n., Mohney, S. E., Toney, M. F., Sood, A. K., Salleo, A. n., Pop, E. n. 2020


    Metal contacts are a key limiter to the electronic performance of two-dimensional (2D) semiconductor devices. Here, we present a comprehensive study of contact interfaces between seven metals (Y, Sc, Ag, Al, Ti, Au, Ni, with work functions from 3.1 to 5.2 eV) and monolayer MoS2 grown by chemical vapor deposition. We evaporate thin metal films onto MoS2 and study the interfaces by Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, and electrical characterization. We uncover that (1) ultrathin oxidized Al dopes MoS2n-type (>2 × 1012 cm-2) without degrading its mobility, (2) Ag, Au, and Ni deposition causes varying levels of damage to MoS2 (e.g. broadening Raman E' peak from <3 to >6 cm-1), and (3) Ti, Sc, and Y react with MoS2. Reactive metals must be avoided in contacts to monolayer MoS2, but control studies reveal the reaction is mostly limited to the top layer of multilayer films. Finally, we find that (4) thin metals do not significantly strain MoS2, as confirmed by X-ray diffraction. These are important findings for metal contacts to MoS2 and broadly applicable to many other 2D semiconductors.

    View details for DOI 10.1021/acsnano.0c03515

    View details for PubMedID 32905703

  • Localized Heating and Switching in MoTe2-Based Resistive Memory Devices. Nano letters Datye, I. M., Rojo, M. M., Yalon, E. n., Deshmukh, S. n., Mleczko, M. J., Pop, E. n. 2020


    Two-dimensional (2D) materials have recently been incorporated into resistive memory devices because of their atomically thin nature, but their switching mechanism is not yet well understood. Here we study bipolar switching in MoTe2-based resistive memory of varying thickness and electrode area. Using scanning thermal microscopy (SThM), we map the surface temperature of the devices under bias, revealing clear evidence of localized heating at conductive "plugs" formed during switching. The SThM measurements are correlated to electro-thermal simulations, yielding a range of plug diameters (250 to 350 nm) and temperatures at constant bias and during switching. Transmission electron microscopy images reveal these plugs result from atomic migration between electrodes, which is a thermally-activated process. However, the initial forming may be caused by defect generation or Te migration within the MoTe2. This study provides the first thermal and localized switching insights into the operation of such resistive memory and demonstrates a thermal microscopy technique that can be applied to a wide variety of traditional and emerging memory devices.

    View details for DOI 10.1021/acs.nanolett.9b05272

    View details for PubMedID 31951419

  • Monolithic mtesla-level magnetic induction by self-rolled-up membrane technology. Science advances Huang, W., Yang, Z., Kraman, M. D., Wang, Q., Ou, Z., Rojo, M. M., Yalamarthy, A. S., Chen, V., Lian, F., Ni, J. H., Liu, S., Yu, H., Sang, L., Michaels, J., Sievers, D. J., Eden, J. G., Braun, P. V., Chen, Q., Gong, S., Senesky, D. G., Pop, E., Li, X. 2020; 6 (3): eaay4508


    Monolithic strong magnetic induction at the mtesla to tesla level provides essential functionalities to physical, chemical, and medical systems. Current design options are constrained by existing capabilities in three-dimensional (3D) structure construction, current handling, and magnetic material integration. We report here geometric transformation of large-area and relatively thick (~100 to 250 nm) 2D nanomembranes into multiturn 3D air-core microtubes by a vapor-phase self-rolled-up membrane (S-RuM) nanotechnology, combined with postrolling integration of ferrofluid magnetic materials by capillary force. Hundreds of S-RuM power inductors on sapphire are designed and tested, with maximum operating frequency exceeding 500 MHz. An inductance of 1.24 muH at 10 kHz has been achieved for a single microtube inductor, with corresponding areal and volumetric inductance densities of 3 muH/mm2 and 23 muH/mm3, respectively. The simulated intensity of the magnetic induction reaches tens of mtesla in fabricated devices at 10 MHz.

    View details for DOI 10.1126/sciadv.aay4508

    View details for PubMedID 32010770

  • Thermal conductivity of crystalline AlN and the influence of atomic-scale defects JOURNAL OF APPLIED PHYSICS Xu, R., Rojo, M., Islam, S. M., Sood, A., Vareskic, B., Katre, A., Mingo, N., Goodson, K. E., Xing, H., Jena, D., Pop, E. 2019; 126 (18)

    View details for DOI 10.1063/1.5097172

    View details for Web of Science ID 000504002600023

  • Temperature-Dependent Contact Resistance to Nonvolatile Memory Materials IEEE TRANSACTIONS ON ELECTRON DEVICES Deshmukh, S., Yalon, E., Lian, F., Schauble, K. E., Xiong, F., Karpov, I. V., Pop, E. 2019; 66 (9): 3816–21
  • Layer-Dependent Interfacial Transport and Optoelectrical Properties of MoS2 on Ultraflat Metals ACS APPLIED MATERIALS & INTERFACES Lee, H., Deshmukh, S., Wen, J., Costa, V. Z., Schuder, J. S., Sanchez, M., Ichimura, A. S., Pop, E., Wang, B., Newaz, A. M. 2019; 11 (34): 31543–50


    Layered materials based on transition-metal dichalcogenides (TMDs) are promising for a wide range of electronic and optoelectronic devices. Realizing such practical applications often requires metal-TMD connections or contacts. Hence, a complete understanding of electronic band alignments and potential barrier heights governing the transport through metal-TMD junctions is critical. However, it is presently unclear how the energy bands of a TMD align while in contact with a metal as a function of the number of layers. In pursuit of removing this knowledge gap, we have performed conductive atomic force microscopy (CAFM) of few-layered (1 to 5 layers) MoS2 immobilized on ultraflat conducting Au surfaces [root-mean-square (rms) surface roughness < 0.2 nm] and indium-tin oxide (ITO) substrates (rms surface roughness < 0.7 nm) forming a vertical metal (CAFM tip)-semiconductor-metal device. We have observed that the current increases with the number of layers up to five layers. By applying Fowler-Nordheim tunneling theory, we have determined the barrier heights for different layers and observed how this barrier decreases as the number of layers increases. Using density functional theory calculations, we successfully demonstrated that the barrier height decreases as the layer number increases. By illuminating TMDs on a transparent ultraflat conducting ITO substrate, we observed a reduction in current when compared to the current measured in the dark, hence demonstrating negative photoconductivity. Our study provides a fundamental understanding of the local electronic and optoelectronic behaviors of the TMD-metal junction, which depends on the numbers of TMD layers and may pave an avenue toward developing nanoscale electronic devices with tailored layer-dependent transport properties.

    View details for DOI 10.1021/acsami.9b09868

    View details for Web of Science ID 000484073400113

    View details for PubMedID 31364836

  • Localized Triggering of the Insulator-Metal Transition in VO2 Using a Single Carbon Nanotube. ACS nano Bohaichuk, S. M., Munoz Rojo, M., Pitner, G., McClellan, C. J., Lian, F., Li, J., Jeong, J., Samant, M. G., Parkin, S. S., Wong, H. P., Pop, E. 2019


    Vanadium dioxide (VO2) has been widely studied for its rich physics and potential applications, undergoing a prominent insulator-metal transition (IMT) near room temperature. The transition mechanism remains highly debated, and little is known about the IMT at nanoscale dimensions. To shed light on this problem, here we use 1 nm-wide carbon nanotube (CNT) heaters to trigger the IMT in VO2. Single metallic CNTs switch the adjacent VO2 at less than half the voltage and power required by control devices without a CNT, with switching power as low as 85 muW at 300 nm device lengths. We also obtain potential and temperature maps of devices during operation using Kelvin probe microscopy and scanning thermal microscopy. Comparing these with three-dimensional electrothermal simulations, we find that the local heating of the VO2 by the CNT plays a key role in the IMT. These results demonstrate the ability to trigger IMT in VO2 using nanoscale heaters and highlight the significance of thermal engineering to improve device behavior.

    View details for DOI 10.1021/acsnano.9b03397

    View details for PubMedID 31393698

  • Thermal boundary conductance of two-dimensional MoS2 interfaces JOURNAL OF APPLIED PHYSICS Suryavanshi, S., Gabourie, A. J., Farimani, A., Pop, E. 2019; 126 (5)

    View details for DOI 10.1063/1.5092287

    View details for Web of Science ID 000484363900002

  • Ultrahigh thermal isolation across heterogeneously layered two-dimensional materials. Science advances Vaziri, S., Yalon, E., Munoz Rojo, M., Suryavanshi, S. V., Zhang, H., McClellan, C. J., Bailey, C. S., Smithe, K. K., Gabourie, A. J., Chen, V., Deshmukh, S., Bendersky, L., Davydov, A. V., Pop, E. 2019; 5 (8): eaax1325


    Heterogeneous integration of nanomaterials has enabled advanced electronics and photonics applications. However, similar progress has been challenging for thermal applications, in part due to shorter wavelengths of heat carriers (phonons) compared to electrons and photons. Here, we demonstrate unusually high thermal isolation across ultrathin heterostructures, achieved by layering atomically thin two-dimensional (2D) materials. We realize artificial stacks of monolayer graphene, MoS2, and WSe2 with thermal resistance greater than 100 times thicker SiO2 and effective thermal conductivity lower than air at room temperature. Using Raman thermometry, we simultaneously identify the thermal resistance between any 2D monolayers in the stack. Ultrahigh thermal isolation is achieved through the mismatch in mass density and phonon density of states between the 2D layers. These thermal metamaterials are an example in the emerging field of phononics and could find applications where ultrathin thermal insulation is desired, in thermal energy harvesting, or for routing heat in ultracompact geometries.

    View details for DOI 10.1126/sciadv.aax1325

    View details for PubMedID 31453337

  • Strain- and Strain-Rate-Invariant Conductance in a Stretchable and Compressible 3D Conducting Polymer Foam MATTER Chen, G., Rastak, R., Wang, Y., Yan, H., Feig, V., Liu, Y., Jiang, Y., Chen, S., Lian, F., Molina-Lopez, F., Jin, L., Cui, K., Chung, J., Pop, E., Linder, C., Bao, Z. 2019; 1 (1): 205–18
  • Significant Phonon Drag Enables High Power Factor in the AlGaN/GaN Two-Dimensional Electron Gas. Nano letters Yalamarthy, A. S., Munoz Rojo, M., Bruefach, A., Boone, D., Dowling, K. M., Satterthwaite, P. F., Goldhaber-Gordon, D., Pop, E., Senesky, D. G. 2019


    In typical thermoelectric energy harvesters and sensors, the Seebeck effect is caused by diffusion of electrons or holes in a temperature gradient. However, the Seebeck effect can also have a phonon drag component, due to momentum exchange between charge carriers and lattice phonons, which is more difficult to quantify. Here, we present the first study of phonon drag in the AlGaN/GaN two-dimensional electron gas (2DEG). We find that phonon drag does not contribute significantly to the thermoelectric behavior of devices with 100 nm GaN thickness, which suppresses the phonon mean free path. However, when the thickness is increased to 1.2 mum, up to 32% (88%) of the Seebeck coefficient at 300 K (50 K) can be attributed to the drag component. In turn, the phonon drag enables state-of-the-art thermoelectric power factor in the thicker GaN film, up to 40 mW m-1 K-2 at 50 K. By measuring the thermal conductivity of these AlGaN/GaN films, we show that the magnitude of the phonon drag can increase even when the thermal conductivity decreases. Decoupling of thermal conductivity and Seebeck coefficient could enable important advancements in thermoelectric power conversion with devices based on 2DEGs.

    View details for DOI 10.1021/acs.nanolett.9b00901

    View details for PubMedID 31088057

  • Understanding the switching mechanism of interfacial phase change memory JOURNAL OF APPLIED PHYSICS Okabe, K. L., Sood, A., Yalon, E., Neumann, C. M., Asheghi, M., Pop, E., Goodson, K. E., Wong, H. 2019; 125 (18)

    View details for DOI 10.1063/1.5093907

    View details for Web of Science ID 000470151800040

  • Quasi-Ballistic Thermal Transport Across MoS2 Thin Films NANO LETTERS Sood, A., Xiong, F., Chen, S., Cheaito, R., Lian, F., Asheghi, M., Cui, Y., Donadio, D., Goodson, K. E., Pop, E. 2019; 19 (4): 2434–42
  • Strongly tunable anisotropic thermal transport in MoS2 by strain and lithium intercalation: first-principles calculations 2D MATERIALS Chen, S., Sood, A., Pop, E., Goodson, K. E., Donadio, D. 2019; 6 (2)
  • Quasi-Ballistic Thermal Transport Across MoS2 Thin Films. Nano letters Sood, A., Xiong, F., Chen, S., Cheaito, R., Lian, F., Asheghi, M., Cui, Y., Donadio, D., Goodson, K. E., Pop, E. 2019


    Layered two-dimensional (2D) materials have highly anisotropic thermal properties between the in-plane and cross-plane directions. Conventionally, it is thought that cross-plane thermal conductivities (kappa z) are low, and therefore c-axis phonon mean free paths (MFPs) are small. Here, we measure kappa z across MoS2 films of varying thickness (20-240 nm) and uncover evidence of very long c-axis phonon MFPs at room temperature in these layered semiconductors. Experimental data obtained using time-domain thermoreflectance (TDTR) are in good agreement with first-principles density functional theory (DFT). These calculations suggest that 50% of the heat is carried by phonons with MFP > 200 nm, exceeding kinetic theory estimates by nearly 2 orders of magnitude. Because of quasi-ballistic effects, the kappa z of nanometer-thin films of MoS2 scales with theirthickness and the volumetric thermal resistance asymptotes to a nonzero value, 10 m2 K GW-1. This contributes as much as 30% to the total thermal resistance of a 20 nm thick film, the rest being limited by thermal interface resistance with the SiO2 substrate and top-side aluminum transducer. These findings are essential for understanding heat flow across nanometer-thin films of MoS2 for optoelectronic and thermoelectric applications.

    View details for PubMedID 30808167

  • Thermal transport in layer-by-layer assembled polycrystalline graphene films NPJ 2D MATERIALS AND APPLICATIONS Estrada, D., Li, Z., Choi, G., Dunham, S. N., Serov, A., Lee, J., Meng, Y., Lian, F., Wang, N. C., Perez, A., Haasch, R. T., Zuo, J., King, W. P., Rogers, J. A., Cahill, D. G., Pop, E. 2019; 3
  • Energy-Efficient Indirectly Heated Phase Change RF Switch IEEE ELECTRON DEVICE LETTERS Yalon, E., Datye, I. M., Moon, J., Son, K., Lee, K., Pop, E. 2019; 40 (3): 455–58
  • Plasmon-Resonant Enhancement of Photocatalysis on Monolayer WSe2 ACS PHOTONICS Chen, J., Bailey, C. S., Hong, Y., Wang, L., Cai, Z., Shen, L., Hou, B., Wang, Y., Shi, H., Sambur, J., Ren, W., Pop, E., Cronin, S. B. 2019; 6 (3): 787–92
  • Ternary content-addressable memory with MoS2 transistors for massively parallel data search NATURE ELECTRONICS Yang, R., Li, H., Smithe, K. H., Kim, T. R., Okabe, K., Pop, E., Fan, J. A., Wong, H. 2019; 2 (3): 108–14
  • Engineering thermal and electrical interface properties of phase change memory with monolayer MoS2 APPLIED PHYSICS LETTERS Neumann, C. M., Okabe, K. L., Yalon, E., Grady, R. W., Wong, H., Pop, E. 2019; 114 (8)

    View details for DOI 10.1063/1.5080959

    View details for Web of Science ID 000460134000016

  • Thermal transport in MoS2 from molecular dynamics using different empirical potentials PHYSICAL REVIEW B Xu, K., Gabourie, A. J., Hashemi, A., Fan, Z., Wei, N., Farimani, A., Komsa, H., Krasheninnikov, A., Pop, E., Ala-Nissila, T. 2019; 99 (5)
  • Spatial Separation of Carrier Spin by the Valley Hall Effect in Monolayer WSe2 Transistors. Nano letters Barre, E., Incorvia, J. A., Kim, S. H., McClellan, C. J., Pop, E., Wong, H. P., Heinz, T. F. 2019


    We investigate the valley Hall effect (VHE) in monolayer WSe2 field-effect transistors using optical Kerr rotation measurements at 20 K. While studies of the VHE have so far focused on n -doped MoS2, we observe the VHE in WSe2 in both the n - and p -doping regimes. Hole doping enables access to the large spin-splitting of the valence band of this material. The Kerr rotation measurements probe the spatial distribution of the valley carrier imbalance induced by the VHE. Under current flow, we observe distinct spin-valley polarization along the edges of the transistor channel. From analysis of the magnitude of the Kerr rotation, we infer a spin-valley density of 44 spins/mum, integrated over the edge region in the p -doped regime. Assuming a spin diffusion length less than 0.1 mum, this corresponds to a spin-valley polarization of the holes exceeding 1%.

    View details for PubMedID 30601667

  • Publisher Correction: An electrochemical thermal transistor. Nature communications Sood, A. n., Xiong, F. n., Chen, S. n., Wang, H. n., Selli, D. n., Zhang, J. n., McClellan, C. J., Sun, J. n., Donadio, D. n., Cui, Y. n., Pop, E. n., Goodson, K. E. 2019; 10 (1): 4465


    An amendment to this paper has been published and can be accessed via a link at the top of the paper.

    View details for DOI 10.1038/s41467-019-12471-4

    View details for PubMedID 31562331

  • Fast Spiking of a Mott VO2-Carbon Nanotube Composite Device. Nano letters Bohaichuk, S. M., Kumar, S. n., Pitner, G. n., McClellan, C. J., Jeong, J. n., Samant, M. G., Wong, H. P., Parkin, S. S., Williams, R. S., Pop, E. n. 2019


    The recent surge of interest in brain-inspired computing and power-efficient electronics has dramatically bolstered development of computation and communication using neuron-like spiking signals. Devices that can produce rapid and energy-efficient spiking could significantly advance these applications. Here we demonstrate direct current or voltage-driven periodic spiking with sub-20 ns pulse widths from a single device composed of a thin VO2 film with a metallic carbon nanotube as a nanoscale heater, without using an external capacitor. Compared with VO2-only devices, adding the nanotube heater dramatically decreases the transient duration and pulse energy, and increases the spiking frequency, by up to 3 orders of magnitude. This is caused by heating and cooling of the VO2 across its insulator-metal transition being localized to a nanoscale conduction channel in an otherwise bulk medium. This result provides an important component of energy-efficient neuromorphic computing systems and a lithography-free technique for energy-scaling of electronic devices that operate via bulk mechanisms.

