A novel class of visible-light-activated TiO2 photocatalysts were prepared by direct hydrolysis of tetrabutyl titanate through iodine-doping. When calcination temperature is at 673 K, these nanoparticles (mean diameter of ∼5 nm) show stronger absorption in the 400−550 nm range with a red shift in the band gap transition and significantly higher photocatalytic activity than pure TiO2 prepared by the same procedure and Degussa P-25 titania nanoparticles in aqueous phenol solution under visible light irradiation (λ > 400 nm). Furthermore, I-doped TiO2 (673 K) still showed pronounced photocatalytic activity under UV and visible light irradiation.

Visible-Light-Activated Nanoparticle Photocatalyst of Iodine-Doped Titanium Dioxide

First-principles density functional theory (DFT) electronic structure calculations were carried out for model halogen-doped anatase TiO2 structures to evaluate the effect of halogen doping on the band edges and the photocatalytic activity of TiO2. The model structures of X-doped TiO2 were constructed by using the 48-atom 2 × 2 × 1 supercell of anatase TiO2 with one O or one Ti atom replaced with X (=F, Cl, Br, I). The unit cell parameters and the atom positions of the resulting X-doped TiO2 with X at an O site (X@O) and that with X at a Ti site (X@Ti) were optimized by performing first-principles DFT calculations. On the basis of the optimized structures of X-doped TiO2 with X@O and X@Ti, the defect formation energies and the plots of the density of states were calculated to analyze the band edges, the band gap states, and the preferred doping sites. Our work shows that the doping becomes more difficult in the order F < Cl < Br < I for X-doped TiO2 with X@O, while the doping becomes less difficult in the ...

Density Functional Characterization of the Band Edges, the Band Gap States, and the Preferred Doping Sites of Halogen-Doped TiO2

High-quality ZnS, ZnSe, and ZnTe epitaxial films were grown on (001)-GaAs-substrates by molecular beam epitaxy. The 1s-exciton peak energy positions have been determined by absorption measurements from 2 K up to about room temperature. For ZnS and ZnSe additional high-temperature 1s-exciton energy data were obtained by reflectance measurements performed from 300 up to about 550 K. These complete E1s(T) data sets are fitted using a recently developed analytical model. The high-temperature slopes of the individual E1s(T) curves and the effective phonon temperatures of ZnS, ZnSe, and ZnTe are found to scale almost linearly with the corresponding zero-temperature energy gaps and the Debye temperatures, respectively. Various ad hoc formulas of Varshni type, which have been invoked in recent articles for numerical simulations of restricted E1s(T) data sets for cubic ZnS, are discussed.

Temperature dependence of exciton peak energies in ZnS, ZnSe, and ZnTe epitaxial films

Despite the development of high-resolution electron microscopy (HREM) that allows imaging of most materials, the extraction of quantitative information at atomic scale still requires considerable additional efforts. This review presents the recent developments on techniques that can be used to determine the local strain, chemical composition or atomic structure retrieval in HREM. The source of noise in images and effective methods for improving the signal-to-noise ratio in direct or Fourier space are discussed. The artefacts of filtering are commented. In all the methods, the sample thickness has to be determined. A detailed analysis of the thin foil relaxation effect on the measured distortion fields is presented as well as the possibilities of using finite element calculations for its modelling. The local composition measurement based on the chemically sensitive reflections; pattern recognition and the measurement of lattice parameters are described. Examples from semiconductor heterostructures are discussed. The current state of the strategies used for the retrieval of the atomic configuration of defects from HRTEM images is briefly presented. The limits of detection and the accuracy of the methods are summarised; and it is pointed out that electron holography, focal series reconstruction and the coming Cs corrected microscopes will help to obtain this information with better accuracy.

Extracting quantitative information from high resolution electron microscopy

The quantum-confined Stark effect in a single self-assembled CdSe/ZnSe quantum dot was studied by means of highly spatially resolved photoluminescence spectroscopy. A nanotechnological approach making use of a capacitor-like geometry enabled us to apply a well-defined lateral electric field on the quantum dots. Stark shifts of up to 1.1 meV were obtained, which can be well fitted by a purely quadratic dependence on an electric field. In quite good agreement with theoretical calculations, an exciton polarizability of 4.9×10−3 meV/(kV/cm)2 can be extracted, while the permanent dipole moment in the lateral direction is found to be negligible.

Stark effect and polarizability in a single CdSe/ZnSe quantum dot

Zinc telluride layers have been grown on (100) GaAs, GaSb and ZnTe substrates at 350 degrees C with atmospheric pressure MOVPE. Diisopropyl-telluride, dimethylzinc-triethylamine and diethylzinc were chosen as metallorganic precursors. The samples were characterized by photoluminescence at 2 K and the Hall effect. In the PL spectra the light hole, heavy hole and bound exciton transitions are well resolved. Most of the transitions have been assigned. The partial pressure ratios of the alkyls were optimized. The influence of composition and stoichiometry of the substrates on the purity of the ZnTe is revealed. Various alkyls as sources of As, Bi, Ga, In and I were investigated for their suitability as p- or n-type dopants. It is shown that tetraethylbiarsine and ethyliodide are promising alkyls for p- and n-type doping of ZnTe.

http://iopscience.iop.org/article/10.1088/0268-1242/6/9A/019/pdf

The MOVPE growth and doping of ZnTe

Resonant excitation of intrinsic and shallow trap luminescence in MOVPE grown ZnTe layers

ZnSexTe1−x epilayers were investigated by means of luminescence, reflectivity, and temperature dependence in the concentration range 0<x<0.4. The studied ZnSexTe1−x epilayers with thicknesses of about 1.5 μm were grown on GaAs substrates by metalorganic vapor phase epitaxy. It was found that the luminescence and reflectivity spectra of the mixed crystals are strongly affected by the compositional disorder. A continuous transition from the recombination through free and bound exciton states to the recombination of excitons localized by the compositional fluctuations of the mixed crystal was observed in the concentration region of about x=0.25. The position of the excitonic band edge was derived from the photoluminescence excitation spectra and from temperature dependence of the emission spectra.

S. Lankes

Papers

The MOVPE growth and doping of ZnTe

Resonant excitation of intrinsic and shallow trap luminescence in MOVPE grown ZnTe layers

Exciton recombination in Te‐rich ZnSexTe1−x epilayers

Bandgap bowing and spontaneous ordering in ZnSexTe1−x

Spectroscopic investigations of donor and acceptor states in n- and p-doped ZnTe epilayers