Browsing by Author "Dunn, S. C."
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Item Open Access Dynamic effects in CdTe quantum-dot LEDs(Cranfield University, 2006-08) Gallardo, Diego E.; Dunn, S. C.In this work the electrical and electroluminescence properties CdTe nanocrystal films were analysed. The structure consisted of a multilayer of CdTe nanocrystals deposited by the layer-by-layer technique, sandwiched between an ITO anode and an aluminium cathode. The first part of this work was dedicated to structural and process improvement. Earlier devices, produced through a layer-by-layer (LbL) manual procedure, had an average thickness of 30nm per nanocrystal monolayer, with a roughness near 30% of the overall thickness. Electrical tests showed current densities over 100 mA/cm^2, with a frequent occurrence of pinholes and short-circuits that caused erratic sample behaviour and device rupture. SEM and AFM microscopic analysis showed that the nanoparticles were aggregated in 15-20nm thick clusters bound by polymer. A high porosity and non- uniformity of the multilayer was observed, explaining the formation of short-circuits. Luminescence was obtained from a very small number of samples, and with a very short duration that did not make spectral analyses possible. A robotic arm was programmed to carry out the LbL deposition, in an attempt to reduce the inhomogeneities of the multilayer. A key factor was the introduction of a special routine to remove the samples from the solutions. The sample withdrawal was designed to be in the vertical at a rate of 1.18 mm/s. The idea was to use gravity and the surface tension of the aqueous solutions to remove all the excess liquid from the surface. Additionally, poly(ethylenimine) (PEI) was eliminated from the process to improve homogeneity. These modifications produced multilayers with a thickness of 3nm per layer, average roughness below 5nm and CdTe packing density of 27%. Electrical measurements showed a stabilisation of the current-voltage (I-V) characteristics. A significant improvement in luminescence occurrence frequency and intensity was also achieved, enabling first spectral analyses. Once a reliable manufacturing procedure was developed, the electrical characterisation commenced with the analyses of samples with a different number of layers, operated in air. A field dependency of the I-V curves was found. Optimal performance was obtained from 30-layer samples, and this number of layers was adopted for subsequent analyses. Best samples showed external quantum efficiencies of 0.51%, with a photometric response of 0.8 lm/W and peak brightness of 1.42 cd/m^2. However, current and electroluminescence (EL) degradation with voltage and operation time were found in the device. Single carrier devices revealed a barrier for electron injection higher than predicted by the band diagram of the structure. The presence of an aluminium oxide layer at the multilayer/cathode interface was postulated, and confirmed through experiments in nitrogen. It was proposed that the growth of this oxide layer is the cause of device degradation during operation in air. However, it was demonstrated that the presence of the oxide favoured radiative recombination prior to degradation, with device efficiencies nearly 10 times higher than in devices without the oxide film. This was justified through three effects: charge accumulation at both sides of the oxide, field concentration across the oxide barrier and a reduction in leakage current. Unequal behaviour of samples with different electrode materials revealed that charge injection was the limiting mechanism for current flow, with a current onset field in the range of 2-3x10^7 V/m. Fowler-Nordheim plots showed that field emission was responsible for hole and electron injection into the device. Also, Fowler-Nordheim plots provided evidence of the dynamic nature of the cathode oxidation process. Dielectric breakdown of the aluminium oxide barrier at a rupture field of 3x10^7 V/m was found as a possible triggering mechanism of oxide layer growth. It was found that a critical field around 4.5x10^7 V/m caused irreversible loss of photoluminescence (PL) of the nanocrystals. This loss was attributed to an avalanche effect within the multilayer. The operational range for the devices is then found to be between 2x10^7 V/m and 4.5x10^7 V/m.Item Open Access The influences of post-preparative treatments on luminescence from CdTe nanoparticles(2008) Gardner, Hannah Catherine; Dunn, S. C.Nanotechnology has seen an explosion of research interest in recent years. Nanoparticles are finding applications in an ever growing list of applications. To further develop these applications an understanding of the properties of a nanoparticle is needed. This thesis