Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session R20: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Modeling and Electrical Characterization |
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Sponsoring Units: DMP Chair: Blanka Magyari-Kope, Stanford University Room: 322 |
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R20.00001: Pseudopotential-based study of electron transport in low-dimensionality nanostructures Invited Speaker: Massimo Fischetti Pseudopotentials-- empirical and \textit{ab initio} -- are now being more commonly used to study not only the atomic and electronic structure of nanometer-scale systems, but also their electronic transport properties. Here we shall give a bird-eye view of the use of density functional theory (DFT) to calibrate empirical pseudopotentials (EPs), of EPs to calculate efficiently the electronic structure of low-dimensionality systems, the most significant electronic scattering processes, and to study semiclassical and quantum electronic transport. Low-dimensionality systems considered here include thin semiconductor layers, graphene, graphene- and silicane-nanoribbons, and silicon nanowires. Regarding graphene, the high electron mobility measured in suspended graphene sheets ($\sim$ 200,000 cm$^{2}$/Vs) is the result of a relatively weak carrier-phonon and the strong dielectric-screening property. However, in practical applications graphene is likely to be supported by an insulating substrate, top-gated, and possibly used in the form of narrow armchair-edge nanoribbons (aGNRs) in order to open a gap. We will discuss several scattering processes which may affect the electron transport properties in these situations. First, we shall present results of the calculation of the intrinsic electron-phonon scattering rates in suspended graphene using empirical pseudopotentials and the rigid-ion approximation, resulting in an electron mobility consistent with the experimental results. We shall then discuss the role of interfacial coupled substrate optical-phonon/graphene-plasmons in depressing the electron mobility in graphene supported by several insulators (SiO$_{2}$, HfO$_{2}$, Al$_{2}$O$_{3}$, and h-BN). We shall also discuss the role of Coulomb scattering with charged defects/impurities in gated graphene sheets and the role of the metal gate in screening this interaction. Finally, we shall review the strong effect of line edge roughness (LER) on electron transport and localization in narrow aGNRs resulting from the ``aromatic'' width dependence of the band-gap of the \textit{sp}$^{2}$-coordinated aGNRs. This will lead us to consider \textit{sp}$^{3}$-coordinate ribbons (silicane) and Si nanowires as possible alternative structures -- less affected by LER scattering -- of interest in nanoelectronics application. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R20.00002: Electron Transport Variability in Armchair Graphene Nanorribbons Shela Aboud, Massimo Fischetti Armchair graphene nanoribbons (AGNR) hold great promise in nano-electronics because of the capability of opening a semiconducting gap in narrow ribbons. However the effective use of AGNR in devices may be limited by structural and chemical modifications from a variety of sources including the support material, edge effects, width variability, and defects which all change the trends in the bandgap scaling. In this work we use density functional theory (DFT) simulations and Empirical Pseudopotentials (EPs) to investigate how structural and chemical variability in the AGNRs influence electron transport through changes in the bandstructure, phonon modes and electron-phonon coupling. Comparisons of the DFT and EPs give the same trend for ribbons with widths of 3N, 3N$+$1 and 3N$+$2 atoms with small differences stemming from the atomic structural relaxation accounted for in the DFT simulations. The dependence of the gap on the ribbon width is attributed to the aromaticity of the graphene that can be understood through the spatial distribution of the Clar resonance structures (Clar sextets) and become more localized because of the formation of the edge states. Chemical functionalization of the edges, defects at the edges and in the bulk of the ribbon, doping, and the type of support material (e.g. h-BN, SiO2, HfO and Al2O3) will all modify the aromaticity of the ribbons. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R20.00003: Photo-induced energy transfer between carbon nanotubes Olena Postupna, Heather Jaeger, Oleg Prezhdo The unique structural, mechanical, and electronic properties of carbon nanotubes (CNTs) have recently been attracting significant attention in academic research and industrial applications. Experimental investigation of the physical properties of CNTs is often hindered by questions that can be answered only with rigorous theoretical approaches, such as ab initio molecular dynamics. Results of time-domain simulations of energy transfer between photo-excited CNTs are reported. Using a system comprised of a pair of CNTs with different chiralities, (6,4) and (8,4), we elucidate the experimental results obtained by Luer at al [1]. Quantification of adiabatic and nonadiabatic contributions to the transfer process clarifies the mechanism of energy transfer. And, the delocalization of the initial exciton is representative of strong donor-acceptor coupling at high energies. Our work contributes to the ever-growing compendium of energy transfer within nanoscale systems and offers valuable insight toward tailoring CNTs for solar energy conversion.