Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L43: Optical Properties of Quantum Dots and Nanowires 
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Sponsoring Units: DCMP Chair: Tom Kennedy, Naval Research Laboratory Room: Colorado Convention Center 506 
Tuesday, March 6, 2007 2:30PM  2:42PM 
L43.00001: MagnetoOptical Studies of PbSe Colloidal Nanostructures J.G. Tischler, T.A. Kennedy, E.R. Glaser, E.E. Foos, T.J. Zega, R.M. Stroud, Al.L. Efros, S.C. Erwin PbSe is an unusual semiconductor material with a direct band gap at the L point of 150 meV at 4 K. The band structure at this symmetry point is fourfold degenerate for both electrons and light holes, and conduction and valence bands possess similar effective masses and gfactors. Since both masses are relatively small, quantum confinement effects are easily achieved by reducing the nanostructure size to dimensions of the order of the large exciton Bohr radius, $a_B$=46 nm. We synthesized high quality PbSe nanocrystals and characterized them using transmission electron microscopy and optical methods. We probed the gfactor and fine structure of excitons in undoped PbSe quantum dots using optically detected magnetic resonance (ODMR) at 24 GHz and polarized photoluminescence in a magnetic field. The ODMR reveals that the gfactor is large for electron and holes (g=7.6) compared to other semiconductor nanocrystal systems. The photoluminescence polarization increases linearly with increasing magnetic fields up to 6 T, indicating that the finestructure splitting is rather small. [Preview Abstract] 
Tuesday, March 6, 2007 2:42PM  2:54PM 
L43.00002: Multiple exciton generation in films of chemically treated lead chalcogenide quantum dots Joseph Luther, Matthew Beard, James Murphy, Randy Ellingson, Matt Law, Kelly Knutsen, Arthur Nozik Multiple exciton generation (MEG) is a unique process, which allows nanocrystals to produce several electronhole pairs if the excitation energy is high compared to the bandgap of the material. Although the exact process that occurs in MEG is still under debate, the existence of the phenomenon is proven for quantum dots (QDs) in solution. This unique process leads to a desire to fabricate photovoltaic devices, among other things, which would benefit from the enhancement in photocurrent produced at short wavelengths. The fabrication of devices is problematic because the QDs must be close enough for charge transport to occur, yet remain confined for the process of MEG. In our work, we discuss how the exciton dynamics including MEG are affected by assembling the QDs into neat ordered arrays using transient absorption spectroscopy. The interQD distance is varied by treating the asprepared array in dilute solutions of shortchained amines. [Preview Abstract] 
Tuesday, March 6, 2007 2:54PM  3:06PM 
L43.00003: Surface Termination Effects on Zinc Oxide Quantum Dots. Steve Whitesell, Joe Spalenka, Christopher Jack, Cary Allen, Reuben Collins, Thomas Furtak We investigate the effects of surface terminations on the optical properties of 26 nm ZnO quantum dots.~ Nanocrystals were grown by wet chemical synthesis in a shortchain alcohol solvent from zinc acetate and sodium hydroxide.~ Quenching of particle growth with various capping agents is necessary to maintain and enhance the unique characteristics of the nanocrystals.~ We reproduce results of previous work and expand on characterization of naked and surface terminated ZnO quantum dots.~ The nanoparticle properties were investigated by UV absorption spectrophotometry, photoluminescence, infrared spectroscopy, scanning electron microscopy , and atomic force microscopy techniques. [Preview Abstract] 
Tuesday, March 6, 2007 3:06PM  3:18PM 
L43.00004: Emission of quantum dots (QDs) and isoelectronic bound excitons (IBEs) from submonolayer ZnTe on ZnSe and dependence on thickness of ZnTe Y. Gong, G. F. Neumark, M. Tamargo, Igor L. Kuskovsky ZnSeTe systems have been of great interest for both lighting applications and their unique optical properties. It is known that the PL of the dilute alloys [1] or quantum wells [2] is usually due to IBEs. ZnTe/ZnSe QDs have been grwon with full monolayer coverage of ZnTe on ZnSe using VolmerWeber growth [3]. We have shown [4] the existence of typeII QDs in ZnSeTe multilayers grown by migration enhanced epitaxy with submonolayer quantities of ZnTe. The multilayers were grown using three ZnTe deposition cycles sandwiched between nominally pure ZnSe barriers. Here, we report ZnTe/ZnSe QDs grown by a similar method, but with only one ZnTe deposition cycle. It is interesting that at T = 10K the PL emission related to these QDs does not shift upon varying the excitation intensity, and attributed to IBEs; however, a large energy shift is observed at T = 80K, suggesting formation of typeII QDs. The presence of typeII QDs is also supported by magnetoPL measurements. \newline [1] Permogorov and Reznitsky, J. Lumin. \textbf{52}, 201 (1992). \newline [2] Suzuki et al., J. Crystal Growth \textbf{184/185}, 882 (1998). \newline [3] Yang et al., JAP \textbf{97}, 033514 (2005). \newline [4] Kuskovsky, et al., Phys. Stat. Sol. (b) \textbf{241}, 527 (2004); Gu et al., Phys. Rev. B \textbf{71}, 045340 (2005). [Preview Abstract] 
