Novel Quantum Phenomena in 2D Materials
Thursday, November 13, 2014 2:20 PM in Room 310
2D+EM+MI+MN+NS+SS+TF-ThA-1 Optoelectronics of Two-Dimensional Semiconductors
Xiaodong Xu (University of Washington)
Two dimensional transition metal dichalcogenides are a recent addition to the 2D electronic materials family. They have shown outstanding electrical and optical properties for new optoelectronic device concepts. In this talk, we will first discuss the unique interplay between spin, valley, and layer pseudospins in bilayer WSe2. Such coupling effects lead to electrical control of spin states and optical generation of valley coherence through interlayer trions, where electrons and holes are localized in different layers. We will then talk about optoelectronic devices based on monolayer WSe2, including p-n junctions as light emitting diodes and hybrid monolayer semiconductor/photonic crystal cavity devices. We will conclude the talk with a discussion of the optoelectronic properties of MoSe2-WSe2 heterostructures.
2D+EM+MI+MN+NS+SS+TF-ThA-3 Theory of Graphene Transport Barriers in the Specular Limit
Daniel Gunlycke, Carter White (Naval Research Laboratory)
Offering room-temperature ballistic electron transport well over one micron, while being atomically thin and planar, graphene is undeniably a promising material for future nanoelectronic devices. Presently, however, switchable devices have normally low on-off ratios, a reflection of the challenge of selectively blocking electron and hole carriers from propagating across the graphene surface. This has stimulated a lot of research on different methods for making graphene nanoribbons that exhibit suitable band gaps. An alternative way to obtain a controllable gap takes advantage of resonant tunneling across a pair of transport barriers. For the latter approach, the key is to find a barrier that is fairly reflective but not so much as to effectively cut off all transport across it.
In this presentation, we present a model for straight transport barriers in graphene in the specular limit. Using the Lippmann-Schwinger equation, we obtain the wave function, from which we derive the reflection and transmission probabilities, as well as the local density of itinerant states. This local density of states exhibits fluctuations arising from quantum interference between incoming and outgoing matter waves that allow the transport properties of a barrier to be estimated without explicitly probing the current across the barrier. Our model is tested against exact multi-channel, tight-binding quantum transport calculations for graphene with weak local potentials, local strain, local adsorption, and a locally defective structure. As the model parameters are related to observable quantities, they could be obtained from theory and/or experiment, allowing the model to be adopted even when the precise details of the barrier are unknown.
2D+EM+MI+MN+NS+SS+TF-ThA-4 Tip-induced Potential Confinement on Graphene in Scanning Tunneling Microscopy Measurement
Yue Zhao, Jungseok Chae, Jonathan Wyrick (NIST/CNST); Fabian Natterer (Ecole Polytechnique Fédérale de Lausanne (EPFL), France); Suyong Jung (Korea Research Institute of Standards and Science (KRISS)); Andrea Young, Cory Dean, Lei Wang, Yuanda Gao (Columbia University); Joao Rodrigues (Graphene Research Centre, NUS, Singapore); Kenji Watanabe, Takashi Taniguchi (National Institute for Materials Science (NIMS), Japan); Shaffique Adam (Graphene Research Centre, NUS, Singapore); James C. Hone, Kenneth Shepard, Philip Kim (Columbia University); Nikolai Zhitenev, Joseph Stroscio (NIST/CNST)
Graphene is a two-dimensional-electron-gas(2DEG) system with exposed surface, which allows scanning tunneling microscopy(STM) to investigate the electron-electron interaction associated with the Dirac nature on a local scale, with a variety of tuning knobs, such as carrier density, spatially varying disorder potential, and applied magnetic field. However, the electron-electron interaction in graphene is sensitive to the disorder details. Moreover, tip induced potential confinement can significantly complicate the interpretation of STM experiment. Utilizing a high mobility graphene device with low residual disorder, we can minimize the effect of local potential fluctuation, to better understand the role tip-induced potential plays in the measurement. We observed the emergency of large spectra gaps, modification to graphene Landau levels (LLs), and quantum dots with changing size due to the spatially inhomogeneous tip gating.
