Charnia-like broadband plasmonic nano-antenna

Charnia was a pre-Cambrian life-form that exhibited a fractal structure to improve the extraction of nutrients from the pre-historic seas. Inspired by its fractal structure, this paper studies the potential application of these self-similarity fractal structures to create a plasmonic nano-antenna that can operate over a large linewidth. These devices are studied both theoretically and experimentally. It is shown that these nano-antennas can produce electric field enhancements above 8 over 200?nm range and surface enhanced Raman scattering (SERS) enhancements higher than 10⁵.     

Tunable light emission from quantum-confined excitons in TiSi2-catalyzed silicon nanowires

Visible and near-infrared photoluminescence (PL) at room temperature is reported from Si nanowires (NWs) grown by chemical vapor deposition from TiSi2 catalyst sites. NWs grown with average diameter of 20 nm were etched and oxidized to thin and passivate the wires. The PL emission blue shifted continuously with decreasing nanowire diameter. Slowed oxidation was observed for small nanowire diameters and provides a high degree of control over the emission wavelength. Transmission electron microscopy, PL, and time-resolved PL data are fully consistent with quantum confinement of charge carriers in the Si nanowire core being the source of luminescence. These light emitting nanowires could find application in future CMOS-compatible photonic devices.     

Si-CdSSe core/shell nanowires with continuously tunable light emission

Uniform Si-CdSSe core/shell nanowires were controllably synthesized by a multisource thermal evaporation route. Both the silicon core and the alloyed CdSSe shell are of high-quality and single crystalline. The silicon core is grown via the gold-catalyzed VLS route with a silicon wafer piece at the high temperature zone as the source. These preferentially grown Si nanowires further serve as templates for the afterward depositions of CdSSe shells using CdS/CdSe powders at the low temperature zone of the furnace as sources. The composition/band gap of the shells can be continuously modulated by the S/Se ratio of the evaporation sources, making these prepared heterostructures have strong and spectral position/color largely tunable light emission at the visible region. These kind of structures may have potential applications in multicolor nanoscaled light-emitting devices. This flexible growth route will also be applicable for controllable synthesis of other Si wire containing heterostructures.     

Coherent broadband light effects in grating spectrometer studies of emission from alkali atoms at surfaces

With ordinary grating spectrometers, strong bands that are due to broadband coherent light emission from samples containing various amounts of alkali atoms can be observed. The coherent light is proposed to be emitted by the alkali Rydberg states that are easily formed in these systems. The edges of the bands are observed at angles corresponding to low numbers of standing waves along the grating surface and perpendicular to it. This type of band is observed both with thermal sources and with broadband light sources created by pulsed laser light, and it is observed only with s-polarized (TE-mode) light. The band intensities are independent of the entrance slit width in the spectrometer, which shows that strong interference effects exist. The number of interference fringes observed on top of the most intense band is directly proportional to the width of the entrance slit. The time-resolved signal shows that large photon peaks from thermal sources are emitted in bursts within 2 mus, probably corresponding to the lifetime of the emitting Rydberg states and Rydberg clusters.     

Field emission from nonaligned zinc oxide nanowires

Zinc oxide (ZnO) nanowires with an average diameter of 15 nm were grown using a vapor phase transport process. Field emission was achieved from these nanowires in spite of their random orientation. The electric field for the extraction of a 10 μA/cm² current density was measured to range from 4.4 to 5.0 V/μm, and that for a 1 mA/cm² current density from 7.6 to 8.7 V/μm, depending on whether the sample was submitted to a heat treatment. The results exhibit the potential application of ZnO nanowires as field emitters in future flat panel displays.     

Measurement and reduction of damping in plasmonic nanowires

We report on a spectroscopic study of surface plasmon damping and group velocity in polycrystalline silver and gold nanowires. By comparing to single-crystalline wires and by using different substrates, we quantitatively deduce the relative damping contributions due to metal crystallinity and absorption in the substrate. Compared to absorbing substrates, we find strongly reduced plasmonic damping for polycrystalline nanowires on quartz substrates, enabling the application of such wires for plasmonic waveguide networks.     

Enhancement of green emission from Sn-doped ZnO nanowires

Oxygen vacancies play a crucial role in the emission characteristics of oxide nanomaterials. In this study, we found that the green emission intensity of ZnO nanowires can be enhanced through a Sn-doping concentration which increases the number of oxygen vacancies. Undoped ZnO nanowires showed blue emission at 380 nm, but as the concentration of Sn was increased, the green emission peak at around 500 nm, which is attributed to oxygen vacancies, showed drastic enhancement. On the basis of XPS compositional analysis, it was confirmed that the green luminescence intensity was closely related to the number of oxygen vacancies in Sn-doped ZnO nanowires. These results pave the way to a greater understanding of tunable light emission from nanowires, which could be a key technology for next-generation display devices, including flexible and transparent displays.     

