Nonlinear optical properties and crystalline orientation of 2-methyl-4-nitroaniline layers grown on nanostructured poly(tetrafluoroethylene) substrates

A new methodology to study the second-order nonlinear optical coefficients of organic materials was developed. The combination of oriented crystal growth (epitaxy or grapho-epitaxy) with grazing incidence X-ray diffraction and second harmonic ellipsometry gives an overview of the relations between the molecular hyperpolarizability and the crystalline nonlinear susceptibility tensor. Thin organic layers of 2-methyl-4-nitroaniline (MNA) were grown on nanostructured poly(tetrafluoroethylene) (PTFE) substrates prepared by the friction transfer method. The investigations show that MNA crystallizes with two definite single-crystallike orientations depending on the substrate temperature. The low- and high-temperature modes of growth correspond to a deposit of the aromatic ring more or less flat ((11-2) orientation) or edge ((010) orientation) on the substrate, respectively, the transition temperature corresponding to the first order-disorder transition of PTFE (similar to 19 degrees C). Two different orientation mechanisms were then proposed. At low temperature, the intermolecular interactions at the overlayer-substrate interface induce a coincident epitaxy that corresponds to a specific physisorption of aromatic molecules on the polymer surface, whereas above the PTFE order-disorder phase transition, the drastic decrease of the nucleation rate strongly suggests that the orientation is achieved via a grapho-epitaxy that occurs on topological defects of the surface. From the second harmonic ellipsometry measurements, the ratio between the nonvanishing components of the first nonlinear susceptibility tensor (chi(ijk) or d(ij)) was measured. Finally, the experimental observations were discussed in relation to the oriented gas model and the components of the molecular hyperpolarizability tensor (B-ijk).