Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session H32: Excitonic Devices from 2D MaterialsFocus
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Sponsoring Units: DMP Chair: Baowen Li, University of Colorado, Boulder Room: 295 |
Tuesday, March 14, 2017 2:30PM - 3:06PM |
H32.00001: Electrons, holes, and excitons in monolayer semiconductors: Magneto-optical studies of polarization dynamics and dielectric screening Invited Speaker: Scott Crooker The burgeoning interest in atomically thin semiconductors such as MoS$_2$ and WSe$_2$ derives in part from the spin-valley locking of resident carriers in \textit{n}- or \textit{p}-type material (for the development of notional spin- and/or valleytronic devices) and also from the strong light-matter coupling of tightly bound excitons (for optoelectronics applications). This talk discusses two recent magneto-optical studies that address how these intrinsic properties of electrons, holes, and excitons in 2D semiconductors can be measured and controlled in real devices. In the first study, the spin and valley dynamics of both resident electrons \textit{and} resident holes are measured and tuned in gated single crystals of monolayer WSe$_2$, using time-resolved Kerr rotation spectroscopy and low magnetic fields [1,2]. Very different relaxation dynamics are observed in the electron- versus hole-doped regime, supporting a picture of robust spin-valley locking in the valence band. In the second study, broadband absorption and very large pulsed magnetic fields to 65T are used to directly reveal how the size and binding energy of 2D excitons are significantly affected by the surrounding dielectric environment [3] -- of particular relevance to future van der Waals heterostructures and devices because excitons in 2D semiconductors necessarily reside close to an interface or surface. [1] P. Dey \textit{et al., submitted}. [2] L. Yang \textit{et al., Nature Physics} \textbf{11}, 830 (2015). [3] A.V. Stier \textit{et al., Nano Letters} \textbf{16}, 7054 (2016). [Preview Abstract] |
Tuesday, March 14, 2017 3:06PM - 3:18PM |
H32.00002: Excitonic Resonant Emission-Absorption of Surface Plasmon in Transition Metal Dichalcogenides for Chip-level Electronic-Photonic Integrated Circuits Zhuan Zhu, Jiangtan Yuan, Haiqing Zhou, Jonathan Hu, Jing Zhang, Chengli Wei, Fang Yu, Shuo Chen, Yucheng Lan, Yao Yang, Yanan Wang, Chao Niu, Zhifeng Ren, Jun Lou, Zhiming Wang, Jiming Bao The monolithic integration of electronics and photonics has attracted enormous attention due to its potential applications. A major challenge to this integration is the identification of suitable materials that can emit and absorb light at the same wavelength. In this paper we utilize unique excitonic transitions in WS2 monolayers and show that WS2 exhibits a perfect spectral overlap between its absorption and photoluminescence spectra. By coupling WS2 to Ag nanowires, we then show that WS2 monolayers are able to excite and absorb surface plasmons of Ag nanowires at the same wavelength of exciton photoluminescence. This resonant absorption by WS2 is distinguished from that of the Ohmic propagation loss of silver nanowires, resulting in a short propagation length of surface plasmons. Our demonstration of resonant optical generation and detection of surface plasmons enables nanoscale optical communication and paves the way for on-chip electronic-photonics integrated circuits. [Preview Abstract] |
Tuesday, March 14, 2017 3:18PM - 3:30PM |
H32.00003: Valley polarized exciton polaritons from two-dimensional atomic crystal in microcavity Zheng Sun, Jie Gu, Areg Ghazaryan, Zav Shotan, Christopher Ryan Considine, Michael Dollar, Pouyan Ghaemi, Vinod Menon Two dimensional (2D) atomic crystals of transition-metal dichalcogenides (TMDs) have become an extremely attractive platform to investigate solid state cavity quantum electrodynamic effects. Indeed, strong coupling between excitons in 2D TMDs and microcavity photons have been demonstrated at room temperature by different groups. One of the most intriguing aspects of 2D TMDs is the valley degree of freedom of the