Role of the Shockley State in Doping of Organic Molecule Monolayer

We present angle-resolved photoemission results of the Shockley state behavior in NTCDA (1,4,5,8-naphthalene tetracarboxylic dianhydride) monolayer covering Na atoms preadsorbed on (111) noble metal surfaces. It is well-know that deposition of low amount of alkali metal atoms on such surfaces induces a shift of the surface state to high binding energies. This shift originates from an electron transfer occurring at the interface. Furthermore, the Shockley state is sensitive to the presence of organic films on the surface. NTCDA monolayer on the (111) surface of noble metals leads to upward displacement of the Shockley state, much stronger in the case of Ag and Cu than Au. Here we observe that in the case of combining both adsorbates on Au(111); deposition of Na atoms followed by growth of NTCDA monolayer, the Shockley state exhibits similar properties as that on pristine Au(111) surface. This behavior is also observed on the (111) surfaces of Cu and Ag and does not depend on the preadsorbed Na amount. We discuss the present phenomenon in terms of Na inducing weakening of the molecule-substrate interaction. Besides that, we demonstrate a new approach to enhancing and controlling the doping process of organic films.