Comparison of the theoretical and experimental band structure of poly(vinylidene fluoride) crystal

– The electronic structure and band symmetries of the ferroelectric phase of poly (vinylidene fluoride) (PVDF) are studied by using the first-principles density-functional method. Our calculated results agree well with the angle-resolved photoemission experiments. We find that the PVDF crystal has an energy gap of about 6 eV at the Γ-point. Large dispersion of the valence band is found only along the chain direction, which is consistent with the quasi– one-dimensional nature of PVDF. In particular, the band symmetries of the valence states deduced theoretically are in good agreement with experiment. Finally, further investigations on the electronic structure of poly(trifluoroethylene) (PTrFE) reveal that the replacement of -(CH2-CF2)by -(CHF-CF2)does not change major features of the band structure. Poly(vinylidene fluoride) [PVDF, -(CH2-CF2)n-] is one of the first known crystalline ferroelectric polymers. Extensive experimental and theoretical efforts have been devoted to the study of their unique piezoelectric, pyroelectric, ferroelectric, and nonlinear optical properties [1–7]. Particularly, ultrathin PVDF and its copolymer films with 30% trifluoroethylene [TrFE, -(CHF-CF2)-] were observed to show a two-dimensional ferroelectric phase transition [8], and also exhibit novel dielectric resonance effects and have large potential application as nonvolatile polymer ferroelectric memories [9]. Thorough photoemission and inverse photoemission studies have been carried out on the high-quality copolymer films, providing an abundance of information concerning the electronic structure, band dispersions and band symmetries [10,11]. However, until now there has been little progress in theoretical efforts to detail and understand the electronic band structure data. The purpose of our work is: first to study the lattice structure and symmetry of PVDF from first principles, then to calculate the electronic structure, which is expected to have strong influence on material properties. In this (∗) E-mail: [email protected].