Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H45: 2D Materials and Graphene: Surface and Interface ConnectionsFocus
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Sponsoring Units: DCMP Chair: Chenggang Tao, Virginia Tech Room: BCEC 211 |
Tuesday, March 5, 2019 2:30PM - 2:42PM |
H45.00001: Experimentally Observed Electronic Coupling between Natively Functionalized 2-D layers of Ti3C2Tx MXenes David Lioi, William Kennedy, Dhriti Nepal, Richard Vaia, James Heckler, Gregory Neher, Ruth Pachter MXenes are a new class of 2-D layered compounds that show promise as a highly tunable material for next generation composites. The electronic properties of MXenes depend on their chemical composition as well as the native surface functional groups that arise during synthesis and processing. Here we present spectroscopic observations of the electronic coupling in few layered MXenes with the topography being verified by atomic force microscopy (AFM). Energy resonances below 2 eV were observed and found to change with laminate thickness. We attribute these to plasmonic resonances that result from the electronic coupling between the different transverse and longitudinal plasmonic modes of individual MXene flakes that have been stacked into larger structures. These resonances were also observed to enhance the Raman spectroscopy response, which proved invaluable for routinely studying the lattice structures of ultrathin MXene flakes. Future applications for MXenes in energy storage and nanoscale optical devices are discussed. |
Tuesday, March 5, 2019 2:42PM - 2:54PM |
H45.00002: Valley-dependent impurity scattering in α-T3 ααlattices and specific role of Lorentz forces Danhong Huang, Andrii Iurov, Ying-Cheng Lai, Hongya Xu, Godfrey Gumbs We have obtained the angular distribution of extrinsic skew-scattering currents related to a set of randomly-distributed impurities in α-T3 lattices using the Boltzmann moment equations. With the help of the screened second-order Born approximation, we chave alculated the inverse momentum-relaxation-time tensor and revealed the anisotropy which appears to be the source of the angle-dependent currents. The static dielectric function was obtained under the random-phase approximation (RPA). The scattering is also modified by Berry-phase variation and local asymmetry in impurity potentials next to the two inequivalent K and K' valleys. The resulting skew current depends on the electron doping level and is determined by the ratio of energy to momentum relaxation times |
Tuesday, March 5, 2019 2:54PM - 3:06PM |
H45.00003: Tuning the opto electronic properties of ultrasmooth large area rGO films grown via Pulsed laser deposition (PLD) technique Muhammed Juvaid We present the growth of large area ultra-smooth reduced graphene oxide (rGO) thin films on a four inch wafer via beam scanning mode of Pulsed laser deposition technique. With the versatility of the PLD technique, the easy tuning of optoelectronic properties is achieved on rGO thin films without employing any buffer layers. The conventional growth mechanism of rGO films involves hazardous chemicals, whereas this physical vapour deposition technique gives high quality rGO films with tunable optoelectronic properties. This wafer-scale growth mechanism of rGO thin films can potentially employ in various optoelectronic applications due to its high transparency and p-type conductivity. We illustrates the optoelectronic tunability of large area rGO films and its use through the evaluation of transparent conducting films, where our rGO films shows highest performance as compared to existing p -type transparent conducting films. |
Tuesday, March 5, 2019 3:06PM - 3:18PM |
H45.00004: Intercalation of Cu and Ru beneath the top graphene layer of graphite surfaces Yong Han, Ann Lii-Rosales, Michael C. Tringides, James William Evans, Patricia A. Thiel STM experiments show that Cu forms encapsulated islands under the top graphene layers of graphite, as a result of vapor deposition of Cu on a sputtered graphite surface [JPCC 122, 4454]. Deposition at 800 K is optimal for formation of encapsulated multilayer Cu islands. Deposition below 600 K favors Cu clusters adsorbed on top of graphite, while deposition above 800 K favors single-layer intercalated Cu islands. To form Ru nanoislands below the top graphene layer requires deposition at 1000 K to 1180 K [Nanotechnology 29, 505601]. We present an extensive study of energetics using density functional theory to compare stabilities of a wide variety of configurations of atoms, clusters, and layers of Cu and Ru on/under the graphite surface. For Cu-graphite systems, the only configuration that is significantly more stable under the graphite surface than on top of it, is a single Cu atom. This analysis leads us to conclude that formation of encapsulated Cu islands is kinetically driven, rather than thermodynamically driven. On the contrary, the embedded Ru islands are thermodynamically favored over on-top clusters. |
Tuesday, March 5, 2019 3:18PM - 3:30PM |
