Formation and Characterisation of CdSe-TiO2 Nanoparticle Films by Electrophoretic Deposition

Electrophoretic deposition (EPD) was used to form a composite layer of mercaptoundeconic acid (MUA) capped CdSe-TiO2 nanoparticle on a fluorine doped indium tin oxide (FTO) substrate. The CdSe-TiO2 layer can be employed to fabricate a quantum dot sensitized solar cells (QDSSC), increased contact between CdSe and TiO2 nanoparticles and leading to improved efficiency of the solar cells. A colloidal suspension of TOPO capped CdSe nanoparticles was prepared by the hot injection method, followed with ligand exchange in order to produce MUA capped CdSe nanoparticles. CdSe particle of diameter in the range of 2.44 nm to 3.26 nm were to be used in this research. The TiO2 nanoparticles were prepared by hydrolysis of titanium isopropoxide in water and produced particles size of 4.66 nm. Both nanoparticles were suspended in ethanolic medium. Electrophoretic deposition parameters were optimized. The results show that an applied voltage of 5 V, was suitable to be used to deposit single layer of MUA capped CdSe, TiO2 nanoparticles and the mixture of MUA capped CdSe-TiO2 nanoparticles. Smooth, uniform and dense layer were produced under this applied voltage. EPD also allows deposition of multilayer structures, in this research two layer structures of MUA capped CdSe on electrophoretically deposited TiO2 on FTO and mixed MUA capped CdSe-TiO2 on electrophoretically deposited TiO2 on FTO were formed. Three layer structures of MUA capped CdSe/MUA capped CdSe-TiO2/ TiO2/FTO were also synthesised. The photocurrent was measured on single layer, two layer and three layers electrodes. The optimum photocurrent parameters for each single layer were studied, in order to measure the photocurrent at the best condition possible. The highest IPCE value recorder was 0.70 % on MUA capped CdSe on FTO, with the MUA capped CdSe size of 2.94 nm. The lowest IPCE, 0.011 %, was obtained from three layer structure of MUA capped CdSe/MUA capped CdSe-TiO2/ TiO2/FTO

Surface modifications and growth of titanium dioxide for photo-electrochemical water splitting

This study investigates photo-anodes based on titanium dioxide (TiO2) that can be used to produce hydrogen by the photo-electrochemical decomposition of water. TiO2 is a wide band gap semiconductor that absorbs only the UV region of the solar spectrum. Sensitization of TiO2 to visible light by the addition of gold nanoparticles (AuNPs) was studied. AuNPs sustain localized surface plasmon resonance (LSPR) that results in the absorption of light at the resonant energy. The evidence for water splitting by Au-TiO2 systems is discussed critically. Fabrication of arrays of AuNPs was done by; annealing sputtered gold thin films, micellar nanolithography, and nano-sphere lithography. The optical characteristics and photo-electrochemical ‘water splitting’ performance of AuNP coated rutile (110) electrodes were determined. Nb-doped crystals coated in AuNPs of ca. 20 nm exhibited a small photocurrent that was not present with the bare rutile electrode. Reduced un-doped rutile (110) with AuNPs did not exhibit the ‘plasmonic photocurrent’. Some Nb-doped electrodes did not exhibit an effect. Batches of Nb-doped and reduced rutile were examined using voltammetry and impedance spectroscopy and it was found that the ‘inactive’ Nb-doped TiO2 was partially reduced. Thin films of TiO2 were fabricated by pulsed laser deposition (PLD) onto amorphous and single crystal substrates. The effect of growth conditions on the phase and orientation of the film were studied, and procedures to grow anatase films oriented with (100), (001), and (101) were developed. The temperature and heating regime of TiO2 films fused silica affected the orientation of film growing. Nb doping of the films also affected the temperature of the anatase-rutile phase transition and the orientation of the films, acting to stabilize anatase at higher temperatures. Surprisingly, highly doped films were found to be non-conductive. The importance of the oxygen partial pressure in producing conductive films for use as electrodes is discussed.Open Acces

Analytical optoacoustic spectrometry

Imperial Users onl

Optical techniques for non-destructive detection of flaws in ceramic components

No abstract availableThis thesis primarily concerns development of a non-destructive inspection method for 3mol% Yttria-Stabilised Zirconia Polycrystal (3Y-TZP) ceramics used for dental applications and a scoping study on applying the technique to other ceramic materials applied in thermal barrier coatings and other fields. Zirconia ceramics are materials of great interest for various engineering applications, primarily due to their stiffness, hardness and wear resistance. These factors in combination with the complex manufacturing processes may reduce the material strength and durability due to induced cracking. Knowledge of the extent of this cracking must be obtained and often, if each part is unique as in biomedicine, the assessment must be carried out for every part non-destructively so the part can be subsequently used. Only a few techniques are known for inspection of Zirconia ceramics, however these techniques are not able to detect flaws in thick (above 500 μm) parts. The main limitation for optical inspection of 3Y-TZP is the highly scattering nature of the material due to its multicrystalline grain structure (grains size of 500 nm) which, particularly in the visible region, reduces imaging capabilities. However, a transmission window in the mid-infrared (between 3 and 8 μm) exists opening up the potential for inspection at these wavelengths. Mid-Infrared Transmission Imaging (MIR-TI) and Confocal Mid-Infrared Transmission Imaging (CMIR-TI) techniques were developed for inspection of 3Y-TZP parts which allow for detecting sub mm scale cracks. The measured imaging resolution for the MIR-TI is 42 ± 5 μm, whereas for the CMIR-TI it is below 38.5 ± 5 μm. The maximum sample thickness inspected with the MIR-TI and CMIR-TI is 6 mm and 3.5 mm respectively, considerably more than currently available inspection methods. The MIRTI technique provides fast inspection of the part due to the large field of view (11 by 7 mm), however the high cost and limited imaging resolution make this technique less attractive. The CMIR-TI technique on the other hand is more cost effective due to reduced cost of the infrared sensor and it provides an enhanced imaging capabilities. The promising results achieved with the MIR-TI and CMIR-TI techniques led to the development of reflection equivalents (Camera-MIRI and Confocal-MIRI) for ceramic coating measurements, however further in-depth experiments to determine and quantify the capabilities of both techniques are required

