Evidence for structural and electronic anisotropy in CdS nanocrystal layers from Langmuir-Blodgett film precursors

Ultrathin semiconductor layers made of CdS nanocrystals synthesized in cadmium arachidate Langmuir-Blodgett films have been investigated by means of polarized optical and infrared absorption spectroscopy, resonance Raman scattering and X-ray diffraction during two different phases of the formation process (before and after the removal of the organic precursor matrix). Optical spectroscopy revealed that CdS nanocrystals are anisotropic, likely ellipsoidal in shape, laying with their major axes in the film plane. Resonance Raman scattering confirmed these results by monitoring, in two different experimental geometries and after different annealing conditions, the coupling to the lattice of the lowest electronic excited state. The reported data allow to describe these samples as made of anisotropic CdS nanocrystals surrounded by a small residual amount of fatty acids. Finally, we have demonstrated the possibility of varying the CdS crystal size from 3 to 10 nm upon the removal of the organic matrix and the annealing procedure. (C) 1998 Elsevier Science Ltd. All rights reserved.