    View details for DOI 10.1021/acs.nanolett.9b01554

    View details for PubMedID 31433663

  • Contact Engineering High-Performance n-Type MoTe2 Transistors. Nano letters Mleczko, M. J., Yu, A. C., Smyth, C. M., Chen, V. n., Shin, Y. C., Chatterjee, S. n., Tsai, Y. C., Nishi, Y. n., Wallace, R. M., Pop, E. n. 2019


    Semiconducting MoTe2 is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains underexplored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe2 transistors with the highest performance to date, including high saturation current (>400 μA/μm at 80 K and >200 μA/μm at 300 K) and relatively low contact resistance (1.2 to 2 kΩ·μm from 80 to 300 K), achieved with Ag contacts and AlO x encapsulation. We also investigate other contact metals (Sc, Ti, Cr, Au, Ni, Pt), extracting their Schottky barrier heights using an analytic subthreshold model. High-resolution X-ray photoelectron spectroscopy reveals that interfacial metal-Te compounds dominate the contact resistance. Among the metals studied, Sc has the lowest work function but is the most reactive, which we counter by inserting monolayer hexagonal boron nitride between MoTe2 and Sc. These metal-insulator-semiconductor (MIS) contacts partly depin the metal Fermi level and lead to the smallest Schottky barrier for electron injection. Overall, this work improves our understanding of n-type contacts to 2D materials, an important advance for low-power electronics.

    View details for DOI 10.1021/acs.nanolett.9b02497

    View details for PubMedID 31314531

  • Reconfigurable Infrared Spectral Imaging with Robust Phase Change Materials Moon, J., Seo, H., Son, K., Yalon, E., Lee, K., Flores, E., Candia, G., Pop, E., George, T., Islam, M. S. SPIE-INT SOC OPTICAL ENGINEERING. 2019

    View details for DOI 10.1117/12.2519492

    View details for Web of Science ID 000484733200015

  • Dry Transfer of van der Waals Crystals to Noble Metal Surfaces To Enable Characterization of Buried Interfaces. ACS applied materials & interfaces Krayev, A. n., Bailey, C. S., Jo, K. n., Wang, S. n., Singh, A. n., Darlington, T. n., Liu, G. Y., Gradecak, S. n., Schuck, P. J., Pop, E. n., Jariwala, D. n. 2019


    Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have been explored for many optoelectronic applications. Most of these applications require them to be on insulating substrates. However, for many fundamental property characterizations, such as mapping surface potential or conductance, insulating substrates are nonideal as they lead to charging and doping effects or impose the inhomogeneity of their charge environment on the atomically thin 2D layers. Here, we report a simple method of residue-free dry transfer of 2D TMDC crystal layers. This method is enabled via noble-metal (gold, silver) thin films and allows comprehensive nanoscale characterization of transferred TMDC crystals with multiple scanning probe microscopy techniques. In particular, intimate contact with underlying metal allows efficient tip-enhanced Raman scattering characterization, providing high spatial resolution (<20 nm) for Raman spectroscopy. Further, scanning Kelvin probe force microscopy allows high-resolution mapping of surface potential on transferred crystals, revealing their spatially varying structural and electronic properties. The layer-dependent contact potential difference is clearly observed and explained by charge transfer from contacts with Au and Ag. The demonstrated sample preparation technique can be generalized to probe many different 2D material surfaces and has broad implications in understanding of the metal contacts and buried interfaces in 2D material-based devices.

    View details for DOI 10.1021/acsami.9b09798

    View details for PubMedID 31512847

  • Reduction of hysteresis in MoS2 transistors using pulsed voltage measurements 2D MATERIALS Datye, I. M., Gabourie, A. J., English, C. D., Smithe, K. H., McClellan, C. J., Wang, N. C., Pop, E. 2019; 6 (1)
  • Thermal transport across graphene step junctions 2D MATERIALS Rojo, M., Li, Z., Sievers, C., Bornstein, A. C., Yalon, E., Deshmukh, S., Vaziri, S., Bae, M., Xiong, F., Donadio, D., Pop, E. 2019; 6 (1)
  • Recommended Methods to Study Resistive Switching Devices ADVANCED ELECTRONIC MATERIALS Lanza, M., Wong, H., Pop, E., Ielmini, D., Strukov, D., Regan, B. C., Larcher, L., Villena, M. A., Yang, J., Goux, L., Belmonte, A., Yang, Y., Puglisi, F. M., Kang, J., Magyari-Kope, B., Yalon, E., Kenyon, A., Buckwell, M., Mehonic, A., Shluger, A., Li, H., Hou, T., Hudec, B., Akinwande, D., Ge, R., Ambrogio, S., Roldan, J. B., Miranda, E., Sune, J., Pey, K., Wu, X., Raghavan, N., Wu, E., Lu, W. D., Navarro, G., Zhang, W., Wu, H., Li, R., Holleitner, A., Wurstbauer, U., Lemme, M. C., Liu, M., Long, S., Liu, Q., Lv, H., Padovani, A., Pavan, P., Valov, I., Jing, X., Han, T., Zhu, K., Chen, S., Hui, F., Shi, Y. 2019; 5 (1)
  • Nanoelectronics and Heterogeneous Integration with 2D Materials Pop, E., IEEE IEEE. 2019
  • Process-induced anomalous current transport in graphene/InAlN/GaN heterostructured diodes Satterthwaite, P. F., Yalamarthy, A., Vaziri, S., Rojo, M., Pop, E., Senesky, D. G., IEEE IEEE. 2019
  • 3D Heterogeneous Integration with 2D Materials McClellan, C., Bailey, C., Datye, I., Gabourie, A., Grady, R., Schauble, K., Vaziri, S., Pop, E., IEEE IEEE. 2019: 89–90
  • An electrochemical thermal transistor NATURE COMMUNICATIONS Sood, A., Xiong, F., Chen, S., Wang, H., Selli, D., Zhang, J., McClellan, C. J., Sun, J., Donadio, D., Cui, Y., Pop, E., Goodson, K. E. 2018; 9
  • An electrochemical thermal transistor. Nature communications Sood, A., Xiong, F., Chen, S., Wang, H., Selli, D., Zhang, J., McClellan, C. J., Sun, J., Donadio, D., Cui, Y., Pop, E., Goodson, K. E. 2018; 9 (1): 4510


    The ability to actively regulate heat flow at the nanoscale could be a game changer for applications in thermal management and energy harvesting. Such a breakthrough could also enable the control of heat flow using thermal circuits, in a manner analogous to electronic circuits. Here we demonstrate switchable thermal transistors with an order of magnitude thermal on/off ratio, based on reversible electrochemical lithium intercalation in MoS2 thin films. We use spatially-resolved time-domain thermoreflectance to map the lithium ion distribution during device operation, and atomic force microscopy to show that the lithiated state correlates with increased thickness and surface roughness. First principles calculations reveal that the thermal conductance modulation is due to phonon scattering by lithium rattler modes, c-axis strain, and stacking disorder. This study lays the foundation for electrochemically-driven nanoscale thermal regulators, and establishes thermal metrology as a useful probe of spatio-temporal intercalant dynamics in nanomaterials.

    View details for PubMedID 30375375

  • Research Update: Recent progress on 2D materials beyond graphene: From ripples, defects, intercalation, and valley dynamics to straintronics and power dissipation APL MATERIALS Lin, Z., Lei, Y., Subramanian, S., Briggs, N., Wang, Y., Lo, C., Yalon, E., Lloyd, D., Wu, S., Koski, K., Clark, R., Das, S., Wallace, R. M., Kuech, T., Bunch, J., Li, X., Chen, Z., Pop, E., Crespi, V. H., Robinson, J. A., Terrones, M. 2018; 6 (8)

    View details for DOI 10.1063/1.5042598

    View details for Web of Science ID 000443756800001

  • Electronic synapses made of layered two-dimensional materials NATURE ELECTRONICS Shi, Y., Liang, X., Yuan, B., Chen, V., Li, H., Hui, F., Yu, Z., Yuan, F., Pop, E., Wong, H., Lanza, M. 2018; 1 (8): 458–65
  • High-Field Transport and Velocity Saturation in Synthetic Monolayer MoS2 NANO LETTERS Smithe, K. H., English, C. D., Suryavanshi, S. V., Pop, E. 2018; 18 (7): 4516–22


    Two-dimensional semiconductors such as monolayer MoS2 are of interest for future applications including flexible electronics and end-of-roadmap technologies. Most research to date has focused on low-field mobility, but the peak current-driving ability of transistors is limited by the high-field saturation drift velocity, vsat. Here, we measure high-field transport as a function of temperature for the first time in high-quality synthetic monolayer MoS2. We find that in typical device geometries (e.g. on SiO2 substrates) self-heating can significantly reduce current drive during high-field operation. However, with measurements at varying ambient temperature (from 100 to 300 K), we extract electron vsat = (3.4 ± 0.4) × 106 cm/s at room temperature in this three-atom-thick semiconductor, which we benchmark against other bulk and layered materials. With these results, we estimate that the saturation current in monolayer MoS2 could exceed 1 mA/μm at room temperature, in digital circuits with near-ideal thermal management.

    View details for DOI 10.1021/acs.nanolett.8b01692

    View details for Web of Science ID 000439008300063

    View details for PubMedID 29927605

  • GST-on-silicon hybrid nanophotonic integrated circuits: a non-volatile quasi-continuously reprogrammable platform OPTICAL MATERIALS EXPRESS Zheng, J., Khanolkar, A., Xu, P., Deshmukh, S., Myers, J., Frantz, J., Pop, E., Hendrickson, J., Doylend, J., Boechler, N., Majumdar, A. 2018; 8 (6): 1551–61
  • Tuning Electrical and Thermal Transport in AlGaN/GaN Heterostructures via Buffer Layer Engineering ADVANCED FUNCTIONAL MATERIALS Yalamarthy, A., So, H., Rojo, M., Suria, A. J., Xu, X., Pop, E., Senesky, D. G. 2018; 28 (22)
  • Unipolar n-Type Black Phosphorus Transistors with Low Work Function Contacts NANO LETTERS Wang, C., Incorvia, J. C., McClellan, C. J., Yu, A. C., Mleczko, M. J., Pop, E., Wong, H. 2018; 18 (5): 2822–27


    Black phosphorus (BP) is a promising two-dimensional (2D) material for nanoscale transistors, due to its expected higher mobility than other 2D semiconductors. While most studies have reported ambipolar BP with a stronger p-type transport, it is important to fabricate both unipolar p- and n-type transistors for low-power digital circuits. Here, we report unipolar n-type BP transistors with low work function Sc and Er contacts, demonstrating a record high n-type current of 200 μA/μm in 6.5 nm thick BP. Intriguingly, the electrical transport of the as-fabricated, capped devices changes from ambipolar to n-type unipolar behavior after a month at room temperature. Transmission electron microscopy analysis of the contact cross-section reveals an intermixing layer consisting of partly oxidized metal at the interface. This intermixing layer results in a low n-type Schottky barrier between Sc and BP, leading to the unipolar behavior of the BP transistor. This unipolar transport with a suppressed p-type current is favorable for digital logic circuits to ensure a lower off-power consumption.

    View details for PubMedID 29620900

  • Probing the Optical Properties and Strain-Tuning of Ultrathin Mo1-&ITx&ITW&ITx&ITTe2 NANO LETTERS Aslan, O., Datye, I. M., Mleczko, M. J., Cheung, K., Krylyuk, S., Bruma, A., Kalish, I., Davydov, A. V., Pop, E., Heinz, T. F. 2018; 18 (4): 2485–91


    Ultrathin transition metal dichalcogenides (TMDCs) have recently been extensively investigated to understand their electronic and optical properties. Here we study ultrathin Mo0.91W0.09Te2, a semiconducting alloy of MoTe2, using Raman, photoluminescence (PL), and optical absorption spectroscopy. Mo0.91W0.09Te2 transitions from an indirect to a direct optical band gap in the limit of monolayer thickness, exhibiting an optical gap of 1.10 eV, very close to its MoTe2 counterpart. We apply tensile strain, for the first time, to monolayer MoTe2 and Mo0.91W0.09Te2 to tune the band structure of these materials; we observe that their optical band gaps decrease by 70 meV at 2.3% uniaxial strain. The spectral widths of the PL peaks decrease with increasing strain, which we attribute to weaker exciton-phonon intervalley scattering. Strained MoTe2 and Mo0.91W0.09Te2 extend the range of band gaps of TMDC monolayers further into the near-infrared, an important attribute for potential applications in optoelectronics.

    View details for DOI 10.1021/acs.nanolett.8b00049

    View details for Web of Science ID 000430155900040

    View details for PubMedID 29561623

  • Ultra-low contact resistance in graphene devices at the Dirac point 2D MATERIALS Anzi, L., Mansouri, A., Pedrinazzi, P., Guerriero, E., Fiocco, M., Pesquera, A., Centeno, A., Zurutuza, A., Behnam, A., Carrion, E. A., Pop, E., Sordan, R. 2018; 5 (2)
  • Flexural resonance mechanism of thermal transport across graphene-SiO2 interfaces JOURNAL OF APPLIED PHYSICS Ong, Z., Qiu, B., Xu, S., Ruan, X., Pop, E. 2018; 123 (11)

    View details for DOI 10.1063/1.5020705

    View details for Web of Science ID 000428070900031

  • Carbon nanomaterials for non-volatile memories NATURE REVIEWS MATERIALS Ahn, E. C., Wong, H., Pop, E. 2018; 3 (3)
  • Microstructural origin of resistance-strain hysteresis in carbon nanotube thin film conductors PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Jin, L., Chortos, A., Lian, F., Pop, E., Linder, C., Bao, Z., Cai, W. 2018; 115 (9): 1986–91


    A basic need in stretchable electronics for wearable and biomedical technologies is conductors that maintain adequate conductivity under large deformation. This challenge can be met by a network of one-dimensional (1D) conductors, such as carbon nanotubes (CNTs) or silver nanowires, as a thin film on top of a stretchable substrate. The electrical resistance of CNT thin films exhibits a hysteretic dependence on strain under cyclic loading, although the microstructural origin of this strain dependence remains unclear. Through numerical simulations, analytic models, and experiments, we show that the hysteretic resistance evolution is governed by a microstructural parameter [Formula: see text] (the ratio of the mean projected CNT length over the film length) by showing that [Formula: see text] is hysteretic with strain and that the resistance is proportional to [Formula: see text] The findings are generally applicable to any stretchable thin film conductors consisting of 1D conductors with much lower resistance than the contact resistance in the high-density regime.

    View details for PubMedID 29440431

  • Detection of Methylation on dsDNA at Single-Molecule Level using Solid State Nanopores Bello, J., Kim, Y., Banerjee, S., Smithe, K., Estrada, D., Myong, S., Nardulli, A., Pop, E., Bashir, R., Shim, J. CELL PRESS. 2018: 216A
  • Theoretical potential for low energy consumption phase change memory utilizing electrostatically-induced structural phase transitions in 2D materials NPJ COMPUTATIONAL MATERIALS Rehn, D. A., Li, Y., Pop, E., Reed, E. J. 2018; 4
  • Energy-Efficient Phase Change Memory Programming by Nanosecond Pulses Yalon, E., Okabe, K., Neumann, C. M., Wong, H., Pop, E., IEEE IEEE. 2018
  • The Heat Conduction Renaissance Sood, A., Pop, E., Asheghi, M., Goodson, K. E., IEEE IEEE. 2018: 1396–1402
  • Investigation of Monolayer MX2 as Sub-Nanometer Copper Diffusion Barriers Smithe, K. H., Zhu, Z., Bailey, C. S., Pop, E., Yoon, A., IEEE IEEE. 2018
  • Localized Heating in MoTe2-Based Resistive Memory Devices Datye, I. M., Rojo, M., Yalon, E., Mleczko, M. J., Pop, E., IEEE IEEE. 2018
  • Probing Self-Heating in RRAM Devices by Sub-100 nm Spatially Resolved Thermometry Deshmukh, S., Rojo, M., Yalon, E., Vaziri, S., Pop, E., IEEE IEEE. 2018
  • Low Power Nanoscale Switching of VO2 using Carbon Nanotube Heaters Bohaichuk, S., Rojo, M., Pitner, G., McClellan, C., Lian, F., Li, J., Jeong, J., Samant, M., Parkin, S., Wong, H., Pop, E., IEEE IEEE. 2018
  • 3D Monolithic Stacked 1T1R cells using Monolayer MoS2 FET and hBN RRAM Fabricated at Low (150 degrees C) Temperature Wang, C., McClellan, C., Shi, Y., Zheng, X., Chen, V., Lanza, M., Pop, E., Wong, H., IEEE IEEE. 2018
  • Sub-Thermionic Steep Switching in Hole-Doped WSe2 Transistors McClellan, C. J., Yalon, E., Cai, L., Suryavanshi, S., Zheng, X., Pop, E., IEEE IEEE. 2018
  • Temperature-Dependent Thermal Boundary Conductance of Monolayer MoS2 by Raman Thermometry ACS APPLIED MATERIALS & INTERFACES Yalon, E., Aslan, O., Smithe, K. H., McClellan, C. J., Suryavanshi, S. V., Xiong, F., Sood, A., Neumann, C. M., Xu, X., Goodson, K. E., Heinz, T. F., Pop, E. 2017; 9 (49): 43013–20
  • Studies of two-dimensional h-BN and MoS2 for potential diffusion barrier application in copper interconnect technology NPJ 2D MATERIALS AND APPLICATIONS Lo, C., Catalano, M., Smithe, K. H., Wang, L., Zhang, S., Pop, E., Kim, M. J., Chen, Z. 2017; 1
  • Dense Vertically Aligned Copper Nanowire Composites as High Performance Thermal Interface Materials ACS APPLIED MATERIALS & INTERFACES Barako, M. T., Isaacson, S. G., Lian, F., Pop, E., Dauskardt, R. H., Goodson, K. E., Tice, J. 2017; 9 (48): 42067–74
  • Dense Vertically Aligned Copper Nanowire Composites as High Performance Thermal Interface Materials. ACS applied materials & interfaces Barako, M. T., Isaacson, S. G., Lian, F., Pop, E., Dauskardt, R. H., Goodson, K. E., Tice, J. 2017; 9 (48): 42067-42074


    Thermal interface materials (TIMs) are essential for managing heat in modern electronics, and nanocomposite TIMs can offer critical improvements. Here, we demonstrate thermally conductive, mechanically compliant TIMs based on dense, vertically aligned copper nanowires (CuNWs) embedded into polymer matrices. We evaluate the thermal and mechanical characteristics of 20-25% dense CuNW arrays with and without polydimethylsiloxane infiltration. The thermal resistance achieved is below 5 mm2 K W-1, over an order of magnitude lower than commercial heat sink compounds. Nanoindentation reveals that the nonlinear deformation mechanics of this TIM are influenced by both the CuNW morphology and the polymer matrix. We also implement a flip-chip bonding protocol to directly attach CuNW composites to copper surfaces, as required in many thermal architectures. Thus, we demonstrate a rational design strategy for nanocomposite TIMs that simultaneously retain the high thermal conductivity of aligned CuNWs and the mechanical compliance of a polymer.

