Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L10: Optical Properties of Nanostructures III: Quantum Dots, Nanowires, and Related Materials |
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Sponsoring Units: DMP Chair: John McGuire, Los Alamos National Laboratory Room: 304 |
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L10.00001: The effect of uniaxial stress on light emitted from GaN/AlN quantum dots Daniel Rich, Ofer Moshe, Benjamin Damilano, Jean Massies We have studied the effect of uniaxial stress on the optical polarization properties of GaN/AlN quantum dots (QDs) grown on Si(111) substrates. Microcracks form as a result of the thermal expansion coefficient mismatch between the GaN/AlN layers and the Si(111) substrate. We show that such microcracks serve as excellent stressors through which the strain tensor of the GaN/AlN QDs can be modified for studies of strain-induced changes in the optical properties using a spatially and temporally resolved probe, such as with cathodoluminescence (CL) imaging and spectroscopy. CL measurements of the ground- state excitonic transition of vertically stacked GaN/AlN quantum dots (QDs) exhibit an in-plane linear polarization anisotropy in close proximity to microcracks, consistent with the presence of uniaxial stress. The spatial dependence of the polarization anisotropy and CL decay time in varying proximity to the microcracks are studied as a function of temperature and excitation conditions in order to assess the influence of thermal stress variations on the oscillator strength between electrons and holes. Three-dimensional 6x6 k.p calculations of the QD eigenstates were performed to examine the influence of stress on the polarization-dependent momentum matrix element in varying proximity to the stressors. [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L10.00002: Computational assessment of stimulated Raman adiabatic passage in embedded germanium nanocrystals Ceyhun Bulutay, Deniz Gunceler All-optical coherent population transfer is one of the key instruments of coherent control phenomena which is much needed for quantum information processing. One powerful scheme is the stimulated Raman adiabatic passage (STIRAP). It provides 100\% population transfer in a $\Lambda$ configuration achieved by two counter-intuitively ordered partially overlapping optical pulses which is robust against parameter variations. The major challenge is to achieve STIRAP in the solid-state. In this respect, Ge nanocrystals (NC) embedded in silica possess atomic-like states which is very convenient for coherent control schemes which is at the same time highly scalable and compatible with the mainstream solid-state technology. In this computational work, we provide a realistic assessment of STIRAP in a 4~nm Ge NC. The electronic structure and optical dipole matrix elements are computed using an atomistic pseudopotential Hamiltonian. A multi-level STIRAP scheme is implemented to incorporate the effects of large number of intermediate states which fall into the quasi-continuum part of the Ge NC states. Our extensive study over the parameter space provides working recipes for achieving STIRAP with the commonly available laser sources. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L10.00003: Optical Aharonov-Bohm Effect for type-II InAs Quantum Dots Vincent R. Whiteside, Ian R. Sellers, Bruce D. McCombe, Huiyun Liu The magnetic field dependence of the ground- and excited-state transitions in InAs quantum dots (QD), capped with a strain reducing layer of GaAs$_{0.76}$Sb$_{0.26}$, which results in a type-II band alignment, has been studied by magneto-photoluminescence (magneto-PL) spectroscopy. Oscillations in the PL intensity of both the excited state and ground state magneto-PL are observed. Similar oscillations in intensity have been previously attributed to the Optical Aharonov-Bohm (OAB) effect in other type-II QD systems. In the present case the ground state oscillations are only observed at low excitation power, for which only the ground state is occupied. At higher excitation power the excited state oscillations are more pronounced than the ground state oscillations. The room temperature emission wavelength of these QDs matches the optical telecommunications window at 1.55 $\mu $m, so the OAB effect in these structures may be useful in future devices. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L10.00004: Quasiparticle Gaps and Exciton Coulomb Energies in Si Nanoshells Kimberly Frey, Juan C. Idrobo, Serdar Ogut, Murilo L. Tiago, Fernando A. Reboredo Quasiparticle gaps and exciton Coulomb energies are calculated in Si nanoshells passivated by H at the inner and outer surfaces. We consider spherical nanoshells with inner radii $R_1$ up to 1 nm and outer radii $R_2$ up to 1.6 nm. Quasiparticle gaps are calculated using $\Delta$SCF and GW methods. While the single-band effective mass approximation predicts that the gap should depend only on the thickness $t=R_2 - R_1$ of the nanoshell, we find from first principles calculations that it depends on both $R_1$ and $R_2$. The dependences of the quasiparticle gap on $R_1$ and $R_2$ are mostly consistent with electrostatics of a charged metallic shell. We also find that the (unscreened) Coulomb energy in Si nanoshells has a somewhat unexpected size dependence at fixed outer radius $R_2$. Namely, the exciton Coulomb energy {\em decreases} as the nanoshell becomes more {\em confining}, contrary to what one would expect from quantum confinement effects. We show that this is a consequence of an increase in the average electron-hole distance, giving rise to reduced exciton Coulomb energies in spite of the reduction in the confining nanoshell volume. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L10.00005: Tunable Absorption Spectroscopy of Individual Nanowires Linyou Cao, Justin White, Joonshik Park, Mark Brongersma The optical properties of semiconductor nanowires have recently emerged as a major topic of research largely motivated by their potential for diverse optoelectronic applications. Here we present a combined experimental and theoretical investigation showing that the absorption individual germanium nanowire is tunable over a broad region from visible to near infrared. Close correspondence between the calculated and experimental results indicates structural resonance as mechanism for the tunable absorption. Similar tunable absorption expects to be also in other nanowires as the mechanism of structural resonance is universal. The large tunability of the light absorption, along with the well-acquired synthetic controllability of nanowires , open up a new pathway for the design of high-efficiency and broadband optoelectronic devices. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L10.00006: Ab-initio calculations of optical spectra of silicon nanowires Yuan Ping, Dario Rocca, Giulia Galli We present ab-initio calculations of absorption spectra of thin silicon nanowires (1-2 nm in diameter) and compare the results of different techniques. In particular we aim at assessing the ability of time dependent Density Functional Theory (TDDFT) to describe trends in the electronic properties of Si nanowires, by comparing results obtained within TDDFT with those of the Bethe-Salpeter Equation (BSE). We also discuss the numerical accuracy of both TDDFT and BSE calculations and the influence on computed spectra of several numerical parameters entering the calculations. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L10.00007: Photoluminescence from core/multiple-shell GaAs/AlGAs Nanowires M.A. Fickenscher, S.D. Perera, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, H.J. Joyce, Y. Kim, Q. Gao, H.H. Tan, C. Jagadish We use photoluminescence (PL) and photoluminescence excitation spectroscopy (PLE) to study the electronic structure of GaAs/AlGaAs core multi-shell NWs. Using Au-catalyst assisted MOCVD, a nominally 2~nm GaAs quantum well tube (QWT) with AlGaAs barriers is formed surrounding a central 50~nm GaAs nanowire core. PL measurements on single nanowires reveal a line at the expected exciton energy for the core and, in addition, several higher energy lines not observed in simple core/shell structures. PLE measurements suggest a coupling of the confined states in the QWT and the core states. A broad PLE response centered at 1.67~eV is suggestive of an AlGaAs shell concentration of approximately 12{\%}. We acknowledge the support of the NSF~(0701703 and 0806700) and the Australia Research Foundation. