Nano-indentation at the surface contact level: applying a harmonic frequency for measuringcontact stiffness of self-assembled monolayers adsorbed on Au

Abstract

In this study, the well-ordered alkanethiolate self-assembled monolayers (SAMs) of variedchain lengths and tail groups were employed as examples for nano-characterization on theirmechanical properties. A novel nano-indentation technique with a constant harmonicfrequency was applied on SAMs chemically adsorbed on Au to explore their contactmechanics, and furthermore to interpret how SAM molecules respond to an infinitesimaloscillation force without pressing them. Experimental results demonstrated that theharmonic contact stiffness along with the measured displacement of SAMs/Au wasdistinguishable using a dynamic contact modulus with the distinct feature of phaseangles. Phase angles resulted from the relaxing continuation of an applied harmonicfrequency and mostly influenced by the outermost tail group of SAM molecules. Theharmonic contact stiffness of SAM molecules obviously increased with the denselypacked alkyl chains and relatively intense agglomeration of the head group at theanchoring site. As a consequence, the result of this work is relevant to contactmechanics at the surface contact level for the distinction of molecular substancesattached on a solid surface. Furthermore it is particularly anticipated to identifybiological molecules of variable qualities under a fluid-like micro-environment.