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.

We present extensive testing in order to find the optimum balance among errors associated with time integration, spatial discretization, and splitting for a fully spectral semi implicit scheme of the phase field crystal model. The scheme solves numerically the equations of dissipative dynamics of the binary phase field crystal model proposed by Elder et al. [Elder et al, 2007]. The fully spectral semi implicit scheme uses the operator splitting method in order to decompose the complex equations in the phase field crystal model into sub-problems that can be solved more efficiently. Using the combination of non-trivial splitting with the spectral approach, the scheme leads to a set of algebraic equations of diagonal matrix form and thus easier to solve. Using this method developed by the BCAST research team we are able to show that it speeds up the computations by orders of magnitude relative to the conventional explicit finite difference scheme, while the costs of the pointwise implicit solution per timestep remains low. Comparing both the finite difference scheme used by Elder et al [Elder et al, 2007] to the spectral semi implicit scheme, we are also able to show that the finite differencing cannot compete with the spectral differencing in regards to accuracy. This is mainly due to numerical dissipation in finite differencing. In addition the results show that this method can efficiently be parallelized for distributed memory systems, where an excellent scalability with the number of CPUs. We have applied the semi-implicit spectral scheme for binary alloys to explore polycrystalline dendritic solidification. The kinetics of transformation has been analysed in terms of Johnson-Mehl-Avrami-Kolmogorov formalism. We show that Avrami plots are not linear, and the respective Avrami-Kolmogorov exponents (PAK) vary with the transformed fraction (or time). Using the semi-implicit spectral scheme we have been able to provide extensive numerical testing of methods in solving the single component case. This has been demonstrated by using unconditional time stepping with comparable simulations using conditional time stepping. We show the accuracy of the solution for unconditional time stepping is not compromised and furthermore computational efficiency can be significantly increased with the introduction of this scheme. Finally we have investigated how the composition of the initial liquid phase influences the eutectic morphology evolving during solidification. This is the first study that addresses this question using the dynamical density functional theory.

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In this thesis we demonstrate how the optical emission from a typical light emitting diode (LED) device can be spatially controlled via the deposition of a periodically structured gold film. A periodically structured gold film was deposited within a 20 um square aperture in the top, p-doped, contact surface of an AlGaInP/GaInP multiple quantum well edge emitting LED. We show that the far-field emission is vastly altered and is spatially controlled by the properties local to the surface plasmon. It is shown that each order of rotational symmetry produces a single emission curve in the far-field which is related directly to the coupling of photon to surface plasmon via the grating vector. We find that the origin of these strong emission curves in the far field is emission of the surface plasmon as photons. These are highly polarised and have a peak emission of 660 nm (as opposed to the peak wavelength from the standard LED device of 654 nm). The k vector for the surface plasmon propagating within the thin gold film with SiO₂ islands is found to be 1.002x10<super> 7</super> m"1 from experimental measurement. It is also shown that a similar gold film with a grating of 592 nm would result in normal emission. Considering the surface plasmon emission as being analogous to emission from a linear dipole antenna array, the far-field emission as a function of angle is modelled using Matlab. From comparison with real profile plots this suggests that a surface plasmon wave spans a distance of 7.5 nm which agrees approximately with the propagation length of a surface plasmon in this gold film.

Single porphyrin molecules and porphyrins isolated in a matrix of alkanethiols were probed by I-V spectroscopy. Repeated I-V measurements on individual molecules showed evidence of temporal fluctuations and changes of the coupling asymmetry in the molecular junctions. Analysis using the transition voltage spectroscopy technique revealed fluctuations in the voltage minimum <italic>V_m</italic>, which we suggest is due to the changing nature of the single molecule contact, and this provides estimates of the degree of contact asymmetry. Statistical analysis suggests more than one preferred value of <italic>V_m</italic> and a molecular energy level alignment to the Fermi energy of <italic>E_F</italic> &ndash; <italic>&epsiv;_HOMO</italic> &amp; 0.4 eV. The behaviour contrasts strongly with results from measurements over the alkanethiol matrix and over porphyrin monolayers, where such fluctuations are absent, in general agreement with previously reported studies. The results can be understood using a simple molecular model of coherent tunnelling and we conclude the electrical properties of single molecular junctions are highly sensitive to the microscopic details of the contacts and molecular conformations or configurations.

