Interband transitions in sol-gel-derived ZrO2 films under different calcination conditions

Su-Min Chang*, Ruey An Doong

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

69 Scopus citations

Abstract

Different types of interband transitions as well as the microstructures and O/Zr ratios in sol-gel-derived ZrO2 films calcined at elevated temperatures in air or N2 were systematically examined to clarify the formation of different band structures in the structural ZrO2 films. Optical absorptions indicate that the structural ZrO2 films all contain direct-band transitions, while the occurrence of indirect-band and band-tail transitions depends on O/Zr ratios. Band tails mainly result from imperfect structures in the grain boundaries and appear in the ZrO2 films with O/Zr ≥ 2. When the ZrO2 films are non-stoichiometric (O/Zr < 2), indirect-band transitions occur due to the folded bands caused by lattice oxygen vacancies. Monoclinic ZrO2 has two direct-band transitions with bandgaps of 5.02-5.08 and 5.83-6.01 eV. The valence-band XPS spectra indicate that an additional occupied sub-band is located above its valence band. Indirect bandgaps in the monoclinic ZrO2 films are in the range of 5.00-5.10 eV. Amorphous ZrO2 films exhibit indirect-band transitions with bandgaps of 5.01-5.47 eV followed by direct-band transitions with bandgaps of 5.90-6.12 eV. In addition, tetragonal ZrO2 contains direct bandgaps of 5.32-5.74 eV and indirect bandgaps of 4.72-5.40 eV. Size-dependent bandgaps are observed in the sol-gel-derived ZrO2 films calcined in air, while the exponential dependence of bandgaps on O/Zr ratios is obtained in the ZrO2 films calcined in N2. The discrepancy is primarily due to different fates of oxygen vacancies resulting from different calcination atmospheres.

Original languageEnglish
Pages (from-to)4804-4810
Number of pages7
JournalChemistry of Materials
Volume19
Issue number19
DOIs
StatePublished - 18 Sep 2007

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