Browsing by Author "Ashwell, G. J."
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Item Open Access Alignment and rectifying properties of donor-electron bridge-acceptor molecules(Cranfield University, 2007-06) Sujka, Marta; Ashwell, G. J.Molecular electronics based on the bottom-up approach appears to be a promising alternative to overcome the limitations of the top-down lithographic fabrication of electronic devices. The ability to manipulate single or small groups of molecules provides a great opportunity to build electronic devices at the molecular level. However, before any device can be constructed, it is vital to understand the parameters that control the device properties such as: molecular structure, conformation and arrangement at the surface, the molecule-substrate and molecule-electrode interactions. This thesis presents an investigation of the alignment of acceptor-electron bridge-donor structures and describes how the molecular structure and arrangement affect rectifying properties of the monolayers. Studies included typical Langmuir-Blodgett (LB), chevron-shaped, and ionically coupled structures that were characterised using various techniques, such as Quartz Crystal Microbalance (QCM), Surface Plasmon Resonance (SPR), Second Harmonic Generation (SHG) and Scanning Tunnelling Spectroscopy (STS). The results obtained showed that to achieve high rectification the molecules must form ordered and stable monolayers that are able to withstand the electric field applied to the junction. It was also shown that due to the disordered monolayer formation and presence of certain ions, it was extremely difficult to state without doubt whether the rectification was a result of the donor-electron bridge-acceptor structure proposed by Aviram and Ratner1. Studies of chevron-shaped molecules confirmed the possibility of depositing them using the LB technique. However, the reduction of long aliphatic chains was very likely balanced by the formation of less ordered or unstable monolayers. The highest rectification ratio of 30 ± 3 at ± 1 V was obtained for 1-butyl-2,6-bis-[2-(4- dibutylamino-phenyl)-vinyl]-pyridinium iodide (dye 7) and the origin of the I-V asymmetry was attributed to back electron transfer from iodide to pyridinium ring. Although dye 1-butyl-2,6-bis-(2-{4-[2-(4-dibutylamino-phenyl)-ethyl]-phenyl}-vinyl)- pyridinium iodide (dye 9) showed electrical asymmetry (RR=16 at plus/minus 1 V) shortly after deposition onto the gold-coated highly oriented pyrolytic graphite (HOPG), it seemed to form an unstable alignment and as a consequence the rectification decayed over a period of a few hours. Improved ordering, stability, and rectification were achieved from ionically coupled structures, where the monolayers were formed using chemisorption and ionic assembly instead of physisorption.Item Open Access Analysis of rectifying molecular thin films(Cranfield University, 2005-09) Miller, James R.; Ashwell, G. J.The mercury-drop electrode technique, when used to measure the current/voltage characteristics of thin films of molecular rectifiers, is a useful, if awed method of analysis. Current/voltage plots taken from a number of samples confirm previous studies taken using the scanning tunnelling microscope (STM), but indicate significant levels of penetration of the mercury into the sample or stabilising monolayer. Quartz crystal microbalance (QCM) studies of the adsorption of 7,7,8,8°- tetracyanoquinodimethane (TCNQ) onto clean gold indicate small but significant adsorption, with possible formation of a salt, via conversion of TCNQ into the radical anion. This has implications for the results of previous experiments, which indicate alternative sources for the rectification observed from donor-bridge-acceptor systems, but also suggest use of the TCNQ radical anion as a electron donor when used to replace the counterion present in a number of self-assembled systems. Previous STM- based measurements confirming this latter suggestion are supported by mercury-drop studies. The results support conclusions previously drawn by a number of authors, removing much of the ambiguity surrounding their interpretation, whilst clarifying the adsorption characteristics of thiol-based molecules that contain a terminal TCNQ group, and to some extent, TCNQ itself. It is suggested that the mercury-drop technique should never be used in isolation to quantify rectification in monolayers of molecular rectifiers, but that it is suitable for use only i confirming and checking results from more established procedures.Item Open Access Electrical rectification from aligned diodesbased on the donor-(π-bridge)-acceptor molecules(Cranfield University, 2007-04) Chwialkowska, Anna; Ashwell, G. J.As traditional devices containing silicon transistors begin to approach their physical limits, new systems composed of organic molecules are being considered for molecularscale devices of the future. The present work reports on the electrical properties of molecular diodes, especially observations of electrical rectification from molecular systems based on donor-(π-bridge)-acceptor molecules. For this purpose three types of molecular assembly were incorporated and their growth was observed with the quartz crystal microbalance (QCM) technique. Covalent self-assembly proved to be the most efficient method of forming well-ordered molecular films compared to those obtained via LB and ESA techniques. SAMs of Q3CNQ molecules yielded higher rectification than their LB analogues and achieved rectification ratio of 30 at ± 1V for every sample. On the other hand ESA films, in which molecular alignment of the physisorbed cationic dye was controlled by selfassembly of the anionic component, were probably more disordered, but exhibited higher (and sample-dependent) rectification ratios with a maximum of 450 at ± 1V. QCM also showed the phenomena of trapped water molecules within the physisorbed ESA monolayer that affected molecular order and also the electrical properties of the samples. Scanning tunnelling microscopy (STM), incorporated for obtaining current-voltage (I-V) characteristics from samples, showed that stearic hindrance has to be taken into consideration when designing donor-(π-bridge)-acceptor rectifiers. Sufficient isolation of donor and acceptor groups by the π-bridge is essential in order to prevent delocalisation of molecular orbitals over the entire molecule. Therefore, implementation of the Aviram-Ratner model of molecular rectification became possible although molecules investigated here did not possess the proposed σ-bridge. Additionally, the rectification effect arising from geometrical asymmetry induced by electrode-linking alkyl chains was shown to be negligible here, which is contrary to other theories of molecular rectification.