    View details for DOI 10.1021/acsami.7b12313

    View details for PubMedID 29119783

  • Improved Hysteresis and Reliability of MoS2 Transistors With High-Quality CVD Growth and Al2O3 Encapsulation IEEE ELECTRON DEVICE LETTERS Illarionov, Y., Smithe, K. H., Waltl, M., Knobloch, T., Pop, E., Grasser, T. 2017; 38 (12): 1763–66
  • Spatially Resolved Thermometry of Resistive Memory Devices SCIENTIFIC REPORTS Yalon, E., Deshmukh, S., Rojo, M., Lian, F., Neumann, C. M., Xiong, F., Pop, E. 2017; 7: 15360


    The operation of resistive and phase-change memory (RRAM and PCM) is controlled by highly localized self-heating effects, yet detailed studies of their temperature are rare due to challenges of nanoscale thermometry. Here we show that the combination of Raman thermometry and scanning thermal microscopy (SThM) can enable such measurements with high spatial resolution. We report temperature-dependent Raman spectra of HfO2, TiO2 and Ge2Sb2Te5 (GST) films, and demonstrate direct measurements of temperature profiles in lateral PCM devices. Our measurements reveal that electrical and thermal interfaces dominate the operation of such devices, uncovering a thermal boundary resistance of 28 ± 8 m2K/GW at GST-SiO2 interfaces and an effective thermopower 350 ± 50 µV/K at GST-Pt interfaces. We also discuss possible pathways to apply Raman thermometry and SThM techniques to nanoscale and vertical resistive memory devices.

    View details for PubMedID 29127371

  • Detection of methylation on dsDNA using nanopores in a MoS2 membrane NANOSCALE Shim, J., Banerjee, S., Qiu, H., Smithe, K. H., Estrada, D., Bello, J., Pop, E., Schulten, K., Bashir, R. 2017; 9 (39): 14836–45


    Methylation at the 5-carbon position of the cytosine nucleotide base in DNA has been shown to be a reliable diagnostic biomarker for carcinogenesis. Early detection of methylation and intervention could drastically increase the effectiveness of therapy and reduce the cancer mortality rate. Current methods for detecting methylation involve bisulfite genomic sequencing, which are cumbersome and demand a large sample size of bodily fluids to yield accurate results. Hence, more efficient and cost effective methods are desired. Based on our previous work, we present a novel nanopore-based assay using a nanopore in a MoS2 membrane, and the methyl-binding protein (MBP), MBD1x, to detect methylation on dsDNA. We show that the dsDNA translocation was effectively slowed down using an asymmetric concentration of buffer and explore the possibility of profiling the position of methylcytosines on the DNA strands as they translocate through the 2D membrane. Our findings advance us one step closer towards the possible use of nanopore sensing technology in medical applications such as cancer detection.

    View details for DOI 10.1039/c7nr03092d

    View details for Web of Science ID 000412940000011

    View details for PubMedID 28795735

    View details for PubMedCentralID PMC5890527

  • HfSe2 and ZrSe2: Two-dimensional semiconductors with native high-κ oxides. Science advances Mleczko, M. J., Zhang, C., Lee, H. R., Kuo, H. H., Magyari-Köpe, B., Moore, R. G., Shen, Z. X., Fisher, I. R., Nishi, Y., Pop, E. 2017; 3 (8): e1700481


    The success of silicon as a dominant semiconductor technology has been enabled by its moderate band gap (1.1 eV), permitting low-voltage operation at reduced leakage current, and the existence of SiO2 as a high-quality "native" insulator. In contrast, other mainstream semiconductors lack stable oxides and must rely on deposited insulators, presenting numerous compatibility challenges. We demonstrate that layered two-dimensional (2D) semiconductors HfSe2 and ZrSe2 have band gaps of 0.9 to 1.2 eV (bulk to monolayer) and technologically desirable "high-κ" native dielectrics HfO2 and ZrO2, respectively. We use spectroscopic and computational studies to elucidate their electronic band structure and then fabricate air-stable transistors down to three-layer thickness with careful processing and dielectric encapsulation. Electronic measurements reveal promising performance (on/off ratio > 106; on current, ~30 μA/μm), with native oxides reducing the effects of interfacial traps. These are the first 2D materials to demonstrate technologically relevant properties of silicon, in addition to unique compatibility with high-κ dielectrics, and scaling benefits from their atomically thin nature.

    View details for DOI 10.1126/sciadv.1700481

    View details for PubMedID 28819644

    View details for PubMedCentralID PMC5553816

  • Low Variability in Synthetic Monolayer MoS2 Devices ACS NANO Smithe, K. H., Suryavanshi, S. V., Rojo, M., Tedjarati, A. D., Pop, E. 2017; 11 (8): 8456–63


    Despite much interest in applications of two-dimensional (2D) fabrics such as MoS2, to date most studies have focused on single or few devices. Here we examine the variability of hundreds of transistors from monolayer MoS2 synthesized by chemical vapor deposition. Ultraclean fabrication yields low surface roughness of ∼3 Å and surprisingly low variability of key device parameters, considering the atomically thin nature of the material. Threshold voltage variation and very low hysteresis suggest variations in charge density and traps as low as ∼1011 cm-2. Three extraction methods (field-effect, Y-function, and effective mobility) independently reveal mobility from 30 to 45 cm2/V/s (10th to 90th percentile; highest value ∼48 cm2/V/s) across areas >1 cm2. Electrical properties are remarkably immune to the presence of bilayer regions, which cause only small conduction band offsets (∼55 meV) measured by scanning Kelvin probe microscopy, an order of magnitude lower than energy variations in Si films of comparable thickness. Data are also used as inputs to Monte Carlo circuit simulations to understand the effects of material variability on circuit variation. These advances address key missing steps required to scale 2D semiconductors into functional systems.

    View details for PubMedID 28697304

  • . Nano letters Cai, L., McClellan, C. J., Koh, A. L., Li, H., Yalon, E., Pop, E., Zheng, X. 2017


    Two-dimensional (2D) molybdenum trioxide (MoO3) with mono- or few-layer thickness can potentially advance many applications, ranging from optoelectronics, catalysis, sensors, and batteries to electrochromic devices. Such ultrathin MoO3 sheets can also be integrated with other 2D materials (e.g., as dopants) to realize new or improved electronic devices. However, there is lack of a rapid and scalable method to controllably grow mono- or few-layer MoO3. Here, we report the first demonstration of using a rapid (<2 min) flame synthesis method to deposit mono- and few-layer MoO3 sheets (several microns in lateral dimension) on a wide variety of layered materials, including mica, MoS2, graphene, and WSe2, based on van der Waals epitaxy. The flame-grown ultrathin MoO3 sheet functions as an efficient hole doping layer for WSe2, enabling WSe2 to reach the lowest sheet and contact resistance reported to date among all the p-type 2D materials (∼6.5 kΩ/□ and ∼0.8 kΩ·μm, respectively). These results demonstrate that flame synthesis is a rapid and scalable pathway to growing atomically thin 2D metal oxides, opening up new opportunities for advancing 2D electronics.

    View details for DOI 10.1021/acs.nanolett.7b01322

    View details for PubMedID 28537732

  • High-Gain Graphene Transistors with a Thin AlOx Top-Gate Oxide. Scientific reports Guerriero, E., Pedrinazzi, P., Mansouri, A., Habibpour, O., Winters, M., Rorsman, N., Behnam, A., Carrion, E. A., Pesquera, A., Centeno, A., Zurutuza, A., Pop, E., Zirath, H., Sordan, R. 2017; 7 (1): 2419-?


    The high-frequency performance of transistors is usually assessed by speed and gain figures of merit, such as the maximum oscillation frequency f max, cutoff frequency f T, ratio f max/f T, forward transmission coefficient S 21, and open-circuit voltage gain A v. All these figures of merit must be as large as possible for transistors to be useful in practical electronics applications. Here we demonstrate high-performance graphene field-effect transistors (GFETs) with a thin AlOx gate dielectric which outperform previous state-of-the-art GFETs: we obtained f max/f T > 3, A v > 30 dB, and S 21 = 12.5 dB (at 10 MHz and depending on the transistor geometry) from S-parameter measurements. A dc characterization of GFETs in ambient conditions reveals good current saturation and relatively large transconductance ~600 S/m. The realized GFETs offer the prospect of using graphene in a much wider range of electronic applications which require substantial gain.

    View details for DOI 10.1038/s41598-017-02541-2

    View details for PubMedID 28546634

  • Photoresponse of Natural van der Waals Heterostructures. ACS nano Ray, K., Yore, A. E., Mou, T., Jha, S., Smithe, K. K., Wang, B., Pop, E., Newaz, A. K. 2017


    Van der Waals heterostructures consisting of two-dimensional materials offer a platform to obtain materials by design and are very attractive owing to unique electronic states. Research on 2D van der Waals heterostructures (vdWH) has so far been focused on fabricating individually stacked atomically thin unary or binary crystals. Such systems include graphene, hexagonal boron nitride, and members of the transition metal dichalcogenide family. Here we present our experimental study of the optoelectronic properties of a naturally occurring vdWH, known as franckeite, which is a complex layered crystal composed of lead, tin, antimony, iron, and sulfur. We present here that thin film franckeite (60 nm < d < 100 nm) behaves as a narrow band gap semiconductor demonstrating a wide-band photoresponse. We have observed the band-edge transition at ∼1500 nm (∼830 meV) and high external quantum efficiency (EQE ≈ 3%) at room temperature. Laser-power-resolved and temperature-resolved photocurrent measurements reveal that the photocarrier generation and recombination are dominated by continuously distributed trap states within the band gap. To understand wavelength-resolved photocurrent, we also calculated the optical absorption properties via density functional theory. Finally, we have shown that the device has a fast photoresponse with a rise time as fast as ∼1 ms. Our study provides a fundamental understanding of the optoelectronic behavior in a complex naturally occurring vdWH, and may pave an avenue toward developing nanoscale optoelectronic devices with tailored properties.

    View details for DOI 10.1021/acsnano.7b01918

    View details for PubMedID 28485958

  • Electronics. Nano letters Yalon, E., McClellan, C. J., Smithe, K. K., Muñoz Rojo, M., Xu, R. L., Suryavanshi, S. V., Gabourie, A. J., Neumann, C. M., Xiong, F., Farimani, A. B., Pop, E. 2017


    The advancement of nanoscale electronics has been limited by energy dissipation challenges for over a decade. Such limitations could be particularly severe for two-dimensional (2D) semiconductors integrated with flexible substrates or multilayered processors, both being critical thermal bottlenecks. To shed light into fundamental aspects of this problem, here we report the first direct measurement of spatially resolved temperature in functioning 2D monolayer MoS2 transistors. Using Raman thermometry, we simultaneously obtain temperature maps of the device channel and its substrate. This differential measurement reveals the thermal boundary conductance of the MoS2 interface with SiO2 (14 ± 4 MW m(-2) K(-1)) is an order magnitude larger than previously thought, yet near the low end of known solid-solid interfaces. Our study also reveals unexpected insight into nonuniformities of the MoS2 transistors (small bilayer regions) which do not cause significant self-heating, suggesting that such semiconductors are less sensitive to inhomogeneity than expected. These results provide key insights into energy dissipation of 2D semiconductors and pave the way for the future design of energy-efficient 2D electronics.

    View details for DOI 10.1021/acs.nanolett.7b00252

    View details for PubMedID 28388845

  • Transistors by Ultra-High Vacuum Metal Deposition. Nano letters English, C. D., Shine, G., Dorgan, V. E., Saraswat, K. C., Pop, E. 2017; 17 (4): 2739-?

    View details for DOI 10.1021/acs.nanolett.7b01337

    View details for PubMedID 28367629

  • Intrinsic electrical transport and performance projections of synthetic monolayer MoS2 devices 2D MATERIALS Smithe, K. K., English, C. D., Suryavanshi, S. V., Pop, E. 2017; 4 (1)
  • Effective n-type Doping of Monolayer MoS2 by AlOx McClellan, C. J., Yalon, E., Smithe, K. H., Suryavanshi, S. V., Pop, E., IEEE IEEE. 2017
  • INVITED: In Quest of the Next Information Processing Substrate Extended Abstract Datta, S., Seabaugh, A., Niemier, M., Raychowdhury, A., Schlom, D., Jena, D., Xing, G., Wong, H., Pop, E., Salahuddin, S., Gupta, S., Guha, S., IEEE IEEE. 2017
  • Metasurfaces Based on Nano-Patterned Phase-Change Memory Materials Colburn, S., Zhan, A., Deshmukh, S., Myers, J., Frantz, J., Pop, E., Majumdar, A., IEEE IEEE. 2017
  • 2D Molybdenum Disulfide (MoS2) Transistors Driving RRAMs with 1T1R Configuration Yang, R., Li, H., Smithe, K. H., Kim, T. R., Okabe, K., Pop, E., Fan, J. A., Wong, H., IEEE IEEE. 2017
  • Thermal Boundary Conductance of the MoS2-SiO2 Interface Suryavanshi, S. V., Gabourie, A. J., Farimani, A., Yalon, E., Pop, E., IEEE IEEE. 2017: 26–29
  • Active metasurfaces based on phase-change memory material digital metamolecules Colburn, S., Zhan, A., Majumdar, A., Deshmukh, S., Pop, E., Myers, J., Frantz, J., IEEE IEEE. 2017: 5–8
  • Electronic, Thermal, and Unconventional Applications of 2D Materials Pop, E., Yalon, E., Munoz-Rojo, M., Mleczko, M., English, C., Wang, N., Smithe, K., Suryavanshi, S., Datye, I., McClellan, C., Gabourie, A., IEEE IEEE. 2017: 916–17
  • Sub-15 nm Nanowires Enabled by Cryo Pulsed Self-Aligned Nanotrench Ablation on Carbon Nanotubes Deshmukh, S., Lian, F., Yalon, E., Pitner, G., Wang, H., Pop, E., IEEE IEEE. 2017: 489–90
  • Invited: A Systems Approach to Computing in Beyond CMOS Fabrics Patil, A., Shanbhag, N., Varshney, L., Pop, E., Wong, H., Mitra, S., Rabaey, J., Weldon, J., Pileggi, L., Manipatruni, S., Nikonov, D., Young, I., IEEE IEEE. 2017
  • S2DS: Physics-based compact model for circuit simulation of two-dimensional semiconductor devices including non-idealities JOURNAL OF APPLIED PHYSICS Suryavanshi, S. V., Pop, E. 2016; 120 (22)

    View details for DOI 10.1063/1.4971404

    View details for Web of Science ID 000391535900011

  • Gate tunneling current and quantum capacitance in metal-oxide-semiconductor devices with graphene gate electrodes APPLIED PHYSICS LETTERS An, Y., Shekhawat, A., Behnam, A., Pop, E., Ural, A. 2016; 109 (22)

    View details for DOI 10.1063/1.4968824

    View details for Web of Science ID 000390243100038

  • Visualization of Defect-Induced Excitonic Properties of the Edges and Grain Boundaries in Synthesized Monolayer Molybdenum Disulfide JOURNAL OF PHYSICAL CHEMISTRY C Yore, A. E., Smithe, K. K., Crumrine, W., Miller, A., TUCK, J. A., Redd, B., Pop, E., Wang, B., Newaz, A. K. 2016; 120 (42): 24080-24087
  • Interfaces. Nano letters Koh, Y. K., Lyons, A. S., Bae, M., Huang, B., Dorgan, V. E., Cahill, D. G., Pop, E. 2016; 16 (10): 6014-6020


    Heat transfer across interfaces of graphene and polar dielectrics (e.g., SiO2) could be mediated by direct phonon coupling, as well as electronic coupling with remote interfacial phonons (RIPs). To understand the relative contribution of each component, we develop a new pump-probe technique called voltage-modulated thermoreflectance (VMTR) to accurately measure the change of interfacial thermal conductance under an electrostatic field. We employed VMTR on top gates of graphene field-effect transistors and find that the thermal conductance of SiO2/graphene/SiO2 interfaces increases by up to ΔG ≈ 0.8 MW m(-2) K(-1) under electrostatic fields of <0.2 V nm(-1). We propose two possible explanations for the small observed ΔG. First, because the applied electrostatic field induces charge carriers in graphene, our VMTR measurements could originate from heat transfer between the charge carriers in graphene and RIPs in SiO2. Second, the increase in heat conduction could be caused by better conformity of graphene interfaces under electrostatic pressure exerted by the induced charge carriers. Regardless of the origins of the observed ΔG, our VMTR measurements establish an upper limit for heat transfer from unbiased graphene to SiO2 substrates via RIP scattering; for example, only <2% of the interfacial heat transport is facilitated by RIP scattering even at a carrier concentration of ∼4 × 10(12) cm(-2).

    View details for PubMedID 27585088

  • SANTA: Self-aligned nanotrench ablation via Joule heating for probing sub-20 nm devices NANO RESEARCH Xiong, F., Deshmukh, S., Hong, S., Dai, Y., Behnam, A., Lian, F., Pop, E. 2016; 9 (10): 2950-2959
  • Role of Remote Interfacial Phonon (RIP) Scattering in Heat Transport Across Graphene/SiO2 Interfaces NANO LETTERS Koh, Y. K., Lyons, A. S., Bae, M., Huang, B., Dorgan, V. E., Cahill, D. G., Pop, E. 2016; 16 (10): 6014-6020
  • High Current Density and Low Thermal Conductivity of Atomically Thin Semimetallic WTe2. ACS nano Mleczko, M. J., Xu, R. L., Okabe, K., Kuo, H., Fisher, I. R., Wong, H. P., Nishi, Y., Pop, E. 2016; 10 (8): 7507-7514


    Two-dimensional (2D) semimetals beyond graphene have been relatively unexplored in the atomically thin limit. Here, we introduce a facile growth mechanism for semimetallic WTe2 crystals and then fabricate few-layer test structures while carefully avoiding degradation from exposure to air. Low-field electrical measurements of 80 nm to 2 μm long devices allow us to separate intrinsic and contact resistance, revealing metallic response in the thinnest encapsulated and stable WTe2 devices studied to date (3-20 layers thick). High-field electrical measurements and electrothermal modeling demonstrate that ultrathin WTe2 can carry remarkably high current density (approaching 50 MA/cm(2), higher than most common interconnect metals) despite a very low thermal conductivity (of the order ∼3 Wm(-1) K(-1)). These results suggest several pathways for air-stable technological viability of this layered semimetal.

