Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E33: 2D Heterostructures and Surface Effects |
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Sponsoring Units: DCMP Chair: Xiaoxiao Zhang, Stanford University Room: 296 |
Tuesday, March 14, 2017 8:00AM - 8:12AM |
E33.00001: Nonlocal plasmon excitations for scaffolded structures of silicene, germanene and molybdenum disulfide Godfrey Gumbs, Andrii Iurov, Danhong Huang We apply our recently developed mean-field theory for a nonlocal plasmon dispersion for a 2D layer-conductor scaffold to buckled honeycomb lattices such as silicene and germanene, as well as group IV dichalcogenides. Numerical solutions for the plasmon branches and particle-hole modes have been obtained for a wide range of experimentally accessible frequencies and wavelengths, different spin-orbit and lattice asymmetry energy bandgaps, as well as various surface plasmon frequencies. Crucial differences are obtained for these plasmon modes in the buckled lattices, compared to that in graphene, were confirmed. We have also discuss the nonlocality of plasmons in molybdenum disulfide and observed several crucial closed-form analytical results within specified approximations. In all these novel two-dimensional lattices, the collective excitations exhibit unusual features which are expected to be of importance for the practical applications. [Preview Abstract] |
Tuesday, March 14, 2017 8:12AM - 8:24AM |
E33.00002: Theory of proximity induced exchange coupling in graphene on hBN/(Co, Ni) Klaus Zollner, Martin Gmitra, Tobias Frank, Jaroslav Fabian A route towards an applicable spintronics device are van der Waals heterostructures\footnote{A. K. Geim and I. V. Grigorieva, Nature 499, 419 (2013).} with two-dimensional materials, such as graphene and hexagonal boron nitride (hBN). We perform systematic first-principles calculations of the proximity exchange coupling, induced by cobalt and nickel in graphene, via a few layers of hBN. We find that the induced spin splitting of the graphene bands is significant, even for two layers of hBN. By employing a pseudospin-dependent exchange model Hamiltonian, we can describe the first-principles data. This model can be used to study transport in graphene with proximity exchange\footnote{K. Zollner, M. Gmitra, T. Frank, and J. Fabian, Phys. Rev. B 94, 155441 (2016).}. We will also present more recent data on the proximity exchange in other two-dimensional materials and topological insulators. [Preview Abstract] |
Tuesday, March 14, 2017 8:24AM - 8:36AM |
E33.00003: Twistronics: Manipulating the Electronic Properties of Two-dimensional Layered Structures through their Twist Angle Stephen Carr, Daniel Massatt, Shiang Fang, Paul Cazeaux, Mitchell Luskin, Efthimios Kaxiras We have introduced a new method for parameter-free computation of electronic properties in incommensurate layered 2D materials with controllable errors. Although here we have only studied bilayer materials, the method is general and extends to any number of layers and of arbitrary heterostructure composition. Viewing the problem on the space of configurations, $\Omega$, allows us to fully characterize the properties of incommensurate (aperiodic) systems. The method allows for the inclusion of external fields and other sources of disorder, such as strain or defects. We present results of applying the method to twisted bilayer graphene and a representative of the TMDC family of semiconductors. The method is accurate enough to correctly calculate quantization of Hall conductivity in tBLG in the presence of magnetic fields, and reproduces the correct Chern number for the $N = 0$ Landau Level. It also predicts that bilayer TMDC's have a twist-dependent band-gap. The method is a promising candidate for the targeted design of electronic properties in layered heterostructures. [Preview Abstract] |
Tuesday, March 14, 2017 8:36AM - 8:48AM |
E33.00004: In-situ electrostatic doping in 2D semiconductor heterostructures determined by micro-ARPES Paul Nguyen, Neil Wilson, Natalie Teutsch, Gabriel Constantinescu, Viktor Kandyba, Alexey Barinov, Nicholas Hine, Xiaodong Xu, David Cobden Understanding the behavior of 2D devices calls for probing the local electronic spectrum and how it is affected by bias. The most powerful technique for determining band structure is angle-resolved photoemission (ARPES), which can now be applied to micron-scale, electrically contacted samples, for example at Spectromicroscopy beamline, Elettra . Using this facility, and by designing samples with electron-transparent monolayer graphene or hBN caps for protection, we have studied heterostructures of WS$_2$, MoS$_2$, MoSe$_2$ and graphene that are back-gated with a thin graphite electrode through an h-BN dielectric. Using the gate we reversibly tuned in-situ in the ARPES chamber the carrier density in graphene up to $\pm$2$\times$ 10$^{13}$ cm$^{-2}$ and measured field induced changes in band alignments in heterostructures. We were also able to electrostatically populate the conduction band in MoS$_2$, revealing a direct gap at the K-point of 2.1 $\pm$ 0.1 eV, in good agreement with recent STM measurements. [Preview Abstract] |