H45.00005: Electrical and mechanical properties of h-BN nanoplates on Ir(111) studied by STM and NC-AFM Mengxi Liu, Xiaohui Qiu Hexagonal boron nitride (h-BN) on metal surfaces is subjected to substrate interactions as well as in-plane strains resulted from the lattice mismatch between h-BN and the underlying substrate. Here, we investigate the electrical and mechanical properties of h-BN nanoplate grown on Ir(111) using a combined approach of scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM) and density functional theory (DFT) calculations. The in situ synthesized h-BN nanoplates have a characteristic triangular shape with zigzag-type edges. Boron and nitrogen atoms can be identified by distinct chemical interaction with metallic tip. The nc-AFM images show a strong tip-dependent contrast, which transforms from honeycomb lattices to hexagonal spots by switching W-tip to Cl-tip. Based on DFT calculations, we interpret the contrast conversion in terms of a combination of tip dipole and charge transfer from Ir(111) surface to BN nanoplates bound to Ir(111). In addition, the elastic deformation of h-BN nanoplates was measured by nc-AFM force map and lateral stiffness of h-BN nanoplates dramatically depend on their size. |
Tuesday, March 5, 2019 3:30PM - 3:42PM |
H45.00006: Scanning tunneling spectroscopy study of incorporated Al atoms in Si(100) substrate Hyun-soo Kim, Aruna Ramanayaka, Ke Tang, Joshua Pomeroy We have studied the structures and electrical properties of incorporated Al atoms in Si(100) using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) at room temperature in ultra-high vacuum. Our recent work shows that, in Si/Al/Si heterostructures, a hole is a dominant charge carrier and we have achieved a comparable mobilities to previous reports on similar two-dimensional hole gas systems at high charge carrier densities. [1,2] However, the electronic states of Al have not been studied well as dopants in Si. STS can provide the local density of states to understand the electrical properties of incorporated Al atoms in Si(100) similar to the Si/Al/Si heterostructure in our recent work. [1] We will discuss the local density of states measured by STS of incorporated Al atoms in a clean (2x1) reconstructed Si(100) substrate at various densities prepared by thermal Al evaporation and annealing in situ at 550 °C. |
Tuesday, March 5, 2019 3:42PM - 3:54PM |
H45.00007: Discovery of a New Layered Crystal, Hexagonal Beryllium Oxide Lifen Wang, Lei Liu, Ji Chen, Shang-Peng Gao, Gong Gu The reported work was prompted by the intriguing question whether sp2-coordinated, layered polymorphs of octet compounds other than BN can exist. While such polymorphs with thicknesses within a thermodynamic limit are predicted, the possibility of their thicker layered crystals has long been dismissed. Here, using high-resolution transmission electron microscopy and electron energy loss spectroscopy, we show that BeO crystallizes in the sp2-coordinated, layered structure in liquid cells formed by sheets of graphene. We further reveal that the layered crystals can be thicker than the thermodynamically determined ultra-thin limit, beyond which the layered phase is energetically unfavored. The discovery counters a long-held dismissal and calls for a reevaluation of the possible existence of sp2-coordinated, layered polymorphs of octet compounds beyond the ultra-thin limit. |
Tuesday, March 5, 2019 3:54PM - 4:06PM |
H45.00008: A Study of Vertical Transport in Silicon-Graphene Junction Sidong Lei, Xiaodan Zhu, Shin-Hung Tsai, Xiang Zhang, Gen Yin, carlos torres, Aryan Navabi, Zehua Jin, Hussam Qasem, Robert Vajtai, Roger Lake, Pulickel M Ajayan, Kang Wang In this report, we explore the important roles of single layer graphene in the vertical tunneling process as a tunneling barrier. Although being a semimetal in the lateral lattice plane, the graphene together with the vdW gap acts as a tunneling barrier, that is nearly transparent to vertically tunneling electrons, due to its atomic thickness and the transverse momenta mismatch between the injected electrons and the graphene band structure. This is accentuated using electron tunneling spectroscopy (ETS) showing a lack of features corresponding to the Dirac cone band structure. Meanwhile, the graphene acts as a lateral conductor through which the potential and charge distribution across the tunneling barrier can be tuned. These unique properties make graphene an excellent 2D atomic grid, transparent to charge carriers, and yet offer the control of the carrier flux via the electrical potential. |
Tuesday, March 5, 2019 4:06PM - 4:18PM |
H45.00009: Construction of graphene/silicene heterostructure by Si intercalation Geng Li Geng Li |
Tuesday, March 5, 2019 4:18PM - 4:30PM |
H45.00010: The mechanism for the stabilization and surfactant properties of epitaxial silicene Alberto Curcella, romain bernard, Yves Borensztein, Michele Lazzeri, geoffroy prevot Using real-time in situ scanning tunneling microscopy and density functional theory simulations, we have studied the growth of Si films on Ag(111) for coverages above the silicene monolayer, evidencing the existence of metastable phases and an original growth mechanism. Above monolayer Si coverage, an initial sqrt(3)Xsqrt(3) structure forms, which is identified as an Ag-free Si bilayer with additional Si adatoms. With further deposition, this structure is replaced by a distinct bilayer structure covered by Si trimers and Ag atoms. The formation of these bilayers follows counterintuitive dynamics: they are partially inserted within the Ag substrate and form by expelling, from the underlying substrate, the atoms that reinsert below the adjacent silicene layer. The growth is therefore characterized by an unexpected “surfactant competition” between Ag and silicene: while silicene is a metastable surfactant for the Ag(111) surface, Ag plays the role of a surfactant for thicker diamond-like Si islands. In spite of being thermodynamically unfavoured, the silicene monolayer is, thus, a remarkably stable structure because of the high kinetic barrier for the growth of thicker layers. |