    View details for DOI 10.1021/acsnano.6b02368

    View details for PubMedID 27434729

  • Electrical and Thermoelectric Transport by Variable Range Hopping in Thin Black Phosphorus Devices NANO LETTERS Choi, S. J., Kim, B., Lee, T., Kim, Y. H., Li, Z., Pop, E., Kim, J., Song, J. H., Bae, M. 2016; 16 (7): 3969-3975


    The moderate band gap of black phosphorus (BP) in the range of 0.3-2 eV, along a high mobility of a few hundred cm(2) V(-1) s(-1) provides a bridge between the gapless graphene and relatively low-mobility transition metal dichalcogenides. Here, we study the mechanism of electrical and thermoelectric transport in 10-30 nm thick BP devices by measurements of electrical conductance and thermopower (S) with various temperatures (T) and gate-electric fields. The T dependences of S and the sheet conductance (σ□) of the BP devices show behaviors of T(1/3) and exp[-(1/T)(1/3)], respectively, where S reaches ∼0.4 mV/K near room T. This result indicates that two-dimensional (2D) Mott's variable range hopping (VRH) is a dominant mechanism in the thermoelectric and electrical transport in our examined thin BP devices. We consider the origin of the 2D Mott's VRH transport in our BPs as trapped charges at the surface of the underlying SiO2 based on the analysis with observed multiple quantum dots.

    View details for DOI 10.1021/acs.nanolett.5b04957

    View details for Web of Science ID 000379794200001

    View details for PubMedID 27223230

  • Role of Pressure in the Growth of Hexagonal Boron Nitride Thin Films from Ammonia-Borane CHEMISTRY OF MATERIALS Koepke, J. C., Wood, J. D., Chen, Y., Schmucker, S. W., Liu, X., Chang, N. N., Nienhaus, L., Do, J. W., Carrion, E. A., Hewaparakrama, J., Ranaarajan, A., Datye, I., Mehta, R., Haasch, R. T., Gruebele, M., Girolami, G. S., Pop, E., Lyding, J. W. 2016; 28 (12): 4169-4179
  • Improved Contacts to MoS2 Transistors by Ultra-High Vacuum Metal Deposition NANO LETTERS English, C. D., Shine, G., Dorgan, V. E., Saraswat, K. C., Pop, E. 2016; 16 (6): 3824-3830


    The scaling of transistors to sub-10 nm dimensions is strongly limited by their contact resistance (RC). Here we present a systematic study of scaling MoS2 devices and contacts with varying electrode metals and controlled deposition conditions, over a wide range of temperatures (80 to 500 K), carrier densities (10(12) to 10(13) cm(-2)), and contact dimensions (20 to 500 nm). We uncover that Au deposited in ultra-high vacuum (∼10(-9) Torr) yields three times lower RC than under normal conditions, reaching 740 Ω·μm and specific contact resistivity 3 × 10(-7) Ω·cm(2), stable for over four months. Modeling reveals separate RC contributions from the Schottky barrier and the series access resistance, providing key insights on how to further improve scaling of MoS2 contacts and transistor dimensions. The contact transfer length is ∼35 nm at 300 K, which is verified experimentally using devices with 20 nm contacts and 70 nm contact pitch (CP), equivalent to the "14 nm" technology node.

    View details for DOI 10.1021/acs.nanolett.6b01309

    View details for Web of Science ID 000377642700060

    View details for PubMedID 27232636

  • Kinetic Study of Hydrogen Evolution Reaction over Strained MoS2 with Sulfur Vacancies Using Scanning Electrochemical Microscopy JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Li, H., Du, M., Mleczko, M. J., Koh, A. L., Nishi, Y., Pop, E., Bard, A. J., Zheng, X. 2016; 138 (15): 5123-5129


    Molybdenum disulfide (MoS2), with its active edge sites, is a proposed alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Recently, the inert basal plane of MoS2 was successfully activated and optimized with excellent intrinsic HER activity by creating and further straining sulfur (S) vacancies. Nevertheless, little is known about the HER kinetics of those S vacancies and the additional effects from elastic tensile strain. Herein, scanning electrochemical microscopy was used to determine the HER kinetic data for both unstrained S vacancies (formal potential Ev0 = −0.53 VAg/AgCl, electron-transfer coefficient αv = 0.4, electron-transfer rate constant kv0 = 2.3 × 10(–4) cm/s) and strained S vacancies (Esv0= −0.53 VAg/AgCl, αsv = 0.4, ksv0 = 1.0 × 10(–3) cm/s) on the basal plane of MoS2 monolayers, and the strained S vacancy has an electron-transfer rate 4 times higher than that of the unstrained S vacancy. This study provides a general platform for measuring the kinetics of two-dimensional material-based catalysts.

    View details for DOI 10.1021/jacs.6b01377

    View details for PubMedID 26997198

  • Thermal conductivity of chirality-sorted carbon nanotube networks APPLIED PHYSICS LETTERS Lian, F., Llinas, J. P., Li, Z., Estrada, D., Pop, E. 2016; 108 (10)

    View details for DOI 10.1063/1.4942968

    View details for Web of Science ID 000372973600034

  • Energy-Efficient Abundant-Data Computing: The N3XT 1,000x COMPUTER Aly, M. M., Gao, M., Hills, G., Lee, C., Pitner, G., Shulaker, M. M., Wu, T. F., Asheghi, M., Bokor, J., Franchetti, F., Goodson, K. E., Kozyrakis, C., Markov, I., Olukotun, K., Pileggi, L., Pop, E., Rabaey, J., Re, C., Wong, H. P., Mitra, S. 2015; 48 (12): 24-33
  • Energy-Efficient Phase-Change Memory with Graphene as a Thermal Barrier. Nano letters Ahn, C., Fong, S. W., Kim, Y., Lee, S., Sood, A., Neumann, C. M., Asheghi, M., Goodson, K. E., Pop, E., Wong, H. P. 2015; 15 (10): 6809-6814


    Phase-change memory (PCM) is an important class of data storage, yet lowering the programming current of individual devices is known to be a significant challenge. Here we improve the energy-efficiency of PCM by placing a graphene layer at the interface between the phase-change material, Ge2Sb2Te5 (GST), and the bottom electrode (W) heater. Graphene-PCM (G-PCM) devices have ∼40% lower RESET current compared to control devices without the graphene. This is attributed to the graphene as an added interfacial thermal resistance which helps confine the generated heat inside the active PCM volume. The G-PCM achieves programming up to 10(5) cycles, and the graphene could further enhance the PCM endurance by limiting atomic migration or material segregation at the bottom electrode interface.

    View details for DOI 10.1021/acs.nanolett.5b02661

    View details for PubMedID 26308280

  • Li Intercalation in MoS2: In Situ Observation of Its Dynamics and Tuning Optical and Electrical Properties NANO LETTERS Xiong, F., Wang, H., Liu, X., Sun, J., Brongersma, M., Pop, E., Cui, Y. 2015; 15 (10): 6777-6784


    Two-dimensional layered materials like MoS2 have shown promise for nanoelectronics and energy storage, both as monolayers and as bulk van der Waals crystals with tunable properties. Here we present a platform to tune the physical and chemical properties of nanoscale MoS2 by electrochemically inserting a foreign species (Li(+) ions) into their interlayer spacing. We discover substantial enhancement of light transmission (up to 90% in 4 nm thick lithiated MoS2) and electrical conductivity (more than 200×) in ultrathin (∼2-50 nm) MoS2 nanosheets after Li intercalation due to changes in band structure that reduce absorption upon intercalation and the injection of large amounts of free carriers. We also capture the first in situ optical observations of Li intercalation in MoS2 nanosheets, shedding light on the dynamics of the intercalation process and the associated spatial inhomogeneity and cycling-induced structural defects.

    View details for DOI 10.1021/acs.nanolett.5b02619

    View details for PubMedID 26352295

  • Engineering Ultra-Low Work Function of Graphene NANO LETTERS Yuan, H., Chang, S., Bargatin, I., Wang, N. C., Riley, D. C., Wang, H., Schwede, J. W., Provine, J., Pop, E., Shen, Z., Pianetta, P. A., Melosh, N. A., Howe, R. T. 2015; 15 (10): 6475-6480


    Low work function materials are critical for energy conversion and electron emission applications. Here, we demonstrate for the first time that an ultralow work function graphene is achieved by combining electrostatic gating with a Cs/O surface coating. A simple device is built from large-area monolayer graphene grown by chemical vapor deposition, transferred onto 20 nm HfO2 on Si, enabling high electric fields capacitive charge accumulation in the graphene. We first observed over 0.7 eV work function change due to electrostatic gating as measured by scanning Kelvin probe force microscopy and confirmed by conductivity measurements. The deposition of Cs/O further reduced the work function, as measured by photoemission in an ultrahigh vacuum environment, which reaches nearly 1 eV, the lowest reported to date for a conductive, nondiamond material.

    View details for DOI 10.1021/acs.nanolett.5b01916

    View details for PubMedID 26401728

  • Energy-Efficient Phase-Change Memory with Graphene as a Thermal Barrier NANO LETTERS Ahn, C., Fong, S. W., Kim, Y., Lee, S., Sood, A., Neumann, C. M., Ashegh, M., Goodson, K. E., Pop, E., Wong, H. P. 2015; 15 (10): 6809-6814
  • Nanoscale phase change memory with graphene ribbon electrodes APPLIED PHYSICS LETTERS Behnam, A., Xiong, F., Cappelli, A., Wang, N. C., Carrion, E. A., Hong, S., Dai, Y., Lyons, A. S., Chow, E. K., Piccinini, E., Jacoboni, C., Pop, E. 2015; 107 (12)

    View details for DOI 10.1063/1.4931491

    View details for Web of Science ID 000361832600072

  • Forward-bias diode parameters, electronic noise, and photoresponse of graphene/silicon Schottky junctions with an interfacial native oxide layer JOURNAL OF APPLIED PHYSICS An, Y., Behnam, A., Pop, E., Bosman, G., Ural, A. 2015; 118 (11)

    View details for DOI 10.1063/1.4931142

    View details for Web of Science ID 000361843300022

  • A Compact Virtual-Source Model for Carbon Nanotube FETs in the Sub-10-nm Regime-Part II: Extrinsic Elements, Performance Assessment, and Design Optimization IEEE TRANSACTIONS ON ELECTRON DEVICES Lee, C., Pop, E., Franklin, A. D., Haensch, W., Wong, H. P. 2015; 62 (9): 3070-3078
  • A Compact Virtual-Source Model for Carbon Nanotube FETs in the Sub-10-nm Regime-Part I: Intrinsic Elements IEEE TRANSACTIONS ON ELECTRON DEVICES Lee, C., Pop, E., Franklin, A. D., Haensch, W., Wong, H. P. 2015; 62 (9): 3061-3069
  • Graphene-Based Platform for Infrared Near-Field Nanospectroscopy of Water and Biological Materials in an Aqueous Environment. ACS nano Khatib, O., Wood, J. D., McLeod, A. S., Goldflam, M. D., Wagner, M., Damhorst, G. L., Koepke, J. C., Doidge, G. P., Rangarajan, A., Bashir, R., Pop, E., Lyding, J. W., Thiemens, M. H., Keilmann, F., Basov, D. N. 2015; 9 (8): 7968-7975


    Scattering scanning near-field optical microscopy (s-SNOM) has emerged as a powerful nanoscale spectroscopic tool capable of characterizing individual biomacromolecules and molecular materials. However, applications of scattering-based near-field techniques in the infrared (IR) to native biosystems still await a solution of how to implement the required aqueous environment. In this work, we demonstrate an IR-compatible liquid cell architecture that enables near-field imaging and nanospectroscopy by taking advantage of the unique properties of graphene. Large-area graphene acts as an impermeable monolayer barrier that allows for nano-IR inspection of underlying molecular materials in liquid. Here, we use s-SNOM to investigate the tobacco mosaic virus (TMV) in water underneath graphene. We resolve individual virus particles and register the amide I and II bands of TMV at ca. 1520 and 1660 cm(-1), respectively, using nanoscale Fourier transform infrared spectroscopy (nano-FTIR). We verify the presence of water in the graphene liquid cell by identifying a spectral feature associated with water absorption at 1610 cm(-1).

    View details for DOI 10.1021/acsnano.5b01184

    View details for PubMedID 26223158

  • Graphene-Based Platform for Infrared Near-Field Nanospectroscopy of Water and Biological Materials in an Aqueous Environment ACS NANO Khatib, O., Wood, J. D., McLeod, A. S., Goldflam, M. D., Wagner, M., Damhorst, G. L., Koepke, J. C., Doidge, G. P., Rangarajan, A., Bashir, R., Pop, E., Lyding, J. W., Thiemens, M. H., Keilmann, F., Basov, D. N. 2015; 9 (8): 7968-7975
  • Bright visible light emission from graphene NATURE NANOTECHNOLOGY Kim, Y. D., Kim, H., Cho, Y., Ryoo, J. H., Park, C., Kim, P., Kim, Y. S., Lee, S., Li, Y., Park, S., Yoo, Y. S., Yoon, D., Dorgan, V. E., Pop, E., Heinz, T. F., Hone, J., Chun, S., Cheong, H., Lee, S. W., Bae, M., Park, Y. D. 2015; 10 (8): 676-681


    Graphene and related two-dimensional materials are promising candidates for atomically thin, flexible and transparent optoelectronics. In particular, the strong light-matter interaction in graphene has allowed for the development of state-of-the-art photodetectors, optical modulators and plasmonic devices. In addition, electrically biased graphene on SiO2 substrates can be used as a low-efficiency emitter in the mid-infrared range. However, emission in the visible range has remained elusive. Here, we report the observation of bright visible light emission from electrically biased suspended graphene devices. In these devices, heat transport is greatly reduced. Hot electrons (∼2,800 K) therefore become spatially localized at the centre of the graphene layer, resulting in a 1,000-fold enhancement in thermal radiation efficiency. Moreover, strong optical interference between the suspended graphene and substrate can be used to tune the emission spectrum. We also demonstrate the scalability of this technique by realizing arrays of chemical-vapour-deposited graphene light emitters. These results pave the way towards the realization of commercially viable large-scale, atomically thin, flexible and transparent light emitters and displays with low operation voltage and graphene-based on-chip ultrafast optical communications.

    View details for DOI 10.1038/NNANO.2015.118

    View details for PubMedID 26076467

  • Solution-Mediated Selective Nanosoldering of Carbon Nanotube Junctions for Improved Device Performance ACS NANO Do, J., Chang, N. N., Estrada, D., Lian, F., Cha, H., Duan, X. J., Haasch, R. T., Pop, E., Girolami, G. S., Lyding, J. W. 2015; 9 (5): 4806-4813


    As-grown randomly aligned networks of carbon nanotubes (CNTs) invariably suffer from limited transport properties due to high resistance at the crossed junctions between CNTs. In this work, Joule heating of the highly resistive CNT junctions is carried out in the presence of a spin-coated layer of a suitable chemical precursor. The heating triggers thermal decomposition of the chemical precursor, tris(dibenzylideneacetone)dipalladium (Pd2(dba)3), and causes local deposition of Pd nanoparticles at the CNT junctions, thereby improving the on/off current ratio and mobility of CNT network devices by an average factor of ∼6. This process can be conducted either in air or under vacuum depending on the characteristics of the precursor species. The solution-mediated nanosoldering process is simple, fast, scalable with manufacturing techniques, and extendable to the nanodeposition of a wide variety of materials.

    View details for DOI 10.1021/nn505552d

    View details for Web of Science ID 000355383000017

    View details for PubMedID 25844819

  • Annealing free, clean graphene transfer using alternative polymer scaffolds. Nanotechnology Wood, J. D., Doidge, G. P., Carrion, E. A., Koepke, J. C., Kaitz, J. A., Datye, I., Behnam, A., Hewaparakrama, J., Aruin, B., Chen, Y., Dong, H., Haasch, R. T., Lyding, J. W., Pop, E. 2015; 26 (5): 055302-?


    We examine the transfer of graphene grown by chemical vapor deposition (CVD) with polymer scaffolds of poly(methyl methacrylate) (PMMA), poly(lactic acid) (PLA), poly(phthalaldehyde) (PPA), and poly(bisphenol A carbonate) (PC). We find that optimally reactive PC scaffolds provide the cleanest graphene transfers without any annealing, after extensive comparison with optical microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Comparatively, films transferred with PLA, PPA, PMMA/PC, and PMMA have a two-fold higher roughness and a five-fold higher chemical doping. Using PC scaffolds, we demonstrate the clean transfer of CVD multilayer graphene, fluorinated graphene, and hexagonal boron nitride. Our annealing free, PC transfers enable the use of atomically-clean nanomaterials in biomolecule encapsulation and flexible electronic applications.

    View details for DOI 10.1088/0957-4484/26/5/055302

    View details for PubMedID 25580991

  • Annealing free, clean graphene transfer using alternative polymer scaffolds. Nanotechnology Wood, J. D., Doidge, G. P., Carrion, E. A., Koepke, J. C., Kaitz, J. A., Datye, I., Behnam, A., Hewaparakrama, J., Aruin, B., Chen, Y., Dong, H., Haasch, R. T., Lyding, J. W., Pop, E. 2015; 26 (5): 055302-?

    View details for DOI 10.1088/0957-4484/26/5/055302

    View details for PubMedID 25580991

  • Scaling of graphene integrated circuits NANOSCALE Bianchi, M., Guerriero, E., Fiocco, M., Alberti, R., Polloni, L., Behnam, A., Carrion, E. A., Pop, E., Sordan, R. 2015; 7 (17): 8076-8083


    The influence of transistor size reduction (scaling) on the speed of realistic multi-stage integrated circuits (ICs) represents the main performance metric of a given transistor technology. Despite extensive interest in graphene electronics, scaling efforts have so far focused on individual transistors rather than multi-stage ICs. Here we study the scaling of graphene ICs based on transistors from 3.3 to 0.5 μm gate lengths and with different channel widths, access lengths, and lead thicknesses. The shortest gate delay of 31 ps per stage was obtained in sub-micron graphene ROs oscillating at 4.3 GHz, which is the highest oscillation frequency obtained in any strictly low-dimensional material to date. We also derived the fundamental Johnson limit, showing that scaled graphene ICs could be used at high frequencies in applications with small voltage swing.

