S.S. Kurbanov

Spatially resolved investigations of the emission around 3.31 eV (A-line) from ZnO nanocrystals

ZnO nanocrystals grown by chemical solution deposition were studied by means of photoluminescence and cathodoluminescence spectroscopy. A postgrown annealing treatment significantly improved the UV emission efficiency and resulted in the clear appearance of a low temperature emission band around 3.31 eV (so-called A-line). Spatially and wavelength resolved cathodoluminescence measurements revealed a spotlike distribution of the A-line emission on a nanocrystal surface. It is found that there is a strong correlation between the emission around 3.31 eV and the specklike defects on the nanocrystal surface that appeared after annealing. The origin of the A-line and the specklike defects are discussed.

Thermo- and Photo-annealing of ZnO Nanocrystals

ZnO nanocrystals grown by chemical solution deposition have been annealed in ambient air using heat and cw He–Cd laser treatments. Both annealing processes increased markedly the visible luminescence intensity. The obtained results indicate that annealing under laser illumination results in effects comparable to those of high temperature annealing.

Impact of visible light illumination on ultraviolet emission from ZnO nanocrystals

The influence of a visible Ar{sup +}-laser (488 nm) illumination on photoluminescence (PL) from ZnO nanocrystals excited by He-Cd laser (325 nm) at various excitation intensities and temperatures was investigated. A reversible quenching of the UV near-band-edge emission under Ar{sup +}-laser illumination was observed. The quenching effect increases with decreasing temperature and He-Cd laser intensity at fixed intensity of Ar{sup +}-laser. It is found that the recovery time of the initial PL intensity depends on temperature and Ar{sup +}-laser exposure time. At room temperature the PL restored the initial intensity instantly after the turning off the Ar{sup +}-laser illumination; however at 10 K this process took several minutes. The proposed mechanism of the observed quenching effect implies an appearance of the additional recombination channel under a visible light illumination due to the recharging of oxygen-vacancy states in the surface depletion zone.

Kelvin Probe Force Microscopy of Defects in ZnO Nanocrystals Associated with Emission at 3.31 eV

The surface potential variations on ZnO nanocrystal surfaces were investigated by using Kelvin probe force microscopy. It was found that the nanocrystal surface contains randomly distributed circular pits with a diameter of ~100 nm and a depth of ~30 nm and that the surface potential within the pits is lower than that in the vicinity of the pits. We discuss the correlation between the observed pits and speck-like defects on a ZnO nanocrystal surface associated with the emission at 3.31 eV. We suggest that the reduced surface potential is caused by the local acceptor-like states concentrated near the structural defects.

Modulation of Excitonic Emission from ZnO Nanocrystals by Visible Light Illumination

We report on the optical modulation of ultraviolet (UV) photoluminescence from ZnO nanocrystals excited by He–Cd laser (325 nm) with visible Ar+ laser radiation (488 nm). The effective reversible quenching of the UV luminescence intensity was achieved. The quenching efficiency was found to depend on temperature, the ratio of He–Cd and Ar+ laser intensities, and the frequency of modulation.

Excitation Intensity Dependent Studies of Photoluminescence from ZnO Nanocrystals Deposited on Different Substrates

The photoluminescence (PL) spectra of ZnO nanocrystals depending on temperature, excitation intensity, and deposited substrate were studied. At a constant cooled cryostat finger temperature of 10 K, under high-power excitation density (He–Cd laser, 325 nm), a temperature induced redshift and distortion of the PL from ZnO nanocrystals deposited on silicon, sapphire, and glass substrates were observed. The effect of the substrate and interface thermal conductance on the observed phenomenon is discussed.

Photoluminescence Properties of ZnO Nanorods Synthesized by Different Methods

The photoluminescence properties of ZnO nanorods synthesized by the low-temperature hydrothermal and high-temperature vapor-phase methods are studied. At room temperature, the photoluminescence of ZnO nanorods synthesized by the high-temperature vapor-phase method exhibits only one highintensity ultraviolet peak at a wavelength of 382 nm. At the same time, the luminescence spectra of ZnO nanorod samples grown by the low-temperature hydrothermal method, but with the use of different chemical reagents exhibit, apart from the ultraviolet peak, a violet band or a yellow-orange band at ~401 and ~574 nm, respectively. The violet luminescence band is attributed to defects or zinc vacancy complexes, and the yellow-orange band to defects associated with interstitial oxygen.

The effect of packing density on luminescence of amorphous SiO2 nanoparticles

Photoluminescence of amorphous SiO2 nanoparticles compressed in the form of tablets is studied under exposure to UV radiation. The observed luminescence spectrum is a broad band extending from the excitation wavelength to 700 nm and with a maximum at ~470 nm. The spectrum can be decomposed into two Gaussian components with maxima at ~460 and ~530 nm. As the pressure applied for sample preparation increases, the integrated intensities of these bands change in opposite directions—the intensity of the short-wavelength band increases, while that of the long-wavelength band decreases. It is concluded that these bands are due to different luminescence centers of silicon dioxide located on the surface and in the bulk of SiO2 nanoparticles.

Luminescence of a Nanocomposite Based on ZnO-Filled Synthetic Opal

The opal-zinc oxide nanocomposite formed by chemical deposition from a solution has been investigated. Zinc oxide nanocrystals deposited on a substrate have a shape of truncated cones, which is characteristic of ZnO single crystals and exhibit good emission characteristics. However, in opal—ZnO nanocomposite, the exciton luminescence is suppressed, while the emission from the opal matrix is enhanced. The possible mechanisms of the observed effect are discussed.

One and Two-Photon Absorption in Multicomponent Glasses and the Measurement of Cubic Nonlinear Susceptibility

One-and two-photon absorption of N2-laser radiation (λ = 337 nm) in multicomponent silicate and phosphate glasses is investigated. A monotonic decrease from pulse to pulse in the intensity of radiation passing through a sample and in the luminescence excited by this radiation is found. The possibility of calculating the imaginary part of the cubic nonlinear susceptibility of glasses using the time dependence of the change in the laser intensity at the sample output is shown.