Investigation of lateral forces in dynamic mode using combined AFM/STM

Abstract

In this Ph.D. work, we constructed a ¯ber optic interferometer based non-contact Atomic Force Microscope (nc-AFM) combined with Scanning Tunneling Micro- scope(STM) to study lateral force interactions on Si(111)-(77) surface. The in- terferometer has been built in such a way that its sensitivity surpasses that of the earlier versions used in normal force measurements. The improvement in the resolution of the interferometer has allowed us to use sub-Angstrom oscillation amplitudes to obtain quantitative lateral force measurements. We have observed single and double atomic steps on Si(111)-(77) surface in topography and lat- eral sti®ness images. This information allowed us to measure the lateral forces directly and quantitatively. We have also carried out lateral force-distance spec- troscopy experiments, in which we simultaneously measured the force gradient and tunneling current, as the sample is approached towards the tip. The lateral force?distance curves exhibit a sharp increase of the force gradient, just after the tunnel current starts to increase, while the sample is approaching to the tip. We observed only positive force gradients. In separate experiments, we imaged the Cu-TBPP molecules deposited on Cu(100) surface in normal and torsional mode in dynamic force microscope us- ing STM feedback, with a homemade tungsten cantilever. Our experiments have shown the possibility of manipulating molecules on surface using a vibrating can- tilever. However the forces involved in these experiments are not quantitatively measured due to limitations of the method.