Browsing by Author "Cavaye, H"
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- ItemThe binding and fluorescence quenching efficiency of nitroaromatic (explosive) vapors in fluorescent carbazole dendrimer thin films(Royal Society of Chemistry, 2013-01-01) Shaw, PE; Cavaye, H; Chen, SSY; James, M; Gentle, IR; Burn, PLWe present a study on three generations of fluorescent carbazole dendrimers that exhibit strong binding with nitroaromatic compounds accompanied by photoluminescence (PL) quenching, making them attractive sensing materials for the detection of explosives such as 2,4,6-trinitrotoluene (TNT). The absorption and release of vapors of the (deuterated) TNT analogue 4-nitrotoluene (pNT) from thin films of the dendrimers were studied with a combination of time-correlated neutron reflectometry and PL spectroscopy. When saturated with pNT the PL of the films was fully quenched and could not be recovered with flowing nitrogen at room temperature but only upon heating to 40-80 [degree]C. Although the majority of the absorbed pNT could be removed with this method the recovered films were found to still contain a residual pNT concentration of [similar]0.1 molecules per cubic nanometer. However, the proportion of the PL recovered increased with generation with the third generation dendrimer exhibiting close to full recovery despite the presence of residual pNT. This result is attributed to a combination of two effects. First, the dendrimer films present a range of binding sites for nitroaromatic molecules with the stronger binding sites surviving the thermal recovery process. Second, there is a large decrease of the exciton diffusion coefficient with dendrimer generation, preventing migration of the excitation to the remaining bound pNT.© 2013, Royal Society of Chemistry
- ItemControlling hierarchy in solution-processed polymer solar cells based on crosslinked P3HT(Wiley-VCH Verlag GmbH & Co. KGaA, 2013-01-01) Tao, C; Aljada, M; Shaw, PE; Lee, KH; Cavaye, H; Balfour, MN; Borthwick, RJ; James, M; Burn, PL; Gentle, IR; Meredith, PUnderstanding and controlling the morphology of donor/acceptor blends is critical for the development of solution processable organic solar cells. By crosslinking a poly(3-n-hexylthiophene-2,5-diyl) (P3HT) film we have been able to spin-coat [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) onto the film to form a structure that is close to a bilayer, thus creating an ideal platform for investigating interdiffusion in this model system. Neutron reflectometry (NR) demonstrates that without any thermal treatment a smaller amount of PCBM percolates throughout the crosslinked P3HT when compared to a non-crosslinked P3HT film. Using time-resolved NR we also show thermal annealing increases the rate of diffusion, resulting in a near-uniform distribution of PCBM throughout the polymer film. XPS measurements confirm the presence of both P3HT and PCBM at the annealed film's surface indicating that the two components are intermixed. Photovoltaic devices fabricated using this bilayer approach and suitable annealing conditions yielded comparable power conversion efficiencies to bulk heterojunction devices made from the same materials. The crosslinking procedure has also enabled the formation of patterned P3HT films by photolithography. Pillars with feature sizes down to 2 μm were produced and after subsequent deposition of PCBM and thermal annealing devices with efficiencies of up to 1.4% were produced.© 2013, Wiley-VCH Verlag GmbH & Co. KGaA
- ItemInvestigating morphology and stability of Fac-tris (2-phenylpyridyl)iridium(III) films for OLEDs(Wiley-Blackwell Publishing, 2011-06-21) Smith, ARG; Ruggles, JL; Cavaye, H; Shaw, PE; Darwish, TA; James, M; Gentle, IR; Burn, PLStable film morphology is critical for long-term high performance organic light-emitting diodes (OLEDs). Neutron reflectometry (NR) is used to study the out-of-plane structure of blended thin films and multilayer structures comprising evaporated small molecules. It is found that as-prepared blended films of fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)(3)] in 4,4'-bis(N-carbazolyl) biphenyl (CBP) are uniformly mixed, but the occurrence of phase separation upon thermal annealing is dependent on the blend ratio. Films comprised of the ratio of 6 wt% of Ir(ppy)(3) in CBP typically used in OLEDs are found to phase separate with moderate heating while a higher weight percent mixture (12 wt%) is found to be stable. Furthermore, it is found that thermal annealing of a multilayer film comprised of typical layers found in efficient devices ([tris(4-carbazoyl-9-ylphenyl)amine (TCTA)/Ir(ppy)(3):CBP/bathocuproine (BCP)]) causes the BCP layer to become mixed with the emissive blend layer, whereas the TCTA interface remains unchanged. This significant structural change causes no appreciable difference in the photo luminescence of the stack although such a change would have a dramatic effect on the charge transport through the device, leading to changes in performance. These results demonstrate the effect of thermal stress on the delicate interplay between the chemical composition and morphology of OLED films. © 2011, Wiley-Blackwell.
