Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F1: Focus Session: Beyond Graphene - Phosphorene II |
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Sponsoring Units: DMP Chair: Xiaomu Wang, Yale University Room: 001A |
Tuesday, March 3, 2015 8:00AM - 8:12AM |
F1.00001: Black Phosphorus Boron Nitride Heterostructures Nathaniel Gillgren, Yafis Barlas, Yanmeng Shi, Jiawei Yang, Takashi Taniguchi, Chun Ning (Jeanie) Lau There has been significant recent interest in black phosphorus as a candidate for~future electronics applications, as it possesses both a~layered-tunable band gap~and a relatively high~mobility (compared to other 2D candidates). However, black phosphorus' degradation in~ambient~conditions constitutes a major road block in future applications. As a~potential~solution for this problem we explore the effects of encapsulating black~phosphorus between hexagonal boron nitride. We will present the effects of this~heterostructure on both the stability and transport properties of thin black~phosphorus~devices. [Preview Abstract] |
Tuesday, March 3, 2015 8:12AM - 8:24AM |
F1.00002: Electronic Structure and Rashba Spin-Orbit Coupling in Black Phosphorus Zoran Popovic, Jamshid Moradi Kurdestany, Sashi Satpathy We investigate the electronic structure of black phosphorus using both the first-principles density-functional methods as well as a tight-binding model. The electronic structure in the gap region is described by a tight-binding Hamiltonian keeping the nearest-neighbor hopping and the $p$ orbitals. The calculated bond-centered Wannier functions lead to the bonding picture in terms of the occupation of the $p_\sigma$ bond orbitals along the phosphorous-phosphorous bonds. We find that a symmetry-breaking external electric field introduces a Rashba spin-orbit coupling; however, its magnitude is small, phosphorous being a small-Z atom. The magnitude is enhanced significantly if the phosphorous is replaced by the larger-Z bismuth. [Preview Abstract] |
Tuesday, March 3, 2015 8:24AM - 8:36AM |
F1.00003: Probing Anistropic Excitonic Wavefunctions in Black Phosphorus using Scanning Tunneling Microscopy Ayelet Notis, Carlos Arguello, Ethan Rosenthal, Abhay Pasupathy Black phosphorus is a layered, van der Waals semiconductor that has several structural similarities to graphite. Ultrathin black phosphorus (phosphorene) is conjectured to have a thickness-tunable bandgap and high carrier mobility, making it attractive for electronic and optical applications. Unlike graphene which is a true planar structure, phosphorene layers have a pronounced c-axis corrugation. This causes the electronic structure to be anisotropic, with different effective masses, carrier velocities and dielectric constants parallel and perpendicular to the direction of the corrugation. This has been predicted to lead to anisotropic wavefunctions for hydrogenic states and excitons in the material. In this talk, we present recent scanning tunneling microscopy and spectroscopy (STM and STS) studies investigating the topographic features and electronic structure of black phosphorus. We show that the primary charge defects are acceptors. By studying the local electronic structure in the vicinity of these defects that create hydrogenic states within the bandgap, we directly probe excitonic wavefunctions and their anisotropy in this material. We will describe spatially-resolved measurements of the bandgap and its inhomogeneity in the presence of defects. [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 8:48AM |
F1.00004: Highly Anisotropic and Robust Excitons in Monolayer Black Phosphorus Xiaomu Wang, Aaron M. Jones, Kyle L. Seyler, Vy Tran, Yichen Jia, Huan Zhao, Han Wang, Li Yang, Xiaodong Xu, Fengnian Xia Recently, black phosphorus emerged as a promising new 2D material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form, and its unique properties at the truly 2D quantum confinement have yet to be demonstrated. Here, we reveal highly anisotropic and tightly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centers around 1.3 eV, a clear signature of emission from highly anisotropic bright excitons. In addition, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2 eV, from which we estimate an exciton binding energy of around 0.9 eV, consistent with theoretical results based on first-principles. The experimental observation of highly anisotropic, bright excitons with exceedingly large binding energy not only opens avenues for the future explorations of many-electron effects in this unusual 2D material, but also suggests a promising future in optoelectronic devices such as on-chip infrared light sources. [Preview Abstract] |
