Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M27: Optical Spectroscopic Measurements of 2D Materials |
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Sponsoring Units: FIAP GIMS DMP Chair: Okan Koksal, Cornell University Room: 404 |
Wednesday, March 4, 2020 11:15AM - 11:27AM |
M27.00001: Photoluminescence of Monolayer MoS2 under Controlled Environment for Optoelectronic Applications Blake Birmingham, Jiantan Yuan, Matthias Filez, Donglong Fu, Jonathan Hu, Jun Lou, Marlan O Scully, Bert M. Weckhuysen, Zhenrong Zhang Monolayer MoS2 has become a very promising two-dimensional materials for photo-related applications. Establishing the impact of individual ambient gas components and chemical dopants on the optical properties of MoS2 is a necessary step toward application development. By using in situ Raman micro-spectroscopy with an environment-controlled reaction cell, the photoluminescence (PL) intensity of CVD-grown MoS2 monolayers is monitored at different intralayer locations under ambient and controlled gas environments, such as N2, O2, H2O, and pyridine. Our study demonstrates that photoreactions with the gaseous environment on MoS2 monolayer flakes should be taken into consideration even upon mild photoirradiation as they strongly impact the flakes’ optical properties. The optical properties of MoS2 at the edges are strongly affected by photoirradiation induced reactions with O2 and H2O. We have also studied the effect of the dopant phase of the same dopants – liquid and gaseous – on the optical properties of ML MoS2. The gaseous n-type dopant, i.e. pyridine, completely quenches the PL intensity of ML MoS2, while liquid pyridine preserved 50% of the original PL intensity attributed to its less effective charge transfer to MoS2 than the gaseous counterpart. |
Wednesday, March 4, 2020 11:27AM - 11:39AM |
M27.00002: Imaging strain-localized exciton states in nanoscale bubbles in monolayer transition metal dichalcogenides at room temperature Thomas Darlington, Christian Carmesin, Mathias Florian, Emanuil Yanev, Demi Ajayi, Jenny Ardelean, Augusto Ghiotto, Andrey Krayev, Jeffrey Kysar, Abhay Pasupathy, James C Hone, Frank Jahnke, Nicholas J Borys, P. James Schuck In monolayer transition metal dichalcogenides (1L-TMD’s), applied strain can deterministically create quantum emitters at arbitrary sites on-demand. Despite a robust empirical correlation with strain, the nanoscopic details of these quantum emitters are poorly understood. Here we combine room-temperature nano-optical imaging of excitons in nanobubbles in 1L-WSe2 with atomistic structural models to elucidate how strain induces the nanoscale confinement potentials that give rise to highly localized exciton states in 2D semiconductors. Nano-optical imaging reveals localized excitons on length scales of ~10 nm at multiple sites along the periphery of individual nanobubbles, which is in stark contrast to predictions of continuum strain models. These results agree with theoretical confinement potentials derived from measured topographies of nanobubbles. We reproduce these findings in nanobubbles of low-defect WSe2 and MoSe2, suggesting our results are applicable to excitons in all the semiconducting 1L-TMD’s. Our results provide one-of-a-kind insight of how strain-induced confinement—without crystalline defects—can localize excitons on length scales commensurate with exciton size, providing key nanoscale structure-property information for quantum emitter phenomena in 1L-TMD’s. |
Wednesday, March 4, 2020 11:39AM - 11:51AM |
M27.00003: Multi-harmonic phonon assisted upconversion in van der Waals heterostructure p-n junction photodidodes Fatemeh Barati, Trevor Arp, Shanshan Su, Roger Lake, Nathaniel Gabor In atomically thin van der Waals heterostructures, the interaction between electrons and phonons is highly restricted compared to bulk materials, suggesting that precise stacking of atomic layers could be used to manipulate vibrational coupling to photoexcited electron-hole pairs. Here, we report on phonon assisted anti-stokes absorption near the interlayer exciton edge of vdW semiconductor heterostructures composed of 2L-WSe2and MoSe2. By carefully tuning the chemical potential of the heterojunction, we find highly rectifying I-Vcharacteristics reminiscent of conventional diode behavior. Using advanced photocurrent spectroscopy measurements, we observe a strong photocurrent peak at near-infrared photon energies with several low energy echoes spaced by 30 meV below this feature. These unusual features occur only when device is precisely tuned to the charge neutrality point. We attribute this behavior to multi-harmonic phonon assisted upconversion, a process by which multiple phonons contribute kinetic energy to the electronic ground state to produce an excited state electron-hole pair. It marks the first observation of strong resonant coupling in a semiconductor system and may herald a new generation of precision optical experiments on laser cooling of vdW heterostructures. |
