The structure of the uppermost layer wasinvestigated by means of AFM, operating inthe tapping mode. Topographic images ofthe surfaces were taken and processed by animage analysis program. Pair distributionfunction analysis was performed based onthe particle centre positions in theuppermost layer. From the pair distributionfunction, g(r), the correlation length ( ) ζwas calculated. The correlation length wasnormalized with respect to the particlediameter, d.