excitons that occupy quantum mechanically distinct valleys in the momentum space resulting in helicity dependent transitions. Here we demonstrate the observation of room temperature strongly coupled microcavity polaritons that are valley polarized due to the coupling of the photons with specific helicity to excitons in the distinct valleys. In a metal mirror based microcavity embedded with 2D WS2, we observe strong coupling with Rabi splitting of 80meV and the emission is found to have average helicity of \textasciitilde 27{\%}. The Fourier space emission shows no angle dependent helicity. The possibility of observing room temperature valley polarized polaritons is a first step towards using valley effects in polariton systems. [Preview Abstract] |
Tuesday, March 14, 2017 3:30PM - 3:42PM |
H32.00004: Interlayer Exciton Optoelectronics in a 2D Heterostructure p-n Junction Pasqual Rivera, Jason Ross, John Schaibley, Eric Lee-Wong, Hongyi Yu, Takashi Taniguchi, Kenji Watanabe, Jiaqiang Yan, David Mandrus, David Cobden, Wang Yao, Xiaodong Xu Semiconductor heterostructures are backbones for solid state based optoelectronic devices, which are now being engineered at the atomically thin limit using 2D semiconductors heterojunctions. In monolayer WSe$_{\mathrm{2}}$-MoSe$_{\mathrm{2}}$ heterobilayers, the type II band alignment causes the formation of interlayer excitons -- with electron and hole confined to different layers -- which have shown promising properties for novel excitonic devices. Here, we demonstrate interlayer exciton optoelectronics in an electrostatically defined p-n junction, which uses tunneling contacts to preferentially inject electrons and holes directly into the n and p type monolayers of the heterobilayer. Wavelength dependent photocurrent measurements provide the first direct observation of resonant optical excitation of the interlayer exciton, which allows estimation of its oscillator strength compared to that of intralayer excitons. Furthermore, comparison of the photocurrent, electroluminescence, and photoluminescence spectra provides evidence for the predicted finite center of velocity light cones in the interlayer exciton dispersion. [Preview Abstract] |
Tuesday, March 14, 2017 3:42PM - 3:54PM |
H32.00005: Internal quantum efficiency measurements of wafer-scale CVD grown $MoS_2$ phototransistors Maxwell Woody, John Robertson, Xue Liu, Jiang Wei, Matthew Escarra Here we perform photocurrent measurements on wafer-scale monolayer and few-layer CVD $MoS_2$ to demonstrate the optoelectronic capabilities of large-scale $MoS_2$ growth. The $MoS_2$ films were grown on $SiO_2$-on-silicon substrates using a Mo-precursor thermal vapor sulfurization technique and used to make back-gated field effect phototransistors. Light from a tunable supercontinuum laser was focused on the devices, while gate or source-drain voltages were varied. Photocurrent spectra of bilayer $MoS_2$ as a function of incident wavelength displays the characteristic A(673nm), B(620nm), and C(438nm) excitons commonly associated with $MoS_2$. By measuring the power of the laser at each wavelength, the external quantum efficiency (EQE) of the device is calculated. The results show a clear band edge at 690nm and corresponding in-band EQE ranging from 1-3$\%$. Internal quantum efficiency (IQE) will be found using absorption data, and optical responsivity will be calculated for different thicknesses of grown $MoS_2$. These results show progress toward $MoS_2$ photodetectors from wafer-scale 2D semiconductors and provide a path toward large area $MoS_2$ to be used as a photovoltaic material. Future experiments intend to synthesize photovoltaic architectures from these materials. [Preview Abstract] |
Tuesday, March 14, 2017 3:54PM - 4:06PM |
H32.00006: Abstract Withdrawn
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Tuesday, March 14, 2017 4:06PM - 4:18PM |
