Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session T12: Optical Spectroscopic Measurements of 2D Materials IIRecordings Available
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Sponsoring Units: FIAP Chair: Jordan Hachtel, Oak Ridge National Lab Room: McCormick Place W-181C |
Thursday, March 17, 2022 11:30AM - 11:42AM |
T12.00001: Inelastic light scattering tuned by correlated states in moiré superlattices Heonjoon Park, William G Holtzmann, John Cenker, Feng-ren Fan, Xi Wang, Di Xiao, Wang Yao, Xiaodong Xu Moiré superlattices formed from twisted transition metal dichalcogenides (TMDCs) have been established as a promising platform to probe strongly correlated physics due to the presence of flat moiré minibands which generate strong Coulomb interactions. In addition, these systems can host significant lattice reconstruction and zone folding which can modify the phonon band structure. Here we report emergent inelastic scattering in dual-gated WS2/WSe2 heterobilayers. Below the melting temperature of the charge ordered states determined by reflection and photoluminescence measurements, we observe additional Raman peaks which are absent in the monolayer spectra. In addition to the strong temperature dependence, these features can be controlled through both doping and electric field. Our study highlights the delicate interplay between correlated electrons and lattice vibrations in moiré systems. |
Thursday, March 17, 2022 11:42AM - 11:54AM |
T12.00002: Layer Hybridized Excitons in Bilayer and Trilayer 2H-MoSe2 Shun Feng, Aidan J Campbell, Mauro Brotons-Gisbert, Iann C Gerber, Bernhard Urbaszek, Brian D Gerardot In bilayer transition metal dichalcogenide (TMD) semiconductors with 2H stacking configuration, the hybridized states between intra- and inter-layer excitons can benefit from both strong oscillator strength and high energy tunability. Such hybridized states remain to be explored in some TMD systems, such as bilayer 2H-MoSe2. Here, using broadband reflectance contrast spectroscopy, we investigate the electric-field dependence of excitonic transitions in a dual-gated bilayer 2H-MoSe2 device. We identify the interlayer exciton transition by its large Stark shift (up to 150 meV), which corresponds to an effective static dipole moment of 0.4 enm. In addition, we report the electric-field-driven coupling between the excited intralayer A2s exciton and the interlayer exciton, featuring an avoided crossing and the corresponding redistribution of oscillator strengths between the different exciton states. We further conducted magneto-reflectance measurements and observe a hybridization driven evolution in the g-factor of the A2s exciton from -4 to 14 as its energy detuning from the interlayer exciton is changed by the applied vertical electric field. The observed spectral evolution and g-factor change can be described with a phenomenological model in which the hybridization between the different exciton states is treated as a coupling between oscillators with spin-selective couplings. Finally, we study the electric-field dependence of excitonic transitions in natural trilayer 2H-MoSe2. Strikingly, we observed an additional interlayer exciton branch with a giant effective dipole moment of 0.77 enm. Our results provide insights for exciton-exciton interaction in TMDs and promote natural bilayer and trilayer MoSe2 as highly tunable system to be further integrated into cavities for enhanced light matter interactions. |
Thursday, March 17, 2022 11:54AM - 12:06PM |
T12.00003: Charge control of excitons in a novel Janus TMD 2D-material device Matthew Feuer, Alejandro Montblanch, Mohammed Sayyad, James Kerfoot, Andrea C Ferrari, Sefaattin Tongay, Dhiren Kara, Mete Atature Janus transition metal dichalcogenides (TMDs) have different chalcogens on the two surfaces of the monolayer, which creates an inherent out-of-plane asymmetry. This provides an inbuilt electric field within a single monolayer and therefore holds the promise of exploiting the long-range interactions between dipolar excitons, without the complexity of building TMD heterostructures. Recent work has shown the synthesis of Janus TMDs, but the optical quality has hindered the identification of the excitonic states. In this talk, I will present our work on the first integration of a Janus WSeS monolayer