Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D26: Focus Session: Computational Nanoscience III: Defects, Doping, and Structure |
Hide Abstracts |
Sponsoring Units: DMP DCOMP Chair: Paul Kent, Oak Ridge National Laboratory Room: 328 |
Monday, March 16, 2009 2:30PM - 3:06PM |
D26.00001: Intrinsic magnetism in nonmagnetic nanostructures: Role of localized states and quantum confinement. Invited Speaker: Manipulation of carrier spins in semiconductors for spintronics applications has received much attention recently driven by the promise of new or improved functionalities. This has stimulated extensive research in the area of magnetic semiconductors. Magnetism is traditionally recognized as arising from unpaired electrons in 3d and 4f materials. However, there has been increasing evidence that localized defect states (and/or surface/edge states) in sp materials, especially in some nanostructures, may form local moments and exhibit collective magnetism. In a recent paper [PRL100, 117204 (2008)], we proposed that the duality (i.e., localized vs extended nature) of defect states in wide-gap nitrides and oxides may promote collective magnetism in these materials without magnetic ions. We have recently extended this study to include unexpected magnetism observed in GaN and ZnO nanowires and other artificial quantum structures. Particular attention will be paid to the role of localized states and quantum confinement in promoting unconventional magnetism in these systems. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D26.00002: Defect induced magnetism in semiconductor nanostructures Hyunwook Kwak, Tzu-Liang Chan, James Chelikowsky It has been proposed that magnetic semiconductors can be designed by using non-magnetic defects, e.g., through the introduction of an extrinsic impurity atom that does not exhibit magnetism by itself (Phys. Rev. Lett. 99, 127201). In order to address such proposals, we have employed a real-space pseudopotential method based on the generalized gradient approximation to determine the magnetic properties of boron and aluminum doped silicon nanocrystals and nanowires. We will discuss theoretical evidence for defect induced magnetism as a function of the nanostructure size. We suggest that defect induced magnetism can be strongly enhanced by quantum confinement. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D26.00003: Ferromagnetism driven by extended defects in nanostructured ZnO Aline L. Schoenhalz, Jeverson T. Arantes, Adalberto Fazzio, Gustavo M. Dalpian Spintronic has a particular interest in diluted magnetic semiconductors because these materials present both semiconducting and magnetic properties at the same time. ZnO-based materials and nanostructures have potential applications in this area because they can present room-temperature ferromagnetism when doped with transition metals and, in some cases, spin polarization can be observed even without magnetic impurities. Following recent experimental results reporting this [Nano Lett. 6, 1489 (2007)], we have analyzed several ZnO nanocrystals, with diameters varying from 0.9 to 1.78 nm. Using DFT, we observed that a large amount of surface reconstructions appear in the non-passivated nanocrystals. Depending on the reconstruction, spin polarization without magnetic impurities can be observed at the surface region, what can lead to long-range spin interactions. Thus, we propose that the referred experimental results for nanostructured ZnO can be originated by extended defects such as surfaces. This can also explain the same magnetic behavior presented by non-doped thin films and other ZnO nanostructures. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D26.00004: Self-doping in Boron Nanostructures Hui Tang, Sohrab Ismail-Beigi Boron nanotubes have attracted much attention since their first fabrication in experiments. Boron nanotubes with large radii (R $\ge$ 10 {\AA}) are predicted to be metallic with large densities of states at their Fermi energies, which may provide excellent conducting systems for one-dimensional electronics. In previous work [1], we have shown a class of stable boron sheets, composed of mixtures of triangular and hexagonal motifs, that are likely to be the precursors of boron nanotubes. These sheets are stabilized by a balance of 2-center and 3-center bonding. Here, using density functional theory and Maximally Localized Wannier Functions, we show that adding a boron atom to a boron sheet is equivalent to doping the boron sheet with all three valence electrons of the added atom. Based on this self-doping picture, we propose a simple counting scheme to construct stable boron nanostructures, e.g. from corresponding carbon ones. We also apply this knowledge to study Mg-doped boron sheets and discuss the possible stable structures of MgB$_2$ nanotubes. [1] H. Tang, and S. Ismail-Beigi, Phys. Rev. Lett. 99, 115501 (2007). [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D26.00005: Computational Studies of Nanostructures of Boron P. Tandy, M. Yu, C. Leahy, W.Q. Tian, S.Y. Wu, C.S. Jayanthi The goal of this work is to develop a reliable semi-empirical Hamiltonian for boron that may be used to predict nanostructures of boron. It is well known that bonding in boron is complicated as it may form three-center, two-electron bonds. The semi-empirical Hamiltonian used here was recently developed by Leahy \textit{et al}. in the framework of linear combination of atomic orbitals[1]. The salient feature of this Hamiltonian is that it treats environment dependency and charge redistributions on equal footing. It will be shown that such a parameterized Hamiltonian can predict the B$_{80}$ cage structure with C$_{1}$ symmetry as found in a recent first-principles study [2]. Having validated our semi-empirical Hamiltonian for boron with small boron clusters and the B$_{80}$ cage, we have performed a systematic study of other boron nanostructures: (i) larger cage structures ($e.g.