H32.00007: Graphene-BN atomic stack microwave direct and heterodyne detector Jiayue Tong, Matthew Conte, Tom Goldstein, Martin Muthee, Joseph Bardin, K. Sigfrid Yngvesson, Jun Yan Due to its high mobility and tunable broadband response, graphene is a promising material for high-speed optoelectronics such as microwave detectors. In this presentation, I will discuss our studies of microwave detection with graphene-BN heterostructure devices. Using a Coplanar Waveguide (CPW) that operates up to 110GHz, we demonstrate that asymmetrically-contacted graphene-BN heterostructure samples can efficiently detect microwave signal. The intrinsic detector responsivity we derived at low MW frequencies is reasonably consistent with that measured from the DC IV curve. Using the same device structure, we also performed the experiment that demonstrates a heterodyne thermoelectric detector at room temperature with very wide IF bandwidth. Our work expands the methodology for making graphene-based microwave detectors. [Preview Abstract] |
Tuesday, March 14, 2017 4:18PM - 4:30PM |
H32.00008: Neutral and Charged Inter-Valley Biexcitons in Monolayer Transition Metal Dichalcogenides Kai Hao, Lixiang Xu, Judith Specht, Philipp Nagler, Kha Tran, Akshay Singh, Chandriker Dass, Christian Schüller, Tobias Korn, Marten Richter, Andreas Knorr, Xiaoqin Li, Galan Moody In monolayer transition metal dichalcogenides, tightly bounded exciton and trion dominate the optical properties. Higher-order correlated states, such as biexciton, are possible but are difficult to unambiguously identified with linear optical spectroscopy alone. With polarization resolved two dimensional coherent spectroscopy, unambiguously signatures of neutral and charged inter-valley biexcitons are observed in monolayer MoSe2 with \textasciitilde 20 meV and \textasciitilde 3meV binding energies, which are consistent with variational and Monte Carlo calculations. These higher--order correlated states consist of quasiparticles formed at opposite valleys with large crystal momentum difference, making them a unique type of bound states with no direct analog in conventional semiconductors. Our findings offer new opportunities for developing ultrathin biexciton lasers andpolarization-entangled photon sources and for creating exotic exciton-polariton condensates. [Preview Abstract] |
Tuesday, March 14, 2017 4:30PM - 4:42PM |
H32.00009: Spectrally narrow neutral and charged exciton emission in MoS2 monolayers : optical doping and superacid treatment Gang Wang, Fabian Cadiz, Cedric Robert, Simon Triccard, Marco Manca, Delphine Lagarde, Pierre Renucci, Thierry Amand, Xavier Marie, Sefaattin Tongay, Bernhard Urbaszek In MoSe2 and WSe2 Monolayers (MLs) photoluminescence (PL) at low temperatures normally exhibits narrow neutral exciton (X) and trion (T) lines (FWHM 10 meV). In contrast, in MoS2 MLs only a single broad peak is observed (FWHM 50 meV) which has been attributed to a possible mixture of X and T emission. This peak is accompanied by a low energy emission attributed to defect-related transitions. Here we present results on MoS2 MLs treated by an inorganic superacid (TFSI) that has been previously shown to passivate defects. In these treated MLs, well identified X and T peaks dominate the optical spectrum at T=4K. Due to the clear X-T separation, we were able to initialize neutral exciton valley coherence (superposition of valley states) with linearly polarized excitation. In addition, we show that at very low laser excitation powers, the X and T peaks can be well identified in MLs even in the absence of any acid treatment, in contrast to what is usually found in the literature. A detailed investigation of the PL as a function of excitation power reveals a non-reversible change in the PL spectrum caused by laser exposure, at power densities that so far have been considered to be non-destructive. [Preview Abstract] |
Tuesday, March 14, 2017 4:42PM - 4:54PM |