Wednesday, March 4, 2020 11:51AM - 12:03PM |
M27.00004: Strong-Coupling of Hybrid Quasiparticles in Excitonic-Dielectric Gratings Bhaskar Abhiraman, Huiqin Zhang, Qing Zhang, Jinshui Miao, Kiyoung Jo, Stefano Roccasecca, Mark Knight, Artur Davoyan, Deep M Jariwala Tightly bound exciton states in van der Waals semiconductors such as MX2 (M= Mo, W; X= S, Se) are promising for optoelectronic applications. However, to-date, strong light-exciton interaction and control of excited states in them has been limited to monolayer samples with external optical cavities. Here, we demonstrate that nanostructured multilayer WS2 provides an ideal cavity-less and exposed surface platform for exciton-photonics where the WS2 serves the role of both the cavity and the excitonic medium. We show that by patterning sub-15 nm thick WS2 into 300 nm wide resonators an avoided crossing of excitons and photon-polaritons with interaction potentials exceeding 410 meV can be engineered with precision. We further observe that inherently strong TMDC exciton absorption may be completely suppressed due to excitation of hybrid photon-exciton states and their interference. By modifying the system with dielectric spacers, we observe a rich platform for coupling and field confinement with bulk and monolayer TMDCs. Our work paves the way to a new class of integrated exciton optoelectronic nano-devices and their applications in light generation and manipulation. |
Wednesday, March 4, 2020 12:03PM - 12:15PM |
M27.00005: Ultrafast exciton dynamics in WSe2 optical waveguides Aaron Sternbach, Simone Latini, Sanghoon Chae, Hannes Huebener, Umberto De Giovannini, Yinming Shao, Lin Xiong, Zhiyuan Sun, Norman Shi, Peter Kissin, Guangxin Ni, Daniel A Rhodes, Brian S Y Kim, Nanfang Yu, Andrew Millis, Michael M Fogler, P. James Schuck, Michal Lipson, Xiaoyang Zhu, James C Hone, Richard Averitt, Angel Rubio, Dmitri Basov We investigated dynamics of excitons in a Van der Waals Semiconducting, WSe2, waveguide. We monitored the electric-field profile of waveguided infrared radiation under intense femtosecond photo-excitation in real space and time. Drastic modifications of the complex wavevector of guided radiation were observed. The non-equilibrium energy momentum dispersion relationship implicates excitons in the photo-induced transformations. Unprecedented coherent dynamics of refraction, on the sub-ps timescale, reveal an optical stark-shift of the A-exciton resonance. Our study establishes that excitons enhance the performance of vdW optical modulators providing a tuning knob unavailable in conventional III-V semiconducting platforms. Our transient images and first-principles theoretical calculations establish fundamental limits of excitons in WSe2optical modulators. |
Wednesday, March 4, 2020 12:15PM - 12:27PM |
M27.00006: Twist-angle dependent optical properties of interlayer excitons in MoSe2/WSe2 heterostructures Junho Choi, Kha Tran, Galan Moody, Akshay K Singh, Jiamin Quan, Liuyang Sun, Rob Claassen, Carter Young, Takashi Taniguchi, Kenji Watanabe, Xiaoqin (Elaine) Li We investigate the interlayer excitons in MoSe2/WSe2 heterostructures, mechanically stacked with an accurately controlled twist angle and capsulated by hexagonal boron nitride. Three R-stacking style samples with different twist angles are measured by polarization-resolved steady state and time-resolved photoluminescence at low-temperature. We observe multiple resonances of interlayer excitons which we attribute to quantized states in the moiré potential. The recombination dynamics of the interlayer excitons are found to change significantly with a small change in the twist angle. |
Wednesday, March 4, 2020 12:27PM - 12:39PM |
M27.00007: Excitonic spectral features of 1L-WSe2 with silicon nitride waveguide coupling Yueh-Chun Wu, Sarath Samudrala, Andrew McClung, Kenji Watanabe, Takashi Taniguchi, Amir Arbabi, Jun Yan In this study, we present excitonic spectral features of 1L-WSe2 coupled to a platform with Silicon nitride (SiNx) waveguides, allowing us to excite and collect signals in different polarizations. In our high quality BN/1L-WSe2/BN/SiNx sample, bright neutral exciton, intervalley/intravalley trions, dark exciton, and bi-exciton and exciton-trion states can be clearly resolved, in agreement with previous reports. The dark exciton, which is known to have out-of-plane dipole moment and thus has been elusive in most standard optical measurements, can be selectively enhanced with out-of-plane light waveguide coupling scheme. Moreover, we study the resonance excitation with 1s bright and dark excitons with different polarizations. Interestingly, as we resonantly excite at dark exciton with out-of-plane light, we observe the up-conversion of bright exciton as well as a down conversion feature lying around 25-27meV lower which indicates dark associate states as being reported to be either dark trions or phonon replica of dark exciton in WSe2. |
