Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y2: Focus Session: Beyond Graphene - New 2D Materials |
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Sponsoring Units: DMP Chair: Ju Li, Massachusetts Institute of Technology Room: 001B |
Friday, March 6, 2015 8:00AM - 8:12AM |
Y2.00001: Enhancing Electron Coherence via Quantum Phonon Confinement in Atomically Thin Nb$_{3}$SiTe$_{6}$ Jin Hu, Xue Liu, Chunlei Yue, Jinyu Liu, Huiwen Zhu, Jibao He, Jiang Wei, Zhiqiang Mao, Liubov Antipina, Zakhar Popov, Pavel Sorokin, Tijiang Liu, Philip Adams, Seyyed Radmanesh, Leonard Spinu, Heng Ji, Douglas Natelson The extraordinary properties of two dimensional (2D) materials, such as the extremely high carrier mobility in graphene and the large direct band gaps in transition metal dichalcogenides MX$_{2}$ (M$=$Mo or W, X$=$S, Se) monolayers, highlight the crucial role quantum confinement can have in producing a wide spectrum of technologically important electronic properties. Currently one of the highest priorities in the field is to search for new 2D crystalline systems with structural and electronic properties that can be exploited for device development. Here we report the discovery and the unusual quantum transport properties of the novel 2D ternary transition metal chalcogenide- Nb$_{3}$SiTe$_{6}$. We show that the micaceous nature of Nb$_{3}$SiTe$_{6}$ allows it to be thinned down to one-unit-cell thick 2D crystals using microexfoliation technique. When the thickness of Nb$_{3}$SiTe$_{6}$ crystal is reduced below a few unit-cells thickness, we observed an unexpected, enhanced weak-antilocalization signature in magnetotransport. This finding provides solid evidence for the long-predicted suppression of electron-phonon interaction caused by the crossover of phonon spectrum from 3D to 2D. [Preview Abstract] |
Friday, March 6, 2015 8:12AM - 8:24AM |
Y2.00002: ARPES and Low-Energy Electron Microscopy Study on Supported, Suspended, and Bilayer Twisted MoS2 Po-Chun Yeh, Wencan Jin, Nader Zaki, Datong Zhang, Jonathan. T. Liou, Jerzy T. Sadowski, Abdullah Al-Mahboob, Jerry I. Dadap, Irving P. Herman, Peter Sutter, Richard M. Osgood, Jr We report on the directly measured electronic structure of exfoliated monolayer molybdenum disulfide (MoS2) using micron-scale angle-resolved photoemission spectroscopy. Measurements of both suspended and supported monolayer MoS2 elucidate the effects of interaction with a substrate. For suspended MoS2, a careful investigation of the measured uppermost valence band gives the effective mass at $\Gamma $ and K, and the estimated value of spin-orbit coupling induced splitting at K. Moreover, we prepare CVD synthesized twisted-bilayer MoS2 flakes on a native-oxide silicon substrate. Band structure of MoS2 for different twist angles is measured by using micro-ARPES, and their crystal orientations are determined by Micro-LEED. We investigate their dispersions, effective mass, and valence band maximum with respect to twist angle. [Preview Abstract] |
Friday, March 6, 2015 8:24AM - 8:36AM |
Y2.00003: Electronic Structure of Epitaxial Single-Layer MoS$_2$ Philip Hofmann, Jill Miwa, S\o ren Ulstrup, Signe G. S{\o}rensen, Maciej Dendzik, Antonija Grubi\v{s}i\'c \v{C}abo, Marco Bianchi, Jeppe Vang Lauritsen The electronic structure of epitaxial single-layer MoS$_2$ on Au(111) is investigated by angle-resolved photoemission spectroscopy. Pristine and potassium-doped layers are studied in order to gain access to the conduction band. The potassium-doped layer is found to have a (1.39$\pm$0.05)~eV direct band gap at $\bar{K}$ with the valence band top at $\bar{\Gamma}$ having a significantly higher binding energy than at $\bar{K}$. The moir\'e superstructure of the epitaxial system does not lead to the presence of observable replica bands or minigaps. The degeneracy of the upper valence band at $\bar{K}$ is found to be lifted by the spin-orbit interaction, leading to a splitting of (145$\pm$4)~meV. This splitting is anisotropic and in excellent agreement with recent calculations. Finally, it is shown that the strength of the potassium doping is $k$-dependent, leading to the possibility of band structure engineering in single-layers of transition metal dichalcogenides. [Preview Abstract] |
