Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session L56: 2D Semiconductors: Heterostructures and Moire SuperlatticesFocus Live
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Sponsoring Units: DMP Chair: Patrick Vora, George Mason University |
Wednesday, March 17, 2021 8:00AM - 8:12AM Live |
L56.00001: Charge Transfer of Twist-Angle-Dependent Phosphorene-Graphene Heterobilayers. A first-principles investigation El-Abed Haidar, Sherif Abdulkader Tawfik Abbas, Catherine Stampfl Twistronics is the study of the changes to the electronic properties of two-dimensional material bilayers due to rotational stacking, or twisting. |
Wednesday, March 17, 2021 8:12AM - 8:24AM Live |
L56.00002: Mechanical Response of Graphene/BN heterostructures Lokanath Patra, Govind Mallick, geeta sachdeva, Ravindra Pandey
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Wednesday, March 17, 2021 8:24AM - 8:36AM Live |
L56.00003: Moiré Exciton Condensates in Twisted Transition Metal Dichalcogenide Bilayers Benjamin Remez, Nigel R Cooper Exciton gases have long been considered an attractive platform for exploring Bose-Einstein condensates (BECs) in the solid state, due to their strong interactions and small mass. Transition metal dichalcogenide (TMD) heterobilayers are promising systems for realizing exciton BECs, thanks to the long-lived, tightly bound interlayer excitons which they can host. Introducing a twist between the layers produces a moiré superlattice, in which the modulated bandgap acts as an energy landscape that localizes excitons to the superlattice sites. We study a Bose-Hubbard model which could describe these systems, and derive the properties of a novel exciton BEC phase that is governed by the interplay of underlying TMD optoelectronic properties: A coexistence of two exciton species corresponding to each of the TMD electronic valleys, distinct optical selection rules, and a moiré momentum mismatch. While the latter renders the condensed excitons optically inactive, we find that interactions generally enable emission to leak from the condensate. Furthermore, excitonic coherence results in a charge-density wave which can lead to interesting interplay of the two components. |
Wednesday, March 17, 2021 8:36AM - 8:48AM Live |
L56.00004: Flatbands in transition-metal dichalocogenides – when and why do we have them? Sumanti Patra, Poonam Kumari, Priya Mahadevan We have examined the evolution of the electronic structure in twisted bilayers of MoSe2 within ab-initio electronic structure calculations, assuming the moire potential to be a small perturbation to the untwisted limit. Its role in modifying the electronic structure is probed by mapping the calculated band structure for the moire cell onto the primitive cell direction which represents the untwisted limit. At large twist angles such as 19.03o, we find that the moire cell band structure is identical to the primitive cell one in the low energy window. This allows certain simplifications of the electronic structure of the twisted bilayers at large angles [1]. There are however significant deviations for small twist angles such as 3.48o which have large patches of high symmetry regions of AA and AB' stackings. These lead to enhanced interlayer hopping interaction strengths in some regions, and hence stronger perturbations leading to sub-band formation of the highest occupied band which has a bandwidth of 19 meV, and is found to be localized both in real space as well as momentum space. |
Wednesday, March 17, 2021 8:48AM - 9:00AM Live |
L56.00005: Resolving the Interlayer electron and hole Transfer processes in a Large-area WS2/MoS2 Heterostructure by Ultrafast Two-dimensional Electronic Spectroscopy Veronica Policht, Mattia Russo, Fang Liu, Chiara Trovatello, Margherita Maiuri, Stefano Dal Conte, Xiaoyang Zhu, Giulio Cerullo Transition metal dichalcogenide heterostructures (TMD HS) with type II band alignment feature an interlayer exciton formed following interlayer charge transfer (ICT)1. Interlayer excitons in TMD HS are of great general interest due enticing properties with a variety of device applications, however it is still necessary to carefully characterize the timescales and efficiencies of ICT. To determine the dynamics of ICT with simultaneously high spectral and temporal resolution, we perform two-dimensional electronic spectroscopy (2DES) on a large-area unaligned WS2/MoS2 HS at room temperature. Utilizing broadband pulses spanning the main excitonic resonances of the HS with a sub-30 fs instrument response function time, we simultaneously measure the interlayer hole transfer (IHT) and the interlayer electron transfer (IET) processes by resolving the build-up dynamics of interlayer cross peaks in the 2DES maps2. We find that both processes are extremely fast and occur on timescale below 100 fs. This work demonstrates the utility of broadband 2DES in studying charge transfer dynamics in TMD HS and serves as a first step in carefully characterizing the dynamics of ICT in TMD HS using 2DES. |
