Interpretation of the Raman spectra of bismuth oxide thin films presenting different crystallographic phases

Bismuth oxide thin films are produced under different deposition conditions (power and substrate temperature) enabling the formation of Bi4O7 and different crystallographic phases of Bi2O3. The crystallographic structure of the films is identified using the X-ray diffraction patterns with the purpose to correlate them to the visible Raman spectra. However, the spectra besides the peaks related to the crystalline structure presented an unknown broad band at 600 cm(-1) that is observed for all the deposition conditions and phases. This broad band vanishes after annealing the films at 500 degrees C in air. A critical analysis of the assignment given in the literature to this broad Raman mode is offered, follow by a different proposal that is consistent with the observation of the band for all the films and supported by theoretical estimations. We propose that the 600 cm(-1) band is due to the presence of residual-weakly bonded [BiOn] units allocated at the grain boundaries of the nanocrystalline films and so, it is not exclusive of the defect fluorite cubic structure of Bi2O3. Moreover, since the Raman spectra of the Bi4O7 phase has not been reported, we use a density functional approximation of the Bi4O7 molecular structure to calculate the Raman active bands, finding a good agreement between theoretical and experimental data. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

Bismuth oxide thin films were studied under different deposition conditions, leading to the identification of various crystallographic phases. An unknown broad band at 600 cm(-1) in the Raman spectra was found for all films, attributed to residual weakly bonded [BiOn] units at grain boundaries. Theoretical estimations and density functional approximation were used to support the experimental data, showing good agreement.