NO Adsorption on Copper Phthalocyanine Functionalized Graphite

NO dosed on a CuPc monolayer deposited on Au(111) and HOPG is observed by scanning tunneling microscopy. After dosing NO with a supersonic molecular beam source onto CuPc/Au(111), about 7% of CuPc molecules form chemisorbates with NO. Conversely, after dosing onto CuPc/HOPG, only about 0.1% CuPc molecules form chemisorbates with NO, even though the reaction sites appear nearly identical. DFT calculations were employed to elucidate the mechanism which causes the >10x difference in saturation coverage between NO/CuPc/Au(111) and NO/CuPc/HOPG. DFT calculations show NO cheinisorption with CuPc/Au(111) induces only negligible perturbation in the density of states (DOS) in Au(111) due to large density of states on Au. Conversely, for NO/CuPc/HOPG, there is a large decrease of DOS in graphene around 1 eV due to NO chemisorption on CuPc/graphene consistent with negative charge transfer from graphene to NO. This DOS perturbation of graphene results in decreased binding energy of NO chemisorption in secondary NO sites, consistent with low saturation coverage. The results suggest that although the saturation coverage of NO chemisorbates is low on CuPc/graphene, the DOS of graphene can be altered by low coverages of adsorbates even onto weakly interacting molecules which chemically functionalize the graphene surface.