prevents an investigation of the photoluminescent properties of CdTe nanoparticles as a function of various post-preparative treatments. Changing the inter-particle distance is found to cause photoluminescence wavelength shifts. These shifts are determined by the amount of energy transferred between nanoparticles. The effects of interparticle distance are seen when a suspension is diluted, re-concentrated or deposited as a thin film. In a thin film format similar effects are seen if the number of layers in a thin film is increased. Changing the temperature of either a thin film or suspension of nanoparticles produces both reversible and irreversible photoluminescence wavelength shifts. The reversible changes are mainly found to be due to the presence of thermally activated trap states within the nanoparticles. Finally, formation of a thin film via the layer-by-layer deposition method is studied. A real time in-situ analysis technique is used to monitor the whole deposition process. The deposition is found to take place in a series of stages with nanoparticles becoming either strongly or loosely bound to the surface. Optical waveguide lightmode spectroscopy is found to be an extremely useful technique for monitoring this thin film deposition as it allows researchers the opportunity to quickly and easily characterize individual experimental setups.Item Open Access An investigation of the photocatalytic properties of lithium niobate and barium titanate(Cranfield University, 2012) Stock, Matt; Dunn, S. C.; Dorey, Robert A.Efficiency of particulate semiconductors for driving photocatalytic reactions is impractically low due to the recombination of excited carriers and intermediate species at the interface. In the literature it has been demonstrated internal depolarisation fields in ferroelectric materials separate electron and hole carriers, this gives rise to spatially distinct reduction and oxidation processes. It is hypothesised this property can supress the rate of back reactions and carrier recombination to improve photocatalytic efficiency. In this thesis the properties of ferroelectric particulates for driving photocatalytic reactions are investigated. Lithium niobate and barium titanate powders were suspended in aqueous solutions of acid black 1 or rhodamine b dye. Adsorption studies compared retention of dye in the double layer by the different powders. Under UV or simulated solar illumination barium titanate or lithium niobate powders photocatalytically decolourised the dye solutions. Powders of lithium niobate powder doped with magnesium or iron showed altered reaction rates and structural selectivity of decolourisation reactions. Photochemical deposition of silver nanoparticles at the surface of the barium titanate or lithium niobate powders increased the rate of photocatalytic decolourisation of rhodamine b solutions under UV or simulated solar illumination. Photochemical reduction of carbon dioxide to form formic acid and formaldehyde over lithium niobate powder was studied under UV illumination. Solid-liquid phase reactions were carried out using aqueous suspensions of powder bubbled with carbon dioxide gas. Solid-gas phase reactions were investigated using a purpose built reaction vessels loaded with carbon dioxide gas and water vapour. Under solid-gas phase conditions the rate of formation of products over lithium niobate powder was greater than over titanium dioxide powder.Item Open Access Investigation of ZnO nanrod solar cells with layer-by-layer deposited CdTe quantum dot absorbers(Cranfield University, 2011-08-03) Briscoe, Joe; Dunn, S. C.; Dorey, Robert A.Innovation in solar cell design is required to reduce cost and compete with traditional power generation. Current innovative solar technologies include nanostructured dye-sensitised solar cells and polymer solar cells, which both contain organic materials with limited lifetime. This project aims to combine the advantages of ZnO nanorods and quantum dot (QD) absorbers in an all-inorganic solar cell, using the layer-by-layer (LbL) process to increase light absorption in the cell. Cont/d.Item Open Access Issues related to the processing of ferroelectric nanostructures via ex situ and in situ methods(Cranfield University, 2008-01) Harada, Scott; Dunn, S. C.This study investigates the impact of several processing parameters on the production of PZT and lead titanate nanostructures, by two different synthesis routes. The first aimed to produce freestanding, nanoscale PZT crystals via a simple, hydrothermal technique. Phase-pure, 52:48 PZT was synthesised within 2 hr at 160 °C in a hydrothermal autoclave. The morphology of the resultant crystals was cubic and micron-sized. Efforts were thus directed at reducing crystal size, by altering several