SCANNING-TUNNELING-MICROSCOPY OF THE CONDUCTING TRANSITION-METAL COMPLEX, COPPER BIS-(2,5-DIMETHYL-N,N'-DICYANOQUINONEDIIMINE)

We have studied the conducting transition metal complex, copper bis-(2 ,5-dimethyl-N,N'-dicyanoquinonediimine) (Cu(2,5-DM-DCNQI)2), using sca nning tunneling microscopy (STM). The crystal face imaged by STM at mo lecular resolution shows' structural periodicties of 11 and 4 angstrom along the orthogonal directions. This surface structure agrees with t he reported bulk crystal structure of Cu(2,5-DM-DCNQI)2, with 11 angst rom periodicity indicative of close-packed planes of Cu(2,5-DM-DCNQI)2 complexes along the crystallographic c axis and the 4 angstrom repeat corresponding to the columnar stacking of 2,5-DM-DCNQI ligands along the crystallographic a axis. We have calculated the lowest unoccupied molecular orbital of a 2,5-DM-DCNQI ligand and have shown that the hig hest electron density resides on the benzene rings, the cyano groups a nd the imino groups. Guided by these MO calculations, distorted rhombi c cells in the STM image could be drawn by connecting four adjacent hi gh contrast sites, possibly the cyano functional groups of surface-ter minating 2,5-DM-DCNQI ligands.