Real-space Imaging of the Multiple Halogen Bonds by Ultrahigh-resolution Scanning Probe Microscopy

Understanding the physical origin of STM/AFM image contrast is of significance for not only promoting surface characterization techniques, but also probing surface nanostructures with the atomic/sub-molecular resolution. Herein, we demonstrate the real-space imaging of halogen bonds acquired by non-contact atomic force microscopy (nc-AFM)/bond-resolution scanning tunneling microscopy (BR-STM) with functional CO-tip, and study the image contrast origin of halogen bonds. The presence of bright line features is associated to the specific site where halogen bond forms, which is experimentally evidenced to be contributed by both CO-tip bending and electron density exchange. Three distinct types of halogen bonds are observed, which origin from the noncovalent interactions of Br-atoms with positive potential H-atom, neutral potential Br-atom and negative potential N-atom, respectively. Our work shows that nc-AFM and BR-STM can directly image halogen bonds and can be used to unambiguously discriminate their bonding features. This work demonstrates the potential use of this technique to image other non-covalent intermolecular bonds and to understand complex supramolecular assemblies at the sub-molecular level.