Visible effects of static electric field on physical vapor growth of lead phthalocyanine crystals

To elucidate the effects of a static electric field on the crystal growth of a molecule with both a molecular dipole and a quadrupole moment, we performed physical vapor growth of monoclinic lead phthalocyanine (PbPc) crystals under quasithermal equilibrium conditions and observed distinct effects of the applied electric field on the drift motion of the molecules that diffused over the substrate surface. The density of crystals grown on the Au electrode with a high electric potential exceeds the crystal density on both an electrically grounded electrode and a glass substrate surface. In addition, this difference in crystal density increases with applied voltage. This biased distribution of crystal growth is explained by the drift motion of diffusing molecules, which is induced by the interaction of the electric field with molecular dipole and quadrupole moments. However, the long-range oriented growth that is clearly seen in the physical vapor growth with an electric field for copper phthalocyanine is not observed in the crystal growth of PbPc; only a locally oriented growth is observed in the vicinity of the Au electrode. For electric-field-sensitive PbPc, it is difficult to maintain quasithermal equilibrium conditions over a wide area because the distribution of the diffusing molecules varies as a function of the spatially nonuniform electric field, which disturbs the long-range oriented growth. (C) 2011 American Institute of Physics. [doi:10.1063/1.3553859]