Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N15: Focus Session: Theory of Nanostructures and Nanowires |
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Sponsoring Units: FIAP Chair: Andrew Williamson, LLNL Room: LACC 405 |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N15.00001: Radiative recombination of charged excitons and multiexcitons in CdSe quantum dots M. Claudia Troparevsky, Alberto Franceschetti Radiative recombination of neutral and charged biexcitons has been recently observed in CdSe nanocrystals using time-resolved, femtosecond spectroscopy. Here we report semi-empirical pseudopotential calculations of charged exciton and multiexciton emission spectra of CdSe nanocrystals. We studied the mono-exciton X (one electron, one hole, 1e-1h), the charged excitons X$^{- }$(2e-1h) and X$^{+}$ (1e-2h), and the charged biexcitons XX$^{- }$(3e-2h) and XX$^{+}$ (2e-3h). For a 3.9 nm-diameter CdSe nanocrystal, we found that the emission peak for the X$^{-}$ recombination overlaps with that of X (at 2.16 eV), while the X$^{+}$ emission peak is slightly blue-shifted (by 0.02 eV). We also found that the main peaks in the XX$^{-}$ and XX$^{+}$ emission spectra are significantly blue-shifted with respect to the exciton peak X (by 0.04 and 0.05 eV, respectively) because of inter-particle interactions. In the case of XX$^{-}$, we observe an additional peak of lower intensity at 2.50 eV originating from the recombination of a 1p electron state with a partially occupied 1p hole state. This work was supported by the US DOE Office of Science LAB3-17 initiative. [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N15.00002: Shape, charge, and alloy fluctuation effects on optical properties of million-atom InGaAs/GaAs dots$^{*}$ Gustavo A. Narvaez, Gabriel Bester, Alex Zunger Single-dot spectroscopy makes it possible to probe in detail dot-to-dot changes in optical properties of self-assembled In$_{1-x}$Ga$_{x}$As/GaAs dots. An atomistic pseudopotential method combined with the configuration interaction approach reveal the role of shape, charge, and alloy fluctuations on the electronic structure, polarization of optical transitions, and excitonic radiative lifetimes of In$_{0.6}$Ga$_{0.4}$As/GaAs quantum dots. Several features emerge. (i) {\em Height fluctuations.} Recombination energies and excitonic binding of $X^0$ (neutral exciton), $X^{-}$, $X^+$, and $XX$ (biexciton) change significantly with height, but not with randomness. (ii) {\em Charge fluctuations.} The lowest transitions of $X^-$ and $X^+$ are naturally unpolarized, whereas those of $X^0$ and $XX$ are polarized in plane. Thus, charge fluctuations affect polarization of an ensemble of dots. (iii) {\em Alloy fluctuations.} Different random realizations (configurations) of the same alloy dot lead to radically different polarizations of transitions in $X^0$ and $XX$. In the light of our simulations, we discuss dramatic changes in polarization recently observed in InAs/GaAs single dots.\newline $^{*}\,$Supported by DOE-SC-BES-DMS [Preview Abstract] |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N15.00003: Radiative lifetime of excitons in CdSe quantum dots M. Califano, A. Franceschetti, A. Zunger Recent experimental measurements have shown that the low-temperature (T $\sim$ 2K) recombination lifetime of excitons in CdSe nanocrystal quantum dots is relatively short, of the order of 10$^{-6}$ s for quantum dots in the 2-4 nm size range. These results are surprising, since the lowest excitonic state of CdSe quantum dots is optically ``dark,'' and the next, ``bright'' state is several meV higher in energy, so it is not thermally populated at low temperature. Using a semi-empirical pseudopotential approach, we have investigated the exciton radiative lifetime of CdSe quantum dots as a function of size and temperature. We find that indeed, in the case of fully passivated CdSe nanocrystals the low-temperature lifetime is at least three orders of magnitude longer than the experimental value. However, we also find that the presence of surface states, such as dangling-bond states, mixes the dark and bright exciton states, dramatically reducing the dark exciton lifetime, and bringing it in agreement with experimental data. We conclude that surface states are the controlling factor of dark-exciton lifetimes in colloidal CdSe dots. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 9:12AM |