Tuesday, March 5, 2019 4:30PM - 4:42PM |
H45.00011: Ab initio study of 2D plasmon enhancement in alkali intercalated graphene on metallic substrates Vito Despoja, Leonardo Marusic Alkali metal (AM) atoms, (e.g. in LiC2 or CsC8) donate electrons to the graphene π band causing the appearance of the strong Dirac plasmon (DP) in the EEL spectra. At the same time, the AM σ band remains partially filled and supports another 2D plasmon, which hybridizes with the Dirac plasmon causing appearance of the weak linearly dispersive plasmon, known as the acoustic plasmon (AP) [1,2]. We present the results of a theoretical simulation of alkali atoms intercalated between the graphene and a metallic substrate (e.g. Ir(111) or Al(111)) and forming a periodic superlattice, which causes a huge enhancement of DP and AP. Moreover the AP intensity and Fermi velocity strongly depend on graphene/substrate separations. This enhancement mechanism, in addition to its very interesting fundamental aspect, suggests many possibilities for plasmonic applications. The theoretical simulation is performed using a state of the art DFT (ground state) + RPA (excited state) technique, adapted for the study of the dielectric properties in large multilayer heterostructures, which completely exclude inter-supercell Coulomb interaction. |
Tuesday, March 5, 2019 4:42PM - 4:54PM |
H45.00012: First-Principles investigation of Epitaxial Pt Layer on Graphene Ji Il Choi, Faisal Alamgir, Seung Soon Jang Platinum is a face centered cubic structured crystal that has been widely known as a superior catalyst for various chemical reactions, while low-dimensional structures, such as mono- or bi-layer platinum, have attracted less attention due to experimental difficulties in synthesizing such 2D structures. In this study, we present a computational research on the unique architecture of epitaxial platinum (mono/multi) layers grown on graphene (Pt_ML/GR), in support of remarkable recent progress in the synthesis of these architectures in simple cubic-like (SC-L) and face-centered cubic-like (FCC-L) phases on the graphene. Recently proposed strongly constrained and appropriately normed (SCAN) density function study (DFT) is employed to investigate the structural and electronic properties of the epitaxial SC-L Pt layered graphene. In these architectures, Pt exhibits registry with the C-C bridge sites along the armchair and zigzag directions. Here, the details of the atomistic/electronic structures and binding energies are discussed. Further, the detailed band structure and the partial/total densities of state (DOS) of the Pt_ML/GR architectures, with Pt in an SC-L registry, are presented. |
Tuesday, March 5, 2019 4:54PM - 5:06PM |
H45.00013: Two dimensional phases of Ag on Ge(111): insights from first-principles calculations Shree Ram Acharya, Duy Le, Shirley Chiang, Ching Fong, Talat S. Rahman Experimental observations using low-energy electron diffraction (LEED) of deposited Ag on the Ge(111) surface have revealed a number of two-dimensional Ag structures which depend on Ag coverage and sample temperature[1]. We have applied density functional theory based calculations to explore the Ag/Ge(111) phase diagram and to obtain insights into the electronic and geometric structures and the vibrational dynamics. In this comparative study of clean Ge(111) with (1x1), reconstructed c(2x8) and (2x1) geometries, and of sub-monolayer Ag covered Ge(111) with (4x4), (v3xv3)R30 and (1x1) overlayers, we find that Ge atoms on the surface prefer three-fold hollow sites and Ag adatoms prefer to form triangular structures. Furthermore, temperature-coverage dependent surface phase diagram constructed by minimizing the surface free energies shows that up to 37% Ag coverage, (4x4) is the stable phase beyond which up to 90% coverage, the phase coexists with (v3xv3)R30 and (1x1) phases at temperature below 600K and higher respectively. Beyond 90% coverage, isolated (v3xv3)R30 and (1x1) phase are the most stable with the same temperature boundary of 600K. |
Tuesday, March 5, 2019 5:06PM - 5:18PM |
H45.00014: WITHDRAWN ABSTRACT
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Tuesday, March 5, 2019 5:18PM - 5:30PM |
H45.00015: Liquid crystal (5CB) adsorption on two-dimensional materials Paul Brown, Sean A Fischer, Jakub Kolacz, Christopher Spillmann, Daniel Gunlycke Liquid crystals are widely used in display technologies. Their collective behavior and use ultimately hinges on single molecules interacting with a supporting surface. Since the isolation of graphene in 2004, the potential for combining two-dimensional nanosheets with liquid crystals has become an area of scientific interest. However, little attention has focused on the local interfacial interactions between the mesogen and two-dimensional materials. In this presentation, we discuss the local mesogenic interactions between the well-known nematic liquid crystal 5CB, and a series of monolayer crystals such as graphene, molybdenum disulfide, phosphorene, and h-BN. In particular, we present electronic ground state properties obtained using density functional theory with nonlocal van der Waals corrections that reveal subtle orientational, energetic, and electronic properties for the characterization of local mesogen-substrate interactions, whereby the anchoring of liquid crystal can be understood more generally across atomically flat surfaces. |
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