Frenkel-like Wannier-Mott Excitons in Few-Layer PbI2

TL;DR

This study combines optical measurements and first-principles calculations to analyze exciton behavior in few-layer PbI2, revealing a Frenkel-like exciton nature and significant electronic property changes with layer thickness.

Contribution

It demonstrates that the lowest exciton in few-layer PbI2 is Frenkel-like and shows how electronic properties evolve with layer number, supported by theoretical and experimental data.

Findings

The n=1 exciton is Frenkel-like with weak thickness dependence.

Band gap and exciton binding energy increase as layers decrease.

Transition from direct to indirect gap occurs at 1-2 layers.

Abstract

Optical measurements and first-principles calculations of the band structure and exciton states in direct-gap bulk and few-layer PbI2 indicate that the n = 1 exciton is Frenkel-like in nature in that its energy exhibits a weak dependence on thickness down to atomic-length scales. Results reveal large increases of the gap and exciton binding energy with decreasing number of layers, and a transition of the fundamental gap, which becomes indirect for 1-2 monolayers. Calculated values are in reasonable agreement with a 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 universal feature of wide-gap layered semiconductors whose effective masses and dielectric constants give bulk Bohr radii that are on the order of the layer spacing.