Tuesday, March 14, 2017 8:48AM - 9:00AM |
E33.00005: Strong Fermi-Level Pinning at Intact Metal/Si Interface Formed with Graphene Diffusion Barrier Kibog Park, Hoon Hahn Yoon, Sungchul Jung, Gahyun Choi, Junhyung Kim, Youngeun Jeon, Yong Soo Kim, Hu Young Jeong, Kwanpyo Kim, Soon-Yong Kwon We report the systematic experimental studies demonstrating that a graphene layer inserted at Metal/n-Si(001) interface can protect the Schottky junction efficiently from unwanted changes in electrical properties. High-resolution transmission electron microscopy (HRTEM) images support the conjecture of the inserted graphene layer preventing the atomic inter-diffusion at interface. Especially, the reverse-bias leakage current of Metal/Graphene/n-Si(001) junction is found to be noticeably smaller than that of Metal/n-Si(001) junction, strongly supporting the role of graphene insertion layer as an efficient diffusion barrier. The internal photoemission (IPE) measurements show unambiguously that the Schottky barrier of Metal/Graphene/n-Si(001) junction is almost independent of metal work-function, implying very strong Fermi-level pinning at interface. The atomically-impermeable and electronically-transparent aspects of the graphene insertion layer can provide a reliable experimental method to form an intact Schottky contact for all semiconductors in general. [Preview Abstract] |
Tuesday, March 14, 2017 9:00AM - 9:12AM |
E33.00006: Semiconductor to Metal Transition in WS2/Ag(111) Charlotte Sanders, Maciej Dendzik, Albert Bruix, Matteo Michiardi, Arlette Ngankeu, Marco Bianchi, Bjørk Hammer, Jill Miwa, Philip Hofmann Substrate effects play an important role in determining electronic structure in two-dimensional materials (2DMATs). A common effect of a metallic substrate on a semiconducting 2DMAT is strong renormalization of the band gap, induced by metallic screening, as recently observed in MoS2/Au(111) [1]. Here we report a substrate effect that goes beyond a band gap change due to screening. For WS2/Ag(111), interaction with the substrate leads to a pronounced change of the band structure of WS2, from a direct band gap semiconductor to a metal. In this transition, the local minimum in the conduction band at the Q point is pulled down in energy relative to the absolute minimum at K. The Q band minimum can even become lower than the K minimum and cross the Fermi level, becoming partially occupied. This is evident in measurements by angle-resolved photoemission spectroscopy (ARPES); the metallicity of WS2 is also indicated by a characteristic asymmetric lineshape observed in core-level photoemission spectra. ARPES and time-resolved ARPES measurements confirm that the conduction band at K remains above the Fermi level. Reasons for this band distortion are investigated, with reference to band structure calculations based on density functional theory. [1] Phys. Rev. B 93, 165422 (2016) [Preview Abstract] |
Tuesday, March 14, 2017 9:12AM - 9:24AM |
E33.00007: Controlling Nanotubes with Phosphorene Aleksandr Rodin, Alexandra Carvalho, Antonio Helio Castro Neto A hundred years after its initial discovery, black phosphorus has become one of the most studied 2D materials. What sets it apart from other members of the 2D family is its highly anisotropic dispersion. While this anisotropy has been studied before in the context of electronic transport, here we investigate its effects from the electro-mechanical standpoint. Dispersion anisotropy results in a direction-dependent polarization which can be utilized to control the arrangements of external charge. Using a combination of analytical and DFT methods, we analyze the interaction of black phosphorus with several different charge configurations. In particular, we address the problem of the orientation of charged carbon nanotubes positioned on top of phosphorene layers. [Preview Abstract] |
Tuesday, March 14, 2017 9:24AM - 9:36AM |