Friday, March 6, 2015 8:36AM - 8:48AM |
Y2.00004: Electronic Properties, Band Gap Renormalization, and Doping Effect in Epitaxial WSe$_{2}$ thin film Yi Zhang, Miguel Ugeda, Su-Fei Shi, Bo Zhou, Yeongkwan Kim, Yulin Chen, Feng Wang, Micchael Crommie, Zahid Hussain, Zhi-Xun Shen, Sung-Kwan Mo As a class of graphene-like two-dimensional materials, the layered metal dichalcogenides MX$_{2}$ (M $=$ Mo, W; X $=$ S, Se, Te) have gained significant interest due to the distinct properties in 2D limit. Examples include the indirect to direct band gap transition in monolayer, and giant spin-splitting of the valence band. These properties give MX$_{2}$ great application potentials in both optoelectronic and spintronics devices. For practically studying and applying the MX$_{2}$, the growth of high-quality MX$_{2}$ thin film with precise control of layers thickness is favorable. Here we report the molecular beam epitaxial growth of WSe$_{2}$ thin film, with controllable thickness from monolayer to 8 monolayer. By using \textit{in-situ} angle-resolved photoemission spectroscopy, we experimentally revealed the valence band evolution with film thickness. By applying the potassium doping on the surface, we observed the indirect to direct band gap transition in monolayer WSe$_{2}$, and the distorted band structure. Combining the \textit{ex-situ} photoluminescence and scanning tunneling spectroscopy, we further presented the giant band gap renormalization and excitonic effects. Our results will enrich the understanding of WSe$_{2}$, and bring it more application potential in practical devices. [Preview Abstract] |
Friday, March 6, 2015 8:48AM - 9:00AM |
Y2.00005: Charge Carrier Transport Properties in Layered Structure of Hexagonal Boron Nitride ($h$-BN) and Thermal Neutron Detection Based on $h$-BN Tri Doan, Samuel Grenadier, Sashikhanth Majety, Jing Li, Jingyu Lin, Hongxing Jiang Hexagonal boron nitride (h-BN) epilayers have been synthesized by MOCVD. It was found that the carrier mobility in h-BN epilayers is strongly dependent on temperature following the power law $\mu \quad \sim $ T$^{\mathrm{-\alpha }}$ with $\alpha \quad \approx $ 3.02, satisfying the 2D carrier transport limit dominated by the polar optical phonon scattering The deduced maximum energy (wave number) of the optical phonon is $\sim $ 192 meV (or 1546 cm$^{\mathrm{-1}})$. The measured carrier mobility-lifetime ($\mu \tau )$ product of $h$-BN thin films grown on sapphire substrate is 2.83 x 10$^{\mathrm{-7}}$ cm$^{\mathrm{2}}$/V for electrons and holes, which is comparable to that of GaN films grown on sapphire. Thermal neutron detectors based on $h$-BN epilayers were fabricated and the reaction product pulse-height spectra were measured under thermal neutron irradiation produced by $^{\mathrm{252}}$Cf source. It was shown that $h$-BN thin film thermal neutron detectors are capable to resolve specific nuclear reaction products with unprecedentedly high energy resolution. [Preview Abstract] |
Friday, March 6, 2015 9:00AM - 9:12AM |
Y2.00006: Polarons in thin Ga$_2$O$_3$ layers Hartwin Peelaers, Joel B. Varley, Chris G. Van de Walle Ga$_2$O$_3$ has a large band gap of 4.9 eV, making it transparent in the UV. It can also be doped n-type, enabling applications as a transparent conductor or in power electronics. The optical properties of Ga$_2$O$_3$ may be affected by the formation of small polarons, i.e., localized holes trapped by a lattice distortion. First-principles calculations have established the stability of such polarons in bulk Ga$_2$O$_3$ [J. B. Varley \textit{et al.