processing conditions. Reaction time and temperature were found to have little effect on crystal size and instead resulted in incomplete reaction below the threshold values shown above. Introducing a 100% excess of Pb precursor led to an improvement in the faceting of the crystals, but again did not lead to a change in average crystal size. However, by using a 2% solution of the surfactant Triton-X, the average crystal size fell from 5 μm to 2 μm. The use of anatase TiO2 in the general hydrothermal procedure imposed a lower limit on the mineraliser concentration that was necessary for dissolution to occur. To overcome this restriction, a hybrid sol-gel/hydrothermal technique was attempted. This had the unexpected result of producing pyramidal microfibres that tapered to nano-sized points. Since the attempts to synthesise freestanding, nanoscale PZT crystals using the hydrothermal method were unsuccessful, work focussed on growing nanoislands on single crystal (100)- SrTiO3 substrates. Heteroepitaxial PZT films up to 1 μm thick, as well as micro- and nanoislands were observed, after extended 24 hr reactions at 160 °C. In some cases, the islands appeared to nucleate along scratches in the SrTiO3 substrate, leading to the formation of microwires up to a millimetre in length. Ferroelectric analysis of the microwires by PFM revealed them to be weakly piezoelectric. The second synthesis route to nanoscale ferroelectrics relied upon the solid phase reaction between Pb and Ti thin films to produce lead titanate. The optimum condition for the crystallisation of the perovskite phase was found to be 1 hr at 650 °C, under an air atmosphere. However, examination of the film morphology revealed large distortions and blistering across the surface. As such, it was not possible to acquire polarisation loop measurements, due to shorting between the top and bottom electrodes. EDAX confirmed that PbO vapour from the film was diffusing into the Si substrate leading to the formation of voids and the production of lead silicate glasses. Alternative adhesion layers for the Pt back electrodes were investigated, in an attempt to limit the diffusion of PbO. Electrical measurements on the films were made possible by substituting Zr for Ti. However, the shape of the hysteresis loops corresponded to a lossy dielectric as opposed to a ferroelectric response from the film. Films annealed on alternative substrates also suffered from blistering, but without the intrusion of PbO. This result suggested that the distortion present in the films was not as a consequence of the interaction between volatile species and the substrate. Instead, blistering and delamination was attributed to stresses generated during the transformation into the PbTiO3 perovskite phase. The solid phase synthesis method was extended to produce PbTiO3 nanoislands by utilising flash thermal evaporations. Deposition times below 1 s resulted in Pb nanoislands with a size range of 5 – 30 nm. Annealing the nanoislands under the same regime as used during the thin film experiments led to the loss of their morphology through melting. To overcome this problem an extended low temperature annealing was adopted. PFM was conducted on various nanoisland and thin film samples produced by the in situ technique. Nanoisland samples subjected to prolonged conventional annealings at 300 °C and brief hot plate annealings at 550 °C exhibited a non-zero piezoresponse. Definitive evidence of ferroelectricity in the nanoisland samples could not be demonstrated, however, as attempts to pole them were unsuccessful. PbTiO3 thin films produced by annealing Pb/Ti bilayers displayed clear c+ and c- ferroelectric domains that were mostly pinned by the grain boundaries. Little evidence of self-polarisation was found, since the average piezoresponse across the image was close to zero. Localised poling resulted in piezoresponse images showing the presence of intermediate contrast. This was interpreted as partial, 90 ° switching or evidence for “tail-to-tail” domain structure formation.Item Open Access Nanoparticle Synthesis via Thin Film Ferroelectric Templates:Surface interactions and effects(Cranfield University, 2008) Jones, Paul M.; Dunn, S. C.An investigation into the processes taking place at the surface interface of ferroelectric Pb(Zr1-x,Tix)O3 immersed in metal salt solution under ultraviolet illumination is presented. The semiconducting and switchable dipolar nature of this material allows the spatial separation and control of photo-induced reduction and oxidation across its surface interface. These properties can be of application in novel techniques such as the controlled growth of metallic nanoparticles across specific polar domains. 