Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session R53: Optical Spectroscopy of Heterostructures of 2D Materials |
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Sponsoring Units: DCMP Chair: Nathan Wilson Room: Mile High Ballroom 1F |
Thursday, March 5, 2020 8:00AM - 8:12AM |
R53.00001: Optical Second-Harmonic Interference in Two-Dimensional Heterostructures Wontaek Kim, Sunmin Ryu Atom-thick two-dimensional (2D) transition metal dichalcogenides (TMDs) with strong excitonic transitions in the NIR and visible range are excellent media for optical second-harmonic generation (SHG). SHG is not only a powerful structural method owing to its sensitivity to crystallographic symmetry, but also a coherent probe because of its instantaneous but non-dissipative response. In this talk, I will present our recent findings on SHG interference occurring in 2D TMD heterostructures. All 2D crystal samples were prepared by mechanical exfoliation and studied with a polarized SHG micro-spectroscopy setup powered by a tunable femtosecond Ti-sapphire laser. Polarization-resolved SHG polar plots of MoS2 homo-bilayers exhibited a six-petal pattern with six angular nodes like those of MoS2 monolayers. Remarkably, the SHG behavior of MoS2/WS2 was much different from that of MoS2 bilayers in that the former lacked angular nodes. This anomalous angular background became larger for more staggered hetero-bilayers but decreased for increased fundamental wavelength. All the results were nicely explained by an SHG interference model considering material-dependent phase delay in their nonlinear response, which was corroborated with phase-resolved interferometric SHG measurements. |
Thursday, March 5, 2020 8:12AM - 8:24AM |
R53.00002: Interlayer Exciton in atomically reconstructed MoSe2/WSe2 van der Waals Heterostructure Hsun jen Chuang, Aubrey T. Hanbicki, Matthew Rosenberger, Vladimir P. Oleshko, Kathleen M McCreary, Saujan V Sivaram, Igor Mazin, Berend Thomas Jonker Interlayer exciton (ILE) can be created by artificially stacking MoSe2 onto WSe2 to form a heterostructure with type II band alignment. With hBN encapsulation and a relative rotational angle close to 60 degrees, well pronounced ILE emission is observed at room temperature and further splits into two distinct peaks (ILE1 and ILE2) at low temp. Furthermore, we demonstrate that the ILE emission peaks have opposite circular polarizations when excited by circularly polarized light. Ab initio calculations provide an explanation of this unique and potentially useful property and indicate that it is a result of the indirect character of both transitions. We further investigate the atomic structure arrangement of MoSe2/WSe2 by TEM and reveal that the heterostructure indeed reconstructs under a small twist angle ( ≤ 1°) between the TMDs. With a small twist angle, periodic domains form with commensurate stacking within the domain. On the other hand, a rigid moiré structure is also observed by TEM when a larger twist angle (≥ 3°) is applied. These results may provide fundamental insights into the mechanical and optical behavior of this exciting class of semiconductor heterostructure. |
Thursday, March 5, 2020 8:24AM - 8:36AM |
R53.00003: Nonradiative energy transfer enhances Raman intensity in layered heterostructure Medha Dandu, Kenji Watanabe, Takashi Taniguchi, Ajay Sood, Kausik Majumdar Since the discovery of Graphene enhanced Raman scattering, layered materials and their heterostructures are emerging as promising candidates for realizing Raman enhancement on flat surfaces. While most of the Raman enhancement studies on these materials are based on charge transfer interaction, here, we experimentally demonstrate for the first time, a strong enhancement of Raman intensity through nonradiative energy transfer (NRET) in a layered heterostructure. We achieve a ten-fold Raman enhancement of a monolayer transition metal dichalcogenide (1L-TMD, such as 1L-MoS2 and 1L-WS2) stacked on a multilayer SnSe2. Spectral resonance, extreme spatial proximity and in-plane orientation of dipoles result in strong dipole-dipole coupling that enables NRET driven Raman enhancement even when a barrier layer like hBN is introduced between 1L-TMD and SnSe2. We corroborate the evidence for NRET driven Raman enhancement by decoupling it from other effects and demonstrating its tunability through modulation of spectral overlap between 1L-TMD and SnSe2 by varying the sample temperature. Observation of such non-local, uniform Raman enhancement over a wide junction area opens new ways to engineer sensing mechanisms using NRET in tandem with other existing enhancement techniques. |