2D+EM+MI+MN+NS+SS+TF-ThA-6 Topological Phase Transitions and Spin-orbit Density Waves
Hugo Dil (Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland)
In recent years systems where the spin-orbit interaction (SOI) is not just a perturbation but the main energy scale have received increasing attention. In combination with a broken inversion symmetry in the crystal structure or at interfaces, SOI will lift the spin degeneracy and induce a complex Fermi surfaces and spin textures with spin momentum locking [1,2]. Furthermore, the SOI can drive the system through a phase transition to a so-called topological insulator. Being an insulator in the bulk these systems are characterized by spin-polarized, topologically protected interface states.
After a short introduction to the role of topology in the band structure of solids I will give an overview of our main spin- and angle-resolved photoemission (SARPES) results on a variety of non-interacting topological insulators . One of the questions is how the spin texture evolves around a topological transition. We explored the occurrence of spin polarized states around a SOI driven topological transition  and around a structure driven topological transition . In both cases we observe spin-polarized precursor states, which indicate that although the topological transition is sharp, the response of the system is more gradual.From a fundamental point of view the truly interesting aspect of non-trivial spin textures lies in their combination with other interactions. This can result in a variety of phenomena, cumulating in the creation of the elusive Majorana Fermion. An example of a combination of interactions is our recent verification with SARPES of SmB6 as a topological Kondo insulator . In topologically trivial systems, interactions can lead to the formation of a so-called spin-orbit density wave. I will show how the combination of a large spin-splitting and Fermi nesting leads to the formation of such a state and can explain the anisotropic behavior of Pb nanowires . Furthermore, I will present our recent SARPES results for transition metal oxide surfaces where a subtle interplay between ferroelectricity and magnetic order results in the formation of a single spin-polarized energy contour. The occurrence of superconductivity in such systems could render it a 2D Majorana platform.
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2D+EM+MI+MN+NS+SS+TF-ThA-8 The Symmetry Dependent Band Structure of MoS2
Duy Le (University of Central Florida); Takashi Komesu (University of Nebraska-Lincoln); Quan Ma (University of California, Riverside); Eike F. Schwier, H. Iwasawa (Hiroshima University, Japan); M. Shimada (Higashi-Hiroshima, Japan); Talat Rahman (University of Central Florida); Ludwig Bartles (University of California, Riverside); Peter Dowben (University of Nebraska-Lincoln)
We will present results of density functional theory (DFT) based calculations of symmetry dependent band structures of single crystal MoS2(0001) surface together with symmetry-polarized angle resolved photoemission spectroscopy (ARPES) derived experimental band structure. The good agreement of the DFT band structure with the experimentally derived bands with even and odd symmetries, attests to the reliability of the results. We performed ARPES at the Hiroshima Synchrotron, determining the MoS2 band structure separately for both p-, and s-, polarized to distinguish even and odd symmetry, and the experimentally determined dispersion, in accordance with expectations and experimental confirmation of C3v symmetry, argues in favor of an experimental band structure obtained from single domains. The comparison of theory and experiment provides strong indications that the bands at the top of the valence band are dominated by Mo 4d states. These states and indeed placement of the valence band can be perturbed by adsorbates. Indeed, we find that, under the effect of Na adsorption, the changing placement of the valence band structure of MoS2 clearly indicate the Na atoms donate electrons to MoS2 and that the Fermi energy level shifts as much as 0.5 eV with respect to the top of MoS2’s valance band. Surprisingly, Na adsorption does not perturb the MoS2 band dispersion significantly. We will discuss these results in the light of those obtained for single layer MoS2 for insights and clarity.
2D+EM+MI+MN+NS+SS+TF-ThA-9 CuInIIIP2S6 - Room Temperature Layered Ferroelectric
Alex Belianinov, Petro Maksymovych (Oak Ridge National Laboratory); Andrius Dziaugys (Vilnius University, Lithuania); Qian He (Oak Ridge National Laboratory); Evgheny Eliseev (National Academy of Sciences of Ukraine); Albina Borisevich (Oak Ridge National Laboratory); Anna Morozovska (NAS of Ukraine); Juras Banys (Vilnius University, Lithuania); Yulian Vysochanskii (Uzhgorod University, Ukraine); Sergei Kalinin (Oak Ridge National Laboratory)
We have utilized ambient and Ultra High Vacuum Scanning Probe Microscopy tools to explore ferroelectric properties in cleaved 2D flakes of copper indium thiophosphate, CuInIIIP2S6 (CITP), and report on size effect and presently achievable limits of ferroelectric phase stability. CITP is an unusual example of a layered, anti-collinear, uncompensated, two-sublattice ferroelectric system. These are the only materials known to display “2-D” ferroelectric semiconductor behavior in a van-der-Waals crystal. The material exhibits a ﬁrst-order phase transition of order–disorder type from the paraelectric to the ferrielectric phase at Tc = 315 K. Our observations suggest the presence of stable ferroelectric polarization as evidenced by domain structures, rewritable polarization, and hysteresis loops. These observations suggest that flakes above 100 nm have bulk-like polarization and domain structures, whereas below 50 nm polarization disappears. Furthermore, the materials have measurable ionic mobility, as evidenced both by macroscopic measurements and by formation of surface damage above tip bias of 4 V, likely due to copper reduction. We ascribe this behavior to well-known instability of polarization due to depolarization field, along with internal screening by mobile Cu ions, as suggested by their high ionic mobility.
