School of Applied Sciences (SAS) (2006-July 2014)
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Browsing School of Applied Sciences (SAS) (2006-July 2014) by Supervisor "Ashwell, G. J."
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Item Open Access Electrical, magnetic and optical characterisation of donor-acceptor systems(Cranfield University, 1991-07) Martin, P. J. P.; Ashwell, G. J.Three novel donor-acceptor systems have been characterised through their spectroscopic properties, magnetic susceptibility, conductivity and second harmonic generation. The magnetic properties of (H2DPE)xx+(DPE)^ x(TCNQ)gX and (H2DPA)xx+(DPA) 1 x(TCNQ)gX' where DPE = 1,2-di(4-pyridyl)ethylene, DPA = 1,2-di(4-pyridyl)ethane, TCNQ = tetracyanoquinodimethane and H2 signifies proton diquaternisation, have been found to depend upon the stoichiometry: For x > \ the magnetic susceptibility decreases with increasing x whereas for x ~ \ it is anomolously large, being ferromagnetically enhanced above the Curie Law value. A possible model involving spin dilution is proposed and its predictions are compared with the experimental results. Langmuir-Blodgett (LB) films of the long-chained analogues of the materials, (C^g)2DPE (TCNQ) 2 and (Cig^DPE^TCNQ)^" where (C16) 2 = bis(hexadecyl) were deposited and characterised. The conductivities of the LB films of the mixed valence system were found to be ~ 1000 times greater than those of the system lacking TCNQ^ molecules. Multiple layer LB films of trans-4-[4- (dimethylamino)styryl]-1- octadecylpyridinium iodide and trans-4-[4-(dimethylamino)styryl]-1- octadecylquinolinium iodide interleaved with 4,4'-dioctadecyl-3,5,3',5' -tetramethyldipyrrylmethene hydrobromide were deposited. The materials form an interlocking bilayer structure which renders their LB films exceptionally well-organised. The films’ second harmonic generation was investigated and one film was found to give the largest second harmonic signal yet seen from an LB structure.Item Open Access Nonlinear optical materials(Cranfield University, 1994-06) Thompson, Peter Anthony; Ashwell, G. J.Twenty different materials have been successfully deposited as Langmuir- Blodgett monolayer films. All exhibit second harmonic generation (SHG) when irradiated with laser light at 1064 nm. E-1-docosyl-4-{2-(4-dimethylami nophenyl)ethenyl}quinolinium bromide (C22H45QHBr) and E-1-docosyl-4-{2-(4-dimethy laminonaphthyl)ethenyl}quinolinium bromide (C22H45QNBr) have been deposited separately as multilayer films. They form Y-type structures when deposition is alternated with the material N-docosyl-4- methylquinolinium bromide. The nonlinear responses are quadratic up to 20 and 10 bilayers respectively and the response from the thick films is only 2 orders less than that produced by a Y-cut quartz plate. Similar results were obtained with C22H45QHBr when interleaved with 4,4'-dioctadecyl-3,5,3', 5'- tetra me thyldipyrrylmethenehydrobromide. Ellipsometry studies of the 10 bilayer film of C H45QNBr indicate that the structure is interdigitated. This explains the stability of the film which gave the same SH response up to 6 months after deposition. A 10 bilayer films has also been fabricated using E-1- docosyl-4-{2-(4-{2-(4-dimethylaminophenyl)ethenyl}benzyl)ethenyl}pyridinium bromide (C22H45PBHBr) alternated with E-1-docosy1-4-{2-(4-methylphenyl)ethenyl}pyridinium bromide (C22H45PT). E-1-octadecyl-4-{2-(4-methyloxyphenyl)ethenyl}pyridinium iodide and E-1- methyl-4-{2-(4-octadecyloxyphenyl)ethenyl}pyridinium iodide have been fabricated into monolayer films that are transparent at 1064 and 532 nm, therefore resonant enhancement does not contribute to their nonlinear response which is attributed solely to charge transfer in the molecule. Mixed solutions of E-1-octadecyl-4-{2-(4-methyloxyphenyl)ethenyl}pyridinium iodide and sodium octadecylsulphate (C1SH37OSO3 Na+) have been deposited as very stable monolayers. The nonlinear response from the mixed film offers a significant improvement upon the performance of the film containing pure hemicyanine. Novel zwitterionic materials have been fabricated as LB monolayers that also exhibit SHG.Item Open Access Zwitterionic materials for photonic applications(Cranfield University, 1992-07) Kuczynski, Andrzej. Piotr.; Ashwell, G. J.A series of zwitterionic materials of general formula R-D*-CH=C(CN)- CÖH4-C(CN)2', where D* is a pyridirium or quinolinium acceptor and R is a hydrophobic alkyl chain or an aryl group, were deposited using the Langmuir- Blodgett (LB) technique and their photochromic and nonlinear optical properties characterised. The materials are highly solvatochromic, exhibiting a broad photochromic charge transfer band in the visible region which bleaches when irradiated. LB films of the zwitterions, Z-ß-(1-hexadecyl-4-pyridinium)-a-cyano-4- styryldicyanomethanide (CMH33-P3CNQ) and the quinolinium analogue, Z-ß-(1- hexadecyl-4-quinolinium)-a-cyano-4-styryldicyanomethanide (CIÖH33-Q3CNQ), are non-centrosymmetric (Z-type). They exhibit sharp charge transfer bands at 495 n ana ses n respecivey with haf widths ai half maximum of 27 and zz mi. Unusually, mixed LB films of CIGH33-P3CNQ and C161-133-Q3CNQ exhibit a single sharp charge transfer band whose position is dependent upon the mole fraction and is finely tunable in the range 495 to 565 nm. These films are photobleached when irradiated at wavelengths which overlap the absorption bands, and may find potential applications a components of a multifrequency optical memory. Also, the unique peak wavelength tuning of the heteromolecular films enabled the effect of the position of the absorption band on second harmonic generation to be investigated for the first time. The zwitterionic materials have exciting norlinear optical properties and the strongest second harmonic intensity from any LB film has been obtained. LB films of the quinolinium zwitterion (CMH33-Q3CNQ) are non-centrosymmetric and the second harmonic intensity increases quadratically with the number of layers deposited to thicknesses of ca. 1 m. It is one of only five known materials to show such behaviour and the second-order susceptibility (xa) = 180 pm V* at 1064 nm) is the highest value obtained for a multilayer structure.