Thursday, March 5, 2020 8:36AM - 8:48AM |
R53.00004: Circular emission under linear excitation from interlayer excitons in WSe2/MoSe2 heterobilayer Xin Lu, Weijie Li, Sudipta Dubey, Ajit Srivastava We report circularly polarized emission from interlayer excitons in WSe2/MoSe2 heterobilayer under linear excitation at zero magnetic field. The magnitude and sign of the degree of circular polarization (DCP) varies spatially over the sample. The DCP is independent of excitation energy when incident laser is linearly polarized, however, can be modulated by circular pumping or magnetic field. This is in stark contrast with WSe2 and MoSe2 intralayer excitons where circular emission only occurs with an applied magnetic field or under circular pumping. The observation of circularly polarized emission, in absence of magnetic field and circular pumping, implies an effective time-reversal symmetry (TRS) breaking in the WSe2/MoSe2 heterobilayer. As different cool downs result in very similar spatial patterns of DCP, we rule out spontaneous TRS breaking in the sample. Due to the momentum space-indirect nature of interlayer excitons, there is a finite center-of mass momentum in the recombining exciton state. This, together with the ubiquitous inhomogeneous strain in exfoliated samples can explain our observations. |
Thursday, March 5, 2020 8:48AM - 9:00AM |
R53.00005: Dipole-dipole interactions between localized interlayer excitons in WSe2/MoSe2 heterobilayer Weijie Li, Xin Lu, Sudipta Dubey, Luka Matej Devenica, Ajit Srivastava We explored localized interlayer excitons in WSe2/MoSe2 heterobilayer by low temperature (~ 4K) photoluminescence spectroscopy. With type-II band alignment, the localized interlayer exciton compromises of an electron from MoSe2 and a hole from WSe2, forming an out-of-plane electric dipole moment. We thus tuned its energy by hundreds of its linewidth via an external electric field. In addition to the field-tuneability, the permanent dipoles have repulsive dipolar interaction scaled as ~ 1/rex3 where rex is the interexcitonic distance. Here we observed a peak blueshifted from the localized interlayer exciton peak by ~ 2 meV with increasing incident power. The blue peak is assigned to be the biexciton peak due to the same synchronized jittering as the exciton and its superlinear power dependence. The similar g factor and gate dependence of biexciton and exciton imply that the second excitation has negligible impact on the exciton wavefunction. The polarization of the biexciton and exciton under finite magnetic field is consistent with the spin-valley singlet nature of the dipolar molecule states. With the singlet configuration and dipolar interaction ~ 2 meV, the confinement length of the localized interlayer exciton is calculated to be ~ 5 nm. |
Thursday, March 5, 2020 9:00AM - 9:12AM |
R53.00006: Confinement of long-lived interlayer excitons in WSe2/WS2 heterobilayers Alejandro Montblanch, Dhiren Kara, Ioannis Paradeisanos, Carola Purser, Gang Wang, Pawel Latawiec, Marko Loncar, Sefaattin Tongay, Andrea Ferrari, Mete Atature Recently, there has been an increased focus on controllable quantum gases with long-range anisotropic interactions to simulate condensed matter problems that have so far remained intractable. Interlayer excitons (IX) in transition metal dichalcogenides (TMDs), which have a permanent electric dipole moment, have emerged as an alternative pathway to realising quantum simulations in the solid state. Here, we create ultra-long-lived IX and demonstrate they can be trapped - an important prerequisite for future investigations. We do this by depositing a heterostructure of monolayer WSe2 and WS2 on a pre-patterned substrate. We observe lifetimes of IX approaching 200 μs at cryogenic temperatures, an order of magnitude longer than any previous IX and three orders for any TMD IX. Power-dependent PL evidences IX-IX interactions and the ability to confine single IX at pre-defined locations. The potential to create arbitrary trapping profiles for long-lived dipolar particles evidences the capability of TMDs to provide a unique avenue for probing exotic states of matter in degenerate gases and artificial lattices. |