Research for (AB, PM, QH, AB, SVK) was supported by the US Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. Research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy.
2D+EM+MI+MN+NS+SS+TF-ThA-10 Doping Efficiency and Mechanisms of Single and Randomly Stacked Bilayer Graphene by Iodine Adsorption
Hokwon Kim, Anastasia Tyurnina (Univ. Grenoble Alpes/ CEA, LETI, France); Jean-François Guillet, Jean-Pierre Simonato, Jean Dijon (Univ. Grenoble Alpes/ CEA, LITEN, France); Denis Rouchon, Denis Mariolle, Nicolas Chevalier, Olivier Renault (Univ. Grenoble Alpes/ CEA, LETI, France)
The precise control of graphene’s conductivity and work function is crucial in developing practical applications of graphene based electronics. In order to enhance the conductivity of graphene, we employed a simple doping method where graphene films produced by chemical vapor deposition and transferred onto SiO2, Al2O3, and WO3 substrates are p-doped with iodine vapor through physisorption at temperature of ~ 100 °C [1-3]. The work function values and iodine to carbon ratios of the one-layer (1L) and two-layer (2L) folded regions were analyzed by high spatial- and energy resolution X-ray photoelectron emission microscopy (XPEEM) on a NanoESCA instrument. After the iodine doping, the work function values were significantly increased up to ~0.4 eV and ~0.5 eV, respectively, for 1L and 2L graphene on SiO2/Si. This higher degree of doping by iodine was corroborated by I 3d5/2 core level imaging of the same area where the 2L graphene exhibited significantly larger concentration of iodine (2 at. % versus 1 at. %) likely due to the intercalation of iodine at the inter-layer space.
The main iodine species identified by high resolution core level X-ray photoemission spectroscopy and Raman spectroscopy were I3- and I5- poly-iodide anionic complexes with slightly higher concentration of I5- in 2L than 1L graphene possibly due to different doping mechanisms. Temperature dependent ultra-high-vacuum, in-situ annealing of the doped films has demonstrated that most of iodine is removed above 300 °C for the both 1L and 2L regions, although a significant removal of iodine is observed for 2L graphene at temperature as low as 100 °C. Surprisingly, after the complete removal of iodine by annealing, the work function value did not return to the original one before the doping treatment and remained at a much higher value. This can be ascribed to the residual hydrocarbon contaminations interacting with the atomic defects within the graphene layer that lead to unintentional n-type doping in our samples.
Acknowledgement: The XPEEM and KFM measurements were performed at the Nanocharacterization Platform (PFNC).
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2D+EM+MI+MN+NS+SS+TF-ThA-11 Use of XPS for Device Characterization
Pinar Aydogan, Emre Polat, Coskun Kocabas, Sefik Suzer (Bilkent University, Turkey)
A noncontact chemical and electrical measurement technique of XPS is utilized to investigate a number of devices made of graphene. The main objective of the technique is to trace chemical and location specific surface potential variations as shifts of the XPS peak positions under operating conditions. Devices consisting of graphene; (i) acting as a simple resistive element between two gold electrodes, (ii) a semiconducting sheet controlled by a back-gate, and (iii) between the source and the drain metal electrodes in a full transistor geometry, have been analyzed by recording the Au4f of the metal electrodes, the C1s of the graphene layer, and the O1s (or N1s) peaks of the silicon oxide (or nitride) of the substrate. The advantage of this technique is its ability to assess element specific surface electrical potentials of devices under operation based on the deviations of the core level peak positions in surface domains/structures. Detection of the variations in electrical potentials and especially their responses to various stimuli gives unprecedented information about the chemical nature as well as the location of structural and/or other types of defects as a result of doping, oxidation, reduction, etc.