Spatial-confinement effect on phonons and excitons in PbI2 microcrystallites

Article abstract of DOI:10.1103/PhysRevB.52.5929

We have measured resonant Raman specta and exciton absorption spectra ofPbI2 microcrystallites embedded in ethylene metacrylic acid (E-MAA) cop olymer at 77 and 2 K, respectively. The microcrystallites are platelike and vary in thickness. In the resonant Raman spectra, a new line is observed in the acoustic-phonon energy region, which is intimately related to the excitonabsorption band in the ultrathin microcrystallite with finite number of layers. The phonon energy as a function of the crystal thickness is explained on the basis of a finite chain model. From this analysis, the relationship between the exciton absorption band and the numberof layers is confirmed. Using this relation, we interpret the dependenc e of the exciton energy on the layer thickness, which has been measured previously. Consequently, the thickness dependence of the exciton energyis well explained by the quantum confinement model of the exciton trans lational motion in the crystallites with more than five layers. In crystallites with thinner layers, however, the exciton energies deviate from the theoretical values.