Optical properties of ultrathin PbI2 microcrystallite in polymer

We have prepared PbI₂ microcrystallites embedded in polymer, which have a layer structure and are ultrathin crystals consisting of two to nine monolayers.

A very large energy shift of the exciton absorption band has been observed in these microcrystallites and interpreted in terms of size confinement of the translational motion of excitons in the c-direction perpendicular to the crystal surface. The simple effective mass approximation is broken down in 2, 3, 4 layer crystallites, because the crystal thickness is smaller compared to the exciton Bohr radius.

Secondly, in the Raman spectrum where the excitation energy is resonant to the exciton energy, there appears a new line in the energy region below 20cm⁻¹ which is characteristic of a ultrathin crystal. The Raman shift increases with decreasing the crystal thickness. This line is assigned as due to a longitudinal mode of a rigid-layer phonon.

Thirdly, the confinement of the internal motion is studied by measuring a diamagnetic shift of the exciton energy in the magnetic field up to 150 T and a bound exciton luminescence in the ultrathin microcrystallites. Much larger binding energy of the exciton compared to the bulk value is estimated. This fact suggests that the envelope function of the exciton shrinks not only by a strong spacial restriction in the c-direction but also by the dielectric screening from the surrounding polymer.

Fourthly, the exciton-phonon interaction is studied by the hole-burning measurement. From the temperature dependence of the hole width, it is suggested that the exciton is scattered by impurities or defects below 40 K and by a LO phonon above 40 K.

Finally, the hydrostatic pressure dependence of the exciton energy and the resonant Raman scattering in the energy region of the optical phonons have been measured and the size effect on the atomic bonding between I and Pb is discussed. It is concluded that covalent bonding between Pb and I atoms changes to ionic bonding in the ultrathin crystallites.