Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M1: Focus Session: Beyond Graphene - Growth II |
Hide Abstracts |
Sponsoring Units: DMP Chair: Gong Gu, University of Tennessee at Knoxville Room: 001A |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M1.00001: Tunable layer-by-layer oxidation of atomically thin WSe$_{2}$ Mahito Yamamoto, Michael Fuhrer, Keiji Ueno, Kazuhito Tsukagoshi Growing a high-quality oxide film with a tunable thickness on atomically thin transition metal dichalcogenides is of great importance for the electronic and optoelectronic applications. Here we demonstrate homogenous surface oxidation of atomically this WSe$_{2}$ with a self-limiting thickness from single- to tri-layers. Atomically thin WSe$_{2}$ films were mechanically exfoliated from bulk crystals onto SiO$_{2}$ and exposed to ozone at various temperatures. Below 100 $^{\circ}$C, the ozone treatment results in lateral growth of tungsten oxide islands on WSe$_{2}$, forming a uniform film on top. However, the oxidation does not progress to the underlying layers. At 200 $^{\circ}$C, the surface layers are oxidized in the layer-by-layer regime, up to trilayers. We find, by using Raman and photoluminescence spectroscopy, that underlying single-layer WSe$_{2}$ is decoupled from the top oxides. These observations have important implications for applications of the oxide film in electronic devices, such as a tunnel barrier and a gate dielectric. [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M1.00002: Layer-by-layer Dielectric Breakdown of Hexagonal Boron Nitride Film in Conductive AFM Measurement Yoshiaki Hattori, Takashi Taniguchi, Kenji Watanabe, Kosuke Nagashio Hexagonal boron nitride (BN) is considered as ideal gate dielectric and substrate for graphene devises. However, many intrinsic properties of BN have not been clarified yet, since many researchers have focused on the electrical properties of graphene. In this study, the dielectric breakdown of BN is systematically studied using the conductive atomic force microscopy. The obtained dielectric field strength is $\sim$ 12 MV/cm, which is comparable to the conventional SiO$_{2}$. After the hard dielectric breakdown, BN fractured like a flower with equilateral triangle fragments. However, when applied voltage is stopped just in the middle of the dielectric breakdown, the formation of hole was clearly observed, which does not penetrate to the bottom metal electrode. Subsequent IV measurement at the hole indicates that the remaining BN layer in the hole is still electrically inactive. Based on these observation, the layer-by-layer breakdown is suggested for BN from the viewpoint of physical fracture and electrical breakdown. Moreover, the statistical analysis on breakdown voltages by Weibull plot suggests the anisotropic formation of defects. These results are unique to the layered materials, unlike the conventional 3D amorphous oxides. [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M1.00003: Fluorinating Single Layer Molydisulfide Masahiro Ishigami, Jyoti Katoch We have investigated the structural and electronic properties of fluorinated (via plasma processing) molydisulfide using scanning tunneling microscopy, x-ray photoelectron spectroscopy, photoluminescence and ultraviolet photoelectron spectroscopy. Fluorine atoms are strongly bound on molydisulfide and the binding leads to p-doping. As such, fluorination can be useful for chemical doping of molydisulfide. We will discuss our experimental results in light of our recent ab-initio calculations. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M1.00004: The effect of defects produced by electron irradiation on the electrical properties of graphene and MoS2 Julio Alejandro Rodriguez-Manzo, Adrian Balan, Carl Nayor, Will Parkin, Matthew Puster, A.T. Charlie Johnson, Marija Drndic We present a study of the effects of the defects produced by electron irradiation on the electrical and crystalline properties of graphene and MoS2 monolayers. We realized back or side gated electrical devices from monolayer MoS2 or graphene crystals (triangles respectively hexagons) suspended on a 50nm SiNx m. The devices are exposed to electron irradiation inside a 200kV transmission electron microscope (TEM) and we perform in situ conductance measurements[1]. The number of defects and the quality of the crystalline lattice obtained by diffraction are correlated with the observed decrease in mobility and conductivity of the devices. We observe a different behavior between MoS2 and graphene, and try to associate this with different models for conduction with defects. Finally, we use the TEM electron beam to tailor the macroscopic layers into ribbons to be used as the sensing