Chemical force microscopy of self-assembled monolayers on sputtered gold films patterned by phase separation

Abstract

Patterned self-assembled monolayers (SAMs) were formed on gold films and observed by friction force microscopy (FFM) and adhesive force mapping with pulsed-force mode atomic force microscopy (PFM–AFM). The substrate gold films were prepared by sputtering gold on flat surfaces of osmium-coated cover glass with surface roughness, R a, of 0.3 nm. The patterned samples with the CH 3 and COOH terminated regions were prepared using the Langmuir–Blodgett (LB) method, partial removal of the LB film by ultrasonication, and SAM formation. The CH 3 and COOH terminated regions of the patterned SAMs in air and in water were observed by mapping friction and adhesive forces with FFM and PFM–AFM, respectively, using gold-coated AFM tips chemically modified with a thiol compound terminating in CH 3 or COOH. The adhesive forces measured in air increased in the order of CH 3/CH 3, CH 3/COOH (or COOH/CH 3), and COOH/COOH, while those in water increased in reverse order. The enormous high adhesive force observed in water for CH 3/CH 3 was attributed to hydrophobic interaction between the CH 3 tip and the CH 3 terminated sample surface. With CH 3 tip, the lower friction force was observed, however, in water on the CH 3 terminated region than on the COOH terminated region. This experimental finding raises a question as to what is the effective normal load in friction measurements in water.