Real-space imaging of {\sigma}-hole by means of Kelvin probe force microscopy with subatomic resolution

An anisotropic charge distribution on individual atoms, such as e.g. {\sigma}-hole, may strongly affect material and structural properties of systems. Nevertheless, subatomic resolution of such anisotropic charge distributions represents a long-standing experimental challenge. In particular, the existence of the {\sigma}-hole on halogen atoms has been demonstrated only indirectly through determination of crystal structures of organic molecules containing halogens or via theoretical calculations. Nevertheless, its direct experimental visualization has not been reported yet. Here we demonstrate that Kelvin probe force microscopy, with a properly functionalized probe, can reach subatomic resolution imaging the {\sigma}-hole or a quadrupolar charge of carbon monoxide molecule. This achievement opens new way to characterize biological and chemical systems where anisotropic atomic charges play decisive role.