RESUMEN
Chalcogenide materials are being considered as some of the most promising systems for energy harvesting and energy conversion. Among them, the orthorhombic family of compounds X2Y3 (with X = Bi, Sb and Y = S, Se) has attracted special attention due to its interesting atomic structure and thermoelectric and optical properties. While Bi2S3 and Sb2Se3 have already been applied to solar cells, practical application of the new metastable Bi2Se3 is still a challenge due to the lack of data and knowledge on its properties. Here, the vibrational and structural properties of the orthorhombic metastable phase of Bi2Se3 are investigated by using Raman spectroscopy and ab initio calculations. We perform Raman spectroscopy measurements along with in situ thermal treatment on samples grown by electrochemical deposition. We show that by increasing the temperature an improved crystallization occurs in the orthorhombic structure, followed by recrystallization to the usual rhombohedral phase. Our results point out specific Raman modes of the orthorhombic phase. First principles computational results based on the density functional theory support the experimental data and describe three singlet Raman active vibrational modes, such as B(2)3g, B(2)2g and A(6)g.
RESUMEN
TiO2 films are grown on LaAlO3 (001), Si (100) and SiO2 substrates by reactive radio frequency sputtering. X-ray diffraction (XRD) pole figures revealed important characteristics about the texture and phase distribution on those films. Combined with spectroscopic ellipsometry, the pole figures allowed the analysis of the growth characteristics over the whole volume of the layers. Details in the microstructure of the films were probed using transmission electron spectroscopy. Anatase is the dominating phase in the films grown on all substrates. On TiO2/LaAlO3 fims, the mosaicity is very low, so that the pole figure closely resembles that of anatase monocrystals. Detailed inspection of XRD pole figures reveals a small amount of rutile in the TiO2/LaAlO3 films. For the growth of TiO2 onto SiO2, rutile and brookite phases are also detected. Transmission electron microscopy and XRD results show the formation of anatase {112} twins in films grown on the different substrates, suggesting that the anatase {112} twin mediates the growth of rutile and brookite phases. Optical results are in agreement with the XRD observations: the optical properties of TiO2/LaAlO3 films are similar to the ordinary values of bulk anatase crystals, indicating the orientation of the film in the [001] direction, whereas results for TiO2/SiO2 are compatible with lower crystalline ordering.