Tip-sample interaction force mediated by water molecules for AFM in water: Three-dimensional reference interaction site model theory

Abstract

Force curves and force maps of atomic force microscopy measurements for hypothetical nanostrucutres in aqueous environment have been simulated using the three-dimensional reference interaction site model (3D-RISM) theory. Various unexpected features of water mediated force are found including the directions of forces opposite to those in vacuum with local charges on both the tip and the sample. From those features and the density distributions of water around the peripheral of nanostructures obtained by the 3D-RISM theory, a close correlation has been found between the free energy of the system and the intactness of water density distributions around the tip and the sample surface. Moreover, it is shown that the 3D-RISM theory is much more precise for AFM simulations in water than the RISM theory from the fact that some of the important features cannot be described by the RISM theory.