Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session A55: Optical Spectroscopy of Transition Metal DichalcogenidesRecordings Available
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Sponsoring Units: DCMP Chair: John Philbin, Harvard Univeristy Room: Hyatt Regency Hotel -Adler |
Monday, March 14, 2022 8:00AM - 8:12AM |
A55.00001: Creating and controlling excitons with structured light in monolayer transition metal dichalcogenides Omadillo Abdurazakov, Yun-Pil Shim Monolayer transition metal dichalcogenide materials host a plethora of quantum many-body phenomena which can be tamed with laser beams for their potential applications in quantum electronics as well as quantum computing devices. In particular, structured light with nontrivial helicity endows an additional knob with which one can selectively populate and control the valley degrees of freedom in a realm dominated by the physics of electron-hole pairs or excitons. Using an effective two-band model coupled to structured light beams, we present theoretical results on the dynamics of long-lived intervalley excitions and their interaction with phonons. |
Monday, March 14, 2022 8:12AM - 8:24AM |
A55.00002: Defect and strain effects on photoluminescence and lifetime of monolayer WS2 and WSe2 Qifan Li, Wei Wang, Qingkai Qian, Wei Zhang, Vicente Torres-Costa, Raúl J Martín-Palma, Xuedan Ma, Shengxi Huang Defect and strain engineering is critical for tailoring the electronic and optical properties of two-dimensional semiconductors for optoelectronics and quantum photonics applications. In this work, atomic defects are created in monolayer WS2 and WSe2 using various particle irradiation, including electron, proton, and helium ion. Strain perturbations are introduced by nanopillars patterned on silica substrates. The effects of defects and strain on photoluminescence (PL) and its dynamics are comprehensively investigated at room and cryogenic temperatures. Irradiation of heavy particles or high dose generates high density of defects, where the low-energy defect-bound excitons exhibit long recombination lifetime of several hundred nanoseconds at cryogenic temperatures. Moreover, the power dependence of the defect-bound exciton and localized exciton can be modulated by irradiation dose and particle. Redshifted PL spectra with a 6-21% enhancement in the trion intensity and an increase in the long decay component of lifetime are observed at the strained regions, indicating an efficient funneling effect. The results indicate that irradiation-induced defects and site-controlled strain are effective and highly beneficial for controllable and high-efficiency quantum optical applications. |
Monday, March 14, 2022 8:24AM - 8:36AM |
A55.00003: Tailoring the Excitonic Features and Valley Polarization in Monolayer MoS2 by Molecule Adsorption Dan Wang, Diana Y Qiu Two-dimensional (2D) materials are the subject of significant ongoing research for exploring exciton physics and device applications. One of the most promising classes of 2D materials, monolayer transition metal dichalcogenides (TMDs), features strong excitonic emission and the locking of the valley and spin degrees of freedom, leading to the selective excitation of states in different valleys by left- and right-hand circularly polarized light. These unique properties make such materials desirable for optical manipulation and enable their application in valleytronics. Here, we explore the modulation of the excitons and valley-selective circular dichroism in monolayer TMDs by proximity effects from the adsorption of chiral molecules, helicene, using ab initio GW and Bethe-Salpeter equation calculations of quasiparticle energy levels and the optical spectra. Our results suggest a pathway for manipulating the valley degree of freedom in 2D materials for valleytronics via molecular functionalization. |
Monday, March 14, 2022 8:36AM - 8:48AM |
A55.00004: Gate-tunable interlayer excitons in WSe2/WS2/WSe2 heterotrilayers Kateryna Pistunova, Leo Yu, Jenny Hu, Kenji Watanabe, Takashi Taniguchi, Tony F Heinz Atomically-thin semiconductors can be stacked with heterogeneous material combinations, enabling the formation of so-called interlayer excitons, where electrons and holes reside in separate layers in systems with staggered band alignment. Such interlayer excitons feature large Stark shifts in photoluminescence energy. However, interlayer excitons have so far been mostly studied in heterobilayers. Here, we investigate interlayer exictons in heterotrilayers, with the material combination of WSe2/WS2/WSe2 and with dual-gated structures. Precise angle alignment between the outer WSe2 layers was achieved via combining two half layers cut from the same original flake, whereas that between WSe2 and WS2 layers achieved via second harmonic generation measurement. From the spatial regions of upper and lower heterobilayers, we observed Stark shifts of opposite slopes, confirming contrasting polarity and thus orientation of interlayer excitons. In the heterotrilayer region, we study photoluminescence, its dependence on gating fields, and the relation of the observed behavior to that seen in the heterobilayer system. We further probe the nature of interactions of the interlayer excitons through study of dependence of the photoluminescence on the laser excitation power and, hence, exciton density. |