70nm thick Pb(Zr0.3,Ti0.7)O3 samples, PZT(30/70), are manufactured using the sol gel methodology, two crystallographic orientations being produced. The orientation being controlled by the substrate used; Si was used for [111] orientation and MgO for [100]. The initial work with wideband ultraviolet light shows that the reduction and growth of silver on the PZT surface is greatly influenced by the structure of the film. The crystallographic orientation of the film affects metal deposition such that on [111] films the metal deposits only on positive domains, where as the [100] films experience deposition on both positive and negative domains. This is shown to be due to the difference in width of the space charge region, Δw = 4.4nm, between the [111] and [100] samples so that the negative domain on [100] samples have 10 19 times higher chance of electron tunnelling compared to the [111]. It is also shown that grain boundaries have the greatest effect on the growth of metal, with a metal cluster growth rate 51 times faster than elsewhere on the surface. This increased rate of growth is due to the effect a grain boundary has on the surrounding area, the energy band bending at the boundary attracting charge carriers from the grains around it. The interface types ranked from greatest to lowest influence are grain boundaries, positive domains, domain boundaries and finally interphase boundaries.ii It is shown that the stern layer, strongly adsorbed charged ions of opposite sign to the surface charge, at the PZT/solution interface act as an insulating layer to metal reduction. The accumulation of photoexcited charge carriers at points along grain boundaries causes the surface potential gradient to alter and allows metal reduction and thus clusters to nucleate. The energy required to cause this variation is investigated by use of narrow band, 5nm bandwidth, ultraviolet. For energy from 4.4eV to 5 eV, it is found there is an increase in the average silver cluster cross sectional area by a ratio of ca 1.6 to 1 for both the [111] and [100] orientations of PZT. Finally it is shown that the type of metal salt used in the photochemical process affects the type of reaction that takes place at the sample surface. For a cation to reduce on positive domains its reduction potential needs to be below the bottom edge of the conduction band of PZT. Chloride salts, that sit above the conduction band, cause decomposition of the negative domains. Use is made of these effects to find the position of the bottom of the conduction band for PZT. It is found that across similar [111] PZT samples FeCl2 can both reduce on positive domains and decompose negative domains, this puts the bottom of the conduction band for PZT(30/70) between 4.06 and 4.36 eV from vacuum. It is also discovered that the type of anion affects the decomposition of the negative domains. Nitrate salts with cations above the conduction band cause no decomposition whereas chlorides do. The decomposition is shown to be the loss of Pb from the negative surface.Item Open Access Nanoscale investigations of surface phenomena in the water teatment industry using the atomic force microscope(Cranfield University, 2007-09) Bargir, Sameer M.; Dunn, S. C.; Jefferson, BruceUnderstanding the interaction between surfaces at the intermolecular level in ambient conditions is not only a fundamental science, but is of increasing value to water treatment systems. Here the uses of the atomic force microscopy (AFM) modified with particles of interest are assessed, and compared to bench-scale experimental techniques. In the first part of this study, the results from force measurements performed with calcite-modified probes in synthetic hard water (SHW) on selected substrates showed there was no correlation with macroscale scaling rate experiments. However, unmodified tips showed some correlation with non-metal substrates, where carbon coatings (Dymon-iC and Graphit-iC) were least adhesive. Although unmodified tips were unlikely to represent one of the surfaces of interest in water treatment systems, the findings suggest they can be used to screen materials with Ra < 50 nm. Contact angle measurements complemented force data, indicating the origin of repulsive forces on carbon coatings was due to hydrophilic repulsion because carbon and calcite were highly basic. Enhanced adhesion was caused by hydrophobic attraction and the presence of acidic surface groups. In the 2nd part of this study, force measurements were performed on natural organic matter (NOM) polyanions such as humic acid fraction (HAF), fulvic acid fraction (FAF) and hydrophilic acid (HPIA) using modified and unmodified tips. The results showed in symmetric NOM-NOM interactions with modified tips, HPIA-HPIA dominated both adhesion and detachment lengths, while FAF-FAF and HAF-HAF gave similar