Tony Heinz is a Professor of Applied Physics and Photon Science at Stanford University and the Associate Laboratory Director for Energy Sciences at the SLAC National Accelerator Laboratory. Heinz received a BS degree in Physics from Stanford University and a PhD degree, also in Physics, from the University of California at Berkeley in 1982. Heinz was subsequently at the IBM Research Division in Yorktown Heights, NY until he joined Columbia University in 1995 as a Professor of Electrical Engineering and Physics. At Columbia, he served as the Chair of the Department of Electrical Engineering from 2003 until 2007. He has also served as a Scientific Director of the Columbia Nanoscale Science and Engineering Center (NSEC) and of the Energy Frontier Research Center (EFRC). He was the President of the Optical Society of America in 2012. Heinz joined Stanford University in 2015, serving as the Director of the Chemical Sciences Division at SLAC from that time until 2019. He was been the Associate Laboratory Director for Energy Sciences at SLAC, including the Material Science, Chemical Science, Computer Science, and the Applied Energy Divisions at SLAC.

Academic Appointments

Administrative Appointments

  • Associate Laboratory Director, Energy Sciences, SLAC (2017 - Present)
  • Professor of Applied Physics and Photon Science, Stanford University (2015 - Present)
  • Director, Chemical Science Division, SLAC National Accelerator Laboratory (2015 - 2019)
  • David M. Rickey Professor, Columbia University (2001 - 2014)
  • Professor of Physics and Electrical Engineering, Columbia University (1995 - 2000)
  • Senior Department Manager, Department Manager, Research Staff Member, IBM Research Division, T. J. Watson Research Center (1983 - 1995)

Honors & Awards

  • William Meggers Award, Optical Society of America (2020)
  • Citation Laureate in Physics, Clarivate Web of Science (2019)
  • Frank Isakson Prize, American Physical Society (2014)
  • Julius Springer Prize for Applied Physics (with Phaedon Avouris), Springer (2008)
  • Great Teacher Award, Columbia University (2005)
  • Alexander von Humboldt Research Award, Alexander von Humboldt-Stiftung Foundation, Germany (1996)
  • Ernst Abbe Medal, International Commission for Optics Prize (1995)
  • IBM Invention Award, IBM (1994)
  • IBM Outstanding Technical Achievement Award, IBM (1992)
  • IBM Graduate Fellow, University of California, Berkeley (1982-83)
  • National Science Foundation Graduate Fellow, University of California, Berkeley (1978-81)
  • Levine Award for Outstanding Studies in Physics, Stanford University (1978)

Boards, Advisory Committees, Professional Organizations

  • Scientific Advisory Boards, Fritz-Haber Institute, Berlin; Max-Born Institute, Berlin (2014 - Present)
  • Editor, North America, 2D Materials journal, The Institute of Physics (2014 - 2018)
  • Chair, Subcommittee on Optics and Photonics, NSF Dir. of Math and Physical Science (2013 - 2015)
  • President, Optical Society of America (2012 - 2012)
  • Chair, Scientific Advisory Board, Center for Integrated Nanotechnologies, Sandia National Laboratories (2011 - 2017)
  • Chair, Gordon Conference on Ultrafast Dynamics of Cooperative Phenomena (2010 - 2010)
  • Scientific Director, DOE Energy Frontier Research Center at Columbia (EFRC) (2009 - 2014)
  • Scientific Director, NSF Nanoscale Science & Engineering Center at Columbia (2006 - 2012)
  • Chair, Board of Editors, Optical Society of America (2006 - 2009)
  • Chair, International Conference on Quantum Electronics (IQEC) (2002 - 2002)
  • Chair, Division of Laser Science, American Physical Society (2001 - 2002)
  • Chair, Review Panel, Optical Technology Division, Physics Laboratory, National Institute of Standards and Technology (NIST) (2000 - 2005)
  • Director, Adriatico Symposium on Laser Applications in Science, Abdus Salam International Centre for Theoretical Physics (ICTP) (2000 - 2000)
  • Chair, Quantum Electronics and Laser Science Conference (QELS) (1995 - 1995)
  • Editor, Journal of the Optical Society of America B (JOSA B) (1994 - 2000)

Professional Education

  • B.S. (with Distinction), Stanford University, Physics (1978)
  • Ph.D., University of California, Berkeley, Physics (1982)

Current Research and Scholarly Interests

Heinz's research has centered on the elucidation of the properties and dynamics of nanoscale materials through the application of a wide range of optical spectroscopies. His research on surfaces, interfaces, and nanoscale materials, such as carbon nanotubes, graphene and other 2D materials, has been recognized by Optics Prize of the International Commission for Optics, a Research Award of the von Humboldt Foundation, the Julius Springer Prize for Applied Physics, and the Isakson Prize of the American Physical Society.

2020-21 Courses

Stanford Advisees

All Publications

  • Signatures of moire trions in WSe2/MoSe2 heterobilayers. Nature Liu, E., Barre, E., van Baren, J., Wilson, M., Taniguchi, T., Watanabe, K., Cui, Y., Gabor, N. M., Heinz, T. F., Chang, Y., Lui, C. H. 2021; 594 (7861): 46-50


    Moire superlattices formed by van der Waals materials can support a wide range of electronic phases, including Mott insulators1-4, superconductors5-10 and generalized Wigner crystals2. When excitons are confined by a moire superlattice, a new class of exciton emerges, which holds promise for realizing artificial excitonic crystals and quantum optical effects11-16. When such moire excitons are coupled to charge carriers, correlated states may arise. However, no experimental evidence exists for charge-coupled moire exciton states, nor have their properties been predicted by theory. Here we report the optical signatures of trions coupled to the moire potential in tungsten diselenide/molybdenum diselenide heterobilayers. The moire trions show multiple sharp emission lines with a complex charge-density dependence, in stark contrast to the behaviour of conventional trions. We infer distinct contributions to the trion emission from radiative decay in which the remaining carrier resides in different moire minibands. Variation of the trion features is observed in different devices and sample areas, indicating high sensitivity to sample inhomogeneity and variability. The observation of these trion features motivates further theoretical and experimental studies of higher-order electron correlation effects in moire superlattices.

    View details for DOI 10.1038/s41586-021-03541-z

    View details for PubMedID 34079140

  • Enhanced nonlinear interaction of polaritons via excitonic Rydberg states in monolayer WSe2. Nature communications Gu, J., Walther, V., Waldecker, L., Rhodes, D., Raja, A., Hone, J. C., Heinz, T. F., Kena-Cohen, S., Pohl, T., Menon, V. M. 2021; 12 (1): 2269


    Strong optical nonlinearities play a central role in realizing quantum photonic technologies. Exciton-polaritons, which result from the hybridization of material excitations and cavity photons, are an attractive candidate to realize such nonlinearities. While the interaction between ground state excitons generates a notable optical nonlinearity, the strength of such interactions is generally not sufficient to reach the regime of quantum nonlinear optics. Excited states, however, feature enhanced interactions and therefore hold promise for accessing the quantum domain of single-photon nonlinearities. Here we demonstrate the formation of exciton-polaritons using excited excitonic states in monolayer tungsten diselenide (WSe2) embedded in a microcavity. The realized excited-state polaritons exhibit an enhanced nonlinear response [Formula: see text] which is 4.6 times that for the ground-state exciton. The demonstration of enhanced nonlinear response from excited exciton-polaritons presents the potential of generatingstrong exciton-polariton interactions, a necessary building block for solid-state quantumphotonic technologies.

    View details for DOI 10.1038/s41467-021-22537-x

    View details for PubMedID 33859179

  • 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

  • Experimental measurement of the intrinsic excitonic wave function. Science advances Man, M. K., Madeo, J., Sahoo, C., Xie, K., Campbell, M., Pareek, V., Karmakar, A., Wong, E. L., Al-Mahboob, A., Chan, N. S., Bacon, D. R., Zhu, X., Abdelrasoul, M. M., Li, X., Heinz, T. F., da Jornada, F. H., Cao, T., Dani, K. M. 2021; 7 (17)


    An exciton, a two-body composite quasiparticle formed of an electron and hole, is a fundamental optical excitation in condensed matter systems. Since its discovery nearly a century ago, a measurement of the excitonic wave function has remained beyond experimental reach. Here, we directly image the excitonic wave function in reciprocal space by measuring the momentum distribution of electrons photoemitted from excitons in monolayer tungsten diselenide. By transforming to real space, we obtain a visual of the distribution of the electron around the hole in an exciton. Further, by also resolving the energy coordinate, we confirm the elusive theoretical prediction that the photoemitted electron exhibits an inverted energy-momentum dispersion relationship reflecting the valence band where the partner hole remains, rather than that of conduction band states of the electron.

    View details for DOI 10.1126/sciadv.abg0192

    View details for PubMedID 33883143

  • Site-Controlled Quantum Emitters in Monolayer MoSe2. Nano letters Yu, L., Deng, M., Zhang, J. L., Borghardt, S., Kardynal, B., Vuckovic, J., Heinz, T. F. 2021


    Atomically thin semiconductors provide a highly attractive platform for quantum emitters (QEs): They can be combined with arbitrary substrates, can be spatially aligned with photonic structures, and can be electrically driven. All QEs reported to date in these materials have, however, relied on nominally spin-forbidden transitions, with radiative rates falling substantially below those of other solid-state QE systems. Here we employ strain confinement in monolayer MoSe2 to produce engineered QEs, as confirmed in photon antibunching measurements. We discuss spin-allowed versus spin-forbidden transitions based on magneto- and time-resolved photoluminescence measurements. We calculate a radiative rate for spin-allowed quantum emission greater than 1 ns-1, which exceeds reported radiative rates of WSe2 QEs by 2 orders of magnitude.

    View details for DOI 10.1021/acs.nanolett.0c04282

    View details for PubMedID 33689386

  • 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

  • Carrier-specific dynamics in 2H-MoTe2 observed by femtosecond soft x-ray absorption spectroscopy using an x-ray free-electron laser. Structural dynamics (Melville, N.Y.) Britz, A., Attar, A. R., Zhang, X., Chang, H., Nyby, C., Krishnamoorthy, A., Park, S. H., Kwon, S., Kim, M., Nordlund, D., Sainio, S., Heinz, T. F., Leone, S. R., Lindenberg, A. M., Nakano, A., Ajayan, P., Vashishta, P., Fritz, D., Lin, M., Bergmann, U. 2021; 8 (1): 014501


    Femtosecond carrier dynamics in layered 2H-MoTe2 semiconductor crystals have been investigated using soft x-ray transient absorption spectroscopy at the x-ray free-electron laser (XFEL) of the Pohang Accelerator Laboratory. Following above-bandgap optical excitation of 2H-MoTe2, the photoexcited hole distribution is directly probed via short-lived transitions from the Te 3d 5/2 core level (M5-edge, 572-577eV) to transiently unoccupied states in the valence band. The optically excited electrons are separately probed via the reduced absorption probability at the Te M5-edge involving partially occupied states of the conduction band. A 400±110 fs delay is observed between this transient electron signal near the conduction band minimum compared to higher-lying states within the conduction band, which we assign to hot electron relaxation. Additionally, the transient absorption signals below and above the Te M5 edge, assigned to photoexcited holes and electrons, respectively, are observed to decay concomitantly on a 1-2 ps timescale, which is interpreted as electron-hole recombination. The present work provides a benchmark for applications of XFELs for soft x-ray absorption studies of carrier-specific dynamics in semiconductors, and future opportunities enabled by this method are discussed.

    View details for DOI 10.1063/4.0000048

    View details for PubMedID 33511247

  • High-resolution optical micro-spectroscopy extending from the near-infrared to the vacuum-ultraviolet REVIEW OF SCIENTIFIC INSTRUMENTS Ma, E., Waldecker, L., Rhodes, D., Hone, J., Watanabe, K., Taniguchi, T., Heinz, T. F. 2020; 91 (7): 073107


    Optical characterization of small samples over a wide spectral range with rapid data acquisition is essential for the analysis of many material systems, such as 2D van der Waals layers and their heterostructures. Here, we present the design and implementation of a tabletop micro-spectroscopy system covering the near-infrared to the vacuum-ultraviolet (1.2 eV-6.8 eV or ∼1.0 μm to 185 nm) using mostly off-the-shelf components. It can measure highly reproducible local reflectance spectra with a total integration time of a few minutes and a full-width-half-maximum spot size of 2.7 by 5.6 μm. For precise positioning, the design also allows simultaneous monitoring of the measurement location and the wide-field image of the sample. We demonstrate ultra-broadband reflectance spectra of exfoliated thin flakes of several wide-gap 2D materials, including ZnPS3, hexagonal BN, and Ca(OH)2.

    View details for DOI 10.1063/5.0010487

    View details for Web of Science ID 000553078600001

    View details for PubMedID 32752845

  • 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
  • Retarded Charge-Carrier Recombination in Photoelectrochemical Cells from Plasmon-Induced Resonance Energy Transfer ADVANCED ENERGY MATERIALS Choi, Y., Lee, B., Jung, M., Han, H., Kim, S., Chen, K., Kim, D., Heinz, T. F., Fan, S., Lee, J., Yi, G., Kim, J., Park, J. 2020
  • Strained bilayer WSe2 with reduced exciton-phonon coupling PHYSICAL REVIEW B Asian, O., Deng, M., Brongersma, M. L., Heinz, T. F. 2020; 101 (11)
  • Revealing multiple classes of stable quantum emitters in hexagonal boron nitride with correlated optical and electron microscopy. Nature materials Hayee, F., Yu, L., Zhang, J. L., Ciccarino, C. J., Nguyen, M., Marshall, A. F., Aharonovich, I., Vuckovic, J., Narang, P., Heinz, T. F., Dionne, J. A. 2020


    Defects in hexagonal boron nitride (hBN) exhibit high-brightness, room-temperature quantum emission, but their large spectral variability and unknown local structure challenge their technological utility. Here, we directly correlate hBN quantum emission with local strain using a combination of photoluminescence (PL), cathodoluminescence (CL) and nanobeam electron diffraction. Across 40 emitters, we observe zero phonon lines (ZPLs) in PL and CL ranging from 540 to 720nm. CL mapping reveals that multiple defects and distinct defect species located within an optically diffraction-limited region can each contribute to the observed PL spectra. Local strain maps indicate that strain is not required to activate the emitters and is not solely responsible for the observed ZPL spectral range. Instead, at least four distinct defect classes are responsible for the observed emission range, and all four classes are stable upon both optical and electron illumination. Our results provide a foundation for future atomic-scale optical characterization of colour centres.

    View details for DOI 10.1038/s41563-020-0616-9

    View details for PubMedID 32094492

  • Directly visualizing the momentum-forbidden dark excitons and their dynamics in atomically thin semiconductors. Science (New York, N.Y.) Madéo, J. n., Man, M. K., Sahoo, C. n., Campbell, M. n., Pareek, V. n., Wong, E. L., Al-Mahboob, A. n., Chan, N. S., Karmakar, A. n., Mariserla, B. M., Li, X. n., Heinz, T. F., Cao, T. n., Dani, K. M. 2020; 370 (6521): 1199–1204


    Resolving momentum degrees of freedom of excitons, which are electron-hole pairs bound by the Coulomb attraction in a photoexcited semiconductor, has remained an elusive goal for decades. In atomically thin semiconductors, such a capability could probe the momentum-forbidden dark excitons, which critically affect proposed opto-electronic technologies but are not directly accessible using optical techniques. Here, we probed the momentum state of excitons in a tungsten diselenide monolayer by photoemitting their constituent electrons and resolving them in time, momentum, and energy. We obtained a direct visual of the momentum-forbidden dark excitons and studied their properties, including their near degeneracy with bright excitons and their formation pathways in the energy-momentum landscape. These dark excitons dominated the excited-state distribution, a surprising finding that highlights their importance in atomically thin semiconductors.

    View details for DOI 10.1126/science.aba1029

    View details for PubMedID 33273099

  • Tunable infrared light emission from MoS2/WSe2 heterostructures Karni, O., Barre, E., Lau, S., Gillen, R., Yue, E., Gal, L., Yaffe, T., Kim, B., Watanabe, K., Taniguchi, T., Orenstein, M., Maultzsch, J., Barmak, K., Page, R. H., Heinz, T. F., IEEE IEEE. 2020
  • Infrared Interlayer Exciton Emission in MoS2/WSe2 Heterostructures PHYSICAL REVIEW LETTERS Karni, O., Barre, E., Lau, S., Gillen, R., Ma, E., Kim, B., Watanabe, K., Taniguchi, T., Maultzsch, J., Barmak, K., Page, R. H., Heinz, T. F. 2019; 123 (24)
  • Rigid Band Shifts in Two-Dimensional Semiconductors through External Dielectric Screening PHYSICAL REVIEW LETTERS Waldecker, L., Raja, A., Rosner, M., Steinke, C., Bostwick, A., Koch, R. J., Jozwiak, C., Taniguchi, T., Watanabe, K., Rotenberg, E., Wehling, T. O., Heinz, T. F. 2019; 123 (20)
  • Anisotropic structural dynamics of monolayer crystals revealed by femtosecond surface X-ray scattering NATURE PHOTONICS Tung, I., Krishnamoorthy, A., Sadasivam, S., Zhou, H., Zhang, Q., Seyler, K. L., Clark, G., Mannebach, E. M., Nyby, C., Ernst, F., Zhu, D., Glownia, J. M., Kozina, M. E., Song, S., Nelson, S., Kumazoe, H., Shimojo, F., Kalia, R. K., Vashishta, P., Darancet, P., Heinz, T. F., Nakano, A., Xu, X., Lindenberg, A. M., Wen, H. 2019; 13 (6): 425-+
  • Disentangling interface and bulk contributions to high-harmonic emission from solids OPTICA Vampa, G., Liu, H., Heinz, T. F., Reis, D. A. 2019; 6 (5): 553–56
  • Zeeman-Induced Valley-Sensitive Photocurrent in Monolayer MoS2 PHYSICAL REVIEW LETTERS Zhang, X., Lai, Y., Dohner, E., Moon, S., Taniguchi, T., Watanabe, K., Smirnov, D., Heinz, T. F. 2019; 122 (12)
  • Recording interfacial currents on the subnanometer length and femtosecond time scale by terahertz emission. Science advances Ma, E. Y., Guzelturk, B., Li, G., Cao, L., Shen, Z., Lindenberg, A. M., Heinz, T. F. 2019; 5 (2): eaau0073


    Electron dynamics at interfaces is a subject of great scientific interest and technological importance. Detailed understanding of such dynamics requires access to the angstrom length scale defining interfaces and the femtosecond time scale characterizing interfacial motion of electrons. In this context, the most precise and general way to remotely measure charge dynamics is through the transient current flow and the associated electromagnetic radiation. Here, we present quantitative measurements of interfacial currents on the subnanometer length and femtosecond time scale by recording the emitted terahertz radiation following ultrafast laser excitation. We apply this method to interlayer charge transfer in heterostructures of two transition metal dichalcogenide monolayers less than 0.7 nm apart. We find that charge relaxation and separation occur in less than 100 fs. This approach allows us to unambiguously determine the direction of current flow, to demonstrate a charge transfer efficiency of order unity, and to characterize saturation effects.

    View details for PubMedID 30783622

  • 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

  • Infrared Interlayer Exciton Emission in MoS_{2}/WSe_{2} Heterostructures. Physical review letters Karni, O. n., Barré, E. n., Lau, S. C., Gillen, R. n., Ma, E. Y., Kim, B. n., Watanabe, K. n., Taniguchi, T. n., Maultzsch, J. n., Barmak, K. n., Page, R. H., Heinz, T. F. 2019; 123 (24): 247402


    We report light emission around 1 eV (1240 nm) from heterostructures of MoS_{2} and WSe_{2} transition metal dichalcogenide monolayers. We identify its origin in an interlayer exciton (ILX) by its wide spectral tunability under an out-of-plane electric field. From the static dipole moment of the state, its temperature and twist-angle dependence, and comparison with electronic structure calculations, we assign this ILX to the fundamental interlayer transition between the K valleys in this system. Our findings gain access to the interlayer physics of the intrinsically incommensurate MoS_{2}/WSe_{2} heterostructure, including moiré and valley pseudospin effects, and its integration with silicon photonics and optical fiber communication systems operating at wavelengths longer than 1150 nm.

    View details for DOI 10.1103/PhysRevLett.123.247402

    View details for PubMedID 31922842

  • THz-Pump UED-Probe on a Topological Weyl Semimetal Sie, E. J., Nyby, C. M., Pemmaraju, C. D., Park, S., Shen, X., Yang, J., Hoffmann, M. C., Ofori-Okai, B. K., Li, R., Reid, A. H., Weathersby, S., Mannebach, E., Finney, N., Rhodes, D., Chenet, D., Antony, A., Balicas, L., Hone, J., Devereaux, T. P., Heinz, T. F., Wang, X., Lindenberg, A. M., IEEE IEEE. 2019
  • Monitoring Charge Separation Dynamics Using THz Emission Spectroscopy Guzelturk, B., Ma, E., Li, G., Cao, L., Shen, Z., Heinz, T., Lindenberg, A., IEEE IEEE. 2019
  • Rigid Band Shifts in Two-Dimensional Semiconductors through External Dielectric Screening. Physical review letters Waldecker, L. n., Raja, A. n., Rösner, M. n., Steinke, C. n., Bostwick, A. n., Koch, R. J., Jozwiak, C. n., Taniguchi, T. n., Watanabe, K. n., Rotenberg, E. n., Wehling, T. O., Heinz, T. F. 2019; 123 (20): 206403


    We investigate the effects of external dielectric screening on the electronic dispersion and the band gap in the atomically thin, quasi-two-dimensional (2D) semiconductor WS_{2} using angle-resolved photoemission and optical spectroscopies, along with first-principles calculations. We find the main effect of increased external dielectric screening to be a reduction of the quasiparticle band gap, with rigid shifts to the bands themselves. Specifically, the band gap of monolayer WS_{2} is decreased by about 140 meV on a graphite substrate as compared to a hexagonal boron nitride substrate, while the electronic dispersion of WS_{2} remains unchanged within our experimental precision of 17 meV. These essentially rigid shifts of the valence and conduction bands result from the special spatial structure of the changes in the Coulomb potential induced by the dielectric environment of the monolayer.