    View details for DOI 10.1039/c5nr01126d

    View details for Web of Science ID 000353981700067

    View details for PubMedID 25873359

  • Direct observation of resistive heating at graphene wrinkles and grain boundaries APPLIED PHYSICS LETTERS Grosse, K. L., Dorgan, V. E., Estrada, D., Wood, J. D., Vlassiouk, I., Eres, G., Lyding, J. W., King, W. P., Pop, E. 2014; 105 (14)

    View details for DOI 10.1063/1.4896676

    View details for Web of Science ID 000344343900053

  • Heterogeneous nanometer-scale Joule and Peltier effects in sub-25 nm thin phase change memory devices JOURNAL OF APPLIED PHYSICS Grosse, K. L., Pop, E., King, W. P. 2014; 116 (12)

    View details for DOI 10.1063/1.4896492

    View details for Web of Science ID 000342840000089

  • Nanometer-scale temperature imaging for independent observation of Joule and Peltier effects in phase change memory devices REVIEW OF SCIENTIFIC INSTRUMENTS Grosse, K. L., Pop, E., King, W. P. 2014; 85 (9)


    This paper reports a technique for independent observation of nanometer-scale Joule heating and thermoelectric effects, using atomic force microscopy (AFM) based measurements of nanometer-scale temperature fields. When electrical current flows through nanoscale devices and contacts the temperature distribution is governed by both Joule and thermoelectric effects. When the device is driven by an electrical current that is both periodic and bipolar, the temperature rise due to the Joule effect is at a different harmonic than the temperature rise due to the Peltier effect. An AFM tip scanning over the device can simultaneously measure all of the relevant harmonic responses, such that the Joule effect and the Peltier effect can be independently measured. Here we demonstrate the efficacy of the technique by measuring Joule and Peltier effects in phase change memory devices. By comparing the observed temperature responses of these working devices, we measure the device thermopower, which is in the range of 30 ± 3 to 250 ± 10 μV K(-1). This technique could facilitate improved measurements of thermoelectric phenomena and properties at the nanometer-scale.

    View details for DOI 10.1063/1.4895715

    View details for Web of Science ID 000342910500062

  • Nanometer-scale temperature imaging for independent observation of Joule and Peltier effects in phase change memory devices. The Review of scientific instruments Grosse, K. L., Pop, E., King, W. P. 2014; 85 (9): 094904


    This paper reports a technique for independent observation of nanometer-scale Joule heating and thermoelectric effects, using atomic force microscopy (AFM) based measurements of nanometer-scale temperature fields. When electrical current flows through nanoscale devices and contacts the temperature distribution is governed by both Joule and thermoelectric effects. When the device is driven by an electrical current that is both periodic and bipolar, the temperature rise due to the Joule effect is at a different harmonic than the temperature rise due to the Peltier effect. An AFM tip scanning over the device can simultaneously measure all of the relevant harmonic responses, such that the Joule effect and the Peltier effect can be independently measured. Here we demonstrate the efficacy of the technique by measuring Joule and Peltier effects in phase change memory devices. By comparing the observed temperature responses of these working devices, we measure the device thermopower, which is in the range of 30 ± 3 to 250 ± 10 μV K(-1). This technique could facilitate improved measurements of thermoelectric phenomena and properties at the nanometer-scale.

    View details for DOI 10.1063/1.4895715

    View details for PubMedID 25273761

  • Phase change materials and phase change memory MRS BULLETIN Raoux, S., Xiong, F., Wuttig, M., Pop, E. 2014; 39 (8): 703-710
  • Theoretical analysis of high-field transport in graphene on a substrate JOURNAL OF APPLIED PHYSICS Serov, A. Y., Ong, Z., Fischetti, M. V., Pop, E. 2014; 116 (3)

    View details for DOI 10.1063/1.4884614

    View details for Web of Science ID 000340710500085

  • Substrate-supported thermometry platform for nanomaterials like graphene, nanotubes, and nanowires APPLIED PHYSICS LETTERS Li, Z., Bae, M., Pop, E. 2014; 105 (2)

    View details for DOI 10.1063/1.4887365

    View details for Web of Science ID 000341151400065

  • Self-aligned Cu etch mask for individually addressable metallic and semiconducting carbon nanotubes. ACS nano Jiang, Y., Xiong, F., Tsai, C. L., Ozel, T., Pop, E., Shim, M. 2014; 8 (6): 6500-8


    Two means to achieve high yield of individually addressable single-walled carbon nanotubes (CNTs) are developed and examined. The first approach matches the effective channel width and the density of horizontally aligned CNTs. This method can provide single CNT devices and also allows control over the average number of CNTs per channel. The second and a more deterministic approach uses self-aligned Cu-filled trenches formed in a photoresist (after Joule heating of the underlying CNT) to protect and obtain a large number of single CNT devices. Unlike electrical breakdown methods, which preserve the least conducting CNT and can leave behind CNT fragments, our approach allows the selection of the single most conducting metallic CNT from an array of as-grown CNTs with average resistance ∼14 times lower than that of as-fabricated single metallic CNTs. This method can also be used to select the best semiconducting CNT from an array and yields, on average, devices that are 15 times more conductive with 40 times higher ON/OFF ratio than those selected through electrical breakdown alone.

    View details for DOI 10.1021/nn502390r

    View details for PubMedID 24848422

  • Ultrafast terahertz-induced response of GeSbTe phase-change materials APPLIED PHYSICS LETTERS Shu, M. J., Zalden, P., Chen, F., Weems, B., Chatzakis, I., Xiong, F., Jeyasingh, R., Hoffmann, M. C., Pop, E., Wong, H. P., Wuttig, M., Lindenberg, A. M. 2014; 104 (25)

    View details for DOI 10.1063/1.4884816

    View details for Web of Science ID 000338515900032

  • Monolithic III-V Nanowire Solar Cells on Graphene via Direct van der Waals Epitaxy. Advanced materials Mohseni, P. K., Behnam, A., Wood, J. D., Zhao, X., Yu, K. J., Wang, N. C., Rockett, A., Rogers, J. A., Lyding, J. W., Pop, E., Li, X. 2014; 26 (22): 3755-3760

    View details for DOI 10.1002/adma.201305909

    View details for PubMedID 24652703

  • Self-Aligned Cu Etch Mask for Individually Addressable Metallic and Semiconducting Carbon Nanotubes ACS NANO Jiang, Y., Xiong, F., Tsai, C., Ozel, T., Pop, E., Shim, M. 2014; 8 (6): 6500-6508

    View details for DOI 10.1021/nn502390r

    View details for Web of Science ID 000338089200120

  • Hysteresis-Free Nanosecond Pulsed Electrical Characterization of Top-Gated Graphene Transistors IEEE TRANSACTIONS ON ELECTRON DEVICES Carrion, E. A., Serov, A. Y., Islam, S., Behnam, A., Malik, A., Xiong, F., Bianchi, M., Sordan, R., Pop, E. 2014; 61 (5): 1583-1589
  • Carbon Nanotube Circuit Integration up to Sub-20 nm Channel Lengths ACS NANO Shulaker, M. M., Van Rethy, J., Wu, T. F., Liyanage, L. S., Wei, H., Li, Z., Pop, E., Gielen, G., Wong, H. P., Mitra, S. 2014; 8 (4): 3434-3443


    Carbon nanotube (CNT) field-effect transistors (CNFETs) are a promising emerging technology projected to achieve over an order of magnitude improvement in energy-delay product, a metric of performance and energy efficiency, compared to silicon-based circuits. However, due to substantial imperfections inherent with CNTs, the promise of CNFETs has yet to be fully realized. Techniques to overcome these imperfections have yielded promising results, but thus far only at large technology nodes (1 μm device size). Here we demonstrate the first very large scale integration (VLSI)-compatible approach to realizing CNFET digital circuits at highly scaled technology nodes, with devices ranging from 90 nm to sub-20 nm channel lengths. We demonstrate inverters functioning at 1 MHz and a fully integrated CNFET infrared light sensor and interface circuit at 32 nm channel length. This demonstrates the feasibility of realizing more complex CNFET circuits at highly scaled technology nodes.

    View details for DOI 10.1021/nn406301r

    View details for Web of Science ID 000334990600034

    View details for PubMedID 24654597

  • Nanoscale thermal transport. II. 2003-2012 APPLIED PHYSICS REVIEWS Cahill, D. G., Braun, P. V., Chen, G., Clarke, D. R., Fan, S., Goodson, K. E., Keblinski, P., King, W. P., Mahan, G. D., Majumdar, A., Maris, H. J., Phillpot, S. R., Pop, E., Shi, L. 2014; 1 (1)

    View details for DOI 10.1063/1.4832615

    View details for Web of Science ID 000334098500010

  • High-Field and Thermal Transport in 2D Atomic Layer Devices Conference on Micro- and Nanotechnology Sensors, Systems, and Applications VI Serov, A., Dorgan, V. E., Behnam, A., English, C. D., Li, Z., Islam, S., Pop, E. SPIE-INT SOC OPTICAL ENGINEERING. 2014

    View details for DOI 10.1117/12.2052093

    View details for Web of Science ID 000342426300004

  • Energy Efficiency and Conversion in 1D and 2D Electronics 44th European Solid-State Device Research Conference (ESSDERC) Pop, E., English, C., Xiong, F., Lian, F., Serov, A., Li, Z., Islam, S., Dorgan, V. IEEE. 2014: 35–37
  • Multi-Valley Hi :h-Field Transport in 2-Dimensional MoS2 Transistors 72nd Annual Device Research Conference (DRC) Serov, A. Y., Dorgan, V. E., English, C. D., Pop, E. IEEE. 2014: 183–184
  • Improving Contact Resistance in MoS2 Field Effect Transistors 72nd Annual Device Research Conference (DRC) English, C. D., Shine, G., Dorgan, V. E., Saraswat, K. C., Pop, E. IEEE. 2014: 193–194
  • Variability of Graphene Mobility and Contacts: Surface Effects, Doping and Strain 72nd Annual Device Research Conference (DRC) Carrion, E. A., Wood, J. D., Behman, A., Tung, M., Lyding, J. W., Pop, E. IEEE. 2014: 199–200
  • Nanosoldering Carbon Nanotube Junctions by Local Chemical Vapor Deposition for Improved Device Performance NANO LETTERS Do, J., Estrada, D., Xie, X., Chang, N. N., Mallek, J., Girolami, G. S., Rogers, J. A., Pop, E., Lyding, J. W. 2013; 13 (12): 5844-5850


    The performance of carbon nanotube network (CNN) devices is usually limited by the high resistance of individual nanotube junctions (NJs). We present a novel method to reduce this resistance through a nanoscale chemical vapor deposition (CVD) process. By passing current through the devices in the presence of a gaseous CVD precursor, localized nanoscale Joule heating induced at the NJs stimulates the selective and self-limiting deposition of metallic nanosolder. The effectiveness of this nanosoldering process depends on the work function of the deposited metal (here Pd or HfB2), and it can improve the on/off current ratio of a CNN device by nearly an order of magnitude. This nanosoldering technique could also be applied to other device types where nanoscale resistance components limit overall device performance.

    View details for DOI 10.1021/nl4026083

    View details for Web of Science ID 000328439200014

    View details for PubMedID 24215439

  • Helical Carbon Nanotubes Enhance the Early Immune Response and Inhibit Macrophage-Mediated Phagocytosis of Pseudomonas aeruginosa PLOS ONE Walling, B. E., Kuang, Z., Hao, Y., Estrada, D., Wood, J. D., Lian, F., Miller, L. A., Shah, A. B., Jeffries, J. L., Haasch, R. T., Lyding, J. W., Pop, E., Lau, G. W. 2013; 8 (11)


    Aerosolized or aspirated manufactured carbon nanotubes have been shown to be cytotoxic, cause pulmonary lesions, and demonstrate immunomodulatory properties. CD-1 mice were used to assess pulmonary toxicity of helical carbon nanotubes (HCNTs) and alterations of the immune response to subsequent infection by Pseudomonas aeruginosa in mice. HCNTs provoked a mild inflammatory response following either a single exposure or 2X/week for three weeks (multiple exposures) but were not significantly toxic. Administering HCNTs 2X/week for three weeks resulted in pulmonary lesions including granulomas and goblet cell hyperplasia. Mice exposed to HCNTs and subsequently infected by P. aeruginosa demonstrated an enhanced inflammatory response to P. aeruginosa and phagocytosis by alveolar macrophages was inhibited. However, clearance of P. aeruginosa was not affected. HCNT exposed mice depleted of neutrophils were more effective in clearing P. aeruginosa compared to neutrophil-depleted control mice, accompanied by an influx of macrophages. Depletion of systemic macrophages resulted in slightly inhibited bacterial clearance by HCNT treated mice. Our data indicate that pulmonary exposure to HCNTs results in lesions similar to those caused by other nanotubes and pre-exposure to HCNTs inhibit alveolar macrophage phagocytosis of P. aeruginosa. However, clearance was not affected as exposure to HCNTs primed the immune system for an enhanced inflammatory response to pulmonary infection consisting of an influx of neutrophils and macrophages.

    View details for DOI 10.1371/journal.pone.0080283

    View details for Web of Science ID 000327308500124

    View details for PubMedID 24324555

    View details for PubMedCentralID PMC3855819

  • High field breakdown characteristics of carbon nanotube thin film transistors NANOTECHNOLOGY Gupta, M. P., Behnam, A., Lian, F., Estrada, D., Pop, E., Kumar, S. 2013; 24 (40)


    The high field properties of carbon nanotube (CNT) network thin film transistors (CN-TFTs) are important for their practical operation, and for understanding their reliability. Using a combination of experimental and computational techniques we show how the channel geometry (length LC and width WC) and network morphology (average CNT length Lt and alignment angle distribution θ) affect heat dissipation and high field breakdown in such devices. The results suggest that when WC ≥ Lt, the breakdown voltage remains independent of WC but varies linearly with LC. The breakdown power varies almost linearly with both WC and LC when WC ≫ Lt. We also find that the breakdown power is more susceptible to the variability in the network morphology compared to the breakdown voltage. The analysis offers new insight into the tunable heat dissipation and thermal reliability of CN-TFTs, which can be significantly improved through optimization of the network morphology and device geometry.

    View details for DOI 10.1088/0957-4484/24/40/405204

    View details for Web of Science ID 000324516300006

    View details for PubMedID 24029606

  • High-field electrical and thermal transport in suspended graphene. Nano letters Dorgan, V. E., Behnam, A., Conley, H. J., Bolotin, K. I., Pop, E. 2013; 13 (10): 4581-4586


    We study the intrinsic transport properties of suspended graphene devices at high fields (≥1 V/μm) and high temperatures (≥1000 K). Across 15 samples, we find peak (average) saturation velocity of 3.6 × 10(7) cm/s (1.7 × 10(7) cm/s) and peak (average) thermal conductivity of 530 W m(-1) K(-1) (310 W m(-1) K(-1)) at 1000 K. The saturation velocity is 2-4 times and the thermal conductivity 10-17 times greater than in silicon at such elevated temperatures. However, the thermal conductivity shows a steeper decrease at high temperature than in graphite, consistent with stronger effects of second-order three-phonon scattering. Our analysis of sample-to-sample variation suggests the behavior of "cleaner" devices most closely approaches the intrinsic high-field properties of graphene. This study reveals key features of charge and heat flow in graphene up to device breakdown at ~2230 K in vacuum, highlighting remaining unknowns under extreme operating conditions.

    View details for DOI 10.1021/nl400197w

    View details for PubMedID 23387323

  • Conductive preferential paths of hot carriers in amorphous phase-change materials APPLIED PHYSICS LETTERS Cappelli, A., Piccinini, E., Xiong, F., Behnam, A., Brunetti, R., Rudan, M., Pop, E., Jacoboni, C. 2013; 103 (8)

    View details for DOI 10.1063/1.4819097

    View details for Web of Science ID 000323788100085

  • Compact Model for Carbon Nanotube Field-Effect Transistors Including Nonidealities and Calibrated With Experimental Data Down to 9-nm Gate Length IEEE TRANSACTIONS ON ELECTRON DEVICES Luo, J., Wei, L., Lee, C., Franklin, A. D., Guan, X., Pop, E., Antoniadis, D. A., Wong, H. P. 2013; 60 (6): 1834-1843
  • Gigahertz Integrated Graphene Ring Oscillators ACS NANO Guerriero, E., Polloni, L., Bianchi, M., Behnam, A., Carrion, E., Rizzi, L. G., Pop, E., Sordan, R. 2013; 7 (6): 5588-5594


    Ring oscillators (ROs) are the most important class of circuits used to evaluate the performance limits of any digital technology. However, ROs based on low-dimensional nanomaterials (e.g., 1-D nanotubes, nanowires, 2-D MoS2) have so far exhibited limited performance due to low current drive or large parasitics. Here we demonstrate integrated ROs fabricated from wafer-scale graphene grown by chemical vapor deposition. The highest oscillation frequency was 1.28 GHz, while the largest output voltage swing was 0.57 V. Both values remain limited by parasitic capacitances in the circuit rather than intrinsic properties of the graphene transistor components, suggesting further improvements are possible. The fabricated ROs are the fastest realized in any low-dimensional nanomaterial to date and also the least sensitive to fluctuations in the supply voltage. They represent the first integrated graphene oscillators of any kind and can also be used in a wide range of applications in analog electronics. As a demonstration, we also realized the first stand-alone graphene mixers that do not require external oscillators for frequency conversion. The first gigahertz multitransistor graphene integrated circuits demonstrated here pave the way for application of graphene in high-speed digital and analog circuits in which high operating speed could be traded off against power consumption.

    View details for DOI 10.1021/nn401933v

    View details for Web of Science ID 000321093800098

    View details for PubMedID 23713626

  • Resistive Random Access Memory Enabled by Carbon Nanotube Crossbar Electrodes ACS NANO Tsai, C., Xiong, F., Pop, E., Shim, M. 2013; 7 (6): 5360-5366


    We use single-walled carbon nanotube (CNT) crossbar electrodes to probe sub-5 nm memory domains of thin AlOx films. Both metallic and semiconducting CNTs effectively switch AlOx bits between memory states with high and low resistance. The low-resistance state scales linearly with CNT series resistance down to ∼10 MΩ, at which point the ON-state resistance of the AlOx filament becomes the limiting factor. Dependence of switching behavior on the number of cross-points suggests a single channel to dominate the overall characteristics in multi-crossbar devices. We demonstrate ON/OFF ratios up to 5 × 10(5) and programming currents of 1 to 100 nA with few-volt set/reset voltages. Remarkably low reset currents enable a switching power of 10-100 nW and estimated switching energy as low as 0.1-10 fJ per bit. These results are essential for understanding the ultimate scaling limits of resistive random access memory at single-nanometer bit dimensions.

