Effect of Cyano Substitution on the Step-Edge Adsorption of Copper Phthalocyanine on Au(111)

Abstract

Step edges of single-crystal surfaces play an important role in tuning the electronic properties of the surfaces and in guiding the application of surfaces as catalytic reaction centers. Modification of step edges by molecular adsorption can be an effective strategy for bottom-up nanofabrication of surfaces. A detailed submolecular level understanding of step-edge adsorption is mandatory to exploit the properties of step edges for a variety of applications. Though a variety of phthalocyanine (Pc) molecules have been investigated on surfaces, there is a huge void in the literature about step-edge behavior of Pcs on surfaces. With this perspective, the adsorption characteristics of copper Pc (CuPc) and copperoctacyano Pc (CuPc(CN)8) have been investigated on Au(111) monoatomic (MA) step edges using low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. At very low coverage, the adsorption of CuPc and CuPc(CN)8 leads to the formation of one-dimensional chains along the step edge. At higher coverage, both CuPc and CuPc(CN)8 guided by tetramer unit cell formation self-assemble on flat terraces and cross over the step edge of Au(111). CuPc adsorption along MA step edge shows only one geometric configuration, whereas two different geometric configurations occur for CuPc(CN)8. The spectroscopic signature of these two configurations, probed using scanning tunneling spectroscopy (STS), manifests in a shift of the peak position of the highest occupied molecular orbital for the CuPc(CN)8 molecule at the MA step edge with respect to the molecule over the flat terrace of Au(111). The STM images simulated on the basis of DFT calculations for specific configurations agree with the experimental results. These findings also advance our understanding of the role played by the pendant groups of the Pc molecules in step-edge adsorption.