Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B18: Graphene and van der Waals Materials I |
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Sponsoring Units: DCMP DMP Chair: Vincent Tung, University of California, Merced Room: LACC 306B |
Monday, March 5, 2018 11:15AM - 11:27AM |
B18.00001: Epitaxial Growth and Properties of 2D Monolayer PtSe2 & In-plane Heterostructures YELIANG WANG In this talk, I will report the fabrication and properties of novel 2D materials like semiconducting transition-metal-dichalcogenide monolayer PtSe2 [1] and its in-plane 1T/1H heterostructure with atom-shape interface [2], as well as superconductor transition-metal-trichalcogenide (HfTe3)[3], grown by direct selenization/tellurization of the Pt/Hf substrate. Their application exploring in nanoelectronics and valleytronics will also be introduced. In addition, the stacking heterolayers based on several these kinds of 2D materials, for instance, a superconductor-topological insulator layered heterostructure (with an HfTe3/HfTe5 layered configuration) for Majorana bound states will be briefly presented [4,5]. We expect that these materials will show intriguing physical property and promising potential applications in nanoscale devices. |
Monday, March 5, 2018 11:27AM - 11:39AM |
B18.00002: Thermally driven structural phase transition in SnSe monolayers in the NVT ensemble Abhiyan Pandit, Salvador Barraza-Lopez SnSe monolayers undergo a thermally driven two dimensional (2D) structural phase transition1-3 that results in a sudden change of lattice parameters that is consistent with experimental findings4. On another theoretical work, a different transition on a system with fixed walls (hence on an NVT ensemble) has been discussed. Here, we show that the phenomena on that other theory can be understood in terms of Car-Parrinello molecular dynamics (MD) calculations based on the NVT (constant number of atoms, volume, and temperature) ensemble. The present results thus contribute to a unified theoretical account to understand 2D structural transitions in these materials. |
Monday, March 5, 2018 11:39AM - 11:51AM |
B18.00003: Phase engineered synthesis of metallic 1T’ MoTe2 and semiconducting 2H MoTe2 Xiaolong Xu, Yu Ye MoTe2 is distinguished from other transition metal dichalcogenides by the existence of an exceptional metallic distorted octahedral structure (1T’) with a small energy difference from its semiconducting (2H) phase, providing the possibility to controllable synthesis of the both phases. Probing the phase engineered synthesis mechanism between these different phases may provide potential applications in large-scale logic circuit. By controlling the growth temperature, growth time and precursors’ pressures, we are able to synthesize millimeter-scale few-layer single-crystalline 2H MoTe2. Combining the density functional theory calculation, dark field transmission electron microscopy and Raman spectroscopy, the nucleation of 2H domains and phase transition from 1T’ to 2H during synthesis are studied, enabling the large-scale controllable synthesis of high-quality 1T’ and 2H MoTe2 few-layers with less grain boundaries. |
Monday, March 5, 2018 11:51AM - 12:03PM |
B18.00004: Telluriding Monolayer MS2 (M=Mo, W) via Sodium Scooter Seok Joon Yun, Hyun Kim The conversion of chalcogen atoms to other types in transition metal dichalcogenides has significant advantages for tuning bandgaps1 and constructing in-plane heterojunctions2; however, difficulty arises from the conversion of sulfur or selenium to tellurium atoms owing to the low decomposition temperature of tellurides3. Here, we propose the use of sodium metal to deliver Te atoms for converting monolayer MoS2 to MoTe2 under a Te-rich vapor. Sodium metal easily anchors Te atoms and reduces the exchange barrier energy by scooting the Te atoms to replace the S atoms. The conversion was initiated at the edges and grain boundaries of monolayer MoS2, followed by a complete conversion in the entire region. By controlling sodium concentration and reaction temperature of monolayer MoS2, various phases such as semiconducting 2H-MoTe2, metallic 1T'-MoTe2, and 2H-MoS2−xTex alloys were obtained with a wide range of electrical bandgap energies from 1.1–2.1 eV. This concept was further extended to WS2. A high valley polarization of ~37% in circularly polarized photoluminescence was obtained in the monolayer WS2−xTex alloy at room temperature. |