element in MoS2 nanoribbon - nanopore devices for DNA detection and sequencing . [1] Towards sensitive graphene nanoribbon-nanopore devices by preventing electron beam induced damage. M. Puster, J. A. Rodriguez- Manzo, A. Balan, M. Drndic. ACS Nano,10.1021/nn405112m. [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M1.00005: Intrinsic disorder in graphene on transition metal dichalcogenide heterostructures Matthew Yankowitz, Stefano Larentis, Kyounghwam Kim, Jiamin Xue, Devin McKenzie, Shengqiang Huang, Marina Paggen, Mazhar Ali, Robert Cava, Emanuel Tutuc, Brian J. LeRoy Recently, semiconducting materials in the transition metal dichalcogenide (TMD) family have gained great popularity for use in novel graphene-based heterostructure devices such as tunneling transistors, highly efficient flexible photovoltaic devices, and nonvolatile memory cells. TMDs have also been explored as alternatives to hexagonal boron nitride (hBN) as substrates for pristine graphene devices. However, their quality has thus far been significantly worse than comparable hBN devices. We examine graphene on numerous TMD substrates (MoS$_2$, WS$_2$, WSe$_2$, MoTe$_2$) with scanning tunneling microscopy and spectroscopy and find that point and line defects intrinsic to all TMD crystals (both of natural and synthetic origin) result in scattering of electrons in graphene. Our findings suggest that the quality of graphene on TMD heterostructures is limited by the intrinsic crystalline quality of the TMDs. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M1.00006: Tailoring the properties of two dimensional molybdenum disulfide Saiful I. Khondaker, Muhammad R. Islam, Narae Kang, Udai Bhanu, Hari P. Paudel, Mikhail Erementchouk, Laurene Tetard, Michael N. Leuenberger The ability to tailor the properties of a material is essential to optimize device functionality. In this talk, I will present evidence that the electrical and optical properties two-dimensional (2D) molybdenum disulfide (MoS$_{2})$ can be tuned by controlled exposure to oxygen plasma. We find that the mobility, on-current and resistance of 2D MoS$_{2}$ FETs vary exponentially by up to four orders of magnitude with respect to the plasma exposure time. Photoluminescence (PL) study show a decrease of PL intensity leading a complete quenching. Raman studies show a significant decrease of intensity of MoS$_{2}$ peaks with the creation of new oxidation induced peak, while X-ray photoelectron spectroscopy (XPS) study show peaks associated with MoO$_{3}$ after plasma exposure. We suggest that during exposure to oxygen plasma, the energetic oxygen molecules interact with MoS$_{2}$ and create MoO$_{3}$ rich defected-regions, which are insulating. MoO$_{3}$ defected-regions act as a tunnel barrier for the injected conduction electrons, giving rise to the exponential increase in resistivity as a function of plasma exposure time. Bandstructure calculation shows that the PL quenching upon plasma exposure is due to the creation of MoO$_{3}$ defected-regions which causes a direct to indirect bandgap transition in monolayer MoS$_{2}$. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 1:03PM |
M1.00007: Synthesis and Characterizations of Two-Dimensional Atomic Layers and Their Heterostructures Invited Speaker: Yi-Hsien Lee Monolayers of van der Waals (vdw) materials, including graphene, h-BN, and MoS$_{2}$, have been highlighted regarding both scientific and industrial aspects due to novel physical phenomenon inherited from the reduced dimensionality. Layered transition metal dichalcogenides (TMD) atomic layers, being considered as the thinnest semiconductor, exhibit great potential for advanced nano-devices. Monolayer in the class of offered a burgeoning field in fundamental physics, energy harvesting, electronics and optoelectronics. Recently, atomically thin heterostructures of TMD monolayer with various geometrical and energy band alignments are expected to be the key materials for next generation flexible optoelectronics. The individual TMD monolayers can be adjoined vertically or laterally to construct diverse heterostructures which are difficult to reach with the laborious pick up-and-transfer method of the exfoliated flakes. The ability to produce copious amounts of high quality layered heterostructures on diverse surfaces is highly desirable but it has remained a challenging issue. Here, we have achieved a direct synthesis of various heterostructures of monolayer TMDs. The synthesis was performed using ambient-pressure CVD with aromatic molecules as seeding promoters. We discuss possible growth behaviors, and we examine the symmetry and the interface of these heterostructures using optical analysis and atomic-resolution scanning TEM. Our method offers a controllable synthesis of to obtain high-quality heterostructures of TMD atomic layers with diverse interface geometry.