}, Phys. Rev. B {\bf 85}, 081109(R) (2012)]. Here we investigate hole polarons in nanomembranes of Ga$_2$O$_3$. We perform first-principles calculations based on density functional theory using a hybrid functional. Since polarons correspond to positive charges, a neutralizing charge needs to be included in supercell calculations. The use of a jellium background leads to divergence problems in low-dimensional systems. To prevent this, we modified the charge of the pseudopotentials, thus providing charge compensation that is confined within the layer. Our results obtained with this technique show that small polarons can indeed be formed in thin layers. Comparisons with polarons in the bulk and with experiments will be discussed. [Preview Abstract] |
Friday, March 6, 2015 9:12AM - 9:48AM |
Y2.00007: Monolayer MoSe$_{2}$/WSe$_{2}$ heterojunctions at the atomic level Invited Speaker: Ana M. Sanchez While graphene is the most studied two-dimensional (2D) material, atomically thin layered transition metal dichalcogenides (TMDs) have recently emerged as a new class of 2D nanomaterials. Due to their band structure, monolayers of direct band gap semiconducting TMD have promise to complement the zero bandgap energy of graphene, offering an extensive range of applications in electronics and optics. The dichalcogenide heterojunctions were grown by physical vapor transport. Lateral heteroepitaxy was visible in an optical microscope and the structures showed enhanced photoluminescence. Atomically resolved transmission electron microscopy using a double-corrected ARM200F (80-200kV) revealed that the MoSe$_{2}$/WSe$_{2}$ heterojunction is an undistorted honeycomb lattice in which substitution of one transition metal by another occurs across the interface [1]. There were no dislocations or grain boundaries, i.e. an atomically seamless MoSe$_{2}$/WSe$_{2}$ semiconductor junction was achieved. Moreover, strain mapping of atomic resolution images demonstrates negligible distortion at the heterojunction, and the analysis of the different atomic species demonstrates that the interface has a finite width similar to 3D heterojunctions. Vertical stacking of MoSe$_{2}$/WSe$_{2}$ bilayers was also analyzed using electron microscopy. An analysis of the intensity in annular dark field images shows that Se atoms of the WSe$_{2}$ layer align with the Mo atoms of the MoSe$_{2}$ layer in some of these heterojunctions. We expect that the growth of these lateral junctions will open new device functionalities, such as in-plane transistors and diodes integrated within a single atomically thin layer [1,2,3]. \\[4pt] [1] C. Huang et al. \textit{Nat. Mater.} \textbf{13} (2014) 1096\\[0pt] [2] Y. Gong et al, Nat. Mater. \textbf{13} (2014) 1135\\[0pt] [3] G.S. Duesberg \textit{Nat. Mater.} \textbf{13} (2014) 1075 [Preview Abstract] |
Friday, March 6, 2015 9:48AM - 10:00AM |
Y2.00008: Energy band structure of bulk and monolayer vanadium pentoxide (V$_{2}$O$_{5}$) beyond the quasiparticle self-consistent GW approximation: lattice polarization effects Churna Bhandari, Walter R.L. Lambrecht, Mark van Schilfgaarde The quasiparticle self-consistent GW method (QSGW) is known to systematically overestimate the band gaps in semiconductors by about 20\% due to the underestimate of screening by the random phase approximation (RPA). We show that for V$_2$O$_5$, a layered oxide material, the overestimate is significantly larger. The smallest direct gap in QSGW is found to be 4.83 eV compared to 2.35 eV experimentally. The evidence for the experimental gap and optical properties are reviewed. We suggest that a major contribution to the self-energy reduction results from the lattice polarization contribution to the dielectric screening. This results from the large LO/TO splittings in this material. We make a simple estimate of the reduction of $W(q{=}0,\omega{=}0)$, and obtain a factor $\sim$0.38, which if assumed to apply for all $W$ and reduces the gap to 2.60 eV. The remainder of the gap overestimate is tentatively ascribed to shortcomings of the RPA. We also consider the band structure of this material in monolayer form. We find that the GW correction depends strongly on the layer separation (L) as 1/L. The lattice polarization itself depends on distance between the layers because of the dependence of the phonons on the long-range Coulomb interactions and hence reduced screening in a 2D system. [Preview Abstract] |
