Session H24: Focus Session: Optical Properties of Nanostructures III: Functional Nanowires

Polarized Photouminescence from Single Wurtzite and Zincblende InP Nanowires

We use polarized photoluminescence spectroscopy of single InP nanowires to compare the optical properties of vapor-liquid-solid growth of single zincblende (ZB) and wurtzite (W) nanowires. Since ZB and W nanowires have different symmetries and selection rules, their optical properties should also be different. The emission from single W nanowires is observed to be $\sim $80 meV higher than for ZB nanowires. Low temperature polarization measurement shows that ZB nanowires are strongly polarized along the nanowire axis, while the W nanowires are polarized perpendicular to the NW axis. The temperature dependence of the ZB and W NW emissions are compared with a bulk InP epilayer. Apart from the 80meV shift in bandgap, the temperature dependencies are similar.   

Investigation of the Electronic Structure of GaAs/AlGaAs Core Multi-Shell Nanowires

We use photoluminescence and PL excitation spectroscopy to study the electronic structure of GaAs/Al$_{x}$Ga$_{x}$As core multi-shell NWs. Using Au-catalyst assisted MOCVD, a 10 nm GaAs quantum well tube (QWT) with AlGaAs barriers is formed surrounding a central $\sim $50 nm GaAs core. With resonant excitation at 780 nm emission is seen from both the core as well as the QWT. The QWT emits in a narrow intense peak $\sim $22 meV above the exciton emission from the core suggesting quantum confinement in a 10 nm quantum well. This QWT emission exhibits a $\sim $600 ps recombination lifetime, while the core decays in $\sim $1 ns. Preliminary PLE measurements exhibit possible excited state structure of this novel quantum-confined nanostructure.   

Photoluminescence Dynamics of GaAs/AlGaAs Core-Shell Nanowires

We use time-resolved PL spectroscopy to study the exciton dynamics of GaAs/AlGaAs core-shell nanowires (NWs) at 20 K. NWs were prepared by Au catalyst-assisted MOCVD. PL emission from single NWs exhibits an excitonic peak at $\sim $1.515~eV. The exciton lifetime depends on the morphology and crystallographic defect density of the GaAs core, which are in turn dependent upon the growth conditions. Nanowires cores grown at higher temperatures (450~~C) give short exciton lifetimes ($<$100~ps). Reducing defects within the nanowire (twinning) or at the interface should increase exciton lifetime and improve luminescence efficiency. Indeed, twin-free minimally tapered nanowires achieved using a low growth temperature (375~~C), exhibit high quantum efficiency with an exciton lifetime approaching 1.6~ns at 20 K.   

Ultrafast dynamics in semiconductor nanowires

Semiconductor nanowires (NW) have recently attracted much interest due to their novel electronic and optical properties along with their potential for device applications in areas including nanoscale lasers and thermoelectrics. However, the further development and optimization of NW-based devices will depend critically on an understanding of carrier relaxation in these unique nanostructures. Here, we present the first all-optical time-resolved measurements of carrier dynamics in free standing semiconductor nanowires. Optical pump-probe measurements on GaN NW reveal a rapid transfer ($<$500 fs) of photoexcited carriers into states responsible for deleterious yellow luminescence, which can be modified by varying the growth and annealing temperatures. Polarization, angle, and wavelength-resolved measurements on vertically aligned Ge NW allow us to independently measure electron and hole dynamics parallel and perpendicular to the NW axis. Carriers propagating parallel to the NW axis have significantly longer lifetimes, clearly demonstrating the influence of two dimensional confinement on carrier dynamics in semiconductor nanowires.   

Optical Switching of Porphyrin-Coated Silicon Nanowire Field Effect Transistors

We study [1] porphyrin coated silicon nanowire field effect transistors, which display a large, stable and reproducible conductance increase upon illumination. The efficiency and the kinetics of the optical switching are studied as a function of gate voltage, illumination wavelength and temperature. The decay kinetics from the high- to the low-conductance state is governed by charge recombination via tunneling, with a rate depending on the state of the SiNW-FET. The comparison to porphyrin sensitized Carbon Nanotube FETs allows to distinguish the environment- and molecule-dependent photoconversion process from the charge-to-current transducing effect of the semiconducting channel. The spectral dependence of the photoconductance agree with the UV-visible absorption spectrum of the isolated molecule [1] C. Winkelmann et al, Nano Lett, vol. 7 , p. 1454 (2007).   