H32.00010: Excitonic properties of hydrogenated single-layer MoS$_{\mathrm{\mathbf{2}}}$ Naseem Ud Din, Volodymyr Turkowski, Talat Rahman The excitation spectrum of hydrogenated single-layer MoS$_{\mathrm{2}}$ are investigated systematically using the first-principle Density-Matrix Time-Dependent Density-Functional Theory, for varying hydrogen coverage. In particular, it is shown that the fully-hydrogenated system is metallic, while in the low-coverage limit the spectrum of single-layer MoS$_{\mathrm{2}}$ acquires spin-polarized partially filled mid-gap states. These states are defined by the orbitals of H atoms which make a tilted bond with the surface S atoms. Our calculated absorption spectrum of the system reveals several excitonic peaks, including states that involve the mid-gap levels. Detailed analysis of the properties of these excitons shows that, similar to the case of pristine single-layer MoS$_{\mathrm{2}}$, binding energies of the excitons of the hydrogenated system are large (few tenths of an eV), making their experimental detection facile and suggesting hydrogenation as a knob for tune the optical properties of single-layer MoS$_{\mathrm{2}}$. Comparisons are made with on-going experimental observations. To gain further insights, we examine the effect of alkali coverage (Li and Na) on the optical properties of single-layer MoS$_{\mathrm{2}}$ and compare them with those of the hydrogenated system. [Preview Abstract] |
Tuesday, March 14, 2017 4:54PM - 5:06PM |
H32.00011: Electrically Controlled Coherent Excitonic Steady States in Semiconductor Bilayers Ming Xie, Allan MacDonald Spatially indirect excitons are long lived bosonic quasiparticles that can form quasi-equilibrium condensed states. Optical access to these excitons has been limited by their small optical matrix elements. Here we propose a promising electrical process that can be used both to populate and to probe fluids of indirect excitons, and is analogous to the crossed Andreev reflection (CAR) process of Cooper pairs in superconductors. We consider vertically stacked multilayer heterostructures containing two transition metal dichalcogenide (TMD) layers that host the indirect excitons, graphene layers on the top and the bottom of the heterostructure, and hBN tunnel barrier layers of variable thickness. When the bias voltage between the graphene leads is smaller than the indirect gap, tunneling between the graphene leads and the TMD hetero-bilayer is possible only through the CAR process. Both DC and low frequency AC bias cases are explored and establish that electrical measurements can be used to determine crucial properties such as the condensate density, interaction strength and CAR tunneling amplitudes. We have also proposed a way to electrically manipulate another type of bosonic quasiparticles, cavity exciton-polaritons, in a laterally contacted structure. [Preview Abstract] |
Tuesday, March 14, 2017 5:06PM - 5:18PM |
H32.00012: Symmetry analysis of 2D four-six-enes (group-IV metal monochalcogenides): electrons, holes, spin and excitons Pengke Li, Ian Appelbaum Band-edge states in group-IV metal monochalcogenide (four-six-enes such as SnS, GeSe, etc.) - a class of 2D indirect-gap semiconductors -~inherit the properties of nearby points of high symmetry at the Brillouin zone boundary. Using group theory and the method of invariants to capture these essential symmetries, we derive concise effective spin-dependent Hamiltonians of the relevant zone edge states at high-symmetry points. Perturbation theory is used to shed light on the nature of the band-edge states, including the selection rules of direct optical transitions across the band gap, interactions responsible for subtle features of the local dispersion relations, momentum and spin transport properties for both conduction electrons and valence holes, and intriguing features of indirect excitons. [Preview Abstract] |
Tuesday, March 14, 2017 5:18PM - 5:30PM |
H32.00013: Dark Excitons in Gapped Chiral Fermion Systems Xiaoou Zhang, Wenyu Shan, Di Xiao The radiative lifetime of excitons puts an intrinsic limit on the operation speed of optoelectronic devices. In this talk, we propose a new mechanism to realize dark exciton state based on gapped 2D chiral fermions. We further show that a gate voltage can be used to tune the lowest exciton state between dark and bright. This provides a pathway to experimental control of optical transitions. [Preview Abstract] |
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