Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F15: 2D Materials (Semiconductors) -- Optical Properties IFocus
|
Hide Abstracts |
Sponsoring Units: DMP DCOMP Chair: Shaowei Li, Northwestern Room: BCEC 154 |
Tuesday, March 5, 2019 11:15AM - 11:27AM |
F15.00001: Optical excitations of transition-metal dichalcogenides with charged adsorbates Martik Aghajanian, Arash A Mostofi, Johannes Lischner Direct band-gap semiconducting monolayer transition-metal dichalcogenides (TMDs) have been previously shown to demonstrate enhanced light absorption, which has promising applications in photovoltaic devices. This work explores the effects of charged adsorbates in MoS2, MoTe2, WS2 and WTe2 on their optical properties. Charged impurities on TMDs can give rise to bound states that extend over tens of nanometers due to weak screening in two-dimensional materials, hence it becomes difficult to capture their effects with conventional electronic structure calculations based on density-functional theory. In our approach, we first determine the electronic structure using large-scale tight-binding calculations which include a screened defect potential obtained from first-principles random-phase approximation calculations. Next, the Bethe-Salpeter equation is solved to include excitonic effects and obtain the optical absorption spectrum to compare with photoluminescence experiments. We indicate how trigonal symmetry present at the valleys of the band extrema results in deviation from simple hydrogenic behaviour of the bound states, and show dependence of excitonic resonances on the defect charge. |
Tuesday, March 5, 2019 11:27AM - 11:39AM |
F15.00002: Nearly 90% circularly polarized emission in monolayer heterogeneous WS2 single crystals by chemical vapor deposition (CVD) Wei-Hsiang Lin, Wei-Shiuan Tseng, Cora Went, George R Rossman, Harry Atwater, Nai-Chang Yeh Monolayer transition metal dichalcogenides (TMDCs) are promising materials for valleytronic applications because the two inequivalent valleys in the Brillouin zone. We report here novel optoelectronic properties of heterogeneous domains in CVD-grown monolayer WS2 single crystals. Spatially resolved PL, Raman, X-ray photoelectron spectroscopy and Kelvin probe force microscopy images revealed the formation of homojunctions in these single crystals, which implied a direct correlation between the chemical stoichiometry and the optoelectronic heterostructure. Conductive atomic force microscopy (AFM) measurements revealed nanoscale distributions of electronically active defects in the heterogeneous WS2, and the local defect density was found to be inversely proportional to the local PL intensity. Additionally, by optically pumping WS2 with CPL and measuring the resulting spatially resolved CP emission (Pcirc) at room temperature (RT) and low temperature (80K), we found significant Pcirc intensities even at RT, and Pcirc was inversely correlated with the defect density. At 80K, the degree of circularly polarized emission in low-defect domains was found to approach ~ 90%, suggesting nearly perfect valley polarization. |
Tuesday, March 5, 2019 11:39AM - 11:51AM |
F15.00003: Exciton regulation of resonant Raman scattering in monolayer MoS2 Yuanxi Wang, Bruno R. Carvalho, Vincent Henry Crespi Strong excitonic effects in 2D semiconductors such as monolayer MoS2 not only downshift its excitation spectrum from a single-particle one, but also redistribute excitation levels and wavefunction characters that profoundly affect exciton-phonon scattering processes, leaving strong signatures in resonant Raman measurements. We present a first-principles GW-Bethe-Salpeter equation (GW-BSE) approach based on perturbation theory to calculate resonant Raman intensities beyond the Placzek approximation. We show how excitonic effects in MoS2 strongly regulate Raman scattering amplitudes and explain two puzzling experimental observations: the near absence of Raman response at the A and B band-edge excitons and the pronounced strength of Raman response near the C exciton. This perturbative approach reduces the number of GW-BSE calculations from two per Raman mode in finite displacement methods to one for all modes and allows a natural extension to higher-order resonant Raman processes [Phys. Rev. B 98, 161405 (2018)]. |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F15.00004: Optical tuning of magnetism with valley polarized excitation Xiao-xiao Zhang, Lizhong Li, Zefang Wang, Jie Shan, Kin Fai Mak Magnetic 2D materials hold great promises to allow a more active control of magnetism. By interfacing other 2D monolayers with the atomically flat magnet, we can create new functional materials that combines different features and properties. In this talk, I would present my work on the heterostructure of ferromagnetic CrBr3 and monolayer transition metal dichalcogenide (TMDC) of WSe2. I would show how the optical excitation within the WSe2 would also affect the magnetism in CrBr3. By employing circularly polarized light, valley and spin polarized carriers are created within the TMDC as a result of the valley optical selection rule. Sequential modification in the magnetic response in CrBr3 indicates that there exists efficient spin transfer from the excited TMDC into the interfaced magnet. Further characterizations including temperature dependence, excitation detuning etc. would also be discussed. The efficient optical tuning of magnetism opens up promising directions to manipulate magnetic domains in an all-optical method. |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F15.00005: Measuring Valley Susceptibility of Transition Metal Dichalcogenides with Second-harmonic Spectroscopy Yi Wei Ho, Henrique Guimaraes Rosa, Ivan Verzhbitskiy, Manuel Jose Ferreira de Lima Rodigues, Takashi Taniguchi, Kenji Watanabe, Goki Eda, Vitor Pereira, José Carlos Viana-Gomes Polarization-resolved (PR) photoluminescence (PL) [1] and Kerr rotation (KR) [2] are commonly used to probe the degree of valley polarization (VP) of transition metal dichalcogenides (TMDs). Recently, second-harmonic (SH) generation was proposed to also probe VP [3], with the advantage over PL and KR of not requiring the material to have a bandgap or strong spin-orbital coupling, and thus extending VP measurements to other materials such as graphene. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F15.00006: Light based control of Exciton Polaritons in Van der Waals Semiconductors Aaron Sternbach, Simone Latini, Hannes Huebener, Umberto De Giovannini, Sanghoon Chae, Lin Xiong, Yinming Shao, Norman Shi, GuangXin Ni, Nanfang Yu, Michael Fogler, James Hone, Angel Rubio, Dimitri Basov In this work we focus on exciton polaritons in the Van der Waals (vdW) Semiconductor, WS2, under intense femto-second (fs) photo-excitation. Photo-excitation initiates a blueshift of the exciton polariton. This corresponds to an extremely large photo-induced change of the dielectric function. Detailed investigation of the dispersion and the fluence dependence indicate the observed effect arises from light-based control of excitons. The change to the dielectric function evolves over two distinct timescales. On the femtosecond timescale a large coherent contribution is observed. On the picosecond (ps) timescale part of the effect is found to persist. The equilibrium state is fully recovered approximately one hundred ps after the photo-excitation event. The information-rich data-sets available from interrogating the exciton-polariton allow us to explore the pathways toward active control of excitons. Additionally, our experimental approach allows us to access key quantities, such as the group velocity and real-space confinement of the exciton polariton, in the non-equilibrium state. Finally, our findings indicate that strong-light matter interactions in vdW semiconductors may be used to achieve all-optical control of significant magnitude. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F15.00007: Spin-Valley dependent optical response at monolayer transition metal dichalcogenide/ferrimagnet interface Takatoshi Akamatsu, Toshiya Ideue, Masaru Onga, Yuki Itahashi, Yuji Nakagawa, Yoshihiro Iwasa Monolayer transition metal dichalcogenides (TMDs) is one of the potential candidates for valley-based electronics, since valley degree of freedom in TMDs couples with orbital and spin degree of freedoms, so that it can be manipulated by optical or magnetic method. So far, valley-optical responses related with orbital degree of freedom have been elucidated in monolayer TMDs. Recently, TMDs/magnet interfaces are widely studied as an ideal platform for studying the spin-valley related phenomena[1][2]. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F15.00008: Ultrafast Dynamics of Excitonic Rydberg Series in Single Layer WSe2 Mehmet B Yilmaz, Emre Ergecen, Nathan P Wilson, Joseph Varghese, Steven Vitale, Xiaodong Xu, Nuh Gedik Single layer transition metal dichalcogenides (TMD) such as MoS2 and WSe2 have been the subject of intense attention for their optically accessible valley polarization. In these materials, circularly polarized light can be used to break the time-reversal symmetry and lift the degeneracy of the K and K’ valleys. Such optical control is of great importance for possible valleytronic applications. In this work, we will present broadband transient reflection microscopy results obtained from hBN capped single layer WSe2 flakes. In these high quality samples, it is possible to observe the excitonic Rydberg states in the reflectivity data. We have studied the dynamics of these states after photo-excitation via pump-probe spectroscopy. When the excitation light energy is tuned below the band gap, high quality of these samples enable observation of novel valley selective coherent response in addition to the previously observed optical Stark shift. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F15.00009: Suspended Excitons in 2D Materials Ozgur Burak Aslan, Yan Joe Lee, Yifei Yu, Linyou Cao, Mark Brongersma Excitons in atomically thin transition metal dichalcogenides (TMDCs, 2D materials) are strongly influenced by their environment. To eliminate that influence, we prepare suspended monolayer (1L) WSe2 samples, enabling suspended excitons. We perform reflectance measurements and calculate the excitonic binding energies from the experimental observables: the energy differences between the 1s, 2s, and 3s states. We employ the recently developed quantum electrostatic heterostructure model for the calculations. We see that the binding energy of the ground state A exciton increases from about 0.3 eV (on substrate) to above 0.4 eV (suspended). A more striking feature of the suspended excitons is the ability to tune them even further by applying mechanical strain. By applying air pressure, we obtain reversible 0.15 eV redshift in the exciton resonance of a suspended 1L sample on a circular hole of 8 μm diameter under a pressure of 40 psi. Interestingly, the linewidth of the A exciton decreases more than half from about 45-50 meV to 20 meV at room temperature, due to the suppression of the intervalley exciton-phonon scattering. Our results show that suspended 2D materials are novel systems for fundamental studies as well as strong and dynamic tuning of the excitons. |