N15.00004: Predictive simulations of semiconductor nanostructures Invited Speaker: \textit{Ab-initio} simulations are playing an increasingly important role in understanding matter at the nanoscale and in predicting with controllable, quantitative accuracy the novel and complex properties of nanomaterials. A microscopic, fundamental understanding of nanoscale phenomena is very much in demand, as experimental investigations are sometimes controversial and usually they cannot be explained on the basis of simple models. In this talk, \textit{ab-initio} molecular dynamics simulations and quantum monte carlo calculations of semiconductor nanoparticles will be presented, with focus on electronic and optical properties and on the microscopic structure of surfaces at the nanoscale. The characterization of nanoscale surfaces and interfaces is of paramount importance to predict the function of nanomaterials, and eventually their assembly into macroscopic solids, and it is still very challenging from an experimental standpoint, due to the lack of appropriate imaging techniques. The presentation will focus on Si, Ge, SiC nanoparticles and nanodiamond, and in addition we will discuss several results for II-VI dots and rods. (*) Work done in collaboration with G.Cicero, E.Draeger, J.Grossman, F.Gygi, D.Prendergast, A.Puzder, J.-Y.Raty, F.Reboredo, E.Schwegler, A.Williamson This work was performed under the auspices of the US Department of Energy by the University of California at the LLNL under contract no W-7405-Eng-48 [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N15.00005: Intrinsic Surface States in Semiconductor Nanocrystals: HgS Quantum Dots James Sims, Garnett Bryant, Howard Hung Confined states in typical nanocrystals are localized to the dot interior. Surface states are extrinsic states localized at unsaturated dangling bonds or surface defects. We show that intrinsic surface states occur in nanocrystals made from negative gap semiconductors such as HgS. We use atomistic tight-binding theory which allows explicit atomic models for the surfaces. We consider spherical HgS nanocrystals with saturated dangling bonds and diameters up the bulk limit. In small HgS dots, the lowest conduction band states are cation-derived and the band-edge valence states are anion-derived, as for finite-gap dots. In bigger HgS dots, valence states and higher conduction band states evolve toward their bulk limits. However, the lowest conduction band state has high density at the surface and slowly decays into the dot. Band mixing is critical for this state. It has mixed cation and anion character and is partly s- and light-hole-like. As the dot size increases, this conduction state crosses the valence band edge, reaching a limit inside the bulk negative gap for very large dots. In this limit, the state is localized to the surface. The optical response of Hgs dots is discussed to identify signatures for intrinsic surface states. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N15.00006: First principles comparison of alkyl terminated Silicon dots with Silicon-Carbide dots. Fernando A. Reboredo, Giulia Galli Using ab-initio methods, we have studied different quantum dots that could be synthesized using the three elements: Si, C and H. In particular, we have compared hydrogen and alkyl passivated Silicon dots with Silicon-Carbide dots. We find that in Si clusters with reconstructed (100) facets a complete alkyl passivation is possible, but steric repulsions prevents full passivation of Si dots with unreconstructed surfaces. In addition, our calculations show that the stability of alkyl passivated Si clusters depends on the length of the carbon chains. Alkyl passivation weakly affects optical gaps of Si quantum dots, while it substantially affects ionization potentials and electron affinities. We also investigate theoretically the possibility to fabricate silicon-carbide quantum dots passivated with H. We find that the optical properties of this type of dots would be weakly dependent on size but strongly influenced by the structure of the surface, which in turn depends on the growth conditions. We discuss the conditions where quantum confinement could be observed in SiC quantum dots. Our results suggest that depending on the experimental conditions either alkyl terminated Si dots or SiC dots could be form being optical gaps strongly dependent on the core and surface structure. [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N15.00007: All-electron and pseudo-potential studies of structural and electronic properties of Si chains and nanowires Jun Li, Arthur Freeman, Andrew Williamson, Jeffrey Grossman, Giulia Galli Recent experiments\footnote{Y. Wu, et.al., Nature 430, 61 (2004); and references therein} invoke Si nanowires as promising materials for nanoscale electronic and optical devices. We carried out electronic structure calculations of silicon chains and nanowires, by using both the full-potential linearized augmented plane wave (FLAPW) method\footnote{E.Wimmer, H.Krakauer, M.Weinert, AJ Freeman, PRB 24, 864 (1981)} and the pseudopotential plane wave method. We studied two sets of H-terminated one nanometer silicon wires, one oriented along (001) and the other along(111); both show direct band gaps, with the (111) oriented wires showing a smaller gap ($\sim$2.1 eV) than (001) ($\sim$2.5 eV). This trend differs from that reported in the literature \footnote{F. Buda, et.al., PRL 69, 1272 (1992); A. M. Saitta, et.al., PRB 53, 1446 (1996)}, but it is the same in both our all-electron and well converged pseudopotential calculations. We also found that structural relaxations induce different effects on the band structure of differently oriented wires; the band gap change is nearly 0.2 eV between the ideal and relaxed models for (001) while it is negligible for (111) wires. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N15.00008: One-dimensional hole gas in Ge/Si nanowire heterostructures Wei Lu, Jie Xiang, Brian Timko, Yue Wu, Charles Lieber Two-dimensional (2D) electron and hole gas systems have played a central role in condensed-matter physics research, as well as high performance electrical and optical devices. In this talk, I will discuss a one-dimensional (1D) hole gas system based on a germanium/silicon core/shell nanowire heterostructure. At room temperature, hole accumulation in the intrinsic germanium channel was observed due to the valence band offset at the Ge/Si interface. At low temperatures, conductance quantization at values close to that expected of a ballistic conductor was observed, and was attributed to the long mean free path in the hole gas and confinement of the hole gas in the radial direction. These effects showed little temperature dependence and suggested that transport in these small diameter nanowires is ballistic even at room temperature. The demonstration of a 1D hole gas in a flexible nanowire heterostructure opens up a number of possibilities for investigating quantum phenomena in low-dimensional systems, as well as applications in both conventional and quantum computing schemes. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N15.00009: Nanowire Photonic Circuit Elements Andrew Greytak, Carl Barrelet, Charles Lieber We report an approach for guiding and manipulating light on sub-wavelength scales using active nanowire waveguides and devices. Semiconducting nanowire structures are distinct from conventional transparent dielectric waveguides since absorption and emission occur for modes with near band edge energies. Quantitative studies show that light propagation in nanowire structures takes place with only moderate losses through sharp and even acute angle bends. The losses measured are compared to those reported recently for photonic crystal structures and plasmon waveguides. Furthermore, a straightforward nanowire based electro-optic modulator (EOM) was demonstrated. The EOM has been shown to yield a substantial field-dependent modulation of the intensity of light propagating through these active waveguides. In addition, an efficient electrically-based injection of light into guided modes of the active nanowire waveguides will be described. Progress towards and challenges for electrical and optical manipulation of light in nanowire waveguides for logic will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N15.00010: ZnO Nanowires: Building Blocks for Nanoscale Electronics, Optoelectronics and Chemical Sensors Zhiyong Fan, Pai-Chun Chang, Dawei Wang, Jia G. Lu Chemical vapor deposition synthesized single crystalline ZnO nanowires are configured as n-channel field effect transistors and their electrical transport properties are studied. It is observed that electron concentration and mobility in nanowires can be modified by varying the synthesis conditions. Photoluminescence and photoconductivity of individual nanowires are investigated. These nanowire field effect transistors demonstrate a broadband and polarization dependent photo- response. Due to the small diameter and tunable electron concentration, ZnO nanowire transistors are implemented as highly sensitive, gate refreshable chemical sensors with potential selectivity. These results open up the future applications of ZnO nanowires as one of the promising materials for nanoscale electronics, optoelectronics and chemical sensing devices. [Preview Abstract] |
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N15.00011: Linear and Nonlinear Optical Properties of GaN Nanoclusters Shashi Karna, Andrew Pineda The linear and nonlinear optical (NLO) properties of III-V binary semiconductors have been a subject of active research since the late 1960s. Recent advancements in (a) experimental techniques to fabricate/produce stable nanometer-size binary atomic clusters composed of group III and group V elements and (b) techniques and tools to probe response properties of nano-scale objects, have attracted a great deal of attention in the linear and NLO properties of III-V nanoclusters due to their potential applications in future technologies. An important issue in a bottom-up approach to fabricating nanoclusters for future technological applications is an understanding of the evolution of response properties with cluster size. In order to develop such an understanding, we have undertaken a systematic study of the electronic and geometrical structures and the optical properties of III-V nanoclusters by first-principles \textit{ab initio} time-dependent Hartree-Fock calculations. In this talk, we present the results of our first-principles quantum mechanical studies of the electronic structure, stability, and linear and NLO properties of Ga$_{m}$N$_{n}$ atomic clusters, with values of $m$ and $n$ ranging between $1 $and $17$. Our calculated results suggest that the linear and NLO properties both exhibit strong dependence on the cluster size and shape. However, the size-dependence is more pronounced for the NLO properties than that for the linear optical properties. [Preview Abstract] |
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