\\[4pt] [1] Larry Luer, Jared Crochet, Tobias Hertel, Giulio Cerullo, Gugliermo Lanzani. ACSNano. Vol.4, No. 7, 4265-4273 [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R20.00004: Realization of High-speed Transport in Low Dimensional Disordered Carbon Films Somnath Bhattacharyya, Mikhail Katkov, Dmitry Churochkin, George Chimowa, Ross McIntosh Developing hybrid super-structures including carbon nanostructures for quantum information science is widely sought after and we show a possible route in carbon superlattice structures based on experimental results as well as theoretical analysis which also incorporates high-speed switching capabilities. We propose a theoretical model of disordered carbon superlattice structures where the local density of electronic states is controlled by changing the $sp^{3}-C$ bond alternation as well as the hopping disorder parameter of the $sp^{2}-C$ regions. In addition the incorporation of nitrogen atoms in carbon networks was modeled as a combination of disorders which vary both in correlated and uncorrelated manners. Resonant peaks associated with $C$ and $N$ sites in these structures show a conductance cross-over under variation of the nitrogen concentration in these structures which can explain the observed negative differential resistance in diamond-like carbon superlattices as well as the conductivity cross-over in nano-crystalline diamond films. Detailed analysis of transport measurements over a wide range of temperatures, magnetic fields and also frequency shows an enhanced characteristic length in these systems that supports switching of complex impedance in the range of 50 GHz. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R20.00005: Preparation and electrical transport property study of MoS$_{2}$ single-layer devices on different substrates Zhiyong Wang, Zhisheng Lin, Ray Sachs, Ji Feng, Jing Shi Micro-exfoliated MoS2 flakes on SiO2/Si substrate are identified with optical microscope first and then atomic force microscopy and Raman spectroscopy. Nanodevices are subsequently prepared by E-beam lithography. The as-prepared MoS2 devices are n-type with a high sheet resistance (typically several MOhms). As a gate voltage is applied, a large gate modulation in sheet resistance is observed. At the highest negative gate voltage, the devices remain n-type but the resistance increases by at least 4 orders of magnitude. In the meantime, the current-voltage characteristics turn from linear to non-linear. The field-effect mobility extracted from the gate voltage dependence is about 10 cm$^{2}$/Vs. To study the effect of the dielectric constant, we have developed a transfer technique that transfers entire working devices from SiO2/Si to any substrates. We have successfully applied the technique to graphene and obtained a relatively high yield. We are currently transferring MoS2 devices from SiO2/Si to strontium titanate (STO) substrate which has a much higher dielectric constant (300 at room temperature). Detailed experimental results and discussions will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R20.00006: Electrical characterization of few-layer MoS2 on HfO2 substrate Jatinder Kumar, Hui-Chun Chien, Hsin-Ying Chiu Due to the realization of graphene transistors but without applicable bandgap, the similar layered structure molybdenum disulfide (MoS2) field effect transistors with nonzero bandgap have been demonstrated and reveal promising potential. Previous experiments showed that carrier mobility could be enhanced by depositing hafnium dioxide (HfO2) on top of MoS2 devices, which was possibly attribute to the suppression of Coulomb scattering by high-$\kappa$ environment and surface polar phonon scattering. In our talk, we will present the electrical transport experiments in few layers of MoS2 on HfO2 dielectrics, including the carrier mobility improvement and electrical transport phenomena in high bias region. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R20.00007: Investigation of $E_{2g}^{1}$ and $A_{1g}$ Raman Modes of Few-Layer MoS$_2$ on HfO$_2$ Substrate Hui-Chun Chien, Jatinder Kumar, Hsin-Ying Chiu The recent research work by Radisavljevic \textit{et al.}[1] shows that the mobilities of monolayer MoS$_{2}$ transistors can be improved by employing a thin layer of hafnium oxide as top-gate dielectric. Dielectric screening has been successfully demonstrated to suppress the Coulomb interactions of charged impurities on the substrate. Therefore, we develop an alternative method of building monolayer MoS$_{2}$ transistors on HfO$_{2}$ substrate. Owing to the low contrast of few-layer MoS$_{2}$ flakes on thin HfO$_{2}$ layer, which makes the realization of such device configuration difficult. By utilizing the thickness dependence of in-plane and out-of-plane Raman peaks of MoS$_{2}$ flakes, $E_{2g}^{1} $and$A_{1g} $, respectively, we establish an efficient approach to improve the identification of MoS$_{2}$ layers by Raman spectrum instead of AFM. Our investigation of Raman spectrum of few-layer MoS$_{2}$ on HfO$_{2}$ shows the significant difference from those on SiO$_{2}$. The substrate dependence of Raman spectrum as well as its further application will be discussed in this talk.\\[4pt] [1] Radisavljevic, \textit{et al., Nat. Nanotech}. \textbf{6}, 147 (2011) [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R20.00008: Liquid-gated ambipolar transistor with ransition-metal dichalcogenides Yijin Zhang, Jianting Ye, Yoshihiro Iwasa Transition-metal dichalcogenides (TMDs) are graphene-like layered materials. In particular, semiconducting group of TMDs are attracting great interests as a post-graphene material since they have a finite band gap which is an important feature for FET applications. We fabricated semiconducting TMD-based FETs using a new type of gate dielectric called electric double layer (EDL). EDL is formed by solid and ions inside liquid at the solid-liquid interface. This nano-scale capacitor provides extremely large charge accumulation capability and realizes high performance FETs and field-effect phase control. We observed ambipolar FET operation of molybdenum disulfide (MoS$_{2})$ for the first time in addition to its well-known n-type operation [1] and field-effect superconducting