X-ray Diffraction Studies Of Multiple Orientation In Poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl) Thin Films

The structural investigation of poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl) (PF2/6) in aligned thin films is presented. Formation of a thickness dependent triaxial texturing is identified in thermotropically aligned films. X-ray reflectivity measurements reveal good macroscopic quality, and polarized photoluminescence and dichroic ratios in absorption indicate clear axial alignment. Grazing-incidence X-ray diffraction shows axially aligned mesomorphic structure with a distinct arrangement of helices and large correlation lengths, indicating a high local lateral order. Theoretical models produced using molecular mechanics methods suggest 5/2-helicity. The polymer chains are parallel to the substrate in the c direction. In particular, the hexagonal-like cells are flattened in the direction of the surface normal and reveal two kinds of coexistent crystallites, a multiple orientation where the greater proportion of the crystallites have one crystal axis a perpendicular to the substrate surface, whereas a smaller proportion is aligned with the crystal axis a parallel to the surface. In thinner films the former class of orientation is usually dominant, while the proportion of the parallel orientation type increases with prolonged annealing

Influence Of Molecular Weight On Self-organization, Uniaxial Alignment, And Surface Morphology Of Hairy-rodlike Polyfluorene In Thin Films

We present investigations of the self-organized structure, overall uniaxial alignment, and larger-scale interface morphology in thin films of low-molecular-weight hairy-rodlike,pi-conjugated poly(9,9-bis(ethylhexyl)-fluorene2-7-diyl), (LMW-PF2/6) (M-n = 7600 g/mol) and a comparison of high-molecular-weight PF2/6 (HMW-PF2/ 6) (M-n = 150 000 g/mol). Photoabsorption, grazing-incidence X-ray diffraction, small/wide-angle X-ray scattering. and X-ray reflectivity methods have been used. The experiments have been supported by molecular mechanics-calculated molecular structure and recursively simulated reflectivity curves and discussed in terms of the semiquantitative theoretical analysis of the self-organization of hairy-rodlike polymers. As with HMW-PF2/6. LMW-PF2/6 is found to be a thermotropic liquid crystal consisting of rodlike 5/2 helices. Blue photoluminescence with an absolute photoluminescence quantum yield of 32% in the solid films is observed. After thermotropic alignment on rubbed substrates, considerably higher dichroic ratios in absorption, >10, are found, indicating a far higher degree of axial alignment compared to the similarly processed HMW-PF2/ 6. The degree of spatial order has been found to be high along the rubbing direction, the z axis but, in contrast to HMW-PF2/6, the structure perpendicular to the z axis on the (ab0) plane is observed to be less ordered, and no multiple orientation (Knaapila, M.; Lyons, B. P.; Kisko, K.; Foreman, J. P.; Vainio, U.; Mihaylova, M.; Seeck, O. H.; Palsson, L.-O.; Serimaa, R.; Torkkeli, M.; Monkman, A. P. J. PhYs. Chem. B 2003, 107, 12425-12430) is seen. Both in bulk and in thin aligned films, LMW-PF2/6 is suggested to approach a nematic instead of a well-defined hexagonal structure. These findings are in agreement with the presented theoretical arguments. LMW-PF2/6 is also shown to form well-developed larger-scale morphology and surface roughness below 2 nm in films over the thickness range from 20 to 200 nm. There is also a difference in macroscopic texture in polarized micrographs between these materials, but the influence of the molecular weight cannot be rigorously established at present

Diffraction analysis of highly ordered smectic supramolecules of conjugated rodlike polymers.

A small/wide-angle X-ray scattering and grazing incidence diffraction study of comb-shaped supramolecules of conjugated poly(2,5-pyridinediyl), acid dopant and hydrogen bonded amphiphilic side chains is reported. In solution, polymers are dissolved rodlike particles. When the side-chains are introduced, polymers self-assemble in hierarchic liquid crystals (LC). Diffraction patterns of aligned LC show h00, 020, and 004 reflections, and additional small-angle reflections along the polymer axis. A triangular correlation function indicating a very large correlation length is seen along the smectic axis. An aligned solid structure can be formed by cleaving side chains from the aligned LC

Lattice parameter accommodation between GaAs(111) nanowires and Si(111) substrate after growth via Au-assisted molecular beam epitaxy

Using out-of-plane and in-plane X-ray diffraction techniques, we have investigated the structure at the interface between GaAs nanowires [NWs] grown by Au-assisted molecular beam epitaxy and the underlying Si(111) substrate. Comparing the diffraction pattern measured at samples grown for 5, 60, and 1,800 s, we find a plastic strain release of about 75% close to the NW-to-substrate interface even at the initial state of growth, probably caused by the formation of a dislocation network at the Si-to-GaAs interface. In detail, we deduce that during the initial stage, zinc-blende structure GaAs islands grow with a gradually increasing lattice parameter over a transition region of several 10 nm in the growth direction. In contrast, accommodation of the in-plane lattice parameter takes place within a thickness of about 10 nm. As a consequence, the ratio between out-of-plane and in-plane lattice parameters is smaller than the unity in the initial state of growth. Finally the wurtzite-type NWs grow on top of the islands and are free of strain