Friction Force Microscopy of AgBr Crystals: Ag⁰ Rods and Adsorbed Gelatin Films

Friction force microscopy was performed on the (111) surface of solution-grown AgBr crystals before and after deposition of gelatin films. Images of the initial surface identify the dissimilar chemical nature of the AgBr and rod-shaped crystallites which grow along the 〈110〉 family of crystal directions during imaging and are attributed to Ag⁰. Pores in gelatin films on AgBr and on highly-oriented pyrolytic graphite are seen to extend to the substrate, based on a reduced frictional force sensed at the bottom of the pores. The images also distinguish frictional contributions of “physical” versus chemical origin. The former are manifest as variations in image contrast seen at subnanometer-scale steps in AgBr(111) and at the edges of the Ag⁰ crystallites and the gelatin pores and reflect additional cantilever torsion at these locations. The quantitative dependence of this effect on the size and shape of surface asperities is discussed, and an expression is derived which relates cantilever torsion to both chemical and physical components of friction.