into a fully encapsulated, charge-controllable device. Encapsulation in hexagonal boron nitride results in the narrowest photoluminescence linewidths reported to date (< 10 meV at 4 K), while gate-dependent reflection contrast measurements allow for the assignment of the neutral exciton and negatively charged trions. We measure a similar exchange energy splitting (7 meV) between the inter- and intra-valley negative trions as for conventional TMDs and magnetic field dependent measurements show that the exciton g-factors agree with our theoretical predictions. These results demonstrate that Janus TMDs are suitable for integration into optoelectronic devices and quantum optics applications. |
Thursday, March 17, 2022 12:06PM - 12:18PM |
T12.00004: Excitonic transport driven by repulsive dipolar interaction in a van der Waals heterostructure ZHE SUN Dipolar bosonic gases are currently the focus of intensive research due to their interesting many-body physics in the quantum regime. Their experimental embodiments range from Rydberg atoms to GaAs double quantum wells and van der Waals heterostructures built from transition metal dichalcogenides. Although quantum gases are very dilute, mutual interactions between particles could lead to exotic many-body phenomena such as Bose-Einstein condensation and high-temperature superfluidity. Here, we report the effect of repulsive dipolar interactions on the dynamics of interlayer excitons in the dilute regime. We image the temporal evolution of an exciton cloud in a van der Waals heterotrilayer, allowing us to reveal repulsive exciton-exciton interactions as the driving force behind exciton transport. This enables direct estimation of the exciton mobility. The presence of interactions significantly modifies the diffusive transport of excitons, effectively enhancing the diffusion coefficient by one order of magnitude. The repulsive dipolar interactions combined with the electrical control of interlayer excitons open up appealing new perspectives for excitonic devices. |
Thursday, March 17, 2022 12:18PM - 12:30PM |
T12.00005: Determination of Twist Angle in Hetero-Bilayer TMDs with Phase-Resolved Second-Harmonic Generation Juseung Oh, Wontaek Kim, Sunmin Ryu Second-harmonic generation (SHG) is a non-linear optical process in which a non-centrosymmetric optical medium interacts with two identical photons to generate another with a doubled frequency. Whereas SHG has been widely used in determining the orientation of various 2D single crystals, it has yet to prove to be applicable to artificially stacked heterocrystals. In this work, we show that the twist angle and orientation of individual layers of hetero-bilayer transition metal dichalcogenides (TMDs) can be directly measured by phase-resolved SHG spectroscopy and imaging. For a complete wave description of SHG signals, the phase and intensity were obtained by spectral phase interferometry for two orthogonally polarized components. For MoS2/WS2 bilayers with negligible interlayer coupling, their twist angle could be determined from the polarized interferometric data and spatially mapped in a raster scanning mode. On the contrary, MoSe2/WS2 bilayers exhibited significant deviation in SHG intensity and phase from what was expected from non-interacting bilayers, and the deviation of each layer could be determined respectively from the data. We will discuss possible causes for these unexpected deviations in SHG behavior. |
Thursday, March 17, 2022 12:30PM - 12:42PM |
T12.00006: Unraveling the Photophysics of Liquid-Phase Exfoliated Two-Dimensional ReS2 Nanoflakes Deepika Poonia, Pieter Schiettecatte, Ivo Tanghe, Sourav Maiti, Michele Failla, Sachin Kinge, Zeger Hens, Laurens Siebbeles, Pieter Geiregat Rhenium disulfide (ReS2) has gained widespread attention in optoelectronic devices such as solar cells, photodetectors, etc., owing to its layer independent optical properties in addition to structural and vibrational anisotropy. For the abovementioned optoelectronic applications, it is crucial to study the generation, mobility, and decay of charge carriers after photoexcitation. In our work, we employed ultrashort laser pulses to photoexcite a thin film of liquid-phase exfoliated ReS2 and studied the time-resolved generation and response of charge carriers. By terahertz conductivity measurements, we successfully detected the presence of free charge carriers despite the high binding energy of excitons in ReS2. During the first few picoseconds, fast decay of free charges due to trapping and/or recombination to excitons was observed. Charges surviving on longer time showed intricate effects on the transient optical spectrum. |
