aThin Film Laboratory, Department of Physics and Astrophysics, University of Delhi, New Delhi 110 007, India bUniversity Scientific Instrumentation Centre, University of Delhi, New Delhi 110 007, India
Received 29 January 2006; revised 9 April 2006; accepted 10 April 2006. Available online 5 July 2006.
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.