Observation of Electron Orbital Signatures of Single Atoms using Atomic Force Microscopy

Resolving the electronic structure of a single atom bonded within a molecular species is of fundamental importance for understanding and predicting chemical and physical properties. However, the observation of the orbital signature of an individual atom is challenging. We report here the direct identification of two adjacent transition-metal atoms, Fe and Co, within phthalocyanine molecules using noncontact atomic force microscopy (nc-AFM). AFM imaging reveals that the Co atom appears bright and presents four distinct lobes, whereas the Fe atom displays a "square" morphology. Pico-force spectroscopy measurements show a larger repulsion force of about 5 pN on the tip exerted by Co in comparison to Fe. Our combined experimental and theoretical results demonstrate that both the distinguishable features in AFM images and the variation in the measured forces arise from Co's higher electron orbital occupation above the molecular plane. The ability of directly observing orbital signatures using nc-AFM is a promising approach to characterizing the electronic structure of an individual atom in a molecular species.