Vacuum-free surface and interface dipole of solids

The electrostatic dipole is among the most fundamental concepts in physics. At the surface or interface of different bulk materials, the dipole which is formed due to local charge density variation is of central importance to the understanding of many phenomena in chemistry, electrical engineering, and materials science. Despite this importance, it has generally been recognized as an ill-defined quantity. Here, we present a universal theory that provides a precise definition of the dipole which is applicable to any type of surface or interface. The key is establishing a vacuum-free surface of bulk solids. In heterojunction interface, this allows us to define a common energy reference among dissimilar materials before the contact. The interface dipole is then explicitly given by the electrostatic potential, rather than, as generally thought, the charge density, and strikingly it is entirely determined by the newly defined intrinsic bulk properties of the constituent materials. Using metal/metal and metal/semiconductor interfaces as examples, we show our theory could provide a completely new perspective into the study of surface and interface sciences.