Wednesday, March 17, 2021 9:00AM - 9:12AM Live |
L56.00006: Twist-Angle Dependent Interlayer Exciton Emission in MoSe2-WSe2 Heterostructures Fateme Mahdikhanysarvejahany, Michael R Koehler, David George Mandrus, Takashi Taniguchi, Kenji Watanabe, Oliver L.A. Monti, Brian J LeRoy, John Schaibley Two-dimension materials can be stacked together to realize van der Waals heterostructures that host novel optical and electronic properties. By stacking two different transition metal dichalcogenide semiconductors together, interlayer excitons (IXs) can be realized, which are composed of electrons and holes located in different layers. Recent studies in MoSe2-WSe2 heterostructures have shown bright IX photoluminescence (PL) when the twist-angle between layers is close to 0 (R-type stacking) or 60 (H-type stacking). Previously it has been shown that R and H type heterostructures exhibit twist-angle dependent polarization properties due to the valley dependent optical selection rules. In this presentation, I will discuss several significant differences between the optical response of R-type and H-type hBN encapsulated MoSe2-WSe2 heterostructures.I will show that the two types of heterostructures (R and H) exhibit qualitatively different dependencies of PL intensity as a function of temperature and excitation power. Time resolved PL measurements further distinguish R and H type IXs, where the latter exhibits a density dependent lifetime which we attribute to intervalley scattering. |
Wednesday, March 17, 2021 9:12AM - 9:24AM Live |
L56.00007: Crystal quality control in 2D TMD structures using polarization-resolved SHG George Kioseoglou, G. Maragkakis, S. Psilodimitrakopoulos 2D transition metal dichalcogenides (TMDs) offer a unique platform for fundamental science and technological applications. They can be assembled in vertically stacked heterostructures with new physical properties that depend on the relative orientation (twist-angle) between the constituent monolayers. While large-area crystal growth techniques such as CVD are successfully used to produce 2D TMDs, the presence of grain boundaries and defects affect their crystal quality. Here, we demonstrate polarization-resolved second harmonic generation (PSHG) microscopy as an all-optical, high-resolution and minimally-invasive technique for the quality control of TMD-based structures. By fitting, pixel-by-pixel, experimental PSHG images of sub-micron resolution into suitable theoretical models, we can spatially-resolved map the main crystallographic axis, as well as the twist-angle in the overlapping region of TMD van der Waals superlattices. By measuring the mean of the corresponding distributions, we define the standard deviation as crystal quality factor. |
Wednesday, March 17, 2021 9:24AM - 9:36AM Live |
L56.00008: Tunable electron-phonon interactions in moire graphene Hiroaki Ishizuka, Ali Fahimniya, Francisco Guinea, Leonid Levitov Understanding strong interactions in moire bands is key for advancing this versatile and highly tunable platform to study strongly-correlated electron physics. While strong electron-electron interactions arise naturally in flat bands, the enhanced electron-phonon interactions, evidenced by many exotic behaviors in moire graphene, have remained a mystery. Drawing inspiration from the Purcell effect in quantum optics, we argue that well-localized Wannier orbitals boost the electron-phonon interactions. Reshaping of Wannier orbitals is a prominent effect in graphene moire superlattices where the orbitals are tunable by the twist angle. Reducing the orbital effective volume leads to an enhancement in the effective el-ph coupling strength, yielding values considerably larger than those known for monolayer graphene. The enhanced coupling boosts the el-ph scattering rates, pushing them above the values predicted from the enhanced spectral density of electronic excitations. The enhanced electron-phonon interaction manifests in the observables such as the electron-lattice cooling and in the resistivity, both of which show an enhancement as large as tenfold. Strengthening interactions by the reshaped Wannier orbitals provides unique means to control the properties of moire bands. |
Wednesday, March 17, 2021 9:36AM - 10:12AM Live |
L56.00009: Twist angle control of excited states in transition metal dichalcogenide bilayers Invited Speaker: Xiaoqin (Elaine) Li A new type of superlattice, known as the moiré superlattice, form when two monolayers of van der Waals materials are vertically stacked. A periodic energy modulation, as well as distinct optical selection rules, gives rise to rich optical properties. In this talk, I will discuss how the twist angle controls the exciton resonances, lifetimes, diffusion in transition metal dichalcogenide heterobilayers. It is possible to control the twist angle with a higher degree of accuracy in homobilayers, we show that lattice reconstruction occurs over a rather large range, as manifested in Raman spectra. |