Thursday, March 17, 2022 12:42PM - 12:54PM |
T12.00007: Emission and Reflectance Spectroscopy Study of Excitonic Behaviors in 2D CrCl3 Hyesun Kim, Sunmin Ryu Two-dimensional chromium(III) chloride (CrCl3) offers the opportunity for applications in various fields including optoelectronic devices and magnetic sensors because of its intrinsic magnetism down to monolayer limit and strong visible absorption of its bulk form. In this work, we studied the excitonic behaviors of few-layer to bulk CrCl3. Absorption/photoluminescence (PL) spectroscopy and time-correlated single-photon counting (TCSPC) were used to elucidate its electronic structure and excitonic lifecycle, respectively. There was no significant spectral change with the number of layers in Raman, absorption, and PL signals. Surprisingly, however, the PL lifetime showed a drastic reduction from 1940 ns to 2 ns with decreasing thickness from 61 to 3 nm. Comparison with the samples sandwiched between hexagonal BN confirmed that the ambient stability of the sample affects the excitonic lifetime. The origins for the accelerated relaxation in 2D forms will be discussed based on the control experiments to address the effects of ambient instability and possible photoinduced reactions. |
Thursday, March 17, 2022 12:54PM - 1:06PM |
T12.00008: Long-lived photogenerated carriers in MoS2 flakes chemically exfoliated. FLORIANA MORABITO, Charles Sayers, Alex Barker, Giulia Folpini, Samuele Martani, Peter Topolovsek, Victor Vega-Majoral, Veronica Policht, Irantzu Landa Garcia, Valentino Jadriško, Giulio Cerullo, Valeria Nicolosi, Guglielmo Lanzani, Christoph Gadermaier Two-dimensional (2D) transition metal dichalcogenides (TMDs) such as MoS2 and WS2 are excellent candidates for next generation energy storage and optoelectronics due to their strong light-matter interaction. |
Thursday, March 17, 2022 1:06PM - 1:18PM |
T12.00009: Anisotropic Interlayer Exciton in GeSe/SnS van der Waals Heterostructure NIKHILESH MAITY The observation of interlayer excitons (ILE), where the electron and hole are confined in different layers, in van der Waals (vdW) type II heterostructures has ignited a new interest in investigating the optical properties of these 2D semiconducting materials. Herein, using GW and Bethe−Salpeter equation simulations, we demonstrate the generation of linearly polarized, anisotropic intra- and interlayer excitonic bound states in the transition metal monochalcogenide (TMC) GeSe/SnS vdW heterostructure. The puckered structure of TMC results in the directional anisotropy in the band structure and in the excitonic bound state. Upon the application of compressive/tensile biaxial strain, a dramatic variation in excitonic energies, the indirect-to-direct semiconductor transition, and the red/blue shift of the optical absorption spectrum are observed. The variations in excitonic energies and optical band gap have been attributed to the change in effective dielectric constant and band dispersion upon the application of biaxial strain. The generation and control over the interlayer excitonic energies will find applications in optoelectronics and optical quantum computers and as a gain medium in lasers. |
Thursday, March 17, 2022 1:18PM - 1:30PM |
T12.00010: Exploring excitonic complexes in large angle twisted bilayer WSe2 Shaili Sett, Rahul Debnath, Sudipta Kundu, Rabindra Biswas, Varun Raghunathan, Manish Jain, Akshay Singh, Arindam Ghosh Twisted bilayers (tbl) of 2D transition metal dichalcogenides show emergent electronic and optical features. The relative rotation in between the two layers, modifies the band structure, band alignment of momentum transitions and gives rise to emergent moiré phonons [1]. Here, we combine photoluminescence spectroscopy (PL) and first principles density functional theory analysis to study the interplay of charge, layer coupling and band structure in tbl WSe2 (twist angle > 150) and compare it with the natural bilayer and monolayer [2]. In the tbl, we observe a blue-shift in energy of interlayer excitons, as well as an asymmetric PL spectrum with electrostatic doping, that corroborate well with band structure calculations. The WSe2 layers show robust higher order excitons (negatively charged biexciton or doubly charged trion) at high temperatures in the heavily doped electron regime. This work provides an improved understanding of complex excitonic states in large angle twisted bilayer WSe2. |
