Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A17: Surface Studies of Two-Dimensional Materials |
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Sponsoring Units: DCMP Chair: Christian Ciobanu, Colorado School of Mines Room: LACC 306A |
Monday, March 5, 2018 8:00AM - 8:12AM |
A17.00001: Vibrational properties of a monolayer silicene sheet studied by tip-enhanced Raman spectroscopy Shaoxiang Sheng, Jiangbin wu, Xin Cong, Wenbin Li, Jian Gou, Qing Zhong, Pinghen Tan, Lan Chen, Kehui Wu Combining ultrahigh sensitivity, spatial resolution and capability to resolve chemical information, tip-enhanced Raman spectroscopy (TERS) is a powerful tool to study molecules or nanoscale objects. Here we show that TERS can also be a powerful tool in studying two-dimensional (2D) materials. We have achieved a 109 Raman signal enhancement and a 0.5 nm spatial resolution using monolayer silicene on Ag(111) as a prototypical 2D material system. Due to the selective enhancement on Raman modes with vertical vibrational components in TERS, our experiment provides a direct evidence of the origination of Raman modes in silicene. Furthermore, the ultrahigh sensitivity of TERS allow us to identify different vibrational properties of silicene phases, which differ only in the bucking direction of the Si-Si bonds. Local vibrational features from defects and domain boundaries in silicene can also be identified. |
Monday, March 5, 2018 8:12AM - 8:24AM |
A17.00002: Gapped Electronic Structure of Epitaxial Stanene on InSb(111) Caizhi Xu, Joseph Hlevyack, Y -H Chan, Peng Chen, Xiaoxiong Wang, David Flötotto, Guang Bian, Sung-Kwan Mo, Mei-Yin Chou, Tai-Chang Chiang Stanene (single-layer grey tin), with an electronic structure akin to that of graphene but exhibiting a much larger spin-orbit gap, offers a promising platform for room-temperature electronics based on the quantum spin Hall (QSH) effect. This material has received much theoretical attention, but a suitable substrate for stanene growth that results in an overall gapped electronic structure has been elusive; a sizable gap is necessary for room-temperature applications. Here, we report a study of stanene epitaxially grown on the (111)B face of indium antimonide (InSb). Angle-resolved photoemission spectroscopy (ARPES) measurements reveal a gap of 0.38 eV, in agreement with our first-principles calculations. The results indicate that stanene on InSb(111) is a strong contender for electronic QSH applications. |
Monday, March 5, 2018 8:24AM - 8:36AM |
A17.00003: Atomic and Electronic Structures of Ultra-thin Epitaxially Grown Te Film Hui Zhang, Siyuan Zhu, Zhenyu Zhang, Chih-Kang Shih
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Monday, March 5, 2018 8:36AM - 8:48AM |
A17.00004: Two-dimensional blue phosphorene grown on Au(111) by molecular-beam epitaxy Jin-Peng Xu, Jun-Qiu Zhang, Hao Tian, Hu Xu, Wingkin Ho, MAOHAI XIE Single layer (SL) phosphorus (phosphorene) has drawn considerable research attention recently. It is a semiconductor showing superior transport and optical properties. In the past several years, few-layer or SL black phosphorus has been successfully isolated by exfoliation and extensively studied for the electronic and optical properties. Blue phosphorus (blueP), an allotrope of black phosphorus but with a more flat atom arrangement, has been predicted to stabilize in the SL form on some substrates. In this work, we report the growth of blueP by molecular-beam epitaxy and reveal an explicit sequential growth behavior involving a dewetting process. The adsorbed P atoms aggregate into one-dimensional (1D) chains, then (√3×√3)R30○ patches and finally SL blueP-like islands. Over a large coverage range, a composite surface consisted of locally high-P coverage blueP islands and locally low-coverage 1D chains or loose (√3×√3)R30○ patches prevails owing to the minimization of the overall system energy. |
Monday, March 5, 2018 8:48AM - 9:00AM |