    View details for DOI 10.1103/PhysRevLett.123.206403

    View details for PubMedID 31809088

  • Nonlinear Interaction of Rydberg Exciton-Polaritons in Two-Dimensional WSe2 Gu, J., Waldecker, L., Rhodes, D., Boehmke, A., Raja, A., Koots, R., Hone, J. C., Heinz, T. F., Menon, V. M., IEEE IEEE. 2019
  • Dielectric disorder in two-dimensional materials. Nature nanotechnology Raja, A. n., Waldecker, L. n., Zipfel, J. n., Cho, Y. n., Brem, S. n., Ziegler, J. D., Kulig, M. n., Taniguchi, T. n., Watanabe, K. n., Malic, E. n., Heinz, T. F., Berkelbach, T. C., Chernikov, A. n. 2019


    Understanding and controlling disorder is key to nanotechnology and materials science. Traditionally, disorder is attributed to local fluctuations of inherent material properties such as chemical and structural composition, doping or strain. Here, we present a fundamentally new source of disorder in nanoscale systems that is based entirely on the local changes of the Coulomb interaction due to fluctuations of the external dielectric environment. Using two-dimensional semiconductors as prototypes, we experimentally monitor dielectric disorder by probing the statistics and correlations of the exciton resonances, and theoretically analyse the influence of external screening and phonon scattering. Even moderate fluctuations of the dielectric environment are shown to induce large variations of the bandgap and exciton binding energies up to the 100 meV range, often making it a dominant source of inhomogeneities. As a consequence, dielectric disorder has strong implications for both the optical and transport properties of nanoscale materials and their heterostructures.

    View details for DOI 10.1038/s41565-019-0520-0

    View details for PubMedID 31427747

  • An ultrafast symmetry switch in a Weyl semimetal. Nature Sie, E. J., Nyby, C. M., Pemmaraju, C. D., Park, S. J., Shen, X. n., Yang, J. n., Hoffmann, M. C., Ofori-Okai, B. K., Li, R. n., Reid, A. H., Weathersby, S. n., Mannebach, E. n., Finney, N. n., Rhodes, D. n., Chenet, D. n., Antony, A. n., Balicas, L. n., Hone, J. n., Devereaux, T. P., Heinz, T. F., Wang, X. n., Lindenberg, A. M. 2019; 565 (7737): 61–66


    Topological quantum materials exhibit fascinating properties1-3, with important applications for dissipationless electronics and fault-tolerant quantum computers4,5. Manipulating the topological invariants in these materials would allow the development of topological switching applications analogous to switching of transistors6. Lattice strain provides the most natural means of tuning these topological invariants because it directly modifies the electron-ion interactions and potentially alters the underlying crystalline symmetry on which the topological properties depend7-9. However, conventional means of applying strain through heteroepitaxial lattice mismatch10 and dislocations11 are not extendable to controllable time-varying protocols, which are required in transistors. Integration into a functional device requires the ability to go beyond the robust, topologically protected properties of materials and to manipulate the topology at high speeds. Here we use crystallographic measurements by relativistic electron diffraction to demonstrate that terahertz light pulses can be used to induce terahertz-frequency interlayer shear strain with large strain amplitude in the Weyl semimetal WTe2, leading to a topologically distinct metastable phase. Separate nonlinear optical measurements indicate that this transition is associated with a symmetry change to a centrosymmetric, topologically trivial phase. We further show that such shear strain provides an ultrafast, energy-efficient way of inducing robust, well separated Weyl points or of annihilating all Weyl points of opposite chirality. This work demonstrates possibilities for ultrafast manipulation of the topological properties of solids and for the development of a topological switch operating at terahertz frequencies.

    View details for PubMedID 30602749

  • Zeeman-Induced Valley-Sensitive Photocurrent in Monolayer MoS_{2}. Physical review letters Zhang, X. X., Lai, Y. n., Dohner, E. n., Moon, S. n., Taniguchi, T. n., Watanabe, K. n., Smirnov, D. n., Heinz, T. F. 2019; 122 (12): 127401


    The control of the valley degree of freedom lies at the core of interest in monolayer transition metal dichalcogenides, where specific valley-spin excitation can be created using circularly polarized light. Measurement and manipulation of the valley index has also been achieved, but mainly with purely optical methods. Here, in monolayer MoS_{2}, we identify a response to the valley polarization of excitons in the longitudinal electrical transport when the valley degeneracy is broken by an out-of-plane magnetic field B_{z}. The spin information is also simultaneously determined with spin-sensitive contacts. In the presence of B_{z}, a significant modulation of the photocurrent is observed as a function of the circular polarization state of the excitation. We attribute this effect to unbalanced transport of valley-polarized trions induced by the opposite Zeeman shifts of two (K and K^{'}) valleys. Our interpretation is supported by the contrasting behavior in bilayer MoS_{2}, as well as the observed doping and spatial dependence of the valley photocurrent.

    View details for PubMedID 30978070

  • Ultrafast dynamics in van der Waals heterostructures NATURE NANOTECHNOLOGY Jin, C., Ma, E., Karni, O., Regan, E. C., Wang, F., Heinz, T. F. 2018; 13 (11): 994–1003
  • Ultrafast dynamics in van der Waals heterostructures. Nature nanotechnology Jin, C., Ma, E. Y., Karni, O., Regan, E. C., Wang, F., Heinz, T. F. 2018; 13 (11): 994–1003


    Van der Waals heterostructures are synthetic quantum materials composed of stacks of atomically thin two-dimensional (2D) layers. Because the electrons in the atomically thin 2D layers are exposed to layer-to-layer coupling, the properties of van der Waals heterostructures are defined not only by the constituent monolayers, but also by the interactions between the layers. Many fascinating electrical, optical and magnetic properties have recently been reported in different types of van der Waals heterostructures. In this Review, we focus on unique excited-state dynamics in transition metal dichalcogenide (TMDC) heterostructures. TMDC monolayers are the most widely studied 2D semiconductors, featuring prominent exciton states and accessibility to the valley degree of freedom. Many TMDC heterostructures are characterized by a staggered band alignment. This band alignment has profound effects on the evolution of the excited states in heterostructures, including ultrafast charge transfer between the layers, the formation of interlayer excitons, and the existence of long-lived spin and valley polarization in resident carriers. Here we review recent experimental and theoretical efforts to elucidate electron dynamics in TMDC heterostructures, extending from timescales of femtoseconds to microseconds, and comment on the relevance of these effects for potential applications in optoelectronic, valleytronic and spintronic devices.

    View details for PubMedID 30397296

  • Resolving Hysteresis in Perovskite Solar Cells with Rapid Flame-Processed Cobalt-Doped TiO2 ADVANCED ENERGY MATERIALS Kim, J., Chai, S., Ji, Y., Levy-Wendt, B., Kim, S., Yi, Y., Heinz, T. F., Norskov, J. K., Park, J., Zheng, X. 2018; 8 (29)
  • Enhancement of Exciton-Phonon Scattering from Monolayer to Bilayer WS2 NANO LETTERS Raja, A., Selig, M., Berghauser, G., Yu, J., Hill, H. M., Rigosi, A. F., Brus, L. E., Knorr, A., Heinz, T. F., Malic, E., Chernikov, A. 2018; 18 (10): 6135–43


    Layered transition metal dichalcogenides exhibit the emergence of a direct bandgap at the monolayer limit along with pronounced excitonic effects. In these materials, interaction with phonons is the dominant mechanism that limits the exciton coherence lifetime. Exciton-phonon interaction also facilitates energy and momentum relaxation, and influences exciton diffusion under most experimental conditions. However, the fundamental changes in the exciton-phonon interaction are not well understood as the material undergoes the transition from a direct to an indirect bandgap semiconductor. Here, we address this question through optical spectroscopy and microscopic theory. In the experiment, we study room-temperature statistics of the exciton line width for a large number of mono- and bilayer WS2 samples. We observe a systematic increase in the room-temperature line width of the bilayer compared to the monolayer of 50 meV, corresponding to an additional scattering rate of ∼0.1 fs-1. We further address both phonon emission and absorption processes by examining the temperature dependence of the width of the exciton resonances. Using a theoretical approach based on many-body formalism, we are able to explain the experimental results and establish a microscopic framework for exciton-phonon interactions that can be applied to naturally occurring and artificially prepared multilayer structures.

    View details for PubMedID 30096239

  • Strain tuning of excitons in monolayer WSe2 PHYSICAL REVIEW B Aslan, O., Deng, M., Heinz, T. F. 2018; 98 (11)
  • Efficient generation of neutral and charged biexcitons in encapsulated WSe2 monolayers. Nature communications Ye, Z., Waldecker, L., Ma, E. Y., Rhodes, D., Antony, A., Kim, B., Zhang, X., Deng, M., Jiang, Y., Lu, Z., Smirnov, D., Watanabe, K., Taniguchi, T., Hone, J., Heinz, T. F. 2018; 9 (1): 3718


    Higher-order correlated excitonic states arise from the mutual interactions of excitons, which generally requires a significant exciton density and therefore high excitation levels. Here, we report the emergence of two biexcitons species, one neutral and one charged, in monolayer tungsten diselenide under moderate continuous-wave excitation. The efficient formation of biexcitons is facilitated by the long lifetime of the dark exciton state associated with a spin-forbidden transition, as well as improved sample quality from encapsulation between hexagonal boron nitride layers. From studies of the polarization and magnetic field dependence of the neutral biexciton, we conclude that this species is composed of a bright and a dark excitons residing in opposite valleys in momentum space. Our observations demonstrate that the distinctive features associated with biexciton states can be accessed at low light intensities and excitation densities.

    View details for PubMedID 30214026

  • Efficient generation of neutral and charged biexcitons in encapsulated WSe2 monolayers NATURE COMMUNICATIONS Ye, Z., Waldecker, L., Ma, E., Rhodes, D., Antony, A., Kim, B., Zhang, X., Deng, M., Jiang, Y., Lu, Z., Smirnov, D., Watanabe, K., Taniguchi, T., Hone, J., Heinz, T. F. 2018; 9
  • Controlling the electronic properties of 2D semiconductors by the external environment Heinz, T. AMER CHEMICAL SOC. 2018
  • Colloquium: Excitons in atomically thin transition metal dichalcogenides REVIEWS OF MODERN PHYSICS Wang, G., Chernikov, A., Glazov, M. M., Heinz, T. F., Marie, X., Amand, T., Urbaszek, B. 2018; 90 (2)
  • Optical Imaging and Spectroscopic Characterization of Self-Assembled Environmental Adsorbates on Graphene NANO LETTERS Gallagher, P., Li, Y., Watanabe, K., Taniguchi, T., Heinz, T. F., Goldhaber-Gordon, D. 2018; 18 (4): 2603–8


    Topographic studies using scanning probes have found that graphene surfaces are often covered by micron-scale domains of periodic stripes with a 4 nm pitch. These stripes have been variously interpreted as structural ripples or as self-assembled adsorbates. We show that the stripe domains are optically anisotropic by imaging them using a polarization-contrast technique. Optical spectra between 1.1 and 2.8 eV reveal that the anisotropy in the in-plane dielectric function is predominantly real, reaching 0.6 for an assumed layer thickness of 0.3 nm. The spectra are incompatible with a rippled graphene sheet but would be quantitatively explained by the self-assembly of chainlike organic molecules into nanoscale stripes.

    View details for DOI 10.1021/acs.nanolett.8b00348

    View details for Web of Science ID 000430155900057

    View details for PubMedID 29589951

  • 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

  • Two-dimensional models for the optical response of thin films 2D MATERIALS Li, Y., Heinz, T. F. 2018; 5 (2)
  • Enhancing Mo:BiVO4 Solar Water Splitting with Patterned Au Nanospheres by Plasmon-Induced Energy Transfer ADVANCED ENERGY MATERIALS Kim, J., Shi, X., Jeong, M., Park, J., Han, H., Kim, S., Guo, Y., Heinz, T. F., Fan, S., Lee, C., Park, J., Zheng, X. 2018; 8 (5)
  • Ultrafast Graphene Light Emitters NANO LETTERS Kim, Y., Gao, Y., Shiue, R., Wang, L., Aslan, O., Bae, M., Kim, H., Seo, D., Choi, H., Kim, S., Nemilentsau, A., Low, T., Tan, C., Efetov, D. K., Taniguchi, T., Watanabe, K., Shepard, K. L., Heinz, T. F., Englund, D., Hone, J. 2018; 18 (2): 934–40


    Ultrafast electrically driven nanoscale light sources are critical components in nanophotonics. Compound semiconductor-based light sources for the nanophotonic platforms have been extensively investigated over the past decades. However, monolithic ultrafast light sources with a small footprint remain a challenge. Here, we demonstrate electrically driven ultrafast graphene light emitters that achieve light pulse generation with up to 10 GHz bandwidth across a broad spectral range from the visible to the near-infrared. The fast response results from ultrafast charge-carrier dynamics in graphene and weak electron-acoustic phonon-mediated coupling between the electronic and lattice degrees of freedom. We also find that encapsulating graphene with hexagonal boron nitride (hBN) layers strongly modifies the emission spectrum by changing the local optical density of states, thus providing up to 460% enhancement compared to the gray-body thermal radiation for a broad peak centered at 720 nm. Furthermore, the hBN encapsulation layers permit stable and bright visible thermal radiation with electronic temperatures up to 2000 K under ambient conditions as well as efficient ultrafast electronic cooling via near-field coupling to hybrid polaritonic modes under electrical excitation. These high-speed graphene light emitters provide a promising path for on-chip light sources for optical communications and other optoelectronic applications.

    View details for PubMedID 29337567

  • Imaging CF3I conical intersection and photodissociation dynamics with ultrafast electron diffraction Science Yang, J., Zhu, X., Wolf, T. J., Li, Z., Nunes, J. F., Coffee, R., Cryan, J. P., Gühr, M., Hegazy, K., Heinz, T. F., Jobe, K., Li, R., Shen, X., Veccione, T., Weathersby, S., Wilkin, K. J., Yoneda, C., Zheng, Q., Martinez, T. J., Centurion, M., Wang, X. 2018; 361 (6397): 64-67

    View details for DOI 10.1126/science.aat0049

  • Imaging CF3I conical intersection and photodissociation dynamics with ultrafast electron diffraction. Science (New York, N.Y.) Yang, J. n., Zhu, X. n., Wolf, T. J., Li, Z. n., Nunes, J. P., Coffee, R. n., Cryan, J. P., Gühr, M. n., Hegazy, K. n., Heinz, T. F., Jobe, K. n., Li, R. n., Shen, X. n., Veccione, T. n., Weathersby, S. n., Wilkin, K. J., Yoneda, C. n., Zheng, Q. n., Martinez, T. J., Centurion, M. n., Wang, X. n. 2018; 361 (6397): 64–67


    Conical intersections play a critical role in excited-state dynamics of polyatomic molecules because they govern the reaction pathways of many nonadiabatic processes. However, ultrafast probes have lacked sufficient spatial resolution to image wave-packet trajectories through these intersections directly. Here, we present the simultaneous experimental characterization of one-photon and two-photon excitation channels in isolated CF3I molecules using ultrafast gas-phase electron diffraction. In the two-photon channel, we have mapped out the real-space trajectories of a coherent nuclear wave packet, which bifurcates onto two potential energy surfaces when passing through a conical intersection. In the one-photon channel, we have resolved excitation of both the umbrella and the breathing vibrational modes in the CF3 fragment in multiple nuclear dimensions. These findings benchmark and validate ab initio nonadiabatic dynamics calculations.

    View details for PubMedID 29976821

  • 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
  • Magnetic brightening and control of dark excitons in monolayer WSe2 NATURE NANOTECHNOLOGY Zhang, X., Cao, T., Lu, Z., Lin, Y., Zhang, F., Wang, Y., Li, Z., Hone, J. C., Robinson, J. A., Smirnov, D., Louie, S. G., Heinz, T. F. 2017; 12 (9): 883-+


    Monolayer transition metal dichalcogenide crystals, as direct-gap materials with strong light-matter interactions, have attracted much recent attention. Because of their spin-polarized valence bands and a predicted spin splitting at the conduction band edges, the lowest-lying excitons in WX2 (X = S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark excitons. Here, we show how an in-plane magnetic field can brighten the dark excitons in monolayer WSe2 and permit their properties to be observed experimentally. Precise energy levels for both the neutral and charged dark excitons are obtained and compared with ab initio calculations using the GW-BSE approach. As a result of their spin configuration, the brightened dark excitons exhibit much-increased emission and valley lifetimes. These studies directly probe the excitonic spin manifold and reveal the fine spin-splitting at the conduction band edges.

    View details for PubMedID 28650442

  • Coulomb engineering of the bandgap and excitons in two-dimensional materials NATURE COMMUNICATIONS Raja, A., Chaves, A., Yu, J., Arefe, G., Hill, H. M., Rigosi, A. F., Berkelbach, T. C., Nagler, P., Schueller, C., Korn, T., Nuckolls, C., Hone, J., Brus, L. E., Heinz, T. F., Reichman, D. R., Chernikov, A. 2017; 8


    The ability to control the size of the electronic bandgap is an integral part of solid-state technology. Atomically thin two-dimensional crystals offer a new approach for tuning the energies of the electronic states based on the unusual strength of the Coulomb interaction in these materials and its environmental sensitivity. Here, we show that by engineering the surrounding dielectric environment, one can tune the electronic bandgap and the exciton binding energy in monolayers of WS2 and WSe2 by hundreds of meV. We exploit this behaviour to present an in-plane dielectric heterostructure with a spatially dependent bandgap, as an initial step towards the creation of diverse lateral junctions with nanoscale resolution.

    View details for DOI 10.1038/ncomms15251

    View details for Web of Science ID 000400561500001

    View details for PubMedID 28469178

  • Local Polar Fluctuations in Lead Halide Perovskite Crystals PHYSICAL REVIEW LETTERS Yaffe, O., Guo, Y., Tan, L. Z., Egger, D. A., Hull, T., Stoumpos, C. C., Zheng, F., Heinz, T. F., Kronik, L., Kanatzidis, M. G., Owen, J. S., Rappe, A. M., Pimenta, M. A., Brus, L. E. 2017; 118 (13)


    Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low-frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in the cubic phase of both hybrid (CH_{3}NH_{3}PbBr_{3}) and all-inorganic (CsPbBr_{3}) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations indicate that head-to-head Cs motion coupled to Br face expansion, occurring on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr_{3}.

    View details for DOI 10.1103/PhysRevLett.118.136001

    View details for Web of Science ID 000397808400007

    View details for PubMedID 28409968

  • High-harmonic generation from an atomically thin semiconductor NATURE PHYSICS Liu, H., Li, Y., You, Y. S., Ghimire, S., Heinz, T. F., Reis, D. A. 2017; 13 (3): 262-?

    View details for DOI 10.1038/NPHYS3946

    View details for Web of Science ID 000395814000017

  • after Ultrafast Excitation. Nano letters Ruppert, C., Chernikov, A., Hill, H. M., Rigosi, A. F., Heinz, T. F. 2017; 17 (2): 644-651


    Transient changes of the optical response of WS2 monolayers are studied by femtosecond broadband pump-probe spectroscopy. Time-dependent absorption spectra are analyzed by tracking the line width broadening, bleaching, and energy shift of the main exciton resonance as a function of time delay after the excitation. Two main sources for the pump-induced changes of the optical response are identified. Specifically, we find an interplay between modifications induced by many-body interactions from photoexcited carriers and by the subsequent transfer of the excitation to the phonon system followed by cooling of the material through the heat transfer to the substrate.

    View details for DOI 10.1021/acs.nanolett.6b03513

    View details for PubMedID 28059520

  • Polaritons in layered two-dimensional materials NATURE MATERIALS Low, T., Chaves, A., Caldwell, J. D., Kumar, A., Fang, N. X., Avouris, P., Heinz, T. F., Guinea, F., Martin-Moreno, L., Koppens, F. 2017; 16 (2): 182-194


    In recent years, enhanced light-matter interactions through a plethora of dipole-type polaritonic excitations have been observed in two-dimensional (2D) layered materials. In graphene, electrically tunable and highly confined plasmon-polaritons were predicted and observed, opening up opportunities for optoelectronics, bio-sensing and other mid-infrared applications. In hexagonal boron nitride, low-loss infrared-active phonon-polaritons exhibit hyperbolic behaviour for some frequencies, allowing for ray-like propagation exhibiting high quality factors and hyperlensing effects. In transition metal dichalcogenides, reduced screening in the 2D limit leads to optically prominent excitons with large binding energy, with these polaritonic modes having been recently observed with scanning near-field optical microscopy. Here, we review recent progress in state-of-the-art experiments, and survey the vast library of polaritonic modes in 2D materials, their optical spectral properties, figures of merit and application space. Taken together, the emerging field of 2D material polaritonics and their hybrids provide enticing avenues for manipulating light-matter interactions across the visible, infrared to terahertz spectral ranges, with new optical control beyond what can be achieved using traditional bulk materials.

    View details for DOI 10.1038/NMAT4792

    View details for Web of Science ID 000393349800009

    View details for PubMedID 27893724

  • The Role of Electronic and Phononic Excitation in the Optical Response of Monolayer WS2 after Ultrafast Excitation NANO LETTERS Ruppert, C., Chernikov, A., Hill, H. M., Rigosi, A. F., Heinz, T. F. 2017; 17 (2): 644-651
  • Optical manipulation of valley pseudospin NATURE PHYSICS Ye, Z., Sun, D., Heinz, T. F. 2017; 13 (1): 26-29

    View details for DOI 10.1038/NPHYS3891

    View details for Web of Science ID 000392043500012

  • 2D Materials Properties and Devices Introduction 2D MATERIALS: PROPERTIES AND DEVICES Avouris, P., Heinz, T. F., Low, T., Avouris, P., Heinz, T., Low, T. 2017: 1–4
  • 2D Materials: Properties and Devices 2D MATERIALS: PROPERTIES AND DEVICES Avouris, P., Heinz, T. F., Low, T. 2017: 1–504
  • Dynamic Optical Tuning of Interlayer Interactions in the Transition Metal Dichalcogenides. Nano letters Mannebach, E. M., Nyby, C. n., Ernst, F. n., Zhou, Y. n., Tolsma, J. n., Li, Y. n., Sher, M. J., Tung, I. C., Zhou, H. n., Zhang, Q. n., Seyler, K. L., Clark, G. n., Lin, Y. n., Zhu, D. n., Glownia, J. M., Kozina, M. E., Song, S. n., Nelson, S. n., Mehta, A. n., Yu, Y. n., Pant, A. n., Aslan, O. B., Raja, A. n., Guo, Y. n., DiChiara, A. n., Mao, W. n., Cao, L. n., Tongay, S. n., Sun, J. n., Singh, D. J., Heinz, T. F., Xu, X. n., MacDonald, A. H., Reed, E. n., Wen, H. n., Lindenberg, A. M. 2017; 17 (12): 7761–66


    Modulation of weak interlayer interactions between quasi-two-dimensional atomic planes in the transition metal dichalcogenides (TMDCs) provides avenues for tuning their functional properties. Here we show that above-gap optical excitation in the TMDCs leads to an unexpected large-amplitude, ultrafast compressive force between the two-dimensional layers, as probed by in situ measurements of the atomic layer spacing at femtosecond time resolution. We show that this compressive response arises from a dynamic modulation of the interlayer van der Waals interaction and that this represents the dominant light-induced stress at low excitation densities. A simple analytic model predicts the magnitude and carrier density dependence of the measured strains. This work establishes a new method for dynamic, nonequilibrium tuning of correlation-driven dispersive interactions and of the optomechanical functionality of TMDC quasi-two-dimensional materials.