Spectral dependence of thermal radiation from metallic nanowires on wire geometry

We report polarization-sensitive thermal radiation measurements of individual platinum nanoheaters (nanowires) as a function of their length, width and temperature. The heaters confine lateral extent of the resistively heated area to dimensions smaller or comparable to the emission wavelengths within our sensitivity range (2-5 $\mu$m). Spectra taken by Fourier Transform Infrared Spectrometry reveal strong suppression of radiation polarized perpendicular to the heater long axis as the heater width shrinks, while at the same time radiation polarized along the long axis of the heater approaches a constant value, resulting in highly polarized emission for heaters with very narrow width. We also observe a $\lambda/2$-like resonance that we believe is associated with surface plasmon oscillations across the heater width. These findings have important implications for nanoscale thermal light generation.   

Anisotropic plasmon excitation and dispersion of Ag nanowires on Cu(110)

Epitaxial Ag nanowires have been found to self-assemble on Cu(110) exceeding 1.2 ML. The plasmon excitation and dispersion of these nanowires have been characterized by low-energy reflection EELS. Previous STM images reveal that the Ag nanowires are approximately 2 nm ($\sim $12 nm) in height (width). However, the nanowires orientate with the long axis parallel to the [\={ }110] substrate direction and posses an anisotropic morphology. EELS reveals that the Ag plasmon excitation of 3.7 eV at the zone-center and is nearly dispersionless perpendicular to the nanowire direction. However, parallel to the Ag nanowires, EELS shows a slight red-shift of the plasmon at q = 0 and disperses to higher energy with increasing momentum transfer. These results will be discussed in light of recent ARPES band-structure measurements, electronic calculations, and anisotropic optical measurements of the Ag nanowires.   

Enhanced Raman Scattering Near the Tip of Semiconducting Nanowires

Results of polarized microRaman scattering experiments are presented on individual $\sim $20$\mu $m long crystalline GaP zinc-blende nanowires (NWs) as a function of the probe beam position along the wire. The probe beam had a spot size of $\sim $0.7$\mu $m. The NWs were characterized by TEM lattice images and selected area diffraction (SAD) patterns. We found enhanced LO and TO phonon scattering near the tip of the nanowire, i.e., the scattering is at least a factor of 5x stronger at the tip than observed at distances many microns away from the tip. The polarized scattering patterns I($\theta )$, where $\theta $ is the angle between the incident electric field and the NW axis, also change as the probe beam approached the tip of the nanowires. The effects observed here should be general and apply to other semiconducting nanowire systems as well.   

Polarized Rayleigh Back Scattering from Individual GaP Nanowires

Results of polarized Rayleigh back-scattering studies are reported for individual $\sim $20 $\mu $m long crystalline GaP Nanowires (NWs) using 514.5 nm excitation. The NWs were supported on Transmission Electron Microscope (TEM) grids. The diameters of the NWs were determined by TEM. Positions of characteristic LO, TO phonon Raman bands were found to agree with bulk GaP. The Rayleigh back-scattering intensity polar pattern I($\theta )$ was measured at room temperature, where $\theta $ is the angle between the incident electric field and the NW axis. The scattered radiation was polarized parallel to the incident electric field. For small NW diameter (d$\sim $70) nm, we observed $\sim $ cos$^{4}\theta $ polar patterns. With increasing NW diameter above 100 nm, the polar scattering patterns rotate by 90\r{ } with respect to those seen in small diameter NWs and then they broaden to a circle. Our experimental data will be compared to the calculated Rayleigh back-scattering efficiency calculated via the Discrete Dipole Approximation (DDA). Our DDA calculations show that the polar patterns are sensitive to both the diameter and the NW length. Although the calculated polar patterns qualitatively support our data, improvement in the modeling is still needed. This work is supported by NSF NIRT, grant DMR-0304178.   

Non-Linear Raman Scattering from Semiconducting GaP Nanowires

Results of polarized micro-Raman scattering from LO and TO phonons in individual GaP nanowires (NWs) with different diameter and length are reported. The NW diameters were determined by Atomic Force Microscope (AFM) and length was measured by Scanning Electron Microscope (SEM). NWs with the same growth direction but variable length were prepared by cutting $\sim $40 $\mu $m long wires into segments using a Focused Ion Beam. The polar plots of the back scattered intensity I$_{TO,LO}(\theta )$ from these segments were collected, where $\theta $ is the angle between the incident electric field and the NW axis. Interestingly, the shapes of these polar patterns depend on both the length and diameter of the NWs. The Raman scattering intensities for short wires (i.e., L$<$1$\mu $m) also exhibit a non-linear dependence on the incident laser power I$_{0}$. The non-linearity increases with decreasing NW length and behaves as $\sim $I$_{0}^{1.5}$ for the shortest wires measured so far (i.e., L$\sim $500 nm). Our results strong suggest strong enhancement in the internal electric field via antenna effects. This work is supported by NSF NIRT, grant DMR-0304178.   