    View details for DOI 10.1021/nn401212p

    View details for Web of Science ID 000321093800072

    View details for PubMedID 23705675

  • Direct observation of nanometer-scale Joule and Peltier effects in phase change memory devices APPLIED PHYSICS LETTERS Grosse, K. L., Xiong, F., Hong, S., King, W. P., Pop, E. 2013; 102 (19)

    View details for DOI 10.1063/1.4803172

    View details for Web of Science ID 000320440800100

  • Signatures of dynamic screening in interfacial thermal transport of graphene PHYSICAL REVIEW B Ong, Z., Fischetti, M. V., Serov, A. Y., Pop, E. 2013; 87 (19)
  • The Role of External Defects in Chemical Sensing of Graphene Field-Effect Transistors NANO LETTERS Kumar, B., Min, K., Bashirzadeh, M., Farimani, A. B., Bae, M., Estrada, D., Kim, Y. D., YASAEI, P., Park, Y. D., Pop, E., Aluru, N. R., Salehi-Khojin, A. 2013; 13 (5): 1962-1968


    A fundamental understanding of chemical sensing mechanisms in graphene-based chemical field-effect transistors (chemFETs) is essential for the development of next generation chemical sensors. Here we explore the hidden sensing modalities responsible for tailoring the gas detection ability of pristine graphene sensors by exposing graphene chemFETs to electron donor and acceptor trace gas vapors. We uncover that the sensitivity (in terms of modulation in electrical conductivity) of pristine graphene chemFETs is not necessarily intrinsic to graphene, but rather it is facilitated by external defects in the insulating substrate, which can modulate the electronic properties of graphene. We disclose a mixing effect caused by partial overlap of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of adsorbed gas molecules to explain graphene's ability to detect adsorbed molecules. Our results open a new design space, suggesting that control of external defects in supporting substrates can lead to tunable graphene chemical sensors, which could be developed without compromising the intrinsic electrical and structural properties of graphene.

    View details for DOI 10.1021/nl304734g

    View details for Web of Science ID 000318892400015

    View details for PubMedID 23586702

  • Using nanoscale thermocapillary flows to create arrays of purely semiconducting single-walled carbon nanotubes NATURE NANOTECHNOLOGY Jin, S. H., Dunham, S. N., Song, J., Xie, X., Kim, J., Lu, C., Islam, A., Du, F., Kim, J., Felts, J., Li, Y., Xiong, F., Wahab, M. A., Menon, M., Cho, E., Grosse, K. L., Lee, D. J., Chung, H. U., Pop, E., Alam, M. A., King, W. P., Huang, Y., Rogers, J. A. 2013; 8 (5): 347-355


    Among the remarkable variety of semiconducting nanomaterials that have been discovered over the past two decades, single-walled carbon nanotubes remain uniquely well suited for applications in high-performance electronics, sensors and other technologies. The most advanced opportunities demand the ability to form perfectly aligned, horizontal arrays of purely semiconducting, chemically pristine carbon nanotubes. Here, we present strategies that offer this capability. Nanoscale thermocapillary flows in thin-film organic coatings followed by reactive ion etching serve as highly efficient means for selectively removing metallic carbon nanotubes from electronically heterogeneous aligned arrays grown on quartz substrates. The low temperatures and unusual physics associated with this process enable robust, scalable operation, with clear potential for practical use. We carry out detailed experimental and theoretical studies to reveal all of the essential attributes of the underlying thermophysical phenomena. We demonstrate use of the purified arrays in transistors that achieve mobilities exceeding 1,000 cm(2) V(-1) s(-1) and on/off switching ratios of ∼10,000 with current outputs in the milliamp range. Simple logic gates built using such devices represent the first steps toward integration into more complex circuits.

    View details for DOI 10.1038/NNANO.2013.56

    View details for Web of Science ID 000318684800017

    View details for PubMedID 23624697

  • Ballistic to diffusive crossover of heat flow in graphene ribbons NATURE COMMUNICATIONS Bae, M., Li, Z., Aksamija, Z., Martin, P. N., Xiong, F., Ong, Z., Knezevic, I., Pop, E. 2013; 4


    Heat flow in nanomaterials is an important area of study, with both fundamental and technological implications. However, little is known about heat flow in two-dimensional devices or interconnects with dimensions comparable to the phonon mean free path. Here we find that short, quarter-micron graphene samples reach ~35% of the ballistic thermal conductance limit up to room temperature, enabled by the relatively large phonon mean free path (~100 nm) in substrate-supported graphene. In contrast, patterning similar samples into nanoribbons leads to a diffusive heat-flow regime that is controlled by ribbon width and edge disorder. In the edge-controlled regime, the graphene nanoribbon thermal conductivity scales with width approximately as ~W(1.8)(0.3), being about 100 W m(-1) K(-1) in 65-nm-wide graphene nanoribbons, at room temperature. These results show how manipulation of two-dimensional device dimensions and edges can be used to achieve full control of their heat-carrying properties, approaching fundamentally limited upper or lower bounds.

    View details for DOI 10.1038/ncomms2755

    View details for Web of Science ID 000318872100091

    View details for PubMedID 23591901

  • InxGa1-xAs Nanowire Growth on Graphene: van der Waals Epitaxy Induced Phase Segregation NANO LETTERS Mohseni, P. K., Behnam, A., Wood, J. D., English, C. D., Lyding, J. W., Pop, E., Li, X. 2013; 13 (3): 1153-1161


    The growth of high-density arrays of vertically oriented, single crystalline InAs NWs on graphene surfaces are realized through the van der Waals (vdW) epitaxy mechanism by metalorganic chemical vapor deposition (MOCVD). However, the growth of InGaAs NWs on graphene results in spontaneous phase separation starting from the beginning of growth, yielding a well-defined InAs-In(x)Ga(1-x)As (0.2 < x < 1) core-shell structure. The core-shell structure then terminates abruptly after about 2 μm in height, and axial growth of uniform composition In(x)Ga(1-x)As takes place without a change in the NW diameter. The In(x)Ga(1-x)As shell composition changes as a function of indium flow, but the core and shell thicknesses and the onset of nonsegregated In(x)Ga(1-x)As axial segment are independent of indium composition. In contrast, no InGaAs phase segregation has been observed when growing on MoS2, another two-dimensional (2D) layered material, or via the Au-assisted vapor-liquid-solid (VLS) mechanism on graphene. This spontaneous phase segregation phenomenon is elucidated as a special case of van der Waals epitaxy on 2D sheets. Considering the near lattice matched registry between InAs and graphene, InGaAs is forced to self-organize into InAs core and InGaAs shell segments since the lack of dangling bonds on graphene does not allow strain sharing through elastic deformation between InGaAs and graphene.

    View details for DOI 10.1021/nl304569d

    View details for Web of Science ID 000316243800045

    View details for PubMedID 23421807

  • Role of Joule Heating on Current Saturation and Transient Behavior of Graphene Transistors IEEE ELECTRON DEVICE LETTERS Islam, S., Li, Z., Dorgan, V. E., Bae, M., Pop, E. 2013; 34 (2): 166-168
  • Self-Aligned Nanotube-Nanowire Phase Change Memory NANO LETTERS Xiong, F., Bae, M., Dai, Y., Liao, A. D., Behnam, A., Carrion, E. A., Hong, S., Ielmini, D., Pop, E. 2013; 13 (2): 464-469


    A central issue of nanoelectronics concerns their fundamental scaling limits, that is, the smallest and most energy-efficient devices that can function reliably. Unlike charge-based electronics that are prone to leakage at nanoscale dimensions, memory devices based on phase change materials (PCMs) are more scalable, storing digital information as the crystalline or amorphous state of a material. Here, we describe a novel approach to self-align PCM nanowires with individual carbon nanotube (CNT) electrodes for the first time. The highly scaled and spatially confined memory devices approach the ultimate scaling limits of PCM technology, achieving ultralow programming currents (~0.1 μA set, ~1.6 μA reset), outstanding on/off ratios (~10(3)), and improved endurance and stability at few-nanometer bit dimensions. In addition, the powerful yet simple nanofabrication approach described here can enable confining and probing many other nanoscale and molecular devices self-aligned with CNT electrodes.

    View details for DOI 10.1021/nl3038097

    View details for Web of Science ID 000315079500022

    View details for PubMedID 23259592

  • Effect of grain boundaries on thermal transport in graphene APPLIED PHYSICS LETTERS Serov, A. Y., Ong, Z., Pop, E. 2013; 102 (3)

    View details for DOI 10.1063/1.4776667

    View details for Web of Science ID 000314032600068

  • Metal-semiconductor-metal photodetectors based on graphene/p-type silicon Schottky junctions APPLIED PHYSICS LETTERS An, Y., Behnam, A., Pop, E., Ural, A. 2013; 102 (1)

    View details for DOI 10.1063/1.4773992

    View details for Web of Science ID 000313646500104

  • Atomic-Scale Evidence for Potential Barriers and Strong Carrier Scattering at Graphene Grain Boundaries: A Scanning Tunneling Microscopy Study ACS NANO Koepke, J. C., Wood, J. D., Estrada, D., Ong, Z., He, K. T., Pop, E., Lyding, J. W. 2013; 7 (1): 75-86


    We use scanning tunneling microscopy and spectroscopy to examine the electronic nature of grain boundaries (GBs) in polycrystalline graphene grown by chemical vapor deposition (CVD) on Cu foil and transferred to SiO(2) substrates. We find no preferential orientation angle between grains, and the GBs are continuous across graphene wrinkles and SiO(2) topography. Scanning tunneling spectroscopy shows enhanced empty states tunneling conductance for most of the GBs and a shift toward more n-type behavior compared to the bulk of the graphene. We also observe standing wave patterns adjacent to GBs propagating in a zigzag direction with a decay length of ~1 nm. Fourier analysis of these patterns indicates that backscattering and intervalley scattering are the dominant mechanisms responsible for the mobility reduction in the presence of GBs in CVD-grown graphene.

    View details for DOI 10.1021/nn302064p

    View details for Web of Science ID 000314082800012

    View details for PubMedID 23237026

  • Substrate Dependent High-Field Transport of Graphene Transistors 71st Device Research Conference (DRC) Islam, S., Serov, A. Y., Meric, I., Shepard, K. L., Pop, E. IEEE. 2013: 35–36
  • Realistic Simulation of Graphene Transistors Including Non-Ideal Electrostatics 71st Device Research Conference (DRC) Serov, A. Y., Islam, S., Pop, E. IEEE. 2013: 31–32
  • Reliability, Failure, and Fundamental Limits of Graphene and Carbon Nanotube Interconnects IEEE International Electron Devices Meeting (IEDM) Liao, A. D., Behnam, A., Dorgan, V. E., Li, Z., Pop, E. IEEE. 2013
  • Novel 3D random-network model for threshold switching of phase-change memories IEEE International Electron Devices Meeting (IEDM) Piccinini, E., Cappelli, A., Xiong, F., Behnam, A., Buscemi, F., Brunetti, R., Rudan, M., Pop, E., Jacoboni, C. IEEE. 2013
  • High-Field Transport and Thermal Reliability of Sorted Carbon Nanotube Network Devices ACS NANO Behnam, A., Sangwan, V. K., Zhong, X., Lian, F., Estrada, D., Jariwala, D., Hoag, A. J., Lauhon, L. J., Marks, T. J., Hersam, M. C., Pop, E. 2013; 7 (1): 482-490


    We examine the high-field operation, power dissipation, and thermal reliability of sorted carbon nanotube network (CNN) devices, with <1% to >99% semiconducting nanotubes. We combine systematic electrical measurements with infrared (IR) thermal imaging and detailed Monte Carlo simulations to study high-field transport up to CNN failure by unzipping-like breakdown. We find that metallic CNNs carry peak current densities up to an order of magnitude greater than semiconducting CNNs at comparable nanotube densities. Metallic CNNs also appear to have a factor of 2 lower intrinsic thermal resistance, suggesting a lower thermal resistance at metallic nanotube junctions. The performance limits and reliability of CNNs depend on their makeup, and could be improved by carefully engineered heat dissipation through the substrate, contacts, and nanotube junctions. These results are essential for optimization of CNN devices on transparent or flexible substrates which typically have very low thermal conductivity.

    View details for DOI 10.1021/nn304570u

    View details for Web of Science ID 000314082800052

    View details for PubMedID 23259715

  • Electrochemistry at the Edge of a Single Graphene Layer in a Nanopore ACS NANO Banerjee, S., Shim, J., Rivera, J., Jin, X., Estrada, D., Solovyeva, V., You, X., Pak, J., Pop, E., Aluru, N., Bashir, R. 2013; 7 (1): 834-843


    We study the electrochemistry of single layer graphene edges using a nanopore-based structure consisting of stacked graphene and Al(2)O(3) dielectric layers. Nanopores, with diameters ranging from 5 to 20 nm, are formed by an electron beam sculpting process on the stacked layers. This leads to a unique edge structure which, along with the atomically thin nature of the embedded graphene electrode, demonstrates electrochemical current densities as high as 1.2 × 10(4) A/cm(2). The graphene edge embedded structure offers a unique capability to study the electrochemical exchange at an individual graphene edge, isolated from the basal plane electrochemical activity. We also report ionic current modulation in the nanopore by biasing the embedded graphene terminal with respect to the electrodes in the fluid. The high electrochemical specific current density for a graphene nanopore-based device can have many applications in sensitive chemical and biological sensing, and energy storage devices.

    View details for DOI 10.1021/nn305400n

    View details for Web of Science ID 000314082800091

    View details for PubMedID 23249127

    View details for PubMedCentralID PMC3551991

  • Impact of thermal boundary conductances on power dissipation and electrical breakdown of carbon nanotube network transistors JOURNAL OF APPLIED PHYSICS Gupta, M. P., Chen, L., Estrada, D., Behnam, A., Pop, E., Kumar, S. 2012; 112 (12)

    View details for DOI 10.1063/1.4767920

    View details for Web of Science ID 000312829400129

  • Thermal properties of graphene: Fundamentals and applications MRS BULLETIN Pop, E., Varshney, V., Roy, A. K. 2012; 37 (12): 1273-1281
  • Quantitative Thermal Imaging of Single-Walled Carbon Nanotube Devices by Scanning Joule Expansion Microscopy ACS NANO Xie, X., Grosse, K. L., Song, J., Lu, C., Dunham, S., Du, F., Islam, A. E., Li, Y., Zhang, Y., Pop, E., Huang, Y., King, W. P., Rogers, J. A. 2012; 6 (11): 10267-10275


    Electrical generation of heat in single-walled carbon nanotubes (SWNTs) and subsequent thermal transport into the surroundings can critically affect the design, operation, and reliability of electronic and optoelectronic devices based on these materials. Here we investigate such heat generation and transport characteristics in perfectly aligned, horizontal arrays of SWNTs integrated into transistor structures. We present quantitative assessments of local thermometry at individual SWNTs in these arrays, evaluated using scanning Joule expansion microscopy. Measurements at different applied voltages reveal electronic behaviors, including metallic and semiconducting responses, spatial variations in diameter or chirality, and localized defect sites. Analytical models, validated by measurements performed on different device structures at various conditions, enable accurate, quantitative extraction of temperature distributions at the level of individual SWNTs. Using current equipment, the spatial resolution and temperature precision are as good as ∼100 nm and ∼0.7 K, respectively.

    View details for DOI 10.1021/nn304083a

    View details for Web of Science ID 000311521700101

    View details for PubMedID 23061768

  • Transport in Nanoribbon Interconnects Obtained from Graphene Grown by Chemical Vapor Deposition NANO LETTERS Behnam, A., Lyons, A. S., Bae, M., Chow, E. K., Islam, S., Neumann, C. M., Pop, E. 2012; 12 (9): 4424-4430


    We study graphene nanoribbon (GNR) interconnects obtained from graphene grown by chemical vapor deposition (CVD). We report low- and high-field electrical measurements over a wide temperature range, from 1.7 to 900 K. Room temperature mobilities range from 100 to 500 cm(2)·V(-1)·s(-1), comparable to GNRs from exfoliated graphene, suggesting that bulk defects or grain boundaries play little role in devices smaller than the CVD graphene crystallite size. At high-field, peak current densities are limited by Joule heating, but a small amount of thermal engineering allows us to reach ∼2 × 10(9) A/cm(2), the highest reported for nanoscale CVD graphene interconnects. At temperatures below ∼5 K, short GNRs act as quantum dots with dimensions comparable to their lengths, highlighting the role of metal contacts in limiting transport. Our study illustrates opportunities for CVD-grown GNRs, while revealing variability and contacts as remaining future challenges.

    View details for DOI 10.1021/nl300584r

    View details for Web of Science ID 000308576000002

    View details for PubMedID 22853618

  • Cascading Wafer-Scale Integrated Graphene Complementary Inverters under Ambient Conditions NANO LETTERS Rizzi, L. G., Bianchi, M., Behnam, A., Carrion, E., Guerriero, E., Polloni, L., Pop, E., Sordan, R. 2012; 12 (8): 3948-3953


    The fundamental building blocks of digital electronics are logic gates which must be capable of cascading such that more complex logic functions can be realized. Here we demonstrate integrated graphene complementary inverters which operate with the same input and output voltage logic levels, thus allowing cascading. We obtain signal matching under ambient conditions with inverters fabricated from wafer-scale graphene grown by chemical vapor deposition (CVD). Monolayer graphene was incorporated in self-aligned field-effect transistors in which the top gate overlaps with the source and drain contacts. This results in full-channel gating and leads to the highest low-frequency voltage gain reported so far in top-gated CVD graphene devices operating in air ambient, A(v) ∼ -5. Such gain enabled logic inverters with the same voltage swing of 0.56 V at their input and output. Graphene inverters could find their way in realistic applications where high-speed operation is desired but power dissipation is not a concern, similar to emitter-coupled logic.

    View details for DOI 10.1021/nl301079r

    View details for Web of Science ID 000307211000013

    View details for PubMedID 22793169

  • Scanning Tunneling Microscopy Study and Nanomanipulation of Graphene-Coated Water on Mica NANO LETTERS He, K. T., Wood, J. D., Doidge, G. P., Pop, E., Lyding, J. W. 2012; 12 (6): 2665-2672


    We study interfacial water trapped between a sheet of graphene and a muscovite (mica) surface using Raman spectroscopy and ultrahigh vacuum scanning tunneling microscopy (UHV-STM) at room temperature. We are able to image the graphene-water interface with atomic resolution, revealing a layered network of water trapped underneath the graphene. We identify water layer numbers with a carbon nanotube height reference. Under normal scanning conditions, the water structures remain stable. However, at greater electron energies, we are able to locally manipulate the water using the STM tip.