Monday, March 14, 2022 8:48AM - 9:00AM |
A55.00005: Temperature dependent interlayer exciton diffusion in a WSe2/WS2 moiré superlattice Antonio Rossi, Jonas Zipfel, Emma Regan, Daria Blach, Luca Francaviglia, Monica Lorenzon, Edward S Barnard, Eli Rotenberg, feng wang, Archana Raja, Alexander Weber-Bargioni The moiré potential arising from the relative twist and lattice mismatch in heterobilayers of two-dimensional materials has led to the discovery of novel excitonic species. Twist-angle dependent transport measurements of these interlayer excitons have been reported for WSe2/WS2 heterobilayers [1]. In our experiment, we use time- and spatially resolved spectroscopies to investigate the temperature dependent lifetime and diffusion length of various interlayer excitons in WSe2/WS2 heterobilayers, where the individual monolayers have been aligned to produce long-range moiré superlattices. We observe that the interlayer exciton diffusion length increases with temperature till it undergoes a phonon-driven transition where the intralayer exciton starts to dominate the photoluminescence spectrum. The temperature dependent formation of the interlayer exciton is reflected in the overall lifetime and diffusion lengths of exciton species in the WSe2/WS2 heterobilayer. |
Monday, March 14, 2022 9:00AM - 9:12AM |
A55.00006: A Charge Density Wave Activated Exciton in TiSe2-MoSe2 Heterostructures Jaydeep Joshi, Benedikt Scharf, Igor Mazin, Sergiy Krylyuk, Daniel J Campbell, Johnpierre Paglione, Albert Davydov, Igor Zutic, Patrick M Vora Van der Waals heterostructures allow for the exploration of interface physics without the constraint of lattice matching. In this study we examine proximity effects in a vdW heterostructure comprised of TiSe2 and monolayer MoSe2. Using temperature-dependent, spatially-resolved photoluminescence (PL) microscopy we discover a new MoSe2 PL emission line at the TiSe2-MoSe2 interface. This PL line appears 30 meV above the MoSe2 neutral exciton which excludes localized or trapped excitons as a viable explanation. Temperature-dependent measurements show a surprising correlation of the high energy PL line with the TiSe2 CDW state, thus suggesting the CDW plays a central role in activating this previously unobserved exciton. We discuss possible CDW-based origins of this feature and outline future opportunities for using proximity effects in vdW heterostructures to engineer novel excitonic states. |
Monday, March 14, 2022 9:12AM - 9:24AM |
A55.00007: Bright and dark excitons in 1L WSe2: a time-dependent density-functional theory analysis Jia Shi, Volodymyr Turkowski, Duy Le, Talat S Rahman Intra-valley bright and intra- and inter-valley dark excitons play an important role in the ultrafast dynamics and photoluminescence (PL) properties of 2D transition metal dichalcogenides (TMDs). Their predictive modeling relies on a microscopic understanding of their properties using accurate ab initio tools. We present results of time-dependent density-functional theory study of the excitonic properties of single layer (1L) WSe2 obtained with local and long-range exchange-correlation kernels. The nanoquanta kernel is found to provide the best agreement for exciton binding energies with those obtained from solutions of the Bethe-Salpeter equation and experimental data. From calculated transition contribution map we establish the weight of different direct electron-hole excitations (defined by the momentum of the excited electron) in each excitonic state. We also analyze possible hybridized excitonic states and estimate the lifetime of each exciton, including effects of brightening of dark excitons that significantly affect the PL spectrum. Obtained results may help to better understand physical properties of 1L WSe2, one of the most actively studied TMDs. |
Monday, March 14, 2022 9:24AM - 9:36AM |
