Noncontact AFM(ncAFM) in aqueous condition is an important method for imaging soft biological nano materials as protein. We are attempting theoretical simulation for the atomic force microscopy(AFM) in water[1,2]. So far we found various remarkable features of tip-surface interaction force mediated by water molecules[2]. They are obtained either by the molecular dynamics (MD) calculation or by the 3D-RISM (Reference Interacting Site Model) calculation.

 As an example of the numerical simulation by MD method, AFM images and 3D force map of mica surface in water were calculated and compared with the experiments by Yamada’s Group. The oscillatory layer structures of water exist around the interface tracing the nano-scale shapes of the sample. The phase and the amplitude of the force oscillation felt by the tip are remarkably changed depending on the lateral tip position, resuting a complicated network structure of the 3D force map. The number of the water layers in the narrow gap between the tip and sample decreased abruptly and finally disappears when the tip approaches to the surface.

 The 3D-RISM method was applied to clarify the local charge separation effect on the interaction force in water. The distribution of water molecules around the atomically charged portion on the substrate is significantly disturbed, and this effect is observed in the 3D-force map from much far region as compared with that in vacuum. We discuss how drastically different the water mediated tip-sample force compared with that in vacuum, and how they influence on the images.

 

[1]M. Tsukada, N. Watanabe, Jpn.J.Appl.Phys., vol.48 No.3 2009

[2] Tsukada, Tagami , Phys. Rev. B submitted.