Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A27: Focus Session: Computational Nanoscience I-Methods and Applications |
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Sponsoring Units: DMP DCOMP Chair: Sergey Stolbov, University of Central Florida Room: Colorado Convention Center 301 |
Monday, March 5, 2007 8:00AM - 8:12AM |
A27.00001: A Hybrid Density Functional Study of SiC Nanotubes Kazi Alam, Asok K. Ray As a continuation of our previous work on SiC nanoclusters,$^{\ast }$ we report here first principles calculations on the electronic and geometric structures of armchair and zigzag silicon carbide nanotubes from (3,3) to (11,11) and (3,0) to (11,0) respectively. The finite cluster approach with dangling bonds terminated with hydrogen has been used. The theoretical formalism used is the hybrid density functional theory incorporating Hartree-Fock exchange with density functional theory exchange-correlation. In particular, we have used the B3LYP hybrid functional and the Los Alamos pseudopotential LANL2DZ$^{ }$as implemented in the Gaussian 03 suite of programs. For silicon, the 1s, 2s, and 2p electrons have been represented by core potentials and the remaining electrons as valence states. For carbon and hydrogen, all electron basis sets have been used. A detailed comparison of the structures and stabilities of the nanotubes has been performed. The dependence of the electronic band gaps on the respective tube diameters and energy density of states have also been investigated. Results will be compared with other published data in the literature where possible. $^{\ast }$ A. K. Ray and M. N. Huda, J. Comp. Th. Nanosci. \textbf{3}, 315 (2006). [Preview Abstract] |
Monday, March 5, 2007 8:12AM - 8:24AM |
A27.00002: A Hybrid Density Functional Study of Si Nanotubes Somilkumar Rathi, Asok Ray First principles calculations have been used to study the electronic and geometric structures of zigzag and chiral silicon nanotubes. The finite cluster approach with dangling bonds terminated with hydrogen has been used. The theoretical formalism used is hybrid density functional theory incorporating Hartree-Fock (HF) exchange with density functional theory (DFT) exchange-correlation. In particular, we have used the B3LYP hybrid functional and the Los Alamos pseudopotential LANL2DZ as implemented in the Gaussian 03 suite of programs. For silicon, the 1s, 2s, and 2p electrons have been represented by core potentials and the remaining electrons as valence states. A detailed comparison of the structures and stabilities of the nanotubes has been performed and the dependence of the electronic band gaps on the respective tube diameters has been investigated. We will also compare our results with previously published data on Si armchair nanotubes published by our group$^{\ast }$ and with other results published in the literature. $^{\ast }$P. Pradhan and A. K. Ray, J. Comp. Th. Nanosci. \textbf{3}, 128 (2006). [Preview Abstract] |
Monday, March 5, 2007 8:24AM - 8:36AM |
A27.00003: Tetra, a modeling system for the generation of the atomic configurations of branched wurtzite/zincblende nanostructures Peter Graf, Kwiseon Kim, Wesley Jones, Lin-Wang Wang The first step in simulating properties of nanostructures is generation of accurate atomic configurations. For complex objects such as the multiply branched heterostructures synthesized by Alivisatos et al.$^{1}$, this is nontrivial. We report here on our code, ``tetra,'' that accomplishes this task. Borrowing from techniques of computer graphics, we represent the complex structure as a tree, each node of which is a shape with fixed crystal structure, and use concatenation of 4 by 4 homogeneous transformation matrices to arrange these fixed building blocks into the final object. A simple text based input ``language'' describes the connectivity and dimensions of the structure. The ultimate purpose of this code is use within a package that will explore and optimize electronic properties of such structures with respect to their geometry$^{2}$. We will present examples of both structures and subsequent semi-empirical pseudopotential-based$^{3}$ electronic structure calculations. [1] A. P. Alivisatos, et al., \textit{Nature,} \textbf{430}, 190 (2004). [2] J. Li and L. W. Wang, \textit{NanoLetters}, \textbf{3}, 10, 1357-1363 (2003). [3] L. W. Wang and A. Zunger, \textit{Phys. Rev. B} \textbf{51}, 17398 (1995). [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 9:12AM |