Wednesday, March 4, 2020 12:39PM - 12:51PM |
M27.00008: The Nature of Trions and Trion-Polaritons in 2D Materials: Theory and Experiments Okan Koksal, Minwoo Jung, Christina Manolatou, Gennady Shvets, Farhan Rana The traditional picture of trions as bound states of two electrons and hole has been called into question by several recent works [1,2]. The traditional model of trions is also inconsistent with coherent trion-polaritons [3]. In this work we present a many body description of trions and trion-polaritons that is valid over a wide range of electron densities. In our work, a negatively charged trion, for example, appears as a bound state of a valence band hole, two conduction band electrons, and a conduction band hole. At small electron densities, this 4-body bound state description of trions is found to be approximately correct. The binding of the conduction band hole allows coherent trion-polaritons. At much larger electron densities, bound trion states don’t exist and a many-body screened-exciton description of trions is found to be more accurate. We present experimental results for the evolution of the exciton and trion oscillator strengths and binding energies with the electron density, and also for the trion-polariton dispersion relations, in 2D-MoSe2, and show very good agreements with our theoretical model. [1] Phys. Rev. B, 95, 035417 (2017), [2] Nat. Phys., 13, 255 (2016). [3] Nat. Phys., 14, 130 (2018) |
Wednesday, March 4, 2020 12:51PM - 1:03PM |
M27.00009: Excitonic Lifetime in CVD-Grown, Few-Layer MoTe2 Aaron Schulzetenberg, James Johns Two-dimensional semiconductors such as transition metal dichalcogenides (TMDCs) with high binding energy excitons provide an ideal platform to study exciton phyics such as exciton-exciton annihilation and chemical defect scattering. The dominant exciton decay mechanisms of chemical vapor deposition (CVD) grown, few-layer MoTe2 are investigated by measuring and statistically analyzing its ultrafast, optical transient reflectivity dynamics under different pump excitation energies, fluences and temperatures. Defect interaction controls the ultrafast exciton decay pathways. Weak intralayer bonding in few-layer, CVD MoTe2 causes it to have a greater defect density than in other mechanically exfoliated 2D transition metal dichalcogenides. Direct charge carrier occupation of trap states is thought to cause an additional transient feature on the A’ exciton which is not present in the absorption spectrum. Additionally, long-term ambient air exposure does not significantly affect few-layer MoTe2 exciton decay time and absorption peak shape, increasing this material’s suitability for many technological applications. |
Wednesday, March 4, 2020 1:03PM - 1:15PM |
M27.00010: Continuous Wave Sum Frequency Generation and Imaging of Monolayer and Heterobilayer 2D Semiconductors Kaiyuan Yao, Emanuil Yanev, Hsun jen Chuang, Matthew Rosenberger, Xinyi Xu, Thomas Darlington, Kathleen M McCreary, Aubrey T. Hanbicki, Berend Thomas Jonker, Xiaoyang Zhu, Dmitri Basov, James C Hone, P. James Schuck Optical nonlinearities in two-dimensional transition metal dichalcogenides (TMDs) are greatly enhanced by excitonic resonances. Here we report continuous-wave second harmonic and sum frequency generation from two-dimensional TMD monolayers and their heterostructures, with pump irradiances several orders of magnitude lower than conventional pulsed experiments. The high nonlinear efficiency originates from above-gap excitons in the band nesting regions, as revealed by wavelength-dependent second order optical susceptibilities quantified in four common monolayer transition metal dichalcogenides. Using sum frequency excitation spectroscopy and imaging, we identify and distinguish one- and two-photon resonances in both monolayers and heterobilayers. Data for heterostructures reveal responses from constituent layers accompanied with nonlinear signal correlated with interlayer transitions. Furthermore, we demonstrate spatial mapping of heterogeneous interlayer coupling by sum frequency and second harmonic confocal microscopy on heterobilayer MoSe2/WSe2. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M27.00011: Second harmonic generation spectroscopy of MoS2/WSe2 heterostructure Jungcheol Kim, Hyeonsik M Cheong Recently, heterostructures of transition metal dichalcogenides (TMDs) have attracted much interest owing to their unique physical properties. One of the most interesting characteristics is the large exciton energy of TMDs. As the electronic band structures of TMD materials are highly dependent on the number of layers, the interlayer interaction between constituent layers in the heterostructure also affects the band structure [1]. Therefore, understanding the changes of exciton states is important to revealing the band structure of the TMD heterostructure. We fabricated MoS2/WSe2 heterostructures by the stamping methods using exfoliated monolayer MoS2 and WSe2. The PL and Raman measurements were carried out to determine the quality of interface in the heterostructures. Finally, using the second harmonic generation spectroscopy, which is a useful experimental method to measure the exciton states in TMD materials [2], we observed a shift of the exciton energy in the heterostructure compare to those in individual constituent layers. |
Wednesday, March 4, 2020 1:27PM - 1:39PM |
M27.00012: Trion valley relaxation dynamics in monolayer WSe2 Keisuke Shinokita, Xiaofan Wang, Yuhei Miyauchi, Kenji Watanabe, Takashi Taniguchi, Satoru Konabe, Kazunari Matsuda Valley degrees of freedom of atomically thin materials have received much attention, with potential in future applications of electronics and optoelectronics as emerging valley-tronics. The exploration of novel valley-dependent phenomena has been boosted by strongly Coulomb-bounded electrons and holes acting as neutral excitons and charged excitons (trions) with finite Berry curvature in monolayers of transition metal dichalcogenides. However, neutral excitons have been reported to undergo fast intervalley scattering on the order of 10 ps, which blurs observations of valley-dependent phenomena. Here we report the valley relaxation dynamics of the bright state of positively charged excitons (bright positive trions) in monolayer WSe2 through polarization- and time-resolved photoluminescence and femtosecond transient reflection measurements. A long valley relaxation time, exceeding 100 ps, is observed for positive trions at low temperature, which is notably prolonged when compared with the characteristic excitonic valley relaxation time of 10 ps. With increasing temperature, the relaxation time decreases to a few ps. This finding implies that phonon-mediated intervalley scattering is important for the relaxation process of the valley-polarized bright state of positive trions. |
Wednesday, March 4, 2020 1:39PM - 1:51PM |
M27.00013: Pressure-tunable stacking order in few-layer WS2 revealed by second harmonic generation Chengrong Wei, Huimin Su, Xuefeng Zhou, Shanmin Wang, Jun-Feng Dai In 2D material, interlayer interaction can be tuning by internal pressure. We experimentally demonstrate a controllable interlayer translation freedom in hBN-encapsulated few-layer WS2 under applying hydrostatic pressure, which is characterized by Raman spectroscopy and the polarization-resolved second harmonic generation (SHG) spectroscopy. The E2g1 and A1g modes of all samples increased with pressure and there is no new peak observed in the Raman spectrum. The SHG patterns in monolayer WS2 have insignificant distortion and the SHG signal in bilayer WS2 is negligible. Meanwhile, SHG patterns in trilayer WS2 show six asymmetric lobes. Such observations suggest that the threefold rotational symmetry is broken under pressure, which origins from a relative sliding between adjacent layers in the zigzag direction. The tunability of translation structural symmetry by using a hydrostatic pressure presented in this work provides a platform to explore novel physical properties in new class of 2D materials. |
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M27.00014: Donor-acceptor-pair transitions in suspended monolayer WS2 Yunkun Wang, Hailan Luo, Xingjiang Zhou, Yuan Huang, Yunan Gao In conventional semiconductors, the donor-acceptor-pair (DAP) transitions could dominate the photoluminescence, when the sample is cooled to low temperature and the excitation is weak. Here we report observation and characterization of DAP transitions in a suspended monolayer WS2 using steady-state and time-resolved photoluminescence spectroscopy. Without substrate induced doping in suspended monolayer WS2, we observed an ultra-broad emission centered at 1.83 eV and with bandwidth of more than 300 meV. The bandwidth is much larger than generally observed bound excitons. These sub-band states show a sub-linear power-law dependence with an index of 0.3 at 12 K. In the DAP transitions, the recombination centers are spatially separated, and their emission energy depends on their relative distances. We observed that the board emission spectra shift to higher energy as the excitation power increased. Time-resolved photoluminescence spectra show the broad emission shifts from higher energy to lower one along the intensity decay. This work provides further deep understanding of trap states and decay channels in transition metal dichalcogenides. |
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