transition [2]. High performance is not only observed in MoS$_{2}$ but also in other semiconducting TMDs like tungsten diselenide (WSe$_{2})$. The ambipolar operation is also important for applications, for example, light-emitting devices like organic materials. We investigated possibilities of EDL-based optical coupling devices. [1] Y. J. Zhang et al. Nano. Lett. 12, 1136 (2012) [2] J. T. Ye, Y. J. Zhang et al. Science in press [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R20.00009: High on/off ratio field effect transistor based on exfoliated crystalline SnS$_{2}$ nano-membrane Debtanu De, John Manongdo, Sean See, Vincent Zhang, Arnold Guloy, Haibing Peng We report the implementation of field effect transistors based on exfoliated nano-membranes of a layered two-dimensional semiconductor SnS$_{2}$, which exhibit an On/Off ratio exceeding 2x10$^{6}$ and a carrier mobility of $\sim$ 1 cm$^{2}$V$^{-1}$s$^{-1}$. The results demonstrate the great potential of SnS$_{2}$, a layered semiconductor with finite band gap, as the building block for future nanoelectronic applications complementary to graphene-based materials with zero or small band gap. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R20.00010: Topological phase transition in hexagonal boron-nitride bilayers modulated by gate voltage Guojun Jin, Xuechao Zhai We study the gate-voltage modulated electronic properties of hexagonal boron-nitride bilayers with two different stacking structures in the presence of intrinsic and Rashba spin-orbit interactions. Our analytical results show that there are striking cooperation effects arising from the spin-orbit interactions and the interlayer bias voltage. For realizing topological phase transition, in contrast to a gated graphene bilayer for increasing its energy gap, the energy gap of a boron-nitride bilayer is significantly reduced by an applied gate voltage. For the AA stacking-bilayer which has the inversion symmetry, a strong topological phase is found, and there is an interesting reentrant behavior from a normal phase to a topological phase and then to a normal phase again, characterized by the topological index. Therefore, the gate voltage modulated AA-boron nitride bilayer can be taken as a newcomer of the topological insulator family. For the AB stacking-bilayer which is lack of the inversion symmetry, it is always topologically trivial, but exhibits an unusual quantum Hall phase with four degenerate low-energy states localized at a single edge. It is suggested that these theoretical findings could be verified experimentally in the transport properties of boron-nitride bylayers. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R20.00011: Surface-Plasmon Assisted Exciton Transport in 1D Nanostructures Charles Cherqui, David Dunlap, Andrei Piryatinski We consider effect of coupling between exciton propagating in a 1D-nanostructure (e.g., carbon nanotube) and localized surface plasmon modes induced by a metal nanoparticle located in close proximity to the nanostructure. Both regimes of weak and strong exciton-plasmon couplings are taken into account leading to the dressed exciton and plasmon states. In this representation, the dynamics of the dressed excitons is mapped on the impurity scattering problem. The analysis of the scattering matrix indicates that the surface-plasmon modes lead to the exciton intraband scattering and possibility to form localized states within the exciton band gap. Surface plasmon induced exciton radiation pattern and the radiative and non-radiative decay rates are calculated and their dependence on the exciton-plasmon coupling is analyzed. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R20.00012: A novel turbulent state of a dipolar exciton Bose-Einstein condensate German V. Kolmakov, Oleg L. Berman, Roman Ya. Kezerashvili We report the formation of a new state in a non-equilibrium Bose-Einstein condensate (BEC) of dipolar excitons: steady turbulence. Two different systems where the BEC is formed are considered: coupled semiconductor quantum wells and two-layer graphene separated by a semiconducting or dielectric barrier. The non-linear dynamics of the systems are studied by using the generalized Gross-Pitaevskii equation. It is demonstrated that in the BEC a steady turbulent state is formed at high enough pumping rates. This state is characterized by oscillations of the spatial distribution of the excitons and fast redistribution of the energy between the oscillatory modes. The dynamics of the system can be explained in terms of the propagation of the fluxes of two quantities -- the energy and the number of particles. The analysis of these excitonic systems as well as the comparison with an atomic condensed state show that the formation of turbulence is a general effect in the BEC. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R20.00013: Theoretical study of electron transport in DNA Bikan Tan, Miroslav Hodak, Wenchang Lu, Jerry Bernholc Many experiments have observed high conductivity of DNA, but its origin has not yet been satisfactorily explained. In this work, we explore the dynamics of solvated B-DNA sandwiched between metallic nanotubes and connected via alkane linkers. The geometries are relaxed using the CHARMM force field. Conductivities of different snapshots of the system are calculated using the non-equilibrium Green's function method within density-functional theory. Our results show that in certain geometries, the DNA conducts significantly better than in others. For the highest conductivity configuration, a HOMO state extends across DNA's guanine sites to the alkane linkers. In general, we find that the conformational changes strongly affect the energy alignment of HOMO states of the DNA and the linker, and thus have a major effect on the conductivity of the entire system. [Preview Abstract] |
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