A27.00004: Global optimization approaches for finding the atomic structure of surfaces and nanowires Invited Speaker: In the cluster structure community, global optimization methods are common tools for seeking the structure of molecular and atomic clusters. The large number of local minima of the potential energy surface (PES) of these clusters, and the fact that these local minima proliferate exponentially with the number of atoms in the cluster simply demands the use of fast stochastic methods to find the optimum atomic configuration. Therefore, most of the development work has come from (and mostly stayed within) the cluster structure community. Partly due to wide availability and landmark successes of scanning tunneling microscopy (STM) and other high resolution microscopy techniques, finding the structure of periodically reconstructed semiconductor surfaces was not generally posed as a problem of stochastic optimization until recently [1], when we have shown that high-index semiconductor surfaces can have a rather large number of local minima with such low surface energies that the identification of the global minimum becomes problematic. We have therefore set out to develop global optimization methods for systems other than clusters, focusing on periodic systems in one- and two- dimensions as such systems currently occupy a central place in the field of nanoscience. In this talk, we review some of our recent work on global optimization methods (the parallel-tempering Monte Carlo method [1] and the genetic algorithm [2]) and show examples/results from two main problem categories: (a) the two-dimensional problem of determining the atomic configuration of clean semiconductor surfaces [1,2], and (b) finding the structure of freestanding nanowires [3]. While focused on mainly on atomic structure, our account will show examples of how these development efforts contributed to elucidating several physical problems and we will attempt to make a case for widespread use of these methods for structural problems in one and two dimenstions. \newline [1]C.V. Ciobanu and C. Predescu, Reconstruction of silicon surfaces: a stochastic optimization problem, Phys. Rev. B 70, 085321 (2004). \newline [2]F.C. Chuang, C.V. Ciobanu, V.B. Shenoy, C.Z. Wang and K.M. Ho, Finding the reconstructions of semiconductor surfaces via a genetic algorithm, Surf. Sci. 573, L375 (2004). \newline [3]T.L. Chan, C.V. Ciobanu, F.C. Chuang, N. Lu, C.Z. Wang and K.M. Ho, Magic structures of H-passivated [110] silicon nanowires, Nano Letters 6, 277 (2006). [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:24AM |
A27.00005: Strained InAs/GaAs quantum structures: non-parabolic simulating model. Branislav Vlahovic, Igor Filikhin, Vladimir Suslov A single sub-band model for InAs/GaAs quantum dot (quantum ring), taking into account the strain and piezoelectric potentials, is applied to study the electron spectral properties of QD(QR). The finite confinement band-gap potential is estimated by the band gap difference of the InAs quantum object and the GaAs substrate. An additional potential Vs(Vs=const for QD, and Vs=0 for a substrate) is included in the model to simulate the total effect of the strain and piezoelectricity. The non-parabolic approximation is defined by dependence of electron effective mass on the confinement energy according to the Kane formula. The 3D confined energy problem is solved numerically by the finite element method. The adequacy of our model is illustrated by comparing electron energy spectra with \textit{ab initio} calculations [1]. The experimental data by A. Lorke, et al.(PRL \textbf{84} 2223 (2000)) for few electrons tunneling into InAs/GaAs QD(QR) are well reproduced within the present model. The non-parabolic effect, which is quite noticeable in our calculations, is also discussed. [1] C. Pryor, PR B \textbf{57} 7190 (1998); O. Stier, M. Grundmann, and D. Bimberg, PR B \textbf{59} 5688 (1999); J.I. Climente, J. Planelles, F. Rajadell, J. Phys.: Condens. Matter \textbf{17} 1573 (2005). [Preview Abstract] |
Monday, March 5, 2007 9:24AM - 9:36AM |
A27.00006: Size reduction in layered semiconducting compounds Tianshu Li, Giulia Galli In the last decade numerous experiments have shown dramatic changes in the optical properties of bulk semiconductors as their size is decreased to nanoscale dimensions. Most investigations have focused on 3D compounds such as II-VI and group IV. A few experiments have also been conducted for layered semiconductors, such as transition-metal dichalcogenides, indicating changes in photoluminescence properties apparently comparable to those found in 3D systems. We present extensive electronic structure calculations of the structural and electronic properties of $\mbox{MoS}_2$ nanostructures showing no appreciable quantum confinement effects in single sheet nanoparticles, whose electronic structure is dominated by the surface and in particular edge states near the Fermi level. On the other hand, a strong dependence of the electronic structure is observed as a function of layer stacking and distance. We suggest that the observed photoluminescence variation as a function of size does not pertain to size reduction in single sheets but rather to the number of planes composing the nanoparticle. We also suggest a way to engineer metallic nanowires taking advantage of edge states in nanosheet composites. [Preview Abstract] |
Monday, March 5, 2007 9:36AM - 9:48AM |