    View details for PubMedID 29119791

  • High-order harmonics from bulk and 2D crystals You, Y., Ndabashimiye, G., Liu, H., Li, Y., Heinz, T. F., Reis, D. A., Ghimire, S., IEEE IEEE. 2017
  • Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides. Nature communications Selig, M., Berghäuser, G., Raja, A., Nagler, P., Schüller, C., Heinz, T. F., Korn, T., Chernikov, A., Malic, E., Knorr, A. 2016; 7: 13279-?


    Atomically thin transition metal dichalcogenides are direct-gap semiconductors with strong light-matter and Coulomb interactions. The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in the optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. Here, we investigate the microscopic origin of the excitonic coherence lifetime in two representative materials (WS2 and MoSe2) through a study combining microscopic theory with spectroscopic measurements. We show that the excitonic coherence lifetime is determined by phonon-induced intravalley scattering and intervalley scattering into dark excitonic states. In particular, in WS2, we identify exciton relaxation processes involving phonon emission into lower-lying dark states that are operative at all temperatures.

    View details for DOI 10.1038/ncomms13279

    View details for PubMedID 27819288

    View details for PubMedCentralID PMC5103057

  • Electronic band gaps and exciton binding energies in monolayer MoxW1-xS2 transition metal dichalcogenide alloys probed by scanning tunneling and optical spectroscopy PHYSICAL REVIEW B Rigosi, A. F., Hill, H. M., Rim, K. T., Flynn, G. W., Heinz, T. F. 2016; 94 (7)
  • Metal-Semiconductor Nanoparticle Hybrids Formed by Self-Organization: A Platform to Address Exciton-Plasmon Coupling. Nano letters Strelow, C., Theuerholz, T. S., Schmidtke, C., Richter, M., Merkl, J., Kloust, H., Ye, Z., Weller, H., Heinz, T. F., Knorr, A., Lange, H. 2016; 16 (8): 4811-4818


    Hybrid nanosystems composed of excitonic and plasmonic constituents can have different properties than the sum of of the two constituents, due to the exciton-plasmon interaction. Here, we report on a flexible model system based on colloidal nanoparticles that can form hybrid combinations by self-organization. The system allows us to tune the interparticle distance and to combine nanoparticles of different sizes and thus enables a systematic investigation of the exciton-plasmon coupling by a combination of optical spectroscopy and quantum-optical theory. We experimentally observe a strong influence of the energy difference between exciton and plasmon, as well as an interplay of nanoparticle size and distance on the coupling. We develop a full quantum theory for the luminescence dynamics and discuss the experimental results in terms of the Purcell effect. As the theory describes excitation as well as coherent and incoherent emission, we also consider possible quantum optical effects. We find a good agreement of the observed and the calculated luminescence dynamics induced by the Purcell effect. This also suggests that the self-organized hybrid system can be used as platform to address quantum optical effects.

    View details for DOI 10.1021/acs.nanolett.6b00982

    View details for PubMedID 27355971

  • Band Alignment in MoS2/WS2 Transition Metal Dichalcogenide Heterostructures Probed by Scanning Tunneling Microscopy and Spectroscopy. Nano letters Hill, H. M., Rigosi, A. F., Rim, K. T., Flynn, G. W., Heinz, T. F. 2016; 16 (8): 4831-4837


    Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), we examine the electronic structure of transition metal dichalcogenide heterostructures (TMDCHs) composed of monolayers of MoS2 and WS2. STS data are obtained for heterostructures of varying stacking configuration as well as the individual monolayers. Analysis of the tunneling spectra includes the influence of finite sample temperature, yield information about the quasi-particle bandgaps, and the band alignment of MoS2 and WS2. We report the band gaps of MoS2 (2.16 ± 0.04 eV) and WS2 (2.38 ± 0.06 eV) in the materials as measured on the heterostructure regions and the general type II band alignment for the heterostructure, which shows an interfacial band gap of 1.45 ± 0.06 eV.

    View details for DOI 10.1021/acs.nanolett.6b01007

    View details for PubMedID 27298270

  • Energy Transfer from Quantum Dots to Graphene and MoS2: The Role of Absorption and Screening in Two-Dimensional Materials. Nano letters Raja, A., Montoya Castillo, A., Zultak, J., Zhang, X., Ye, Z., Roquelet, C., Chenet, D. A., van der Zande, A. M., Huang, P., Jockusch, S., Hone, J., Reichman, D. R., Brus, L. E., Heinz, T. F. 2016; 16 (4): 2328-2333


    We report efficient nonradiative energy transfer (NRET) from core-shell, semiconducting quantum dots to adjacent two-dimensional sheets of graphene and MoS2 of single- and few-layer thickness. We observe quenching of the photoluminescence (PL) from individual quantum dots and enhanced PL decay rates in time-resolved PL, corresponding to energy transfer rates of 1-10 ns(-1). Our measurements reveal contrasting trends in the NRET rate from the quantum dot to the van der Waals material as a function of thickness. The rate increases significantly with increasing layer thickness of graphene, but decreases with increasing thickness of MoS2 layers. A classical electromagnetic theory accounts for both the trends and absolute rates observed for the NRET. The countervailing trends arise from the competition between screening and absorption of the electric field of the quantum dot dipole inside the acceptor layers. We extend our analysis to predict the type of NRET behavior for the near-field coupling of a chromophore to a range of semiconducting and metallic thin film materials.

    View details for DOI 10.1021/acs.nanolett.5b05012

    View details for PubMedID 26928675

  • Ultrasensitive Plasmonic Detection of Molecules with Graphene ACS PHOTONICS Farmer, D. B., Avouris, P., Li, Y., Heinz, T. F., Han, S. 2016; 3 (4): 553-557
  • Linearly Polarized Excitons in Single- and Few-Layer ReS2 Crystals ACS PHOTONICS Aslan, O. B., Chenet, D. A., van der Zande, A. M., Hone, J. C., Heinz, T. F. 2016; 3 (1): 96-101
  • Experimental Evidence for Dark Excitons in Monolayer WSe2 PHYSICAL REVIEW LETTERS Zhang, X., You, Y., Zhao, S. Y., Heinz, T. F. 2015; 115 (25)

    View details for DOI 10.1103/PhysRevLett.115.257403

    View details for PubMedID 26722944

  • Measurement of Lateral and Interfacial Thermal Conductivity of Single- and Bilayer MoS2 and MoSe2 Using Refined Optothermal Raman Technique. ACS applied materials & interfaces Zhang, X., Sun, D., Li, Y., Lee, G., Cui, X., Chenet, D., You, Y., Heinz, T. F., Hone, J. C. 2015; 7 (46): 25923-25929


    Atomically thin materials such as graphene and semiconducting transition metal dichalcogenides (TMDCs) have attracted extensive interest in recent years, motivating investigation into multiple properties. In this work, we demonstrate a refined version of the optothermal Raman technique to measure the thermal transport properties of two TMDC materials, MoS2 and MoSe2, in single-layer (1L) and bilayer (2L) forms. This new version incorporates two crucial improvements over previous implementations. First, we utilize more direct measurements of the optical absorption of the suspended samples under study and find values ∼40% lower than previously assumed. Second, by comparing the response of fully supported and suspended samples using different laser spot sizes, we are able to independently measure the interfacial thermal conductance to the substrate and the lateral thermal conductivity of the supported and suspended materials. The approach is validated by examining the response of a suspended film illuminated in different radial positions. For 1L MoS2 and MoSe2, the room-temperature thermal conductivities are 84 ± 17 and 59 ± 18 W/(m·K), respectively. For 2L MoS2 and MoSe2, we obtain values of 77 ± 25 W and 42 ± 13 W/(m·K). Crucially, the interfacial thermal conductance is found to be of order 0.1-1 MW/m(2) K, substantially smaller than previously assumed, a finding that has important implications for design and modeling of electronic devices.

    View details for DOI 10.1021/acsami.5b08580

    View details for PubMedID 26517143

  • Tunable electronic correlation effects in nanotube-light interactions PHYSICAL REVIEW B Miyauchi, Y., Zhang, Z., Takekoshi, M., Tomio, Y., Suzuura, H., Perebeinos, V., Deshpande, V. V., Lu, C., Berciaud, S., Kim, P., Hone, J., Heinz, T. F. 2015; 92 (20)
  • Photonic and Plasmonic Guided Modes in Graphene-Silicon Photonic Crystals ACS PHOTONICS Gu, T., Andryieuski, A., Hao, Y., Li, Y., Hone, J., Wong, C. W., Lavrinenko, A., Low, T., Heinz, T. F. 2015; 2 (11): 1552-1558
  • Impedance spectroscopy studies of moisture uptake in low-k dielectrics and its relation to reliability MICROELECTRONIC ENGINEERING Raja, A., Laibowitz, R., Liniger, E. G., Shaw, T. M., Heinz, T. F. 2015; 147: 100-103
  • Dynamic Structural Response and Deformations of Monolayer MoS2 Visualized by Femtosecond Electron Diffraction NANO LETTERS Mannebach, E. M., Li, R., Duerloo, K., Nyby, C., Zalden, P., Vecchione, T., Ernst, F., Reid, A. H., Chase, T., Shen, X., Weathersby, S., Hast, C., Hettel, R., Coffee, R., Hartmann, N., Fry, A. R., Yu, Y., Cao, L., Heinz, T. F., Reed, E. J., Duerr, H. A., Wang, X., Lindenberg, A. M. 2015; 15 (10): 6889-6895


    Two-dimensional materials are subject to intrinsic and dynamic rippling that modulates their optoelectronic and electromechanical properties. Here, we directly visualize the dynamics of these processes within monolayer transition metal dichalcogenide MoS2 using femtosecond electron scattering techniques as a real-time probe with atomic-scale resolution. We show that optical excitation induces large-amplitude in-plane displacements and ultrafast wrinkling of the monolayer on nanometer length-scales, developing on picosecond time-scales. These deformations are associated with several percent peak strains that are fully reversible over tens of millions of cycles. Direct measurements of electron-phonon coupling times and the subsequent interfacial thermal heat flow between the monolayer and substrate are also obtained. These measurements, coupled with first-principles modeling, provide a new understanding of the dynamic structural processes that underlie the functionality of two-dimensional materials and open up new opportunities for ultrafast strain engineering using all-optical methods.

    View details for DOI 10.1021/acs.nanolett.5b02805

    View details for PubMedID 26322659

  • Electrical Tuning of Exciton Binding Energies in Monolayer WS2 PHYSICAL REVIEW LETTERS Chernikov, A., van der Zande, A. M., Hill, H. M., Rigosi, A. F., Velauthapillai, A., Hone, J., Heinz, T. F. 2015; 115 (12)


    We demonstrate continuous tuning of the exciton binding energy in monolayer WS_{2} by means of an externally applied voltage in a field-effect transistor device. Using optical spectroscopy, we monitor the ground and excited excitonic states as a function of gate voltage and track the evolution of the quasiparticle band gap. The observed decrease of the exciton binding energy over the range of about 100 meV, accompanied by the renormalization of the quasiparticle band gap, is associated with screening of the Coulomb interaction by the electrically injected free charge carriers at densities up to 8×10^{12}  cm^{-2}. Complete ionization of the excitons due to the electrical doping is estimated to occur at a carrier density of several 10^{13}  cm^{-2}.

    View details for DOI 10.1103/PhysRevLett.115.126802

    View details for Web of Science ID 000361316500014

    View details for PubMedID 26431003

  • In-Plane Anisotropy in Mono- and Few-Layer ReS2 Probed by Raman Spectroscopy and Scanning Transmission Electron Microscopy NANO LETTERS Chenet, D. A., Aslan, O. B., Huang, P. Y., Fan, C., van der Zande, A. M., Heinz, T. F., Hone, J. C. 2015; 15 (9): 5667-5672


    Rhenium disulfide (ReS2) is a semiconducting layered transition metal dichalcogenide that exhibits a stable distorted 1T phase. The reduced symmetry of this system leads to in-plane anisotropy in various material properties. Here, we demonstrate the strong anisotropy in the Raman scattering response for linearly polarized excitation. Polarized Raman scattering is shown to permit a determination of the crystallographic orientation of ReS2 through comparison with direct structural analysis by scanning transmission electron microscopy (STEM). Analysis of the frequency difference of appropriate Raman modes is also shown to provide a means of precisely determining layer thickness up to four layers.

    View details for DOI 10.1021/acs.nanolett.5b00910

    View details for Web of Science ID 000361252700001

    View details for PubMedID 26280493

  • Probing Interlayer Interactions in Transition Metal Dichalcogenide Heterostructures by Optical Spectroscopy: MoS2/WS2 and MoSe2/WSe2 NANO LETTERS Rigos, A. F., Hill, H. M., Li, Y., Chernikov, A., Heinz, T. F. 2015; 15 (8): 5033-5038


    We have applied optical absorption spectroscopy to investigate van der Waals heterostructures formed of pairs of monolayer transition metal dichalcogenide crystals, choosing MoS2/WS2 and MoSe2/WSe2 as test cases. In the heterostructure spectra, we observe a significant broadening of the excitonic transitions compared to the corresponding features in the isolated layers. The broadening is interpreted as a lifetime effect arising from decay of excitons initially created in either layer through charge transfer processes expected for a staggered band alignment. The measured spectral broadening of 20 meV - 35 meV implies lifetimes for charge separation of the near band-edge A and B excitons in the range of 20-35 fs. Higher-lying transitions exhibit still greater broadening.

    View details for DOI 10.1021/acs.nanolett.5b01055

    View details for Web of Science ID 000359613700028

    View details for PubMedID 26186085

  • Probing the Dynamics of the Metallic-to-Semiconducting Structural Phase Transformation in MoS2 Crystals NANO LETTERS Guo, Y., Sun, D., Ouyang, B., Raja, A., Song, J., Heinz, T. F., Brus, L. E. 2015; 15 (8): 5081-5088


    We have investigated the phase transformation of bulk MoS2 crystals from the metastable metallic 1T/1T' phase to the thermodynamically stable semiconducting 2H phase. The metastable 1T/1T' material was prepared by Li intercalation and deintercalation. The thermally driven kinetics of the phase transformation were studied with in situ Raman and optical reflection spectroscopies and yield an activation energy of 400 ± 60 meV (38 ± 6 kJ/mol). We calculate the expected minimum energy pathways for these transformations using DFT methods. The experimental activation energy corresponds approximately to the theoretical barrier for a single formula unit, suggesting that nucleation of the phase transformation is quite local. We also report that femtosecond laser writing converts 1T/1T' to 2H in a single laser pass. The mechanisms for the phase transformation are discussed.

    View details for DOI 10.1021/acs.nanolett.5b01196

    View details for Web of Science ID 000359613700035

    View details for PubMedID 26134736

  • 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

  • Excitons in ultrathin organic-inorganic perovskite crystals PHYSICAL REVIEW B Yaffe, O., Chernikov, A., Norman, Z. M., Zhong, Y., Velauthapillai, A., van der Zande, A., Owen, J. S., Heinz, T. F. 2015; 92 (4)
  • Population inversion and giant bandgap renormalization in atomically thin WS2 layers NATURE PHOTONICS Chernikov, A., Ruppert, C., Hill, H. M., Rigosi, A. F., Heinz, T. F. 2015; 9 (7): 466-U69
  • Observation of Ground- and Excited-State Charge Transfer at the C-60/Graphene Interface ACS NANO Jnawali, G., Rao, Y., Beck, J. H., Petrone, N., Kymissis, I., Hone, J., Heinz, T. F. 2015; 9 (7): 7175-7185


    We examine charge transfer interactions in the hybrid system of a film of C60 molecules deposited on single-layer graphene using Raman spectroscopy and Terahertz (THz) time-domain spectroscopy. In the absence of photoexcitation, we find that the C60 molecules in the deposited film act as electron acceptors for graphene, yielding increased hole doping in the graphene layer. Hole doping of the graphene film by a uniform C60 film at a level of 5.6 × 10(12)/cm(2) or 0.04 holes per interfacial C60 molecule was determined by the use of both Raman and THz spectroscopy. We also investigate transient charge transfer occurring upon photoexcitation by femtosecond laser pulses with a photon energy of 3.1 eV. The C60/graphene hybrid exhibits a short-lived (ps) decrease in THz conductivity, followed by a long-lived increase in conductivity. The initial negative photoconductivity transient, which decays within 2 ps, reflects the intrinsic photoresponse of graphene. The longer-lived positive conductivity transient, with a lifetime on the order of 100 ps, is attributed to photoinduced hole doping of graphene by interfacial charge transfer. We discuss possible microscopic pathways for hot carrier processes in the hybrid system.

    View details for DOI 10.1021/acsnano.5b01896

    View details for Web of Science ID 000358823200054

    View details for PubMedID 26072947

  • Observation of biexcitons in monolayer WSe2 NATURE PHYSICS You, Y., Zhang, X., Berkelbach, T. C., Hybertsen, M. S., Reichman, D. R., Heinz, T. F. 2015; 11 (6): 477-U138

    View details for DOI 10.1038/NPHYS3324

    View details for Web of Science ID 000355552200013

  • Observation of Excitonic Rydberg States in Monolayer MoS2 and WS2 by Photoluminescence Excitation Spectroscopy NANO LETTERS Hill, H. M., Rigosi, A. F., Roquelet, C., Chernikov, A., Berkelbach, T. C., Reichman, D. R., Hybertsen, M. S., Brus, L. E., Heinz, T. F. 2015; 15 (5): 2992-2997


    We have identified excited exciton states in monolayers of MoS2 and WS2 supported on fused silica by means of photoluminescence excitation spectroscopy. In monolayer WS2, the positions of the excited A exciton states imply an exciton binding energy of 0.32 eV. In monolayer MoS2, excited exciton transitions are observed at energies of 2.24 and 2.34 eV. Assigning these states to the B exciton Rydberg series yields an exciton binding energy of 0.44 eV.

    View details for DOI 10.1021/nl504868p

    View details for Web of Science ID 000354906000034

    View details for PubMedID 25816155

  • Valley Splitting and Polarization by the Zeeman Effect in Monolayer MoSe2 PHYSICAL REVIEW LETTERS Li, Y., Ludwig, J., Low, T., Chernikov, A., Cui, X., Arefe, G., Kim, Y. D., van der Zande, A. M., Rigosi, A., Hill, H. M., Kim, S. H., Hone, J., Li, Z., Smirnov, D., Heinz, T. F. 2014; 113 (26)


    We have measured circularly polarized photoluminescence in monolayer MoSe2 under perpendicular magnetic fields up to 10 T. At low doping densities, the neutral and charged excitons shift linearly with field strength at a rate of ∓0.12  meV/T for emission arising, respectively, from the K and K' valleys. The opposite sign for emission from different valleys demonstrates lifting of the valley degeneracy. The magnitude of the Zeeman shift agrees with predicted magnetic moments for carriers in the conduction and valence bands. The relative intensity of neutral and charged exciton emission is modified by the magnetic field, reflecting the creation of field-induced valley polarization. At high doping levels, the Zeeman shift of the charged exciton increases to ∓0.18  meV/T. This enhancement is attributed to many-body effects on the binding energy of the charged excitons.

    View details for DOI 10.1103/PhysRevLett.113.266804

    View details for Web of Science ID 000348364000007

    View details for PubMedID 25615372

  • Measurement of the optical dielectric function of monolayer transition-metal dichalcogenides: MoS2, MoSe2, WS2, and WSe2 PHYSICAL REVIEW B Li, Y., Chernikov, A., Zhang, X., Rigosi, A., Hill, H. M., van der Zande, A. M., Chenet, D. A., Shih, E., Hone, J., Heinz, T. F. 2014; 90 (20)
  • Spectroscopic Study of Anisotropic Excitons in Single Crystal Hexacene JOURNAL OF PHYSICAL CHEMISTRY LETTERS Chernikov, A., Yaffe, O., Kumar, B., Zhong, Y., Nuckolls, C., Heinz, T. F. 2014; 5 (21): 3632-3635


    The linear optical response of hexacene single crystals over a spectral range of 1.3-1.9 eV was studied using polarization-resolved reflectance spectroscopy at cryogenic temperatures. We observe strong polarization anisotropy for all optical transitions. Pronounced deviations from the single-molecule, solution-phase spectra are present, with a measured Davydov splitting of 180 meV, indicating strong intermolecular coupling. The energies and oscillator strengths of the relevant optical transitions and polarization-dependent absorption coefficients are extracted from quantitative analysis of the data.

    View details for DOI 10.1021/jz501693g

    View details for Web of Science ID 000344579500010

    View details for PubMedID 26278730

  • Evolution of the Raman spectrum of graphene grown on copper upon oxidation of the substrate NANO RESEARCH Yin, X., Li, Y., Ke, F., Lin, C., Zhao, H., Gan, L., Luo, Z., Zhao, R., Heinz, T. F., Hu, Z. 2014; 7 (11): 1613-1622
  • Optical Properties and Band Gap of Single- and Few-Layer MoTe2 Crystals NANO LETTERS Ruppert, C., Aslan, O. B., Heinz, T. F. 2014; 14 (11): 6231-6236


    Single- and few-layer crystals of exfoliated MoTe2 have been characterized spectroscopically by photoluminescence, Raman scattering, and optical absorption measurements. We find that MoTe2 in the monolayer limit displays strong photoluminescence. On the basis of complementary optical absorption results, we conclude that monolayer MoTe2 is a direct-gap semiconductor with an optical band gap of 1.10 eV. This new monolayer material extends the spectral range of atomically thin direct-gap materials from the visible to the near-infrared.

