Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session W57: Physical Engineering of 2D MaterialsFocus
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Sponsoring Units: DMP Chair: Nasim Alem Room: Mile High Ballroom 3A |
Friday, March 6, 2020 8:00AM - 8:12AM |
W57.00001: Anomalous phonon vibrations of monolayer MoS2 revealed by polarization degree of freedom of light Teng-De Huang, Kristan Bryan Simbulan, YannWen Lan, Ting-Hua Lu Light of linear and circular polarization is broadly used to explore interesting light-matter interaction in layered two-dimensional transition metal dichalcogenides (TMDs) such through Raman scattering and photoluminescence. Little attention has been given to the interaction of TMD materials and light with the elliptical polarization state. We applied elliptically polarized light in micro-Raman spectroscopy to observe variations in the relative intensity of the two most dominant Raman peaks of MoS2. This provides an additional degree of freedom affecting the relationship between the two orthogonal components of the material’s in-plane lattice vibrations. Different magnitudes of the in-plane lattice vibrations along two orthogonal directions were induced by varying the light from circularly polarized to elliptically polarized states. Besides, with changing the incident linear polarization angle, the out-of-plane A1g phonon mode red shifts, while the in-plane E2g phonon mode blue shifts. The anomalous lattice vibrations of monolayer MoS2 originate from the built-in strain introduced by the SiO2/Si substrate. This work investigates the material's unexplored fundamental phonon property which may enlighten past and future studies. |
Friday, March 6, 2020 8:12AM - 8:24AM |
W57.00002: A first-principles study of the thermodynamic and vibrational properties of ReS2 under pressure Natalya Sheremetyeva, Damien Tristant, Anthony Yoshimura, Jason Gray, Liangbo Liang, Vincent Meunier Two-dimensional layered materials (2DMs) are promising candidates for novel devices due to their tunable properties. The properties can be tuned by e.g. controlling the number of layers or applying external pressure on the structures. In order to control the (tuned) properties exactly, precise characterization of 2DMs' is needed. Here, the effect of hydrostatic pressure on the structural, energetic, electronic, and vibrational properties of layered bulk ReS2 was studied using density functional theory. The electronic band gap of the 1T phase is shown to undergo a nearly-direct to indirect transition at about 9 GPa, while the 1T' phase is found to remain a robust nearly-direct band gap material under pressure. The computational analysis of the vibrational properties of both ReS2 phases reproduced existing experimental Raman spectroscopy data for ω vs. P trends and provided a path towards an accurate phase discrimination using infrared spectroscopy, inelastic neutron, and X-ray scattering. |
Friday, March 6, 2020 8:24AM - 8:36AM |
W57.00003: Van der Waals and Electron Stimulated Covalent Hydrogenation of Borophene Shaowei Li, Matthew S Rahn, Qiucheng Li, Mark C Hersam The recent experimental realization of atomically thin, crystalline boron sheets has opened a new direction in the study of synthetic 2D materials. Chemical modification of borophene could further enable fine-tuning of its band structure for nanoelectronic applications. Here, we present the first experimental study of both the non-covalent and covalent hydrogenation of borophene. Adsorbed hydrogen molecules are observed to form a triangular lattice on both v1/6 and v1/5 borophene polymorphs. The rotational excitation of H2 is detected at 36 meV, indicating the adsorption of hydrogen is dominated by its van der Waals interaction with borophene. The formation of this H2-borophene vertical heterostructure increases the work function of borophene. Moreover, covalent modification of borophene is achieved by inducing a hydrogen-borophene reaction at elevated tip bias voltage. This reaction significantly modifies the electronic structure of borophene and further increases its work function. Overall, this work illustrates the sensitivity of the electronic properties of borophene to chemical modification, which is expected to stimulate further exploration of boron-based chemistry in the two-dimensional limit. |
Friday, March 6, 2020 8:36AM - 8:48AM |
W57.00004: Native point defects and impurities in WSe2 Darshana Wickramaratne Tungsten diselenide (WSe2) is being actively pursued for two-dimensional electronics and as a host for single-photon emitters. We have studied the properties of native defects and impurities in WSe2 using density functional theory with a hybrid functional. We find that all native defects give rise to levels that are deep in the band gap, and thus will not contribute to conductivity. The defect chemistry of WSe2 is likely dominated by common background impurities. In particular, substitutional oxygen, carbon, and transition metal impurities such as vanadium, are low energy defects. Our results allow us to comment on the origin of the frequently observed p-type conductivity and sub-band gap emission lines in as-grown WSe2 |
Friday, March 6, 2020 8:48AM - 9:00AM |