E33.00008: Understanding the electronic states of one-atom-thick materials on metal substrates Changwon Park, Mina Yoon Using density functional theory calculations, we investigated how a coating of one-atom-thick materials modifies the metallic states in space and energy. The most prominent modification is governed by the Pauli exclusion principle, and the accompanying spatial modification of the metallic states causes them to follow the orbital shape of the coating material. The energy level of the metallic surface states also changes through the subtle interplay between dipole layer formation and strong substrate interaction. We chose two-dimensional boron nitride on Cu substrate (2D BN on Cu) as an example system for demonstrating the role of one-atom-thick materials on the substrate’s metallic states. Specifically, we revealed the nature of some counterintuitive features of STM images of 2D BN on Cu. Implications on the workfunction change and STM interpretation of molecular assemblies are also presented. [Preview Abstract] |
Tuesday, March 14, 2017 9:36AM - 9:48AM |
E33.00009: Bonding Directionality Matters: Direct-Indirect Transition in Few-Layer SnSe. Hansika Sirikumara, Thushari Jayasekera SnSe is one of the best thermoelectric materials reported to date. The possibility of growing few-layer SnSe helped boost the interest in SnSe, and paves the path for various other applications such as photovoltaics and optoelectronics. However, indirect band gap of SnSe hinders its success in such fields. Based on the results from first principles Density Functional Theory, we carefully analyzed electronic band structures of bulk, mono and few-layer SnSe with various interlayer stackings. Our results reveal that it is the directionality of interlayer interactions, which leads to the indirect electronic band gap. In fact, by modifying the interface between layers, there is a possibility of achieving few-layer SnSe with direct electronic band gap. Moreover, the fundamental understanding of interlayer interactions at the atomic level also paves the path for designing Van der Waals heterostructures based on SnSe with prescribed electronic properties. [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:00AM |
E33.00010: Strain, stabilities and electronic properties of hexagonal BN bilayers Yoshitaka Fujimoto, Susumu Saito Hexagonal boron nitride (h-BN) atomic layers have been regarded as fascinating materials both scientifically and technologically due to the sizable band gap. This sizable band-gap nature of the h-BN atomic layers would provide not only new physical properties but also novel nano- and/or opto-electronics applications [1]. Here, we study the first-principles density-functional study that clarifies the biaxial strain effects on the energetics and the electronic properties of h-BN bilayers [2].$^{\mathrm{\thinspace }}$We show that the band gaps of the h-BN bilayers are tunable by applying strains. Furthermore, we show that the biaxial strains can produce a transition from indirect to direct band gaps of the h-BN bilayer. We also discuss that both AA and AB stacking patterns of h-BN bilayer become feasible structures because h-BN bilayers possess two different directions in the stacking patterns. [1] Y. Fujimoto and S. Saito, Phys. Rev. B 93 (2016) 045402. [2] Y. Fujimoto and S. Saito, submitted to Phys. Rev. B. [Preview Abstract] |
Tuesday, March 14, 2017 10:00AM - 10:12AM |
E33.00011: Band gap closing in multilayer Nickel Bis(ditiolene) induced by interlayer interactions Suchun Li, Shen Li, Gang Wu, Shuo-Wang Yang Nickel Bis(dithiolene) shows the highest experimentally recorded electrical conductivity for coordination polymeric materials [JACS 2014, 136, 14357]. On the contrary contrast, the thermal conductivity of such coordination polymeric materials is expected to be very low due to the pores structure. Therefore NiBis and similar coordination polymeric materials are expected to show high thermoelectric figure of merit. Motived by the high potential of this type of materials in thermoelectric applications, we studied Nickel Bis(dithiolene) using first principles calculations. We find very interestingly that monolayer NiBis is a semiconductor with an indirect gap of \textasciitilde 0.15eV while all multilayer NiBis become metallic. We analyze the underline mechanism using bilayer NiBis with varying interlayer distance, and find that the gap can be gradually opened again by gradually increase the interlayer distance. We reveal that interlayer interactions in multilayer NiBis are more than simple van der Waals interactions. Multilayer NiBis become metallic because of covalent-like interactions between Sulfur atoms across different layers. Making use of this interlayer-interaction mechanism, we can use interlayer distance (pressure) to tune the gap and many related properties such as electrical conductivity, Seebeck coefficient, optical properties, etc. [Preview Abstract] |