Tuesday, March 5, 2019 1:03PM - 1:39PM |
F15.00010: Exciton Manipulation in 2D TMDC Heterostructures Invited Speaker: Andras Kis The discovery of graphene marked the start of research in 2D electronic materials which was expanded in new directions with MoS2 and other layered semiconducting materials. They have a wide range of interesting fundamental properties and potential applications. New opportunities are enabled by the band structure of transition metal dichalcogenides (TMDCs) in which we could harness the valley degree of freedom for valleytronics and next-generation photonics. Long-lived interlayer excitons in van der Waals heterostructures based on TMDCs have recently emerged as a promising platform for this, allowing control over exciton diffusion length, energy and polarization. I will show here how by using MoS2/WSe2 van der Waals heterostructures, we can realize excitonic transistors with switching action, confinement and control over diffusion length at room temperature in a reconfigurable potential landscape. Heterostructures with a long-range moiré potential such as in MoSe2/WSe2, on the other hand, offer the way to control polarization, emission and wavelength emitted by different optically active regions in the moiré. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F15.00011: Optical spectroscopy of excited Rydberg excitons to 65 tesla in monolayer semiconductors Andreas Stier, Mateusz Goryca, Jing Li, Scott Crooker, Nathan P Wilson, Xiaodong Xu, Emmanuel Courtade, Cedric Robert, Xavier Marie, Bernhard Urbaszek Monolayer transition-metal dichalcogenide (TMD) semiconductors, such as MoS2, host very tightly-bound excitons due to reduced dielectric screening and relatively heavy electron / hole masses. Advances in the encapsulation of monolayer TMDs in atomically-flat hexagonal boron nitride (hBN) result in narrow neutral exciton resonances, as well as spectral features associated with excited Rydberg states. Optical spectroscopy in high magnetic fields was recently demonstrated to be a powerful way to uniquely identify these states in WSe2 and to determine fundamental properties such as the exciton size, mass and binding energy [1]. Here, we report 65 T magneto-absorption spectroscopy of excited Rydberg excitons in hBN-encapsulated WS2, MoS2 and MoSe2 monolayers. The distinct diamagnetic shifts of these excited states (2s, 3s, …, ns) permits their unambiguous identification, and provide a direct measure of the reduced exciton masses. We compare our experimental results with numerical simulations of the non-hydrogenic Rytova-Keldysh potential for strictly 2D semiconductors, and more general models describing Coulomb interactions in thin-film semiconductors. [1]Stier et al., PRL 120, 057405 (2018). |
Tuesday, March 5, 2019 1:51PM - 2:03PM |
F15.00012: Berry Curvature Induces Exciton Fine-structure and Valley-dependent Autler-Townes Doublet in Molybdenum Diselenide Monolayer Chaw Keong Yong, Iqbal B Utama, Feng Wang The geometry phase of Bloch states in the momentum space, characterized by the Berry curvature, can strongly modify the electron dynamics and lead to novel transport phenomena, such as the anomalous Hall effect. Recently it was predicted that the nontrivial Bloch band geometry can also modify the collective optical excitations in transition metal dichalcogenide monolayers, and lift the energy degeneracy of exciton states with opposite angular momentum through an effective valley-orbital effect. Here we report the first experimental observation of the Berry curvature signature in the exciton spectrum of MoSe2 monolayer using novel techniques based on intraexciton optical Stark spectroscopy. We demonstrate the time-reversal-symmetric analog of the orbital Zeeman effect resulting from the valley-dependent Berry curvature, which leads to energy difference of +14 (-14) meV between the 2p+ and 2p- exciton fine-structure in the K (K’) valley. The coherent light-matter coupling between intraexciton states are remarkably strong, leading to a prominent valley-dependent Autler-Townes doublet. Our study opens up new pathways to manipulate the quantum states with infrared radiation and suggests the possibility to control the light-matter interaction via topological quantum phase engineering. |
Tuesday, March 5, 2019 2:03PM - 2:15PM |
F15.00013: Understanding thickness-dependent electronic and optical transitions of Ruddlesden-Popper halide perovskites Yeongsu Cho, Timothy Berkelbach Ruddlesden-Popper halide perovskites are layered heterostructures that are promising candidates for effective optoelectronic devices due to their photoefficiency and stability. Despite their high tunability through substitution and stoichiometry, the physics that control the resulting electronic and optical properties are still not fully understood. We investigate the dependence of the band gap and exciton binding energy on the inorganic layer thickness through the tight binding approximation and nonlocal screened Coulomb potential, making use of material properties of three-dimensional perovskites. Our theory provides an analysis of the optical gap measured by absorption and photoluminescence. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700