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
Here, we report large magnetoresistance and magnetocapacitance response of undoped TiO2 quantum dots weighting the contribution of both grain and grain boundaries by means of impedance spectroscopy. We also performed a complete characterization of the TiO2 quantum dots (~5 nm) prepared by sol-gel via water vapor diffusion method, using X-ray diffraction, small angle X-ray scattering, transmission electron microscopy and Raman spectroscopy. In addition, we showed a complete theoretical study on the electronic properties of TiO2 surface and subsurface oxygen and titanium vacancies to shed some light in their electronic and magnetic properties. Based in our study, we can conclude that the presence of defects, mainly at the grain boundary of these undoped TiO2 quantum dots, could be responsible for the large positive magnetoresistance (+1200%) and negative magnetocapacitance (-115%) responses at low applied magnetic fields (1.8 kOe) and room temperature.
RESUMEN
Bi2Se3 is a prominent narrow gap semiconductor with a rhombohedral crystal structure and potential applications in thermoelectric and spintronic technologies. Its electrical conduction is ruled by native point defects inducing an n-type degenerate behavior. Here, we present a first principles study of the point defects in Bi2Se3, focusing on the relevance of the interstitial sites. A density functional methodology was employed with van der Waals correction and spin-orbit coupling in order to achieve a better description of the defects. The results indicate that interstitial Bi atoms in octahedral sites between two consecutive quintuple layers have a lower formation energy than selenium vacancies and that these interstitials could act as a possible source of free electron carriers. In addition, we show that the utilization of an experimental or strained lattice constant in the calculations may lead to an under- or overestimation of the defect formation energies.