Metal-induced gap states in epitaxial organic-insulator/metal interfaces

Abstract

We have shown, both experimentally and theoretically, that the metal-induced gap states ͑MIGS͒ can exist in epitaxially grown organic insulator/metal interfaces. The experiment is done for alkane/Cu͑001͒ with an element-selective near edge x-ray absorption fine structure ͑NEXAFS͒, which exhibits a prepeak indicative of MIGS. An ab initio electronic structure calculation supports the existence of the MIGS. When the Cu substrate is replaced with Ni, an interface magnetism may be possible with a carrier doping.

Electronic structure of octane on Cu(1 1 1) and Ni(1 1 1) studied by near edge X-ray absorption fine structure

“…Using the polarization dependence of the σ*(C-C) peak, the mean inclination angle of the octane molecules from the surface was determined to be 15(±10)° on both substrates (lying-down configuration). The lying-down configuration agreed with our previous results of the n-C 44 H 90 (TTC) film on Cu(001) [8].…”

“…Therefore, the pre-peak observed by NEXAFS originated not from a strong chemical bond at the interface, but from the states formed by the proximity to a metal. The states could be qualitatively understood as metal induced gap states, MIGS [7,8]. The formation of MIGS at alkane/metal interfaces agreed with the previous study of octane/Cu(110) [5] and TTC/Cu(001) [8].…”

“…The intensity of the MIGS was larger for the grazing X-ray incidence than normal X-ray incidence on both Ni(111) and Cu(111), indicating that the MIGS had a p z like structure. This character of MIGS agreed with other insulator/metal systems, such as alkali halides/metals and organic insulator crystals/metals [7,8]. Then, we compared the intensity of the pre-peak observed for the 1 ML thick octane films on Cu(111) and Ni(111).…”

“…We discuss the reason why there was little difference in the intensity of the pre-peak between Ni(111) and Cu(111). Figure 4 shows the schematic energy diagram for MIGS in octane/Cu(111) and Ni(111) obtained by first-principles (density functional theory) electronic-structure calculations [8]. The MIGS band of octane/Ni lies below the Fermi level, so the density of states in the unoccupied side is small, while the density of occupied states is large.…”

“…NEXAFS is based on the X-ray absorption by atoms, and thus, it is atomic species selective. We can study the electronic structure of the molecule itself with negligible influence from the substrate [7,8]. In the present study, we have investigated the atomic and electronic structure of the octane films grown on metals substrates with NEXAFS.…”