Electron emission from individual indium arsenide semiconductor nanowires

A procedure was developed to mount individual semiconductor indium arsenide nanowires onto tungsten support tips to serve as electron field-emission sources. The electron emission properties of the single nanowires were precisely determined by measuring the emission pattern, current-voltage curve, and the energy spectrum of the emitted electron beam. The two investigated nanowires showed stable, Fowler-Nordheim-like emission behavior and a small energy spread. Their morphology was characterized afterward using transmission electron microscopy. The experimentally derived field enhancement factor corresponded to the one calculated using the basic structural information. The observed emission behavior contrasts the often unstable emission and large energy spread found for semiconductor emitters and supports the concept of Fermi-level pinning in indium arsenide nanowires. Indium arsenide nanowires may thus present a new type of semiconductor electron sources.     

Tuning gold nanorod-nanoparticle hybrids into plasmonic Fano resonance for dramatically enhanced light emission and transmission

We investigate the optical response of a gold nanorod array coupled with a semicontinuous nanoparticle film. We find that, as the gold nanoparticle film is adjusted to the percolating regime, the nanorod-film hybrids are tuned into plasmonic Fano resonance, characterized by the coherent coupling of discrete plasmonic modes of the nanorod array with the continuum band of the percolating film. Consequently, optical transmission of the percolating film is substantially enhanced. Even more strikingly, electromagnetic fields around the nanorod array become much stronger, as reflected by 2 orders of magnitude enhancement in the avalanche multiphoton luminescence. These findings may prove instrumental in the design of various plasmonic nanodevices.     

Investigation on light emission in light-emitting diodes constructed with n-ZnO and p-Si nanowires

The light emission was investigated in light-emitting diodes (LEDs) constructed with n-ZnO and p-Si nanowires (NWs). ZnO NWs were synthesized by thermal chemical vapor deposition and Si NWs were formed by crystallographic wet etching of a Si wafer. The LEDs were fabricated using the NWs via dielectrophoresis (DEP) and direct transfer methods. The DEP method enabled to align the ZnO NW at the position that led to p-n heterojunction diodes by crossing with the transferred Si NW. The I-V curve of the p-n heterojunction diode showed the well-defined current-rectifying characteristic, with a turn-on voltage of 3 V. The electroluminescence spectrum in the dark showed the strong emission at approximately 385 nm and the broad emission centered at approximately 510 nm, at a forward bias of 30 V. Under the illumination of 325-nm-wavelength light, the luminescence intensity at 385 nm was dramatically enhanced, compared to that in the dark, probably due to the electric-field-induced enhancement of luminescence.     

Directional emission from plasmonic Yagi-Uda antennas probed by angle-resolved cathodoluminescence spectroscopy

Optical nanoantennas mediate optical coupling between single emitters and the far field, making both light emission and reception more effective. Probing the response of a nanoantenna as a function of position requires accurate positioning of a subwavelength sized emitter with known orientation. Here we present a novel experimental technique that uses a high-energy electron beam as broad band point dipole source of visible radiation, to study the emission properties of a Yagi-Uda antenna composed of a linear array of Au nanoparticles. We show angle-resolved emission spectra for different wavelengths and find evidence for directional emission of light that depends strongly on where the antenna is excited. We demonstrate that the experimental results can be explained by a coupled point dipole model which includes the effect of the dielectric substrate. This work establishes angle-resolved cathodoluminescence spectroscopy as a powerful technique tool to characterize single optical nanoantennas.     

High quantum efficiency of band-edge emission from ZnO nanowires

External quantum efficiency (EQE) of photoluminescence as high as 20% from isolated ZnO nanowires were measured at room temperature. The EQE was found to be highly dependent on photoexcitation density, which underscores the importance of uniform optical excitation during the EQE measurement. An integrating sphere coupled to a microscopic imaging system was used in this work, which enabled the EQE measurement on isolated ZnO nanowires. The EQE values obtained here are significantly higher than those reported for ZnO materials in forms of bulk, thin films or powders. Additional insight on the radiative extraction factor of one-dimensional nanostructures was gained by measuring the internal quantum efficiency of individual nanowires. Such quantitative EQE measurements provide a sensitive, noninvasive method to characterize the optical properties of low-dimensional nanostructures and allow tuning of synthesis parameters for optimization of nanoscale materials.     

Directional emission from beryllium doped GaAs/AlGaAs nanowires

The emission directionality of self-catalytic GaAs nanowires in an AlGaAs shell, produced by molecular-beam epitaxy with a varied level of beryllium doping, is studied. It is shown that an undoped sample possesses pronounced waveguide properties along the growth direction. With increasing doping level, the intensity of the emission directed perpendicular to the lateral nanowire walls grows.     