Measurements of the magnetocrystalline anisotropy constants of gadolinium have been made using a torque method. Torque observations were made on a single crystal oblate spheroid of the material which contained the hexagonal axis in its plane. The system for measuring torque was null-reading and a counter - torque was provided by passing currents through a small coil running in an instrument magnet. Balancing was done automatically with a photocell feed-back amplifier system. Values of the anisotropy constants have been derived from observed torque curves by a least - squares fit to the theoretical curves. The process was carried out on the Perranti Pegasus computer. The anisotropy constants were obtained over the temperature range from the Curie point (~289 K) to 20 K and they are presented graphically. K(_1) found to be negative below ~ 235 K indicating an easy direction in the basal plane and the crystal only exhibits a uniaxial character above this temperature. The change in sign of prevents any comparison with the Zener theory.

The properties of cad sulphide are such that the material has some potential for use in the semiconductor industry. The potential has so far not been realised because of the difficulty of preparing crystals with controlled properties. In order to improve the material available, much work has been carried out to identify and measure the parameters of the defects in the material. The study of double injection phenomena is one tool which enables defects to be investigated. During the work which forms the basis of this thesis, a large number of double injection devices have been fabricated from crystals grown by sublimation of CdS powder in a stream of argon. The devices have been studied at temperatures in the range 90 to 300 ºk and at temperatures below about 150 ºk visible green light is emitted when currents above about 5 mA flow. The light is due to recombination in the bulk and on the surface of the crystals, A comparison of the photo-, electro-, and cathodo- luminescent spectra has been made. The electrical measurements reveal that the current flow is governed by a class IT centre located 0.98 eV above the valence band. The presence of this centre leads to a switching effect which is such that the device can be turned on when a voltage pulse or a pulse of light is applied to the device when biased below threshold. When pulsed voltages are applied, the current pulse has a step in it due to a fraction of the class II centres capturing electrons while the pulse is off. The maximum frequency of sine wave modulation of the light has been found to be less than 1 megacycle so that the CdS device is unlikely to compete with injection lasers in other materials as a source of light for optical telecommunication systems.

The Stokes (U.V.) and anti-Stokes (A.S.) photo-excited emissions of large, doped and undoped CdS single crystals, grown under controlled partial pressures of cadmium and sulphur, at liquid helium temperatures were examined to establish a correlation between the crystal growth conditions and the spectral distribution of the green edge and exciton emissions. Anti-Stokes excitation spectra were also obtained. Two longitudinal optical phonon assisted series constituted the green emission. The “high energy series" (H.E.S.) was attributed to the recombination of free electrons with holes bound to acceptors some 0.17eV above the valence band, the "low energy series" (L.E.S.) to a distant-pair recombination process involving electrons bound to donors some 0.03eV below the conduction band and holes bound to the same acceptor. The mean separation between the donors and acceptors was about 100 Å. Only the L.E.S. was observed in A.S. excited green emission. The I(_1)and I(_2) bound excitons which dominated the blue emissions are associated with exciton recombination at neutral acceptors and neutral donors respectively. I*(_2) emission, associated with excitons bound to neutral donors loosing some of their recombination energy in raising the donor electron to an excited state of the donor, was observed and used to evaluate a donor ionisation energy of 0.026eV. Blue emission was excited by A.S. radiation in several crystals and ascribed tentatively to I(_1) emission. A model is developed to explain the variation of the emission characteristics with crystal growth conditions. A cadmium vacancy- donor impurity complex is suggested as the acceptor involved in the green and exciton emissions, with the hole in an excited state of the complex, and as the centre through which the two-step A.S. excitation process proceeds. Sulphur vacancy-acceptor impurity complexes and donor impurities are suggested to explain the donors associated with the L.E.S. and 1(_2) emissions.