Effect of thickness on the electronic structure of poly(vinylidene fluoride) molecular films from first-principles calculations

We present first-principles calculations on layered molecular poly(vinylidene fluoride) (PVDF) films. Upon structural relaxation, the largest relaxation of the bond angles of H-C-H and F-C-F are found in films with an F terminated surface. The relaxation leads the subsurface layer atoms to move toward the surface, consistent with observation in experiments. However, for the film of five molecular layers with an H terminated surface, the relaxation causes motion of the subsurface atoms toward the interior. Moreover, the energy gap of the molecular film decreases with an increase in the number of molecular layers. The electric character of the molecular film tends to change from insulator to semiconductor and then to semimetal. The partial density of states of the atoms in the F terminated surface shifts to higher energy than that of bulk PVDF, while that in the H terminated surface shifts to lower energy.