SPATIAL-CONFINEMENT EFFECT ON PHONONS AND EXCITONS IN PBI2 MICROCRYSTALLITES

We have measured resonant Raman spectra and exciton absorption spectra of PbI2 microcrystallites embedded in ethylene metacrylic acid (E-MAA) copolymer at 77 and 2 K, respectively. The microcrystallites are plateletlike 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 exciton absorption 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 number of layers is confirmed. Using this relation, we reinterpret the dependence of the exciton energy on the layer thickness, which has been measured previously. Consequently, the thickness dependence of the exciton energy is well explained by the quantum confinement model of the exciton translational motion in the crystallites with more than five layers. In crystallites with thinner layers, however, the exciton energies deviate from the theoretical values.