A17.00005: Giant Ferroelectric Polarization in Ultrathin Ferroelectrics via Boundary-condition Engineering Lin Xie, Linze Li, Xiaoqing Pan Tailoring and enhancing the functional properties of materials at reduced dimension is critical for continuous advancements of modern electronic devices. Here, we report the discovery of local surface induced giant spontaneous polarization in ultrathin BiFeO3 ferroelectric films. Using aberration-corrected scanning transmission electron microscopy (STEM), it is found that the spontaneous polarization in a 2 nm thick ultrathin BiFeO3 film is abnormally increased up to ~90-100 μC/cm2 in the out-of-plane direction and a pecuiliar rumpled nanodomain structure with very large variation in c/a ratios, which is in analog to morphotropic phase boundaries (MPB), is formed. By a combination of density functional and phase-field calculations, it’s shown that it’s the unique single atomic Bi2O3-x layer at the surface that leads to the enhanced polarization and appearance of MPB-like nanodomain structure. This finding clearly demonstrates a novel route to the enhanced functional properties in the material system with reduced dimension via engineering the surface boundary conditions. |
Monday, March 5, 2018 9:00AM - 9:12AM |
A17.00006: Epitaxial Growth and Electronic structure measurement of a New Two-Dimensional Binary Alloy with Honeycomb Lattice: Sn2Bi Hang Li, Lan Chen, Kehui Wu, Jian Gou, Yong Xu, Tian Qian, Hui Li, Longjuan Kong, Peng Cheng, bingyu xia, Hong Ding, Yukiaki Ishida After the discovery of graphene and it’s exotic physical properties such as Dirac cones in electronic structures due to specific honeycomb lattice, several new mono-elemental 2D materials with honeycomb lattice such as silicene, germanene, stanene, borophene, antimonene have been realized. Most of these 2D materials are grown on metal substrates which limits further studies of transport and applications. In this work, we successfully grow a new novel 2D binary alloy Sn2Bi on Si(111) surface. As the most famous semiconductor, using Silicon as substrate can both meet the requirements of intrinsic property preservation and transport measurement. Also Si(111) surface has the same symmetry (C3v) with honey comb lattice. Combined with STM and DFT calculations, we confirm a honeycomb lattice in this 2D alloy with one hexagonal ring consisting two Bi and four Sn atoms. The STS measurements reveal a 0.8 eV gap and near-free electronic states on this 2D alloy, which are further reproduced by ARPES and tr-ARPES measurements and DFT calculations. |
Monday, March 5, 2018 9:12AM - 9:24AM |
A17.00007: Improved Performance of TiO2 based MOS Capacitor with Ultra-thin Al2O3 Layer Arvind Kumar, Sandip Mondal, KSR Koteswara Rao In this work, we report the improved performance of TiO2 based MOS capacitor by inserting a thin layer of Al2O3 (5 nm) in between TiO2 dielectric layer and Si substrate. Al2O3 is a stable dielectric material and act as an efficient barrier to reduce the down diffusion of O2 into Si substrate and possible formation of uncontrolled SiOx layer. This is useful in two ways, first it will provide a large conduction and valence band offset to Si, and second TiO2 stoichiometry will be maintained. The dielectric constant of such bilayer MOS gate stack was estimated 31 from accumulation capacitance with improved leakage performance. The leakage current density was found 1.69×10-8 A/cm2 at + 1 V. The interface states density were extracted from DLTS method and found 2.9x1011 eV-1cm-2, which is reasonably low for Si/high-k system. In addition to that, various other parameters such as flat band voltage (VFB), change in flat band voltage during bidirectional sweep (△VFB), oxide trapped charges (Qot), and equivalent oxide thickness (EOT) are found -0.48 V, 50 mV, 2.97x1011/cm2, and 5.66 nm, respectively. In conclusion, bilayer gate stacks based on TiO2 and Al2O3 can be useful in future CMOS technology. |
Monday, March 5, 2018 9:24AM - 9:36AM |
A17.00008: Observation of adsorption-induced expansion in the interlayer spacing of graphene oxide frameworks during supercritical adsorption Joe Schaeperkoetter, Helmut Kaiser, Haskell Taub, Carlos Wexler During physisorption it is generally assumed that the solid remains inert. However, as the adsorbate molecules go into the pores it is possible that the adsorbant will experience structural changes. This is intriguing, as well as possibly important for improvements in the sorption characteristics of the material. In the past, changes have been observed in subcritical conditions, e.g. the Metal Organic Frameworks "gate opening transitions". Here we focus on structural changes under supercritical conditions in Graphene Oxide Frameworks (GOF). Using in situ neutron scattering, we have observed an increase of the interlayer spacing of GOFs during adsorption of three supercritical gases (hydrogen, methane, xenon) in the 0-150 bar range. (The GOFs were synthesized by the insertion of diboronic acid (DBA) molecules between layers of graphene oxide.) We observe an approximate law of corresponding states where the layer expansion can be mapped into a quasi universal curve (vs. molar excess or absolute adsorption) when adjusted by the critical temperature of the adsorbed gas. Comparison of the experimental findings with molecular dynamics simulations suggest possible structures for the pores. |
