Atomic force microscopy studies of mesoscopic membranous bubbles on monolayers derived from SiCl3-terminated...

Monolayers of dendrimers were prepared on mica by spin-coating of the second generation carbosilane dendrons with 9 SiCl(3) periphery groups. AFM images of the films showed the presence of soft yet robust, dome-shaped features with a base diameter of 100-2000 nm. The apparent height of the features, ranging from 10 to 200 nm, rapidly reduced under increasing compression force, eventually to the same value ( approximately 2.5 nm) corresponding to a bilayer of the flattened dendrons. The change in shape of the features in response to the compression force from the AFM tip was fully reversible, indicating that the features were robust. The contrast of the features in the tapping mode AFM (TMAFM) phase images flipped at a setpoint ratio of approximately 0.55. In contrast to the reported amplitude vs displacement (A/z) curves for compliant materials, A/z curves of the features showed that the reduction of amplitude was larger than the tip displacement as if the cantilever tip were repelled by the soft features. This result cautions the use of amplitude/phase vs displacement (APD) curves for interpreting TMAFM images and for optimizing conditions for TMAFM imaging of very soft and "sticky" surfaces. On the basis of the AFM studies, we believe that the dome-shaped features are membranous air bubbles. The membranes of the bubbles were probably composed of a bilayer of the dendron molecules bound through the peripheral silanol groups. The bilayer could be formed by self-assembly of the molecules on top of the air bubbles entrapped at the monolayer/solution interface during spin-coating.