Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B15: 2D Materials (Semiconductors) -- Multilayers & Heterostructures IFocus
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Sponsoring Units: DMP DCOMP Chair: Morteza Kayyalha, Pennsylvania State University Room: BCEC 154 |
Monday, March 4, 2019 11:15AM - 11:27AM |
B15.00001: Twist-angle dependent Optical Second-Harmonic Generation from Transition Metal Dichalcogenides (TMD) Heterostructures Wontaek Kim, Sunmin Ryu Optical second-harmonic generation (SHG) by single and few-layer transition metal dichalcogenides (TMD) is highly efficient and sensitive to their structural symmetry. The SHG signal from artificially-stacked MoS2 bilayers can be understood as a superposition of SHG fields from the individual layers. However, this model has not been verified for TMD heterostructures. Here, we studied the SHG behavior of TMD hetero-bilayers of varying twist angle in comparison with homo-bilayers. When homo-bilayers were rotated over 2pi-azimuth with a fundamental beam normally incident on their basal planes, the polarization-resolved SHG signal exhibited a six-petal pattern with six azimuthal nodes, which agreed with the superposition model. The heterostructures, however, lacked the angular nodes because of an azimuth-independent signal in addition to the six-petal pattern. The fractional contribution of the constant signal was larger for more staggered hetero-bilayers. To explain these results, we propose that the anomaly originates from a non-zero phase difference between SHG fields generated in the individual monolayers. In addition, we show that the degree of the anomaly is sensitive to the fundamental’s wavelength, as the phase depends on photon energy. |
Monday, March 4, 2019 11:27AM - 11:39AM |
B15.00002: Observation of negative photoconductance in van der Waals heterostructures Yu Wang, Erfu Liu, weida Hu, shijun liang, Feng Miao Van der Waals (vdW) heterostructures made of different two-dimensional (2D) materials exhibit interesting optoelectronic properties. For example, photodetection and photo-controllable memory devices are two of intriguing applications based on vdW heterostructures. However, the operating principle for most of these devices rely on the positive photoconductance (PPC) effect. Negative photoconductance (NPC) has not yet been observed in vdW heterostructures, which holds promise in constructing optoelectronic devices with low power consumption and high-speed frequency response. In this work [1], we for the first time observed the NPC phenomenon in ReS2/h-BN/MoS2 vdW heterostructures-based floating gate phototransistor. The devices show excellent memory properties. More importantly, we realized the transition between the PPC and the NPC in the floating gate phototransistor. With these findings, the vdW heterostructures may open up a novel avenue for the fabrication of photoelectronic nonvolatile memory devices and multifunctional photoconductive devices. |
Monday, March 4, 2019 11:39AM - 11:51AM |
B15.00003: Interlayer translations in commensurately stacked transition metal dichalcogenide bilayers Madeleine Phillips, C Stephen Hellberg Transition metal dichalcogenide (TMD) bilayers are promising candidates for optoelectronic device components. In addition to being semiconductors that are optically active in the visible light range, they can exist in a vast array of stacking orientations due to the weak van der Waals bonding between layers. To take advantage of this geometric flexibility, it is essential to understand how changes in stacking orientation affect the properties of TMD bilayers. In this work, we use density functional theory (DFT) to study the stacking variations brought about by interlayer translations in 0 and 180-degree stacked WSe2/MoSe2 and WS2/MoS2 bilayers. We show that the 180-degree stacked bilayers have a single ground state geometry, while the 0-degree stacked bilayers have two degenerate ground state stackings. We also describe how stacking orientation correlates with interlayer distance and electronic structure, and we suggest ways to probe stacking geometry in photoluminescence experiments. |
Monday, March 4, 2019 11:51AM - 12:03PM |
B15.00004: Interlayer excitons in bilayer MoS2 with strong oscillator strength up to room temperature Bernhard Urbaszek, Iann Gerber, Emmanuel Courtade, Shivangi Shree, Cedric Robert, Xavier Marie, Kenji Watanabe, Takashi Taniguchi Coulomb bound electron-hole pairs, excitons, govern the optical properties of transition metal dichalcogenides (TMDs) like MoS2 and WSe2. TMD bilayers allow studying several exciton species : intralayer excitons with electrons and holes in the same layer and also interlayer excitons with the electron and the hole residing in different layers. |
Monday, March 4, 2019 12:03PM - 12:15PM |
