Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N44: Focus Session: Optical Properties of Nanowires and Nanocrystals |
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Sponsoring Units: DMP Chair: Greg Salamo, University of Arkansas Room: Colorado Convention Center 507 |
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N44.00001: Electronic and optical properties of single-walled GaN nanotubes from first principles Sohrab Ismail-Beigi There is current experimental interest in fabricating GaN nanotubes for possible optoelectronics/luminescence applications. To date, {\it ab initio} studies of these potentially interesting systems have used ground-state density functional theory which has well-known shortcomings when used to predict electronic excitations. We report on our {\it ab initio} predictions of the electronic and optical properties of single- walled GaN nanotubes using electronic Green's functions within the GW-Bethe Salpeter Equation formalism. We present results the nanotube band structures, optical spectra, excitonic states, and likely luminescence properties. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N44.00002: Optical Second-Harmonic Generation from Single GaN Nanowires. J.P. Long, B.S. Simpkins, D.J. Rowenhorst, P.E. Pehrsson The nonlinear optical response of nanostructured materials is of interest because of the need for active elements in nanophotonic applications, and because the nonlinear response can provide information about the nanostructure itself. Here we report measurements of second-harmonic generation (SHG) from individual GaN nanowires (NWs) based on far-field optical microscopy. By correlating the polarization behavior of the SHG signal from each NW with its orientation as determined with electron backscattered diffraction, we show that far-field methods can provide a flexible approach for distinguishing the crystallographic orientations of wurtzite NWs lying on a substrate. Analysis is based on the quasi-static approximation, which assumes that a NW's transverse dimension (75 nm) is less than the relevant wavelengths and thus permits treating the optical electric-fields as spatially uniform. This approach proves sufficient to explain the main SHG polarization features of these NWs, once one accounts for internal depolarization effects for both the excitation and SH electric fields, and for the collection-aperture of the microscope objective. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N44.00003: Non-Equilibrium Exciton Spin Dynamics in Resonantly Pumped Single Core-Shell GaAs-AlGaAs Nanowires Thang B. Hoang, L.V. Titova, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, A.O. Govorov, Y. Kim, H.J. Joyce, H.H. Tan, C. Jagadish We use spatially-resolved photoluminescence (PL) imaging in combination with polarized resonant excitation to investigate the non-equilibrium exciton spin states in single core-shell GaAs-AlGaAs nanowires ($\sim $40 nm core diameter) at low temperature. The large dielectric mismatch between the nanowire and the vacuum results in a strong polarization of excitonic dipoles in the nanowire. This leads to strong polarization of both exciton excitation and emission along the nanowire. Resonant excitation shows two resonances at 1-LO and 2-LO phonons of GaAs and a third resonance likely from electronic states of the AlGaAs. More interestingly, we observe that the polarization of the PL emission is strongly enhanced as the excitation energy comes closer to resonance with the exciton emission. This strong polarization enhancement indicates that resonant excitation creates non-equilibrium exciton spin distributions near resonance. Rate equation modeling allows us to estimate the spin relaxation times which range from $\sim $5ps at high energies to $\sim $50ps at energies close to resonance. Financial support for this work was provided by the University of Cincinnati, Ohio University and the Australian Research Council. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N44.00004: Photoluminescence dynamics of single InP nanowires L.V. Titova, A. Mishra, Thang B. Hoang, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, H.J. Joyce, Y. Kim, Q. Gao, H.H. Tan, C. Jagadish We use time-resolved photoluminescence (PL) spectroscopy to study exciton dynamics in single InP nanowires prepared by catalyst-assisted vapor-liquid-solid growth. In contrast to other III-V materials like GaAs, InP has a lower surface recombination velocity, which should result in longer excitonic lifetimes and higher quantum efficiencies. Indeed, the InP nanowires exhibit emission lifetimes ranging from 80 ps to 2 ns compared to $<$80 ps lifetimes observed for GaAs/AlGaAs nanowires. The large variation in the lifetimes from nanowire to nanowire may be the result of structural inhomogeneities and defects that act as nonradiative recombination centers, thus limiting the excitonic lifetimes. In addition, we have observed changes in the recombination dynamics for single InP nanowires as a function of energy. On the high energy side of the PL peak, the recombination rate is rapid ($\sim $50 ps), while on the low energy side it is significantly slower (up to 1 ns) due to the spectral diffusion of carriers. Preliminary polarization measurements show rapid depolarization of the PL during the $\sim $75 ps emission risetime due to spin scattering of excitons. Financial support for this work was provided by the University of Cincinnati and the Australian Research Council. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N44.00005: Low temperature photoluminescence of single InP nanowires A. Mishra, D. Shereen, Thang B. Hoang, L.V. Titova, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, H.J. Joyce, Y. Kim, Q. Gao, H.H. Tan, C. Jagadish We investigate low-temperature optical emission properties of a number of single InP nanowires prepared by catalyst-assisted vapor-liquid-solid growth. Photoluminescence (PL) spectra of the nanowires display a broad (full width at half maximum of 27 $\pm $ 3 meV) peak centered at 1.414 $\pm $ 0.008 eV, often accompanied by a broader lower energy shoulder at $\sim $ 1.378 $\pm $ 0.008 eV. The variability in energy of the main peak, which may correspond to bandgap emission, may be explained by structural and compositional variations and non-uniformities of the nanowires. The origin of the lower energy emission is not clear but is likely related to the defect states. We find the bandgap emission as well as excitation of InP nanowires to be strongly linearly polarized along the nanowire axis with a degree of polarization which varies significantly (45{\%} - 85{\%}) from wire to wire. We anticipate that polarization-sensitive single nanowire excitation spectroscopy will reveal information about spin dynamics in these nanostructures. Financial support for this work was provided by the University of Cincinnati and the Australian Research Council. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N44.00006: Temperature dependence of photoluminescence from single InP nanowires D. Shereen, A. Mishra, Thang B. Hoang, L.V. Titova, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, H.J. Joyce, Y. Kim, Q. Gao, H.H. Tan, C. Jagadish Temperature-dependent micro-photoluminescence (PL) measurements have been carried out to study electronic states of InP nanowires prepared by catalyst-assisted vapor-liquid-solid growth. Low temperature PL spectra of the nanowires show a broad (full width at half maximum of 27 $\pm $ 3 meV) near bandgap peak centered at 1.414 $\pm $ 0.008 eV, in some cases accompanied by a weak defect-related lower energy shoulder at $\sim $ 1.378 $\pm $0.008 eV. Unlike the emission from GaAs/AlGaAs nanowires, which quenches at temperature 120 K due to the presence of large number of non-radiative centers, the PL from the single InP nanowires persists up to room temperature. With increasing temperature, the emission broadens and redshifts. The emission intensity is nearly constant at low temperatures, and begins to quench at 50K. We compare this data with data obtained from epitaxial InP layers grown under similar conditions. Strong linear polarization of the nanowire emission in the direction of nanowire axis is observed in the entire temperature region (8 K -- 270 K). Financial support for this work was provided by the University of Cincinnati and the Australian Research Council. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N44.00007: Synthesis and Raman Scattering from DMS Zn1-xMnxS Nanowires Jian Wu, Humberto Gutierrez, Peter Eklund Diluted Magnetic Semiconductor (DMS) have attracted a lot of attention in the field of Spintronics. Here, we report on our progress to grow DMS Zn1-xMnxS (0$\le $x$<$0.6) nanowires using the vapor-liquid-solid growth mechanism and CVD source techniques based on sublimation of ZnS and MnCl2 powder. Ar/H2 carrier gas was passed over ZnS and MnCl2 maintained at specific temperatures to control the Zn/Mn ratio in the stream. The Zn/Mn concentration also is found to determine the structure, i.e., wurtzite vs zinc blende. HRTEM measurements show that the nanowires are single crystal and the composition was determined by EDS. Raman scattering was performed to study the vibrational properties of these nanowires vs. x. With increasing x, the TO band disappears quickly and cannot be observed; the LO band, however, is found insensitive to x. Between the TO and LO bands, there are three additional peaks appear. Two bands associated with zone edge modes (LA branch) are observed; one upshifts and the other downshifts with x. When possible, our Raman data on nanowires will be compared to that of the bulk. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N44.00008: Optical imaging of oscillating nanowires Dan Hessman, Monica Lexholm, Kimberly Dick, Sara Ghatnekar-Nilsson, Lars Samuelson Systematic investigations of the mechanical properties of semiconductor nanowires require new measurement techniques. In this paper, we present a stroboscopic imaging technique using an optical microscope, capable of tracking the bending and oscillation of a nanowire in space and time. Due to the ideal shape of the nanowires, their position within an image may be determined with a precision given by the signal-to-noise ratio rather than by the optical resolution. We demonstrate an accuracy below 1 nm, more than two orders of magnitude better than the diffraction limit. Temporal information is obtained stroboscopically using a pulsed LED as a light source. The time-resolution is given by the width of the light pulses which in our experiments is below 100 ns. The nanowires are electrostatically bent by applying a voltage between the nanowire and a nearby W-needle. By applying voltage pulses we induce damped oscillations and by applying a sinus voltage we drive the nanowire at varying frequency. In both cases we get resonance frequencies of a few MHz for nanowires about 100 nm in diameter and 5 $\mu$m long. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N44.00009: Optical Absorption in Small Diameter Si Nanowires Kofi Adu, Gugang Chen, Humberto Guti\'errez, Peter Eklund Optical absorption spectra on 3 sets of Si nanowires with most probable diameter 3.5 nm, 5.5 nm and 9 nm are presented. In the optical absorption, apart from the direct gap absorption at E$_{\Gamma 1} \quad \sim $ 3.4 eV and E$_{\Gamma 2 }\sim $ 4.2 eV, we observed two additional strong absorption bands near 1.5 eV and $\sim $2.5 eV. Interestingly, these lower energy features are not expected on the basis of the bulk dielectric function of Si. They are observed experimentally to increase in intensity with decreasing nanowire diameter. It is also interesting that the 1.5eV peak does not shift with decreasing wire diameter. This behavior leads us to tentatively assign this structure to SiO2:Si interface states. On the other hand, the $\sim $2.5 eV absorption band exhibits a systematic blue shift with decreasing diameter. Many of the features in the experimental absorption spectrum will be explained on the basis of dielectric function calculations carried out in the discrete dipole approximation (DDA). The DDA model results are shown to depend on the nanowire diameter/length and the thickness of amorphous SiO2 shell. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N44.00010: Short-wavelength visible light emission from silicon nanocrystals Xiaodong Pi, Rick Liptak, Stephen Campbell, Uwe Kortshagen Si is the material of choice for modern microelectronics but, as an indirect-bandgap semiconductor, it is not an efficient light emitter. An electrically pumped Si laser would present a breakthrough for optoelectronic integration that may enable optical interconnect to make computers faster. Si light emitting diodes may revolutionize solid-state lighting and displays because of the low cost and environmental friendliness of Si. One of the most challenging problems of Si-based lighting and displays is the lack of a reliable and efficient full visible spectrum emission. Si nanocrystals (Si-NCs) have so far been the most promising form of Si to emit light. Most of the synthesis approaches of Si-NCs, however, only lead to red light emission. Our recent work on Si-NCs synthesized by non-thermal plasmas has focused on extending their light emission into the short-wavelength range. Firstly, the process of oxidation-etching-oxidation of Si-NCs is investigated. This process causes the size of Si-NCs to decrease, leading to shorter wavelength light emission from Si-NCs. Yellow or green photoluminescence (PL) has been observed from initially oxidized red light emitting Si-NCs after HF vapour etching and atmospheric oxidation. The intensity of PL from Si-NCs, however, decreases by a factor up to 100. It is found that HF etching restructures the surface of Si-NCs. This