Energy level alignment at metal-octaethylporphyrin interfaces.

We studied the electronic structure of copper-octaethylporphyrin (CuEOP) adsorbed on three metal surfaces--Ag(001), Ag(111), and Cu(111)--by means of ultraviolet photoelectron spectroscopy (UPS). The adsorption-induced work function shifts saturate roughly beyond two monolayers. The saturation values are substrate dependent, negative, and range from -1.30 to -0.85 eV. This shift is larger than that for tetraphenylporphyrins. The two highest occupied molecular orbitals (HOMO and HOMO-1) of the organic are clearly resolved in the UPS spectra. The origin of the negative work function shift is discussed.