Tuesday, March 14, 2017 10:12AM - 10:24AM |
E33.00012: Stacking-dependent electronic structure of TaS2 Pierre Darancet Among transition metal dichalcogenides, 1T-TaS2 is known for its so-called “Star of David” charge density wave (CDW) conducive to a transition from a high symmetry metal into an odd-electron-number insulator, suggesting a Mott transition. While DFT correctly captures the lattice distortion and the opening of an in-plane gap, it also predicts that interlayer coupling destroys the Mott state leading to the delocalization of electrons out-of-plane [1]. In the past two years, new experimental findings have challenged the DFT description by observing a ground state Mott insulating phase that can be transformed into a long-lived metallic state by light [2], and voltage pulses [3]. Here, we explain these findings using DFT calculations that takes into account the previously theoretically ignored 13-layer periodicity of the CDW. We determine the thermally accessible configurations for the 13-layer-thick unit cell of TaS2 and compute the electronic structure of these systems as well as the conditions of emergence of an insulating state. [1] Ge and Liu, Phys. Rev. B (2010); Liu, Phys. Rev. B (2009); Darancet et al., Phys. Rev. B (2014); [2] Yu et al., Nat Nano (2015);[3] Tsen et al., PNAS (2015); Vaskivskyi et al., Science Adv. (2015); Cho et al., Nat. Comm. (2015); Ma et al. Nat Comm [Preview Abstract] |
Tuesday, March 14, 2017 10:24AM - 10:36AM |
E33.00013: Substrate-induced modification of band parameters of graphene Shi Che, Petr Stepanov, Supeng Ge, Yafis Barlas, Kenji Watanabe, Takashi Taniguchi, Chun Ning Lau Graphene and its few-layer counterparts have emerged as an attractive platform for investigating physical and chemical processes in reduced dimensions. Their band structures are conventionally calculated based on the Slonczewski-Weiss-McClure hopping parameters of graphite. Here we present experimental and computational evidence that these hopping parameters are potentially modified by the substrates. Our work may pave the path in the band structure engineering. [Preview Abstract] |
Tuesday, March 14, 2017 10:36AM - 10:48AM |
E33.00014: Bound states in nanoscale graphene quantum dots in a continuous graphene sheet Jiabin Qiao, Lin He Considerable efforts have been made to trap massless Dirac fermions in graphene monolayer, but only quasi-bound states are realized in continuous graphene sheets up to now. Here, we demonstrate the realization of bound states in nanoscale graphene quantum dots (GQDs) in a continuous graphene sheet. The GQDs are electronically isolated from the surrounding continuous graphene sheet by circular atomically sharp boundaries, which are generated by the strong interaction between graphene and substrate. By using scanning tunneling microscopy (STM), we observe single-electron charge states of the GQDs, seen as Coulomb oscillations in the tunneling conductance. Evolution of single-electron transport of the GQDs between the Coulomb blockade regime and the Coulomb staircase regime is observed by tuning the STM tip-sample distances. Spatial maps of the local electronic properties reveal concentric rings inside the GQDs with each ring corresponding to a single Coulomb oscillation of the tunneling spectra. These results indicate explicitly that the electrons are completely trapped inside the nanoscale GQDs. [Preview Abstract] |
Tuesday, March 14, 2017 10:48AM - 11:00AM |
E33.00015: Quantum oscillations in inverted insulators: gated bilayer graphene Simonas Grubinskas, Lars Fritz It was recently demonstrated that a band insulator, i.e., a system without a Fermi surface can exhibit quantum oscillations with well-defined frequencies. We consider a concrete model that has a valence band of the shape of the Goldstone potential which subsequently is subjected to a magnetic field. The quantum oscillations come from the fact that as the magnetic field is varied, different Landau levels come closest to the chemical potential without crossing it. We derive an analytic expression for the Lifshitz-Kosevich formula in such a system and show that the role of 'the area of the Fermi surface' is taken by the area enclosed by the circular minimum of the Goldstone potential. Furthermore, we find that the damping is governed by a complicated interplay of the gap and the finite temperature. We propose to measure this effect in gated bilayer graphene. [Preview Abstract] |
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