Thursday, March 5, 2020 9:12AM - 9:24AM |
R53.00007: Mechanically tunable nonlinear optics from a van der Waals interface in twisted hexagonal boron nitride heterostructures Nathan Finney, Kaiyuan Yao, Samuel Moore, Fang Liu, Jenny Ardelean, Xinyi Xu, Kenji Watanabe, Takashi Taniguchi, Xiaoyang Zhu, Dmitri Basov, Cory Dean, P. James Schuck, James C Hone In the bulk limit, the second harmonic generation (SHG) response from hexagonal boron nitride (BN) includes electric quadrupole and dipole contributions. Both the total layer number and layer parity play a critical role in determining the overall nonlinear optical response. Here we investigate micromechanical devices consisting of a rotatable crystal of bulk BN placed on top of another crystal of BN, where the relative rotational alignment of the two crystal lattices can be dynamically tuned via an atomic force microscope (AFM) in contact mode. We find significant angle dependence of the unpolarized SHG response where the SHG signal is strongly suppressed or enhanced depending on the stacking order (AA’ or AA, respectively) at the homojunction between the top and bottom BN. At intermediate rotation angles of the top BN crystal between 0 and 60 degrees, we observe continuous modulation of both the SHG intensity and polarization. The non-symmetry-selective third harmonic generation (THG) signal remains unchanged over all rotation angles, suggesting that the observed modulation in nonlinear optical response is induced by mechanically tuning the symmetry at the van der Waals interface between the crystals. |
Thursday, March 5, 2020 9:24AM - 9:36AM |
R53.00008: Interlayer Exciton Fine Structure in MoSe2/WSe2 Heterostructure Shengnan Miao, Tianmeng Wang, Zhipeng Li, Zhengguang Lu, Sufei Shi The realization of the valleytronics based on transitional metal dichalcogenides (TMDCs) is hindered by the short lifetime of the exciton. Recently, it is predicted that the long-lived interlayer exciton in aligned TMDC hetero-bilayer possesses fine structures with distinctive valley polarization selection rules, which can be utilized to lift this limitation. However, the revealing of the details of the interlayer exciton is hindered by the sample quality. Further, the Moiré effect due to small twist angle also complicates the spectroscopy signature of the interlayer exciton. In this work, we present a comprehensive study of the interlayer excitons through gate-dependent, magnetic-field dependent spectroscopy study. The valley polarization is investigated through photoexcitation spectroscopy (PLE) study, and the high valley polarization of interlayer exciton is also explored. Our work paves the way for future exploration and understanding of the fascinating interlayer excitonic physics and Moiré physics in TMDC heterostructures. |
Thursday, March 5, 2020 9:36AM - 9:48AM |
R53.00009: Evidence of Purely Electronic Lattice at Interface of TMD/Bi2Se3 2D Heterostructures Induced by Strong Interlayer Coupling Zachariah Hennighausen, Christopher Lane, Ioana G Buda, Vineet K Mathur, Arun Bansil, Swastik Kar Vertically-stacked 2D heterostructures are more than a sum of the individual layers, but a product of the interlayer coupling and twist angle. New properties emerge from interlayer orbital interactions and charge redistribution, further modulated by the interlayer atomic registry and moiré superlattice. This talk shows experimental and theoretical indications of a real-space, non-atomic lattice formed by significant charge redistribution in vertically stacked Bi2Se3/Transition Metal Dichalcogenide (TMD) 2D heterostructures. High-energy (200keV) selected area electron diffraction (SAED) patterns correspond excellently with simulations from the moiré superlattices, suggesting substantial charge redistribution at sites of high interlayer atomic registry. Density functional theory (DFT) predicts concentrated charge pools reside in the interlayer region, located at sites of high nearest-neighbor atomic registry, suggesting the non-atomic lattices are standalone, reside in the interlayer region, and are purely electronic. |