Friday, March 6, 2015 10:00AM - 10:12AM |
Y2.00009: Observation of Piezoelectricity in Free-standing Monolayer Molybdenum Disulfide Hanyu Zhu, Yuan Wang, Jun Xiao, Ming Liu, Shaomin Xiong, Zi Jing Wong, Ziliang Ye, Yu Ye, Xiaobo Yin, Xiang Zhang Piezoelectricity offers precise and robust conversion between electricity and mechanical force, which originates from the broken inversion symmetry of atomic structure. Yet reducing the size of bulk piezoelectric materials to single molecular layer was challenging, since the surface energy can cause piezoelectric structures to be thermodynamically unstable. Here we report experimental evidence of piezoelectricity in free-standing single layer of molybdenum disulfide (MoS$_{\mathrm{2}})$ crystal, with measured piezoelectric coefficient e$_{\mathrm{11}}=$2.9*10$^{\mathrm{-10}}$ C/m. The free-standing measurement of the intrinsic piezoelectricity is free from the substrate effects, such as doping and parasitic charge. We observed oscillation of piezoelectric response in MoS$_{\mathrm{2}}$ in odd and even number of layers due to breaking and recovery of inversion symmetry, respectively, in sharp contrast to bulk piezoelectric materials. Through the angular dependence of electro-mechanical coupling, we uniquely determined the 2D crystal orientation. The piezoelectricity discovered in single molecular membrane promises new applications in low-power logic switch and ultrasensitive sensors scaled down to single atomic unit cell -- the ultimate material limit. [Preview Abstract] |
Friday, March 6, 2015 10:12AM - 10:24AM |
Y2.00010: Air Stability of Two-Dimensional Transition Metal Dichalcogenides Santosh KC, Roberto Longo, Rafik Addou, Diego Barrera, Julia W.P. Hsu, Robert M. Wallace, Kyeongjae Cho Layered transition metal dichalcogenides (TMDs) have emerged as a potential alternative channel material for ultra-thin and low power nanoelectronics. Highly tunable and unique electronic properties of TMDs made them promising novel materials for various other applications as well. However, in order to realize the superior performance of TMD based devices, the physical and chemical properties need to be understood, in particular their stability under different chemical environments. A detailed comparative analysis of the air stability (i.e., oxygen interaction) of different TMDs is still lacking. We have examined various TMD stabilities in air and found them different from graphene which is stable in air. The changes in the electronic properties with air exposure were studied using density functional theory (DFT), Kelvin probe, and photoelectron emission in air. The results reveal that transition metal sulfides are kinetically more stable than selenides in air, but all TMDs are thermodynamically unstable against oxidation. Furthermore, it is shown that TMD surface defects function as facile oxidation sites impacting their air stabilities. These findings provide helpful guidance to controlled exfoliation and device fabrication processes. [Preview Abstract] |
Friday, March 6, 2015 10:24AM - 10:36AM |
Y2.00011: Air Stable Doping of MoS2 FETs Using TiOx Sol-Gel Amritesh Rai, Rudresh Ghosh, Anupam Roy, Amithraj Valsaraj, Hema CP Movva, Sangwoo Kang, Emanuel Tutuc, Leonard Register, Sanjay Banerjee Field effect transistors based on ultra-thin transition metal dichalcogenides suffer from high contact resistances due to the Schottky barrier formed between the metal and the semiconducting channel. An effective way to overcome this issue is to dope the semiconducting channel in order to reduce the Schottky barrier width, thereby enabling efficient electron injection via tunneling. Previously used charge transfer doping techniques employed