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L10.00008: Polarization Dynamics of Twin Free GaAs/AlGaAs Core-Shell Nanowires S. Perera, L.M. Smith, H.E. Jackson, J.M. Yarrison-Rice, H.J. Joyce, Y. Kim, Q. Gao, H.H. Tan, C. Jagadish, X. Zhang, J. Zou We use polarized time-resolved photoluminescence to study exciton dynamics in GaAs/AlGaAs core-shell nanowires (NWs) at 20~K. By pumping the nanowire with lasers polarized parallel and perpendicular to the nanowire, the polarization dynamics reflect the exciton dipole distributions within the nanowires. The NWs were prepared by Au catalyzed MOCVD and excited by a pulsed titanium-sapphire laser at 798~nm. The polarization of the emitted PL was monitored at the exciton emission peak (1.515~eV) as a function of time after excitation by a polarized pulse. The diameter of the nanowire is much larger than the exciton Bohr radius so that the exciton dipoles are degenerate regardless of orientation; thus in thermal equilibrium the density of excitons parallel and perpendicular dipoles should be equal. At low excitation intensities we find that the excitons are created out of thermal equilibrium, but relax within several hundred picoseconds. At higher excitation powers, the exciton dipoles relax much more rapidly within a time. This suggests that exciton dipole relaxation is very sensitive to carrier-carrier scattering. We acknowledge the support of the NSF (0701703 and 0806700) as well as the Australia Research Foundation. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L10.00009: Polarization dependent photoluminescence studies of InP nanowires Lei Fang, Xianwei Zhao, Fengyuan Yang, Ezekiel Johnston-Halperin Control of the polarization anisotropy observed in measurements of single NWs has the potential to enable both fundamental studies of polarization-sensitive electronic states and potential applications in polarization-sensitive photodetectors. This anisotropy is caused by the large dielectric mismatch between the semiconductor nanowire and the environment (air), which suggests that with appropriate dielectric matching it is possible to minimize or eliminate polarization anisotropy. In order to explore this possibility, we measure the polarization dependence of ensembles of InP nanowires grown by pulsed laser deposition. The measured polarization response of these ensembles correlates well with the straightforward extension of previous models developed to describe single wire measurements. Further, initial studies involving coating InP nanowires with tantalum oxide, whose dielectric constant (5.76 to 8.41) is close to that of InP (9.61), reduces the polarization anisotropy by 20{\%}. These preliminary results will be presented and proposed strategies for more dramatic suppression will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L10.00010: Photocurrent spectroscopy of single InP nanowires A. Maharjan, H.E. Jackson, L.M. Smith, A. Kogan, J.M. Yarrison-Rice, S. Paiman, Q. Gao, H.H. Tan, C. Jagadish We use photocurrent spectroscopy of single InP nanowires at room temperature to study nanowires having either zinc-blende (ZB) or wurtzite(WZ) crystal structures. Photolithography is used to fabricate the Ti/Al metal contact pads separated by 3 microns on several ZB or WZ InP nanowires. The metal- semiconductor-metal contacts are modeled based on thermionic emission and field emission theory. Analysis of the dark I-V characteristics of these devices determines important intrinsic properties including donor density, barrier heights and electrical conductivity. Current-voltage (I-V) photocurrent curves for a nanowire are obtained by broad illumination of the device from a Ti-Sapphire laser with energies ranging from 1.30 eV to 1.55 eV.The photocurrent at a given bias voltage is plotted as a function of photon energy to determine the band edge of given semiconductor nanowire. The photocurrent drops exponentially below the band edge reflecting Urbach's rule. We find that the energy band gaps of wurzite and zinc blende nanowires are 1.42 eV and 1.34 eV respectively at room temperature showing that the energy band gap of wurtzite structure is about $\sim$80 meV higher than zinc blende structure. Supported by the NSF (\# 0701703, \# 0806700 and \# 0804199) and the Australian Research Council. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L10.00011: Photoluminescence Study of Type II ZB-WZ InP nanowire homostructures K. Pemasiri, M. Montazeri, R. Gass, H.E. Jackson, L.M. Smith, J.M. Yarrison, S. Paiman, Q. Gao, H.H. Tan, C. Jagadish, X. Zhang, J. Zou We use CW and time-resolved photoluminescence (TRPL) from single InP nanowires containing both wurtzite (WZ) and zincblende (ZB) crystalline phases to study the quantum confinement of excitons in a Type-II homostructure. We observe strong excitation power dependence, with a change in PL emission energy from the ZB edge to the WZ edge suggesting that the nanowires have mixed phases. TRPL shows a dramatic increase of recombination lifetime from 170 ps for excitons in the continuum above the conduction and valence band barriers to more than 8400 ps for electrons and holes which are strongly confined in quantum potential wells defined by monolayer-scale ZB sections in a predominantly WZ nanowire. Using detailed HR TEM measurements from a 600nm length of a single nanowire, we calculate a complete set of electrons and hole confined states using an eigenfunction expansion method. Analysis of the distribution of electron and hole confined states demonstrates that the observed energy-dependent dynamics are consistent with the type II nature of the confined electron and hole wavefunctions. Supported by the NSF ({\#}0701703 and {\#}0806700) and the Australian Research Council. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L10.00012: Scanning Photocurrent Imaging in CdS Nanosheets P. Kumar, A. Maharjan, M. Fickenscher, H.E. Jackson, L.M. Smith, A. Kogan, J.M. Yarrison-Rice, H. Rho, Y. Lee, Y.J. Choi, J. Choi, J.G. Park We study photocurrent from photoexcitation of charged carriers in a CdS nanosheet (NS) at room temperature. Photolithography followed by Ti/Al (20nm/200nm) metal evaporation was used to fabricate Schottky type contact pads separated by $\sim $4 microns across a $\sim $3$\mu $m wide single nanosheet. Dark current measurements and a model constructed using thermionic emission for the forward biased contact and thermionic field emission for the reverse biased contact is used to extract an intrinsic donor carrier density (N$_{d}$~$\approx $10$^{16}$~cm$^{-3})$, barrier height ($\phi _{b}\approx $~0.8-0.9eV) and depletion layer ($\sim $400nm) for each device. Spatial imaging of the photocurrent exhibits peak photocurrents near the reverse bias contact confirming the confinement of the electric field within the space charge region due to the applied bias voltage. Photogenerated electrons and holes are collected at forward and reverse biased contacts, respectively. Polarization analysis shows that the photocurrent is maximized for laser excitation polarized perpendicular to the c-axis of the nanosheet. Supporte by the NSF ({\#}0701703 and {\#}0806700), Korea Research Foundation and KIST. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L10.00013: Raman Stress Mapping of CdS Nanosheets M. Montazeri, J.M. Yarrison-Rice, L.M. Smith, H.E. Jackson, H. Rho, Y. Lee, Y.J. Choi, J. Choi, J.G. Park We present results of spatially-resolved room temperature second order Raman scattering measurements for single $\sim $3 micron wide CdS nanosheets. The sheets, grown by pulsed laser deposition using vapor-phase transport, are uniform in size and shape and exhibit hexagonal wurtzite structure. The orientation of the c-axis is determined by Raman polarization analysis. Spatially-resolved Raman scattering reveals a stress gradient across the nanosheets, with the 2LO phonon energy at the center of nanosheet being higher by $\sim $2~cm$^{-1}$ with respect to the edges which indicates that nanosheets are relaxed at the edges with a strain gradient toward the center. Support provided by NSF ({\#}0701703 and {\#}0806700), Korea Research Foundation and KIST. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L10.00014: Ultrafast spatially-resolved carrier dynamics in single CdSSe nanobelts Lars Gundlach, Piotr Piotrowiak A recently constructed Kerr-gated microscope was applied to spatially, temporally, and spectrally resolve the ultrafast non-linear excitation relaxation dynamics in single CdSSe nanobelts. Luminescence movies with a 100 fs frame resolution were constructed. The ability to spatially resolve the femtosecond dynamics in a single emitting object gives insights which would be impossible to obtain in an ensemble measurement. By applying the Kerr-gated microscope we are able to monitor the dynamics in a single nanobelt with a sufficient time resolution to reveal the different pathways that compete with the dissociation of multiple excitons. We will show that ensemble averaging methods give results that are complicated because of ensemble inhomogeneities. Indeed, already a different orientation of the nanoparticles with respect to the light-field leads to different dynamic response and difficult to interpret results. The onset of nonlinear behavior and the subsequent dynamics are particularly sensitive to even the most subtle inhomogeneities in composition and morphology and hence most difficult to study under the condition of ensemble averaging making time-resolved wide-field fluorescence microscopy a perfect aid in disentangling the complex response. [Preview Abstract] |
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