$, B$_{215})$, (ii) boron clusters cut from the bulk alpha boron, and (iii) boron sheets (triangular sheets with and without holes). We will discuss the ground state structures of these boron nanostructures as well as the energetics and HOMO-LUMO gaps of different families of boron clusters as a function their diameters. 1. C. Leahy \textit{et al.} Phys. Rev. B74, 155408 (2006). 2. N. G. Szwacki et al. PRL 100, 159901 (2008). [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D26.00006: An efficient method to use \textit{ab-initio} calculations to study substitutional order in nanoparticles Gerbrand Ceder, Tim Mueller To study systems with substitutional disorder researchers commonly use effective Hamiltonians known as cluster expansions.~ In a cluster expansion the phase space of a system is coarse-grained over a fixed set of crystal sites and the energy is expressed as a linear combination of interactions between these sites.~ The coefficients of the linear expansion are typically fit to training data generated using \textit{ab-initio} methods.~ Low-symmetry systems such as nanoparticles require the determination of a large number of distinct coefficients.~ A large amount of training data must be generated for such problems, and the cost of calculating the energy of each training structure is high due to the low symmetry of the system.~ For these reasons it has been impractical to use the cluster expansion to study low-symmetry materials with the same level of accuracy as bulk materials.~ We address this problem by demonstrating new methods that significantly reduce the prediction error of a cluster expansion for a given training set size.~ Our approach makes it possible to study atomic ordering in nanoparticles at a fraction of the current computational cost. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D26.00007: Graph-based global optimization of fully-coordinated cluster geometries Edwin Flikkema, Stefan Bromley We present a detailed global optimization study of cluster geometries with silica nano-clusters (SiO$_2$)$_N$ as a specific example. In an earlier study (Phys. Rev. Lett., 95: 185505, 2005) we used the Basin Hopping methodology combined with an empirical potential to find low-energy cluster geometries. These often exhibit defects such as dangling oxygens. In this contribution we will present an algorithm for global optimization of cluster geometries, which limits the search specifically to fully-coordinated cluster geometries, i.e. defectless clusters where each silicon atom is bonded to 4 oxygen atoms and each oxygen atom is bonded to 2 silicon atoms. This algorithm is based on performing Monte Carlo moves on the set of graphs rather than in coordinate space, the graph being the network of silicon-oxygen bonds. Promising low-energy geometries are selected for refinement using Density Functional Theory calculations. Clusters of a size of up to 30 SiO$_2$ units have been studied. The properties of low-energy fully-coordinated clusters will be compared to those of clusters with defects. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D26.00008: Nanostructure determination from the atomic pair distribution function Luke Granlund, Pavol Juhas, Saurabh Gujarathi, Phil Duxbury, Simon Billinge Many materials at the nanoscale cannot benefit from crystallographic analysis and are unsuitable for refinement techniques that require an initial guess at the structure. One approach to overcoming these difficulties is the Liga algorithm, which generates structures relying solely on distances extracted from the atomic pair distribution function[1,2]. This method is shown to successfully reconstruct the buckyball from experimental data. Recent extensions to multi-component and periodic systems have also allowed reconstruction of common crystals from experimental data. The ability to handle both periodic and nonperiodic cases may make the algorithm a useful tool in the study of local structure deviations in nanocrystals in addition to noncrystalline nanomaterials. [1] P. Juhas, D. M. Cherba, P. M. Duxbury, W. F. Punch, S. J. L. Billinge. Ab initio determination of solid-state nanostructure. Nature, 440, 655-658 (2006). [2] P. Juhas, L. Granlund, P. M. Duxbury, W. F. Punch, S. J. L. Billinge. The Liga algorithm for ab initio determination of nanostructure. Acta Cryst., A64, 631-640 (2008). [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D26.00009: The role of confinement on the diffusion barriers in semiconductor nanocrystals Tzu-Liang Chan, Alexey Zayak, Gustavo Dalpian, James Chelikowsky We find that quantum size effects not only play an important role in the electronic properties of defects in semiconductor nanocrystals, but also strongly affect the incorporation of defect atoms into the nanocrystals. In particular, using ab initio methods based on density functional theory, we predict that Mn defects will be energetically expelled to the surface of CdSe and ZnSe nanocrystals, and that the diffusion barrier of a Mn interstitial defect in a CdSe nanocrystal will be significantly lower than that in the bulk. This can be ascribed to the large surface to volume ratio of nanocrystals, which can effectively release the strain during diffusion. By calculating the vibrational spectrum of the CdSe nanocrystal, we estimated the diffusion rate within the nanocrystal. Our results suggest that energetics can play a role in the self purification of small CdSe and ZnSe nanocrystals, as diffusion of the defect atom can readily occur inside such small nanocrystals. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D26.00010: Accelerated Kinetic Monte Carlo Simulations of Vacancy-Mediated Arsenic Diffusion and Clustering in Silicon Brian Puchala, Michael Falk, Krishna Garikipati During semiconductor device fabrication, ion implantation of dopants creates large populations of defects, vacancies and interstitials, which mediate dopant diffusion. Experiments have shown large changes in dopant diffusivity in silicon as a function of annealing time and dopant concentration. We perform kinetic Monte Carlo (KMC) simulations of vacancy-mediated arsenic diffusion in silicon to investigate the effect of dopant concentration, distribution and clustering on diffusivity. In order to follow the diffusion and breakup of clusters, on the order of minutes, our KMC simulations are accelerated using absorbing Markov chain analysis on states intelligently chosen on-the-fly to fill trapping basins in the local energy landscape. At lower dopant concentrations, we calculate the diffusivity and breakup rates of different cluster types and a mean field approach can be used to describe the overall cluster population evolution and dopant diffusivity. Above a critical concentration this mean field description fails as dopants become close enough to form a percolating structure throughout the material. At all concentrations, diffusivity decreases significantly over time as larger, less mobile clusters form. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D26.00011: First Principles Study of the Effect of Lattice Strain on Diffusion Barriers Handan Yildirim, Talat S. Rahman To understand the three times larger values of the Erlich Schoewebel barriers for Ag on Cu(100) as compared to that for Cu on Ag(100) as obtained from our density functional theory calculations, we performed the diffusion of Ag adatom on Ag(100) and Cu adatom on Cu(100) under uniform lattice strain of 0-5{\%}. We find the origin of the differences in the energetics to be the combination of Cu-Ag electronic coupling, relative atomic sizes and adsorbate-substrate lattice mismatch. The diffusion barriers on the ideal surfaces are found to decrease with increasing compressive strain and to increase with increasing tensile strain and almost a linear function of strain up to 5{\%}. We show that this trend is universal and transferable to other metals. We will discuss the consequences of the modifications in the height of the ES barriers on growth modes. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D26.00012: Quantum confinement in P-doped Si[110] nanowires Jiaxin Han, Tzu-Liang Chan, James Chelikowsky Recently, doped Si nanowires have been synthesized and demonstrated experimentally that they can be used as interconnects in electronic circuits or building blocks for semiconductor nanodevices. In order to understand how doping operates at the nanoscale, we used a real-space first-principles pseudopotential method to study P-doped Si[110] nanowires. We examined the size dependence of the electronic binding energy for the P donor level. We found the donor electron to be more strongly bound to the P atom with decreasing nanowire diameter owing to quantum confinement. We also examined the energetically favorable position of the P atom in Si nanowires. For nanowires with diameter less than 1 nm, the P atom is expelled to the surface owing to the stress introduced by the defect, which suggests that doping will be difficult for small-diameter Si nanowires. In addition, we calculated the core-level binding energy shift as the P atom moves from the surface towards the center of the nanowire. We will compare our results with experimental measurements. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D26.00013: Effect of structure, surface passivation, and doping on the electronic and optical properties of GaAs nanowires: A first principles study. S. V. Khare, V. Gade, N. Shi, R. Ramprasad We investigate the structural, energetic, electronic, and optical properties of hydrogen-passivated doped and undoped gallium arsenide nanowires along [001], [110], and [111] directions with diameter d up to 3 nm, using \textit{ab initio} methods. A critical diameter d$_{c}\approx $2 nm is found above which all wires have faceted cross sections determined by the symmetry of their axis. The wires possess several electronic properties relevant for sensing and other nanoelectronic applications: (i) Quantum confinement has a substantial effect on the electronic band structure and hence the band gap (E$_{g})$, which increases with decreasing diameter. (ii) Unlike Si or Ge wires, GaAs wires oriented along all axes are found to have a direct E$_{g}$.$_{ }$(iii) The electronic band structure shows a significant response to changes in surface passivation with hydrogen. (iv) Doping of wires with n and p type atoms produced a response in the band structure similar to that in a doped bulk crystal. (v) However, the dielectric function shows differences in absorption peaks with p type versus n type doping. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700