Field emission from copper phthalocyanine and copper hexadecafluorophthalocyanine nanowires

Copper phthalocyanine (CuPc) and copper hexadecafluorophthalocyanine (F₁₆CuPc) nanowires were fabricated by vapor deposition. The nanowires were studied by scanning electron microscopy and transmission electron microscopy. Field emission properties of these nanowires were studied. The field emission properties were strongly dependent on the substrate temperature and the material used, and the best results are obtained for β-phase CuPc nanoribbons. Different dependences of field emission properties on the substrate temperature were obtained for the two materials investigated. The obtained results are discussed.     

Synthesis of plasmonic Ag@AgBr nanowires as highly efficient sunlight photocatalyst

Ag@AgBr core–shell nanowires have been synthesized through a simple method. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electronmicroscopy, photoluminescence spectroscopy, X-ray photoelectron spectroscopy and UV–Vis diffuse reflectance spectroscopy, were used to characterize the as-synthesized nanowires. The as-prepared Ag@AgBr photocatalyst can be used to remove the pollutants under direct sunlight. The as-prepared Ag@AgBr plasmonic photocatalysts show excellent visible-light photocatalytic performance and good reusability for decomposing organic pollutant of Rhodamine B and methyl orange dyes due to the surface plasmon resonance effect of Ag and AgBr nanoparticles. Additionally, the recycling experiment of Ag@AgBr nanowires has been done, demonstrating that Ag@AgBr nanowires have high efficiency and stability.     

Infrared light emission from porous silicon

Very intense broad sub-bandgap infrared (IR) light emission around 1,550 nm was observed on porous silicon by photoluminescence (PL) measurements. The integrated intensity of the IR signal is two orders of magnitude higher than that of the band–band emission in Cz silicon. PL measurements with the sample immersed in different media, e.g., in HF and H₂O₂, confirmed that the broad IR band originates from the Si/SiO _x interface. Electroluminescence spectroscopy was carried out on a porous silicon p–n junction sample contacted with indium-tin oxide. The IR band was detected at room temperature at both forward and reverse bias. The results indicate that radiative recombination through interface states is very efficient at room temperature.     

Optical emission of InAs nanowires

Wurtzite InAs nanowire samples grown by chemical beam epitaxy have been analyzed by photoluminescence spectroscopy. The nanowires exhibit two main optical emission bands at low temperatures. They are attributed to the recombination of carriers in quantum well structures, formed by zincblende-wurtzite alternating layers, and to the donor-acceptor pair. The blue-shift observed in the former emission band when the excitation power is increased is in good agreement with the type-II band alignment between the wurtzite and zincblende sections predicted by previous theoretical works. When increasing the temperature and the excitation power successively, an additional band attributed to the band-to-band recombination from wurtzite InAs appears. We estimated a lower bound for the wurtzite band gap energy of approximately 0.46?eV at low temperature.     

Wave-particle duality of broadband light

We describe and analyse a simple experiment based on interference of thermally distributed broadband photons which demonstrates quantitatively the gradual trade-off between knowledge of the trajectory of a quantum particle and its wave-like character as expressed via the inequality V ^?2?+?K ^?2?≤?1. The experiment relies upon colour-selective detection of light passing through each slit of a Young's double-slit arrangement: path information K is deduced from the overlapping bandwidths of the interfering light and the classical first-order degree of coherence, and wave information V is obtained from the fringe visibility. A classical Fourier analysis of the experiment is given which reproduces the observed interferograms.     

Surface exciton-plasmon polariton enhanced light emission via integration of single semiconductor nanowires with metal nanostructures

The light emission enhancement behavior from single ZnO nanowires integrated with metallic contacts is investigated by micro-photoluminescence measurements. Apart from surface plasmon polaritons at the air/metal interface, the emission of a single ZnO nanowire can be coupled into guided modes of surface excitonplasmon polaritons (SEPPs). The out-coupling avenues of SEPP guided modes are modeled in the presence of nanostructures, such as corrugation and gratings, on the metal surface. The guided modes of SEPPs in metalcontacted ZnO nanowires are calculated using the effective index method. The enhanced light emission from single semiconductor nanowires shows promise for use in highly efficient nano-emitters and nano-lasers, as well as macroscopic solid state light sources with very high efficiency. This article is published with open access at Springerlink.com     

Enhanced field emission from the ZnO nanowires by hydrogen gas exposure

ZnO nanowires (ZNWs) were synthesized on Co-coated Si wafer via a carbon thermal reduction vapor transport method. Scanning electron microscopy, X-ray diffraction and transmission electron microscopy investigations show that these ZNWs present a high-quality single-crystalline hexagonal structure. Field emission (FE) characteristics of the ZNWs film were measured. A low turn-on voltage for driving a current density of 0.1 μA/cm² is about 3.9 V/μm. The field enhancement factor was determined to be ∼ 1180 for ZNWs film. Exposure of H₂ during FE causes a permanent increase in the FE current and a decrease in the turn-on field. Also, the field enhancement factor γ was finally increased from 1180 ± 20 to 1510 ± 20 after FE saturation.