Scanning tunneling microscopy of the conducting transition metal complex, copper bis-(2,5-dimethyl- N, N′-dicyanoquinonediimine)

Abstract

We have studied the conducting transition metal complex, copper bis-(2,5-dimethyl- N, N′-dicyanoquinonediimine) (Cu(2,5-DM-DCNQI) 2), using scanning tunneling microscopy (STM). The crystal face imaged by STM at molecular resolution shows structural periodicities of 11 and 4 Å along the orthogonal directions. This surface structure agrees with the reported bulk crystal structure of Cu(2,5-DM-DCNQI) 2, with 11 Å periodicity indicative of close-packed planes of Cu(2,5-DM-DCNQI) 2 complexes along the crystallographic c axis and the 4 Å 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 highest electron density resides on the benzene rings, the cyano groups and the imino groups. Guided by these MO calculations, distorted rhombic cells in the STM image could be drawn by connecting four adjacent high contrast sites, possibly the cyano functional groups of surface-terminating 2,5-DM-DCNQI ligands.