A27.00007: The design of a nanocontainer for high pressure storage of hydrogen Zhi-Feng Liu, Deyan Sun, Xiang Ye, Xingao Gong Molecule hydrogen is known to have a weak van der Waals potential, which makes it difficult to raise its storage efficiency for physisorption based methods. In this report, we explore the other side of such a weak potential, the well-known compressibility of hydrogen. A (20,0) single wall carbon nanotube based nanocontainer is designed, in which a C$_{60}$ ``peapod'' at the cap section of the nanotube serves as a molecular valve. Diffusion barriers through such a valve is examined by molecular dynamics simulations under various conditions. It is demonstrated that H$_2$ can first be filled into the container upon compression at low temperature, and then be locked inside it after the release of external pressure. The internal pressure that can be achieved in this design is in the GPa range at room temperature, which is much higher than the typical pressure of a few hundred bar currently employed for hydrogen storage. At 2.5 GPa, the storage weight ratio approaches a promising 7.7$\%$. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:00AM |
A27.00008: Surface Green functions in molecular transport junctions: The generalization to interacting electrons in the leads Aleksey Kletsov, Yuri Dahnovsky The expression for current in transport junctions is generalized to interacting electrons in the leads. We derive a formula for the current where in the expression for line-width matrices the lead density of states is replaced by the surface spectral density matrix for arbitrary {\em e-e} interactions in the leads and in the bridge, respectively. This expression is only valid for small lead-bridge interactions. A novel computational method for a surface Green function matrix is introduced to find the surface spectral density ($\sim$ the trace of the imaginary part of the surface Green function matrix). The proposed non-recursive approach results in the solution of the second order matrix equation for the spectral density matrix (the density of states for noninteracting electrons). The single and double principle layer models are studied for {\em aluminum} surfaces. We find that the peak in the spectral function is rather narrow ($\sim 2\;eV$). and can cause a peak in the $\Gamma$ matrices resulting in a peak in the current-voltage characteristics. Beside the {\em aluminum} surface with {\em fcc}-structures, we study a {\em hexagonal} structure as well. Such surfaces exhibit a gap and two bands in the spectral density. The gap and the band widths depend on the parameters of the lead Hamiltonian. We show that the narrow gap and the narrow bands can result in large negative resistances in the conduction. [Preview Abstract] |
Monday, March 5, 2007 10:00AM - 10:12AM |
A27.00009: Adiabatic quantum pumping in an Aharonov-Bohm loop and in a Si-like nanowire: interference in real space and in k-space Sungjun Kim, Kunal Das, Ari Mizel We study interference effects in the current generated by adiabatic quantum pumping in two extended chain models. The first model contains an Aharonov-Bohm loop within a tight-binding chain of sites. It exhibits interference between the two arms of the loop. We investigate the effect of magnetic field reversal on the pumped current. Our second model is a tight-binding chain of sites with next-nearest-neighbor hopping terms. The resulting Si-like indirect band structure can have 4 degenerate Fermi wave vectors $\pm k_{1F}$ and $\pm k_{2F}$ rather than the usual 2 Fermi wave vectors $\pm k_{F}$. It exhibits signatures of interference between these degenerate conduction band states. [Preview Abstract] |
Monday, March 5, 2007 10:12AM - 10:24AM |
A27.00010: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 10:24AM - 10:36AM |
A27.00011: Hypervelocity impact on carbon nanotube reinforced a-SiC composite targets: An atomistic simulation study Maxim Makeev, Deepak Srivastava Atomistic simulation studies, employing the Tersoff many-body reactive potential, have been performed to investigate the hypersonic velocity impact protection properties of carbon nanotube (CNT) reinforced a-SiC composites, for a diamond spherical projectile velocities ranging from 1 km/s to 20 km/s. The scaling relations and analytical forms are derived to describe the penetration depth as a function of the velocity and radius of the projectile. A theoretical framework has been developed to describe the penetration depth behavior in the case of impact of hard projectile on hard target material. The atomistic simulation results are found to compare well with the obtained analytical forms. The effects of diamond nanoparticle impact on the a-SiC composites, with CNTs aligned parallel and perpendicular to the impact direction, caused by impact induced shock absorption and damage creation, will be described in this presentation. [Preview Abstract] |
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