Oriented crystallization of NLO organic materials

A new methodology to study the second order NLO coefficients of organic materials was developed. We have demonstrated that the combination of oriented crystal growth on structured surfaces, grazing incidence X-ray diffraction and second harmonic ellipsometry gives an insight of the relations between the molecular hyperpolarizability and the crystalline non-linear susceptibility tensor. Thin organic layers of 2-methyl-4-nitroaniline (MNA), N-(4-nitrophenyl)-l-prolinol (NPP) and 2-(N,N-dimethylamino)-5-nitroacetanilide (DAN) were grown on poly(tetrafluoroethylene) (PTFE) substrates prepared by the friction transfer method. From the observed crystallographic orientations, a growth mechanism based on a “soft-epitaxy” was proposed. At low substrate temperature, the overlayer-substrate interactions dominate and induce a two-dimensional epitaxial growth which corresponds to a specific physisorption of aromatic molecules on the polymer surface. At high substrate temperature, the aromatic crystals nucleate via a grapho-epitaxy mechanism which occurs on topological defects of the surface. The formation of specific interactions with the PTFE substrates is not a requisite to orient the crystals in this model. Finally, we will show that the ratio between the non-vanishing components of the first non-linear susceptibility tensor (χ_ijk or d_ij) can be estimated from second harmonic ellipsometry measurements.