Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U22: Optical Properties of Nanowires |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Joe Tischler, Naval Research Laboratory Room: 324 |
Thursday, March 21, 2013 11:15AM - 11:27AM |
U22.00001: Exciton Dynamics in Hexagonal InP Nanowires Masoud Kaveh-Baghbadorani, Wolfgang Langbein, Qiang Gao, Chennupati Jagadish, Hans-Peter Wagner We study the exciton dynamics in InP nanowire ensembles by intensity- and temperature-dependent photoluminescence (PL) measurements, time-correlated-single-photon-counting (TCSPC) and heterodyne detected four-wave-mixing experiments (HFWM). The InP nanowires were grown on fused silica substrate by 50 nm gold catalyst metal-organic-vapor-phase-epitaxy at a temperature of 450 $^{\circ}$C resulting in nearly wurtzite type nanowires. The PL measurements at 15 K show a strong emission band at 837 nm and two weak side bands at nearly 820 and 860 nm. The bands are tentatively attributed to trapped, free and zinc-blende related exciton transitions, respectively. With increasing temperature the free-exciton band gains importance relative to the dominating trapped exciton band while the low energy band vanishes. TCSPC measurements show an increasing PL decay rate of all emission bands with increasing temperature most pronounced for the low energy band. The result agrees with the exciton population dynamics obtained from three-beam HFWM measurements. Photon echo experiments at 80 K reveal an ultrafast exciton dephasing time of less than 100 fs which is attributed to scattering with a high carrier background in these nanowires. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U22.00002: Time Resolved Photoluminescence Studies of ZnO and Zn$_{2}$SnO$_{4}$ Nanowires for Solar Cells Applications Baichhabi Raj Yakami, Meg Mahat, Jiajun Chen, Liyou Lu, Wenyong Wang, Jon M. Pikal Sensitized nanowires (NWs) are a promising option for solar cells. They serve as the support structure for the absorbing centers, provide interfacial charge separation, and transport to the anode. Most work has focused on binary oxides, but ternary oxides have advantages due to flexibility in the properties of the oxide. Here we report on the photoluminescence (PL) and Time Resolved PL (TRPL) of Zinc oxide (ZnO) and Zinc Tin Oxide (ZTO) NWs grown by Chemical Vapor Deposition. The ZnO NWs show strong band gap emission and weak but resolvable defect emission peaks. The PL from the ZTO NWs does not show any band gap emission and absorption measurements confirm that these NWs have a direct forbidden transition. The ZTO NWs do have a board visible emission peak, which is usually attributed defects and oxygen vacancies. TRPL of the band gap emission in ZnO NWs yield a carrier lifetime of 1.4ns. The TRPL of the defect peaks in ZTO NWs are more complicated, showing a multi-exponential decay but with an overall decay rate similar to the ZnO NWs. This indicates that the expected increase in carrier lifetime in the ZTO NWs is not currently realized likely due to defect recombination, and additional optimization of the NW growth process may yield improved performance. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U22.00003: Picosecond carrier dynamics within the band structure of single InP nanowires with zincblende and wurtzite symmetries M. Montazeri, Y. Wang, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, T. Burgess, H.H. Tan, Q. Gao, C. Jagadish Low temperature transient Rayleigh scattering spectroscopy (TRS) is used to probe the carrier dynamics of single zincblende (ZB) and wurtzite (WZ) InP nanowires (NW). The NWs were MOCVD grown using 50 nm Au-nanoparticles. For ZB NWs, the TRS signal reveals various dynamical processes of the electrons within the conduction band as well as the holes in the degenerate heavy/light bands and the split-off band. The fundamental and the split-off band gaps are measured at 1.423eV and 1.529eV. For WZ NWs, we observe three excitonic resonances associated with the hole bands A at 1.501eV, B at 1.534eV and C at 1.66eV. We also observe clear transitions between the same A and B bands and the second conduction band, resulted from zone folding of the L-valley, which is measured at $\sim$ 230meV higher than the first. The lifetimes of the A, B and C excitons at $\sim$ 800ps, $\sim$ 400ps and $\sim$ 50ps respectively. In addition, a type II transition between electrons confined to small zincblende inclusions and holes confined to the wurtzite is identified which marks the ZB-WZ band-offset. We acknowledge the NSF (DMR-1105362, 1105121), ECCS-1100489 and the ARC. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U22.00004: Electronic and optical properties of InN nanowires from first principles Dylan Bayerl, Emmanouil Kioupakis Group-III-nitride nanowires are promising materials for photovoltaic and solid-state-lighting applications. We use first-principles calculations to investigate the electronic and optical properties of InN nanowires. Density functional theory provides the ground-state properties to which we subsequently apply quasiparticle corrections with the GW method. We thereby accurately predict the electronic band gaps, effective masses, and band dispersions of these nanostructured materials. We further solve the Bethe-Salpeter equation to predict the optical absorption spectra of InN nanowires as a function of cross-sectional dimension and geometry. We demonstrate that quantum confinement can increase the fundamental gap in InN nanowires as high as near-ultraviolet energies. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U22.00005: Stark Effect of Excitons in a Quantum Nano-rod with Parabolic Confinement S.K. Lyo We study the exciton binding energy and the oscillator strength as a function of a DC electric field in a quasi-one-dimensional quantum dot (\emph{i.e.}, nano rod) with parabolic confinements in the conduction and valence bands. The relative importance of the quantum confinement and electron-hole interaction is examined by varying the the linear confinement length (\emph{i.e.}, rod length). We find an abrupt decrease of the oscillator strength, loss of exciton binding energy, and a sudden increase of the root-mean-square average of electron-hole separation as the excitons are dissociated at the threshold field. The field dependence of the effects are also investigated as a function of the rod length and the radius of the nano rod. The numerical results are applied to GaAs and CdSe rods. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U22.00006: Observation of quantum dots in GaAs/AlGaAs core-multishell nanowire quantum well tubes Teng Shi, Howard Jackson, Leigh Smith, Jan Yarrison-Rice, Changlin Zheng, Peter Miller, Joanne Etheridge, Bryan Wong, Qiang Gao, Hark Tan, Chennupati Jagadish We use photoluminescence excitation (PLE) spectroscopy to study the electronic structure of GaAs/Al$_{x}$Ga$_{1-x}$As core-multishell nanowires (NW) which define 4 nm GaAs quantum well tubes (QWTs) embedded inside AlGaAs barriers wrapped around a central 50 nm GaAs core. HAADF-STEM images of NW cross-sections show a GaAs layer wrapped around the hexagonal facets with some tapering. Numerical calculations of this structure show the ground states are localized along the corners of the hexagonal QWT. Because of the strong quantum confinement, localized states can easily be formed through width or alloy concentration fluctuations. By using a hemispherical solid immersion lens, we are able directly observe such localized quantum dots (QDs) and map the emission of QDs with a spatial resolution of 600 nm in a single NW. Excitation and emission light polarized parallel and perpendicular to the NW long axis show multiple QDs along the NW long axis with $\sim$100 micro-eV emission lines. PLE measurements on single dots reveal excited state transitions between confined light or heavy holes to electrons at or above the AlGaAs conduction band barrier. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U22.00007: Photocurrent Spectroscopy of ZB and WZ InP Nanowire Ohmic devices K. Pemasiri, S. Perera, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rise, S. Paiman, Q. Gao, H.H. Tan, C. Jagadish We use photocurrent spectroscopy to study InP nanowire Ohmic devices having either Zincblende (ZB) or wurtzite (WZ) crystal structures at 10K. Photolithography is used to fabricate Ohmic Ti/Al contact pads separated by 5 $\mu$m. Using a tunable Ti-Sapphire laser, photocurrent is measured as a function of bias voltage and excitation energy. At low temperatures (10 K), the ZB device shows strong evidence for excitonic resonance peaks at 1.425eV and 1.539eV relevant to the degenerate heavy and light holes band and the split-off band. The WZ device shows three excitonic peaks at 1.504eV, 1.530eV, and 1.655eV corresponding to the A, B and C valence band energies, respectively. These energies coincide with recent photoluminescence excitation measurements. In some WZ InP nanowire devices, the A, B and C peaks have been observed at 20-30meV higher energies compared to above, suggesting a possible quantum confinement in the nanowires. The polarization dependence of photocurrent spectra measured from 275nm ZB nanowire and 20nm WZ nanowire shows very good agreement with theoretical absorption of light by nanowires as a function of diameter and photon energy. We acknowledge the NSF through DMR-1105362, 1105121 and ECCS-1100489, and the ARC. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U22.00008: Photocurrent spectroscopy of GaAs/GaP hetero-structured nanowires P. Kumar, H.E. Jackson, L.M. Smith, J. Yarrison Rice, J.H. Kang, Q. Gao, H.H. Tan, C. Jagadish We study the photocurrent from photoexcited charge carriers in GaAs/GaP axial and radial hetero-structured nanowires (NWs). These NWs are grown using Metal-Organic Chemical Vapor Deposition (MOCVD) in [111]B direction with Au nano-particles as catalysts. As grown axial GaAs/GaP NWs are sonicated in methanol and dispersed on Si-SiO insulated substrate. Photolithography followed by Ti/Al (20nm/300nm) metal evaporation and lift-off is used to fabricate contacts in Metal-semiconductor-metal across single NW. Spatial imaging of photocurrent at different wavelengths distinguishes the GaP and GaAs regions in these NWs. Peak photocurrent is observed around GaP region for light wavelengths $\sim$ 458nm whereas peak photocurrent is shift towards GaAs region for light wavelength $\sim$ 800nm. Photocurrent measurements in GaAs/GaP strained core-shell NWs are in progress. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U22.00009: Photocurrent spectroscopy of single ZB GaAs and GaAs/AlGaAs core-shell nanowires Bekele Badada, Leigh Smith, Howard Jackson, Jan Yarrison-Rice, Tim Burgess, Chennupati Jagadish We investigate the band structure of single ZB GaAs nanowires and GaAs/Al$_{0.5}$Ga$_{0.5}$As core shell nanowires using photocurrent spectroscopy at room and low temperatures. The single nanowire devices were fabricated photolithographically to define Ti (20nm)/Al (300nm) metal contacts on either end the nanowire. Photocurrent measurements were performed using CW excitation from a tunable CW Ti-Sapphire laser (775nm-890nm) and a broadly tunable (550-960 nm) pulsed excitation from a coherent super continuum photonic crystal fiber. At room temperature we observe an Urbach tail near the absorption edge at 1.42 eV for both GaAs and GaAs/ Al$_{0.5}$Ga$_{0.5}$As core-shell nanowires. In the core shell structure, we also observe the exponential tail from the Al$_{0.5}$Ga$_{0.5}$As superimposed on the GaAs absorption in the core. The 2eV onset is consisant with 50{\%}. At low temperature, 10K, similar measurements were performed and a peak is observed near the band edge $\sim$ 1.50-1.51 eV for both bare and core-shell structure for GaAs reflecting the contribution of excitons to the photocurrent. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U22.00010: Strain-induced piezoelectric field effects on the optical properties of ZnO nanowires Wenhao Guo, Shuigang Xu, Ning Wang, M.M.T. Loy, Shengwang Du In our work, we report the evidence of piezoelectric effects which modifies the spatial distribution of the photo-generated carriers in bent ZnO nanowires. This piezoelectric effect, together with strain-induced changes of the energy band structure, results in redshift of free exction photo-luminescence emission in strained ZnO nanowires. The net redshift is only dependent on the strain, independent on the diameter of the nanowire unless the depth of depletion layer is comparable to the size of nanowire. The experimental results obtained by the near-field scanning microscopy agree well with our numerical simulation. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U22.00011: Tip-enhanced Raman scattering of an InGaN/GaN quantum well on a single GaN nanorod Emanuele Poliani, Markus Wagner, Axel Hoffmann, Janina Maultzsch, Juan Sebastian Reparaz, Martin Mandl, Werner Bergbauer, Martin Strassburg Vertical GaN nanorods with double In0.2Ga0.8N/GaN quantum well were studied by tip-enhanced Raman spectroscopy (TERS). Exploiting the spatial resolution below the diffraction limit, we were able to perform a Raman map of the nanorod top part with 35 nm spatial resolution. Undetectable in the far field, enhanced phonons belonging to InGaN, InN rich regions and GaN were detected and analyzed as Raman shift map. These enhanced spatially resolved phonons revealed an Indium cluster region nucleated at the end of a planar dislocation in the GaN core. The dislocation continues inside the cluster area as an interface between zinclende and wurtzite modification. A narrow localized strain zone was found close to the interface on the zinc blende side surrounding material. On the wurtzite side instead, the Raman map of the GaN surface optical phonon revealed a more extended charge depletion region. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U22.00012: Raman Spectroscopy on GaAs/GaP Nanowire Axial Heterostructures Yuda Wang, Mohammad Montazari, Leigh Smith, Howard Jackson, Jan Yarrison-Rice, Qiang Gao, Jung-Hyun Kang, Chennupati Jagadish We use Raman scattering to study the spatially-resolved strain and stress in Zinc Blende GaAs/GaP axial heterostructure nanowires at room temperature. The nanowires are grown by Metal-Organic Chemical Vapor Deposition in the [111] direction with Au nano particles as catalysts. After initial growth of a 6 $\mu $m-long GaP wire, a short GaAs segment is grown. Since Raman scattering reflects the phonon energies that are in turn related to the stress, we control the polarization of the incident and scattered light to acquire and resolve the TO1 (Transverse Optical) and TO2 phonon modes of both GaAs and GaP. High spatial resolution Raman scans along the nanowires show that the GaAs/GaP interface is clearly identifiable. Within the GaP section of the wire, GaP TO modes are observed at lower energies compared to bulk GaP since it is under tension, while GaAs shell TO modes are at higher energies than bulk GaAs since it is under compression. A strain gradient exists across the interface so the GaP phonon energies shift to lower and GaAs phonon shift to higher energies as one approaches the interface. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U22.00013: Anisotropic surface plasma resonance in self-assembled ErSb quantum nanostructures of tunable shape and orientation Daniel Ouellette, Hong Lu, Sascha Preu, Justin Watts, Ben Zaks, Mark Sherwin, Arthur Gossard Incorporation of erbium during MBE growth of GaSb leads to various self-assembled, semi-metallic ErSb nanostructures. At the lowest concentration, spheres of diameter 4-5 nm are observed. By contrast, at 7-10$\%$ Er, $\sim$5 nm diameter nanowires self-align along the $<001>$ growth direction, and at 15-20$\%$, the nanowires align in the growth plane along the $<\overline{1}10>$ direction. Light polarized along the wires is strongly attenuated over a broad range from THz to near-IR. By contrast, light polarized perpendicular to the wires experiences minimal attenuation apart from a very strong surface plasma resonance at 0.46 eV. Surprisingly, the resonant frequency of the nanospheres is slightly higher than that of the wires, despite the smaller depolarization factor. Motivated by this observation and estimates of the confinement energy, we construct an effective medium theory for the nanostructures which includes a single characteristic intersubband transition. This model provides an excellent description of the IR reflectance and transmittance over the whole range of Er concentration, in contrast to a model which excludes the effect of quantum confinement. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U22.00014: Continuous frequency multiplication in a strongly driven modulated nanowire Kathleen Hamilton, Alexey Kovalev, Amrit De, Leonid Pryadko High-order harmonic generation in a bulk solid strongly driven by a few-cycle pulsed infrared laser has recently been observed [1]. We consider the possibility of observing an analogous effect using a continuously driven, single-band one-dimensional metal. In the absence of phonon scattering, the quantum efficiency of frequency tripling for such a system can be as high as $93\%$. Combining the Floquet quasi-energy spectrum with the Keldysh Green's function technique, we derive the quantum transport equation for strongly and rapidly driven electrons in the presence of weak scattering by phonons. The power absorbed from the driving field is continuously dissipated by phonon modes, leading to a quasi-equilibrium in the electron distribution. We assume terahertz frequency range, and use the Kronig-Penny model with varying effective mass to establish dimensions and modulation periodicity of an InAs/InP nanowhisker. Driving such nanowhiskers could lead to efficient third and higher-harmonic generation. [1] S. Ghimire et al., Nature Physics 7, 138-141 (2011). [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U22.00015: Generation of core-shell structures and segregation of dopants in Si/SiO$_2$ nanowires Sunghyun Kim, Ji-Sang Park, K.J. Chang Oxidized Si nanowires (SiNWs) are usually synthesized by subsequent thermal annealing of as-grown SiNWs. It has been observed that B diffusivity is enhanced during thermal annealing in SiNWs, similar to the phenomena called transient enhanced diffusion or oxidation enhanced diffusion in planar Si/SiO$_2$ interfaces. However, previous theoretical studies have been focused on hydrogen or hydroxyl terminated SiNWs. In this work, we generate realistic atomic models for oxidized SiNWs in which crystalline Si core is sheathed by amorphous SiO$_2$ by using a combined approach of classical molecular dynamics simulations with first-principles density functional calculations. For realistic core-shell structures, we investigate the stability and segregation behavior of B and P dopants. A single substitutional B is more stable in the Si core, with a very small energy variation with the radial position of B. On the other hand, B dopants easily segregate to the oxide shell with the aid of Si self-interstitials generated during thermal oxidation. In contrast to B dopants, P dopants prefer to reside in the Si core even in the presence of Si self-interstitials but tend to aggregate in the Si region near the interface, forming nearest-neighbor donor pairs which are electrically inactive. [Preview Abstract] |
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