Monday, March 5, 2018 12:03PM - 12:15PM |
B18.00005: GaS/ZnS heterojunctions via sulfurization: thin film deposition and characterizations Zong Wu, Quark Chen, Wanchen Hsieh, P. V. Wadekar, Yi Liang, Shih Huang, Che Min Lin, Wei Ciou, Po Lin, Ming Chou, Li-Wei Tu, Ikai Lo, Chien-Cheng Kuo, Shin-Ming Huang, Yi Lu, Wei-Kan Chu, Chien-Chun Chen, Hye-Won Seo Thin films of Ga(II)S as produced by sulfurization of gallium oxides are investigated concerning their structures, electrical and optical properties. The weak interlayer coupling of layer compounds facilitates ease of growth into 2D materials or heterojunction structures with other 3D counterparts. In this work, we focused on the superlattices of GaS/ZnS through substitution of oxygen with sulfur atoms starting from their pristine form of GaO/ZnO superlattices mostly put on sapphire substrates, though other substrates have also been used. The chemical substitution was conducted by vaporizing sulfur powders at a proper temperature to achieve a suitable vapor pressure by which the reaction could reach completion. The samples thus obtained were characterized by various techniques of X-ray diffraction (XRD) and grazing angle reflectivity (XRR) measurements from which the crystal structure, interface smoothness and mass density of the constituent layers can be determined. Variable-temperature magneto-transport behaviors and luminescence properties will be discussed in relation to the material processing conditions of the sulfide-based quantum-well structures. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B18.00006: MoS/ZnS heterojunctions via sulfurization: thin film deposition and characterizations Shih Huang, Wanchen Hsieh, P. V. Wadekar, Che Min Lin, Chun Fu Chang, H. C. Huang, Hye-Won Seo, Yi Liang, Zong Wu, Ting Lin, Wei Ciou, Po Lin, Ming Chou, Li-Wei Tu, Ikai Lo, Chien-Cheng Kuo, Shin-Ming Huang, Yi Lu, Chi Liao, Wei-Kan Chu, Quark Chen Sulfurization of molybdenum oxides to produce thin films of Mo(II)S was investigated concerning the structures, electrical and optical properties of teh resulted materials. The weak interlayer coupling of layer compounds facilitates ease of growth into 2D materials or heterojunction structures with other 3D counterparts. In this work, we focused on the superlattices of MoS/ZnS through substitution of oxygen with sulfur atoms starting from their pristine form of MoO/ZnO superlattices mostly put on sapphire substrates, though other substrates have also been used. The chemical substitution was conducted by vaporizing sulfur powders at a proper temperature to achieve a suitable vapor pressure by which the reaction could reach completion. The samples thus obtained were characterized by various techniques of X-ray diffraction (XRD) and grazing incidence reflectivity (XRR) measurements from which the crystal structure, interface smoothness and mass density of the constituent layers can be determined. Variable-temperature magneto-transport behaviors and luminescence properties will be discussed in relation to the material processing conditions of the sulfide-based quantum-well structures. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B18.00007: ZnS/MoSx superlattices: growth and characterizations of physical properties Yi Liang, Wanchen Hsieh, P. V. Wadekar, Chun Chang, Hui Huang, Che Min Lin, Chih Chung Ke, Yu Hsuan Yu, Hye-Won Seo, Shih Huang, Zong Wu, Wei Ciou, Po Lin, Ming Chou, Li-Wei Tu, Ikai Lo, Chien-Cheng Kuo, Yi Lu, Wei-Kan Chu, Chien-Chun Chen, Chi Liao, Quark Chen While layered compound of molybdenum disulfide (MoS2) has re-emerged as a two-dimensional channel material of high carrier mobility for gated devices, its ultimate integration with the silicon technology would require good epitaxial quality and the path to maturity for this adoption is still at a beginning stage. Concept of superlattice heterojunction structures with other compound materials provide an additional route around the challenges. In this work, we had an attempt at ZnS/MoSx superlattices hoping to acquire different stoichiometry of the molybdenum sulfides in different configurations of molecular coordination so that stable crystal structures with desired electronic characteristics, high carrier mobility included, would be found. Atomic Layer Deposition (ALD) method was used for this purpose in the preliminary effort. The properties of the film studied with X-ray diffraction (XRD), X-ray reflection (XRR), grazing incident angle X-ray diffraction (GIXRD), transmission electron microscopy (TEM), and photoluminescence (PL) will be presented and their applications as a channel semiconductor in a device will also be discussed. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B18.00008: Identification of crystallographic structure of ReS2 by polarized Raman spectroscopy Yun Choi, Jungcheol Kim, Jung Hwa Kim, Zonghoon Lee, Hyeonsik Cheong ReS2 is attracting interest recently as a 2-dimensional semiconducting material. Unlike MoX2 and WX2(X=S, Se), ReS2 has a direct bandgap from single-layer to bulk. In addition, due to the structural anisotropy, the physical properties are different depending on the orientation, like black phosphorus. Since the front and back sides are also distinct, a method for determining not only the in-plane direction but also vertical orientation of a ReS2 is required. We performed polarized Raman spectroscopy to investigate the polarization dependence on crystallographic orientation of ReS2. From the measurements, the polarization dependence of ReS2 is different on the front and back side due to the anisotropic structure. In addition, we confirmed the lattice structure of the sample by high-resolution scanning transmission electron microscopy(HRSTEM) measurements. From the correlation between polarized Raman measurements and HRSTEM results, we developed a method to determine the crystallographic orientation of ReS2 both the in-plane direction and the front and back side. |
Monday, March 5, 2018 12:51PM - 1:03PM |
B18.00009: Imaging the In-Plane Anisotropy of ReS2 Using Scanning Tunneling Microscopy Ryan Plumadore, Justin Boddison-Chouinard, Florence Grenapin, Ahmed Rezk, Gregory Lopinski, Adina Luican-Mayer Among the layered transition metal dichalcogenides, the compounds that exhibit in-plane anisotropy are of particular interest as they offer an additional tuning knob for their novel properties. In this talk we focus on studying the nanoscale lattice structure of semiconducting ReS2 by using an ultrahigh vacuum, room temperature scanning tunneling microscope. We demonstrate that rhenium atoms form diamond-shaped clusters, organized in disjointed chains. We further characterize the structure and properties of lattice defects in the anisotropic planes of ReS2. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B18.00010: Experimentally Validated Interface Structures of Pt Nanoclusters on MoS2 Yongliang Shi, Boao Song, Reza Shahbazian Yassar, Jin Zhao, Wissam Saidi We systematically predict the structures and energetics of nanoclusters on (001) using a genetic algorithm utilizing atomistic force fields and density functional theory, and predictive classical models for large clusters based on symmetry and Wulff-Kaischew construction. The supported n≤20 structures are validated atom-by-atom using aberration-corrected scanning transmission electron microscopy that can yield precise atomistic positions. We find that relatively small clusters grow with (111) orientation such that Pt is parallel to MoS2, which is different from predictions based on lattice-match for thinfilms epitaxy. The underpinning of this growth mode is the tendency of the NPs to maximize the metal-sulfur interactions rather than to minimize lattice strain. We show that these findings are general and apply to other 4d and 5d metals despite the different levels of lattice misfit. This study underscores that a coordinated theoretical and experimental approach can determine the interfacial atomic structure and energetics with high fidelity, which is particularly valuable to quantitatively ascertain the structure-property relationships of supported clusters. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B18.00011: Controlled layer growth of WS2 grains by chemical vapor deposition Amina Zafar, Zhenhua Ni We produced single, bi and few layered WS2 single crystals by controlled growth condition. The single step growth with better controllability allows the fine tuning of layer number and crystal quality which has been confirmed by AFM, Raman and PL spectroscopies. Moreover, the calculated field effect mobilities based on layer number and demonstration of high performance phototransistor with prominent photoresponse facilitate the practical application of our growth recipe to other TMDs. |
Monday, March 5, 2018 1:27PM - 1:39PM |