Thursday, March 17, 2022 1:30PM - 1:42PM |
T12.00011: The Role of Free Carriers and Defects in Exciton Recombination in Few-layer MoS2 Nanosheets RIYANKA KARMAKAR, Pravrati Taank, K V Adarsh Understanding the carrier recombination of transition metal dichalcogenides (TMDCs) is a fundamental importance in many-body physics and various optoelectronic applications. Although the electronic and optical properties are well understood in TMDCs, much less is known about the role of defects and free carriers in exciton recombination. To this aim here, we investigate the photoexcited carrier recombination mechanism in few-layer (4-6 L) MoS2 nanosheets by employing pump energy and fluence dependent femtosecond transient absorption spectroscopy. We demonstrate that multi-particle (excitons and free carriers) generated by 3.1 eV excitation well above the electronic bandgap exhibit distinct recombination times. For instance, free carriers slow down the recombination by orders of magnitude relative to excitons. In contrast, the recombination time of excitons generated upon near quasi-particle excitation (1.94-2.2 eV) drops to ~3 ps, which is associated with fast exciton capture to defects. To understand the nature of defects in MoS2, we have investigated different defect configurations with various possible densities of sulfur vacancies from density functional theory (DFT) and time-dependent DFT. Our systematic studies in few-layer MoS2 nanosheets reveal crucial information on the exciton recombination in TMDCs for several optoelectronic applications. |
Thursday, March 17, 2022 1:42PM - 1:54PM |
T12.00012: Fine Structure Splitting in van der Waals Layered Transition Metal Dichalcogenide Single Crystal Pravrati Taank, Aravind R, Ravi Shankar Singh, K.V. Adarsh Alloying of van der Waals layered transition metal dichalcogenides (TMDCs) MoS(Se)2 and WS(Se)2 provides materials containing versatile and tunable properties with great potential in optoelectronics. Alloying by substituting metal atoms causes an insignificant effect on the lattice strain but significantly affects the optical and electrical properties. Here, using ground-state optical reflectance and femtosecond transient differential reflectance (DR) spectroscopy measurement, we demonstrate fine structure splitting in MoWSe2 single crystal in addition to MoWS2. Our studies reveal the vital role of the size of anion atoms, which results in the additional features in the DR spectra. Moreover, density functional theory and Bethe-Salpeter equation calculations support our experimental findings. Our systematic studies offer critical insights into the unexplored domain of alloy TMDCs for improving optoelectronic devices. |
Thursday, March 17, 2022 1:54PM - 2:06PM |
T12.00013: Polarization tuned light-matter coupling in exciton-polariton manifold Avijit Dhara, Devarshi Chakrabarty, Kritika Ghosh, Aswini K Pattanayak, Shreyashi Mukherjee, Ayan Roy Chaudhuri, Sajal Dhara Group VII TMDs like ReS2 show strongly anisotropic electronic and optical properties from bulk to monolayer limit without any drastic change, making it ideal for multilayer photonic devices. Here, we demonstrate highly tunable strong light-matter coupling in ReS2, with Rabi splitting of up to 68 meV, utilizing the interaction of anisotropic polarized excitons and the photonic mode supported in the ReS2 layer placed on a DBR structure. The coupling strength of such highly anisotropic exciton-polaritons can be tuned by rotation of incident beam polarization. It also can be used to switch between two independent three-body coupled oscillators in this system. In addition, layer thickness and temperature variation provide a way to manipulate the coupling strength and polariton detuning. Our result paves the way to engineer devices for application in polarization-controlled photonics. |
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