Coverage-dependent anisotropy of the NTCDA/Ag(111) interface state dispersion

Abstract

We employ density functional theory (DFT) to analyze the dispersion of the electronic state that exists at the commensurate interface between a monolayer of 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) and the Ag(111) surface. First, we present and verify a hydrogen-termination approach which allows a meaningful DFT description of the interface state with relatively thin silver slabs. Complemented with a projection technique which maps the interface electronic structure onto the original Ag(111) Shockley state, the DFT calculations enable us to analyze the evolution of the dispersion of the NTCDA/Ag(111) interface state when changing of the molecular coverage. Our calculations yield a difference between the interface state energy and the Shockley state energy that scales linearly with coverage. Furthermore, they predict a pronounced anisotropy of the dispersion of the interface state at long wavelengths which also depends linearly on the molecular coverage. The dispersion anisotropy is fully confirmed by our Fourier transform (FT) scanning tunneling spectroscopy (STS) experiments performed on a relaxed phase NTCDA/Ag(111) monolayer. Using feature detection STS (FD-STS), we moreover measure a band gap in the interface state band structure at the Brillouin zone boundary which indicates Bragg scattering of the interface state electrons in the periodic potential of the molecular layer. We thus observe an influence of the molecular layer on the interface state both at long (DFT, STS) and short wavelengths (STS).