Optical Antenna Effect in Semiconducting Nanowires

Using Raman scattering, we have observed strong optical antenna effects which we identify with internal standing wave photon modes of the wire. The antenna effects were probed in individual GaP NWs whose diameters are in the range 40$<$d$<$300 nm. The data and our calculations show that the nature of the backscattered light is critically dependent on the interplay between a photon confinement effect and bulk Raman scattering. At small diameter, d$<$65 nm, the NWs are found to act like a nearly perfect dipole antenna and the bulk Raman selection rules are masked leading to a polarized scattering intensity function I$_{R }\sim $ cos$^{4}\theta $. For larger wires, many other different polar patterns are found. Underscoring the importance of this work is the realization that a fundamental understanding of the ``optical antenna effect'' in semiconducting NWs is essential to the analysis of all electro-optic effects in small diameter filaments.   

Spatially-Resolved Photoluminescence Mapping of Single CdS Nanosheets

We present results of spatially-resolved low temperature photoluminescence of single 5 micron wide CdS nanosheets. The sheets, grown by pulsed laser deposition using vapor-phase transport, are uniform in size and shape and exhibit a hexagonal wurtzite structure. The orientation of the c-axis determined by PL polarization analysis and HR TEM varies from sheet to sheet. The spatially-resolved PL reveals spectral variation across the sheet, with A-like excitons at the edges showing a spectral peak at 2.547eV, and B like excitons at the center showing a peak at 2.563eV. Exciton lifetimes of $\sim $200 ps are observed, which are significantly longer than CdS nanowires of identical diameter, but shorter than measured in bulk CdS.   

Anomalous Photoluminescence in CdSe Quantum Dot Solids at High Pressure due to Non-uniform Stress.

The application of static high pressure provides a means to precisely control and investigate many fundamental and unique nanoparticle properties. CdSe is a model quantum dot (QD) system whose behavior under high pressure has been extensively studied; however, the effect of non-uniform stresses on this system has not been fully appreciated. In order to model these experiments and account for the behavior of the photoluminescence we carried out electronic structure simulations of wurtzite CdSe QDs with a core diameter of 2 to 5 nm using the Semi-Empirical Pseudopotential Method (SEPM). These calculations guide the interpretation of the photoluminescence data obtained from CdSe QD solids in different stress environments varying from purely uniform to highly non-uniform. Small deviations from a uniform stress distribution are found to profoundly affect the electronic properties of this system. In non-uniform stress environments, we observe a pronounced photoluminescence energy flattening above 3 GPa. The importance of this effect must be considered when investigating other potentially pressure mediated phenomena.   

Dynamics of Photo-Excited Carriers in Single InP Nanowires Under High Excitation Density

The dynamics of photo-excited carriers in single InP nanowires at low temperature is investigated using time-resolved photoluminescence spectroscopy. Under highly intensity excitation, the photoluminescence spectrum from a single nanowire shows a broad emission band at early times after the excitation pulse indicating the presence of a degenerate, high density electron-hole plasma. At later times ($>$ 600 ps) when the density of carriers decreases, the emission spectrum becomes narrower and converges toward the free exciton emission band. The lifetime of free excitons in a single nanowire is measured to be close to the lifetime of excitons in high quality InP epilayers, indicating the relative insensitive of the carriers to the InP nanowire surface. These results indicate that significant state filling and band gap renormalization occur in single InP nanowires.   

Near Infrared Photoresponse in Annealed CdSe Nanocrystal Films

We found unexpected near infrared (NIR) photo response in CdSe nanocrystal superlattice film annealed above 400 C in air. The current voltage characteristic measured in a planer device geometry show a large increase in NIR current over dark current. The calculated external quantum efficiency of the device is up to 10.6 {\%} at -5V and the responsivity is 0.7A/W obtained under 1.32 $\mu $W IR irradiation. UV-VIS absorption of annealed CdSe shows the redshifting and broadening of exciton peak and a decrease of band gap as the annealing temperature is increased. TEM image show that CdSe nanocrystals have been melted to fuse to different size distribution nanoparticles during annealing. We discuss possible reason for this unexpected behavior.