Optical and Theoretical Studies of Excitons in Few-Layer Lead Iodide.

Abstract

Optical measurements and first-principles calculations of few-layer lead iodide (PbI2) crystals are presented in this work. Results indicate that the band structure and exciton energies change dramatically from direct-gap bulk to few-layer PbI2. Moreover, the n = 1 exciton appears to be Frenkel-like in nature, in that its energy exhibits a weak dependence on thickness down to atomic length scales. Calculations predict large increases of the gap and exciton binding energy with decreasing number of layers, as well as a transition of the fundamental gap from direct to indirect for 1 - 2 layers. Results are in reasonable agreement with a simple particle-in-a-box model relying on the Wannier-Mott theory of exciton formation. General arguments and existing data suggest that the Frenkel-like character of the lowest exciton is a common feature of wide-gap layered semiconductors whose effective masses and dielectric constants yield bulk Bohr radii on the order of the layer separation. In addition, experimental observations of exciton mediated multiphonon resonant Raman scattering in few-layer PbI2 are presented. Results demonstrate that the resonance and overtone scattering strength can be tuned with varying crystal thickness. This is shown to be an effective technique for the characterization of few-layer polar semiconductors such as PbI2.