Surface potential studies using Kelvin force spectroscopy

Abstract

A home-built Kelvin force microscope combined with a spectroscopic method is dedicated to local measure- ments of topography and contact potential differences (CPD). This technique is based on a simple modification of a non- contact atomic force microscopy (nc-AFM), where a bias voltage is applied between the sample and tip during the CPD measurement. The changes in CPD between the tip and vari- ous sample materials have been measured. Kelvin force spec- tra of Pd=Si and of self-assembling monolayer films (SAMs) are presented. The Kelvin method (1) is a well-established technique for measuring the contact potential differences (CPD) between a reference electrode and a sample. The CPD for clean metal surfaces is given by the difference in the work functions of the two materials. A common method of measuring the con- tact potential difference is the vibrating capacitor method or Kelvin method. In this method, the probe, which is made of a material with a known work function (commonly tungsten or platinum), is arranged close to the surface to be measured as a parallel plate capacitor with small spacing. In a sim- ple model, the contact potential between the two materials is VCPDD 1=e.2 1/ ,w here 1 and 2 are the work func- tions of the conductors, including changes due to adsorption layers on the surface. A periodic vibration between the two plates at frequency ! with amplitude d1 results in a current i.t/ given by i.t/D @C