TiO2 is well known as a low-cost, highly active photocatalyst showing good environmental compatibility. Recently it was found that TiO2 nanotubes promise to enable for high photocatalytic activity (PCA). In our experiments, we studied the PCA and spectroscopic properties of TiO2 nanotube arrays formed by the anodization of Ti. The PCA efficiency related to the decomposition of methylene-blue was measured. To obtain reliable data, the results were calibrated by comparing with standard materials like Pilkington Activ™ which is a commercially available self cleaning glass. The studies included a search strategy for finding optimum conditions for the nanotube formation and the investigation of the relationship between PCA and annealing temperature. TiO2 nanotubes of different shapes and sizes were prepared by an anodization of Ti foil in different electrolytes, at variable applied voltages and concentrations. The photo-dissociation of methylene-blue was detected spectroscopically. For the optimized material, an enhancement factor of 2 in comparison to the standard reference material was found. Furthermore, femtosecond-laser induced photoluminescence and nonlinear absorption of the material were investigated. Possibilities for further enhancements of the PCA are discussed.
Photoluminescence (PL) studies of the surface exciton peak in ZnO nanostructures at ~3.367 eV are
reported to elucidate the nature and origin of the emission and its relationship to nanostructure
morphology. Localised voltage application in high vacuum and different gas atmospheres show a
consistent PL variation (and recovery), allowing an association of the PL to a bound excitonic
transition at the ZnO surface modified by an adsorbate. Studies of samples treated by plasma and of
samples exposed to UV light under high vacuum conditions show no consistent effects on the
surface exciton peak indicating no involvement of oxygen species. X-ray photoelectron spectroscopy
data indicate involvement of adsorbed OH species. The relationship of the surface exciton peak to
the nanostructure morphology is discussed in light of x-ray diffraction, scanning and transmission
electron microscopy data.
We report growth of ZnO nanostructures on Au-coated Si substrates using vapor phase transport in the temperature range
from 800°C to 1150°C. Nanostructures grown at 800°C are rod-like with diameters of ~ 200 nm. Growth at higher
temperature shows a more complex behaviour with 2-D structures connecting 1-D nanorods at intermediate temperatures
and 3-D growth at the highest temperatures. Our work indicates that it may be possible to systematically control the
growth mode and morphology of ZnO nanostructures by tuning the growth temperature.
Nitrogen- and phosphorus-doped ZnO thin films were grown by pulsed laser deposition using an electron cyclotron
resonance (ECR) nitrogen plasma ion source or a ZnO:P2O5 doped target, as the dopant source, respectively. Both types
of films were grown on sapphire substrates first coated at low temperature with a ZnO buffer layer. For the N-doped
ZnO thin films, temperature-dependent Van der Pauw measurements showed consistent p-type behavior over the
measured temperature range of 200-450 K, with typical room temperature acceptor concentrations and mobilities of 5 x
1015 cm-3 and 5.61 cm2/Vs, respectively. The room-temperature photoluminescence spectrum of a N-doped ZnO thin
film featured a broad near band-edge emission at about 3.1 eV photon energy with a width of 0.5 eV. XPS studies
confirmed the incorporation of nitrogen in the samples. The ZnO:P layers (with phosphorus concentrations of between
0.01 and 1 wt %) typically showed weak n-type conduction in the dark, with a resistivity of 70 &OHgr;.cm, a Hall mobility of
&mgr;n ~ 0.5 cm2V-1s-1 and a carrier concentration of n ~ 3 x 1017 cm-3 at room temperature. After exposure to an incandescent
light source, the samples underwent a change from n- to p-type conduction, with an increase in mobility and a decrease
in concentration for temperatures below 300K. Electrical measurements showed noticeable differences for both types of
doped films when carried out in air or in vacuum. The results are discussed in terms of both the presence of surface
conducting channels and the influence of photoconductive effects.
A heterogeneous system composed of zinc oxide nanorods and ruthenium bipyridyl complex dye molecules is described. The photophysical and spectroscopic properties of the system are reported. It has been found that the ruthenium complex binds well to the zinc oxide nanorods and exhibits strong emission properties characteristic of ruthenium complexes. Photoluminescence and emission studies suggest that the ruthenium dye influences the emission and lifetime of the zinc oxide.
Lateral growth of ZnO nanowall arrays with subsequent growth of vertical nanowires using a two-step vapour phase transport method on a-plane sapphire are reported. X-ray diffraction and scanning electron microscopy data show that the nanostructures are aligned with c-axis normal to the substrate. Photoluminescence data demonstrate the exceptionally high optical quality of these structures, with intense emission and narrow bound exciton linewidths. We observe high energy excitonic emission at low temperatures close to the band-edge which we assign to the surface exciton in ZnO at ~3.366 eV. This assignment is consistent with the large surface to volume ratio of the nanowire systems and indicates that this large ratio has a significant effect on the luminescence even at low temperatures. The band-edge intensity decays rapidly with increasing temperature compared to bulk single crystal material, indicating a strong temperature-activated non-radiative mechanism peculiar to the nanostructures. No evidence is seen of the free exciton emission due to exciton delocalisation in the nanostructures with increased temperature, unlike the behaviour in bulk material. The use of such nanostructures in room temperature optoelectronic devices appears to be dependent on the control or elimination of such surface effects.
Thin films of ZnO and GaN have been deposited by pulsed laser deposition in atmospheres of oxygen and nitrgoen respectively. A time-of-flight ion probe and optical spectroscopy were used to study the interaction of the ablation plasma with the background gas. The deposition rate was measured using in situ optical reflectivity, and the thin film quality was assessed using x-ray diffraction and photoluminescence. By correlating the plasma measurements and the thin film characterization it was possible to identify the plasma regime required for the deposition of good quality films.
A facility for producing photoluminescence (PL) images of semiconductor wafers at cryogenic temperatures is described. A custom-designed liquid helium cryostat capable of accepting 50-mm-diam wafers is used together with a scanning optical table for PL excitation and collection. Spectral analysis is achieved by the use of a CCD detector mounted on a standard grating spectrometer. The configuration used enables a full spectrum to be recorded for each point on a wafer with a spatial resolution of approximately 100 μm. The spectral information may be used to produce images of the total PL intensity, the peak PL intensity, and the wavelength at which the peak intensity occurs. Typical PL maps are presented to illustrate the quality of the information.
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