adhesion profiles. It is thought these intermolecular interactions can be transferred to floc size data, where HPIA flocs were bigger than FAF flocs. In non-symmetric systems adhesion between FAF-NOM was indiscriminate, compared to HAF and HPIA polyanions, indicating FAF polyanions were most likely to control coagulation performance during NOM removal.Item Open Access Photochemical growth of metal nanoparticles on domain patterned ferroelectric surfaces(Cranfield University, 2009) Tiwari, Divya; Dunn, S. C.In this work, the growth of metal nanoparticles on domain patterned ferroelectric Pb(ZrxTi1-x)O3 and LiNbO3 by photochemical reaction is demonstrated. The photochemical properties and phenomena occurring on the surface of Pb(ZrxTi1-x)O3 and LiNbO3 under ultraviolet illumination are investigated. Ferroelectric materials possess a reversible spontaneous polarisation that has an effect on photochemical reactivity of a surface. Since the spontaneous polarisation is reversible, a desired pattern can be drawn on a ferroelectric surface in the form of domains. A combination of domain patterning and domain specific surface reactions can lead to fabrication of complex nanostructures. It is found that on a PZT (30/70) thin film, under UV irradiation, metal deposition occurred only on C + domains and no deposition occurred on C - domains. Hence, the chemical reactivity of ferroelectric surface was found to be dependent on the polarisation of domain that is underlying the surface. Annealing of PZT samples at high temperatures alters the defect concentration of the PZT as shown by an increase in the deposition of silver on the surface. When the PZT samples were annealed in air at temperatures ranging from 530-690°C the silver deposition increased by more than 150% and the size of deposited silver clusters increased by four times. The photochemical properties of PZT thin films of different compositions PbZr0.3Ti0.7O3, PbZr0.52Ti0.48O3 and PbZr0.7Ti0.3O3 were investigated by undertaking silver nanocluster deposition experiments. The composition of PZT film (Zr/Ti ratio) affects silver deposition such that on PbZr0.3Ti0.7O3 silver deposits only on C + domains, whereas PbZr0.52Ti0.48O3 and PbZr0.7Ti0.3O3 experience deposition on both C + and C - domains. This difference in silver deposition pattern is shown to be due to the difference in width of the space charge region and bandii gap of the three samples. The impact of size of poled pattern on silver deposition at the surface is shown. It is found that for smaller size of C + domains (smaller than 1 μm) the amount and size of deposited silver decreased with the decrease in domain size. On a PZT surface, formation of spherical, triangular and hexagonal nanoplates of gold via photoreduction has also been demonstrated. Silver cation reduction on C + and C - domains of ferroelectric lithium niobate (LN) by photochemical and photoelectric process is demonstrated. The interaction of photoelectric and domain dependent influences can be observed in LiNbO3 due to the low electron affinity (ca 1.1-1.5eV). The impact of composition of LN on the photoreduction of silver has been shown. It is found that the photochemical reactivity of MgO-doped LN is significantly higher as compared to non-doped LN. The energy and intensity of incident photon is also found to have an impact on the photoreduction of silver on LN surface. In addition to this, deposition of Al and Mn nanoparticles on LN surface by means of photoreduction reaction has been demonstrated. It is shown that for a metal to photoreduce on C + domain of a ferroelectric material, the reduction potential of the metal has to be within the band-gap of the material. It is shown that under atmospheric conditions, it is possible to selectively adsorb molecules of opposite charge on the surface of LN due to uncompensated polarisation charge.Item Open Access Synthesis and characterisation of nanostructured BiFeO3 for photodecolourisation of azo dyes using visible light(Cranfield University, 2013) Chang, Hengky; Dorey, Robert A.; Dunn, S. C.In this work, effort is being made to synthesize a narrow band gap ferroelectric perovskite nanostructure semiconductor that is BiFeO3 (BFO). The BFO nanopowders were synthesized at 650ºC using a self-combustion method with glycine as the fuel. The effect of the different fuel concentrations, annealing temperatures and the duration of annealing are all demonstrated to influence the phase and crystallography of the synthesized nanoparticles. The author has demonstrated that the self-combustion process can be used to produce high purity BFO nanopowders which exhibit good absorption in the visible-light regime as determined by the UV-Vis-NIR spectroscopy with a measured optical band gap of 2.22 eV. Cont/d.