Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C30: Emerging 2D Materials Beyond GrapheneFocus
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Sponsoring Units: DMP Chair: Jose M. Gomez-Rodriguez, Universidad Autonoma de Madrid Room: 293 |
Monday, March 13, 2017 2:30PM - 2:42PM |
C30.00001: Electronic structure of two-dimensional boron sheets Baojie Feng, Iwao Matsuda Boron is the fifth element in the periodic table and hosts rich physical and chemical properties. Inspired by the fruitful results of graphene, the possibility for the existence of two-dimensional boron sheets have been extensively explored in the recent years. Here, we synthesized two types of monolayer boron sheets on Ag(111) substrate. Angle-resolved photoemission spectroscopy measurements reveal the existence of metallic bands from these boron sheets, distinct from the semiconducting behavior of other boron allotropes. Combined with first-principles calculations, we demonstrate that the interaction between the boron layer and the substrate is relatively weak and the band structure of free-standing boron sheets remain largely intact after being adsorbed on Ag(111). References: 1. B. Feng, \quad et al., Experimental realization of two-dimensional boron sheets. Nat. Chem. 8, 563(2016). 2. B. Feng, et al., Direct evidence of metallic bands in a monolayer boron sheet. Phys. Rev. B 94, 041408(R)(2016). [Preview Abstract] |
Monday, March 13, 2017 2:42PM - 2:54PM |
C30.00002: Borophene: Synthesis and Emergent Electronic Phenomena Andrew Mannix, Brian Kiraly, Joshua Wood, Mark Hersam, Nathan Guisinger Boron is the lightest metalloid element, and exhibits unusual physical characteristics derived from electron deficient, highly delocalized covalent bonds. Although bulk boron shows great structural complexity, nanoscale boron clusters form simple, aromatic planar molecules similar to those of carbon. Theoretical studies suggest that these clusters could form the basis for metallic nanostructured boron allotropes (e.g., nanotubes and sheets). Recently, we have reported the synthesis of borophenes on Ag(111) under ultra-high vacuum [Science~350,~1513--1516 (2015)]. Atomic-scale scanning tunneling microscopy shows the growth of two distinct phases, both of which exhibit anisotropic, chain-like structures. We confirm the atomically thin character of these sheets through multiple, orthogonal techniques. Furthermore, in situ scanning tunneling spectroscopy of the borophene sheets shows metallic characteristics consistent with theoretical predictions, in contrast to semiconducting bulk boron. The structure and electronic properties of borophenes are further studied at cryogenic temperatures (2.5 K). We observe no evidence for a significant structural phase change, but tunneling spectra show features suggestive of an electronic phase change. Several possible explanations for these spectral features will be discussed. [Preview Abstract] |
Monday, March 13, 2017 2:54PM - 3:06PM |
C30.00003: Borophene synthesis on Au(111) Nathan Guisinger, Brian Kiraly, Zhuhua Zhang, Andrew Mannix, Mark C. Hersam, Boris I. Yakobson The recent experimental discovery of borophene, the metallic 2-dimensional allotrope of boron, has sparked tremendous interest in further exploration of this unique material. The initial synthesis of borophene was accomplished on Ag substrates and serves as a quintessential example of predictive modeling to experimental realization. In this talk, we expand the phase-space of borophene synthesis to Au. Borophene synthesis was accomplished by evaporating elemental boron onto a Au(111) substrate. The synthesized borophene retains its metallic character on Au as verified with scanning tunneling spectroscopy. Most fascinating is the difference in growth dynamics on the Au(111) substrate where the reconstructed surface presents a unique energy landscape for borophene nucleation and growth. We find that the initial low-coverage growth of borophene modifies the herringbone reconstruction into a ``trigonal'' network, where the 2D boron islands are uniformly templated across the surface. Increasing coverage results in the increasing size of the templated borophene islands until they coalesce into larger sheets. The observed growth dynamics are supported by the computational modeling of boron nucleation on Au. [Preview Abstract] |
Monday, March 13, 2017 3:06PM - 3:18PM |
