Theoretical and Experimental Study of Modulation of the Fermi Level of CuPc/HOPG Via NO Adsorption

Metal phthalocyanines (MPc) have been widely used for chemical vapor sensing due to their novel gas adsorption properties. In this work, we present both theoretical and experimental study of copper phthalocyanine (CuPc) interactions with a nonreactive substrate, HOPG, showing ground-state electronic modulation of the substrate near the Fermi level. In order to study the NO interactions onto CuPc molecules, we performed both van der Waals corrected spin-polarized periodic density functional theory (DFT) calculations and site specific scanning tunneling spectroscopy (STS) experiments. Both calculations and experimental results suggest that NO interacts weakly (physisorption) with CuPc/HOPG surface. The low coverage of NO adsorbates on HOPG in STM images is consistent with the small perturbation of neighboring density of states of CuPc. DFT calculations show that as NO adsorbs on metal center of CuPc, negative charge carriers are injected from CuPc molecules to NO oxidant. During this charge transfer process, excess electrons can be transfer to CuPc from the underneath layer of the substrate, HOPG, and HOPG substrates may be doped to p-type. The binding of NO to CuPc/HOPG also leads to the excess opening of the band gap by 0.18 eV as compare to the experimental value of 0.25 eV. The results of this work have shown that CuPc/HOPG could be a potential sensor device for oxidative analytes.