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 solutio n growth using atomic force and scanning electron microscopy was perfo rmed. The process that was studied involved the slow generation of sul fide ions via the controlled hydrolysis of thiourea in the presence of cadmium nitrate and triethanolamine, a chelating ligand. The formatio n of two types of films was observed, a mirrorlike primary film and a secondary layer of larger particles that was loosely attached to the f irst film. The substrates used for the deposition were either bare mic a or mica with a partial self-assembled monolayer of octadecylphosphon ic acid, CH3(CH2)(17)PO3H, which served as an internal height standard due to the formation of submonolayer islands 18 +/- 2 Angstrom tall. Islands of CdS were observed to nucleate on the remaining bare patches of mica suggesting a potential route to lateral surface patterning. T he CdS grew to a height of 22-23 nm, leveled off by 6.5 h, and then pr oceeded 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 attrib uted to either involve attachment of colloidal particles to the surfac e or an ion-by-ion deposition process. Atomic force microscopy has pro vided compelling evidence that the latter mechanism is the correct one .