Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session X15: 2D Materials (Semiconductors) -- Synthesis, Defects, and StrainFocus
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Sponsoring Units: DMP DCOMP Chair: Guillaume Gervais, McGill University Room: BCEC 154 |
Friday, March 8, 2019 8:00AM - 8:12AM |
X15.00001: Spinodal Dewetting Instabilities on Graphene and two-dimensional atomic crystals Valeri Kotov, David Peterson, Juan Vanegas We demonstrate theoretically, for the first time, the possibility of spinodal dewetting in heterostructures made of light-atom liquids (hydrogen, helium, and nitrogen) deposited on graphene and other 2D substrates. Extending our theory of film growth on 2D materials [S. Sengupta et al., Phys. Rev. Lett. 120, 236802 (2018)] to include analysis of surface instabilities via the hydrodynamic Cahn-Hilliard – type equation, we characterize in detail the resulting periodic patterns. Both linear stability analysis and advanced computational treatment of the surface hydrodynamics show unconventional, micron-sized (generally material dependent) patterns of “dry” regions. The physical reason for the development of such instabilities on graphene can be traced back to the inherently weak van der Waals interactions between atomically-thin materials and atoms in the liquid, causing the arrest of film growth under normal equilibrium conditions and triggering the associated surface instabilities. These phenomena are robust to some mechanical deformations and are also universally present in doped graphene and other 2D materials, such as monolayer dichalcogenides. Thus, two-dimensional materials represent a universal theoretical and technological platform for studies of spinodal dewetting. |
Friday, March 8, 2019 8:12AM - 8:24AM |
X15.00002: Phase transition in MoX2 (X = S, Se, Te) monolayers with hydrogenation Priyanka Manchanda, Pratibha Dev Surface functionalization of two-dimensional monolayers can produce materials with different structural and electronic properties as compared to their pure counterparts. The lowest energy phase for molybdenum (Mo) based transition metal dichalcogenides is the semiconducting hexagonal H phase. However, the alternative T phase with Mo atoms in octahedral coordination and the distorted version of the T phase (T') can be stabilized by intercalation, chemical doping, and charge mediation. In this work, we use density functional theory calculations to demonstrate that H to T phase transition can be achieved in MoX2 monolayer with hydrogenation. We find that the phase stability of the T phase increases as a function of hydrogenation coverage. This phase transition results in the semiconductor to metallic transition in these monolayers. Our results suggest that these MoX2 monolayers with controllable structural and electronic properties can find applications in catalysts and nanodevices. |
Friday, March 8, 2019 8:24AM - 8:36AM |
X15.00003: The Effect of Strain and Strain Relaxation on the Atomic-Scale in Monolayer MoS2 Films Dan Trainer, Yuan Zhang, Fabrizio Bobba, Xiaoxing Xi, Saw W Hla, Maria Iavarone The ability to control nanoscale electronic properties by introducing macroscopic strain is of critical importance for the implementation of 2D materials into flexible electronics and next generation strain engineering devices. In this work we use scanning tunneling microscopy and spectroscopy (STM/STS) to correlate the atomic-scale lattice deformation and local electronic properties with a systematic macroscopic bending of monolayer molybdenum disulfide (MoS2) films, using a custom-built sample holder. Using this technique, we find a reduction of the quasiparticle band gap with increasing strain. In addition, nanoscale strain relaxation of van der Waals monolayer sheets has been investigated and resulted in 1D ripples and 2D wrinkles which alter the local strain fields as well as the local electronic density of states. |
Friday, March 8, 2019 8:36AM - 8:48AM |
X15.00004: Role of Strain in Quantum-Confined States of Heterobilayers of 2D Materials Dacen Waters, Felix Lukas Lüpke, Yifan Nie, Yi Pan, Stefan Fölsch, Yu-Chuan Lin, Bhakti Jariwala, Kehao Zhang, Kyeongjae Cho, Joshua Robinson, Randall Feenstra In a recent report [1], the present authors demonstrated the existence of quantum-confined states arising from spatially varying band edge energies in a MoS2/WSe2 vertical heterostructure. Here, this system is studied in more detail, combining results from low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements and density functional theory (DFT) calculations. It is experimentally observed that in addition to the band edge shifts, the moiré pattern, which forms due to a 3.7% lattice mismatch, results in spatially varying biaxial strain in the MoS2 of ~2% tension (compression) at the corrugation minima (maxima). Including band edge shifts due to strain and corrugation of the underlying WSe2 leads to a consistent explanation for the observed spatial locations of the quantum-confined states. |
Friday, March 8, 2019 8:48AM - 9:00AM |
X15.00005: Spin effects at mirror twin grain boundaries in monolayered dichalcogenides Miguel Pruneda First-principles calculations are used to study the origin of the charge density wave observed in 1D inversion domain grain boundaries in monolayered MoSe2. A small structural (Peierls) distortion with the same periodicity observed in the STM experiments can be reproduced. However, an opening of the gap does not require this distortion, which can be obtained with a suitable spin configuration for the edge electronic states. Spin-orbit effects on these peculiar interfacial states are also discussed. Remarkably, similar spin configurations can also be obtained for edge states in zigzag-edged domains. |
Friday, March 8, 2019 9:00AM - 9:12AM |
X15.00006: Point defects, impurities, and single-photon emitters in hexagonal boron nitride Leigh Weston, Darshana Wickramaratne, Mark E. Turiansky, Mazena Mackoit, Audrius Alkauskas, Chris Van de Walle Hexagonal BN (h-BN) is attracting a lot of attention for two-dimensional electronics and as a host for single-photon emitters. We have studied the properties of native defects and impurities in h-BN using density functional theory with a hybrid functional. Native vacancy and antisite defects have high formation energies. Self-interstitials can have low formation energies, but their low migration barriers render them highly mobile, and they are unlikely to be present as isolated defects. We find that the defect chemistry of h-BN is likely dominated by unintentional impurities rather than native point defects. Substitutional carbon and oxygen, as well as interstitial hydrogen and boron vacancy–hydrogen complexes, are low-energy defects in h-BN. Our results allow us to comment on frequently observed emission lines and on the microscopic origins of single-photon emitters. |
