ATOMIC FORCE MICROSCOPY IMAGING OF SUBSTRATE AND PH EFFECTS ON LANGMUIR-BLODGETT MONOLAYERS

The atomic force microscope (AFM) provides a nondestructive way to image Langmuir-Blodgett (LB) films to determine film quality at length scales from nanometers to micrometers. Monolayers of cadmium arachidate deposited on mica and silicon at pH values of 5.5 and 6.5 were imaged by the AFM. Holes 10-100 nm in diameter were present in the monolayer on mica made at a pH of 5.5. The depth of the holes was roughly 3 nm, corresponding to the thickness of a monolayer. However, an image of a monolayer deposited on mica at a subphase pH of 6.5 yielded a uniform surface, although the film was more easily damaged by the AFM than the pH 5.5 monolayers. Monolayers on polished silicon wafers with native oxide were homogeneous and hole-free for films made at pH 5.5 and 6.5. The differences in film quality were ascribed to the less extensive hydration of the mica at pH 5.5, with secondary effects due to the pH-dependent complexation of cadmium with the arachidic acid. The microscopic roughness of the monolayers was not measurably different from that of the substrates. No lattice structure was observed for the monolayers, regardless of substrate, suggesting a disordered, fluidlike surface.