B18.00012: A Route to Atomically Precise Hexagonal Boron Nitride Nanopores Stephen Gilbert, Gabriel Dunn, Amin Azizi, Thang Pham, Brian Shevitski, Stanley Liu, Shaul Aloni, Alex Zettl In this presentation, we discuss the fabrication of individual nanopores in hexagonal boron nitride (h-BN) with atomically precise control of the pore shape and size using only a conventional TEM. By careful tuning of the beam conditions, we show that individual perfect triangular pores are fabricated in h-BN and their size can be controlled with near-atomic precision. While much progress has been made, most previous methods of pore production in other 2D materials still typically create pores with irregular geometry and imprecise diameters. In contrast, other studies have shown that with careful control of electron irradiation, defects in h-BN grow with pristine zig-zag edges at quantized triangular sizes, but they have never before demonstrated production and control of isolated defects. In this work, we combine these techniques to yield a method in which we can create and control individual size-quantized triangular nanopores through multilayer h-BN sheets. |
Monday, March 5, 2018 1:39PM - 1:51PM |
B18.00013: Atomic Bonding and Chemical Identification of Hexagonal Boron Nitride-Graphene Interface Mengxi Liu, Xiaohui Qiu Interface structure and atomic bonding characteristics in heterostructures dominate many intriguing physical properties. Here we studied the interfacial structure of in-plane h-BNC hybrid monolayer in atomic scale using combined scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM).[1,2] The atomic resolved images reveal seamless boundaries for both boron and nitrogen atoms terminated zigzag edges, showing an immiscible connection at the atomic level. The boron, nitrogen and carbon atoms were exactly distinguished by the site-specific force spectroscopy. First-principles calculations further reveal the different growth behaviors and the selective formation of different interface configurations at the two kinds of nucleation center of graphene. The bonding and structure information of the interfacewould facilitate the understanding of the atomic process of films growth and the interplay of geometry and electronic properties. |
Monday, March 5, 2018 1:51PM - 2:03PM |
B18.00014: A Kinetic Pathway towards High-Density Ordered N Doping of Epitaxial Graphene on Cu(111) Using C5NCl5 Precursors Ping Cui, Jin-Ho Choi, Changgan Zeng, Zhenyu Li, Jinlong Yang, Zhenyu Zhang Pristine graphene possesses high electrical mobility, but its low charge carrier density severely limits its technological significance. Past efforts to increase graphene’s carrier density via chemical doping have shown limited successes, accompanied by substantial reductions in the mobility caused by disordered dopants. Here, based on first-principles calculations, we propose to grow graphene on Cu(111) via self-assembly of C5NCl5 molecular precursors to achieve high-density (1/6) and highly ordered nitrogen doping. Such a process relies on the elegant concerted roles played by the London dispersion, chemical, and screened Coulomb repulsive forces in enhancing molecular adsorption, facilitating easy dechlorination, and dictating the overall orientation of the C5N radicals, respectively. Further growth from the orientationally correlated graphene islands is accompanied by significantly minimized density of grain boundaries as the grains coalesce to form larger N-doped graphene sheets, which are further shown to possess superb electronic properties for future device applications. Initial kinetic processes involved in N-doped graphene growth using C5NH5 precursors are also investigated and contrasted with that of C5NCl5. |
Monday, March 5, 2018 2:03PM - 2:15PM |
B18.00015: Graphene Microstructure Fabrication by Femtosecond Laser Ablation Ethan Richman, Mac Selesnick, Christopher LaFratta, Paul Cadden-Zimansky We report on the development of a resist-free graphene microstructure fabrication technique using a femtosecond Ti:Sapphire laser. By coupling the laser into an inverted microscope fitted with a programmable x-y translation stage, we can create sub-micron structures of 2-dimensional materials by using a one-step process of direct-writing laser ablation. This technique also has the advantage that one can observe and adjust the fabrication in real time. Possible applications include the fabrication of graphene nanoribbons and Hall bars for research into quantum confinement. |
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