    View details for DOI 10.1021/nl502557g

    View details for Web of Science ID 000345723800032

    View details for PubMedID 25302768

  • Piezoelectricity of single-atomic-layer MoS2 for energy conversion and piezotronics NATURE Wu, W., Wang, L., Li, Y., Zhang, F., Lin, L., Niu, S., Chenet, D., Zhang, X., Hao, Y., Heinz, T. F., Hone, J., Wang, Z. L. 2014; 514 (7523): 470-?


    The piezoelectric characteristics of nanowires, thin films and bulk crystals have been closely studied for potential applications in sensors, transducers, energy conversion and electronics. With their high crystallinity and ability to withstand enormous strain, two-dimensional materials are of great interest as high-performance piezoelectric materials. Monolayer MoS2 is predicted to be strongly piezoelectric, an effect that disappears in the bulk owing to the opposite orientations of adjacent atomic layers. Here we report the first experimental study of the piezoelectric properties of two-dimensional MoS2 and show that cyclic stretching and releasing of thin MoS2 flakes with an odd number of atomic layers produces oscillating piezoelectric voltage and current outputs, whereas no output is observed for flakes with an even number of layers. A single monolayer flake strained by 0.53% generates a peak output of 15 mV and 20 pA, corresponding to a power density of 2 mW m(-2) and a 5.08% mechanical-to-electrical energy conversion efficiency. In agreement with theoretical predictions, the output increases with decreasing thickness and reverses sign when the strain direction is rotated by 90°. Transport measurements show a strong piezotronic effect in single-layer MoS2, but not in bilayer and bulk MoS2. The coupling between piezoelectricity and semiconducting properties in two-dimensional nanomaterials may enable the development of applications in powering nanodevices, adaptive bioprobes and tunable/stretchable electronics/optoelectronics.

    View details for DOI 10.1038/nature13792

    View details for Web of Science ID 000343775900035

    View details for PubMedID 25317560

  • Observation of Rapid Exciton-Exciton Annihilation in Monolayer Molybdenum Disulfide NANO LETTERS Sun, D., Rao, Y., Reider, G. A., Chen, G., You, Y., Brezin, L., Harutyunyan, A. R., Heinz, T. F. 2014; 14 (10): 5625-5629


    Monolayer MoS2 is a direct-gap two-dimensional semiconductor that exhibits strong electron-hole interactions, leading to the formation of stable excitons and trions. Here we report the existence of efficient exciton-exciton annihilation, a four-body interaction, in this material. Exciton-exciton annihilation was identified experimentally in ultrafast transient absorption measurements through the emergence of a decay channel varying quadratically with exciton density. The rate of exciton-exciton annihilation was determined to be (4.3 ± 1.1) × 10(-2) cm(2)/s at room temperature.

    View details for DOI 10.1021/nl5021975

    View details for Web of Science ID 000343016400023

    View details for PubMedID 25171389

  • Heterostructures based on inorganic and organic van der Waals systems APL MATERIALS Lee, G., Lee, C., van der Zande, A. M., Han, M., Cui, X., Arefe, G., Nuckolls, C., Heinz, T. F., Hone, J., Kim, P. 2014; 2 (9)

    View details for DOI 10.1063/1.4894435

    View details for Web of Science ID 000342568000014

  • Atomically thin p-n junctions with van der Waals heterointerfaces NATURE NANOTECHNOLOGY Lee, C., Lee, G., van der Zande, A. M., Chen, W., Li, Y., Han, M., Cui, X., Arefe, G., Nuckolls, C., Heinz, T. F., Guo, J., Hone, J., Kim, P. 2014; 9 (9): 676-681


    Semiconductor p-n junctions are essential building blocks for electronic and optoelectronic devices. In conventional p-n junctions, regions depleted of free charge carriers form on either side of the junction, generating built-in potentials associated with uncompensated dopant atoms. Carrier transport across the junction occurs by diffusion and drift processes influenced by the spatial extent of this depletion region. With the advent of atomically thin van der Waals materials and their heterostructures, it is now possible to realize a p-n junction at the ultimate thickness limit. Van der Waals junctions composed of p- and n-type semiconductors--each just one unit cell thick--are predicted to exhibit completely different charge transport characteristics than bulk heterojunctions. Here, we report the characterization of the electronic and optoelectronic properties of atomically thin p-n heterojunctions fabricated using van der Waals assembly of transition-metal dichalcogenides. We observe gate-tunable diode-like current rectification and a photovoltaic response across the p-n interface. We find that the tunnelling-assisted interlayer recombination of the majority carriers is responsible for the tunability of the electronic and optoelectronic processes. Sandwiching an atomic p-n junction between graphene layers enhances the collection of the photoexcited carriers. The atomically scaled van der Waals p-n heterostructures presented here constitute the ultimate functional unit for nanoscale electronic and optoelectronic devices.

    View details for DOI 10.1038/NNANO.2014.150

    View details for Web of Science ID 000341814400009

    View details for PubMedID 25108809

  • Exciton Binding Energy and Nonhydrogenic Rydberg Series in Monolayer WS2 PHYSICAL REVIEW LETTERS Chernikov, A., Berkelbach, T. C., Hill, H. M., Rigosi, A., Li, Y., Aslan, O. B., Reichman, D. R., Hybertsen, M. S., Heinz, T. F. 2014; 113 (7)


    We have experimentally determined the energies of the ground and first four excited excitonic states of the fundamental optical transition in monolayer WS_{2}, a model system for the growing class of atomically thin two-dimensional semiconductor crystals. From the spectra, we establish a large exciton binding energy of 0.32 eV and a pronounced deviation from the usual hydrogenic Rydberg series of energy levels of the excitonic states. We explain both of these results using a microscopic theory in which the nonlocal nature of the effective dielectric screening modifies the functional form of the Coulomb interaction. These strong but unconventional electron-hole interactions are expected to be ubiquitous in atomically thin materials.

    View details for DOI 10.1103/PhysRevLett.113.076802

    View details for Web of Science ID 000341115700020

    View details for PubMedID 25170725

  • Multiphonon Relaxation Slows Singlet Fission in Crystalline Hexacene JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Busby, E., Berkelbach, T. C., Kumar, B., Chernikov, A., Zhong, Y., Hlaing, H., Zhu, X., Heinz, T. F., Hybertsen, M. S., Sfeir, M. Y., Reichman, D. R., Nuckolls, C., Yaffe, O. 2014; 136 (30): 10654-10660


    Singlet fission, the conversion of a singlet excitation into two triplet excitations, is a viable route to improved solar-cell efficiency. Despite active efforts to understand the singlet fission mechanism, which would aid in the rational design of new materials, a comprehensive understanding of mechanistic principles is still lacking. Here, we present the first study of singlet fission in crystalline hexacene which, together with tetracene and pentacene, enables the elucidation of mechanistic trends. We characterize the static and transient optical absorption and combine our findings with a theoretical analysis of the relevant electronic couplings and rates. We find a singlet fission time scale of 530 fs, which is orders of magnitude faster than tetracene (10-100 ps) but significantly slower than pentacene (80-110 fs). We interpret this increased time scale as a multiphonon relaxation effect originating from a large exothermicity and present a microscopic theory that quantitatively reproduces the rates in the acene family.

    View details for DOI 10.1021/ja503980c

    View details for Web of Science ID 000339693900022

    View details for PubMedID 24983697

  • Tailoring the Electronic Structure in Bilayer Molybdenum Disulfide via Interlayer Twist NANO LETTERS van der Zande, A. M., Kunstrnann, J., Chernikov, A., Chenet, D. A., You, Y., Zhang, X., Huang, P. Y., Berkelbach, T. C., Wang, L., Zhang, F., Hybertsen, M. S., Muller, D. A., Reichman, D. R., Heinz, T. F., Hone, J. C. 2014; 14 (7): 3869-3875


    Molybdenum disulfide bilayers with well-defined interlayer twist angle were constructed by stacking single-crystal monolayers. Varying interlayer twist angle results in strong tuning of the indirect optical transition energy and second-harmonic generation and weak tuning of direct optical transition energies and Raman mode frequencies. Electronic structure calculations show the interlayer separation changes with twist due to repulsion between sulfur atoms, resulting in shifts of the indirect optical transition energies. These results show that interlayer alignment is a crucial variable in tailoring the properties of two-dimensional heterostructures.

    View details for DOI 10.1021/nl501077m

    View details for Web of Science ID 000338979700026

    View details for PubMedID 24933687

  • Edge Structures for Nanoscale Graphene Islands on Co(0001) Surfaces ACS NANO Prezzi, D., Eom, D., Rim, K. T., Zhou, H., Xiao, S., Nuckolls, C., Heinz, T. F., Flynn, G. W., Hybertsen, M. S. 2014; 8 (6): 5765-5773


    Low-temperature scanning tunneling microscopy measurements and first-principles calculations are employed to characterize edge structures observed for graphene nanoislands grown on the Co(0001) surface. Images of these nanostructures reveal straight well-ordered edges with zigzag orientation, which are characterized by a distinct peak at low bias in tunneling spectra. Density functional theory based calculations are used to discriminate between candidate edge structures. Several zigzag-oriented edge structures have lower formation energy than armchair-oriented edges. Of these, the lowest formation energy configurations are a zigzag and a Klein edge structure, each with the final carbon atom over the hollow site in the Co(0001) surface. In the absence of hydrogen, the interaction with the Co(0001) substrate plays a key role in stabilizing these edge structures and determines their local conformation and electronic properties. The calculated electronic properties for the low-energy edge structures are consistent with the measured scanning tunneling images.

    View details for DOI 10.1021/nn500583a

    View details for Web of Science ID 000338089200040

    View details for PubMedID 24830340

  • Postgrowth tuning of the bandgap of single-layer molybdenum disulfide films by sulfur/selenium exchange. ACS nano Ma, Q., Isarraraz, M., Wang, C. S., Preciado, E., Klee, V., Bobek, S., Yamaguchi, K., Li, E., Odenthal, P. M., Nguyen, A., Barroso, D., Sun, D., von Son Palacio, G., Gomez, M., Nguyen, A., Le, D., Pawin, G., Mann, J., Heinz, T. F., Rahman, T. S., Bartels, L. 2014; 8 (5): 4672-4677


    We demonstrate bandgap tuning of a single-layer MoS2 film on SiO2/Si via substitution of its sulfur atoms by selenium through a process of gentle sputtering, exposure to a selenium precursor, and annealing. We characterize the substitution process both for S/S and S/Se replacement. Photoluminescence and, in the latter case, X-ray photoelectron spectroscopy provide direct evidence of optical band gap shift and selenium incorporation, respectively. We discuss our experimental observations, including the limit of the achievable bandgap shift, in terms of the role of stress in the film as elucidated by computational studies, based on density functional theory. The resultant films are stable in vacuum, but deteriorate under optical excitation in air.

    View details for DOI 10.1021/nn5004327

    View details for PubMedID 24684434

  • Tuning Many-Body Interactions in Graphene: The Effects of Doping on Excitons and Carrier Lifetimes PHYSICAL REVIEW LETTERS Mak, K. F., da Jornada, F. H., He, K., Deslippe, J., Petrone, N., Hone, J., Shan, J., Louie, S. G., Heinz, T. F. 2014; 112 (20)
  • Postgrowth Tuning of the Bandgap of Single-Layer Molybdenum Disulfide Films by Sulfur/Selenium Exchange ACS NANO Ma, Q., Isarraraz, M., Wang, C. S., Preciado, E., Klee, V., Bobek, S., Yamaguchi, K., Li, E., Odenthal, P. M., Ariana Nguyen, A., Barroso, D., Sun, D., Palacio, G. v., Gomez, M., Andrew Nguyen, A., Le, D., Pawin, G., Mann, J., Heinz, T. F., Rahman, T. S., Bartels, L. 2014; 8 (5): 4672-4677

    View details for DOI 10.1021/nn5004327

    View details for Web of Science ID 000336640600056

  • Spin and pseudospins in layered transition metal dichalcogenides NATURE PHYSICS Xu, X., Yao, W., Xiao, D., Heinz, T. F. 2014; 10 (5): 343-350

    View details for DOI 10.1038/NPHYS2942

    View details for Web of Science ID 000335371200011

  • 2-Dimensional Transition Metal Dichalcogenides with Tunable Direct Band Gaps: MoS2(1-x)Se2x Monolayers ADVANCED MATERIALS Mann, J., Ma, Q., Odenthal, P. M., Isarraraz, M., Le, D., Preciado, E., Barroso, D., Yamaguchi, K., Palacio, G. v., Andrew Nguyen, A., Tai Tran, T., Wurch, M., Ariana Nguyen, A., Klee, V., Bobek, S., Sun, D., Heinz, T. F., Rahman, T. S., Kawakami, R., Bartels, L. 2014; 26 (9): 1399-1404


    MoS2(1-x) Se2x single-layer films are prepared using a mixture of organic selenium and sulfur precursors as well as a solid molybdenum source. The direct bandgaps are found to scale nearly linearly with composition in the range of 1.87 eV (pure single-layer MoS2 ) to 1.55 eV (pure single-layer MoSe2 ) permitting straightforward bandgap engineering.

    View details for DOI 10.1002/adma.201304389

    View details for Web of Science ID 000332330400002

    View details for PubMedID 24339159

  • Graphene Plasmon Enhanced Vibrational Sensing of Surface-Adsorbed Layers NANO LETTERS Li, Y., Yan, H., Farmer, D. B., Meng, X., Zhu, W., Osgood, R. M., Heinz, T. F., Avouris, P. 2014; 14 (3): 1573-1577


    We characterize the influence of graphene nanoribbon plasmon excitation on the vibrational spectra of surface-absorbed polymers. As the detuning between the graphene plasmon frequency and a vibrational frequency of the polymer decreases, the vibrational peak intensity first increases and is then transformed into a region of narrow optical transparency as the frequencies overlap. Examples of this are provided by the carbonyl vibration in thin films of poly(methyl methacrylate) and polyvinylpyrrolidone. The signal depth of the plasmon-induced transparency is found to be 5 times larger than that of light attenuated by the carbonyl vibration alone. The plasmon-vibrational mode coupling and the resulting fields are analyzed using both a phenomenological model of electromagnetically coupled oscillators and finite-difference time-domain simulations. It is shown that this coupling and the resulting absorption enhancement can be understood in terms of near-field electromagnetic interactions.

    View details for DOI 10.1021/nl404824w

    View details for Web of Science ID 000335720300074

    View details for PubMedID 24528250

  • Competing Thermodynamic and Dynamic Factors Select Molecular Assemblies on a Gold Surface PHYSICAL REVIEW LETTERS Haxton, T. K., Zhou, H., Tamblyn, I., Eom, D., Hu, Z., Neaton, J. B., Heinz, T. F., Whitelam, S. 2013; 111 (26)


    Controlling the self-assembly of surface-adsorbed molecules into nanostructures requires understanding physical mechanisms that act across multiple length and time scales. By combining scanning tunneling microscopy with hierarchical ab initio and statistical mechanical modeling of 1,4-substituted benzenediamine (BDA) molecules adsorbed on a gold (111) surface, we demonstrate that apparently simple nanostructures are selected by a subtle competition of thermodynamics and dynamics. Of the collection of possible BDA nanostructures mechanically stabilized by hydrogen bonding, the interplay of intermolecular forces, surface modulation, and assembly dynamics select at low temperature a particular subset: low free energy oriented linear chains of monomers and high free energy branched chains.

    View details for DOI 10.1103/PhysRevLett.111.265701

    View details for Web of Science ID 000331934500012

    View details for PubMedID 24483804

  • Chip-integrated ultrafast graphene photodetector with high responsivity NATURE PHOTONICS Gan, X., Shiue, R., Gao, Y., Meric, I., Heinz, T. F., Shepard, K., Hone, J., Assefa, S., Englund, D. 2013; 7 (11): 883-887
  • Controlling the spontaneous emission rate of monolayer MoS2 in a photonic crystal nanocavity APPLIED PHYSICS LETTERS Gan, X., Gao, Y., Mak, K. F., Yao, X., Shiue, R., van der Zande, A., Trusheim, M. E., Hatami, F., Heinz, T. F., Hone, J., Englund, D. 2013; 103 (18)

    View details for DOI 10.1063/1.4826679

    View details for Web of Science ID 000327816000018

  • in a photonic crystal nanocavity. Applied physics letters Gan, X., Gao, Y., Fai Mak, K., Yao, X., Shiue, R., van der Zande, A., Trusheim, M. E., Hatami, F., Heinz, T. F., Hone, J., Englund, D. 2013; 103 (18): 181119-?


    We report on controlling the spontaneous emission (SE) rate of a molybdenum disulfide (MoS2) monolayer coupled with a planar photonic crystal (PPC) nanocavity. Spatially resolved photoluminescence (PL) mapping shows strong variations of emission when the MoS2monolayer is on the PPC cavity, on the PPC lattice, on the air gap, and on the unpatterned gallium phosphide substrate. Polarization dependences of the cavity-coupled MoS2emission show a more than 5 times stronger extracted PL intensity than the un-coupled emission, which indicates an underlying cavity mode Purcell enhancement of the MoS2SE rate exceeding a factor of 70.

    View details for PubMedID 24273329

  • Raman study of 2,7-bis(biphenyl-4-yl-)2 ',7 '-ditertbutyl-9,9 '-spirobifluorene adsorbed on oxide surfaces CHEMICAL PHYSICS LETTERS Staehler, J., Hofmann, O. T., Rinke, P., Blumstengel, S., Henneberger, F., Li, Y., Heinz, T. F. 2013; 584: 74-78
  • Real-Time Observation of Interlayer Vibrations in Bilayer and Few-Layer Graphene NANO LETTERS Boschetto, D., Malard, L., Lui, C. H., Mak, K. F., Li, Z., Yan, H., Heinz, T. F. 2013; 13 (10): 4620-4623


    We report real-time observation of the interlayer shearing mode, corresponding to the lateral oscillation of graphene planes, for bi- and few-layer graphene. Using a femtosecond pump-probe technique, we have followed coherent oscillations of this vibrational mode directly in the time domain. The shearing-mode frequency, as expected for an interlayer mode, exhibits a strong and systematic dependence on the number of layers, varying from 1.32 THz for the bulk limit to 0.85 THz for bilayer graphene. We explored the role of interactions with the external environment on this vibrational mode by comparing the response observed for graphene layers supported by different substrates and suspended in free space. No significant frequency shifts were observed.

    View details for DOI 10.1021/nl401713h

    View details for Web of Science ID 000326356300006

    View details for PubMedID 24047242

  • Intrinsic Line Shape of the Raman 2D-Mode in Freestanding Graphene Monolayers NANO LETTERS Berciaud, S., Li, X., Htoon, H., Brus, L. E., Doorn, S. K., Heinz, T. F. 2013; 13 (8): 3517-3523


    We report a comprehensive study of the two-phonon intervalley (2D) Raman mode in graphene monolayers, motivated by recent reports of asymmetric 2D-mode line shapes in freestanding graphene. For photon energies in the range 1.53-2.71 eV, the 2D-mode Raman response of freestanding samples appears as bimodal, in stark contrast with the Lorentzian approximation that is commonly used for supported monolayers. The transition between the freestanding and supported cases is mimicked by electrostatically doping freestanding graphene at carrier densities above 2 × 10(11) cm(-2). This result quantitatively demonstrates that low levels of charging can obscure the intrinsically bimodal 2D-mode line shape of monolayer graphene. In pristine freestanding graphene, we observe a broadening of the 2D-mode feature with decreasing photon energy that cannot be rationalized using a simple one-dimensional model based on resonant inner and outer processes. This indicates that phonon wavevectors away from the high-symmetry lines of the Brillouin zone must contribute to the 2D-mode, so that a full two-dimensional calculation is required to properly describe multiphonon-resonant Raman processes.

    View details for DOI 10.1021/nl400917e

    View details for Web of Science ID 000323241000011

    View details for PubMedID 23799800

  • Probing Symmetry Properties of Few-Layer MoS2 and h-BN by Optical Second-Harmonic Generation NANO LETTERS Li, Y., Rao, Y., Mak, K. F., You, Y., Wang, S., Dean, C. R., Heinz, T. F. 2013; 13 (7): 3329-3333


    We have measured optical second-harmonic generation (SHG) from atomically thin samples of MoS2 and h-BN with one to five layers. We observe strong SHG from materials with odd layer thickness, for which a noncentrosymmetric structure is expected, while the centrosymmetric materials with even layer thickness do not yield appreciable SHG. SHG for materials with odd layer thickness was measured as a function of crystal orientation. This dependence reveals the rotational symmetry of the lattice and is shown to provide a purely optical method of determining the orientation of crystallographic axes. We report values for the nonlinearity of monolayers and odd-layers of MoS2 and h-BN and compare the variation as a function of layer thickness with a model that accounts for wave propagation effects.

    View details for DOI 10.1021/nl401561r

    View details for Web of Science ID 000321884300053

    View details for PubMedID 23718906

  • Controlled argon beam-induced desulfurization of monolayer molybdenum disulfide JOURNAL OF PHYSICS-CONDENSED MATTER Ma, Q., Odenthal, P. M., Mann, J., Le, D., Wang, C. S., Zhu, Y., Chen, T., Sun, D., Yamaguchi, K., Tran, T., Wurch, M., McKinley, J. L., Wyrick, J., Magnone, K., Heinz, T. F., Rahman, T. S., Kawakami, R., Bartels, L. 2013; 25 (25)


    Sputtering of MoS2 films of single-layer thickness by low-energy argon ions selectively reduces the sulfur content of the material without significant depletion of molybdenum. X-ray photoelectron spectroscopy shows little modification of the Mo 3d states during this process, suggesting the absence of significant reorganization or damage to the overall structure of the MoS2 film. Accompanying ab initio molecular dynamics simulations find clusters of sulfur vacancies in the top plane of single-layer MoS2 to be structurally stable. Measurements of the photoluminescence at temperatures between 175 and 300 K show quenching of almost 80% for an ~10% decrease in sulfur content.