\\[4pt] [1] Yi-Hsien Lee, et al., Adv. Mater., 24 (17), p.2320-2325 (2012)\\[0pt] [2] Yi-Hsien Lee, et al. Nano Lett., 13 (4), 1852--1857 (2013)\\[0pt] [3] Xi Ling, Yi-Hsien Lee*, et al., Nano Lett., 14, p.464--472 (2014)\\[0pt] [4] Lili Yu, Yi-Hsien Lee, et al, Nano Lett, 14, p.3055-3063 (2014)\\[0pt] [5] X Zhang, C Lin, Y Tseng, K Huang,Yi-Hsien. Lee*, Nano Letters. (revised) [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M1.00008: Spatial Progression of Thermal Oxidation in Layered WSe2 Nano-sheets Yingnan Liu, Cheng Tan, Harry Chou, Avinash Nayak, Di Wu, Joonseok Kim, Rodney Ruoff, Deji Akinwande, Keji Lai Owing to the extremely different bonding strengths between intralayer covalent bonds and interlayer van der Waals interaction, many physical and chemical properties of layered transition metal dichalcogenides are expected to be highly anisotropic in nature. Using a number of compositional, structural, and electrical characterization tools, we have studied the spatial progression of the thermal oxidation of exfoliated WSe2 nano-sheets, which primarily starts at the sample edges and propagates laterally towards the center. As revealed by microwave impedance microscopy and transport measurements, the partially oxidized regions show much higher conductivity than either the WSe2 itself or the completely oxidized WO3. The ability to electrically map out how chemical reactions are taking place in the nanoscale could be of particular importance for 2D materials that hold promise for future applications. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M1.00009: Bulk Direct Band Gap MoS$_{2}$ by Plasma Induced Layer Decoupling Rohan Dhall, Mahesh Neupane, Darshana Wickramaratne, Matthew Mecklenburg, Zhen Li, Cameron Moore, Roger Lake, Stephen Cronin We report a robust method for engineering the optoelectronic properties of few layer MoS$_{2}$ using low energy oxygen plasma treatment. Gas phase treatment of MoS$_{2}$ with an upstream N$_{2}$-O$_{2}$ plasma is shown to enhance the photoluminescence (PL) of few layer MoS$_{2}$ flakes by up to 20 times, without reducing the layer thickness. A blue shift in the photoluminescence spectra and narrowing of linewidth is consistent with a transition of MoS$_{2}$ from indirect to direct band gap material. Atomic force microscopy and Raman spectra reveal that the flake thickness actually increases as a result of the plasma treatment, indicating an increase in the interlayer separation in MoS$_{2}$. Ab-initio calculations reveal that the increased interlayer separation is sufficient to decouple the electronic states in individual layers, leading to a transition from an indirect to direct gap semiconductor. With optimized plasma treatment parameters, we observed enhanced PL signals for 32 out of 35 few layer MoS$_{2}$ flakes tested, indicating this method is robust and scalable. Monolayer MoS$_{2}$, while direct band gap, has a small optical density, which limits its potential use in practical devices. The results presented here provide a material with the direct band gap of monolayer MoS$_{2}$, without reducing sample thickness, and hence optical density. [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M1.00010: High-Throughput Screening of Substrates for Synthesis of Two-Dimensional Materials Arunima K. Singh, Houlong L. Zhuang, Francesca Tavazza, Richard G. Hennig Since the discovery of graphene, several two-dimensional (2D) materials have been synthesized experimentally, but many theoretically predicted 2D materials are yet to be synthesized. Common synthesis techniques such as chemical-vapor deposition and molecular-beam epitaxy require suitable substrates. We are developing a strategy to enable high-throughput searches for suitable substrates for 2D materials by automatically identifying suitable substrate candidates and characterize their stabilizing properties and doping effects using density-functional theory. As first steps, we have found that several transition-metal, rare-earth-metal, and refractory-diboride substrates sufficiently reduce the formation energies of 2D group-III-V materials, making them thermodynamically stable on these substrates [1,2]. Additionally, these substrates lead to variable amount of doping of the 2D materials depending on the work functions of the 2D materials and the substrates. We observe large adsorption energies and strong doping of the 2D materials which indicates that these substrates can provide good electrical contact to enable transport measurements and electronic applications.\\[4pt] [1] Physical Review B 89 (24), 245431 (2014).