B15.00005: Saturation and photon-energy dependence of ultrafast charge transfer in vdW heterostructures Eric Yue Ma, Burak Guzelturk, Guoqing Li, Linyou Cao, Zhi-Xun Shen, Aaron M Lindenberg, Tony F Heinz We have directly monitored the interlayer charge transfer after photoexcitation in type-II van der Waals heterostructures of monolayer transition metal dichalcogenides. Charge transfer is found to occur on the fs time scale by recording the electromagnetic transient emitted at THz frequencies using THz time-domain spectroscopy. This approach, insensitive to spectral changes of the materials and competing charge-neutral processes, is ideal for studying the dynamics at high excitation fluences and as a function of excitation photon energy. At low excitation fluence, we observe a linear increase in the interlayer current with fluence. However, as the fluence is increased, we see a clear saturation. We analyze this effect in terms of band re-alignment induced by the electric field from charge transfer itself. Such unavoidable saturation puts a limit on the power handling capability of these heterostructures and needs to be carefully characterized for practical applications. With further calibration, such measurements also offer a new route to determine band offsets. We further report the insensitivity of charge transfer dynamics to excitation photon energy within the instrument response. We discuss its implication on the mechanism of interlayer charge transfer. |
Monday, March 4, 2019 12:15PM - 12:27PM |
B15.00006: MoS2/Si and WS2/Si 2D/3D heterojunction photodiodes fabricated by low-temperature plasma enhanced CVD Soyeong Kwon, Yonghun Kim, Jungeun Song, Dongrye Choi, Eunah Kim, Jung-Dae Kwon, Byungjin Cho, Dongwook Kim We prepared MoS2 and WS2 tri-layers directly on Si substrates using atmospheric-pressure plasma enhanced chemical vapor deposition (AP-PECVD). High pressure (over 100 Torr) plasma formation and low-temperature (under 200°C) growth of AP-PECVD enabled high-quality 2D/3D semiconductor (SC) heterojunctions on a large area (over 4 inch) at a high deposition rate (10 minutes for 2 inch). The fabricated devices showed clear rectifying behaviors and large shunt resistance, indicating formation of uniform heterojunction diodes. Relatively large dark current of the heterojunctions under reverse bias could be attributed to band-to-band tunneling and avalanche multiplication process. Temperature dependence of ideality factor was studied to understand the major recombination processes, based on the conventional 3D SC model. Both of the MoS2/Si and WS2/Si heterojunctions exhibited large and fast photocurrent responses to the laser illumination (wavelength: 532 nm). The measured photocurrent was linear to the laser power, indicating interfacial defect states could not suppress photo-generated carrier collection. All the results demonstrated high quality 2D/3D SC heterojunction photodiodes with clean interfaces were prepared by AP-PECVD. |
Monday, March 4, 2019 12:27PM - 12:39PM |
B15.00007: High Degree Valley-Polarization from Interlayer Exciton in van der Waals Heterostructure Long Zhang, Rahul Gogna, Will W Burg, Jason Horng, Eunice Paik, Yu-Hsun Chou, Kyounghwan Kim, Emanuel Tutuc, Hui Deng Two-dimensional semiconductors promise valleytronics applications, where the valleys of the electronic bands, in addition to spin and charge, become a new degree of freedom for quantum engineering and information processing. Yet electron-hole exchange interactions lead to rapid valley depolarization and low valley polarizations (VPs) in monolayer materials. Here, by precisely aligning the momentum valleys of a WSe2 monolayer to a MoSe2 monolayer, we form a hetero-bilayer with bright interlayer excitons that inherit and preserve the VP from the intralayer ones while at the same time with greatly suppressed electron-hole exchange interactions. Using these hetero-bilayers, We demonstrate brightened spin triplet inter-layer excitons localized at the potential minimum of the moiré lattice with a VP up to 80% and a valley lifetime of 33 ns. Spin singlet transition localized at the same registry is also measured, with an as high but opposite VP. Our work leads the way of using van der Waals semiconductor heterostructures to control the charge transfer, VP, optical selection rules and other novel optoelectronic properties. |
Monday, March 4, 2019 12:39PM - 12:51PM |
B15.00008: Van der Waals TMDC Heterostructures: electronic structure dependence on number of layers Muhammad Sufyan Ramzan, Agnieszka Beeta Kuc Two-dimensional (2D) transition metal dichalcogenides (TMDCs) of type MX2 (M = Mo, W; X = S, Se) have attracted considerable attention in electronics and optoelectronics due to their extraordinary properties, which differ strongly between single layers and bulk materials. Moreover, weak van der Waals interactions between layers make it possible to exfoliate them and stack with different TMDC layers to form heterostructures, which offer remarkable electronic properties. In this work, we have studied interlayer coupling between various TMDC layers, which differ either in chalcogen or transition-metal atoms. Furthermore, we have investigated stacking on 3, 4, and 5 layers. The property we were interested in was the electronic structure, in particular, which stacking yields direct band gap materials. For instance, stacking a monolayer of WSe2 on top of MoS2 multi-layer (2 or more layers) can yield direct (type-II) band gap for up to 6-layers system (i.e., 1L WSe2 and 5L MoS2) whereas vice versa yields in-direct gap for even 5-layers system (i.e., 1L MoS2 and 4L WSe2). We believe that our findings could greatly contribute to the potential applications of heterostructure TMDCs in photovoltaics, energy harvesting, and solar cells. |
Monday, March 4, 2019 12:51PM - 1:03PM |
