Observation of electron orbital signatures of single atoms within metal-phthalocyanines using atomic force microscopy

Resolving the electronic structure of a single atom within a molecule is of fundamental importance for understanding and predicting chemical and physical properties of functional molecules such as molecular catalysts. 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 high-resolution noncontact atomic force microscopy (HR-AFM). HR-AFM imaging reveals that the Co atom is brighter and presents four distinct lobes on the horizontal plane 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 to directly observe orbital signatures using HR-AFM should provide a promising approach to characterizing the electronic structure of an individual atom in a molecular species and to understand mechanisms of certain chemical reactions. Resolving the orbital structure of single atoms is challenging and of great importance for understanding basic chemistry. Here, the authors demonstrate that the orbital occupation difference of single Fe/Co atoms within molecules can be distinguished with high resolution AFM imaging and spectroscopy.

Automated summary

By using high-resolution noncontact atomic force microscopy, the authors successfully identified two adjacent transition-metal atoms (Fe and Co) within phthalocyanine molecules. The AFM imaging showed that the Co atom was brighter and had four distinct lobes on the horizontal plane, while the Fe atom had a square morphology. Pico-force spectroscopy measurements revealed a larger repulsion force exerted by Co compared to Fe. The experimental and theoretical results demonstrated that the distinguishable features in AFM images and the variation in measured forces were due to Co's higher electron orbital occupation above the molecular plane. The ability to directly observe orbital signatures using HR-AFM provides a promising approach for characterizing the electronic structure of individual atoms in molecular species and understanding mechanisms of certain chemical reactions.