Characterization of CdS nanoparticles formed and aggregated in stearic acid Langmuir-Blodgett films by atomic force microscopy

Alternate films, which are composed of stearic acid and CdS nanoparticles w ere synthesized by exposing Langmuir-Blodgett (LB) films of cadmium stearat e (CdSt(2)) to H2S gas at a pressure of 1 Torr. The changes of surface morp hology of film with the increased reaction time were directly observed by a tomic force microscopy for the first time. Before being exposed to H2S, the surface of CdSt(2) LB film was homogeneous from microscale down to nanosca le, and it was observed that CdSt(2) molecules formed a well orderly rectan gular herringbone lattice structure on the molecular scale. However, after being exposed to H2S the ordered CdSt(2) molecules gradually changed into a disordered state, and eventually the LB film surface became rough with the apparent feature of bulk structures on the nanoscale. This change in the m orphology can be attributed to the aggregation of buried CdS nanoparticles within LB films, which has been confirmed by a structured UV-visible absorp tion spectrum where the absorption edge is red-shifted about 0.7 eV with re spect to bulk CdS. Finally, the aggregation mechanism of CdS in the LB film was analyzed. (C) 1998 Elsevier Science Limited. All rights reserved.