Tuesday, March 3, 2015 8:48AM - 9:00AM |
F1.00005: Anisotropic transient reflection spectrum of Black Phosphorus thin films Shaofeng Ge, Zhiming Zhang, Qiu Jun, Junku Liu, Xuefeng Liu, Qinsheng Wang, Dong Sun We present an experimental investigation on the ultrafast dynamics of the black phosphorus film, which is studied by femtosecond transient reflection spectrum. The results show that the transient reflection spectrum is polarization sensitive to both pump and probe laser pulse. The pump polarization has effect on the absorption of photons which determines the magnitude of the signal while the probe polarization has effect on the shape of the signal which indicates it correspond to different dynamics for different probe polarization. Moreover, the temperature dependent and pump power dependent has been performed. [Preview Abstract] |
Tuesday, March 3, 2015 9:00AM - 9:12AM |
F1.00006: Tiling Phosphorene Zhen Zhu, David Tomanek, Jie Guan We introduce a scheme to categorize the structure of different layered phosphorene allotropes by mapping their non-planar atomic structure onto a two-color 2D triangular tiling pattern. In the puckered structure of a phosphorene monolayer, we assign atoms in ``top'' positions to dark tiles and atoms in ``bottom'' positions to light tiles. Optimum $sp^3$ bonding is maintained throughout the structure when each triangular tile is surrounded by the same number $N$ of like-colored tiles, with $0{\le}N{\le}2$. Our {\em ab initio} density functional calculations indicate that both the relative stability and electronic properties depend primarily on the structural index $N$. The proposed mapping approach may also be applied to phosphorene structures with non-hexagonal rings and 2D quasicrystals with no translational symmetry, which we predict to be nearly as stable as the hexagonal network. [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F1.00007: Few-Layer Phosphorene and Arsenene Allotropes: A Computational Study David Tomanek, Zhen Zhu, Jie Guan There has been rising interest in layered compounds of group V elements including phosphorus and arsenic as 2D semiconductors with a substantial band gap and a high carrier mobility. Our {\em ab initio} density functional calculations suggest the existence of multiple $sp^3$ bonded phosphorene and arsenene allotropes that are stable as free-standing monolayers. We have found that $\alpha$-P (black), $\beta$-P (blue), $\gamma$-P and $\delta$-P allotropes of phosphorus are similarly stable, but display a different electronic structure. The monolayer of grey arsenic has a very similar structure as blue phosphorene and also has a wide band gap. The fundamental band gap of the compounds depends sensitively not only on the allotrope, but also the number of layers, the stacking arrangement, and in-layer strain. The energy penalty to interconnect different allotropes of the same element is unusually low, which becomes particularly valuable in assembling heterostructures with well-defined metallic and semiconducting regions in one contiguous layer. [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F1.00008: Lattice Stacking Interactions: Comparisons between bilayer graphene and silicene David Carey, Nathanael Roome The stacking arrangement of atoms in elemental 2D materials, such as graphene and silicene, plays a crucial role in determining their structural, electronic and vibrational properties. The weaker $\pi $ bonding in silicene results in atomic buckling, and previously we have found linear band dispersion in a low atom buckling geometry with a Fermi velocity about 2/3 that of graphene and electron-phonon matrix elements are about a factor of 25 times smaller than in graphene [1]. Here we investigate the properties of different stacking configurations of bilayer silicene with those of bilayer graphene (BLG). In the case of BLG there are two stable configurations AA and AB stacking, with no atomic buckling present. In the case of bilayer silicene the presence of buckling and the different stacking arrangements results in a range of stable configurations. We calculate the frequencies of the IR and Raman active modes as a means to identify the different bonding and stacking configurations. This approach of fingerprint identification is applicable to other elemental layered materials. \\[4pt] [1] Beyond Graphene: Stable Elemental Monolayers of Silicene and Germanene, Nathanael Roome and J David Carey, ACS Applied Materials {\&} Interfaces, 6, 7743 -- 7750 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F1.00009: SnS$_{2}$: An Emerging Layered Metal Dichalcogenide Semiconductor Yuan Huang, Peter Sutter Layered materials are of interest for new physics and due to their promise for device applications. Recent research has extended from graphene to transition metal metal dichalcogenides, with a strong focus on MoS$_{2}$. Here, we report a comprehensive study of a new group IV metal dichalcogenide, tin disulfide (SnS$_{2})$ [1]. Using flakes exfoliated from bulk crystals, we establish the characteristics of single- and few-layer SnS$_{2}$ in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy. Band structure study show that SnS$_{2}$ is an indirect gap semiconductor over the entire thickness range from bulk to a single layer. Ultrathin transistors screened by a liquid gate show promising characteristics, such as on-off current ratios \textgreater 10$^{6}$, high carrier mobilities (up to 230 cm$^{2}$ V$^{-1}$s$^{-1})$, minimal hysteresis and near-ideal subthreshold swing. SnS$_{2}$ transistors are efficient photodetectors, but similar to other dichalcogenides show a relatively slow response to pulsed irradiation, likely due to adsorbate-induced long-lived extrinsic trap states. \\[4pt] [1] Y. Huang et al., ACS Nano 8, 10343 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F1.00010: Semiconducting Behavior, Schottky Barriers and Field Effect Transistors in Ultrathin Rhenium DiSulfide Chris Corbet, Connor McClellan, Amritesh Rai, Sushant Sonde, Emanuel Tutuc, Sanjay K. Banerjee We report the fabrication, characterization, and device characteristics of exfoliated dual-gated ReS$_{2}$ Field Effect Transistors (FETs). All devices were created on few-layer crystals isolated using micromechanical exfoliation of source material grown by molecular beam epitaxy. X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy found the composition of the source material to be 34{\%} Re and 66{\%} S. A combination of atomic force microscopy, optical microscopy, and photoluminescence (PL) measurements were used to estimate the number of ReS$_{2}$ layers (2-7) in all fabricated devices. Source and drain contacts were created using a combination of electron beam lithography and e-beam evaporation of 10 nm Cr / 40 nm Au. The ReS$_{\mathrm{2}}$ FETs showed n-type behavior with an on-off ratio of 10$^{\mathrm{5}}$ and a maximum field-effect mobility of 16 cm$^{\mathrm{2}}$\textbullet V$^{\mathrm{-1}}$\textbullet s$^{\mathrm{-1}}$ at room temperature. The contact resistance was determined using the transfer length method and was found to be gate bias dependent ranging from 175 k$\Omega $\textbullet $\mu $m to 5 k$\Omega $\textbullet $\mu $m. Additionally, the contact resistance showed an exponential dependence on back-gate voltage, indicating Schottky barriers at the source and drain contacts. Dual-gated FETs were fabricated with an e-beam-evaporated alumina gate dielectric and a Cr/Au top-gate. The dual-gated FETs demonstrated current saturation and voltage gain with a subthreshold swing of 148 mV/decade. [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F1.00011: Rhenium Disulfide Depletion-Load Inverter Connor McClellan, Chris Corbet, Amritesh Rai, Hema C.P. Movva, Emanuel Tutuc, Sanjay K. Banerjee Many semiconducting Transition Metal Dichalcogenide (TMD) materials have been effectively used to create Field-Effect Transistor (FET) devices but have yet to be used in logic designs. We constructed a depletion-load voltage inverter using ultrathin layers of Rhenium Disulfide (ReS$_{2})$ as the semiconducting channel. This ReS$_{2}$ inverter was fabricated on a single micromechanically-exfoliated flake of ReS$_{2}$. Electron beam lithography and physical vapor deposition were used to construct Cr/Au electrical contacts, an Alumina top-gate dielectric, and metal top-gate electrodes. By using both low (Aluminum) and high (Palladium) work-function metals as two separate top-gates on a single ReS$_{2}$ flake, we create a dual-gated depletion mode (D-mode) and enhancement mode (E-mode) FETs in series. Both FETs displayed current saturation in the output characteristics as a result of the FET ``pinch-off'' mechanism and On/Off current ratios of 10$^{5}$. Field-effect mobilities of 23 and 17 cm$^{2}$V$^{-1}$s$^{-1}$ and subthreshold swings of 97 and 551 mV/decade were calculated for