Direct Evidence for an Ion-by-Ion Deposition Mechanism in Solution Growth of CdS Thin Films

A systematic investigation of the mechanism of cadmium sulfide solution growth using atomic force and scanning electron microscopy was performed. The process that was studied involved the slow generation of sulfide ions via the controlled hydrolysis of thiourea in the presence of cadmium nitrate and triethanolamine, a chelating ligand. The formation of two types of films was observed, a mirrorlike primary film and a secondary layer of larger particles that was loosely attached to the first film. The substrates used for the deposition were either bare mica or mica with a partial self-assembled monolayer of octadecylphosphonic acid, CH 3 (CH 2 ) 17 PO 3 H, which served as an internal height standard due to the formation of submonolayer islands 18 ± 2 Å tall. Islands of CdS were observed to nucleate on the remaining bare patches of mica suggesting a potential route to lateral surface patterning. The CdS grew to a height of 22−23 nm, leveled off by 6.5 h, and then proceeded to grow in scattered domains 60−70 nm tall with a few spots as high as 80−100 nm after 10 h. The mechanism of growth has been attributed to either involve attachment of colloidal particles to the surface or an ion-by-ion deposition process. Atomic force microscopy has provided compelling evidence that the latter mechanism is the correct one.