- ItemMorphology of all-solution-processed “bilayer” organic solar cells(Wiley-VCH Verlag Berlin, 2011-02-08) Lee, KH; Schwenn, PE; Smith, ARG; Cavaye, H; Shaw, PE; James, M; Krueger, KB; Gentle, IR; Meredith, P; Burn, PLWe investigate the evolution of the vertical morphology in a solution-processed P3HT/PCBM “bilayer” organic solar cell using a combination of techniques, including neutron reflectometry. By correlating the device performance with the active layer morphology, we establish that the solution processed bilayer concept is a misnomer and sequential solution processing is an elegant way to make bulk heterojunction organic solar cells with high efficiency. © 2011, Wiley-VCH Verlag Berlin
- ItemSolid state dendrimer sensors: effect of dendrimer dimensionality on detection and sequestration of 2,4-dinitrotoluene(American Chemical Society, 2011-09-22) Cavaye, H; Shaw, PE; Smith, ARG; Burn, PL; Gentle, IR; James, M; Lo, SC; Meredith, PWe compare two dendrimers, which contain the same luminescent chromophores but differ in dimensionality, for the detection of an explosive analyte via PL quenching. Each dendrimer has first generation biphenyl dendrons with 2-ethylhexyloxy surface groups but differ in the core units. One dendrimer has a bifluorene core and hence has a “planar†structure, whereas the second has four bifluorene units tetrahedrally arranged around an adamantyl center and hence has a “three-dimensional†structure. Solution Stern–Volmer measurements have previously been reported to show that the three-dimensional dendrimer has a higher binding constant than that of the more planar compound. Films of the dendrimers rapidly detect 2,4-dinitrotoluene (DNT) with thinner films (∼25 nm) being more responsive than thicker films (∼85 nm). Neutron reflectometry measurements show that the analyte can diffuse completely through the films with the three-dimensional dendrimer absorbing more of the analyte. The rate of recovery of the PL was faster for the planar dendrimer than the three-dimensional material showing that large binding constants are not necessary for reversible detection of analytes.© 2011, American Chemical Society
- ItemSolid-state dendrimer sensors: probing the diffusion of an explosive analogue using neutron reflectometry(American Chemical Society, 2009-11-03) Cavaye, H; Smith, ARG; James, M; Nelson, A; Burn, PL; Gentle, IR; Lo, SC; Meredith, PDetermining how analytes are sequestered into thin films is important for solid-state sensors that detect the presence of the analyte by oxidative luminescence quenching. We show that thin (230 ± 30 Å) and thick (750 ± 50 Å) films of a first-generation dendrimer comprised of 2-ethylhexyloxy surface groups, biphenyl-based dendrons, and a 9,9,9′,9′-tetra-n-propyl-2,2′-bifluorene core, can rapidly and reversibly detect p-nitrotoluene by oxidative luminescence quenching. For both the thin and thick films the photoluminescence (PL) is quenched by p-nitrotoluene by 90% in 4 s, which is much faster than that reported for luminescent polymer films. Combined PL and neutron reflectometry measurements on pristine and analyte-saturated films gave important insight into the analyte adsorption process. It was found that during the adsorption process the films swelled, being on average 4% thicker for both the thin and thick dendrimer films. At the same time the PL was completely quenched. On removal of the analyte the films returned to their original thickness and scattering length density, and the PL was restored, showing that the sensing process was fully reversible. © 2009, American Chemical Society