PhD and Masters by research theses (SoE)
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Browsing PhD and Masters by research theses (SoE) by Supervisor "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 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 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.