Topography and surface potential in Kelvin force microscopy of perfluoroalkyl alkanes self-assemblies

Abstract

Self-assemblies of two perfluoroalkyl alkanes [F(CF2)n(CH2)mH with n=14, m=20 and n=12, m=8] on different substrates (mica, Si, and graphite) were examined with Kelvin force microscopy (KFM) by detecting either the electrostatic tip-sample force or its gradient. Both measurements were realized in the single pass operation using different frequencies for monitoring topography and surface potential changes. Furthermore, the KFM imaging was performed in the intermittent contact regime that allowed high-resolution mapping of surface potential changes without noticeable cross-talk between signals related to mechanical and electric forces. The use of force gradient for electrostatic force measurements led to high-resolution surface potential images of the perfluoroalkyl alkane adsorbates with strong contrast of the particular self-assemblies (spirals, ribbons, and toroids). The obtained potential data are close to ones recorded with macroscopic Kelvin probe and to theoretical estimates. The surface potential correlates with the orientation of molecular dipoles of –CF3 and –CF2–CH2– groups with respect to the substrate. Therefore, topography and surface potential data indicate that the fluorocarbon parts are mostly oriented vertically, whereas hydrocarbon segments are partially or fully spread on the substrate. In addition, the ability of KFM to monitor a transformation of the self-assemblies was demonstrated by studies of these objects in humid air.