  We report an ab initio study of the electronic properties of surface
dangling-bond (SDB) states in hydrogen-terminated Si and Ge nanowires with
diameters between 1 and 2 nm, Ge/Si nanowire heterostructures, and Si and Ge
(111) surfaces. We find that the charge transition levels e(+/-) of SDB states
behave as a common energy reference among Si and Ge wires and Si/Ge
heterostructures, at 4.3 +/- 0.1 eV below the vacuum level. Calculations of
e(+/-) for isolated atoms indicate that this nearly constant value is a
periodic-table atomic property.