    View details for DOI 10.1088/0953-8984/25/25/252201

    View details for Web of Science ID 000320110200001

    View details for PubMedID 23708055

  • Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide NATURE MATERIALS van der Zande, A. M., Huang, P. Y., Chenet, D. A., Berkelbach, T. C., You, Y., Lee, G., Heinz, T. F., Reichman, D. R., Muller, D. A., Hone, J. C. 2013; 12 (6): 554-561


    Recent progress in large-area synthesis of monolayer molybdenum disulphide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chemical vapour deposition synthesis to grow highly crystalline islands of monolayer molybdenum disulphide up to 120 μm in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror twin boundaries that are stitched together by lines of 8- and 4-membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. We find that mirror twin boundaries cause strong photoluminescence quenching whereas tilt boundaries cause strong enhancement. Meanwhile, mirror twin boundaries slightly increase the measured in-plane electrical conductivity, whereas tilt boundaries slightly decrease the conductivity.

    View details for DOI 10.1038/nmat3633

    View details for Web of Science ID 000319402200026

    View details for PubMedID 23644523

  • Graphene as an atomically thin interface for growth of vertically aligned carbon nanotubes SCIENTIFIC REPORTS Rao, R., Chen, G., Arava, L. M., Kalaga, K., Ishigami, M., Heinz, T. F., Ajayan, P. M., Harutyunyan, A. R. 2013; 3


    Growth of vertically aligned carbon nanotube (CNT) forests is highly sensitive to the nature of the substrate. This constraint narrows the range of available materials to just a few oxide-based dielectrics and presents a major obstacle for applications. Using a suspended monolayer, we show here that graphene is an excellent conductive substrate for CNT forest growth. Furthermore, graphene is shown to intermediate growth on key substrates, such as Cu, Pt, and diamond, which had not previously been compatible with nanotube forest growth. We find that growth depends on the degree of crystallinity of graphene and is best on mono- or few-layer graphene. The synergistic effects of graphene are revealed by its endurance after CNT growth and low contact resistances between the nanotubes and Cu. Our results establish graphene as a unique interface that extends the class of substrate materials for CNT growth and opens up important new prospects for applications.

    View details for DOI 10.1038/srep01891

    View details for Web of Science ID 000319495700005

    View details for PubMedID 23712556

  • Facile growth of monolayer MoS2 film areas on SiO2 EUROPEAN PHYSICAL JOURNAL B Mann, J., Sun, D., Ma, Q., Chen, J., Preciado, E., Ohta, T., Diaconescu, B., Yamaguchi, K., Tai Tran, T., Wurch, M., Magnone, K., Heinz, T. F., Kellogg, G. L., Kawakami, R., Bartels, L. 2013; 86 (5)
  • Tunable Infrared Phonon Anomalies in Trilayer Graphene PHYSICAL REVIEW LETTERS Lui, C. H., Cappelluti, E., Li, Z., Heinz, T. F. 2013; 110 (18)


    Trilayer graphene in both ABA (Bernal) and ABC (rhombohedral) stacking sequences is shown to exhibit intense infrared absorption from in-plane optical phonons. The phonon feature, lying at ~1580 cm(-1), changes strongly with electrostatic gating. For ABC-stacked graphene trilayers, we observed a large enhancement in phonon absorption amplitude, as well as softening of the phonon mode, as the Fermi level is tuned away from charge neutrality. A similar, but substantially weaker, effect is seen in samples with the more common ABA stacking order. The strong infrared response of the optical phonons and the pronounced variation with electrostatic gating and stacking order reflect the interactions of the phonons and electronic excitations in the two systems. The key experimental findings can be reproduced within a simplified charged-phonon model that considers the influence of charging through Pauli blocking of the electronic transitions.

    View details for DOI 10.1103/PhysRevLett.110.185504

    View details for Web of Science ID 000319019300010

    View details for PubMedID 23683217

  • Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene ACS NANO Butler, S. Z., Hollen, S. M., Cao, L., Cui, Y., Gupta, J. A., Gutierrez, H. R., Heinz, T. F., Hong, S. S., Huang, J., Ismach, A. F., Johnston-Halperin, E., Kuno, M., Plashnitsa, V. V., Robinson, R. D., Ruoff, R. S., Salahuddin, S., Shan, J., Shi, L., Spencer, M. G., Terrones, M., Windl, W., Goldberger, J. E. 2013; 7 (4): 2898-2926


    Graphene's success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in field-effect transistors, spin- and valley-tronics, thermoelectrics, and topological insulators, among many other applications.

    View details for DOI 10.1021/nn400280c

    View details for Web of Science ID 000318143300005

    View details for PubMedID 23464873

  • Measurement of layer breathing mode vibrations in few-layer graphene PHYSICAL REVIEW B Lui, C. H., Heinz, T. F. 2013; 87 (12)
  • Tightly bound trions in monolayer MoS2 NATURE MATERIALS Mak, K. F., He, K., Lee, C., Lee, G. H., Hone, J., Heinz, T. F., Shan, J. 2013; 12 (3): 207-211


    Two-dimensional (2D) atomic crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable physical properties. In contrast to graphene, monolayer MoS(2) is a non-centrosymmetric material with a direct energy gap. Strong photoluminescence, a current on/off ratio exceeding 10(8) in field-effect transistors, and efficient valley and spin control by optical helicity have recently been demonstrated in this material. Here we report the spectroscopic identification in a monolayer MoS(2) field-effect transistor of tightly bound negative trions, a quasiparticle composed of two electrons and a hole. These quasiparticles, which can be optically created with valley and spin polarized holes, have no analogue in conventional semiconductors. They also possess a large binding energy (~ 20 meV), rendering them significant even at room temperature. Our results open up possibilities both for fundamental studies of many-body interactions and for optoelectronic and valleytronic applications in 2D atomic crystals.

    View details for DOI 10.1038/NMAT3505

    View details for Web of Science ID 000315707200017

    View details for PubMedID 23202371

  • High-Contrast Electrooptic Modulation of a Photonic Crystal Nanocavity by Electrical Gating of Graphene NANO LETTERS Gan, X., Shiue, R., Gao, Y., Mak, K. F., Yao, X., Li, L., Szep, A., Walker, D., Hone, J., Heinz, T. F., Englund, D. 2013; 13 (2): 691-696


    We demonstrate high-contrast electro-optic modulation of a photonic crystal nanocavity integrated with an electrically gated monolayer graphene. A silicon air-slot nanocavity provides strong overlap between the resonant optical field and graphene. Tuning the Fermi energy of the graphene layer to 0.85 eV enables strong control of its optical conductivity at telecom wavelengths, which allows modulation of cavity reflection in excess of 10 dB for a swing voltage of only 1.5 V. The cavity resonance at 1570 nm is found to undergo a shift in wavelength of nearly 2 nm, together with a 3-fold increase in quality factor. These observations enable a cavity-enhanced determination of graphene's complex optical sheet conductivity at different doping levels. Our simple device demonstrates the feasibility of high-contrast, low-power, and frequency-selective electro-optic modulators in graphene-integrated silicon photonic integrated circuits.

    View details for DOI 10.1021/nl304357u

    View details for Web of Science ID 000315079500061

    View details for PubMedID 23327445

  • Observation of a Transient Decrease in Terahertz Conductivity of Single-Layer Graphene Induced by Ultrafast Optical Excitation NANO LETTERS Jnawali, G., Rao, Y., Yan, H., Heinz, T. F. 2013; 13 (2): 524-530


    We have measured the terahertz frequency-dependent sheet conductivity and its transient response following femtosecond optical excitation for single-layer graphene samples grown by chemical vapor deposition. The conductivity of the unexcited graphene sheet, which was spontaneously doped, showed a strong free-carrier response. The THz conductivity matched a Drude model over the available THz spectral range and yielded an average carrier scattering time of 70 fs. Upon photoexcitation, we observed a transient decrease in graphene conductivity. The THz frequency-dependence of the graphene photoresponse differs from that of the unexcited material but remains compatible with a Drude form. We show that the negative photoconductive response arises from an increase in the carrier scattering rate, with a minor offsetting increase in the Drude weight. This behavior, which differs in sign from that reported previously for epitaxial graphene, is expected for samples with relatively high mobilities and doping levels. The photoinduced conductivity transient has a picosecond lifetime and is associated with nonequilibrium excitation conditions in the graphene.

    View details for DOI 10.1021/nl303988q

    View details for Web of Science ID 000315079500033

    View details for PubMedID 23330567

  • Observation of intra- and inter-band transitions in the transient optical response of graphene NEW JOURNAL OF PHYSICS Malard, L. M., Mak, K. F., Castro Neto, A. H., Peres, N. M., Heinz, T. F. 2013; 15
  • All-optical structure assignment of individual single-walled carbon nanotubes from Rayleigh and Raman scattering measurements PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS Berciaud, S., Deshpande, V. V., Caldwell, R., Miyauchi, Y., Voisin, C., Kim, P., Hone, J., Heinz, T. F. 2012; 249 (12): 2436-2441
  • Observation of Layer-Breathing Mode Vibrations in Few-Layer Graphene through Combination Raman Scattering NANO LETTERS Lui, C. H., Malard, L. M., Kim, S., Lantz, G., Laverge, F. E., Saito, R., Heinz, T. F. 2012; 12 (11): 5539-5544


    We report the observation of layer-breathing mode (LBM) vibrations in few-layer graphene (FLG) samples of thickness from two to six layers, exhibiting both Bernal (AB) and rhombohedral (ABC) stacking order. The LBM vibrations are identified using a Raman combination band lying around 1720 cm(-1). From double resonance theory, we assign the feature as the LO+ZO' combination mode of the out-of-plane LBM (ZO') and the in-plane longitudinal optical mode (LO). The LOZO' Raman band is found to exhibit multiple peaks with a unique line shape for each layer thickness and stacking order. These complex line shapes of the LOZO'-mode arise both from the material-dependent selection of different phonons in the double-resonance Raman process and from the detailed structure of the different branches of LBM in FLG.

    View details for DOI 10.1021/nl302450s

    View details for Web of Science ID 000311244400016

    View details for PubMedID 22963681

  • Strong Enhancement of Light-Matter Interaction in Graphene Coupled to a Photonic Crystal Nanocavity NANO LETTERS Gan, X., Mak, K. F., Gao, Y., You, Y., Hatami, F., Hone, J., Heinz, T. F., Englund, D. 2012; 12 (11): 5626-5631


    We demonstrate a large enhancement in the interaction of light with graphene through coupling with localized modes in a photonic crystal nanocavity. Spectroscopic studies show that a single atomic layer of graphene reduces the cavity reflection by more than a factor of one hundred, while also sharply reducing the cavity quality factor. The strong interaction allows for cavity-enhanced Raman spectroscopy on subwavelength regions of a graphene sample. A coupled-mode theory model matches experimental observations and indicates significantly increased light absorption in the graphene layer. The coupled graphene-cavity system also enables precise measurements of graphene's complex refractive index.

    View details for DOI 10.1021/nl302746n

    View details for Web of Science ID 000311244400031

    View details for PubMedID 23043452

  • Optical spectroscopy of graphene: From the far infrared to the ultraviolet SOLID STATE COMMUNICATIONS Mak, K. F., Ju, L., Wang, F., Heinz, T. F. 2012; 152 (15): 1341-1349
  • Control of valley polarization in monolayer MoS2 by optical helicity NATURE NANOTECHNOLOGY Mak, K. F., He, K., Shan, J., Heinz, T. F. 2012; 7 (8): 494-498


    Electronic and spintronic devices rely on the fact that free charge carriers in solids carry electric charge and spin. There are, however, other properties of charge carriers that might be exploited in new families of devices. In particular, if there are two or more minima in the conduction band (or maxima in the valence band) in momentum space, and if it is possible to confine charge carriers in one of these valleys, then it should be possible to make a valleytronic device. Valley polarization, as the selective population of one valley is designated, has been demonstrated using strain and magnetic fields, but neither of these approaches allows dynamic control. Here, we demonstrate that optical pumping with circularly polarized light can achieve complete dynamic valley polarization in monolayer MoS(2) (refs 11, 12), a two-dimensional non-centrosymmetric crystal with direct energy gaps at two valleys. Moreover, this polarization is retained for longer than 1 ns. Our results, and similar results by Zeng et al., demonstrate the viability of optical valley control and suggest the possibility of valley-based electronic and optoelectronic applications in MoS(2) monolayers.

    View details for DOI 10.1038/NNANO.2012.96

    View details for Web of Science ID 000307359600007

    View details for PubMedID 22706698

  • Ultrafast Supercontinuum Spectroscopy of Carrier Multiplication and Biexcitonic Effects in Excited States of PbS Quantum Dots NANO LETTERS Gesuele, F., Sfeir, M. Y., Koh, W., Murray, C. B., Heinz, T. F., Wong, C. W. 2012; 12 (6): 2658-2664


    We examine the population dynamics of multiple excitons in PbS quantum dots using spectrally resolved ultrafast supercontinuum transient absorption (SC-TA) measurements. We simultaneously probe the first three excitonic transitions. The transient spectra show the presence of bleaching of absorption for the 1S(h)-1S(e) transition, as well as transients associated with the 1P(h)-1P(e) transition. We examine signatures of carrier multiplication (multiple excitons arising from a single absorbed photon) from analysis of the bleaching features in the limit of low absorbed photon numbers (left angle bracket N(abs) right angle bracket ∼ 10(-2)) for pump photon energies from two to four times that of the band gap. The efficiency of multiple-exciton generation is discussed both in terms of the ratio between early- to long-time transient absorption signals and of a broadband global fit to the data. Analysis of the population dynamics shows that bleaching associated with biexciton population is red shifted with respect to the single exciton feature, which is in accordance with a positive binding energy for the biexciton.

    View details for DOI 10.1021/nl2021224

    View details for Web of Science ID 000305106400002

    View details for PubMedID 22149990

  • Excitonic signatures in the optical response of single-wall carbon nanotubes PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS Voisin, C., Berger, S., Berciaud, S., Yan, H., Lauret, J., Cassabois, G., Roussignol, P., Hone, J., Heinz, T. F. 2012; 249 (5): 900-906
  • Structure-Dependent Fano Resonances in the Infrared Spectra of Phonons in Few-Layer Graphene PHYSICAL REVIEW LETTERS Li, Z., Lui, C. H., Cappelluti, E., Benfatto, L., Mak, K. F., Carr, G. L., Shan, J., Heinz, T. F. 2012; 108 (15)


    The in-plane optical phonons around 200 meV in few-layer graphene are investigated utilizing infrared absorption spectroscopy. The phonon spectra exhibit unusual asymmetric features characteristic of Fano resonances, which depend critically on the layer thickness and stacking order of the sample. The phonon intensities in samples with rhombohedral (ABC) stacking are significantly higher than those with Bernal (AB) stacking. These observations reflect the strong coupling between phonons and interband electronic transitions in these systems and the distinctive variation in the joint density of electronic states in samples of differing thickness and stacking order.

    View details for DOI 10.1103/PhysRevLett.108.156801

    View details for Web of Science ID 000302638600008

    View details for PubMedID 22587273

  • Water-Gated Charge Doping of Graphene Induced by Mica Substrates NANO LETTERS Shim, J., Lui, C. H., Ko, T. Y., Yu, Y., Kim, P., Heinz, T. F., Ryu, S. 2012; 12 (2): 648-654


    We report on the existence of water-gated charge doping of graphene deposited on atomically flat mica substrates. Molecular films of water in units of ~0.4 nm thick bilayers were found to be present in regions of the interface of graphene/mica heterostacks prepared by micromechanical exfoliation of kish graphite. The spectral variation of the G and 2D bands, as visualized by Raman mapping, shows that mica substrates induce strong p-type doping in graphene with hole densities of (9 ± 2) × 10(12) cm(-2). The ultrathin water films, however, effectively block interfacial charge transfer, rendering graphene significantly less hole-doped. Scanning Kelvin probe microscopy independently confirmed a water-gated modulation of the Fermi level by 0.35 eV, which is in agreement with the optically determined hole density. The manipulation of the electronic properties of graphene demonstrated in this study should serve as a useful tool in realizing future graphene applications.

    View details for DOI 10.1021/nl2034317

    View details for Web of Science ID 000299967800020

    View details for PubMedID 22260483

  • Observation of an electrically tunable band gap in trilayer graphene NATURE PHYSICS Lui, C. H., Li, Z., Mak, K. F., Cappelluti, E., Heinz, T. F. 2011; 7 (12): 944-947

    View details for DOI 10.1038/NPHYS2102

    View details for Web of Science ID 000298186100016

  • High-resolution spatial mapping of the temperature distribution of a Joule self-heated graphene nanoribbon APPLIED PHYSICS LETTERS Yu, Y., Han, M. Y., Berciaud, S., Georgescu, A. B., Heinz, T. F., Brus, L. E., Kim, K. S., Kim, P. 2011; 99 (18)

    View details for DOI 10.1063/1.3657515

    View details for Web of Science ID 000296659400078

  • Observation of Electronic Raman Scattering in Metallic Carbon Nanotubes PHYSICAL REVIEW LETTERS Farhat, H., Berciaud, S., Kalbac, M., Saito, R., Heinz, T. F., Dresselhaus, M. S., Kong, J. 2011; 107 (15)


    We present experimental measurements of the electronic contribution to the Raman spectra of individual metallic single-walled carbon nanotubes (MSWNTs). Photoexcited carriers are inelastically scattered by a continuum of low-energy electron-hole pairs created across the graphenelike linear electronic subbands of the MSWNTs. The optical resonances in MSWNTs give rise to well-defined electronic Raman peaks. This resonant electronic Raman scattering is a unique feature of the electronic structure of these one-dimensional quasimetals.

    View details for DOI 10.1103/PhysRevLett.107.157401

    View details for Web of Science ID 000296286300001

    View details for PubMedID 22107317

  • Low Bias Electron Scattering in Structure-Identified Single Wall Carbon Nanotubes: Role of Substrate Polar Phonons PHYSICAL REVIEW LETTERS Chandra, B., Perebeinos, V., Berciaud, S., Katoch, J., Ishigami, M., Kim, P., Heinz, T. F., Hone, J. 2011; 107 (14)


    We have performed temperature-dependent electrical transport measurements on known structure single wall carbon nanotubes at low bias. The experiments show a superlinear increase in nanotube resistivity with temperature, which is in contradiction with the linear dependence expected from nanotube acoustic-phonon scattering. The measured electron mean free path is also much lower than expected, especially at medium to high temperatures (>100  K). A theoretical model that includes scattering due to surface polar phonon modes of the substrates reproduces the experiments very well. The role of surface phonons is further confirmed by resistivity measurements of nanotubes on aluminum nitride.

    View details for DOI 10.1103/PhysRevLett.107.146601

    View details for Web of Science ID 000295328200009

    View details for PubMedID 22107221

  • Visualizing Individual Nitrogen Dopants in Monolayer Graphene SCIENCE Zhao, L., He, R., Rim, K. T., Schiros, T., Kim, K. S., Zhou, H., Gutierrez, C., Chockalingam, S. P., Arguello, C. J., Palova, L., Nordlund, D., Hybertsen, M. S., Reichman, D. R., Heinz, T. F., Kim, P., Pinczuk, A., Flynn, G. W., Pasupathy, A. N. 2011; 333 (6045): 999-1003


    In monolayer graphene, substitutional doping during growth can be used to alter its electronic properties. We used scanning tunneling microscopy, Raman spectroscopy, x-ray spectroscopy, and first principles calculations to characterize individual nitrogen dopants in monolayer graphene grown on a copper substrate. Individual nitrogen atoms were incorporated as graphitic dopants, and a fraction of the extra electron on each nitrogen atom was delocalized into the graphene lattice. The electronic structure of nitrogen-doped graphene was strongly modified only within a few lattice spacings of the site of the nitrogen dopant. These findings show that chemical doping is a promising route to achieving high-quality graphene films with a large carrier concentration.

    View details for DOI 10.1126/science.1208759

    View details for Web of Science ID 000294000400051

    View details for PubMedID 21852495

  • Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy Studies of Graphene Films Prepared by Sonication-Assisted Dispersion ACS NANO Polyakova (Stolyarova), E. Y., Rim, K. T., Eom, D., Douglass, K., Opila, R. L., Heinz, T. F., Teplyakov, A. V., Flynn, G. W. 2011; 5 (8): 6102-6108


    We describe scanning tunneling microscopy and X-ray photoelectron spectroscopy studies of graphene films produced by sonication-assisted dispersion. Defects in these samples are not randomly distributed, and the graphene films exhibit a "patchwork" structure where unperturbed graphene areas are adjacent to heavily functionalized ones. Adjacent graphene layers are likely in poor mechanical contact due to adventitious species trapped between the carbon sheets of the sample.

    View details for DOI 10.1021/nn1009352

    View details for Web of Science ID 000294085400006

    View details for PubMedID 21726071

  • Raman spectra of out-of-plane phonons in bilayer graphene PHYSICAL REVIEW B Sato, K., Park, J. S., Saito, R., Cong, C., Yu, T., Lui, C. H., Heinz, T. F., Dresselhaus, G., Dresselhaus, M. S. 2011; 84 (3)
  • Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy REVIEWS OF MODERN PHYSICS Ulbricht, R., Hendry, E., Shan, J., Heinz, T. F., Bonn, M. 2011; 83 (2): 543-586
  • Temperature dependence of the anharmonic decay of optical phonons in carbon nanotubes and graphite PHYSICAL REVIEW B Chatzakis, I., Yan, H., Song, D., Berciaud, S., Heinz, T. F. 2011; 83 (20)
  • Influence of copper crystal surface on the CVD growth of large area monolayer graphene SOLID STATE COMMUNICATIONS Zhao, L., Rim, K. T., Zhou, H., He, R., Heinz, T. F., Pinczuk, A., Flynn, G. W., Pasupathy, A. N. 2011; 151 (7): 509-513
  • Seeing Many-Body Effects in Single- and Few-Layer Graphene: Observation of Two-Dimensional Saddle-Point Excitons PHYSICAL REVIEW LETTERS Mak, K. F., Shan, J., Heinz, T. F. 2011; 106 (4)


    Significant excitonic effects were observed in graphene by measuring its optical conductivity in a broad spectral range including the two-dimensional π-band saddle-point singularities in the electronic structure. The strong electron-hole interactions manifest themselves in an asymmetric resonance peaked at 4.62 eV, which is redshifted by nearly 600 meV from the value predicted by ab initio GW calculations for the band-to-band transitions. The observed excitonic resonance is explained within a phenomenological model as a Fano interference of a strongly coupled excitonic state and a band continuum. Our experiment also showed a weak dependence of the excitonic resonance in few-layer graphene on layer thickness. This result reflects the effective cancellation of the increasingly screened repulsive electron-electron (e-e) and attractive electron-hole (e-h) interactions.