Monday, March 5, 2018 9:36AM - 9:48AM |
A17.00009: Effects of Intrinsic Surface Defects on Morphology and Electronic Structure of a Thin PtSe2 Film from First-principles Yichul Choi, Husong Zheng, Fazel Baniasadi, Chenggang Tao, Kyungwha Park Among transition-metal dichalcogenide families, PtSe2 is relatively new. In a bulk form, it was confirmed to be a Dirac semimetal, whereas as a monolayer, it was shown to be semiconducting with a small band gap. Recently, a PtSe2 monolayer was grown on a Pt substrate and revealed interesting layer-selective Rashba splitting. Compared to MoS2 family, PtSe2 has a stronger bonding between neighboring monolayers, which allows to observe defects slightly deeper into the surface. Very recently, a scanning tunneling microscopy and spectroscopy (STM/S) study revealed interesting morphology changes depending on types of intrinsic surface defects on a thin PtSe2 film grown on sapphire. Motivated by this, we investigate morphology, formation energies and electronic structures of five distinct surface defects in a thin PtSe2 film, by using supercells within density-functional theory. Our results will be compared with the STM/S experimental data. |
Monday, March 5, 2018 9:48AM - 10:00AM |
A17.00010: STM characterization of point defects in few-layer PtSe2 Husong Zheng, Fazel Baniasadi, Yichui Choi, Kyungwha Park, Chenggang Tao To enable effective applications of atomically thin transition metal dichalcogenides (TMDs) grown via chemical vapor deposition (CVD), it is essential to characterize and understand the defects within such materials, including atomic point defects and grain boundaries. By using scanning tunneling microscopy/spectroscopy (STM/S), we investigated characteristic point defects in few-layer platinum diselenide (PtSe2). Combining density functional theory (DFT) calculations, we were able to identify these point defects, such as Pt and Se vacancy defects. The results could be potentially important for electronic, spintronic, optical and catalytic applications of two-dimensional PtSe2 and other TMDs. |
Monday, March 5, 2018 10:00AM - 10:12AM |
A17.00011: The Atomic Structure of a Metal-Supported Two-Dimensional Germania Film Kristen Burson, Adrian Lewandowski, Philomena Schlexer, Christin Buechner, Hannah Burrall, Wolf-Dieter Schneider, Gianfranco Pacchioni, Markus Heyde, H Freund Scanning tunneling microscopy (STM) images of two-dimensional silica films have recently provided new insight into the structural configurations of amorphous networks [1]. Similar investigations of other known glass formers, such as germania, are needed to establish a more complete understanding of amorphous network structures. Here we present the growth and characterization of two-dimensional germania films. Germanium oxide monolayer films were grown on Ru(0001) by physical vapor deposition and subsequent annealing in oxygen. We obtain a comprehensive image of the germania film structure by combining LEED-IV (Low energy electron diffraction) and ab initio density functional theory (DFT) analysis with high-resolution scanning tunneling microscopy (STM) imaging. STM images show the hexagonal network and domain boundary structures in atomically flat germania films. For quantitative LEED, the best agreement has been achieved with DFT structures where the germanium atoms sit preferentially on the top and fcc hollow sites of the Ru(0001) substrate. |
Monday, March 5, 2018 10:12AM - 10:24AM |
