Heterogeneous nucleation on a substrate

Nucleation theory in the case of condensation on a substrate is reviewed. The thermodynamics of adsorbed layer formation is briefly discussed. It was shown that three distinctly different condensation regions exist : a) that of medium supersaturation and an adsorbed layer which is less than a monolayer, b) very high supersaturation and an adsorbed layer which is much less than a monolayer, c) supersaturation very low and adsorbed atoms in one or more monolayers before condensation begins. Nucleation theory is very suitable for application to case a). The kinetics of nucleii formation was derived and the frequency of nucleation was obtained as a function of supersaturation. Collision between nucleii was taken into account and saturation density of the number of nucleii was obtained analytically. Experimental results are shown confirming the theoretical predictions ; in particular ad-atom concentration is shown to exist as well as a saturation density of nucleii controlled by collision process.