Thursday, March 5, 2020 9:48AM - 10:00AM |
R53.00010: Investigation of interfacial charge transfer in hybrid system of Graphene-MoS2 Sajedeh Pourianejad, Frederick Aryeetey, Olubunmi Ayodele, Shyam Aravamudhan, Tetyana Ignatova Heterostructures of two dimensional layered materials have exciting optical properties that encourage scientists to further study of their photodetector applications. Interactions and, in particular, charge transfer between the constituting layers plays a critical role in the electronic and optical properties. Heterostructure of a few layers of graphene/MoS2 has been demonstrated as a promising candidate for diverse unique optoelectronic devices owing to the high transparency of graphene, the tunability of its Fermi level and, the magnificent optical properties of MoS2. Here, the magnitude and the direction (i.e. electron or hole) of the charge transfer was investigated in a graphene/MoS2 heterostructure. Confocal Raman microscopy and photoluminescence were used to study doping and strain configuration. The surface morphologies of MoS2 and graphene were investigated by scanning electron microscopy. To correlate work function difference to charge-related phenomena, KPFM and EFM were carried out. We speculate that the tunable Fermi level in graphene allows excellent work-function match with MoS2, resulting in low contact resistance. |
Thursday, March 5, 2020 10:00AM - 10:12AM |
R53.00011: Trion light emission in type-I WSe2/MoTe2 van der Waals heterostructures Hyemin Bae, SUK HYUN KIM, Seungmin Lee, Ouri Karni, Aidan O'Beirne, Elyse Barré, Sangwan Sim, Tony F Heinz, Hyunyong Choi Two-dimensional (2D) transition metal dichalcogenides (TMDs) are attractive materials to investigate the light-induced many-body excitons. The reduced Coulomb screening leads to a large exciton binding energy, and stacking two or more dissimilar TMDs provides easy way to build van der Waals (vdW) heterostructures. With this features TMDs enables us to study a variety body of excitons such as neutral exciton, charged excitons, type-II interlayer excitons, moiré excitons, and localized excitons. Here we perform photoluminescence (PL), reflection contrast (RC) and photoluminescence excitation (PLE) to investigate vdW type-I heterostructures. We report the spectroscopic identification in type-I heterostructure of trions, a quasiparticle composed of two electrons and a hole. In the heterostructure spectra, we observe a significant enhancement of the MoTe2 A trion PL emission under the excitation in resonance with the WSe2 trion resonances. The enhancement is interpreted as a transfer effect arising from trions initially created in either layer. We suggest that the trions generated in WSe2 have a great influence on the MoTe2 trion emission. Our results show possibilities for fundamental studies of many-body interactions in 2D TMDs heterostructures. |
Thursday, March 5, 2020 10:12AM - 10:24AM |
R53.00012: The bright side of defects in MoS2 and WS2 and a generalizable chemical treatment protocol for defect passivation Hope Bretscher, Zhaojun Li, James Xiao, Diana Qiu, Sivan Refaely-Abramson, Jack Alexander-Webber, Arelo Tanoh, Ye Fan, Geraud Delport, Cyan Williams, Silvia Vignolini, Sam Stranks, Stephan Hofmann, Steven Louie, Jeffrey Neaton, Akshay Rao Defects in transition metal dichalcogenides, such as MoS2 and WS2, are frequently considered responsible for quenching photoluminescence (PL) and lowering mobility, limiting many of the proposed applications. However, while many chemical treatments have been proposed to passivate defects, primarily assumed to be sulfur vacancies, the mechanism of this passivation is poorly understood. In this work, we illustrate how TFSI superacid treatment reveals an optical subgap state associated with sulfur vacancies. At room temperature, this subgap state contributes to enhanced quantum yields and lengthened emission lifetimes, compared to untreated samples, rather than quenching PL. Building on this understanding, we propose a generalizable treatment protocol to passivate defects in monolayer MoS2 and WS2, increasing photoluminescence and maintaining mobilities. This protocol opens up a route for solution-based, post-processing of samples, which could not only passivate defects, but also simultaneously tune properties and functionalize materials. |
