Quantitative determination of a model organic/insulator/metal interface structure

Abstract

By combining X-ray photoelectron spectroscopy, X-ray standing waves and scanning tunneling microscopy, we investigate the geometric and electronic structure of a prototypical organic/insulator/metal interface, namely cobalt porphine on monolayer hexagonal boron nitride (h-BN) on Cu(111). Specifically, we determine the adsorption height of the organic molecule and show that the original planar molecular conformation is preserved in contrast to the adsorption on Cu(111). In addition, we highlight the electronic decoupling provided by the h-BN spacer layer and find that the h-BN-metal separation is not significantly modified by the molecular adsorption. Finally, we find indication of a temperature dependence of the adsorption height, which might be a signature of strongly-anisotropic thermal vibrations of the weakly bonded molecules.

Fig. 1. Co–P on h-BN/Cu(111). STM images at 6 K display the preferred adsorption of Co–P molecules in the pores of the h-BN layer on Cu(111) for coverages of (a) ∼0.15 ML, and (b) ∼0.3 ML. (c) The XP spectrum of the Co 2p_3/2 core level (recorded at 50 K, coverage ∼0.15 ML) exhibits a complex line shape due to the occurrence of multiplet splitting and a satellite structure. Scan parameters: (a) U_b = 1.0 V, I_t = 41 pA, (b) U_b = 1.23 V, I_t = 120 pA. The inset shows a structural model of the Co–P molecule (gray: carbon, blue: nitrogen, orange: cobalt, and white: hydrogen).

Fig. 2. Normal incidence XSW Co 2p_3/2 absorption profiles of Co–P on h-BN/Cu(111) in the low coverage regime (∼0.15 ML) at the (111) Bragg reflection, acquired at (a) 300 K and (b) 50 K, respectively, integrated over both the Co₁ and Co₂ species (see Fig. 1c). Solid lines are fits to the data. Black data points are the reflectivity curve.

Fig. 3. Schematic model of the Co–P molecule on h-BN/Cu(111) with the indicated average adsorption heights of the h-BN layer and the Co center for low coverage (∼0.15 ML) at 300 K and at 50 K, respectively. Note that the vertical corrugation of the h-BN layer was not considered in the sketch and only the average position is taken.