leads to a decrease in the incorporation of O during subsequent oxidation, which finally results in silicon suboxide SiO$_{1.9}$. Such an understoichiometry indicates a high density of defects such as Si dangling bonds at the Si-NC/oxide interface. Therefore, the PL efficiency is extremely low for short-wavelength light emitting Si-NCs obtained by the process of oxidation-etching-oxidation. Secondly, an integrated two-stage plasma system is employed to achieve the light emission from Si-NCs in the full visible spectrum range. Red-light-emitting Si-NCs are produced in the first stage by the plasma decomposition of SiH$_{4}$. In the second stage a tetrafluoromethane (CF$_{4})$-based plasma etches Si-NCs and at the same time passivates them with carbon and fluorine. After the two-stage process Si-NCs emit light in the short-wavelength region from yellow to blue. We find that a self-limited oxidation process blueshifts the light emission until saturation is reached. Significantly, relatively high quantum yields of short-wavelength light emission from Si-NCs are obtained in spite of oxidation. It is interesting to note that Si-NCs treated by CF$_{4}$-based plasma are hydrophilic while those without CF$_{4}$-based plasma treatment are hydrophobic. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N44.00011: Dynamical and Optical Properties of Si and Ge Nanocrystals Kelly Knutsen, Matt Beard, P.R. Yu, Qing Song, Wyatt Metzger, Art Nozik, Randy Ellingson Si nanocrystals exhibit the unusual property of having a high photoluminescence quantum yield as well as a long first exciton lifetime. This implies that the decay rates for the non-radiative channels have decreased compared to the bulk. We explore this phenomenon by first characterizing the optical properties of the Si nanocrystals by measuring their linear absorption and photoluminescence spectra as a function of nanocrystal size, which show an expected shift to the blue for the transition onset with decreased particle size. The nanoparticles exhibit indirect transition characteristics, and emit roughly 1eV to the red of the absorption onset. We also employ time-resolved photoluminescence (TRPL) and transient absorption (TA) spectroscopy to investigate the Auger dynamics of the single and biexcitons. Initial results for 9nm Si nanoparticles show that the biexciton lifetime is roughly 200 ps and the single exciton lifetime is greater than 200 microseconds. The size dependence on the single and biexciton lifetimes, as well as the potential presence of multiple exciton generation (MEG) in these materials will be presented. Initial optical studies of Ge nanocrystal charge carrier dynamics will also be presented. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N44.00012: Visible light absorption and photodarkening in Te-modified TiO$_{2}$ nanocrystals Steven Phillips, Ian James, Bret Hess Applications of titanium dioxide nanocrystals in solar cells and solar photocatalysis are limited by the lack of visible light absorption. We have created TiO$_{2}$ nanocrystals modified by tellurium, which causes absorption in the visible. In TiO$_{2}$:Te nanocrystals annealed between 300 and 600 C, light exposure quickly causes the visible absorption to increase until the sample is reddish-brown. The presence of Te stabilizes the anatase structure, while the rutile phase is found in undoped nanocrystals. We discuss possible mechanisms for the photosensitivity, and explore whether this visible absorption is useful in photocatalysis. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N44.00013: Density Matrix Approach for Valence Band Optical Properties M.P. Prange, J.J. Rehr We present an extension of the {\it ab initio} real-space multiple-scattering (RSMS) theory currently used for core-level spectra (e.g. EELS, XAS and NRIXS) to calculations of the valence band optical response. The method is based on RSMS calculations of the occupied and unoccupied density matrices and transition matrix elements between the two resulting in an efficient way to calculate various optical constants in aperiodic materials. In contrast to bandstructure or basis-set methods, the calculation can be applied to a large class of materials including both insulators and metals up to the nanoscale. By combining the method with the RSMS approach for core level response, we obtain an approach applicable for spectra from the far IR to x-rays. Results are compared with experiment and with other theoretical techniques. Possible extensions are also discussed. [Preview Abstract] |
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