C30.00004: IV-VI monochalcogenide SnSe nanostructures: synthesis, doping and thermoelectric properties Shuhao Liu, Naikun Sun, Sukrit Sucharitakul, Mei Liu, Xuan Gao Recently IV-VI monochalcogenide SnSe or SnS has been proposed as a promising two-dimensional (2D) material for valleytronics and thermoelectrics. Despite much theoretical interest and many experimental reports on the thermoelectric characterizations of SnSe single crystal, experimental studies on SnSe in the nanostructured form are still limited. We report the synthesis of SnSe nanoflakes and thin films with chemical vapor deposition (CVD) and their thermoelectric properties. As grown SnSe nanostructures are found to be intrinsically p-type and different types of dopants (In, Pb and Ag) were explored to control the carrier density. We will present the electrical transport property of SnSe nanoflake field effect transistor devices and the effects of doping on the electrical conductivity, Seebeck coefficient, power factor and anisotropy in SnSe films. By doping, the power factor of SnSe films can be improved by at least one order of magnitude compared to the $"$intrinsic$"$ as grown materials. Our work provides an initial step in the pursuit of IV-VI monochalcogenides as novel 2D semiconductors for electronics and thermoelectrics. [Preview Abstract] |
Monday, March 13, 2017 3:18PM - 3:30PM |
C30.00005: Surface atomic structure characterization of SnSe and black phosphorus using selected area uLEED-IV via LEEM Zhongwei Dai, Maxwell Grady, Jiexiang Yu, Jiadong Zang, Karsten Pohl, Wencan Jin, Young Duck Kim, James Hone, Jerry Dadap, Richard Osgood, Jerzy Sadowski, Suresh Vishwanath, Huili Xing Selected area diffraction intensity-voltage ($\mu$LEED-\textit{IV}) analysis via low energy electron microscopy (LEEM) has the combined functionality of atomic surface structure determination and $\mu$m area selectivity, making it ideal for structural investigations of 2-D materials. SnSe thin films have been predicted and observed to be topological crystalline insulators. Previous studies suggested that SnSe has a preferred Se-terminated surface configuration. Using $\mu$LEED-\textit{IV}, we determined that SnSe has, on the contrary, a stable Sn termination. This surface is stabilized through an oscillatory interlayer relaxation, which agrees with previous DFT predictions. Black phosphorus (BP) has an intrinsic layer-dependent bandgap ranging from 0.3 eV to 2 eV. Previous STM and DFT studies suggested BP surfaces have a buckling of 0.02 \AA\ to 0.06 \AA. We experimentally determined that the surface buckling of BP to be near 0.2 \AA. We further propose, using DFT calculations, that this large surface buckling is induced by the presence of surface defects. The influence of this surface buckling on the electronic structures of BP is under investigation. [Preview Abstract] |
Monday, March 13, 2017 3:30PM - 3:42PM |
C30.00006: STM study on the surface structures and defects of SnSe Jungdae Kim, Ganbat Duvjir, Trinh Thi Ly, Taewon Min, Taehoon Kim, Sang Hwa Kim, Anh-Tuan Duong, S. H. Rhim, Sunglae Cho, Jaekwang Lee Tin selenide (SnSe) is a IV-VI semiconductor with a band gap of 1.0 eV, and also one of layered chalcogenide materials (LCMs) where each layer is coupled by weak van der Waals interactions. SnSe has been widely studied due to its many potential applications that take advantage of its excellent thermoelectric, photovoltaic, and optoelectronic properties. However, experimental investigations into the microscopic structure of SnSe remain largely unexplored. The atomic and electronic structures of SnSe surfaces are studied by a home-built low temperature scanning tunneling microscope (STM). The cleaved surface of SnSe is comprised of covalently bonded Se and Sn atoms in zigzag patterns. However, rectangular periodicity was observed in the atomic images of SnSe surfaces for filled and empty state probing. Detailed atomic structures are analyzed by density functional theory (DFT) calculations, indicating that the bright extrusions of both filled and empty state images are mostly located at the positions of Sn atoms. We also report the origin of p-type behavior in SnSe by investigating three dominant intrinsic defects (Sn, Se, and Se-Sn-Se vacancies) using STM and DFT calculations. [Preview Abstract] |
Monday, March 13, 2017 3:42PM - 3:54PM |