B15.00009: Characterization of the contact resistance in transition metal dichalcogenide heterojunctions Adam Pfeifle, Marcelo Kuroda Semiconducting (2H) transition metal dichalcogenides (TMDs) are known to form high resistance contacts with most metals. Recently, lateral heterojunctions formed by 2H and metallic (1T’) phases of TMDs have been proposed to reduce the contact resistance in these systems. Here we combine first principles and quantum transport calculations to rationalize the contact resistance of heterojunctions accounting for their phases (2H and 1T’), composition (WTe2, MoTe2, WSe2, and MoSe2), and channel length. We find that telluride 1T’ phases in metal/metal junctions are nearly ideal close to the Fermi level as Bloch states remain delocalized through the metal electrode and channel. Mixtures of 1T’ selenides and tellurides depart from this ideal scenario due to the momentum mismatch of Bloch states that limit carrier injection. The coupling between metallic (1T’) and semiconducting (2H) channels also shows large barriers (> 0.3 eV). The crossover between transport regimes governed by thermionic emission and tunneling is analyzed for the different compositions. We also discuss the presence of edge states in these heterostructures. This work may prove valuable to attaining low contact resistance suitable for optoelectronic applications based on two-dimensional materials. |
Monday, March 4, 2019 1:03PM - 1:39PM |
B15.00010: Quantum Transport in Transition Metal Dichalcogenide Monolayers and Twisted Bilayers Invited Speaker: James Hone This presentation will review our recent progress in studies of the magneto-transport properties of transition metal dichalcogenides. This work utilizes materials exfoliated from ultrahigh-purity single crystals grown by a flux synthesis technique, and encapsulation by h-BN to minimize external disorder. Ohmic contacts are achieved by work function control and dual-gating. These samples are studied in high magnetic fields by a combination of electrical transport measurements and probing the density of states using capacitance. I will describe a number of systems studied in this way, including WSe2 monolayers, bilayers, and bilayers with small interlayer twist angles. In addition, recent studies of semiconducting and metallic forms MoTe2 will be described. |
Monday, March 4, 2019 1:39PM - 1:51PM |
B15.00011: Photon-Induced Suppression of Interlayer Tunneling in Van Der Waals Heterostructures Wang Kong Tse, Woo-Ram Lee We present a theory for interlayer tunneling in van der Waals heterostructures driven under a strong electromagnetic field, using graphene/hBN/graphene as a paradigmatic example. Our theory predicts that strong anti-resonances appear at bias voltage values equal to an integer multiple of the light frequency. These features are found to originate from photon-assisted resonant tunneling transitions between Floquet sidebands of different graphene layers, and are unique to two-band systems due to the interplay of both intraband and interband tunneling transitions. Our results point to the possibility of tunneling localization in van der Waals heterostructures using strong electromagnetic fields. |
Monday, March 4, 2019 1:51PM - 2:03PM |
B15.00012: Ab initio Study and Search of Chalcogenide-oxide Interfaces Yuxin Yin, Jennifer Coulter, Prineha Narang The discovery of interface enhanced superconductivity has sparked interest in studying interfacial properties of quantum materials. Experimental and theoretical efforts highlight the relevance of spin fluctuation, orbital order and electron-phonon coupling to the onset enhanced superconducting behavior. However, the exact mechanism responsible for increased Tc is still not well-understood. Inspired by recent observations of interface enhanced superconductivity in FeSe/SrTiO3 system and ultrathin film CuS2, we use ab initio theoretical tools to investigate interfacial properties. We investigate interfacial effects on electronic structure, electron-electron and electron-phonon interactions, as well as study the effect of interlayer coupling in chalcogenide-oxide structures. Additionally, we aim to use our methods to identify superconducting interfaces in other chalcogenide material and substrate combinations. These theoretical investigations provide avenues to design and search for superconducting chalcogenide-oxide interfaces. |
Monday, March 4, 2019 2:03PM - 2:15PM |
B15.00013: Effect of stacking orientation on the electronic and optical properties of 2D nitride heterostructures Nocona Sanders, Emmanouil Kioupakis Given the successful synthesis of 2D GaN and investigations into the properties of freestanding 2D nitrides, heterostructures of these materials are now of particular interest. Extreme quantum confinement is a viable method to shift light emission to shorter wavelengths, but in 2D nitrides this is counteracted by the quantum-confined Stark shift due to the strong inherent polarization perpendicular to the 2D plane. We report the electronic and optical properties of 2D BN, GaN, AlN, and InN in various stacking orientations, such that the electric fields are either aligned or anti-parallel in two possible configurations. We employ density functional theory and quasiparticle corrections with the GW method, as well as the Bethe-Salpeter Equation, to derive accurate band structures, exciton binding energies, and luminescence energies. Through understanding how the stacking arrangement influences the underlying electronic and optical properties, critical insight will be gained in how to improve 2D III-nitride-based optoelectronics through accessing the additional degree of freedom provided by polarization. |
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