A17.00012: First Principle Study of SiO2 Bilayers on Ni-Pd Substrates Xin Liang, Chao Zhou, Gregory Hutchings, Eric I. Altman, Sohrab Ismail-Beigi Silicates in zeolite form have many applications in catalysis. They have porous structures with large internal surface area where chemical reactions can occur. However, it is hard to characterize such internal surfaces with atomic resolution in real-space experimentally. Therefore, a two-dimensional (2D) form of silica has been created experimentally to serve as a model system that can be probed using surface science techniques such as scanning tunneling microscopy (STM). Silica can either form monolayer or bilayer forms depending on the substrate on which it is grown. When 2D silica forms a bilayer, it interacts weakly with substrates due to the fully saturated Si-O chemical bond. Therefore, bilayer SiO2 can be used to simulate the internal surface of zeolites. Experimentally, one can create Ni-Pd random alloys which have tunable composition-dependent lattice constants and can serve as a set of substrates for 2D silica. In this work, we investigate the stable morphologies of 2D SiO2 on Ni-Pd using density functional theory (DFT). We also describe the role of the weak interaction between bilayer silica and Ni-Pd in terms of how much epitaxial strain it can enforce before the silica overlayer becomes incommensurate to the substrate. |
Monday, March 5, 2018 10:24AM - 10:36AM |
A17.00013: The power law suppression of the density of states of Au/Si(001) is not a unique characteristic of the quasi one-dimensional superstructure. Petros Thomas, Shinichiro Hatta, Hiroshi Okuyama, Tetsuya Aruga Metallic atoms that induce self-assembly of quasi one- dimensional (1D) chains on semiconductor surfaces have been extensively investigated in search of Tomonaga-Luttinger liquid (TLL) physics. Of the many investigated self assembled quasi 1D systems on semiconductor surfaces, only Au/Ge(001), and Bi/InSb(001) were claimed to host TLL physics; the power law suppression of the density of states (DoS) towards the chemical potential being one of the hallmarks of TLL physics. However, the TLL nature of Au/Ge(001) has been contested by other studies. |
Monday, March 5, 2018 10:36AM - 10:48AM |
A17.00014: Tunable two-dimensional interfacial coupling in molecular heterostructures Himanshu Chakraborty, Beibei Xu, Vivek K. Yadav, Zhuolei Zhang, Michael L. Klein, Shenqiang Ren Two-dimensional van der Waals heterostructures are of considerable interest for the next generation nanoelectronics because of their unique interlayer coupling and optoelectronic properties. Here, we report a modified Langmuir–Blodgett method to organize two- dimensional molecular charge transfer crystals into arbitrarily and vertically stacked heterostructures, consisting of bis(ethylenedithio)tetrathiafulvalene (BEDT–TTF)/C60 and poly (3-dodecylthiophene-2,5-diyl) (P3DDT)/C60 nanosheets. A strong and anisotropic interfacial coupling between the charge transfer pairs is demonstrated. Density functional theory calculations confirm charge transfer between the n-orbitals of the S atoms in BEDT–TTF of the BEDT–TTF/C60 layer and the π* orbitals of C atoms in C60 of the P3DDT/C60 layer contribute to the inter-complex CT. The two-dimensional molecular van der Waals heterostructures with tunable optical–electronic–magnetic coupling properties are promising for flexible electronic applications. |
Monday, March 5, 2018 10:48AM - 11:00AM |
A17.00015: Scanning Tunneling Microscopy Study of 2D Silicon Nanosheets on Ag(111) Thin Film Heterostructures Madisen Holbrook, Hui Zhang, Fei Cheng, Qiang Zhang, Chih-Kang Shih Two dimensional (2D) materials have gained interest as promising building blocks for novel 2D electronic devices. Silicon has ruled the electronics industry to date, therefore it is natural to consider 2D silicon nanosheets for next generation device design. Silicene, a single atomic sheet of Si, was proposed and successfully fabricated on single crystalline Ag surfaces. Here we report silicon nanosheet synthesis on a thin film Ag-Si(111) substrate using molecular being epitaxy (MBE). Using scanning tunneling microscopy and spectroscopy, the electronic and atomic structure of this 2D silicon nanosheet is clearly shown. The electronic structure surprisingly shows an insulating band gap for the silicon nanosheet, contrary to the electronic behavior of silicene and bulk silicon counterparts. Interestingly, the dI/dV spectra within the bandgap of the silicon nanosheet displays Ag(111) surface states. In addition, we report novel moiré and LEED patterns for this Si-Ag(111) heterostructure. Importantly, the bandgap of the silicon nanosheet is clearly modulated by the moiré pattern. Finally, we explore growth of different coverage nanosheets and multilayer silicon nanostructures using a two-step MBE growth method. |
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