Estudo de espectroscopia Raman dependente de temperatura e pressão de nanofios de AgVO3 e Nb2O5.

Abstract

Among nanostructured materials, nanowires have attracted significant interest from the scientific community due to their versatile functional properties and potential for various technological applications. In this thesis, we conducted a detailed investigation of temperature- and pressure-dependent Raman spectroscopy in AgVO3 and Nb2O5 nanowires. For β-AgVO3 nanowires, we combined classical lattice dynamics calculations with Raman spectroscopy to identify the vibrational modes and their corresponding eigenvectors. These modes exhibited a blue shift as pressure increased (from 0 to 29.4 GPa), reflecting the effects of crystal lattice compression. Additionally, temperature-dependent Raman measurements, performed in the range of 10 K to 363 K, revealed a linear behavior in most vibrational modes, with some showing an anomalous blue shift, attributed to slight atomic rearrangements. The integration of experimental data with calculations based on Density Functional Theory (DFT) allowed the estimation of the thermal expansion coefficient, bulk modulus, and Grüneisen parameters for the nanowires. The results highlight the structural stability and vibrational dynamics of the β-AgVO3 phase under extreme conditions, providing insight into this material and enabling the estimation of key physical parameters for applications requiring mechanical property control. For Nb2O5 nanowires, the investigation was conducted through an experimental approach based on a systematic analysis of Raman spectra under varying temperature and pressure conditions. The low number of observed modes, compared to the literature, suggests phonon confinement effects or structural disorder induced by the one-dimensional morphology. With increasing pressure, a significant broadening of the Raman bands and a progressive decrease in their intensities were observed, along with inversions in the relative intensities of modes associated with NbO6 units, possibly indicating subtle changes in local symmetry. In contrast, temperature variations revealed structural stability between 93 K and 300 K, while changes observed from 300 K to 773 K may be attributed to local rearrangements or a possible phase transition. The findings presented in this thesis not only expand the understanding of the structural stability and vibrational dynamics of AgVO3 and Nb2O5 nanowires under extreme conditions but also provide fundamental physical parameters for technological applications in optoelectronic devices and sensors, where knowledge of the material’s response to different operating conditions is crucial to ensure performance and reliability.