Search

This paper describes methods for synthesizing different nanostructures of ZnO in aqueous solutions at low temperatures and examines the effect of the size of the synthesized samples on the photoluminescence (PL) spectra. As the diameters of the nanostructures increased, a decrease in the ratio of the intensity of the ultraviolet radiation band to the intensity of the yellow radiation band in the PL spectrum observed. It is found that changing the diameters of nanostructures from 15 nm to 1000 nm leads to a decrease in their bandgap energy (Eg) from 3.28 to 3.21 eV. When the diameters of the nanostructures were less than ~400 nm, it was found that the energy of the band gap was strongly depend on the size of the nanostructure, and that the bond was weaker as the size of the nanostructures exceeded 400 nm.

Room temperature photoluminescence (PL) properties of vertically aligned and spindle-shaped, randomly oriented ZnO nanorods synthesized by using a low temperature hydrothermal method are studied. In air, the vertically aligned ZnO nanorods oriented mainly parallel to the luminescencerecording axis exhibited only one, very strong UV emission peak at 382 nm. This band is assigned to emission of free excitons. A new violet PL band near 400 nm arises with increasing angle between the nanorod growth direction and the luminescence-recording axis. The violet band also appears under UV illumination in vacuum and vanishes after exposure to air. The randomly oriented ZnO nanorods along with free exciton related PL band reveal a broad yellow-orange emission band around 590 nm. The violet band is attributed to Zn vacancy related defects or their complexes, while the yellow-orange emission band is ascribed to oxygen interstitial related defects.

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.

Zinc oxide (ZnO) nanorods were grown on glass substrates coated with a conducting indium tin oxide film using the hydrothermal method. The nanorods are 2–2.5 μm long and 70–200 nm in diameter. Under UV irradiation the nanorods exhibit photoluminescence with a maximum at 382 nm. It is found that changes in angle between the nanorods growth direction and the emission recording direction give rise to an appearance of a violet emission band centered at ∼400 nm. It is possible dependence of the luminescence spectrum on the ZnO nanorods’ spatial orientation is due to localization of the violet emission centers in the surface layer.

We present photoluminescence (PL) studies of ZnO nanorods synthesized by high-temperature vapor-phase transport method under pulse laser excitation. The PL spectrum contains a strong near band edge (NBE) emission located at around 387 nm and no emission bands in the visible range were observed. Our studies show that an increase in power density of pulse UV laser results in appearance of a so-called P-band centered at 389 nm and it overlaps the NBE emission. The variations in excitation intensity and detection angles revealed that the band at 389 nm can be deconvoluted into Lorentzian components at ~382 and ~395 nm. Excitation intensity dependence of the band at 389 nm exhibits a clear threshold behavior while the intensity of the band at ~395 nm grows almost linearly with the increase in the excitation intensity. It is possible that the emission band at 395 nm is associated with emission centers caused by zinc vacancy defects or their complexes in ZnO nanorods.

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.

Porous textured ZnO films have been fabricated using the low temperature bath chemical deposition method. The photoluminescence (PL) spectra of the ZnO films show a broad emission band peaked at ~400 nm. The PL exhibits sensitivity to the emission recording conditions. A decrease in temperature leads to separation of the PL into the UV and violet emission bands; at 10 K they are located at 368.4 and 412 nm, respectively. The observed violet emission is attributed to zinc vacancy related defects. The observed sensitivity of the emission to the recording conditions is due to oxygen desorption processes and variations in the width of a depletion region on the porous textured ZnO film surface.

Under high power, nanosecond duration pulse UV radiation spectral narrowing and temporal pulse shortening of an amplified spontaneous emission (ASE) from ordered ZnO nanorods have been studied. The dependence of the emission intensity on the excitation power density shows a clear threshold behavior. At low excitation intensities, full-width at half-maximum (FWHM) of the ASE pulse is equal to the N2 laser radiation pulse’s FWHM (~5.9 ns). With increasing the excitation intensity, the FWHM of the ASE pulse substantially reduced to 2.72 ns. The observed ASE pulse shortening is explained by reducing the excited-state population when the ASE intensity riches the certain level.

A red shift of the photonic band gap and stress-induced birefringence have been observed in the bare and ZnO infilled artificial opal samples under UV laser irradiation. The stress-induced birefringence appeared between the irradiated and non-irradiated parts of the opal. These effects were related to the irradiation-stimulated increase in the density and the refractive index of the opal. Using the obtained results, the UV illumination-induced changes in the degree of polarization, volume and refractive index of the opal were estimated.

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.

The effects of washing and heat treatment on photoluminescence (PL) behavior from artificial opals are studied. The PL spectra excited by N2-laser radiation (337 nm) display a broad maximum at around 435 nm and range from ∼340 to ∼700 nm. It is found that washing of the opal in different solvents and liquids leads to suppression of the long wavelength PL band at ∼500 nm, whereas heat treatment in air and vacuum results in a decrease in intensity of both of the PL bands. The obtained results indicate that the emission centers located on the surface of SiO2 spheres constructing artificial opal may be responsible for the PL band around ∼500 nm.

The effect of the Rhodamine 6G concentration on photoluminescence (PL) properties of opal-dye and anodized aluminum oxide-dye nanocomposites has been investigated. It has been shown that in the wide range of the dye concentration both nanocomposites exhibit monomer-like luminescence spectrum. The interaction between dye molecules and dangling bonds on the surface of SiO 2 balls and Al 2 O 3 affects the native PL matrices.