Tuesday, March 6, 2007 3:18PM  3:30PM 
L43.00005: \textit{Abinitio} studies of semiconductor quantum dots Brent Walker, Shaun Hendy, Richard Tilley Quantum dots (QDs) in the form of semiconductor nanocrystals have considerable potential as cell markers/disease trackers in medical physics due to their favorable lightemitting properties and long lifetimes in the cellular environment. Quantum confinement is believed to be responsible for the optical properties of semiconductor QDs (for instance, the direct band gap transition in H or C terminated Si dots is allowed); size and surface functionalization both affect the degree of quantum confinement in the QD, and hence its electronic and optical properties. We present \textit{abinitio} computational studies of Si and Ge based nanocrystals made using density functional theory (DFT); in particular, we obtain optical absorption spectra by application of Lanczos algorithms to the central equations of linearresponse timedependent DFT [1]. We examine how the atomic geometries, electronic structure and optical absorption spectra are affected by the nanocrystal size and surface functionalization. [1] B. Walker, A. M. Saitta, R. Gebauer, and S. Baroni, \textit{Phys. Rev. Lett.}, \textbf{96}, 113001, (2006). [Preview Abstract] 
Tuesday, March 6, 2007 3:30PM  3:42PM 
L43.00006: Optimizing Solar Conversion Efficiency Using Semiconductor Nanocrystals Randy Ellingson, Kelly Knutsen, Qing Song, Justin Johnson, Matt Law, Wyatt Metzger, Joseph Luther, Kathrine Gerth, Mark Hanna, Matt Beard, Arthur Nozik The increasing need for large quantities of CO$_{2}$free energy sources may be partially met through the development of novel highlyefficient solar photoconversion approaches. The efficient generation of multiple excitons following absorption of a single photon in semiconductor nanocrystals (NCs) represents one possible route towards inexpensive, highefficiency solar cells. The multiple exciton state, which has been observed for absorption of a single photon with energy above twice the NC effective bandgap in various colloidal semiconductors, can be detected through the Auger recombination signature in the decay of the charge carrier population. Exciton population decays can be measured by studying the timedependence of either intra or interband photoinduced absorption, or via the decay of photoluminescence for emissive samples. We report on work in our laboratory to study multiple exciton generation, and its potential for solar energy conversion applications, in various leadsalt as well as Si and Ge NCs. [Preview Abstract] 
Tuesday, March 6, 2007 3:42PM  3:54PM 
L43.00007: First Principles Computation of Optical Response in Silicon Nanostructures Sebastien Hamel, Andrew Williamson, Hugh Wilson, Francois Gigy, Giulia Galli, Ed Ratner, Dan Wack In the next few years, the typical size of finFET devices used in the microchip industry is expected to be of the order of a few nanometers. This poses formidable challenges, including for optical metrology, i.e. for the development of appropriate tools and techniques based on optical response, to monitor and validate the growth of silicon nanostructures. Current optical metrology tools are based on the assumption that in Si finFETs the dielectric response is piecewise constant and equal to the bulk value. Such an assumption is expected to break down for sizes smaller then 10 nm, where the dielectric response of Si nanostructures may substantially deviate from that of the bulk. We present an analysis of the dielectric properties of Si slabs, spheres and rods as a function of size and shape, based on first principles, Density Functional Theory calculations. In particular, we discuss the relative influence of quantum confinement and surface effects, and propose a way to monitor dielectric properties changes at the nanoscale, based on the definition of local dielectric response functions. [Preview Abstract] 
Tuesday, March 6, 2007 3:54PM  4:06PM 
