All Publications

  • 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

  • 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)
  • 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
  • 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

  • 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

  • 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

  • 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
  • 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

  • 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

  • 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

  • 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
  • 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