  We report modulation of a reversible phase transition in VO2 films by
hydrogen doping. A metallic phase and a new insulating phase are successively
observed at room temperature as the doping concentration increases. It is
suggested that the polarized charges from doped hydrogens play an important
role. These charges gradually occupy V3d-O2p hybridized orbitals and
consequently modulate the filling of the VO2 crystal conduction band-edge
states, which eventually evolve into new valence band-edge states. This
demonstrates the exceptional sensitivity of VO2 electronic properties to
electron concentration and orbital occupancy, providing key information for the
phase transition mechanism.
