STM IMAGES OF GUANINE ON GRAPHITE SURFACE AND THE ROLE OF TIP-SAMPLE INTERACTION

A combined experimental and theoretical study is conducted on guanine molecules adsorbed on graphite substrate. The main effort is to unders tand the scanning tunneling microscopic images and the effect of scann ing tip force on guanine molecules and their images. The force on guan ine molecule is determined from the tip-induced deformation in the sub strate and is found to be a few nano-Newtons per guanine molecule. A j ellium substrate is used to model the graphite substrate in ab initio calculations of electronic density of states. Both the equilibrium hei ght of guanine and tip force on guanine are determined using pseudopot ential density functional calculations of system's total energy. The c alculated height of guanine molecule above the substrate is in good ag reement with measurements. Electronic states are calculated and it is shown that features in surface density of states are mostly due to adm ixture with the guanine LUMO and the admixture increases with increasi ng scanning tip force. (C) 1996 American Institute of Physics.