W57.00005: Discontinuous Evolution of the Structure of Stretching Polycrystalline Graphene. Federico D'Ambrosio, Vladimir Juricic, Gerard T. Barkema Polycrystalline graphene has an inherent tendency to buckle, i.e. develop out-of-plane, three-dimensional structure, yielding a rich landscape of configurations. A force applied to stretch a piece of polycrystalline graphene influences the out-of-plane structure. In this talk, we show that if the graphene sheet is well-relaxed, as long as is not completely crystalline, this happens in non-linear fashion: occasionally, a tiny increase in stretching force induces a significant displacement, an avalanche-like event in which ridges and vertices are created and annihilated around the defects, which in turn can create vibrations in the surrounding medium [1]. We establish this effect in computer simulations: by continuously changing the strain, we follow the non-affine displacements of the atoms that turn out to exhibit a discontinuous evolution. Furthermore, the displacements exhibit a hysteretic behavior upon the change from low to high stress and back. Our results motivate further studies of dynamical elasticity of polycrystalline quasi-two-dimensional systems, and in particular the implications on their mechanical and thermal properties. |
Friday, March 6, 2020 9:00AM - 9:12AM |
W57.00006: Control of thermoelectric properties in MoSe2 thin films by using He+ irradiation Hyuk Jin Kim, Van Quang Nguyen, Thi Huong Nguyen, Yangjin Lee, Sera Kim, Maeng-Je Seong, Kwanpyo Kim, Sunglae Cho, Young Jun Chang Defect engineering have been widely used via chemical doping and ion irradiation techniques for modifying electrical and thermal properties. For increasing figure of merit, ZT=α2σT/κ in thermoelectric materials, tuning of electrical and thermal characteristics plays an important role, where α is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity. Here, we present our experimental control of both electrical conductivity and Seebeck coefficient of MoSe2 thin films by irradiating high-energy He+ ion beams. From different doses, we observed enhancement of electrical conductivity despite small decrease of Seebeck coefficient, which results in improvement of thermoelectric performance, i.e. power factor (α2σ), for temperature range of 300K – 420K. To understand the irradiation-induced defects, we discuss complementary experimental analysis, such as X-ray diffraction, transmission electron microscopy and Raman spectroscopy. |
Friday, March 6, 2020 9:12AM - 9:48AM |
W57.00007: Shape - Composition - Defect Interplay in 2D Transition Metal Dichalcogenides Invited Speaker: David Srolovitz We focus on the interplay of the 2D sheet shape via local variations in composition and/or variations in topological defect density through a combination of strain engineering and shape programming. This includes patterning composition and defect profiles via growth on non-flat substrates and patterning profiles via composition patterning. We implement these ideas theoretically and within phase field and finite element method computations. We then explore applications of these ideas to create a diverse set of composition, defect, and shape patterns and programming bilayer twist. We then exploit Janus structures to program bilayer twist and to actuate dynamic shape change. |
Friday, March 6, 2020 9:48AM - 10:00AM |
W57.00008: Study of the Shape Single Crystal Graphene Growth by Chemical Vapor Deposition on Copper Foil Tharanga Nanayakkara, Sajith Withanage, U. Kushan Wijewardena, Annika Kriisa, Rasanga L Samaraweera, Ramesh Mani The chemical vapor deposition (CVD) growth of single-crystal graphene on polycrystalline copper foils is a complex process affected by thermodynamics, kinetic, and growth parameters. Moreover, these factors lead to the diversity of island shapes of single-crystal graphene, including hexagons, flowers, squares, stars, dendrites, butterflies, hourglass, and lobes. Here, we present experimental observations of the different shapes of the micrometer-sized single-crystal graphene on copper foil obtained by the CVD technique. We applied Atomic Force Microscopy (AFM) and optical microscopy techniques to examine the diverse growth morphologies of the graphene shapes in different copper domains with various crystal orientations and the evolution of the nuclei shapes over the time. |
Friday, March 6, 2020 10:00AM - 10:12AM |
W57.00009: Coupling of Quantum Valley Hall Edge States between CVD Bilayer Graphene Layer Stacking Domain Walls Qicheng Zhang, Sheng Wang, Zhaoli Gao, Sebastian Hurtado Parra, Paul Masih Das, Joel M Berry, Zachariah M Addison, William Parkin, Marija Drndic, James Makato Kikkawa, Eugene John Mele, Feng Wang, Zhengtang Luo, Alan T Johnson Quantum valley Hall (QVH) edge states are topologically protected in the absence of valley mixing. In addition to the long mean free path from the protection, their inter-wall coupling enables novel properties such as current partition and valley filtering, when arranged in specific configurations. Stacking order alternation across a boundary in Bernal stacking bilayer graphene creates a layer stacking wall (LSW) where QVH edge states are bound. However, the uncontrollable strain of LSWs upon their creation places a big challenge to study their inter-wall coupling. Here we use different strain mechanisms during chemical vapor deposition (CVD) and subsequent transfer processess to create well-separated LSWs and close packing LSW bundles, and report inter-wall coupling of QVH edge states in the latter case through electron transport measurements. |