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 Fibre optic chemical sensing using Langmuir-Blodgett overlay waveguides(Cranfield University, 1998-04) Flannery, D.; Tatum, R. P.; Ashwell, G. J.Fibre optic chemical sensing has been demonstrated using side-polished single mode optical bre, evanescently coupled to chemically sensitive Langmuir-Blodgett overlay waveguides. The sensors exhibit a channel dropping response centred on a wavelength dependent upon the thickness and refractive index of the overlay waveguide. It has been shown that chemically sensitive organic dyes prove to be suitable materials for forming the overlay waveguide whereas the Langmuir-Blodgett deposition technique provides the required overlay thickness control. A brief optic pH sensor has been demonstrated by depositing a merocyanine dye onto a side-polished optical fibre which exhibits a change in transmission of 9.7 ± 0.8 dB pH" at a working wavelength of 750 n and 4.2 ± 0.5 dB pH`1 at 780 nm, with a shift in the channel dropping centre wavelength of 18.8 ± 0.8 n pH`1. The response time of the sensor to a step change in pH has been measured to be ~ 20 s. The advantage of being able to chemically engineer the organic materials used to form the overlay waveguides has been demonstrated by manufacturing a second pH sensor designed to operate at longer wavelengths using a second, modified merocyanine dye. The overall wavelength response of this second sensor to pH has been shown to be non- linear but a sensitivity of 23.3 ± 0.3 n pH`l is observed over a linear region. Finally, i order to investigate the non-linear response, a computer model based on the Kramers- Kronig relations has been devised and shows good agreement with experimental values. lItem Open Access Langmuir-Blodgett films of porphyrins and phthalocyanines(Cranfield University, 2002) Portus, Dan; Ashwell, G. J.Phthalocyanines and porphyrins have been studied for many years as bulk, thick and thin films. Their use in Langmuir and Langmuir-Blodgett films is governed their peripheral substituents. These can enhance or reduce their ability to form "quality" ultra-thin films. There are a number of potential and current applications for thin films porphyrins and phthalocyanines, which include CD-R discs and gas-sensors. It is latter that this PhD has focussed on. Ultra-thin films of phthalocyanines, porphyrins and a porphyrin/phthalocyanine hybrid dye were deposited onto glass microscope slides, gold-coated glass microscope slides and quartz crystals. These assemblies were then characterised using Ultraviolet- Visible spectroscopy, pressure-area isotherms, surface plasmon resonance and a quartz crystal microbalance to try and determine the nature of the molecules on the surface the substrate. The thin films were exposed to chlorine gas and the change in their absorption spectrum and (in some cases) their surface plasmon resonance curve monitored. A short series of phthalocyanines showed that the central metal atom can play important role in the orientation of the macrocycles - with copper adopting an edge-configuration, zinc, face-down and metal-free, random. This in turn affected sensitivity of the gas-sensor with the facedown orientation offering a significantly greater response than the other two. A series of substituted tetraphenylporphyrins also illustrated that the sensitivity of the dye molecules is dependent on the orientation and spacing of molecules in the film. Generally the response to the chlorine was quite low, but porphyrin (stearamido) demonstrated a 2"d order decay in the absorbance due to oxidation of the macrocycle, the time constants being 31 and 425 seconds for surface and bulk diffusion responses respectively. The use of differentiation on the Pressure-Area Isotherms of the deposited Langmuir-Blodgett films is a new concept developed in this thesis. It proved a useful tool for looking at the effect of the quality of the film on the gas sensitivity.Item Open Access 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.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 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.Item Open Access Self-assembled films for molecular rectification(Cranfield University, 2004-10) Tyrrell, Wayne David.; Ashwell, G. J.The original model for molecular rectification was proposed by Aviram and Ratner (Chemical Physics Letters, 1974, 29, 277-284.), who suggested that donor-electron bridge-acceptor molecules could be organic counterparts of the inorganic p-n junction. Experimental verification was difficult and the first unambiguous study was reported only three years ago. However, other researchers were still sceptical of the data, but results in this thesis now confirm that molecular rectification is a real effect. Thirteen compounds have been studied of which seven have shown Aviram-Ratner type rectifying behaviour. Self-assembled monolayer films were studied by scanning tunnelling spectroscopy (STS) which provided their current-voltage characteristics. The principal group of materials studied that exhibited electrical asymmetry, involved hemicyanine dyes where the length of the group that connected the chromophore to the gold-coated substrate was varied from S-C3H6 to S-C10H20. Interestingly, the different alkyl analogues showed almost indistinguishable current-voltage characteristics. This suggested that recent theoretical models were incorrect. Furthermore, the chromophore is readily protonated resulting in disruption of the donor-acceptor combination, and it was found that exposure to acid switched the rectification off, which was reversible by subsequent exposure to base. This provided proof that the electrical asymmetry resulted from the donor-bridge-acceptor chromophore and not from any extrinsic effect. It is believed that this is the first conclusive study of molecular rectification.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.