    View details for DOI 10.1103/PhysRevLett.106.046401

    View details for Web of Science ID 000286736000012

    View details for PubMedID 21405342

  • Imaging Stacking Order in Few-Layer Graphene NANO LETTERS Lui, C. H., Li, Z., Chen, Z., Klimov, P. V., Brus, L. E., Heinz, T. F. 2011; 11 (1): 164-169


    Few-layer graphene (FLG) has been predicted to exist in various crystallographic stacking sequences, which can strongly influence the material's electronic properties. We demonstrate an accurate and efficient method to characterize stacking order in FLG using the distinctive features of the Raman 2D-mode. Raman imaging allows us to visualize directly the spatial distribution of Bernal (ABA) and rhombohedral (ABC) stacking in tri- and tetralayer graphene. We find that 15% of exfoliated graphene tri- and tetralayers is composed of micrometer-sized domains of rhombohedral stacking, rather than of usual Bernal stacking. These domains are stable and remain unchanged for temperatures exceeding 800 °C.

    View details for DOI 10.1021/nl1032827

    View details for Web of Science ID 000286029400028

    View details for PubMedID 21121668

  • Measurement of the thermal conductance of the graphene/SiO2 interface APPLIED PHYSICS LETTERS Mak, K. F., Lui, C. H., Heinz, T. F. 2010; 97 (22)

    View details for DOI 10.1063/1.3511537

    View details for Web of Science ID 000284965000020

  • Infrared spectra of individual semiconducting single-walled carbon nanotubes: Testing the scaling of transition energies for large diameter nanotubes PHYSICAL REVIEW B Sfeir, M. Y., Misewich, J. A., Rosenblatt, S., Wu, Y., Voisin, C., Yan, H., Berciaud, S., Heinz, T. F., Chandra, B., Caldwell, R., Shan, Y., Hone, J., Carr, G. L. 2010; 82 (19)
  • Probing Strain-Induced Electronic Structure Change in Graphene by Raman Spectroscopy NANO LETTERS Huang, M., Yan, H., Heinz, T. F., Hone, J. 2010; 10 (10): 4074-4079


    Two-phonon Raman scattering in graphitic materials provides a distinctive approach to probing the material's electronic structure through the spectroscopy of phonons. Here we report studies of Raman scattering of the two-dimensional mode of single-layer graphene under uniaxial stress and which implicates two types of modification of the low-energy electronic structure of graphene: a deformation of the Dirac cone and its displacement away from the K point.

    View details for DOI 10.1021/nl102123c

    View details for Web of Science ID 000282727600046

    View details for PubMedID 20735024

  • Atomically Thin MoS2: A New Direct-Gap Semiconductor PHYSICAL REVIEW LETTERS Mak, K. F., Lee, C., Hone, J., Shan, J., Heinz, T. F. 2010; 105 (13)


    The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N=1,2,…,6 S-Mo-S monolayers have been investigated by optical spectroscopy. Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure. With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 0.6 eV. This leads to a crossover to a direct-gap material in the limit of the single monolayer. Unlike the bulk material, the MoS₂ monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 10⁴ compared with the bulk material.

    View details for DOI 10.1103/PhysRevLett.105.136805

    View details for Web of Science ID 000282136900010

    View details for PubMedID 21230799

  • Ultrafast Photoluminescence from Graphene PHYSICAL REVIEW LETTERS Lui, C. H., Mak, K. F., Shan, J., Heinz, T. F. 2010; 105 (12)


    Since graphene has no band gap, photoluminescence is not expected from relaxed charge carriers. We have, however, observed significant light emission from graphene under excitation by ultrashort (30-fs) laser pulses. Light emission was found to occur across the visible spectral range (1.7-3.5 eV), with emitted photon energies exceeding that of the excitation laser (1.5 eV). The emission exhibits a nonlinear dependence on the laser fluence. In two-pulse correlation measurements, a dominant relaxation time of tens of femtoseconds is observed. A two-temperature model describing the electrons and their interaction with strongly coupled optical phonons can account for the experimental observations.

    View details for DOI 10.1103/PhysRevLett.105.127404

    View details for Web of Science ID 000282137400009

    View details for PubMedID 20867672

  • The evolution of electronic structure in few-layer graphene revealed by optical spectroscopy PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Mak, K. F., Sfeir, M. Y., Misewich, J. A., Heinz, T. F. 2010; 107 (34): 14999-15004


    The massless Dirac spectrum of electrons in single-layer graphene has been thoroughly studied both theoretically and experimentally. Although a subject of considerable theoretical interest, experimental investigations of the richer electronic structure of few-layer graphene (FLG) have been limited. Here we examine FLG graphene crystals with Bernal stacking of layer thicknesses N = 1,2,3,...8 prepared using the mechanical exfoliation technique. For each layer thickness N, infrared conductivity measurements over the spectral range of 0.2-1.0 eV have been performed and reveal a distinctive band structure, with different conductivity peaks present below 0.5 eV and a relatively flat spectrum at higher photon energies. The principal transitions exhibit a systematic energy-scaling behavior with N. These observations are explained within a unified zone-folding scheme that generates the electronic states for all FLG materials from that of the bulk 3D graphite crystal through imposition of appropriate boundary conditions. Using the Kubo formula, we find that the complete infrared conductivity spectra for the different FLG crystals can be reproduced reasonably well within the framework a tight-binding model.

    View details for DOI 10.1073/pnas.1004595107

    View details for Web of Science ID 000281311500014

    View details for PubMedID 20696939

  • Electron and Optical Phonon Temperatures in Electrically Biased Graphene PHYSICAL REVIEW LETTERS Berciaud, S., Han, M. Y., Mak, K. F., Brus, L. E., Kim, P., Heinz, T. F. 2010; 104 (22)


    We examine the intrinsic energy dissipation steps in electrically biased graphene channels. By combining in-situ measurements of the spontaneous optical emission with a Raman spectroscopy study of the graphene sample under conditions of current flow, we obtain independent information on the energy distribution of the electrons and phonons. The electrons and holes contributing to light emission are found to obey a thermal distribution, with temperatures in excess of 1500 K in the regime of current saturation. The zone-center optical phonons are also highly excited and are found to be in equilibrium with the electrons. For a given optical phonon temperature, the anharmonic downshift of the Raman G mode is smaller than expected under equilibrium conditions, suggesting that the electrons and high-energy optical phonons are not fully equilibrated with all of the phonon modes.

    View details for DOI 10.1103/PhysRevLett.104.227401

    View details for Web of Science ID 000278477400007

    View details for PubMedID 20867202

  • Energy Transfer from Individual Semiconductor Nanocrystals to Graphene ACS NANO Chen, Z., Berciaud, S., Nuckolls, C., Heinz, T. F., Brus, L. E. 2010; 4 (5): 2964-2968


    Energy transfer from photoexcited zero-dimensional systems to metallic systems plays a prominent role in modern day materials science. A situation of particular interest concerns the interaction between a photoexcited dipole and an atomically thin metal. The recent discovery of graphene layers permits investigation of this phenomenon. Here we report a study of fluorescence from individual CdSe/ZnS nanocrystals in contact with single- and few-layer graphene sheets. The rate of energy transfer is determined from the strong quenching of the nanocrystal fluorescence. For single-layer graphene, we find a rate of approximately 4 ns(-1), in agreement with a model based on the dipole approximation and a tight-binding description of graphene. This rate increases significantly with the number of graphene layers, before approaching the bulk limit. Our study quantifies energy transfer to and fluorescence quenching by graphene, critical properties for novel applications in photovoltaic devices and as a molecular ruler.

    View details for DOI 10.1021/nn1005107

    View details for Web of Science ID 000277976900058

    View details for PubMedID 20402475

  • Anomalous Lattice Vibrations of Single- and Few-Layer MoS2 ACS NANO Lee, C., Yan, H., Brus, L. E., Heinz, T. F., Hone, J., Ryu, S. 2010; 4 (5): 2695-2700


    Molybdenum disulfide (MoS(2)) of single- and few-layer thickness was exfoliated on SiO(2)/Si substrate and characterized by Raman spectroscopy. The number of S-Mo-S layers of the samples was independently determined by contact-mode atomic force microscopy. Two Raman modes, E(1)(2g) and A(1g), exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime.

    View details for DOI 10.1021/nn1003937

    View details for Web of Science ID 000277976900028

    View details for PubMedID 20392077

  • Electronic Structure of Few-Layer Graphene: Experimental Demonstration of Strong Dependence on Stacking Sequence PHYSICAL REVIEW LETTERS Mak, K. F., Shan, J., Heinz, T. F. 2010; 104 (17)


    The electronic structure of few-layer graphene (FLG) samples with crystalline order was investigated experimentally by infrared absorption spectroscopy for photon energies ranging from 0.2-1 eV. Distinct optical conductivity spectra were observed for different samples having precisely the same number of layers. The different spectra arise from the existence of two stable polytypes of FLG, namely, Bernal (AB) stacking and rhombohedral (ABC) stacking. The observed absorption features, reflecting the underlying symmetry of the two polytypes and the nature of the associated van Hone singularities, were reproduced by explicit calculations within a tight-binding model. The findings demonstrate the pronounced effect of stacking order on the electronic structure of FLG.

    View details for DOI 10.1103/PhysRevLett.104.176404

    View details for Web of Science ID 000277210600030

    View details for PubMedID 20482122

  • Excitons and high-order optical transitions in individual carbon nanotubes: A Rayleigh scattering spectroscopy study PHYSICAL REVIEW B Berciaud, S., Voisin, C., Yan, H., Chandra, B., Caldwell, R., Shan, Y., Brus, L. E., Hone, J., Heinz, T. F. 2010; 81 (4)
  • Performance of monolayer graphene nanomechanical resonators with electrical readout NATURE NANOTECHNOLOGY Chen, C., Rosenblatt, S., Bolotin, K. I., Kalb, W., Kim, P., Kymissis, I., Stormer, H. L., Heinz, T. F., Hone, J. 2009; 4 (12): 861-867


    The enormous stiffness and low density of graphene make it an ideal material for nanoelectromechanical applications. Here, we demonstrate the fabrication and electrical readout of monolayer graphene resonators, and test their response to changes in mass and temperature. The devices show resonances in the megahertz range, and the strong dependence of resonant frequency on applied gate voltage can be fitted to a membrane model to yield the mass density and built-in strain of the graphene. Following the removal and addition of mass, changes in both density and strain are observed, indicating that adsorbates impart tension to the graphene. On cooling, the frequency increases, and the shift rate can be used to measure the unusual negative thermal expansion coefficient of graphene. The quality factor increases with decreasing temperature, reaching approximately 1 x 10(4) at 5 K. By establishing many of the basic attributes of monolayer graphene resonators, the groundwork for applications of these devices, including high-sensitivity mass detectors, is put in place.

    View details for DOI 10.1038/NNANO.2009.267

    View details for Web of Science ID 000272415600021

    View details for PubMedID 19893525

  • Ultraflat graphene NATURE Lui, C. H., Liu, L., Mak, K. F., Flynn, G. W., Heinz, T. F. 2009; 462 (7271): 339-341


    Graphene, a single atomic layer of carbon connected by sp(2) hybridized bonds, has attracted intense scientific interest since its recent discovery. Much of the research on graphene has been directed towards exploration of its novel electronic properties, but the structural aspects of this model two-dimensional system are also of great interest and importance. In particular, microscopic corrugations have been observed on all suspended and supported graphene sheets studied so far. This rippling has been invoked to explain the thermodynamic stability of free-standing graphene sheets. Many distinctive electronic and chemical properties of graphene have been attributed to the presence of ripples, which are also predicted to give rise to new physical phenomena that would be absent in a planar two-dimensional material. Direct experimental study of such novel ripple physics has, however, been hindered by the lack of flat graphene layers. Here we demonstrate the fabrication of graphene monolayers that are flat down to the atomic level. These samples are produced by deposition on the atomically flat terraces of cleaved mica surfaces. The apparent height variation in the graphene layers observed by high-resolution atomic force microscopy (AFM) is less than 25 picometres, indicating the suppression of any existing intrinsic ripples in graphene. The availability of such ultraflat samples will permit rigorous testing of the impact of ripples on various physical and chemical properties of graphene.

    View details for DOI 10.1038/nature08569

    View details for Web of Science ID 000271899300041

    View details for PubMedID 19924211

  • Time-resolved Raman spectroscopy of optical phonons in graphite: Phonon anharmonic coupling and anomalous stiffening PHYSICAL REVIEW B Yan, H., Song, D., Mak, K. F., Chatzakis, I., Maultzsch, J., Heinz, T. F. 2009; 80 (12)
  • Measurement of the optical Stark effect in semiconducting carbon nanotubes APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING Song, D., Wang, F., Dukovic, G., Zheng, M., Semke, E. D., Brus, L. E., Heinz, T. F. 2009; 96 (2): 283-287
  • Structure and Electronic Properties of Graphene Nanoislands on Co(0001) NANO LETTERS Eom, D., Prezzi, D., Rim, K. T., Zhou, H., Lefenfeld, M., Xiao, S., Nuckolls, C., Hybertsen, M. S., Heinz, T. F., Flynn, G. W. 2009; 9 (8): 2844-2848


    We have grown well-ordered graphene adlayers on the lattice-matched Co(0001) surface. Low-temperature scanning tunneling microscopy measurements demonstrate an on-top registry of the carbon atoms with respect to the Co(0001) surface. The tunneling conductance spectrum shows that the electronic structure is substantially altered from that of isolated graphene, implying a strong coupling between graphene and cobalt states. Calculations using density functional theory confirm that structures with on-top registry have the lowest energy and provide clear evidence for strong electronic coupling between the graphene pi-states and Co d-states at the interface.

    View details for DOI 10.1021/nl900927f

    View details for Web of Science ID 000268797200008

    View details for PubMedID 19630380

  • The evolution of optical and electrical properties of low-k dielectrics under bias stress MICROELECTRONIC ENGINEERING Atkin, J. M., CARTIER, E., Shaw, T. M., Lloyd, J. R., Laibowitz, R. B., Heinz, T. F. 2009; 86 (7-9): 1891-1893
  • Observation of an Electric-Field-Induced Band Gap in Bilayer Graphene by Infrared Spectroscopy PHYSICAL REVIEW LETTERS Mak, K. F., Lui, C. H., Shan, J., Heinz, T. F. 2009; 102 (25)


    It has been predicted that application of a strong electric field perpendicular to the plane of bilayer graphene can induce a significant band gap. We have measured the optical conductivity of bilayer graphene with an efficient electrolyte top gate for a photon energy range of 0.2-0.7 eV. We see the emergence of new transitions as a band gap opens. A band gap approaching 200 meV is observed when an electric field approximately 1 V/nm is applied, inducing a carrier density of about 10(13) cm(-2)}. The magnitude of the band gap and the features observed in the infrared conductivity spectra are broadly compatible with calculations within a tight-binding model.

    View details for DOI 10.1103/PhysRevLett.102.256405

    View details for Web of Science ID 000267432000049

    View details for PubMedID 19659105

  • Phonon softening and crystallographic orientation of strained graphene studied by Raman spectroscopy PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Huang, M., Yan, H., Chen, C., Song, D., Heinz, T. F., Hone, J. 2009; 106 (18): 7304-7308


    We present a systematic study of the Raman spectra of optical phonons in graphene monolayers under tunable uniaxial tensile stress. Both the G and 2D bands exhibit significant red shifts. The G band splits into 2 distinct subbands (G(+), G(-)) because of the strain-induced symmetry breaking. Raman scattering from the G(+) and G(-) bands shows a distinctive polarization dependence that reflects the angle between the axis of the stress and the underlying graphene crystal axes. Polarized Raman spectroscopy therefore constitutes a purely optical method for the determination of the crystallographic orientation of graphene.

    View details for DOI 10.1073/pnas.0811754106

    View details for Web of Science ID 000265783600009

    View details for PubMedID 19380746

  • Circularly polarized light in the single-cycle limit: the nature of highly polychromatic radiation of defined polarization OPTICS EXPRESS Shan, J., Dadap, J. I., Heinz, T. F. 2009; 17 (9): 7431-7439


    We have developed a general analytic description of polarized light pulses and explored the properties of circularly polarized single-cycle pulses. The temporal evolution of the electric-field vector of such spectrally broad pulses, which may be described in terms of a Hilbert transform relationship, differs significantly from the well-known behavior of quasi-monochromatic radiation. Single-cycle circularly polarized pulses are produced and characterized experimentally in the terahertz spectral region.

    View details for DOI 10.1364/OE.17.007431

    View details for Web of Science ID 000266381700058

    View details for PubMedID 19399121

  • Molecular-Scale Quantum Dots from Carbon Nanotube Heterojunctions NANO LETTERS Chandra, B., Bhattacharjee, J., Purewal, M., Son, Y., Wu, Y., Huang, M., Yan, H., Heinz, T. F., Kim, P., Neaton, J. B., Hone, J. 2009; 9 (4): 1544-1548


    Carbon nanotube heterojunctions (HJs), which seamlessly connect nanotubes of different chiral structure using a small number of atomic-scale defects, represent the ultimate scaling of electronic interfaces. Here we report the first electrical transport measurements on a HJ formed between semiconducting and metallic nanotubes of known chiralities. These measurements reveal asymmetric IV-characteristics and the presence of a quantum dot (QD) with approximately 60 meV charging energy and approximately 75 meV level spacing. A detailed atomistic and electronic model of the HJ enables the identification of specific defect arrangements that lead to the QD behavior consistent with the experiment.

    View details for DOI 10.1021/nl803639h

    View details for Web of Science ID 000265030000049

    View details for PubMedID 19278212

  • Longitudinal Optical Phonons in Metallic and Semiconducting Carbon Nanotubes PHYSICAL REVIEW LETTERS Fouquet, M., Telg, H., Maultzsch, J., Wu, Y., Chandra, B., Hone, J., Heinz, T. F., Thomsen, C. 2009; 102 (7)


    We analyze the high-energy Raman modes, G+ and G-, in a pair of one metallic and one semiconducting nanotubes. By combining Rayleigh scattering with Raman resonance profiles of the radial breathing mode and the high-energy modes, we show that the observed G- and G+ peaks can originate from longitudinal optical phonons of different tubes. The G- peak is the longitudinal mode of the metallic tube; it is broadened and downshifted due to strong electron-phonon coupling in the metallic nanotube. The G+ peak is due to the longitudinal mode in the semiconducting tube. This result resolves an ongoing debate in the literature.

    View details for DOI 10.1103/PhysRevLett.102.075501

    View details for Web of Science ID 000263599500037

    View details for PubMedID 19257684

  • Effects of photoinduced carrier injection on time-dependent dielectric breakdown 2009 IEEE INTERNATIONAL RELIABILITY PHYSICS SYMPOSIUM, VOLS 1 AND 2 Atkin, J. M., Laibowitz, R. B., Heinz, T. F., Lloyd, J. R., Shaw, T. M., CARTIER, E. 2009: 851-?
  • Probing the Intrinsic Properties of Exfoliated Graphene: Raman Spectroscopy of Free-Standing Monolayers NANO LETTERS Berciaud, S., Ryu, S., Brus, L. E., Heinz, T. F. 2009; 9 (1): 346-352


    The properties of pristine, free-standing graphene monolayers prepared by mechanical exfoliation of graphite are investigated. The graphene monolayers, suspended over open trenches, are examined by means of spatially resolved Raman spectroscopy of the G-, D-, and 2D-phonon modes. The G-mode phonons exhibit reduced energies (1580 cm(-1)) and increased widths (14 cm(-1)) compared to the response of graphene monolayers supported on the SiO(2)-covered substrate. From analysis of the G-mode Raman spectra, we deduce that the free-standing graphene monolayers are essentially undoped, with an upper bound of 2 x 10(11) cm(-2) for the residual carrier concentration. On the supported regions, significantly higher and spatially inhomogeneous doping is observed. The free-standing graphene monolayers show little local disorder, based on the very weak Raman D-mode response. The two-phonon 2D mode of the free-standing graphene monolayers is downshifted in frequency compared to that of the supported region of the samples and exhibits a narrowed, positively skewed line shape.

    View details for DOI 10.1021/nl8031444

    View details for Web of Science ID 000262519100065

    View details for PubMedID 19099462

  • Determination of the Young's Modulus of Structurally Defined Carbon Nanotubes NANO LETTERS Wu, Y., Huang, M., Wang, F., Huang, X. M., Rosenblatt, S., Huang, L., Yan, H., O'Brien, S. P., Hone, J., Heinz, T. F. 2008; 8 (12): 4158-4161


    We have combined optical characterization with a magnetic actuation technique to measure the stiffness of single-walled carbon nanotubes of defined crystal structure. The measured stiffnesses correspond to an average Young's modulus of E = 0.97 +/- 0.16 TPa. For the structures investigated, no dependence on the nanotube chiral index was observed within the indicated experimental accuracy.

    View details for DOI 10.1021/nl801563q

    View details for Web of Science ID 000261630700012

    View details for PubMedID 19367839

  • Reversible Basal Plane Hydrogenation of Graphene NANO LETTERS Ryu, S., Han, M. Y., Maultzsch, J., Heinz, T. F., Kim, P., Steigerwald, M. L., Brus, L. E. 2008; 8 (12): 4597-4602


    We report the chemical reaction of single-layer graphene with hydrogen atoms, generated in situ by electron-induced dissociation of hydrogen silsesquioxane (HSQ). Hydrogenation, forming sp3 C--H functionality on the basal plane of graphene, proceeds at a higher rate for single than for double layers, demonstrating the enhanced chemical reactivity of single sheet graphene. The net H atom sticking probability on single layers at 300 K is at least 0.03, which exceeds that of double layers by at least a factor of 15. Chemisorbed hydrogen atoms, which give rise to a prominent Raman D band, can be detached by thermal annealing at 100-200 degrees C. The resulting dehydrogenated graphene is "activated" when photothermally heated it reversibly binds ambient oxygen, leading to hole doping of the graphene. This functionalization of graphene can be exploited to manipulate electronic and charge transport properties of graphene devices.