Friday, March 6, 2020 10:12AM - 10:24AM |
W57.00010: Preferential hole formation in WSe2 by electron beam irradiation Donghan Shin, Andrew O'Hara, Junhao Lin, Sokrates T Pantelides In recent years, the trigonal-prismatic transition-metal dichalcogenides (TMDs) have been extensively studied due to a wide range of physical properties and applications. Scanning transmission electron microscopy (STEM) is a powerful tool for the investigation of material systems at the atomic level and the imaging of defect configurations. Moreover, STEM can be used to induce and modify defects. We will show that, using STEM, round multivacancy holes of various diameters and densities can be easily formed and stabilized in WSe2, but not in other TMDs like WS2. We report density-functional-theory (DFT) calculations to investigate the structure and stability of the observed defects in WSe2. We construct a comprehensive model to explain how the observed stable defects form and grow in different TMDs by calculating formation energies, displacement thresholds, and electronic structures. The demonstrated control of high-density uniform multivacancies has potential for applications relating to molecular translocation. |
Friday, March 6, 2020 10:24AM - 10:36AM |
W57.00011: Structural Evolution of Many-Layer Epitaxial Graphene on 4H-SiC During Low-Energy Ion Implantation Paul Miceli, Alessandro Mazza, Anna L Miettinen, Timothy R Charlton, Thomas Zac Ward, Xiaoqing He, Alex A Daykin, Suchismita Guha, Guang Bian, Edward Conrad The ability to modify graphene with defects and chemisorption is of interest for its potential to tailor graphene’s physical properties, including induced p-orbital magnetism that could be useful for spintronics. As a pathway towards these goals, we have investigated the behavior of multilayer epitaxial graphene implanted by low-energy ions (360-2000 eV) of H, D and He. In situ x-ray scattering reveals that the ions expand the spacing between graphene layers, which depends on the type of ion, the fluence, as well as whether the range of the ion distribution resides at the buried interface or within the graphene. Neutron reflectivity measurements show that H remains chemisorbed in the graphene. We show that the apparently different behavior among the ions and their distributions can be understood by a single concept and by the appropriate scaling of the data. |
Friday, March 6, 2020 10:36AM - 10:48AM |
W57.00012: Controlling Nanoscale Thermal Expansion of Monolayer Transition Metal Dichalcogenides by Alloy Engineering Xuan Hu, Zahra Hemmat, Leily Majidi, John Cavin, Rohan Mishra, Serdar Ogut, Amin Salehi-Khojin, Robert Klie Two dimensional materials, such as transition metal dichalcogenides (TMDs), graphene, boron nitride (BN) are seen as promising materials for future high power/high frequency electronics. However, the large difference in the thermal expansion coefficient (TEC) between many of these 2D materials could impose a serious challenge for the design of monolayer-materials-based nano-devices. To address this challenge, alloy engineering of TMDs is used to tailor their TECs. Here, in-situ heating experiments in a scanning transmission electron microscope are combined with electron energy-loss spectroscopy and first-principles modeling of monolayer Mo1-xWxS2 with different alloying concentrations to determine the TEC. Significant changes in the TEC are seen as a function of chemical composition in Mo1-xWxS2, with the smallest TEC being reported for a configuration with the highest entropy. This study provides key insights into understanding of nanoscale phenomena that control TEC values of 2D materials. |
Friday, March 6, 2020 10:48AM - 11:00AM |
W57.00013: Superelasticity and anomalous mechanical response of two-dimensional SiO2 under tensile and bending deformations Ricardo Nunes, Nestor Reina, Helio Chacham Silica displays a rich phenomenology of mechanical response to deformations in its 3D form. It recrystallizes by dislocation creep and shows non-linear elastic behavior under large strains. In this study, we discuss results from first-principles calculations, that indicate that the recently synthetized 2D form of silica (2D-SiO2) displays as rich a reponse to mechanical deformations as its 3D counterpart. Under tensile uniaxial strain of up to 30% in both armchair (AC) and zigzag (ZZ) directions, 2D-SiO2 shows a non-linear elastic response, and the stress-strain curves do not display the plastic deformation platoo. At larger than 30% strains, the response is highly anisotropic: while in the AC direction the lattice breaks, in the ZZ direction it recovers the broken bonds and undergoes a process akin to a 15o rotation of the lattice. For even larger strains another effective rotation (by 30o) of the lattice takes place. The effective rotation is shown to occur also in the presence of vacancies in the SiO2 layer. Moreover, 2D-SiO2 shows an intrinsic non-linear response to bending deformations even for small deformations. We identify a metastable structure of a 2D-SiO2 layer that is connected with the anomalous bending response of 2D silica. |
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