Force measurements and tip shape approximation with the atomic force microscope

Abstract

The Atomic Force Microscope (AFM) is an instrument with huge impact on modern research in the nanosciences and in nanotechnology. In AFM, a tip which is sharp on a scale of nanometers or tens of nanometers, is scanning along the surface which is imaged. A few among the large numbers of factors that potentially affect the interaction between the tip and the surface were studied in the experimental work that form this Thesis. Friction and elasticity measurements with AFM and the approximation of the tip shape are discussed. Also the contribution of the imaging environment is studied. A commercial AFM was modified into a so called environmental-AFM (env- AFM) , where the imaging conditions, such as pressure and humidity, can be controlled. The modification was done so that a minimal intrusion to the original AFM was made. The original setup of the AFM can be easily restored. The elasticity of multiwalled carbon nanotubes (MWCNT) was measured by studying the bending due to the van der Waals force of nanotubes that were placed on microfabricated structures on silicon surfaces. The measurements were in agreement with the theoretical model that takes into account the competition between the van der Waals force between carbon nanotube (CNT) and surface and the restoring elastic force of the nanotube. Lateral force (LF) and pull-off distance measurements as a function of humidity on a hydrophilic silicon showed a maximum at the relative humidity (RH) slightly above 30 %. The coincidence of the maximum for both measurements was shown for the first time in this work. The precise shape of the AFM tip was determined with MWCNTs and micro fabricated steps for different kind of tips in contact and in tapping mode.