    View details for DOI 10.1021/nl802940s

    View details for Web of Science ID 000261630700088

    View details for PubMedID 19053793

  • Measurement of the Optical Conductivity of Graphene PHYSICAL REVIEW LETTERS Mak, K. F., Sfeir, M. Y., Wu, Y., Lui, C. H., Misewich, J. A., Heinz, T. F. 2008; 101 (19)


    Optical reflectivity and transmission measurements over photon energies between 0.2 and 1.2 eV were performed on single-crystal graphene samples on a SiO2 substrate. For photon energies above 0.5 eV, graphene yielded a spectrally flat optical absorbance of (2.3+/-0.2)%. This result is in agreement with a constant absorbance of pialpha, or a sheet conductivity of pie2/2h, predicted within a model of noninteracting massless Dirac fermions. This simple result breaks down at lower photon energies, where both spectral and sample-to-sample variations were observed. This "nonuniversal" behavior is explained by including the effects of doping and finite temperature, as well as contributions from intraband transitions.

    View details for DOI 10.1103/PhysRevLett.101.196405

    View details for Web of Science ID 000260776300048

    View details for PubMedID 19113291

  • G(-) and G(+) in the Raman spectrum of isolated nanotube: a study on resonance conditions and lineshape PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS Telg, H., Fouquet, M., Maultzsch, J., Wu, Y., Chandra, B., Hone, J., Heinz, T. F., Thomsen, C. 2008; 245 (10): 2189-2192
  • Charge trapping at the low-k dielectric-silicon interface probed by the conductance and capacitance techniques APPLIED PHYSICS LETTERS Atkin, J. M., CARTIER, E., Shaw, T. M., Laibowitz, R. B., Heinz, T. F. 2008; 93 (12)

    View details for DOI 10.1063/1.2990648

    View details for Web of Science ID 000259799100055

  • Direct observation of atomic scale graphitic layer growth NANO LETTERS Liu, L., Rim, K. T., Eom, D., Heinz, T. F., Flynn, G. W. 2008; 8 (7): 1872-1878


    The demand for better understanding of the mechanism of soot formation is driven by the negative environmental and health impact brought about by the burning of fossil fuels. While soot particles accumulate most of their mass from surface reactions, the mechanism for surface growth has so far been characterized primarily by measurements of the kinetics. Here we provide atomic-scale scanning tunneling microscope images of carbon growth by chemistry similar to that of importance in soot formation. At a temperature of 625 K, exposure of the surface of highly ordered pyrolytic graphite to 1 Langmuir of acetylene leads to the formation of both graphitic and amorphous carbonaceous material at the edges of nanoscale pits. Given the similarity of the electronic structure at these graphite defect sites to that of soot material growing in flames at higher temperatures, the present studies shed light on the mechanism for soot growth. These experiments also suggest that healing of defect sites in graphene nanostructures, which are of considerable interest as novel electronic devices, should be possible at modest surface temperatures by exposure of defected graphene to unsaturated hydrocarbons.

    View details for DOI 10.1021/nl0804046

    View details for Web of Science ID 000257504500015

    View details for PubMedID 18563944

  • Direct measurement of the lifetime of optical phonons in single-walled carbon nanotubes PHYSICAL REVIEW LETTERS Song, D., Wang, F., Dukovic, G., Zheng, M., Semke, E. D., Brus, L. E., Heinz, T. F. 2008; 100 (22)


    Time-resolved anti-Stokes Raman spectroscopy has been applied to probe the dynamics of optical phonons created in single-walled carbon nanotubes by femtosecond laser excitation. From measurement of the decay of the anti-Stokes Raman signal in semiconducting nanotubes of (6,5) chiral index, a room-temperature lifetime for G-mode phonons of 1.1+/-0.2 ps has been determined. This lifetime, which reflects the anharmonic coupling of the G-mode phonons to lower-frequency phonons, is important in assessing the role of nonequilibrium phonon populations in high-field transport phenomena.

    View details for DOI 10.1103/PhysRevLett.100.225503

    View details for Web of Science ID 000256528400026

    View details for PubMedID 18643430

  • Photocurrent spectroscopy of low-k dielectric materials: Barrier heights and trap densities JOURNAL OF APPLIED PHYSICS Atkin, J. M., Song, D., Shaw, T. M., CARTIER, E., Laibowitz, R. B., Heinz, T. F. 2008; 103 (9)

    View details for DOI 10.1063/1.2907958

    View details for Web of Science ID 000255983200110

  • Direct measurement of strain-induced changes in the band structure of carbon nanotubes PHYSICAL REVIEW LETTERS Huang, M., Wu, Y., Chandra, B., Yan, H., Shan, Y., Heinz, T. F., Hone, J. 2008; 100 (13)


    The effect of uniaxial strain on the optical transition energies of single-walled carbon nanotubes with known chiral indices was measured by Rayleigh scattering spectroscopy. Existing theory accurately predicts the trends in the measured strain-induced shifts, but overestimates their magnitude. Modification of the analysis to account for internal sublattice relaxation results in quantitative agreement with experiment.

    View details for DOI 10.1103/PhysRevLett.100.136803

    View details for Web of Science ID 000254670300064

    View details for PubMedID 18517983

  • Rayleigh scattering Spectroscopy CARBON NA NOTUBES Heinz, T. F. 2008; 111: 353-369
  • Theory of Rayleigh scattering from metallic carbon nanotubes PHYSICAL REVIEW B Malic, E., Hirtschulz, M., Milde, F., Wu, Y., Maultzsch, J., Heinz, T. F., Knorr, A., Reich, S. 2008; 77 (4)
  • Observation of excitons in one-dimensional metallic single-walled carbon nanotubes PHYSICAL REVIEW LETTERS Wang, F., Cho, D. J., Kessler, B., Deslippe, J., Schuck, P. J., Louie, S. G., Zettl, A., Heinz, T. F., Shen, Y. R. 2007; 99 (22)


    Excitons are generally believed not to exist in metals because of strong screening by free carriers. Here we demonstrate that excitonic states can in fact be produced in metallic systems of a one-dimensional character. Using metallic single-walled carbon nanotubes as a model system, we show both experimentally and theoretically that electron-hole pairs form tightly bound excitons. The exciton binding energy of 50 meV, deduced from optical absorption spectra of individual metallic nanotubes, significantly exceeds that of excitons in most bulk semiconductors and agrees well with ab initio theoretical predictions.

    View details for DOI 10.1103/PhysRevLett.99.227401

    View details for Web of Science ID 000251327500062

    View details for PubMedID 18233325

  • Theoretical approach to Rayleigh and absorption spectra of semiconducting carbon nanotubes PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS Malic, E., Hirtschulz, M., Milde, F., Wu, Y., Maultzsh, J., Heinz, T. F., Knorr, A., Reich, S. 2007; 244 (11): 4240-4243
  • Variable electron-phonon coupling in isolated metallic carbon nanotubes observed by Raman scattering PHYSICAL REVIEW LETTERS Wu, Y., Maultzsch, J., Knoesel, E., Chandra, B., Huang, M., Sfeir, M. Y., Brus, L. E., Hone, J., Heinz, T. F. 2007; 99 (2)


    We report the existence of broad and weakly asymmetric features in the high-energy (G) Raman modes of freely suspended metallic carbon nanotubes of defined chiral index. A significant variation in peak width (from 12 cm(-1) to 110 cm(-1)) is observed as a function of the nanotube's chiral structure. When the nanotubes are electrostatically gated, the peak widths decrease. The broadness of the Raman features is understood as the consequence of coupling of the phonon to electron-hole pairs, the strength of which varies with the nanotube chiral index and the position of the Fermi energy.

    View details for DOI 10.1103/PhysRevLett.99.027402

    View details for Web of Science ID 000248021000059

    View details for PubMedID 17678258

  • High-resolution scanning tunneling microscopy imaging of mesoscopic graphene sheets on an insulating surface PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Stolyarova, E., Rim, K. T., Ryu, S., Maultzsch, J., Kim, P., Brus, L. E., Heinz, T. F., Hybertsen, M. S., Flynn, G. W. 2007; 104 (22): 9209-9212


    We present scanning tunneling microscopy (STM) images of single-layer graphene crystals examined under ultrahigh vacuum conditions. The samples, with lateral dimensions on the micrometer scale, were prepared on a silicon dioxide surface by direct exfoliation of crystalline graphite. The single-layer films were identified by using Raman spectroscopy. Topographic images of single-layer samples display the honeycomb structure expected for the full hexagonal symmetry of an isolated graphene monolayer. The absence of observable defects in the STM images is indicative of the high quality of these films. Crystals composed of a few layers of graphene also were examined. They exhibited dramatically different STM topography, displaying the reduced threefold symmetry characteristic of the surface of bulk graphite.

    View details for DOI 10.1073/pnas.0703337104

    View details for Web of Science ID 000246935700024

    View details for PubMedID 17517635

  • Multiphonon raman scattering from individual single-walled carbon nanotubes PHYSICAL REVIEW LETTERS Wang, F., Liu, W., Wu, Y., Sfeir, M. Y., Huang, L., Hone, J., O'Brien, S., Brus, L. E., Heinz, T. F., Shen, Y. R. 2007; 98 (4)


    Combinations of up to 6 zone-edge and zone-center optical phonons are observed in the Raman spectra of individual single-walled carbon nanotubes (SWNTs). These multiphonon Raman modes exhibit distinct signatures of the one-dimensional nature of SWNTs and provide information on the phonon structure, exciton-phonon coupling, and excitonic transitions in nanotubes.

    View details for DOI 10.1103/PhysRevLett.98.047402

    View details for Web of Science ID 000243789700066

    View details for PubMedID 17358810

  • Optical Studies of Individual Single-Walled Carbon Nanotubes under Axial Strain 2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS AND LASER SCIENCE CONFERENCE (CLEO/QELS 2007), VOLS 1-5 Wu, Y., Huang, M., Voisin, C., Yan, H., Chandra, B., Hone, J., Heinz, T. F. 2007: 2520-2521
  • Auger recombination of excitons in semiconducting carbon nanotubes ULTRAFAST PHENOMENA XV Wang, F., Dukovic, G., Wu, Y., Hybertsen, M. S., Brus, L. E., Heinz, T. F. 2007; 88: 683-?
  • Observation of the optical stark effect in semiconducting carbon nanotubes ULTRAFAST PHENOMENA XV Song, D., Wang, F., Dukovic, G., Zheng, M., Semke, E. D., Brus, L. E., Heinz, T. F. 2007; 88: 674-?
  • Electrical transport measurements of nanotubes with known (n, m) indices PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS Chandra, B., Caldwell, R., Huang, M., Huang, L., Sfeir, M. Y., O'Brien, S. P., Heinz, T. F., Hone, J. 2006; 243 (13): 3359-3364
  • Exciton polarizability in semiconductor nanocrystals NATURE MATERIALS Wang, F., Shan, J., Islam, M. A., Herman, I. P., Bonn, M., Heinz, T. F. 2006; 5 (11): 861-864


    The response of charge to externally applied electric fields is an important basic property of any material system, as well as one critical for many applications. Here, we examine the behaviour and dynamics of charges fully confined on the nanometre length scale. This is accomplished using CdSe nanocrystals of controlled radius (1-2.5 nm) as prototype quantum systems. Individual electron-hole pairs are created at room temperature within these structures by photoexcitation and are probed by terahertz (THz) electromagnetic pulses. The electronic response is found to be instantaneous even for THz frequencies, in contrast to the behaviour reported in related measurements for larger nanocrystals and nanocrystal assemblies. The measured polarizability of an electron-hole pair (exciton) amounts to approximately 10(4) A(3) and scales approximately as the fourth power of the nanocrystal radius. This size dependence and the instantaneous response reflect the presence of well-separated electronic energy levels induced in the system by strong quantum-confinement effects.

    View details for DOI 10.1038/nmat1739

    View details for Web of Science ID 000241732000018

    View details for PubMedID 17028577

  • Auger recombination of excitons in one-dimensional systems PHYSICAL REVIEW B Wang, F., Wu, Y., Hybertsen, M. S., Heinz, T. F. 2006; 73 (24)
  • Interactions between individual carbon nanotubes studied by Rayleigh scattering spectroscopy PHYSICAL REVIEW LETTERS Wang, F., Sfeir, M. Y., Huang, L. M., Huang, X. M., Wu, Y., Kim, J. H., Hone, J., O'Brien, S., Brus, L. E., Heinz, T. F. 2006; 96 (16)


    The electronic properties of single-walled carbon nanotubes (SWNTs) are altered by intertube coupling whenever bundles are formed. These effects are examined experimentally by applying Rayleigh scattering spectroscopy to probe the optical transitions of given individual SWNTs in their isolated and bundled forms. The transition energies of SWNTs are observed to undergo redshifts of tens of meVs upon bundling with other SWNTs. These intertube coupling effects can be understood as arising from the mutual dielectric screening of SWNTs in a bundle.

    View details for DOI 10.1103/PhysRevLett.96.167401

    View details for Web of Science ID 000237156700068

    View details for PubMedID 16712273

  • Optical spectroscopy of individual single-walled carbon nanotubes of defined chiral structure SCIENCE Sfeir, M. Y., Beetz, T., Wang, F., Huang, L. M., Huang, X. M., Huang, M. Y., Hone, J., O'Brien, S., Misewich, J. A., Heinz, T. F., Wu, L. J., Zhu, Y. M., Brus, L. E. 2006; 312 (5773): 554-556


    We simultaneously determined the physical structure and optical transition energies of individual single-walled carbon nanotubes by combining electron diffraction with Rayleigh scattering spectroscopy. These results test fundamental features of the excited electronic states of carbon nanotubes. We directly verified the systematic changes in transition energies of semiconducting nanotubes as a function of their chirality and observed predicted energy splittings of optical transitions in metallic nanotubes.

    View details for DOI 10.1126/science.1124602

    View details for Web of Science ID 000237296700038

    View details for PubMedID 16645089

  • Experimental study of optical second-harmonic scattering from spherical nanoparticles PHYSICAL REVIEW A Shan, J., Dadap, J. I., Stiopkin, I., Reider, G. A., Heinz, T. F. 2006; 73 (2)
  • Structural dependence of excitonic optical transitions and band-gap energies in carbon nanotubes NANO LETTERS Dukovic, G., Wang, F., Song, D. H., Sfeir, M. Y., Heinz, T. F., Brus, L. E. 2005; 5 (11): 2314-2318


    The optical transitions of semiconducting carbon nanotubes have been ascribed to excitons. Here we use two-photon excitation spectroscopy to measure exciton binding energies, as well as band-gap energies, in a range of individual species of semiconducting SWNTs. Exciton binding energies are large and vary inversely with nanotube diameter, as predicted by theory. Band-gap energies are significantly blue-shifted from values predicted by tight-binding calculations.

    View details for DOI 10.1021/nl058122

    View details for Web of Science ID 000233481700039

    View details for PubMedID 16277475

  • Extracting subnanometer single shells from ultralong multiwalled carbon nanotubes PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Hong, B. H., Small, J. P., Purewal, M. S., Mullokandov, A., Sfeir, M. Y., Wang, F., Lee, J. Y., Heinz, T. F., Brus, L. E., Kim, P., Kim, K. S. 2005; 102 (40): 14155-14158


    We report a simple but powerful method for engineering multi-walled carbon nanotubes (MWNTs) by using manipulation by an atomic-force microscope. The successive shell-by-shell extraction process of ultralong MWNTs allows the exposure of the innermost single-walled carbon nanotubes (SWNTs), which have diameters as small as approximately 0.4 nm. The inner-shell extraction process changes the electrical characteristics of the MWNTs. Whereas the outer hollowed-out nanotubes show either metallic or semiconducting character, the innermost SWNTs of small diameter exhibit predominantly metallic transport properties.

    View details for DOI 10.1073/pnas.0505219102

    View details for Web of Science ID 000232392900008

    View details for PubMedID 16186505

  • Second-harmonic generation and theoretical studies of protonation at the water/alpha-TiO2 (110) interface CHEMICAL PHYSICS LETTERS Fitts, J. P., Machesky, M. L., Wesolowski, D. J., Shang, X. M., Kubicki, J. D., Flynn, G. W., Heinz, T. F., Eisenthal, K. B. 2005; 411 (4-6): 399-403
  • The optical resonances in carbon nanotubes arise from excitons SCIENCE Wang, F., Dukovic, G., Brus, L. E., Heinz, T. F. 2005; 308 (5723): 838-841


    Optical transitions in carbon nanotubes are of central importance for nanotube characterization. They also provide insight into the nature of excited states in these one-dimensional systems. Recent work suggests that light absorption produces strongly correlated electron-hole states in the form of excitons. However, it has been difficult to rule out a simpler model in which resonances arise from the van Hove singularities associated with the one-dimensional band [corrected] structure of the nanotubes. Here, two-photon excitation spectroscopy bolsters the exciton picture. We found binding energies of approximately 400 millielectron volts for semiconducting single-walled nanotubes with 0.8-nanometer diameters. The results demonstrate the dominant role of many-body interactions in the excited-state properties of one-dimensional systems.

    View details for DOI 10.1126/science.1110265

    View details for Web of Science ID 000228986500043

    View details for PubMedID 15879212

  • Electrostatic surface charge at aqueous/alpha-Al2O3 single-crystal interfaces as probed by optical second-harmonic generation JOURNAL OF PHYSICAL CHEMISTRY B Fitts, J. P., Shang, X. M., Flynn, G. W., Heinz, T. F., Eisenthal, K. B. 2005; 109 (16): 7981-7986


    Second harmonic generation (SHG) spectroscopy was used to characterize the pH-dependent electrostatic charging behavior of (0001) and (102) crystallographic surfaces of corundum (alpha-Al2O3) single-crystal substrates. The pH value of the point of zero charge (pH(pzc)) for each surface was determined by monitoring the SH response during three consecutive pH titrations conducted with 1, 10, and 100 mM NaNO3 carbonate-free aqueous solutions. The crossing point of the three titration curves, which corresponds to the pH(pzc), occurs at pH 4.1 +/- 0.4 for the (0001) surface and pH 5.2 +/- 0.4 for the (102) surface. SHG measurements that were recorded as a function of NaNO3 concentration at fixed pH values were found to corroborate the pH(pzc) values identified in the pH titrations. A comparison of the SHG results with surface protonation constants calculated using a simple electrostatic model suggests that surface relaxation and bonding changes resulting from surface hydration do not account for differences between experimental observations and model predictions. The measured pH(pzc) values for the alpha-Al2O3 single-crystal surfaces are significantly more acidic than published values for Al-(hydr)oxide particles which typically range from pH 8 to 10. This discrepancy suggests that the charging behavior of Al-(hydr)oxide particles is determined by surface sites associated with defects assuming that differences in surface acidity reflect differences in the coordination environment and local structure of the potential-determining surface groups.

    View details for DOI 10.1021/jp040297d

    View details for Web of Science ID 000228603700057

    View details for PubMedID 16851933

  • Probing the electronic response of nanostructures by THz time-domain spectroscopy IRMMW-THZ2005: THE JOINT 30TH INTERNATIONAL CONFERENCE ON INFRARED AND MILLIMETER WAVES AND 13TH INTERNATIONAL CONFERENCE ON TERAHERTZ ELECTRONICS, VOLS 1 AND 2 Heinz, T. F. 2005: 473-474
  • Observation of rapid Auger recombination in optically excited semiconducting carbon nanotubes PHYSICAL REVIEW B Wang, F., Dukovic, G., Knoesel, E., Brus, L. E., Heinz, T. F. 2004; 70 (24)
  • Probing electronic transitions in individual carbon nanotubes by Rayleigh scattering SCIENCE Sfeir, M. Y., Wang, F., Huang, L. M., Chuang, C. C., Hone, J., O'Brien, S. P., Heinz, T. F., Brus, L. E. 2004; 306 (5701): 1540-1543


    Rayleigh scattering spectra were obtained from individual single-walled carbon nanotubes with the use of a laser-generated visible and near-infrared supercontinuum. This diagnostic method is noninvasive and general for nanoscale objects. The approach permits clear identification of excited states in arbitrary metallic and semiconducting nanotubes. We analyzed spectral lineshapes in relation to the role of excitonic effects and correlated the results with Raman scattering data on individual tubes. The nanotube structure remained the same over distances of tens of micrometers. Small nanotube bundles retained distinct Rayleigh spectroscopic signatures of their component nanotubes, thus allowing the probing of nanotube-nanotube interactions.

    View details for DOI 10.1126/science.1103294

    View details for Web of Science ID 000225442700054

    View details for PubMedID 15514117

  • Reversible surface oxidation and efficient luminescence quenching in semiconductor single-wall carbon nanotubes JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Dukovic, G., WHITE, B. E., Zhou, Z. Y., Wang, F., Jockusch, S., Steigerwald, M. L., Heinz, T. F., Friesner, R. A., Turro, N. J., Brus, L. E. 2004; 126 (46): 15269-15276


    We have investigated reversible single-wall carbon nanotube (SWNT) oxidation by quantitative analysis of the oxide-induced absorption bleaching and luminescence quenching at low pH. These data, in combination with DFT structure calculations, suggest that the nanotube oxide is a 1,4-endoperoxide. At low pH, the endoperoxide protonates to create a hydroperoxide carbocation, introducing a hole in the SWNT valence band. Nanotube luminescence is extremely sensitive to quenching by hole-doping, while the absorption is relatively robust.

    View details for DOI 10.1021/ja046526r

    View details for Web of Science ID 000225233600049

    View details for PubMedID 15548024

  • Real-space observation of molecular motion induced by femtosecond laser pulses SCIENCE Bartels, L., Wang, F., Moller, D., Knoesel, E., Heinz, T. F. 2004; 305 (5684): 648-651


    Femtosecond laser irradiation is used to excite adsorbed CO molecules on a Cu110 surface; the ensuing motion of individual molecules across the surface is characterized on a site-to-site basis by in situ scanning tunneling microscopy. Adsorbate motion both along and perpendicular to the rows of the Cu110 surface occurs readily, in marked contrast to the behavior seen for equilibrium diffusion processes. The experimental findings for the probability and direction of the molecular motion can be understood as a manifestation of strong coupling between the adsorbates' lateral degrees of freedom and the substrate electronic excitation produced by the femtosecond laser radiation.