the E-mode and D-mode FETs, respectively. With a supply voltage of 1V, at low/negative input voltages the inverter output was at a high logic state of 900 mV. Conversely with high/positive input voltages, the inverter output was at a low logic state of 500 mV. The inversion of the input signal demonstrates the potential for using ReS$_{2}$ in future integrated circuit designs and the versatility of depletion-load logic devices for TMD research. [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F1.00012: ABSTRACT WITHDRAWN |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F1.00013: Field effect vs. Hall mobility in back-gated multilayered InSe FETs Sukrit Sucharitakul, Nicholas Goble, U. Rajesh Kumar, Raman Sankar, Fang Cheng Chou, Yit-Tsong Chen, Xuan Gao 2D graphene-like materials, not only are interesting for their exotic transport behavior but also, hold promises for their mechanical robustness and many possibilities in miniaturization. As one material belonging to this category, InSe is not only a promising candidate for optoelectronic devices [1] but also has potential for ultrathin field effect transistor (FET) with high mobility transport [2]. Recent investigation [2] showed that exfoliated InSe FET device on PMMA substrate can yield field effect mobility as high as 1000 cm$^{2}$/Vs at room temperature. In this work, various substrates such as PMMA, bare SiO2, passivated SiO2, and Si3N4 were used to fabricate InSe FET devices. Through back gating and Hall measurement, the devices' field effect mobility and intrinsic Hall mobility were extracted at various temperatures to study the dielectric effect on the material's intrinsic transport behavior. Overall trend of the devices' mobility was found to increase as the temperature is reduced due to reduced phonon scattering. The sample's field effect and Hall mobilities over the range of 77-300K fall in the range of 0.5-2.0 $\times$ 10$^{3}$ cm$^{2}$/Vs, better than the transition metal-dichalcogenides.\\[4pt] [1] Tamalampudi, S. R. \textit{et al}. \textit{Nano Lett.} \textbf{14,} 2800--2806 (2014).\\[0pt] [2] Feng, W. \textit{et al}, P. DOI:10.1002/adma.201402427. \textit{Adv. Mater.} (2014). [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F1.00014: Quasiparticle and Optical Properties of Mono- and Bi-layer SnS2: A First-Principles GW and GW$+$BSE Study Meng Wu, Diana Qiu, Steven G. Louie Unlike most semiconducting transition metal dichalcogenides, SnS2, another layered metal dichalcogenide, is calculated within density functional theory to be an indirect bandgap semiconductor in both its bulk and monolayer forms. Experimental characterization of mono- and bi-layer SnS2 has been performed, but the details of its quasiparticle and excitonic properties remain unclear. Thus, we employ ab initio GW and GW$+$BSE calculations to study the quasiparticle band structure and optical absorption spectrum, respectively, of mono- and bi-layer SnS2 with spin-orbit coupling included throughout the calculations. We further investigate the character of excitonic states contributing to the optical spectrum. [Preview Abstract] |
Tuesday, March 3, 2015 10:48AM - 11:00AM |
F1.00015: Thickness-dependent Dielectric Constant of Few-layer In$_2$Se$_3$ Nano-flakes Di Wu, Alexander Pak, Yingnan Liu, Xiaoyu Wu, Yuan Ren, Yu-Hao Tsai, Min Lin, Hailin Peng, Gyeong Hwang, Keji Lai The dielectric constant or relative permittivity of active materials in electronic devices is a critical parameter for charging and screening effects. For layered two-dimensional (2D) materials, it is of great interest to understand how their dielectric constants depend on dimensionalities and the arrangement of crystal lattices. Here we present both experimental and theoretical investigations on the dielectric constant of few-layer In$_2$Se$_3$ nano-flakes grown on mica substrates by van der Waals epitaxy. A nondestructive microwave impedance microscope (MIM) is employed to simultaneously quantify the number of layers and local electrical and optical properties. The measured effective dielectric constant increases monotonically as a function of the thickness and saturates to the bulk value at around 6-8 quintuple layers. The same trend of layer-dependent dielectric constant is also revealed through a density functional theory approach. Our results of the dielectric response are expected to be significant for the applications of layered materials in nano-devices. [Preview Abstract] |
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