Excitons in Ultrathin PbI$_{2}$ Crystals

Due to their weak inter-layer van der Waals bonding, layered materials offer the unique possibility to produce natural quantum wells in the form of single and few atomic layer samples. A technique known as micromechanical cleavage, involving repeated cleaving, is used to isolate samples of all thicknesses [1]. Here, we present a combined experimental and theoretical study of band-edge excitons in the layered compound PbI$_{2}$ and, in particular, on their behavior as a function of sample thickness. Results of photoluminescence and reflection experiments are reported on samples with thicknesses ranging from a few micrometers down to a few monolayers, as determined by atomic force microscopy measurements. The data display striking and well reproducible changes in the transition from three to two-dimensions, which will be compared with results of first-principles calculations of the electronic band structure based on density functional and many-body perturbation theory. Computational resources were provided by the DOE NERSC facility. [1] A. K. Geim et. al. Nature Materials 6, 183 (2007)