    View details for DOI 10.1126/science.1099770

    View details for Web of Science ID 000222992100041

    View details for PubMedID 15218095

  • Theory of optical second-harmonic generation from a sphere of centrosymmetric material: small-particle limit JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS Dadap, J. I., Shan, J., Heinz, T. F. 2004; 21 (7): 1328-1347
  • Conductivity of solvated electrons in hexane investigated with terahertz time-domain spectroscopy JOURNAL OF CHEMICAL PHYSICS Knoesel, E., Bonn, M., Shan, J., Wang, F., Heinz, T. F. 2004; 121 (1): 394-404


    We present investigations of the transient photoconductivity and recombination dynamics of quasifree electrons in liquid n-hexane and cyclohexane performed using terahertz time-domain spectroscopy (THz-TDS). Quasifree electrons are generated by two-photon photoionization of the liquid using a femtosecond ultraviolet pulse, and the resulting changes in the complex conductivity are probed by a THz electromagnetic pulse at a variable delay. The detection of time-domain wave forms of the THz electric field permits the direct determination of both the real and the imaginary part of the conductivity of the electrons over a wide frequency range. The change in conductivity can be described by the Drude model, thus yielding the quasifree electron density and scattering time. The electron density is found to decay on a time scale of a few hundred picoseconds, which becomes shorter with increasing excitation density. The dynamics can be described by a model that assumes nongeminate recombination between electrons and positive ions. In addition, a strong dependence of the quasifree electron density on temperature is observed, in agreement with a two-state model in which the electron may exist in either a quasifree or a bound state.

    View details for DOI 10.1063/1.1757442

    View details for Web of Science ID 000222112100044

    View details for PubMedID 15260559

  • Time-resolved fluorescence of carbon nanotubes and its implication for radiative lifetimes PHYSICAL REVIEW LETTERS Wang, F., Dukovic, G., Brus, L. E., Heinz, T. F. 2004; 92 (17)


    The temporal evolution of fluorescence from isolated single-wall carbon nanotubes (SWNTs) has been investigated using optical Kerr gating. The fluorescence emission is found to decay on a time scale of 10 ps. This fast relaxation arises from nonradiative processes, the existence of which explains the relatively low observed fluorescence efficiency in isolated SWNTs. From the measured decay rate and a determination of fluorescence quantum efficiency, we deduce a radiative lifetime of 110 ns.

    View details for DOI 10.1103/PhysRevLett.92.177401

    View details for Web of Science ID 000221179200058

    View details for PubMedID 15169189

  • Electron transport in TiO2 probed by THz time-domain spectroscopy PHYSICAL REVIEW B Hendry, E., Wang, F., Shan, J., Heinz, T. F., Bonn, M. 2004; 69 (8)
  • Electronic charge transport in sapphire studied by optical-pump/THz-probe spectroscopy ULTRAFAST PHENOMENA IN SEMICONDUCTORS AND NANOSTRUCTURE MATERIALS VIII Wang, F., Shan, J., Knoesel, E., Bonn, M., Heinz, T. F. 2004; 5352: 216-221

    View details for DOI 10.1117/12.532505

    View details for Web of Science ID 000222660600022

  • Terahertz radiation from semiconductors ULTRAFAST DYNAMICAL PROCESSES IN SEMICONDUCTORS Shan, J., Heinz, T. F. 2004; 92: 1-56
  • Ultrafast scattering of electrons in TiO2 FEMTOCHEMISTRY AND FEMTOBIOLOGY: ULTRAFAST EVENTS IN MOLECULAR SCIENCE Bonn, M., Wang, F., Shan, J., Heinz, T. F., Hendry, E. 2004: 517-520
  • Quantitative study of adsorbate-adsorbate interactions of hydrogen on the Si(100) surface PHYSICAL REVIEW B Hu, Z. H., Biedermann, A., Knoesel, E., Heinz, T. F. 2003; 68 (15)
  • Observation of an isotope effect in femtosecond laser-induced desorption of O-2/Pd(111) JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A Quinn, D. P., Heinz, T. F. 2003; 21 (4): 1312-1316

    View details for DOI 10.1116/1.1580486

    View details for Web of Science ID 000184409200080

  • Measurement of the frequency-dependent conductivity in sapphire PHYSICAL REVIEW LETTERS Shan, J., Wang, F., Knoesel, E., Bonn, M., Heinz, T. F. 2003; 90 (24)


    Electron transport of photoexcited single-crystal sapphire (alpha-Al2O3) is characterized by terahertz time-domain spectroscopy. The complex conductivity displays a Drude-type frequency dependence, which yields carrier scattering rates and densities. Carrier scattering is dominated by interactions with acoustical and optical phonons at low and high temperatures, respectively, and follows Matthiessen's law over the measured temperature range of 40-350 K. The results, including low-temperature mobilities >10000 cm(2)/V s, are compatible with a large-polaron description of the conduction electrons.

    View details for DOI 10.1103/PhysRevLett.90.247401

    View details for Web of Science ID 000183642800046

    View details for PubMedID 12857225

  • Transient conductivity in single-crystal Al-2(3)O probed by THz time-domain spectroscopy ULTRAFAST PHENOMENA XIII Shan, J., Wang, F., Knoesel, E., Bonn, M., Wolf, M., Heinz, T. F. 2003; 71: 262-264
  • Two-dimensional imaging of continuous-wave terahertz radiation using electro-optic detection APPLIED PHYSICS LETTERS Nahata, A., Yardley, J. T., Heinz, T. F. 2002; 81 (6): 963-965

    View details for DOI 10.1063/1.1497190

    View details for Web of Science ID 000177171400005

  • Probing high-barrier pathways of surface reactions by scanning tunneling microscopy SCIENCE Durr, M., Biedermann, A., Hu, Z., Hofer, U., Heinz, T. F. 2002; 296 (5574): 1838-1841


    The ability of scanning tunneling microscopy to probe the pathways of thermally activated high-barrier surface processes is frequently limited by competing low-barrier processes that can confuse measurement of the true initial and final configuration. We introduce an approach to circumvent this difficulty by driving the surface process with nanosecond laser heating. The method is applied to determine the pathway of recombinative desorption in the H/Si(001) system. The observed configuration of dangling bonds after laser heating reveals that the desorbed hydrogen molecules are not formed on single dimers, but rather from neighboring silicon dimers via an interdimer reaction pathway.

    View details for Web of Science ID 000176054300039

    View details for PubMedID 12052951

  • Deeper into the (infra)red IEEE CIRCUITS & DEVICES Nahata, A., Cao, H., Heinz, T. F. 2002; 18 (3): 32-39
  • Electro-optic detection of femtosecond electromagnetic pulses by use of poled polymers OPTICS LETTERS Cao, H., Heinz, T. F., Nahata, A. 2002; 27 (9): 775-777


    We report the generation and coherent detection of freely propagating ultrashort baseband electromagnetic pulses. Using optical rectification in ?110? GaAs for wideband emission and electro-optic sampling in a poled polymer for wideband detection, we demonstrate spectral sensitivity that extends from the far infrared (lambda~100 mum) to ~33 THz(lambda = 9 mum) . Over a band of nearly 20 THz, a relatively flat frequency response is observed. We discuss issues that limit the response bandwidth.

    View details for Web of Science ID 000175284000034

    View details for PubMedID 18007929

  • Terahertz time-domain spectroscopy based on nonlinear optics JOURNAL OF NONLINEAR OPTICAL PHYSICS & MATERIALS Shan, J., Nahata, A., Heinz, T. F. 2002; 11 (1): 31-48
  • Real-space study of the pathway for dissociative adsorption of H-2 on Si(001) PHYSICAL REVIEW LETTERS Durr, M., Hu, Z., Biedermann, A., Hofer, U., Heinz, T. F. 2002; 88 (4)


    Dissociative adsorption of molecular hydrogen on clean Si(001) surfaces has been investigated by means of scanning tunneling microscopy. The dissociated hydrogen atoms are found to occupy Si atoms of adjacent dimers. In addition to this interdimer configuration associated with the adsorption of isolated hydrogen molecules, pairs of adjacent doubly occupied dimers are readily formed. They arise from the enhanced reactivity of partially occupied dimers following the initial H2 adsorption step. The results are considered in light of recent adsorption and desorption measurements.

    View details for DOI 10.1103/PhysRevLett.88.046104

    View details for Web of Science ID 000173907500051

    View details for PubMedID 11801144

  • Generation and coherent detection of far-infrared and mid-infrared radiation 2002 IEEE/LEOS ANNUAL MEETING CONFERENCE PROCEEDINGS, VOLS 1 AND 2 Nahata, A., Cao, H., Linke, R. A., Heinz, T. F. 2002: 865-866
  • Magnetic-field enhancement of terahertz emission from semiconductor surfaces: A comparison of experiment with a semiclassical model Conference on Ultrafast Phenomena in Semiconductors VI Shan, J., Weiss, C., Wallenstein, R., Beigang, R., Heinz, T. F. SPIE-INT SOC OPTICAL ENGINEERING. 2002: 1–11
  • Origin of magnetic field enhancement in the generation of terahertz radiation from semiconductor surfaces OPTICS LETTERS Shan, J., Weiss, C., Wallenstein, R., Beigang, R., Heinz, T. F. 2001; 26 (11): 849-851


    We present a theory of the magnetic field enhancement of terahertz (THz) emission from photogenerated carriers in the surface depletion region of a semiconductor. A combination of the Drude-Lorentz model for the carrier dynamics with an appropriate solution of the radiation problem is sufficient to explain the strong B -field enhancement in THz radiation that has been observed experimentally. The effect arises primarily from the increased radiation efficiency of transient currents flowing in the plane of the surface. The model provides quantitative agreement with experiment for the pronounced angular dependence of the enhancement and predicts the correct trend for the enhancement in a variety of materials.

    View details for Web of Science ID 000168924200031

    View details for PubMedID 18040471

  • Real-space investigation of hydrogen dissociation at step sites of vicinal Si(001) surfaces PHYSICAL REVIEW B Durr, M., Hu, Z., Biedermann, A., Hofer, U., Heinz, T. F. 2001; 63 (12)
  • Novel surface vibrational spectroscopy: Infrared-infrared-visible sum-frequency generation PHYSICAL REVIEW LETTERS Bonn, M., Hess, C., Miners, J. H., Heinz, T. F., Bakker, H. J., Cho, M. 2001; 86 (8): 1566-1569


    A novel type of surface vibrational sum-frequency generation spectroscopy is presented that enables a highly specific measurement of the coupling of molecules on surfaces. With this doubly vibrationally resonant technique, two-dimensional vibrational spectroscopy of molecules on surfaces becomes possible. The technique is demonstrated for the C-O stretch vibration of CO on a ruthenium (001) surface. It allows for the determination of the intermolecular coupling strength of dipole-coupled CO molecules on the surface.

    View details for Web of Science ID 000167010000043

    View details for PubMedID 11290194

  • Charge transport and carrier dynamics in liquids probed by THz time-domain spectroscopy PHYSICAL REVIEW LETTERS Knoesel, E., Bonn, M., Shan, J., Heinz, T. F. 2001; 86 (2): 340-343


    We examine the transport properties and the dynamics of free electrons in n-hexane by means of femtosecond spectroscopy using an ultraviolet pump pulse to create the electrons and a THz electromagnetic pulse as a probe. The complex dielectric response of the photogenerated electrons is determined over a broad range of frequencies, from which we infer the electron scattering time and density through the Drude model. The time evolution of the carrier density reveals nongeminate electron-ion recombination within hundreds of picoseconds at high ion concentration.

    View details for Web of Science ID 000166231400039

    View details for PubMedID 11177826

  • Single-shot measurement of terahertz electromagnetic pulses by use of electro-optic sampling OPTICS LETTERS Shan, J., Weling, A. S., Knoesel, E., Bartels, L., Bonn, M., Nahata, A., Reider, G. A., Heinz, T. F. 2000; 25 (6): 426-428


    We demonstrate a simple scheme for capturing the temporal waveforms of a freely propagating terahertz electromagnetic transient in a single shot. The method relies on electro-optic sampling in a noncollinear geometry for the terahertz radiation and the visible probe beam, coupled with multichannel detection. The approach provides time resolution that is comparable to that of conventional electro-optic sampling measurements.

    View details for Web of Science ID 000085846500022

    View details for PubMedID 18059901

  • Second-harmonic Rayleigh scattering from a sphere of centrosymmetric material PHYSICAL REVIEW LETTERS Dadap, J. I., Shan, J., Eisenthal, K. B., Heinz, T. F. 1999; 83 (20): 4045-4048
  • Free-space electro-optic detection of continuous-wave terahertz radiation APPLIED PHYSICS LETTERS Nahata, A., Yardley, J. T., Heinz, T. F. 1999; 75 (17): 2524-2526
  • Enhancement in the spectral irradiance of photoconducting terahertz emitters by chirped-pulse mixing JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS Weling, A. S., Heinz, T. F. 1999; 16 (9): 1455-1467
  • Dissociative adsorption of H-2 on Si(100) induced by atomic H PHYSICAL REVIEW LETTERS Biedermann, A., Knoesel, E., Hu, Z., Heinz, T. F. 1999; 83 (9): 1810-1813
  • Measurement of the vector character of electric fields by optical second-harmonic generation OPTICS LETTERS Dadap, J. I., Shan, J., Weling, A. S., Misewich, J. A., Nahata, A., Heinz, T. F. 1999; 24 (15): 1059-1061


    We present a scheme for the determination of the vector nature of an electric field by optical second-harmonic generation. We demonstrate the technique by mapping the two-dimensional electric-field vector of a biased transmission line structure on silicon with a spatial resolution of ~10mum .

    View details for Web of Science ID 000081700800019

    View details for PubMedID 18073940

  • Anisotropic orientational motion of molecular adsorbates at the air-water interface JOURNAL OF PHYSICAL CHEMISTRY B Zimdars, D., Dadap, J. I., Eisenthal, K. B., Heinz, T. F. 1999; 103 (17): 3425-3433
  • Homodyne detection of second-harmonic generation as a probe of electric fields APPLIED PHYSICS B-LASERS AND OPTICS Dadap, J. I., Shan, J., Weling, A. S., Misewich, J. A., Heinz, T. F. 1999; 68 (3): 333-341
  • Femtosecond dynamics of solvation at the air/water interface CHEMICAL PHYSICS LETTERS Zimdars, D., Dadap, J. I., Eisenthal, K. B., Heinz, T. F. 1999; 301 (1-2): 112-120
  • Generation of subpicosecond electrical pulses by optical rectification OPTICS LETTERS Nahata, A., Heinz, T. F. 1998; 23 (11): 867-869


    We describe the generation of subpicosecond electrical pulses by optical rectification of ultrashort optical pulses. The electrical pulses are generated by the second-order nonlinear response of a LiTaO(3) crystal bonded to a coplanar transmission line. A bipolar temporal waveform with a width of 875 fs was measured after a propagation distance of 175mum . This pulse width was limited by the response time of the photoconductive sampler. We observed both broadening and amplitude reduction in the temporal waveform owing to propagation.

    View details for Web of Science ID 000074069500018

    View details for PubMedID 18087368

  • Ultrafast measurements of electric fields in semiconductors by optical harmonic generation Conference on Ultrafast Phenomena in Semiconductors II Nahata, A., Heinz, T. F., Misewich, J. A. SPIE - INT SOC OPTICAL ENGINEERING. 1998: 238–243
  • Detection of freely propagating terahertz radiation by use of optical second-harmonic generation OPTICS LETTERS Nahata, A., Heinz, T. F. 1998; 23 (1): 67-69


    We report the application of electric-field-induced optical second-harmonic generation as a new technique for measuring the field of freely propagating terahertz radiation. Using silicon as the nonlinear medium, we demonstrate subpicosecond time resolution and a sampling signal that varies linearly with the terahertz electric field. This approach, which is attractive for centrosymmetric media, permits a significantly broadened class of materials to be exploited for free-space sampling measurements.

    View details for Web of Science ID 000071271100023

    View details for PubMedID 18084414

  • Physisorbed template for spatial patterning of adsorbates PHYSICAL REVIEW LETTERS Williams, P. A., Reider, G. A., Li, L. P., Hofer, U., Suzuki, T., Heinz, T. F. 1997; 79 (18): 3459-3462
  • A wideband coherent terahertz spectroscopy system using optical rectification and electro-optic sampling APPLIED PHYSICS LETTERS Nahata, A., Weling, A. S., Heinz, T. F. 1996; 69 (16): 2321-2323
  • Reshaping of freely propagating terahertz pulses by diffraction IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS Nahata, A., Heinz, T. F. 1996; 2 (3): 701-708
  • High-speed electrical sampling using optical second-harmonic generation APPLIED PHYSICS LETTERS Nahata, A., Heinz, T. F., Misewich, J. A. 1996; 69 (6): 746-748
  • Anomalous branching ratio in the femtosecond surface chemistry of O-2/Pd(111) International Symposium on Dynamical Quantum Processes on Solid Surfaces Misewich, J. A., Nakabayashi, S., Weigand, P., Wolf, M., Heinz, T. F. ELSEVIER SCIENCE BV. 1996: 204–13
  • Ionization probabilities of A(2)Sigma(+)(v'=0,1,2) and B-2 Pi(v'=0,2) states of NO JOURNAL OF CHEMICAL PHYSICS Zacharias, H., DEROUGEMONT, F., Heinz, T. F., Loy, M. M. 1996; 105 (1): 111-117
  • Coherent detection of freely propagating terahertz radiation by electro-optic sampling APPLIED PHYSICS LETTERS Nahata, A., AUSTON, D. H., Heinz, T. F., Wu, C. J. 1996; 68 (2): 150-152

    View details for Web of Science ID A1995RR28800084

    View details for PubMedID 9981795


    View details for Web of Science ID A1995RR50200093

    View details for PubMedID 9981712

  • ULTRAFAST STUDIES OF SURFACE DYNAMICS - O2/PD(111) 11th International Conference on Laser Spectroscopy Misewich, J. A., Heinz, T. F., Hofer, U., KALAMARIDES, A., Loy, M. M. AIP PRESS. 1994: 165–70
  • FEMTOSECOND LASER-INDUCED PROCESSES - ULTRAFAST DYNAMICS AND REACTION PATHWAYS FOR O2/PD(111) Conference on Laser Techniques for Surface Science Heinz, T. F., Misewich, J. A., Hofer, U., KALAMARIDES, A., Nakabayashi, S., Weigand, P., Wolf, M. SPIE - INT SOC OPTICAL ENGINEERING. 1994: 276–284
  • GENERAL DISCUSSION FARADAY DISCUSSIONS Hopkinson, A., Harris, J., POLANYI, J. C., Auerbach, D. J., Rettner, C. T., Michelsen, H. A., Holloway, S., Buckingham, A. D., Marcus, R. A., Darling, G. R., Holloway, S., Tetenyi, P., Kunimori, K., Billing, G. D., Kolasinski, K. W., Rocca, M., Stolte, S., King, D. A., Hasselbrink, E., Urena, A. G., Zacharias, H., Jansen, A. P., Gross, A., Harris, A., Heinz, T. F., Kosloff, R., Kleyn, A. W., Uzer, T. 1993; 96: 67-93

    View details for Web of Science ID A1992HZ98000021

    View details for PubMedID 10045784

  • FEMTOSECOND TIME-RESOLVED MEASUREMENT OF DESORPTION PHYSICAL REVIEW LETTERS Budde, F., Heinz, T. F., Loy, M. M., Misewich, J. A., DEROUGEMONT, F., Zacharias, H. 1991; 66 (23): 3024-3027

    View details for Web of Science ID A1991FP87100022

    View details for PubMedID 10043679

  • SURFACE-DIFFUSION OF HYDROGEN ON SI(111)7 X 7 PHYSICAL REVIEW LETTERS Reider, G. A., Hofer, U., Heinz, T. F. 1991; 66 (15): 1994-1997

    View details for Web of Science ID A1991FG03300013

    View details for PubMedID 10043363

  • DESORPTION INDUCED BY FEMTOSECOND LASER-PULSES PHYSICAL REVIEW LETTERS Prybyla, J. A., Heinz, T. F., Misewich, J. A., Loy, M. M., Glownia, J. H. 1990; 64 (13): 1537-1540

    View details for Web of Science ID A1990CW33000016

    View details for PubMedID 10041423

  • BONDING AT SILICON INSULATOR INTERFACES APPLIED SURFACE SCIENCE Himpsel, F. J., Heinz, T. F., McLean, A. B., Palange, E. 1989; 41-2: 346-351


    Diffracted surface second-harmonic radiation emerging in several orders has been observed from a periodically modulated monolayer of adsorbed dye molecules. The molecular grating was produced by laser-induced desorption in the field of two crossed beams. An elementary theory is presented that relates the characteristics of the second-harmonic diffraction pattern to the spatially varying properties of the surface and is applied to infer the adsorbate density profile of the spatially modulated grating. The density profile is compared with the predictions of a model of grating formation based on thermal desorption.

    View details for Web of Science ID A1989AX17300013

    View details for PubMedID 19759634


    View details for Web of Science ID A1989AJ69400016

    View details for PubMedID 10041135

  • TWO-DIMENSIONAL ENERGY-BANDS AT THE CAF2/SI(111) INTERFACE JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B Himpsel, F. J., Heinz, T. F., McLean, A. B., Palange, E., Burstein, E. 1989; 7 (4): 879-881

    View details for Web of Science ID A1985TY38500018

    View details for PubMedID 10030885

  • SURFACE STUDIES BY OPTICAL 2ND-HARMONIC GENERATION - THE ADSORPTION OF O2, CO, AND SODIUM ON THE RH(111) SURFACE PHYSICAL REVIEW LETTERS Tom, H. W., Mate, C. M., Zhu, X. D., Crowell, J. E., Heinz, T. F., Somorjai, G. A., Shen, Y. R. 1984; 52 (5): 348-351


    We demonstrate the existence of coherent coupling effects in pump-probe measurements with collinear, copropagating beams, despite the absence of any induced spatial gratings in this geometry. The coherent interaction, which is found to be similar but not identical to that for crossed beams, must be taken into account in analyzing relaxation processes occurring on the time scale of the laser pulse. These coherent effects cannot generally be eliminated by detecting the total change in energy in both the pump and probe beams.

    View details for Web of Science ID A1984TD10300015

    View details for PubMedID 19721598

  • SURFACE-ENHANCED 2ND-HARMONIC GENERATION AND RAMAN-SCATTERING PHYSICAL REVIEW B Chen, C. K., Heinz, T. F., Ricard, D., Shen, Y. R. 1983; 27 (4): 1965-1979