Quantum dot-like behavior of ultrathin PbI2 films

Vacuum-evaporated stoichiometric ultrathin discontinuous PbI2 films (in the range 2.5-30 nm) show a linear grain size (4-16 nm) growth with film thickness as revealed by transmission electron microscopy/X-ray diffraction analyses. A single peak around 3 eV in the optical absorption spectra shows a systematic blue shift with decreasing grain size (d). The optical spectra show additional peaks for films thicker than 20 nm. The 3 eV peak has a linear dependence on 1/d2 indicating a typical quantum dot-like behavior yielding a corresponding bulk value of the transition as 2.98 eV and the reduced effective mass of the electron-hole pair involved as 0.14. Relevant data from similar earlier studies agree quite well with our analysis. This study shows that an appropriate choice of the bulk optical transition concerned (not merely the minimum energy gap) is necessary for correctly interpreting the grain size-induced blue shift of optical structures for anisotropic and complex band structure materials like PbI2.