Cranfield University at Silsoe (1975-2008)

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Browsing Cranfield University at Silsoe (1975-2008) by Supervisor "Ashwell, G. J."

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    The non-linear optical and gas sensing properties of ultra-thin films
    (Cranfield University, 1997-12-17) Roberts, M. P. S.; Ashwell, G. J.
    It is not often that a single experiment opens up a new direction of research. In this report, the optical properties of a series of organic molecules, of the form 2,4-bis((N-methyl-N-alkylamino)phenyl)squarine, which are symmetrical about their centre point, are investigated. By conventional thinking, these molecules should not show any even order non-linear optical effects; the generation of second, fourth or sixth harmonics of the incident light. In fact, significant second harmonics are generated by the materials, x(2)=25-pm v-1. This fact in itself is noteworthy, but the additional fact that the efficiency of the conversion is over two times greater than any other previously reported material is exceptional. The door is now open to an entirely new class of non-linear optical materials. In this report a thorough characterisation of the organic molecules is made by a variety of techniques; polarised second harmonic generation, quartz crystal micro balance and grazing angle X-ray diffraction. A mechanism for this new phenomenon, based on intermolecular charge transfer is developed. In a world of ever tightening pollution control legislation the need for sensors which can accurately measure the concentrations of various pollutant gases is becoming more important. In this report the pollutant sensing properties of a series of organic molecules, 2,4-bis((N-methyl-N-alkylamino)phenyl)squarine, are investigated. The series of compounds was found to be sensitive to nitrogen dioxide in concentrations of as little as 1 part per million in air, but the changes produced in the molecules were non-reversible. It is very rare to accidentally stumble across a molecule with ideal properties. A molecule must usually be modified to achieve optimum properties. In this report the steps taken in the molecular engineering of a new selective and reversible gas sending molecule are also reported. 1-(4-hydroxyphenyl)-2-(1-octadecyl-4-quinolinium iodide) ethene shows sensitivity to ammonia gas in concentrations as low as 25 parts per million in air. The response is fully reversible and can be detected using a surface plasmon resonance based sensor. In the search for new technical solutions to old problems simple solutions are often overlooked. The final section off this report describes a simple gas sensing technique, which had been available in many research institutions, but overlooked in favour of more complex solutions.
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    ItemOpen Access
    Optical and structural characterisation of ultra-thin films
    (Cranfield University, 2000-07) Skjonnemand, K.; Ashwell, G. J.
    Chloride, bromide, pyridinium and quinolinium homologues of 4-(N- hexadecylpyridinium-4-ylmethylidene-amino)-2,6-dichlorophenolate have been investigated in solution, Langmuir and Langmuir-Blodgett films. Techniques including spectroscopy, surface potential measurement, quartz crystal microbalance, surface plasmon resonance, atomic force microscopy, reectometry and X-ray diffraction have been used to characterise these molecular systems. In solution, solvatochroism was observed and Benisi-Hildebrand analysis revealed dimeric aggregation. Langmuir monolayers were compressed at the air/water interface and chromophore rotation was observed by surface potential measurement. Langmuir- Blodgett monolayers showed lm-thickness dependence on the deposition-pressure. Monolayer thicknesses between 6-24Ä were measured using SPR and molecular areas between 40-l25Ä2 were measured using a quartz crystal microbalance. Both the molecular/s/area)and monolayer thicknesses were deposition-pressure dependent. The high tilt phases were visually distinguishable from the low tilt phases using atomic force microscopy, The compounds showed phase behaviour that was predominantly alike for the bromide and chloride homologues but different for the pyridinium and quinolinum homologues. Multilayer Y-type films of the merocyanine dyes were analysed using reectometry and deposition-pressure dependent thicknesses were found. Alternate layer structures of NLO-active hemicyanine amphiphiles were used to achieve homogeneous. orientation ordering using active and inactive spacer layers. Ordering was achieved but the optical efficiency was reduced by high proportions of inactive material and interlayer dipole formation. Double chained hemicyanine molecules were used to form Z-type structures and subsequent layers were found to significantly interdigitate. Different chain lengths were found to interdigitate by the length of the shortest chain. Gas detection experiments were undertaken on the quinolinium, dichloro merocyanine using three optical geometries. The absorption method showed slow switching and poor sensitivity. The Kretschmann SPR geometry showed high sensitivity and rapid switching. The grating SPR geometry showed rapid switching but was less sensitive than the ATR method. Protonation of the monolayers was investigated using hydrochloric acid gas, acetic acid vapour and stearic acid immobilised within the lm.