    View details for DOI 10.1021/nl202613t

    View details for Web of Science ID 000305106400003

    View details for PubMedID 22612064

  • Effect of Carbon Nanotube Network Morphology on Thin Film Transistor Performance NANO RESEARCH Timmermans, M. Y., Estrada, D., Nasibulin, A. G., Wood, J. D., Behnam, A., Sun, D., Ohno, Y., Lyding, J. W., Hassanien, A., Pop, E., Kauppinen, E. I. 2012; 5 (5): 307-319
  • Effects of tip-nanotube interactions on atomic force microscopy imaging of carbon nanotubes NANO RESEARCH Alizadegan, R., Liao, A. D., Xiong, F., Pop, E., Hsia, K. J. 2012; 5 (4): 235-247
  • Nanometalization of single-wall carbon nanotubes and graphene quantum dots Symposium on Ionic Liquids - Science and Applications / 243rd National Spring Meeting of the American-Chemical-Society Ye, W., Martin, P. A., Kumar, N., Estrada, D., Daly, S. R., Rockett, A. A., Abelson, J. R., Pop, E., Girolami, G. S., Lyding, J. W. AMER CHEMICAL SOC. 2012
  • New Technique of DNA Sensing: Nanoribbon Transverse Electrodes 56th Annual Meeting of the Biophysical-Society Solovyeva, V., Chow, E., Bae, M., Estrada, D., Banerjee, S., Behnam, A., Dorgan, V. E., Chang, W., Pop, E., Bashir, R. CELL PRESS. 2012: 428A–428A
  • Chemical sensors based on randomly stacked graphene flakes APPLIED PHYSICS LETTERS Salehi-Khojin, A., Estrada, D., Lin, K. Y., Ran, K., Haasch, R. T., Zuo, J., Pop, E., Masel, R. I. 2012; 100 (3)

    View details for DOI 10.1063/1.3676276

    View details for Web of Science ID 000299386800049

  • Atomic-scale Study of Scattering and Electronic Properties of CVD Graphene Grain Boundaries 12th IEEE International Conference on Nanotechnology (IEEE-NANO) Koepke, J. C., Wood, J. D., Estrada, D., Ong, Z., Xiong, F., Pop, E., Lyding, J. W. IEEE. 2012
  • IMPACT OF CONTACT RESISTANCES ON ELECTRICAL AND THERMAL TRANSPORT IN CARBON NANOTUBE NETWORK TRANSISTORS 3rd ASME Micro/Nanoscale Heat and Mass Transfer International Conference (MNHMT2012) Gupta, M. P., Estrada, D., Pop, E., Kumar, S. AMER SOC MECHANICAL ENGINEERS. 2012: 769–776
  • Scanning Tunneling Microscopy Characterization of Graphene-coated Few-layered Water on Mica 12th IEEE International Conference on Nanotechnology (IEEE-NANO) He, K. T., Wood, J. D., Doidge, G. P., Pop, E., Lyding, J. W. IEEE. 2012
  • Nanosoldering Carbon Nanotube Junctions with Metal via Local Chemical Vapor Deposition for Improved Device Performance 12th IEEE International Conference on Nanotechnology (IEEE-NANO) Do, J., Estrada, D., Xie, X., Chang, N. N., Girolami, G. S., Rogers, J. A., Pop, E., Lyding, J. W. IEEE. 2012
  • Graphene Nanopores for Nucleic Acid Analysis 12th IEEE International Conference on Nanotechnology (IEEE-NANO) Shim, J., Solovyeva, V., Estrada, D., Banerjee, S., Rivera, J., Pop, E., Bashir, R. IEEE. 2012
  • Improved Graphene Growth and Fluorination on Cu with Clean Transfer to Surfaces 12th IEEE International Conference on Nanotechnology (IEEE-NANO) Wood, J. D., Schmucker, S. W., Raasch, R. T., Doidge, G. P., Nienhaus, L., Damhorst, G. L., Lyons, A. S., Gruebele, M., Bashir, R., Pop, E., Lyding, J. W. IEEE. 2012
  • Stacked Graphene-Al2O3 Nanopore Sensors for Sensitive Detection of DNA and DNA-Protein Complexes ACS NANO Venkatesan, B. M., Estrada, D., Banerjee, S., Jin, X., Dorgan, V. E., Bae, M., Aluru, N. R., Pop, E., Bashir, R. 2012; 6 (1): 441-450


    We report the development of a multilayered graphene-Al(2)O(3) nanopore platform for the sensitive detection of DNA and DNA-protein complexes. Graphene-Al(2)O(3) nanolaminate membranes are formed by sequentially depositing layers of graphene and Al(2)O(3), with nanopores being formed in these membranes using an electron-beam sculpting process. The resulting nanopores are highly robust, exhibit low electrical noise (significantly lower than nanopores in pure graphene), are highly sensitive to electrolyte pH at low KCl concentrations (attributed to the high buffer capacity of Al(2)O(3)), and permit the electrical biasing of the embedded graphene electrode, thereby allowing for three terminal nanopore measurements. In proof-of-principle biomolecule sensing experiments, the folded and unfolded transport of single DNA molecules and RecA-coated DNA complexes could be discerned with high temporal resolution. The process described here also enables nanopore integration with new graphene-based structures, including nanoribbons and nanogaps, for single-molecule DNA sequencing and medical diagnostic applications.

    View details for DOI 10.1021/nn203769e

    View details for Web of Science ID 000299368300054

    View details for PubMedID 22165962

    View details for PubMedCentralID PMC3265664

  • Polycrystalline Graphene Ribbons as Chemiresistors ADVANCED MATERIALS Salehi-Khojin, A., Estrada, D., Lin, K. Y., Bae, M., Xiong, F., Pop, E., Masel, R. I. 2012; 24 (1): 53-?

    View details for DOI 10.1002/adma.201102663

    View details for Web of Science ID 000298602300004

    View details for PubMedID 22113971

  • Pressure tuning of the thermal conductance of weak interfaces PHYSICAL REVIEW B Hsieh, W., Lyons, A. S., Pop, E., Keblinski, P., Cahill, D. G. 2011; 84 (18)
  • A Web Service and Interface for Remote Electronic Device Characterization IEEE TRANSACTIONS ON EDUCATION Dutta, S., Prakash, S., Estrada, D., Pop, E. 2011; 54 (4): 646-651
  • Effects of Polycrystalline Cu Substrate on Graphene Growth by Chemical Vapor Deposition NANO LETTERS Wood, J. D., Schmucker, S. W., Lyons, A. S., Pop, E., Lyding, J. W. 2011; 11 (11): 4547-4554


    Chemical vapor deposition of graphene on Cu often employs polycrystalline Cu substrates with diverse facets, grain boundaries (GBs), annealing twins, and rough sites. Using scanning electron microscopy (SEM), electron-backscatter diffraction (EBSD), and Raman spectroscopy on graphene and Cu, we find that Cu substrate crystallography affects graphene growth more than facet roughness. We determine that (111) containing facets produce pristine monolayer graphene with higher growth rate than (100) containing facets, especially Cu(100). The number of graphene defects and nucleation sites appears Cu facet invariant at growth temperatures above 900 °C. Engineering Cu to have (111) surfaces will cause monolayer, uniform graphene growth.

    View details for DOI 10.1021/nl201566c

    View details for Web of Science ID 000296674700008

    View details for PubMedID 21942318

  • Reduction of phonon lifetimes and thermal conductivity of a carbon nanotube on amorphous silica PHYSICAL REVIEW B Ong, Z., Pop, E., Shiomi, J. 2011; 84 (16)
  • Scaling of High-Field Transport and Localized Heating in Graphene Transistors ACS NANO Bae, M., Islam, S., Dorgan, V. E., Pop, E. 2011; 5 (10): 7936-7944


    We use infrared thermal imaging and electrothermal simulations to find that localized Joule heating in graphene field-effect transistors on SiO(2) is primarily governed by device electrostatics. Hot spots become more localized (i.e., sharper) as the underlying oxide thickness is reduced, such that the average and peak device temperatures scale differently, with significant long-term reliability implications. The average temperature is proportional to oxide thickness, but the peak temperature is minimized at an oxide thickness of ∼90 nm due to competing electrostatic and thermal effects. We also find that careful comparison of high-field transport models with thermal imaging can be used to shed light on velocity saturation effects. The results shed light on optimizing heat dissipation and reliability of graphene devices and interconnects.

    View details for DOI 10.1021/nn202239y

    View details for Web of Science ID 000296208700042

    View details for PubMedID 21913673

  • Electronic, optical and thermal properties of the hexagonal and rocksalt-like Ge2Sb2Te5 chalcogenide from first-principle calculations JOURNAL OF APPLIED PHYSICS Tsafack, T., Piccinini, E., Lee, B., Pop, E., Rudan, M. 2011; 110 (6)

    View details for DOI 10.1063/1.3639279

    View details for Web of Science ID 000295619300071

  • Stretchable, Transparent Graphene Interconnects for Arrays of Microscale Inorganic Light Emitting Diodes on Rubber Substrates NANO LETTERS Kim, R., Bae, M., Kim, D. G., Cheng, H., Kim, B. H., Kim, D., Li, M., Wu, J., Du, F., Kim, H., Kim, S., Estrada, D., Hong, S. W., Huang, Y., Pop, E., Rogers, J. A. 2011; 11 (9): 3881-3886


    This paper describes the fabrication and design principles for using transparent graphene interconnects in stretchable arrays of microscale inorganic light emitting diodes (LEDs) on rubber substrates. We demonstrate several appealing properties of graphene for this purpose, including its ability to spontaneously conform to significant surface topography, in a manner that yields effective contacts even to deep, recessed device regions. Mechanics modeling reveals the fundamental aspects of this process, as well as the use of the same layers of graphene for interconnects designed to accommodate strains of 100% or more, in a completely reversible fashion. These attributes are compatible with conventional thin film processing and can yield high-performance devices in transparent layouts. Graphene interconnects possess attractive features for both existing and emerging applications of LEDs in information display, biomedical systems, and other environments.

    View details for DOI 10.1021/nl202000u

    View details for Web of Science ID 000294790200064

    View details for PubMedID 21790143

  • Effect of substrate modes on thermal transport in supported graphene PHYSICAL REVIEW B Ong, Z., Pop, E. 2011; 84 (7)
  • Electrical power dissipation in semiconducting carbon nanotubes on single crystal quartz and amorphous SiO2 APPLIED PHYSICS LETTERS Tsai, C., liao, A., Pop, E., Shim, M. 2011; 99 (5)

    View details for DOI 10.1063/1.3622769

    View details for Web of Science ID 000293617300082

  • Thermally Limited Current Carrying Ability of Graphene Nanoribbons PHYSICAL REVIEW LETTERS Liao, A. D., Wu, J. Z., Wang, X., Tahy, K., Jena, D., Dai, H., Pop, E. 2011; 106 (25)


    We investigate high-field transport in graphene nanoribbons (GNRs) on SiO(2), up to breakdown. The maximum current density is limited by self-heating, but can reach >3 mA/μm for GNRs ~15 nm wide. Comparison with larger, micron-sized graphene devices reveals that narrow GNRs benefit from 3D heat spreading into the SiO(2), which enables their higher current density. GNRs also benefit from lateral heat flow to the contacts in short devices (<~0.3 μm), which allows extraction of a median GNR thermal conductivity (TC), ~80 W m(-1)K(-1) at 20 °C across our samples, dominated by phonons. The TC of GNRs is an order of magnitude lower than that of micron-sized graphene on SiO(2), suggesting strong roles of edge and defect scattering, and the importance of thermal dissipation in small GNR devices.

    View details for DOI 10.1103/PhysRevLett.106.256801

    View details for Web of Science ID 000291801900010

    View details for PubMedID 21770659

  • Nanoscale Joule heating, Peltier cooling and current crowding at graphene-metal contacts NATURE NANOTECHNOLOGY Grosse, K. L., Bae, M., Lian, F., Pop, E., King, W. P. 2011; 6 (5): 287-290


    The performance and scaling of graphene-based electronics is limited by the quality of contacts between the graphene and metal electrodes. However, the nature of graphene-metal contacts remains incompletely understood. Here, we use atomic force microscopy to measure the temperature distributions at the contacts of working graphene transistors with a spatial resolution of ~ 10 nm (refs 5-8), allowing us to identify the presence of Joule heating, current crowding and thermoelectric heating and cooling. Comparison with simulation enables extraction of the contact resistivity (150-200 Ω µm²) and transfer length (0.2-0.5 µm) in our devices; these generally limit performance and must be minimized. Our data indicate that thermoelectric effects account for up to one-third of the contact temperature changes, and that current crowding accounts for most of the remainder. Modelling predicts that the role of current crowding will diminish and the role of thermoelectric effects will increase as contacts improve.

    View details for DOI 10.1038/NNANO.2011.39

    View details for Web of Science ID 000290301300008

    View details for PubMedID 21460825

  • Low-Power Switching of Phase-Change Materials with Carbon Nanotube Electrodes SCIENCE Xiong, F., Liao, A. D., Estrada, D., Pop, E. 2011; 332 (6029): 568-570


    Phase-change materials (PCMs) are promising candidates for nonvolatile data storage and reconfigurable electronics, but high programming currents have presented a challenge to realize low-power operation. We controlled PCM bits with single-wall and small-diameter multi-wall carbon nanotubes. This configuration achieves programming currents of 0.5 microampere (set) and 5 microamperes (reset), two orders of magnitude lower than present state-of-the-art devices. Pulsed measurements enable memory switching with very low energy consumption. Analysis of over 100 devices finds that the programming voltage and energy are highly scalable and could be below 1 volt and single femtojoules per bit, respectively.

    View details for DOI 10.1126/science.1201938

    View details for Web of Science ID 000289991100042

    View details for PubMedID 21393510

  • Imaging dissipation and hot spots in carbon nanotube network transistors APPLIED PHYSICS LETTERS Estrada, D., Pop, E. 2011; 98 (7)

    View details for DOI 10.1063/1.3549297

    View details for Web of Science ID 000287507200058

  • Reliably Counting Atomic Planes of Few-Layer Graphene (n > 4) ACS NANO Koh, Y. K., Bae, M., Cahill, D. G., Pop, E. 2011; 5 (1): 269-274


    We demonstrate a reliable technique for counting atomic planes (n) of few-layer graphene (FLG) on SiO(2)/Si substrates by Raman spectroscopy. Our approach is based on measuring the ratio of the integrated intensity of the G graphene peak and the optical phonon peak of Si, I(G)/I(Si), and is particularly useful in the range n > 4 where few methods exist. We compare our results with atomic force microscopy (AFM) measurements and Fresnel equation calculations. Then, we apply our method to unambiguously identify n of FLG devices on SiO(2) and find that the mobility (μ ≈ 2000 cm(2) V(-1) s(-1)) is independent of layer thickness for n > 4. Our findings suggest that electrical transport in gated FLG devices is dominated by carriers near the FLG/SiO(2) interface and is thus limited by the environment, even for n > 4.

    View details for DOI 10.1021/nn102658a

    View details for Web of Science ID 000286487300034

    View details for PubMedID 21138311

  • Imaging, Simulation, and Electrostatic Control of Power Dissipation in Graphene Devices NANO LETTERS Bae, M., Ong, Z., Estrada, D., Pop, E. 2010; 10 (12): 4787-4793

    View details for DOI 10.1021/nl1011596

    View details for Web of Science ID 000284990900003

  • Frequency and polarization dependence of thermal coupling between carbon nanotubes and SiO2 JOURNAL OF APPLIED PHYSICS Ong, Z., Pop, E. 2010; 108 (10)

    View details for DOI 10.1063/1.3484494

    View details for Web of Science ID 000285005000042

  • Thermal dissipation and variability in electrical breakdown of carbon nanotube devices PHYSICAL REVIEW B liao, A., Alizadegan, R., Ong, Z., Dutta, S., Xiong, F., Hsia, K. J., Pop, E. 2010; 82 (20)
  • Heat Conduction across Monolayer and Few-Layer Graphenes NANO LETTERS Koh, Y. K., Bae, M., Cahill, D. G., Pop, E. 2010; 10 (11): 4363-4368


    We report the thermal conductance G of Au/Ti/graphene/SiO(2) interfaces (graphene layers 1 ≤ n ≤ 10) typical of graphene transistor contacts. We find G ≈ 25 MW m(-2) K(-1) at room temperature, four times smaller than the thermal conductance of a Au/Ti/SiO(2) interface, even when n = 1. We attribute this reduction to the thermal resistance of Au/Ti/graphene and graphene/SiO(2) interfaces acting in series. The temperature dependence of G from 50 ≤ T ≤ 500 K also indicates that heat is predominantly carried by phonons through these interfaces. Our findings suggest that metal contacts can limit not only electrical transport but also thermal dissipation from submicrometer graphene devices.

    View details for DOI 10.1021/nl101790k

    View details for Web of Science ID 000283907600012

    View details for PubMedID 20923234

  • Mobility and saturation velocity in graphene on SiO2 APPLIED PHYSICS LETTERS Dorgan, V. E., Bae, M., Pop, E. 2010; 97 (8)

    View details for DOI 10.1063/1.3483130

    View details for Web of Science ID 000281306500042

  • Molecular dynamics simulation of thermal boundary conductance between carbon nanotubes and SiO2 PHYSICAL REVIEW B Ong, Z., Pop, E. 2010; 81 (15)
  • Covalent Functionalization and Electron-Transfer Properties of Vertically Aligned Carbon Nanofibers: The Importance of Edge-Plane Sites CHEMISTRY OF MATERIALS Landis, E. C., Klein, K. L., liao, A., Pop, E., Hensley, D. K., Melechko, A. V., Hamers, R. J. 2010; 22 (7): 2357-2366

    View details for DOI 10.1021/cm9036132

    View details for Web of Science ID 000276394800025

  • Reduced Thermal Conductivity in Nanoengineered Rough Ge and GaAs Nanowires NANO LETTERS Martin, P. N., Aksamija, Z., Pop, E., Ravaioli, U. 2010; 10 (4): 1120-1124


    We model and compare the thermal conductivity of rough semiconductor nanowires (NWs) of Si, Ge, and GaAs for thermoelectric devices. On the basis of full phonon dispersion relations, the effect of NW surface roughness on thermal conductivity is derived from perturbation theory and appears as an efficient way to scatter phonons in Si, Ge, and GaAs NWs with diameter D < 200 nm. For small diameters and large root-mean-square roughness Delta, thermal conductivity is limited by surface asperities and varies quadratically as (D/Delta)(2). At room temperature, our model previously agreed with experimental observations of thermal conductivity down to 2 W m(-1) K(-1) in rough 56 nm Si NWs with Delta = 3 nm. In comparison to Si, we predict here remarkably low thermal conductivity in Ge and GaAs NWs of 0.1 and 0.4 W m(-1) K(-1), respectively, at similar roughness and diameter.