A55.00008: Nonlinear Response of Trion-Polaritons in Two-Dimensional Materials Kok Wee Song, Salvatore Chiavazzo, Oleksandr Kyriienko The emergence of strongly bound composite particles of electrons (e) and holes (h) in 2D materials gives rise to many remarkable optical properties. In particular, the strong interactions between these composite particles are essential for the material’s nonlinear optical response. In contrast to excitons (e-h), trions (e-e-h) have more extended wavefunction, potentially leading to the increased interaction between these quasiparticles. When coupled to light in a microcavity, trions hybridize with cavity photons and form polaritonic states. The trion-trion scattering thus promotes strong effective nonlinearity for photons and facilitates nonlinear optical effects at the few-photon level. To investigate these effects, we calculate trion wavefunctions by solving the 3-body Wannier equation using a rich Gaussian-type basis. With this solution, we examine the effects of the direct and exchange scattering processes on the nonlinear response of trion-polaritons in the microcavity. The developed theory represents the first study of trion nonlinearity due to Coulomb processes and is directly applicable for describing the trion-polaritons in monolayers of transition metal dichalcogenides. |
Monday, March 14, 2022 9:36AM - 9:48AM |
A55.00009: Nonlinear, Ultra-fast photocurrent generation using charged biexciton Sarthak Das, Suman Chatterjee, Garima Gupta, Kausik Majumdar Monolayer transition metal dichalcogenides host several many-body systems under suitable excitation and doping condition. We probe the formation of charged biexciton, a five-particle state in monolayer WS2, through the photoluminescence excitation spectroscopy, and demonstrate that the formation is favoured when the excitation laser is in resonance with the neutral exciton line. We further probe the generation by using a dual-wavelength excitation, demonstrating a favoured generation when the two excitations match with the exciton and trion lines. Using a few-layer graphene capped WS2 sample, we demonstrate a photocurrent generation mechanism that is highly nonlinear in nature. Further, we show that the lifetime of the charged biexciton reduces to sub-5 ps due to graphene capping, suggesting an ultra-fast photocurrent generation rate. |
Monday, March 14, 2022 9:48AM - 10:00AM |
A55.00010: Ultrafast dynamics of Rydberg excitons in monolayer WSe2 Chiara Trovatello, Giulio Cerullo, Stefano Dal Conte, Andrea C Ferrari, Armando Genco, James Andrew Kerfoot, Tanweer Ahmed, Oscar Balci, Evgeny Alexeev The extreme confinement in 2D Transition Metal Dichalcogenides (TMDs) leads to the appearance of excitons with high binding energies (up to 0.5 eV). Low temperature and encapsulation in hBN narrow the exciton linewidths and reveal the full hydrogen-like exciton Rydberg series below the free particle gap [Phys. Rev. Lett. 113, 076802, 2014]. Rydberg excitons can also couple to free charges in the TMD, forming trions. Recently, ultrafast spectroscopy has been extensively used to explore exciton formation and relaxation dynamics of the lowest excitonic state (1s). Here we use pump-probe optical microscopy to measure the ultrafast dynamics of the 1s- and 2s-excitons and their relative trions in hBN-encapsulated monolayer WSe2. We clearly observe a faster formation of the 2s-exciton over the 1s state and different relaxation times. The difference in the formation timescale is the result of the exciton cascade process upon carrier photoexcitation at high-energy [Nat. Commun. 11, 5277, 2020], while longer decay dynamics for 2s excitons can be ascribed to a reduced exciton–phonon scattering due to their higher Bohr radius [Nanoscale, 11, 12381, 2019]. Our results show novel insights on the many-body physics of TMDs, opening up interesting opportunities for exploring Rydberg excitons for future opto-electronic applications. |
Monday, March 14, 2022 10:00AM - 10:12AM |
A55.00011: Electronic properties and inter- and intralayer excitons in MoSe2 / WSe2 heterostructure Katarzyna Sadecka, Maciej Bieniek, Arkadiusz Wojs, Pawel Hawrylak We describe the electronic and optical properties of MoSe2 / WSe2 heterostructure using a combination of density functional theory, tight-binding (TB) approximation and Bethe-Salpeter equation (BSE) for excitons. We start with determining the electronic structure of MoSe2 / WSe2 from first principles, using the PBE parametrization of the GGA for the exchange-correlation potentials and including spin-orbit interaction. We obtain type-II band alignment and conduction band minima at Q points in agreement with previous work. We include analysis of Kohn-Sham wavefunctions allowing to describe the leading layer and spin contribution for a given band. Following our previous work on monolayers we construct the ab initio based TB model [1] for MoSe2 / WSe2 heterostructure, which allows to understand orbital contributions to Bloch states and study of both twisted transition metal dichalcogenide (TMD) heterostructures and TMD-based multi-million atom nanostructures. To validate the model we compute the exciton spectrum using dielectrically modified Keldysz electron-hole interaction. We accurately solve BSE and determine the exciton fine structure due to type-II spin-split band arrangement and topological moments, considering both A/B, spin bright/dark and intra-/interlayer exciton series. |
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