C30.00007: Defects in Monolayer Titanium Carbide (Ti$_{\mathrm{3}}$C$_{\mathrm{2}}$T$_{\mathrm{x}})$ MXene Xiahan Sang, Yu Xie, Ming-Wei Lin, Mohamed Alhabeb, Katherine Van Aken, Yury Gogotsi, Paul Kent, Kai Xiao, Raymond Unocic Mxene materials, transition metal carbides or nitrides, have recently gained interest as a developing class of 2D materials with applications geared towards energy storage, catalysis, and electronic devices. To better understand the physiochemical and electronic properties, detailed atomic resolution structural analysis of monolayer MXene was investigated using a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and density functional theory (DFT). Large area Ti$_{\mathrm{3}}$C$_{\mathrm{2}}$T$_{\mathrm{x}}$ MXene flakes, were synthesized and the type and concentration of atomic scaled defects were analyzed. Ti vacancies and Ti vacancy clusters were found to be the most prevalent defects. The edge defects, although not intrinsic to the single-layer flakes, can be created using beam irradiation. The formation energy and electronic structure of point defects and edge defects have been calculated using DFT. The influence of the defects on the conductivity is also studied using DFT. Our results thus shed light on the future nano-electronic application using 2D metallic MXene single layers. [Preview Abstract] |
Monday, March 13, 2017 3:54PM - 4:06PM |
C30.00008: Atomically-Precise Layer Controlled Synthesis and Characterization of cm-Scale Hexagonal Boron Nitride. W.-H. Lin, V.W. Brar, D. Jariwala, M.C. Sherrott, W.-S. Tseng, C.-I Wu, N.-C. Yeh, H.A. Atwater Hexagonal boron nitride is the most promising two-dimensional insulator for device applications because of its large band gap and low density of charged impurities in addition to being isostructural and isoelectronic with graphene. Here we report the synthesis of h-BN films over cm$^{\mathrm{2}}$ area on Cu foils via chemical vapor deposition, with layer control from 1 to 20 layers. We have characterized these large-area h-BN films at both atomic and macroscopic scales. Raman and infrared spectroscopy indicate the presence of B-N bonds and reveal a linear dependence of thickness with growth time. X-ray photoelectron spectroscopy provides the film stoichiometry, showing the B/N atom ratio to be 1 for all thicknesses. Atomically resolved STM images of monolayer h-BN films on graphene and Au substrates display both the atomic h-BN honeycomb lattice and a Moir\'{e} superlattice between h-BN and graphene. Electrical current transport in Au/h-BN/Au heterostructures indicates that these h-BN films behave like excellent tunnel barriers and also possess a high value of the hard-breakdown field strength. Our large-area h-BN films are therefore structurally, chemically and electronically uniform over cm$^{\mathrm{2}}$ areas. [Preview Abstract] |
Monday, March 13, 2017 4:06PM - 4:18PM |
C30.00009: Role of defects in the adsorption of small molecules on single-layer hexagonal boron nitride Tao Jiang, Takat B. Rawal, Duy Le, Talat S. Rahman In this work, we have investigated the adsorption of small molecules (CO, CO$_{\mathrm{2}}$, H$_{\mathrm{2}})$ on single-layer hexagonal boron nitride (h-BN) with point defects employing \textit{ab initio} density functional theory (DFT) with incorporation of non-local van der Waals functional. We find that N vacancy (V$_{\mathrm{N}})$ and N substituted by B (B$_{\mathrm{N}})$ facilitate the adsorption of CO and CO$_{\mathrm{2}}$ molecules on h-BN. CO molecularly chemisorbs on h-BN with these defects with adsorption energy of -1.01 eV and -2.56 eV, whereas CO$_{\mathrm{2}}$ molecularly chemisorbs with adsorption energy of -1.66 eV and -0.094 eV. In contrast H$_{\mathrm{2}}$ does not chemisorb on these defects. We will analyze the geometric and electronic structure of these systems to establish the rationale for the differences in behavior for these adsorbates. We will also present results of the phonon dispersion of the systems, and discuss the vibrational modes of those adsorbed molecules. These results provide atomistic understanding of the physical processes involved in, occurring at the reaction sites, the conversion of synthetic gases into higher alcohols as observed in recent experiments [1]. [1] R. Blair, and L. Tetrad, private communication (2016) [Preview Abstract] |
Monday, March 13, 2017 4:18PM - 4:30PM |