Friday, March 8, 2019 9:12AM - 9:24AM |
X15.00007: Localized strain tuning of exciton emissions on suspended two-dimensional materials Hyowon Moon, Chitraleema Chakraborty, Gabriele Grosso, Dirk R. Englund Two-dimensional materials can endure extraordinarily large strain which strongly influences its electronic and optical properties1. Strain tuning has been reported for free excitons in transition metal dichalcogenides (TMDs) 2,3 as well as for single photon emissions in TMDs4 or in hexagonal boron nitrides (hBN)5. However, strain applied on the entire substrate or over few micrometers has a limited ability to produce or control individual strain-induced quantum emissions in TMDs. Here we demonstrate extremely localized strain tuning of exciton emissions in suspended 2D materials induced by sub-10nm radius tip mechanical probe, which can be potentially used to create and tune nonclassical light sources in atomically thin materials. |
Friday, March 8, 2019 9:24AM - 9:36AM |
X15.00008: Strain-tunable single-photon emission from an atomically thin semiconductor Chitraleema Chakraborty, Kumarasiri Konthasinghe, Hyowon Moon, Dirk R. Englund, Nick Vamivakas One of the most unique mechanical properties of two-dimensional (2D) materials over conventional semiconductors is their high stretchability of more than 20% before fracture1. Recently strain induced single photon emitters have been observed in 2D transition metal dichalcogenides2-6. Tuning their emission energy is highly desired for efficient coupling to photonic devices. We demonstrate such tunability by engineering van der Waals heterostructure consisting of graphene/h-BN/WSe2 on a piezoelectric actuator. Application of an electric field across the piezoelectric substrate induces a biaxial strain on the emitters hosted by the monolayer WSe2 in the heterostructure. We have demonstrated a strain tunable energy shift of tens of meV using this approach. |
Friday, March 8, 2019 9:36AM - 9:48AM |
X15.00009: Electrical Control of Defect Assisted Trapped Excitons and trions States in Monolayer MoS2 Pradeepa H L, Praloy Mondal, Aveek Bid, Jaydeep K Basu Unique optical properties of monolayer MoS2 such as strong binding energy of excitons and trions at room temperature make it a suitable material to study the dynamics of these quasi particles. We report a transition between quasiparticle states using Time resolved Photoluminescence (TRPL) and Photoluminescence (PL) spectroscopy by tuning the carrier concentration of MoS2 monolayer in FET configuration. Since the defect assisted trapping potential is electrically sensitive, we are able to control the trapping states of quasiparticle by applying gate voltage in FET. This control of defect assisted states could influence the exciton exciton annihilation mechanism which is one of the dominant nonradiative decay channel of excitons in two dimensional materials. Our study helps in understanding the exciton dynamics in 2D materials. |
Friday, March 8, 2019 9:48AM - 10:24AM |
X15.00010: Synthesis of large-scale transition metal dichalcogenide films and their applications in hydrogen production Invited Speaker: Judy Cha Transition metal dichalcogenides (TMDCs) have been studied extensively for their attractive mechanical, electrical, optical, and electrochemical properties. A large scale synthesis of these materials in thin films down to monolayers with thickness control and high crystalline quality remains a major hurdle to utilize them in applications. In this talk, I will discuss our efforts to synthesize centimeter-scale thin films and heterostructures of TMDCs including tellurides such as WTe2 and MoTe2. I will discuss how the morphology and grain size of the thin films is intimately linked to the strain that builds during synthesis and ways to mitigate such strain. With our synthesized films, we explore thermal conductivity of WTe2 thin films and aspects of hydrogen evolution reaction that is often overlooked, such as the Schottky barrier at the interface between the chalcogenide catalyst (for example MoS2) and the electrode and the dielectric substrate effect. Finally, I will discuss the detrimental surface oxidation effect on the transport properties of the TMDCs, which must be overcome for large-scale applications. |
Friday, March 8, 2019 10:24AM - 10:36AM |
X15.00011: ABSTRACT WITHDRAWN
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Friday, March 8, 2019 10:36AM - 10:48AM |
X15.00012: ABSTRACT WITHDRAWN
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Friday, March 8, 2019 10:48AM - 11:00AM |
X15.00013: Mechanical control of valley magnetization and Berry curvature dipole in monolayer MoS2 Joolee Son, Kyung-Han Kim, Younghwan Ahn, Hyun-Woo Lee, Jieun Lee Monolayer transition metal dichalcogenides (TMDs) are known to have the valley degree of freedom of electrons in momentum space, called K and K’. The control of valley degree of freedom utilizes the valley-dependent Berry curvatures which have the opposite signs at the K and K’ valleys. Here we report a new type of valley control using the Berry curvature dipole. By applying strain to monolayer MoS2, we show that Berry curvature distributions about K and K’ valleys become asymmetric, leading to the emergence of the Berry curvature dipole. The Berry curvature dipole is manifested by the valley magnetization arising as functions of the Berry curvature dipole and an in-plane electric field. We fabricated several flexible monolayer MoS2 devices and measured the valley magnetization by scanning Kerr rotation microscopy. We will discuss the dependence of the valley magnetization on the direction and magnitude of strain and in-plane electric fields. Our results obtained at room-temperature pave a way for practical valley-based electronic devices and information processing. |
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