Tip- Surface Interactions

In the earlier studies of Scanning Tunneling Microscopy the tip-sample distance was generally assumed to be large enough to disregard the tip-surface interaction effects, and the electrodes were considered to be independent. However, modifications of the electronic structure depending upon the tip-surface separation have led to the identification of different regimes (i.e. independent electrode, electronic contact and point contact regimes) in the operation of STM. As the tip approaches the sample surface, the tip-sample interaction gradually increases and the potential barrier is lowered. The charge density is rearranged and the ions of the tip and sample are displaced to attain the minimum of the total energy at the preset tip-sample distance. Owing to the overlap of the tip and sample states site-specific localized states appear and provide a net binding interaction, whereby the tip and sample are connected electronically. Upon further approach of the tip to the sample surface the tunneling barrier is perforated even before the tip enters in the strongly attractive force region. Eventually, the point contact regime is initiated, and new channels of conduction are opened through the electronic states localized in the gap. The conductance undergoes a qualitative change and its commonly accepted proportionality to the local density of states of the unperturbed sample surface is invalidated. In this lecture note, a microscopic analysis of the tip-surface interaction is presented and recent STM studies are examined within the framework of this analysis.