C30.00010: Optical Characterization of Few-Layer Ferromagnetic Insulator Chromium(III) Iodide Crystals Dahlia Klein, Efrén Navarro-Moratalla, Kyle Seyler, Daniel Larson, Efthimios Kaxiras, Xiaodong Xu, Pablo Jarillo-Herrero Van der Waals heterostructures of layered 2D materials have been widely explored for the combination of conductors, semiconductors, and insulators. However, there has been little development of magnetic 2D crystalline layers, which could lead to interesting new physical states when coupled with other atomically thin materials. Thus, we have expanded this field through the study of an insulating ferromagnetic layered material: chromium(III) iodide. We have optimized the growth and characterized the crystal structure of the bulk crystals. Moreover, we have successfully exfoliated few-layer flakes down to the monolayer. In light of their sensitivity to ambient moisture, we have also developed the necessary techniques to assure the integrity of the flakes during their manipulation. The characterization via optical contrast data and Raman spectroscopy as a function of flake thickness allows for the reliable identification of few-layer samples. In particular, we have constructed models based on Fresnel's laws to quantify the number of layers of exfoliated flakes from their optical contrast values. [Preview Abstract] |
Monday, March 13, 2017 4:30PM - 4:42PM |
C30.00011: Exciton Dynamics of 2D Hybrid Perovskite Nanocrystal Rui Guo, Zhuan Zhu, Abdelaziz Boulesbaa, Swaminathan Venkatesan, Kai Xiao, Jiming Bao, Yan Yao, Wenzhi Li Organic-inorganic hybrid perovskites have emerged as promising materials for applications in photovoltaic and optoelectronic devices. Among the perovskites, two dimensional (2D) perovskites are of great interests due to their remarkable optical and electrical properties as well as the flexibility of material selection for the organic and inorganic moieties. In this study, we demonstrate the solution-phase growth of large square-shaped single-crystalline 2D hybrid perovskites of $(C_6H_5C_2H_4NH_3)_2PbBr_4 $ with a few unit cells thickness. Compared to the bulk crystal, a band gap shift and new photoluminescence (PL) peak are observed from the hybrid perovskite sheets. Color of the 2D crystals can be tuned by adjusting the sheet thickness. Pump-probe spectroscopy is used to investigate the exciton dynamics and exhibits a biexponential decay with an amplitude-weighted lifetime of 16.7 ps. Such high-quality $(C_6H_5C_2H_4NH_3)_2PbBr_4 $ sheets are expected to have high PL quantum efficiency which can be adopted for light-emitting devices. [Preview Abstract] |
Monday, March 13, 2017 4:42PM - 4:54PM |
C30.00012: Solution manufacturing of 2D piezoelectric semiconductors for smart wearable devices Yixiu Wang, Gang Qiu, Peide Ye, Wenzhuo Wu Due to two-dimensional (2D) nanomaterials, such as graphene and transition metal dichalcogenide (TMD) nanosheets, with single- or few-layer thickness have shown some extraordinary properties in contrast to their bulk counter parts. For example, some of the 2D nanomaterials exhibit layer-dependent bandgap. Besides, the high flexibility, ultrahigh surface area and good mechanical strength make them promising for electronics/optoelectronics and sensors. Here, we report a low-temperature, solution-based method to produce a new class of 2D piezoelectric semiconductors with controlled thickness and lateral dimensions at large scale. Such material shows non-centrosymmetric crystal structure, which enable us to not only explore its basic piezoelectric and semiconductor properties but also its application in wearable devices. [Preview Abstract] |
Monday, March 13, 2017 4:54PM - 5:30PM |
C30.00013: Atomistic Control of the Dynamical Electronic Properties of 2D Materials and Beyond Invited Speaker: Elton Santos Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides and recently found 2D-perovskites, have attracted intense interest for their fascinating electronic properties, and exhibit strikingly different optoelectronic and mechanical features from their 3D-bulk counterparts. They are promising for a wide range of applications, including flexible, mechanically strong electronics such as transistors, memories, logic circuits, light emitters and photodetectors. In this presentation I will discuss different alternatives ranging from electric fields, strain, Ar-plasma treatment, to synthesis processes, to precisely control intrinsic material properties. I will describe several case studies where the synergy between in-silico predictions and experiments has driven smart devices with novel set of functional chemical and physical properties. Moreover, I will discuss some challenges at the forefront of 2D-perovskite materials as the few-layer limit is reached and new device-platforms for energy conversion applications. [Preview Abstract] |
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