\\[0pt] [2] Applied Physics Letters 105 (5), 051604 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M1.00011: Direct patterning and characterization of large area, single layer MoS$_{2}$ film synthesized by chemical vapor deposition Woanseo Park, Jaeyoon Baik, Tae-Young Kim, Kyungjune Cho, Woong-Ki Hong, Hyun-Joon Shin, Takhee Lee Molybdenum disulfide (MoS$_{2})$ has gained a significant amount of attention due to a great potential for atomic-film electronics. Recently chemical vapor deposition (CVD) method has been utilized to synthesize MoS$_{2}$ films, however, the synthesis of large area MoS$_{2}$ films still remains a challenge for practical device development. For the further utilization, existing synthetic approaches that can be used to fabricate large-area MoS$_{2}$ films require additional patterning processes, which may introduce unintentional contamination from other chemicals during the various processes. Therefore, it is required to directly prepare patterned, MoS$_{2}$ films during the CVD synthesis. In this presentation, we report a simple method for the synthesis of MoS$_{2}$ films that can be directly patterned during the synthesis, so that post-patterning processes can be avoided and device fabrication can be made simultaneously This study suggests that large-area, single-layer MoS$_{2}$ films can be synthesized by CVD and directly patterned for atomic-film electronic devices. [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M1.00012: Chemical Exfoliation of Layered Superconductors: An Avenue to Synthesize Boron-rich Quasi Two Dimensional Nanostructures Saroj Kumar Das, Asha Liza James, Kabeer Jasuja Zero-dimensional and one-dimensional boron based nanostructures have presented excellent avenues in the past for utilizing the fascinating science of boron at the atomic level. The research on synthesizing two-dimensional (2-D) boron-based nanostructures is currently in its incipient stages. In this talk, we demonstrate two chemical approaches that yield quasi 2-D boron-rich nanostructures by enabling an exfoliation of a layered boron-based superconductor. While one approach employs the simple tool of ultrasonication in an aqueous phase, the other approach utilizes a chelation mediated strategy based on coordination of metal ions and organic ligands. Both these synthetic routes are shown to result in a processable colloidal dispersion of nanosheets. This talk will present details of the two exfoliation approaches and a comprehensive study of the morphological, chemical and optical properties of the dispersed nanosheets. We will demonstrate that the exfoliated nanosheets undergo an in-situ chemical modification with ionizable functional groups derived from solvent that enable electrostatic stabilization. We will further shown that this functionalization modifies the band structure of the nanosheets which gives rise to photoluminescence and result in physico-chemical properties distinct from the parent superconductor. This ability to synthesize quasi 2-D boron rich nanostructures significantly adds to the current state of literature on born-based quasi-planar nanostructures. [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M1.00013: The formation of Colloidal 2D/3D MoS2 Nanostructures in Organic Liquid Environment Engin Durgun, H. Sener Sen, Tugba Oztas, Bulend Ortac 2D MoS2 nanosheets (2D MoS2 NS) and fullerene-like MoS2 nanostructures (3D MoS2 NS) with varying sizes are synthesized by nanosecond laser ablation of hexagonal crystalline 2H-MoS2 powder in methanol. Structural, chemical, and optical properties of MoS2 NS are characterized by optical microscopy, SEM, TEM, XRD, Raman and UV/VIS/NIR absorption spectroscopy techniques. Results of structural analysis show that the obtained MoS2 NS mainly present layered morphology from micron to nanometer surface area. Detailed analysis of the product also proves the existence of inorganic polyhedral fullerene-like 3D MoS2 NS generated by pulsed laser ablation in methanol. The possible factors which may lead to formation of both 2D and 3D MoS2 NS in methanol are examined by ab initio calculations and shown that it is correlated with vacancy formation. The hexagonal crystalline structure of MoS2 NS was determined by XRD analysis. The colloidal MoS2 NS solution presents broadband absorption edge tailoring from UV region to NIR region. Investigations of MoS2 NS show that the one step physical process of pulsed laser ablation-bulk MoS2 powder interaction in organic solution opens doors to the formation of ``two scales'' micron- and nanometer-sized layered and fullerene-like morphology MoS2 structures. [Preview Abstract] |
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