Wednesday, March 17, 2021 10:12AM - 10:24AM Live |
L56.00010: Moiré Structure and Band Alignment in Gate-Tunable WSe2/MoSe2 Heterostructures Anna Roche, Rachel Myers, Takashi Taniguchi, Kenji Watanabe, Brian J LeRoy Monolayer transition metal dichalcogenide (e.g., MoSe2, WSe2, MoS2, WS2) semiconductors have received massive attention since they were discovered to form moiré superlattices. When stacked, a ~0.1% lattice mismatch causes periodic modulation of the bands resulting in trapped moiré excitons in the potential minima. Twist angles near 0 and 60 degrees create moiré flat bands and the interplay of these bands with long-range Coulomb interactions is predicted to create novel correlated quantum phenomena. To date, there is limited work exploring moiré physics in MoSe2/WSe2 heterostructures and the investigations show conflicting results. Here we use fabricated graphene nanoribbons placed underneath the heterostructure as contact electrodes and a silicon substrate as a back gate to control the device carrier density. We probe a WSe2/MoSe2 heterostructure near a 0-degree twist angle using scanning tunnel microscopy and spectroscopy (STM/S) and report changes of the moiré structure and band alignment with carrier density. |
Wednesday, March 17, 2021 10:24AM - 10:36AM Live |
L56.00011: MoS2 and MoSSe Based van der Waals Heterostructures as an Emerging Direct Z-scheme Photocatalysts Arunima Singh, Saswata Bhattacharya MoS2 monolayer, a prototypical transition metal dichalcogenide (TMD), with high carrier mobility and apt optical properties have often been used in photocatalysis. Janus (MoSSe) too follows the same. However, their usage is limited by the carrier (e--h+) recombination, making it imperative to examine their bilayer van der Waals heterostructure (vdW HTS), where the spatial separation of e--h+ on two layers reduce recombination. The design of bilayer vdW HTSs here, consists of TMDs (HS2, ZrS2, TiS2, WS2) and transition metal oxides (TMOs) (HfO2, T-SnO2, T-PtO2). The present work displays curiosity in their Z-scheme photocatalytic capability, where faster interlayer e--h+ recombination is required for efficient charge separation. We have performed first-principles based calculations under the framework of (hybrid) density functional theory (DFT) and many-body perturbation theory (GW approximation), to obtain the band edge levels and optical spectra, respectively. The comparative study of vdW HTSs and the constituent monolayers, have been undertaken by analyzing their electrostatic potential, work function and e--h+ recombination. Finally, with the promising optical response, we find MoSSe/HfS2, MoSSe/TiS2, MoS2 /T-SnO2, MoS2 /ZrS2 and MoSSe/ZrS2 as probable Z-scheme photocatalysts. |
Wednesday, March 17, 2021 10:36AM - 10:48AM Live |
L56.00012: Observation of strain transfer in vertical van-der-Waals heterostructures Jenny Hu, Leo Yu, Tony Heinz Strain is a powerful method to tune the properties of 2D materials. It has been widely applied both to study 2D monolayers and to modify their properties. The same approach also has great potential for van-der-Waals (vdW) heterostructures. To this end, however, we need to understand how strain can be applied to vertically stacked vdW structures, for which strain transfer from one layer to the next remains poorly understood. |
Wednesday, March 17, 2021 10:48AM - 11:00AM Live |
L56.00013: Band structure calculations of van der Waals heterostructures Georgios Vailakis, Georgios Kopidakis Layer by layer stacking of two-dimensional materials gives rise to “van der Waals” (vdW) heterostructures of nanometer thickness and clean interfaces. These systems often exhibit extraordinary properties and present novel challenges for theory. Superconductivity of twisted bilayer graphene at the magic angle, interlayer excitons in transition metal dichalcogenide (TMD) heterostructures, and optoelectronic properties of TMD/graphene heterostructures, are examples, among others, where vdW heterostructures significantly differ from their monolayer constituents. We present density functional theory (DFT) results for the atomic and electronic structure of vdW heterostructures consisting of combinations of TMD monolayers and graphene. Due to the large size of the supercells used, DFT calculations are demanding and need careful interpretation. We have developed a methodology for unfolding and analyzing their electronic band structure. Interlayer interactions have been carefully included. Our results show that composition of monolayers, twist angles, and stress in the heterostructures have a strong influence on their observable optoelectronic properties. |
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