Absorption Due to Quasi-Two-Dimensional Electronic Transition in Very Thin Films of PbI2

Absorption spectra in a very thin film of PbI 2 have been measured by using multiple total reflections at 90 K. A new band appearing at 21.14 kcm -1 is assumed to be associated with an exciton located in a few layers adjacent to vacuum, and the exciton binding energy of 650 ±50 cm -1 is larger than that of the bulk exciton. This larger binding energy suggests quasi-two dimensionality of this exciton. Moreover, a step and a plateau appear at 20.8 kcm -1 and in 22–23 kcm -1 , respectively. These structures are concluded to be due to the band-to-band transition in the quasi-two dimensional electronic structure.