the use of potassium ions and PEI. However, these doping reagents are unstable in air. Here we report the use of an air stable, self encapsulating, spin on n-type doping technique on MoS2 utilizing TiOx sol-gel. The doping of the channel is confirmed by the broadening of the A1g Raman mode of MoS2. High performance field effect transistors are demonstrated which show three times improvement in the field effect mobility as well as a two-fold increase in the intrinsic mobility of the MoS2 channel. The enhancement of intrinsic mobility can be attributed to the suppression of the A1g phonon modes of MoS2 as well as screening of charged impurities by the TiOX layer. The devices show extended air stability over two to three weeks. The use of TiOx sol-gel can be a promising way to enhance the performance of TMD based transistors. [Preview Abstract] |
Friday, March 6, 2015 10:36AM - 10:48AM |
Y2.00012: Scanning tunneling microscopy study of a new charge density wave phase in VSe$_{2}$ thin films Duming Zhang, Jeonghoon Ha, Hongwoo Baek, Fabian Natterer, Young Kuk, Nikolai Zhitenev, Joseph Stroscio Ultra-thin two-dimensional materials of transition metal dichalcogenides have recently attracted great interest due to their diverse electronic properties and potential applications. Upon cooling to low temperature, some materials exhibit interesting phenomena of collective electronic states such as superconductivity and charge density waves. While charge density waves in bulk materials of transition metal dichalcogenides have been extensively studied in the past few decades, the understanding of this collective electronic state in materials with reduced dimensionality is still in its infancy. Here, we report \textit{in-situ} ultra-low temperature scanning tunneling microscopy and spectroscopy measurements on VSe$_{2}$ thin films synthesized by molecular beam epitaxy. We observed an unconventional charge density wave which does not follow previous reports of hexagonal symmetry of VSe$_{2}$. Spectroscopy results will be discussed in relation to other characterizations using electrical transport and transmission electron microscopy. [Preview Abstract] |
Friday, March 6, 2015 10:48AM - 11:00AM |
Y2.00013: Two-dimensional materials based transparent flexible electronics Lili Yu, Sungjae Ha, Dina El-Damak, Elaine McVay, Xi Ling, Anantha Chandrakasan, Jing Kong, Tomas Palacios Two-dimensional (2D) materials have generated great interest recently as a set of tools for electronics, as these materials can push electronics beyond traditional boundaries. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency, and favorable transport properties for realizing electronic, sensing, and optical systems on arbitrary surfaces. These thin, lightweight, bendable, highly rugged and low-power devices may bring dramatic changes in information processing, communications and human-electronic interaction. In this report, for the first time, we demonstrate two complex transparent flexible systems based on molybdenum disulfide (MoS$_{2})$ grown by chemical vapor method: a transparent active-matrix organic light-emitting diode (AMOLED) display and a MoS$_{2}$ wireless link for sensor nodes. The 1/2 x 1/2 square inch, 4 x 5 pixels AMOLED structures are built on transparent substrates, containing MoS$_{2}$ back plane circuit and OLEDs integrated on top of it. The back plane circuit turns on and off the individual pixel with two MoS$_{2}$ transistors and a capacitor. The device is designed and fabricated based on SPICE simulation to achieve desired DC and transient performance. We have also demonstrated a MoS$_{2}$ wireless self-powered sensor node. The system consists of as energy harvester, rectifier, sensor node and logic units. AC signals from the environment, such as near-field wireless power transfer, piezoelectric film and RF signal, are harvested, then rectified into DC signal by a MoS$_{2}$ diode. [Preview Abstract] |
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