Photo-induced charge generation in mono- and bimetallic nanoparticles deposited on titanium dioxide

Metal nanoparticles/TiO2 systems can generate photo-charge carriers to drive chemical reactions or induce photocurrents in photovoltaic systems. Gold and silver nanoparticles can exhibit localized surface plasmon resonance and enhance the charge carrier density by several energy transfer mechanisms. This thesis combined magnetron sputtering thin film deposition and thermal annealing to synthesize mono-metallic gold and bimetallic gold-core silver-shell nanoparticles to correlate the activity of the particles with their properties

Studies of composite metal oxide based ETA solar cells

The drive to produce low cost and efficient solar cells to replace solid state silicon cells has led to the rapid growth of nanotechnology in the PV sector. The extremely thin absorber (ETA) layer solar cell is a device that relies on the use of nanostructured anodes. The very high surface area of the metal oxides enhances the efficiency of the devices by increasing light harvesting in the cell. TiO2 has been the most common material of choice in these cells. However, alternative materials such as composite electrodes ZnO/TiO2, ZnO/SnO2, ZnO/Al2O3 have been considered. These systems also have the ability to improve charge carrier separation and broaden their photoresponse region. In addition to selecting materials with the correct energetics, the morphology of the metal oxide particles plays an important role in these devices. The ability to manipulate the shape, size, and surface to volume ratio of these oxides is critical in influencing the materials chemical, electronic and optical properties. In this thesis the fabrication of composite (ZnO,SnO2) electrodes by aerosol assisted chemical vapor deposition (AACVD) was investigated. By simply varying the Zn:Sn ratio in the precursor solution, a range of (ZnO,SnO2) composite materials along with single phase ZnO and SnO2 has been fabricated. It has been found that the morphology of the deposited electrodes is highly dependent on the Zn content with electrodes with morphologies ranging from nanoplates, to nanocolumns, to highly compact structures have been deposited. The dependence of the Zn content in the deposition solution on the photoelectrochemical (PEC), optoelectronic, photon to electron conversion efficiency (APCE) and photovoltaic characterization was investigated. ETA solar cells with FTO/(ZnO,SnO2)/In2S3/PbS/PEDOT:PSS/Cgraphite/FTO structures were successfully fabricated to demonstrate the suitability of (ZnO,SnO2) anodes in these devices. This work has shown that AACVD is a useful technique for engineering the properties of semiconducting electrodes for PV applications.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Novel Semiconductor Materials for Mid-Infrared Optoelectronic Application

Thin film gallium antimony bismuth (GaSbBi) grown on GaSb substrate via molecular beam epitaxy (MBE) have been explored in this thesis by characterising the films electrical, physical and optical properties. From these wafers Schottky diodes, and metal-semiconductor-metal photodetectors (MSM-PDs) have been investigated. Another promising material for infrared photodetectors is InAs, in this work the use of high-k based dielectrics has been investigated to improve the passivation of InAs avalanche photodiodes (APDs). Initially, the influence of postgrowth thermal annealing on GaSbBi Schottky barrier diodes has been investigated. The I-V characteristics indicated a better ideality factor and less leakage current at the reverse bias, as the annealing temperature increased up to 500 °C for a duration of 30 min. X-ray diffraction and scanning transmission electron microscope measurements were performed to verify that the bismuth composition was unaffected during the annealing process. Energy dispersive x-ray analysis indicated that Sb clustering occurs at high annealing temperatures, resulting in a concomitant degradation in the electrical performance. The optimum electrical characteristics of the diode were obtained with an annealing temperature of 500 °C for 30 min, resulting in an ideality factor of 1.3 being achieved. The optimised GaSbBi and GaSbN samples were then fabricated into MSM-PDs and verify the viability of using GaSbBi and GaSbN as active layers in photodetector. The cut-off wavelength extended to 1950 nm (2.9% Bi), 1990 nm (3.8% Bi), 2080 nm (4.5% Bi) and 2190 nm (1.5% N) have been observed, demonstrating the viability of using Bi and N for mid-infrared sensing. The comparison of different geometry on the photo spectral response indicating Bi incorporation increase the lattice expansion, which reduce the carrier concentration of the devices. Optimization of InAs based APDs are reported in this thesis. The use of high-k dielectric material as a passivation layer to improve the performance of InAs APDs are discussed. Three potential passivation layers, including ZnO, Al2O3 and MgO have been identified, all of which enables the suppression of surface leakage in smaller sized InAs APDs with a radius of 50 ìm and at lower temperatures of 175 K compared to a reference SU8 device. The influence of repeated temperature cycling on these layers has also been investigated with ZnO observed clear degradation after ten cycle, MgO shows almost a 10% higher current at a constant voltage after ten temperature cycles and Al2O3 passivated device, exhibiting no change in performance after temperature cycles. Suggesting Al2O3 as an effective and stable material for InAs APDs