    View details for DOI 10.1021/nl902720v

    View details for Web of Science ID 000276557100004

    View details for PubMedID 20222669

  • Energy Dissipation and Transport in Nanoscale Devices NANO RESEARCH Pop, E. 2010; 3 (3): 147-169
  • Reduction of hysteresis for carbon nanotube mobility measurements using pulsed characterization NANOTECHNOLOGY Estrada, D., Dutta, S., liao, A., Pop, E. 2010; 21 (8)


    We describe a pulsed measurement technique for suppressing hysteresis for carbon nanotube (CNT) device measurements in air, vacuum, and over a wide temperature range (80-453 K). Varying the gate pulse width and duty cycle probes the relaxation times associated with charge trapping near the CNT, found to be up to the 0.1-10 s range. Longer off times between voltage pulses enable consistent, hysteresis-free measurements of CNT mobility. A tunneling front model for charge trapping and relaxation is also described, suggesting trap depths up to 4-8 nm for CNTs on SiO2. Pulsed measurements will also be applicable for other nanoscale devices such as graphene, nanowires, or molecular electronics, and could enable probing trap relaxation times in a variety of material system interfaces.

    View details for DOI 10.1088/0957-4484/21/8/085702

    View details for Web of Science ID 000273965000012

    View details for PubMedID 20097980

  • Topography and refractometry of nanostructures using spatial light interference microscopy OPTICS LETTERS Wang, Z., Chun, I. S., Li, X., Ong, Z., Pop, E., Millet, L., Gillette, M., Popescu, G. 2010; 35 (2): 208-210


    Spatial light interference microscopy (SLIM) is a novel method developed in our laboratory that provides quantitative phase images of transparent structures with a 0.3 nm spatial and 0.03 nm temporal accuracy owing to the white light illumination and its common path interferometric geometry. We exploit these features and demonstrate SLIM's ability to perform topography at a single atomic layer in graphene. Further, using a decoupling procedure that we developed for cylindrical structures, we extract the axially averaged refractive index of semiconductor nanotubes and a neurite of a live hippocampal neuron in culture. We believe that this study will set the basis for novel high-throughput topography and refractometry of man-made and biological nanostructures.

    View details for Web of Science ID 000273879200039

    View details for PubMedID 20081970

  • Infrared Imaging of Heat Dissipation in Graphene Transistors 2nd International Symposium on Graphene, Ge/III-V and Emerging Materials For Post-CMOS Applications / 217th Meeting of the Electrochemical Society (ECS) Bae, M., Ong, Z., Estrada, D., Pop, E. ELECTROCHEMICAL SOC INC. 2010: 51–62

    View details for DOI 10.1149/1.3367936

    View details for Web of Science ID 000313494400006

  • Modeling of the Voltage Snap-Back in Amorphous-GST Memory Devices 15th International Conference on Simulation of Semiconductor Processes and Devices (SISPAD 2010) Rudan, M., Giovanardi, F., Tsafack, T., Xiong, F., Piccinini, E., Buscemi, F., Liao, A., Pop, E., Brunetti, R., JACOBONI, C. IEEE. 2010: 257–260
  • Inducing chalcogenide phase change with ultra-narrow carbon nanotube heaters APPLIED PHYSICS LETTERS Xiong, F., liao, A., Pop, E. 2009; 95 (24)

    View details for DOI 10.1063/1.3273370

    View details for Web of Science ID 000272954900050

  • Multiband Mobility in Semiconducting Carbon Nanotubes IEEE ELECTRON DEVICE LETTERS Zhao, Y., Liao, A., Pop, E. 2009; 30 (10): 1078-1080
  • Compact Thermal Model for Vertical Nanowire Phase-Change Memory Cells IEEE TRANSACTIONS ON ELECTRON DEVICES Chen, I., Pop, E. 2009; 56 (7): 1523-1528
  • Electrical and Thermal Coupling to a Single-Wall Carbon Nanotube Device Using an Electrothermal Nanoprobe NANO LETTERS Lee, J., liao, A., Pop, E., King, W. P. 2009; 9 (4): 1356-1361


    We utilize a multifunctional atomic force microscope (AFM) cantilever applying highly localized temperature and electric fields to interrogate transport in single-wall carbon nanotube field-effect transistors (CNTFETs). The probe can be operated either in contact with the CNT, in intermittent contact, or as a Kelvin probe, and can independently control the electric field, mechanical force, and temperature applied to the CNT. We modulate current flow in the CNT with tip-applied electric field, and find this field-effect depends upon both cantilever heating and CNT self-heating. CNT transport is also investigated with AFM tip temperature up to 1170 degrees C. Tip-CNT thermal resistance is estimated at 1.6 x 10(7) K/W and decreases with increasing temperature. Threshold force (<100 nN) for reliable contact mode imaging is extracted and used to determine set points for nanotube manipulation, such as displacement or cutting. The ability to measure thermal coupling to a single-molecule electronic device could offer new insights into nanoelectronic devices.

    View details for DOI 10.1021/nl803024p

    View details for Web of Science ID 000265030000014

    View details for PubMedID 19245239

  • Impact of Phonon-Surface Roughness Scattering on Thermal Conductivity of Thin Si Nanowires PHYSICAL REVIEW LETTERS Martin, P., Aksamija, Z., Pop, E., Ravaioli, U. 2009; 102 (12)


    We present a novel approach for computing the surface roughness-limited thermal conductivity of silicon nanowires with diameter D<100 nm. A frequency-dependent phonon scattering rate is computed from perturbation theory and related to a description of the surface through the root-mean-square roughness height Delta and autocovariance length L. Using a full phonon dispersion relation, we find a quadratic dependence of thermal conductivity on diameter and roughness as (D/Delta)(2). Computed results show excellent agreement with experimental data for a wide diameter and temperature range (25-350 K), and successfully predict the extraordinarily low thermal conductivity of 2 W m(-1) K-1 at room temperature in rough-etched 50 nm silicon nanowires.

    View details for DOI 10.1103/PhysRevLett.102.125503

    View details for Web of Science ID 000264632100040

    View details for PubMedID 19392295

  • Infrared Microscopy of Joule Heating in Graphene Field Effect Transistors 9th IEEE Conference on Nanotechnology (IEEE-NANO) Bae, M., Ong, Z., Estrada, D., Pop, E. IEEE. 2009: 818–821
  • Avalanche, Joule Breakdown and Hysteresis in Carbon Nanotube Transistors 47th Annual IEEE International Reliability Physics Symposium Pop, E., Dutta, S., Estrada, D., liao, A. IEEE. 2009: 405–408
  • Prediction of Reduced Thermal Conductivity in Nano-Engineered Rough Semiconductor Nanowires 16th International Conference on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures (EDISON 16) Martin, P. N., Aksamija, Z., Pop, E., Ravaioli, U. IOP PUBLISHING LTD. 2009
  • Comparison of Energy Relaxation in One-Dimensional Thermionic and Tunneling Transistors 9th IEEE Conference on Nanotechnology (IEEE-NANO) Ramasubramanian, B., Pop, E. IEEE. 2009: 496–499
  • Avalanche-Induced Current Enhancement in Semiconducting Carbon Nanotubes PHYSICAL REVIEW LETTERS liao, A., Zhao, Y., Pop, E. 2008; 101 (25)


    Semiconducting single-wall carbon nanotubes under high electric field stress ( approximately 10 V/mum) display a remarkable current increase due to avalanche generation of free electrons and holes. Unlike other materials, the avalanche process in such 1D quantum wires involves access to the third subband and is insensitive to temperature but strongly dependent on diameter approximately exp(-1/d;{2}). Comparison with a theoretical model yields a novel approach to obtain the inelastic optical phonon emission length lambda{OP,ems} approximately 15d nm. The new results underscore the importance of multiband transport in 1D molecular wires.

    View details for DOI 10.1103/PhysRevLett.101.256804

    View details for Web of Science ID 000261891200059

    View details for PubMedID 19113739

  • The role of electrical and thermal contact resistance for Joule breakdown of single-wall carbon nanotubes NANOTECHNOLOGY Pop, E. 2008; 19 (29)


    Several data sets for the electrical breakdown in air of single-wall carbon nanotubes (SWNTs) on insulating substrates are collected and analyzed. A universal scaling of the Joule breakdown power with nanotube length is found, which appears to be independent of the substrate thermal properties of their thickness. This suggests that the thermal resistances at SWNT-insulator and at SWNT-electrode interfaces govern heat sinking from the nanotube. Analytical models for the breakdown power scaling are presented, providing an intuitive, physical understanding of the breakdown process. The electrical and thermal resistances at the electrode contacts limit the breakdown behavior for sub-micron SWNTs; the breakdown power scales linearly with length for tubes that are microns long, and a minimum breakdown power (∼0.05 mW) is observed for the intermediate (∼0.5 µm) length range.

    View details for DOI 10.1088/0957-4484/19/29/295202

    View details for Web of Science ID 000256838300006

    View details for PubMedID 21730598

  • Thermal properties of metal-coated vertically aligned single-wall nanotube arrays JOURNAL OF HEAT TRANSFER-TRANSACTIONS OF THE ASME Panzer, M. A., Zhang, G., Mann, D., Hu, X., Pop, E., Dai, H., Goodson, K. E. 2008; 130 (5)

    View details for DOI 10.1115/1.2885159

    View details for Web of Science ID 000255880300006

  • Operational voltage reduction of flash memory using high-kappa composite tunnel barriers IEEE ELECTRON DEVICE LETTERS Verma, S., Pop, E., Kapur, P., Parat, K., Saraswat, K. C. 2008; 29 (3): 252-254
  • Electrically driven light emission from hot single-walled carbon nanotubes at various temperatures and ambient pressures APPLIED PHYSICS LETTERS Wang, X., Zhang, L., Lu, Y., Dai, H., Kato, Y. K., Pop, E. 2007; 91 (26)

    View details for DOI 10.1063/1.2827281

    View details for Web of Science ID 000251987400002

  • Thickness and stoichiometry dependence of the thermal conductivity of GeSbTe films APPLIED PHYSICS LETTERS Reifenberg, J. P., Panzer, M. A., Kim, S., Gibby, A. M., Zhang, Y., Wong, S., Wong, H. P., Pop, E., Goodson, K. E. 2007; 91 (11)

    View details for DOI 10.1063/1.2784169

    View details for Web of Science ID 000249474000022

  • Electrical and thermal transport in metallic single-wall carbon nanotubes on insulating substrates JOURNAL OF APPLIED PHYSICS Pop, E., Mann, D. A., Goodson, K. E., Dai, H. 2007; 101 (9)

    View details for DOI 10.1063/1.2717855

    View details for Web of Science ID 000246567900049

  • Electrically driven thermal light emission from individual single-walled carbon nanotubes NATURE NANOTECHNOLOGY Mann, D., Kato, Y. K., Kinkhabwala, A., Pop, E., Cao, J., Wang, X., Zhang, L., Wang, Q., Guo, J., Dai, H. 2007; 2 (1): 33-38

    View details for DOI 10.1038/nnano.2006.169

    View details for Web of Science ID 000243902900012

    View details for PubMedID 18654204

  • Electrical and thermal transport in metallic single-wall carbon nanotubes International Semiconductor Device Research Symposium Pop, E. IEEE. 2007: 401–402
  • Heat generation and transport in nanometer-scale transistors PROCEEDINGS OF THE IEEE Pop, E., Sinha, S., Goodson, K. E. 2006; 94 (8): 1587-1601
  • Non-equilibrium phonon distributions in sub-100 nm silicon transistors JOURNAL OF HEAT TRANSFER-TRANSACTIONS OF THE ASME Sinha, S., Pop, E., DUTTON, R. W., Goodson, K. E. 2006; 128 (7): 638-647

    View details for DOI 10.1115/1.2194041

    View details for Web of Science ID 000239047600003

  • Electrical transport properties and field effect transistors of carbon nanotubes NANO Dai, H., Javey, A., Pop, E., Mann, D., Kim, W., Lu, Y. 2006; 1 (1): 1-13
  • Thermally and molecularly stimulated relaxation of hot phonons in suspended carbon nanotubes JOURNAL OF PHYSICAL CHEMISTRY B Mann, D., Pop, E., Cao, J., Wang, Q., Goodson, K. E., Dai, H. J. 2006; 110 (4): 1502-1505


    The high-bias electrical transport properties of suspended metallic single-walled carbon nanotubes (SWNTs) are investigated at various temperatures in vacuum, in various gases, and when coated with molecular solids. It is revealed that nonequilibrium optical phonon effects in suspended nanotubes decrease as the ambient temperature increases. Gas molecules surrounding suspended SWNTs assist the relaxation of hot phonons and afford enhanced current flow along nanotubes. Molecular solids of carbon dioxide frozen onto suspended SWNTs quench the nonequilibrium phonon effect. The discovery of strong environmental effects on high current transport in nanotubes is important to high performance nanoelectronics applications of 1D nanowires in general.

    View details for DOI 10.1021/jp0563991

    View details for Web of Science ID 000235198300002

    View details for PubMedID 16471703

  • Advanced cooling technologies for microprocessors Workshop on Frontiers in Electronics (WOFE-04) Kenny, T. W., Goodson, K. E., Santiago, J. G., Wang, E., Koo, J., Jiang, L., Pop, E., Sinha, S., Zhang, L., Fogg, D., Yao, S., Flynn, R., Chang, C., Hidrovo, C. H. WORLD SCIENTIFIC PUBL CO PTE LTD. 2006: 301–313
  • Electro-thermal transport in silicon and carbon nanotube devices 14th International Conference on Nonequilibrium Carrier Dynamics in Semiconductors Pop, E., Mann, D., ROWLETTE, J., Goodson, K., Dai, H. SPRINGER-VERLAG BERLIN. 2006: 195–199
  • Multiphysics modeling and impact of thermal boundary resistance in phase change memory devices 10th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems Reifenberg, J., Pop, E., Gibby, A., Wong, S., Goodson, K. IEEE. 2006: 106–113
  • Thermal properties of metal-coated vertically-aligned single wall nanotube films 10th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems Panzer, M., Zhang, G., Mann, D., Hu, X., Pop, E., Dai, H., Goodson, K. E. IEEE. 2006: 1306–1313
  • Thermal conductance of an individual single-wall carbon nanotube above room temperature NANO LETTERS Pop, E., Mann, D., Wang, Q., Goodson, K. E., Dai, H. J. 2006; 6 (1): 96-100


    The thermal properties of a suspended metallic single-wall carbon nanotube (SWNT) are extracted from its high-bias (I-V) electrical characteristics over the 300-800 K temperature range, achieved by Joule self-heating. The thermal conductance is approximately 2.4 nW/K, and the thermal conductivity is nearly 3500 Wm(-1)K(-1) at room temperature for a SWNT of length 2.6 mum and diameter 1.7 nm. A subtle decrease in thermal conductivity steeper than 1/T is observed at the upper end of the temperature range, which is attributed to second-order three-phonon scattering between two acoustic modes and one optical mode. We discuss sources of uncertainty and propose a simple analytical model for the SWNT thermal conductivity including length and temperature dependence.

    View details for DOI 10.1021/nl052145f

    View details for Web of Science ID 000235532400018

    View details for PubMedID 16402794

  • Negative differential conductance and hot phonons in suspended nanotube molecular wires PHYSICAL REVIEW LETTERS Pop, E., Mann, D., Cao, J., Wang, Q., Goodson, K. E., Dai, H. J. 2005; 95 (15)


    Freely suspended metallic single-walled carbon nanotubes (SWNTs) exhibit reduced current carrying ability compared to those lying on substrates, and striking negative differential conductance at low electric fields. Theoretical analysis reveals significant self-heating effects including electron scattering by hot nonequilibrium optical phonons. Electron transport characteristics under strong self-heating are exploited for the first time to probe the thermal conductivity of individual SWNTs (approximately 3600 W m-1 K-1 at T=300 K) up to approximately 700 K, and reveal a 1/T dependence expected for umklapp phonon scattering at high temperatures.

    View details for DOI 10.1103/PhysRevLett.95.155505

    View details for Web of Science ID 000232443400039

    View details for PubMedID 16241738

  • Monte Carlo simulation of Joule heating in bulk and strained silicon APPLIED PHYSICS LETTERS Pop, E., DUTTON, R. W., Goodson, K. E. 2005; 86 (8)

    View details for DOI 10.1063/1.1870106

    View details for Web of Science ID 000227609000032

  • Thermal phenomena in deeply scaled MOSFETs IEEE International Electron Devices Meeting Rowlette, J., Pop, E., Sinha, S., Panzer, M., Goodson, K. IEEE. 2005: 1005–1008
  • Joule heating under quasi-ballistic transport conditions in bulk and strained silicon devices International Conference on Simulation of Semiconductor Processes and Devices Pop, E., Rowlette, J. A., DUTTON, R. W., Goodson, K. E. JAPAN SOCIETY APPLIED PHYSICS. 2005: 307–310
  • Electro-thermal transport in metallic single-wall carbon nanotubes for interconnect applications IEEE International Electron Devices Meeting Pop, E., Mann, D., Reifenberg, J., Goodson, K., Dai, H. J. IEEE. 2005: 261–264
  • Thermal simulation techniques for nanoscale transistors IEEE/ACM International Conference on Computer Aided Design Rowlette, J., Pop, E., Sinha, S., Panzer, M., Goodson, K. IEEE. 2005: 225–228
  • Analytic band Monte Carlo model for electron transport in Si including acoustic and optical phonon dispersion JOURNAL OF APPLIED PHYSICS Pop, E., DUTTON, R. W., Goodson, K. E. 2004; 96 (9): 4998-5005

    View details for DOI 10.1063/1.1788838

    View details for Web of Science ID 000224799300042

  • Electro-thermal comparison and performance optimization of thin-body SOI and GOI MOSFETs 50th IEEE International Electron Devices Meeting Pop, E., Chui, C. O., Sinha, S., Dutton, R., Goodson, K. IEEE. 2004: 411–414
  • Thermal phenomena in nanoscale transistors 9th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems Pop, E., Goodson, K. E. IEEE. 2004: 1–7
  • Thermal analysis of ultra-thin body device scaling IEEE International Electron Devices Meeting Pop, E., Dutton, R., Goodson, K. IEEE. 2003: 883–886
  • Detailed heat generation simulations via the Monte Carlo method IEEE International Conference on Simulation of Semiconductor Processes and Devices Pop, E., Dutton, R., Goodson, K. IEEE. 2003: 121–124