Estudo das propriedades ópticas em filmes finos de BiVO4

Abstract

In this work the optical properties were investigated in BiVO4 semiconductor samples, using the low temperatures photoluminescence technique, in the range of 16 K to 300 K. The bismuth vanadate solutions (BiVO4) were prepared by the organic metal decom position method (MOD) and the deposition of the thin films was done by dip-coating method. The deposition process was repeated several times, obtaining samples with 5, 10, 15, 17, 20, 30 and 40 depositions. The samples were submitted to a heat treatment in a furnace at 350◦ C for 10 minutes, in order to reach the monoclinic phase of the semiconductor. Samples with no heat treatment were also prepared, obtaining the tetra gonal phase. By morphological characterization it was possible to observe the presence of defects similar to pores with approximately 4 µm diameter average. From optical mi croscopy measurements performed on the substrate, it was found that these pore-like structures were possibly induced by the substrate. MEV measurements were performed using a scanning electron microscope, where it was possible to observe different shapes and sizes of nanoparticles agglomerates, distributed in an inhomogeneous way and sizes varying from 0.03 µm to 0.15 µm. Profilometry measurements were performed near the film edges. The probe behavior can be divided into 3 regions. By the extrapolation of the behavior of these regions the thickness of the films was determined. The thin film thick nesses obtained for the 1, 5 and 40 BiVO4 depositions were approximately 0.023 µm, 0.41 µm and 0.97 µm, respectively. X - ray diffraction measurements were performed in order to determine the crystalline structure. For the sample with 20 BiVO4 depositions Bragg diffraction peaks related to the monoclinic crystallographic phase were found, although it was also observed the presence of a peak related to tetragonal phase of low intensity. Similar results were obtained for the 15 and 17 BiVO4 depositions. Photoluminescence measurements were performed in a sample with 20 consecutive depositions of BiVO4 at low temperatures. The observed peaks in the photoluminescence spectrum are associated to a radioactive electron recombination and gap in the BiVO4 film. The width of the gap found in the literature for BiVO4, with a monoclinic structure, is approximately 2.4-2,5 eV. The highest energy peak, around 2.28 eV, is very close to the gap value found in the literature. Energy emissions around 1.65 eV and 1.68 eV occurred at temperatures below 100 K and 50 K, respectively. For temperatures above these, a recombination process gives rise to a phonon, suggesting that the energy of the excitons is dissipated in the crystalline lattice. The energy peaks around 3.2 eV, 3.3 eV and 3.35 eV, have energies higher than the expected semiconductor gap for any of the crystallographic structures of BiVO4. The results obtained lead us to assume that these peaks may be related to the presence of a secondary phase formed by a compound other than bismuth vanadate, suggesting being a bismuth oxide, such as BiOCOOH.