Harding University Course Catalog 1988-1989

Catalog of Harding University 1988-1989https://scholarworks.harding.edu/catalogs/1050/thumbnail.jp

Efficient Blue-Light-Emitting Polymer Heterostructure Devices: The Fabrication of Multilayer Structures from Orthogonal Solvents

An all-solution processed organic light-emitting diode with enhanced device efficiency based on an additional methanol-soluble polyfluorene layer (see figure) with nonionic ethylene glycol side chains is presented. Due to an asymmetric shift of the energy levels at the polymer/polymer interface, significant efficiency enhancements were obtained

Grazing-incidence in-plane X-ray diffraction on ultra-thin organic films using standard laboratory equipment

A novel grazing incidence in plane X ray diffraction setup based on a commercial four circle diffractometer with a sealed ceramic copper X ray tube, upgraded with parabolic graded multilayer X ray optics and a one dimensional position sensitive detector, is presented. The high potential of this setup is demonstrated by a phase analysis study of pentacene thin films and the determination of in plane lattice constants of pentacene mono and multilayers. The quality of the results compare well to studies performed at synchrotron radiation facilitie

A disordered layered phase in thin films of sexithiophene

This Letter reports the impact of the evaporation rate on the crystallographic phase formation of vacuum deposited alpha sexithiophene thin films studied by X ray diffraction methods. The experiments reveal the formation of two crystal phases, one of which is a thermodynamically stable phase occurring at low rates, while the second is favored by high rates. This second phase exhibits an increased layer spacing and diffraction features typical for two dimensional crystals which are laterally ordered but without interlayer correlations of the molecular positions. This disordered layered phase comprises molecules of nonuniform conformations, and is kinetically induce

Microstructure and Phase Behavior of a Quinquethiophene-Based Self-Assembled Monolayer as a Function of Temperature

The self-assembly of monolayers is a highly promising approach in organic electronics, but most systems show weak device performances, probably because of a lack of long-range order of the molecules. The present self-assembled monolayer was formed by a molecule that contains a dimethylchlorosilyl group combined with a quinquethiophene unit through an undecane spacer. This system is the first reported self-assembled monolayer on silicon oxide surfaces that forms two-dimensional crystals. A detailed structural solution is presented based on grazing-incidence X-ray scattering experiments and theoretical packing analysis. By transverse shear microscopy, the shape and size of the crystallites were determined: polygonal shapes with lateral sizes of several micrometers were observed. In situ temperature studies revealed gradual changes of the molecular packing that were irreversible. Melting of the crystal structure was found at 520 K, whereas the self-assembled monolayer remained stable up to 620 K. This work presents unknown structural properties of a self-assembled monolayer revealing insights into layer formation and irreversible evolution upon temperature treatment.

Color Tuning of Nanofibers by Periodic Organic–Organic Hetero-Epitaxy

We report on the epitaxial growth of periodic para-hexaphenyl (<i>p</i>-6P)/α-sexi-thiophene (6T) multilayer heterostructures on top of <i>p</i>-6P nanotemplates. By the chosen approach, 6T molecules are forced to align parallel to the <i>p</i>-6P template molecules, which yields highly polarized photoluminescence (PL)-emission of both species. The PL spectra show that the fabricated multilayer structures provide optical emission from two different 6T phases, interfacial 6T molecules, and 3-dimensional crystallites. By a periodical deposition of 6T monolayers and <i>p</i>-6P spacers it is demonstrated that the strongly polarized spectral contribution of interfacial 6T can be precisely controlled and amplified. By analyzing the PL emission of both 6T phases as a function of <i>p</i>-6P spacer thickness (Δ<i>d</i>_{<i>p</i>–6P}) we have determined a critical value of Δ<i>d</i>_{<i>p</i>–6P}≈ 2.73 nm where interfacial 6T runs into saturation and the surplus of 6T starts to cluster in 3-dimensional crystallites. These results are further substantiated by UPS and XRD measurements. Moreover, it is demonstrated by morphological investigations, provided by scanning force microscopy and fluorescence microscopy, that periodical deposition of 6T and <i>p</i>-6P leads to a significant improvement of homogeneity in PL-emission and morphology of nanofibers. Photoluminescence excitation experiments in combination with time-resolved photoluminescence demonstrate that the spectral emission of the organic multilayer nanofibers is dominated by a resonant energy transfer from <i>p</i>-6P host- to 6T guest-molecules. The sensitization time of the 6T emission in the 6T/<i>p</i>-6P multilayer structures depends on the <i>p</i>-6P spacer thickness, and can be explained by well separated layers of host–guest molecules obtained by organic–organic heteroepitaxy. The spectral emission and consequently the fluorescent color of the nanofibers can be efficiently tuned from the blue <i>via</i> white to the yellow-green spectral range