Surface morphology, optical properties and Urbach tail of spray deposited Co3O4 thin films

Abstract

Cobalt oxide (Co3O4) thin films were deposited onto glass substrates by a simple spray pyrolysis technique using Co (CH3COO)2 .4H2O as a precursor material. The as-deposited films were characterized using field emission scanning electron microscope (FESEM) and UV–visible spectrophotometer. FESEM images showed uniform and well-aligned nanofiber growth of Co3O4 thin film. The average crystallite size was found to increase from 15 to 25 nm with the increase of substrate temperature. The increase in average crystallite size may be due to the agglomeration and recrystallization of cobalt oxide nanoparticles. The absorption peaks obtained at 355 nm and 540 nm were attributed to the transitions taking place between oxygen and cobalt charge transfer (O2− → Co3+ and O2− → Co2+). The energy band gap was found to decrease from 2.35 to 2.03 eV and Urbach energy (Eu) increased from 0.73 eV to 0.84 eV with the increased substrate temperatures from 250 °C to 350 °C. The optical absorption coefficient exhibited an exponential dependence of photon energy following Urbach’s rule, and Urbach tail was dependent on lattice defects. The direct band gap of the Co3O4 thin film clearly showed a red shift with increasing temperature and provided an absorption favorable for bio-sensing applications.

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Acknowledgements

One of the authors, Muslima Zahan is grateful to the Ministry of Science and Technology, Government of the People’s Republic of Bangladesh for granting NST fellowship for her PhD program.

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Correspondence to Jiban Podder.

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Zahan, M., Podder, J. Surface morphology, optical properties and Urbach tail of spray deposited Co3O4 thin films. J Mater Sci: Mater Electron 30, 4259–4269 (2019). https://doi.org/10.1007/s10854-019-00717-2

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