Thursday, March 5, 2020 10:24AM - 10:36AM |
R53.00013: Probing the nonlinear response of strongly coupled plasmon-WSe2 system Chentao Li, Xin Lu, Ajit Srivastava, Hayk Harutyunyan Plasmonic nanostructures have been previously shown to facilitate strong light-matter interactions in a wide variety of systems. With the recent progress in our understanding of 2D transition metal dichalcogenides (TMDs), studies on plasmon-exciton coupling in these systems have increasingly attracted considerable attention, demonstrating evidence of strong light-matter coupling in scattering spectra. However, more convincing evidences of strong coupling are still needed, including probing in nonlinear optical interactions, to give a direct access for studying near-field properties of the coupled system. |
Thursday, March 5, 2020 10:36AM - 10:48AM |
R53.00014: Homogeneous Broadening of Excitons in Bilayer MoSe2 Kevin Sampson, Sophia Helmrich, Carter Young, Nina Owschimikow, Jacob Embley, Kai Hao, Ulrike Woggon, Xiaoqin (Elaine) Li In transition metal dichalcogenides (TMDs), the electronic band structure shifts dramatically when the thickness of a sample is reduced from bilayer to monolayer, producing a concurrent change in their optical properties. Exciton resonances form at the K points on the Brillouin zone boundary in both the monolayers and bilayers, however, we find that their quantum dynamics differ drastically. We perform two-dimensional electronic coherent spectroscopy (2DECS) on a MoSe2 monolayer and bilayer to investigate the quantum decoherence (homogeneous linewidth) of various exciton resonances. We first identify the A-exciton and trion resonances in the monolayer. The bilayer A-exciton is shifted to lower energy, consistent with previous studies. Despite the presence of an inhomogeneous broadening, we were able to extract the homogeneous linewidth using 2DECS. Most notably, the homogeneous linewidth of the bilayer exciton exceeds that of the monolayer exciton and trion significantly. Because the band structure of a MoSe2 bilayer evolves to an indirect bandgap, we expect that scattering with additional interlayer phonon modes would lead to additional dephasing, consistent with the increased homogeneous linewidths observed. |
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R53.00015: Electron relaxation and motion at coplanar 1T'/2H MoTe2 homojunction imaged by time-resolved photoemission electron microscopy Aiqin Hu, Xiaolong Xu, Wei Liu, Quan Sun, Qihuang Gong, Yu Ye, Guowei Lu One critical aspect of optoelectronic devices, such as transistors, diodes, and solar cells, is the internal motion of electrons through the interface between semiconductor and electrode. High spatial and temporal resolution imaging the movement of these electrons would provide unprecedented insight into this critical phenomenon. Here, employing the femtosecond time-resolved photoemission electron microscopy with an energy analyzer, we demonstrate imaging of carrier dynamics in space and time after photoexcitation at 1T'/2H MoTe2 coplanar homojunction, a seamlessly contacted metal/semiconductor interface. The energy-resolved photoelectron images revealed a highly non-equilibrium distribution of photocarriers in space and energy. Combing the time-resolved images, we visualize the motion of electrons from metallic 1T′-MoTe2 to semiconducting 2H-MoTe2 within about 1 picosecond. And the carriers lifetime and transfer rate can be obtained from the modified rate equations set for the homojunction. Our findings provide a new insight for the in-plane semiconductor-metal heterostructure, and it will aid the development of future high-performance devices based on 2D materials. |
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