L43.00008: Optical properties of doping effects in Si quantum dots Jingbo Li, SuHuai Wei Si quantum dots (QDs) have many applications such as in highefficient solar cells and light emitting diodes. Understanding the doping properties in Si QDs are very important for both theory and experiment. Using firstprinciples methods, we have systematically calculated the defect formation energies and transition energy levels of groupIII and groupV impurities doped in Si QDs as functions of the QD size. The general chemical trends found in the QDs are similar to those found in bulk Si. We show that defect formation energy and transition energy level increase when the size of the QD decreases, thus doping in small Si QDs becomes more difficult. We explain the general chemical trends and the variation as a function of QDs size in terms of the atomic eigenvalues and quantum confinement effects. We also calculate the absorption spectrum of sizedependent Si QDs and quantum rods by largescale ``charge patching method''. We show that the band gap and optical transitions of Si nanocrystals can be tailored continuously by size or shape. These results provide guidelines for future device designs that require the knowledge of the size/shapedependence of the nanocrystal's electronic and optical properties. [Preview Abstract] 
Tuesday, March 6, 2007 4:06PM  4:18PM 
L43.00009: Study of Charged Exciton in Silicon Quantum Dot. Gian Franco Sacco, Paul von Allmen, Seungwon Lee The trion system, a bound state of two electrons and a hole or two holes and one electron, has received particular attention in the past few years due to the possible practical applications, ranging from single photon emitter which can be used in quantum cryptography, ultahigh density memory devices, mobile light emitters and probe for semiconductor material. Moreover the trion has been proposed as a key element in the fabrication of quantum gates allowing fast spin flip of an electron in a charged quantum dot. In this study we compute the lowest energies of a trion system for two Silicon quantum dots of radius 1.09 and 1.36 nm respectively by using the configuration interaction method. We first obtain the electron and hole energies states using the tight binding method and then we construct a basis set of trion states with good spin quantum number and diagonalize the trion Hamiltonian. We find that the spin three half state is more bound then the onehalf and we confirmed that the binding energy decreases as the size of the system increases. We get that the trion binding energy of the dot with radius 1.09 nm is 0.242 eV and 0.227 eV for the spin threehalf and onehalf respectively, while that of the dot with radius 1.36 nm is 0.295 eV for the spin threehalf state and 0.261 eV for the spin one halfstate. [Preview Abstract] 
Tuesday, March 6, 2007 4:18PM  4:30PM 
L43.00010: Internal transitions of singlet and triplet charged excitons in quantum dots A. Dzyubenko, A. Sivachenko We theoretically studied the spectra of internal transitions of spinsinglet and spintriplet charged excitons X$^{ }$ in quantum dots in finite magnetic fields. The lateral confinement in quantum dots was modeled by a parabolic potential. Both equal and different oscillator lengths for electrons and holes were considered. The states of charged excitons were found using the expansion in FockDarwin inplane states and in sizequantization levels in quantum wells of different widths. We performed systematic studies of the spectra dependencies on the quantum well width, on the regime of lateral confinement, and on the magnetic field strength. In the absence of lateral confinement, the spectra of internal transitions are governed by an exact optical selection rule that follows from the rotational symmetry and magnetic translations. The selection rule prohibits, in particular, a triplet boundtobound interLandau level transition with energies below the electron cyclotron resonance. When the magnetic translational symmetry is broken by the lateral confinement, the previously prohibited transitions become allowed and acquire finite oscillator strengths. We compare our theoretical results with the existing experimental data on internal excitonic transitions in interface fluctuation quantum dots in GaAs QWs. [Preview Abstract] 

L43.00011: ABSTRACT WITHDRAWN 
Tuesday, March 6, 2007 4:42PM  4:54PM 
