Physical vapour deposition of alkyl phosphonic acid on mica and HOPG investigated by NC-AFM

We investigate the growth of octadecylphosphonic acid ( OPA) self- assembly molecules prepared by physical vapour deposition ( PVD) on mica and highly oriented pyrolytic graphite ( HOPG) under ultrahigh- vacuum conditions. On samples prepared by immersion from diluted solution on mica self- assembled monolayers are formed, whereas by PVD and subsequent annealing we observe the formation of almost perfect self- assembled bilayers slightly tilted with respect to the surface normal. On the non- polar surface of HOPG, the vapour- deposited molecules adsorb in bilayers parallel to the surface, similar to the films produced by spread coating ( Fontes and Neves 2005 Langmuir 21 11113). On all samples we deduce the molecular ordering by means of noncontact atomic force microscopy; on HOPG, even submolecular resolution is obtained. A comparison of our vapour- deposited films with samples prepared by other techniques mentioned in the literature demonstrates that PVD yields excellent film quality, and in many applications might be preferred due to its clean, solution- free environment and the possibility for exact dosage, e. g. for multilayer formation with a defined thickness.