DFT analysis of the role of the number of layers: accounting for the multiple energy gaps in iron oxychloride

matrix (mass ratio of 1 : 40), confirms the onset of these transitions at 1.68 and 2.38 eV. Although the presence of two optical energy gaps has been reported for this layered n-type semiconductor, its origin has not yet been explained. A scrutiny of the previous reports reveals a possible correlation between the number of layers and the presence of multiple energy gaps. DFT analysis of the band structures of bulk, monolayer and few-layered FeOCl validates this hypothesis. Furthermore, the analysis reveals the indirect nature and a progressive decrease in the energy gap with an increase in the number of layers. The energy gaps for monolayer, five-layered and bulk FeOCl are calculated to be 2.25, 2.16 and 1.86 eV, respectively. Electron microscopy reveals the presence of both bulk and few-layered FeOCl in the specimen, supporting the DFT analysis. The energy gap estimated from the combined results of valence band X-ray photoelectron spectroscopy and Mott-Schottky analysis is 2.2 eV. Due to the surface sensitive nature of these two techniques, the value is closer to both the higher energy gap estimated from UV-visible spectroscopy and the value obtained for few-layered FeOCl from DFT analysis.