L43.00012: Influence of the substrate orientation on the electronic and optical properties of InAs/GaAs quantum dots Vladan Mlinar, Francois Peeters Variation of the electronic and optical properties of InAs/GaAs quantum dots (QD) as a function of the substrate orientation is studied in the framework of 3D eightband k.p model. The QD transition energies are obtained for high index surfaces [11k], where k = 1, 2, 3 and are compared with [001]. We show that the QD size in the growth direction determines the degree of the influence of the substrate orientation, whereas the influence of the shape is of secondary importance. The effects of an external magnetic field applied parallel and perpendicular to the QD growth direction are analyzed taking into account the Zeeman effect and employing the gauge invariant discretization scheme. The available experimental data are successfully described by one of the optically active exciton states of the lowest lying exciton quartet. We also discuss the experimentally observed negative exciton diamagnetic shift for small values of the magnetic field: (1) for samples grown on a (001) substrate and magnetic field applied perpendicular to the growth direction, and (2) for samples grown on a (311) substrate and magnetic field applied parallel to the growth direction. [Preview Abstract] 
Tuesday, March 6, 2007 4:54PM  5:06PM 
L43.00013: Selfassembled (In,Ga)As Quantum Posts on GaAs. Jun He, Hubert Krenner, Craig Pryor, Jingpin Zhang, Yuan Wu, Dan Allen, Chris Morris, Mark Sherwin, Pierre Petroff We demonstrate a method for the MBE growth of height controlled (In,Ga)As quantum posts (QPs). Its main axis is along the growth direction. they are dislocation free and have a dimensions $\approx $ 20nm and 40nm. From EDX measurements, The Ga$_{.55}$In$_{.45}$As QPs are embedded laterally in a Ga$_{.9}$In$_{.1}$As layer. We have computed the electron and hole levels using an 8 bands, stain dependent, \underline {k.p} effective mass model. Comparison of calculated single particle electron and hole levels with the observed microPL spectra single QPs indicates that electrons are spread over the entire QP whereas holes are localized near the seed QD. * This research is sponsored through an NSFNIRT grant No. CCF 0507295. HJK acknowledges support by the Alexander von Humboldt Foundation. [Preview Abstract] 
Tuesday, March 6, 2007 5:06PM  5:18PM 
L43.00014: Measurements of the bandgap of wurtzite InAs$_{1x}$P$_x$ nanowires using photocurrent spectroscopy J Tragardh, A.I. Persson, J.B. Wagner, D. Hessman, L. Samuelson We report measurements of the bandgap of InAs$_{1x}$P$_x$
nanowires with wurtzite crystal structure as a function of the
composition. The bandgap was measured using photocurrent
spectroscopy (performed at 5 K) on single InAs nanowires with a
centrally placed InAs$_{1x}$P$_x$ segment, contacted at the InAs
ends. The nanowires were grown with chemical beam epitaxy (CBE).
The measured bandgap was larger than the bandgap of zincblende
InAs$_{1x}$P$_x$ by about 120 meV over the measured composition
range, $0.15 
Tuesday, March 6, 2007 5:18PM  5:30PM 
L43.00015: Magnetoplasmon excitations in Rashba spintronic quantum wires: Maxons, rotons, and negative energy dispersion M.S. Kushwaha We investigate the plasmon excitations in a quasionedimensional electron gas (Q1DEG) in the presence of a perpendicular magnetic field ($B$) and spinorbit interaction (SOI) induced by the Rashba effect. The problem involves three length scales: $\ell_0=\sqrt{\hbar/m^*\omega_0}$, $\ell_c=\sqrt{\hbar/m^*\omega_c}$, and $\ell_s=\hbar^2/(2m^*\alpha)$ associated with, respectively, the confining potential, the magnetic field, and the Rashba SOI. The resulting Schrodingerlike equations satisfied by the wave function $\phi_{\uparrow \downarrow}$ are two coupled equations, which cannot be solved in an explicit analytical form. However, the limit of a strong magnetic field ($\ell_c \ll \ell_0$) and $k\ell_0\ll 1$ helps solve this set of coupled equations exactly. We derive and discuss the dispersion relations for chargedensity excitations within the framework of BohmPines' RPA. The intrasubband and intersabband magnetoplasmons (MPs)in a Q1DEG are characterized by, respectively, the negative energy dispersion with increasing $B$ and the magnetoroton excitations . Here we scrutinize the effect of the SOI on these characteristics in depth. We observe that the SOI modifies drastically the behavior of both the intrasubband and intersubband MPs in the LW limit and renders them relatively more prone to the Landau damping in the SW limit. We discuss the dependence of the MPs energy on the propagation vector, the magnetic field, the 1D chargedensity, and the Rashba parameter characterizing the SOI. [Preview Abstract] 
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