Atomic force microscopy of subsurface structures

Abstract

Atomic force microscopy techniques of rapid indentation make it possible to obtain physical and mechanical properties simultaneously with the topography of the surface. In the general case, the obtained relief depends on the load with which the probe indents the surface. The paper presents a method of treatment of the force curves that highlights the changes in the relief at each individual step of the applied load: the surface topography (the moment of the probe touching the material), the transition region, and the sub-surface structures are separated (standard software only provides the topography corresponding to the maximum of the applied load). The proposed approach is illustrated by studying both soft and relatively stiff materials. In the first case, a two-phase polymer (polyurethane) was considered – the fibrillar structure of the polymer hidden by the soft nanolayer was shown; a study of the protein layer deposited on the surface of this polymer revealed a dendritic nanostructure of the protein film under the homogeneous surface layer. In the second case, a polymer modified by plasma implantation of carbon ions was investigated – subsurface inhomogeneities of the implanted carbon layer were shown; deformation-induced nanocracks were found to be first formed under the surface in the stiffest part of the modified layer. The paper proposes a method for estimating the thickness of the soft surface layer. It is also shown that the most complete picture of the subsurface structure does not correspond to the maximum of the applied load. The paper raises the problem of investigating adhesion interactions and the validity of using adhesion in calculating the elastic modulus in cases where the surface of an inhomogeneous material is covered by some nanolayer. The considered methods not only allow one to determine the surface relief and the features of the subsurface structures but also give the knowledge of what the researcher sees when using standard software of the device.