First-principles quantum-mechanical techniques, based on density functional theory ͑B3LYP level͒ were employed to study the electronic structure of ordered and deformed asymmetric models for Ba 0.5 Sr 0.5 TiO 3 . Electronic properties are analyzed and the relevance of the present theoretical and experimental results on the photoluminescence behavior is discussed. The presence of localized electronic levels in the band gap, due to the symmetry break, would be responsible for the visible photoluminescence of the amorphous state at room temperature. Thin films were synthesized following a soft chemical processing. Their structure was confirmed by x-ray data and the corresponding photoluminescence properties measured.

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This letter reports on a process to prepare nanostructured PbTiO3 (PT) at room temperature with photoluminescence (PL) emission in the visible range. This process is based on the high-energy mechanical milling of ultrafine PbTiO3 powder. The results suggest that high-energy mechanical milling modifies the particle’s structure, resulting in localized states in an interfacial region between the crystalline PT and the amorphous PT. These localized states are believed to be responsible for the PL obtained with short milling times. When long milling times are employed, the amorphous phase that is formed causes PL behavior. An alternative method to process nanostructured wide-band-gap semiconductors with active optical properties such as PL is described in this letter.

Intense photoluminescence in highly disordered strontium titanate amorphous thin films prepared by the polymeric precursor method was observed at room temperature (300 K). The luminescence spectra of SrTiO 3 amorphous thin films at room temperature revealed an intense single-emission band in the visible region. X-ray absorption near edge structure was used to probe the local atomic structure of SrTiO 3 amorphous and crystalline thin films. Photoluminescence intensity in the 535 nm range was found to be correlated with the presence of non-bridging oxygen defects. A discussion is presented of the nature of this photoluminescence, which may be related to the disordered structure in SrTiO 3 amorphous thin films.

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The photoluminescence observed in ABO3 type perovskite in their highly structural disordered state can be explained by a model in which is assumed a distribution of electronic states localized within the energy band gap coupled to lattice local vibrational states. The model fits very well the experimental results and indicates that photoluminescence in the visible region can be considered as a general behavior of disordered solids.

Barium strontium titanate ͑Ba 0.8 Sr 0.2 TiO 3 ͒ thin films have been prepared on Pt/Ti/SiO 2 /Si substrates using a soft solution processing. X-ray diffraction and also micro-Raman spectroscopy showed that the Ba 0.8 Sr 0.2 TiO 3 thin films exhibited a tetragonal structure at room temperature. The presence of Raman active modes was clearly shown at the 299 and 725 cm Ϫ1 peaks. The tetragonal-to-cubic phase transition in the Ba 0.8 Sr 0.2 TiO 3 thin films is broadened, and suppressed at about 35°C, with a maximum dielectric constant of 948 ͑100 kHz͒. Electrical measurements for the prepared Ba 0.8 Sr 0.2 TiO 3 thin films showed a remnant polarization ͑P r ͒ of 6.5 C/cm 2 , a coercive field ͑E c ͒ of 41 kV/cm, and good insulating properties. The dispersion of the refractive index is interpreted in terms of a single electronic oscillator at 6.97 eV. The direct band gap energy ͑E g ͒ and the refractive index ͑n͒ are estimated to be 3.3 eV and n ϭ 2.27-2.10, respectively.

This communication describes, for the first time, the growth of SnO2 nanoribbons by a controlled carbothermal reduction process. An analysis of the transmission electron microscopy image revealed that these nanoribbons have a well-defined shape, with a typical width in the range of 70-300 nm. In general, the nanostructured ribbons were more than 100 microns in length. The results reported here support the hypothesis that this ribbon-like nanostructured material grows by a vapor-solid process. This study introduces two hypotheses to explain the SnO2 nanoribbon growth process.