Orbital resolution of molecules covalently attached to a clean semiconductor surface

Understanding the chemical and electronic nature of molecules attached to semiconductors is of great importance in the study of molecule-based electronic devices. Resolving individual molecular orbitals using scanning tunnelling microscopy is a straightforward approach but remains challenging on the semiconductor surfaces because of their highly reactive dangling bonds. Here we show that hybridized molecular orbitals of pyridazine molecules covalently attached to Ge(100) surfaces can be resolved by scanning tunnelling microscopy. Pyridazine binds to Ge(100) through single/double dative bond(s) and presents two types of features with three and four lobes. These features resemble the lowest unoccupied molecular orbitals of free pyridazine, which are hybridized by the surface states in the adsorbed state. The adsorbing sites, binding mechanisms, orientations and electronic properties of the adsorbed molecules are convincingly determined. Our results indicate that orbital resolution of molecules covalently attached to semiconductors is accessible despite of their high reactivity.