Materials Physics PhD Projects

The following are typical PhD projects offered in Materials Physics. Specific projects may vary from year to year, and related projects may be offered in consultation with the institute.

For further information please contact us at imaps@aber.ac.uk.

Surface of Hard Material

Supervisor: Professor Andy Evans

Investigations of the surface physics and chemistry of hard materials such as diamond and boron nitride have been established at UWA over several years. In addition to their mechanical properties, these materials have received much recent interest in their electronic properties since they can behave as wide-gap semiconductors. The surfaces of these solids often exhibit enhanced electron emission due to a low electron affinity, which can be very sensitive to the presence of thin metal overlayers. In addition, as such materials become increasingly attractive for high temperature electronics, a knowledge of the interfacial electronic properties of interfaces and metal contacts is important, especially since the formation of low resistance contacts becomes increasingly difficult as the band gap is increased. This project will investigate a range of metal contacts on cubic surfaces, using techniques available at UWA (XPS, I-V, Raman Spectroscopy) and at synchrotron radiation facilities.

Electron Spectroscopy of Organic Semiconductors

Supervisors: Professor Andy Evans and Dr Dave Langstaff

The application of organic materials such as polymers and molecular crystals as alternatives to conventional inorganic semiconductors is already leading to improvement in the performance of a range of optoelectronic and electronic devices. This project involves the development of a new integrated multi-channel electron detector for fast electron spectroscopy of organic semiconductor thin film growth on inorganic semiconductor surfaces. The project will also involve the fabrication of organic semiconductor diode structures and the in-situ measurement of their electrical characteristics. While most of the experimental work will be performed in the new Materials Physics laboratory at Aberystwyth, some periods will be spent at partner institutions (CLRC at Daresbury and Rutherford).

Metrology of ultra-thin films on semiconductors

Supervisor: TBC

Thin films on semiconductor substrates are important in fundamental science and in technology. Films of current interest are approaching values of the order of a few nanometres or so. In these conditions, the physical properties of the thin films, such as optical or electrical properties, are far different from those in the bulk. Non-destructive characterisation of such very thin films is of crucial importance in understanding the properties of the films and the ability to predict their behaviour under operating conditions.

In order that an experimental technique can be useful in the metrology of thin film devices, it needs to be able to:

determine accurately thin film thicknesses;
determine the structure of the film and the interface between film and semiconductor;
understand the relationship between physical, electrical and optical determination of film properties.

Spectroscopic ellipsometry is a well-established for thin film dielectric measurement. It is non-destructive, reproducible and sensitive to sub-monolayer surface layers. It is the principal optical technique used at Aberystwyth. However, with such films rapidly approaching thicknesses of around 2nm with a need for process control tolerances of about 4%, ellipsometry becomes unreliable in the extraction of both thin film and refractive index data simultaneously.

In order for spectroscopic ellipsometry to meet process control requirements of thin films and determine film composition and morphology accurately, a multi-technique study of oxide films on SiC, with thicknesses ranging from about 1nm to about 20 nm, is proposed. Ellispometry will be used at Abersytwyth, together with medium energy ion scattering (MEIS) at Daresbury, and Neutron and X-ray reflectometry at to provide a coherent description of the structure of the film and its interface with the semiconductor substrate, and the subsequent development of an optical model which will enable spectroscopic ellipsometry to be used as a metrology tool for these films.

Initial experiments will concentrate on silicon dioxide films on silicon carbide, but experiments on organic molecules, particularly those of biological importance, will occupy much of the project.

Imaging Ellipsometry

Supervisor: TBC

Ellispometry is a well-established, non-destructive and highly accurate technique for the measurement of the optical properties of materials. In particular, it can determine the optical properties and thickness of very thin films (nms in thickness). Conventional ellipsometers measure properties at one point only on a surface. In this project, we wish to develop an imaging ellipsometer, where optical properties can be determined simultaneously at thousands of points on a surface with micron or sub-micron resolution. Such an imaging ellispometer will be employed to monitor the growth and development in real time of thin films on solid substrates in a variety of controlled environments.

The ability of imaging ellipsometers to capture images with high spatial and temporal resolution is currently hampered by a technical problem related to to the focus of the images. As the sample surface is viewed from an oblique angle, it is not possible to obtain an image that is in focus over the whole field of view with a conventional camera. Numerous solutions to this problem have been devised, but all have drawbacks which limit the spatial and temporal resolution of the instrument. In this project, a principle of capturing sharp images of surfaces viewed from oblique angles that has been known to large format photographers for many years, the Scheimpflug principle, will be developed to provide an effective solution to the focus problem. This will provide ellipsometric images with high spatial and temporal resolution. Ther ellipsometer will be used to study the interaction of proteins with semiconductor surface, such as silicon and porous silicon.

Real-Time Electron Energy Spectroscopy (REES) using Multichannel Detectors

Supervisor: Dr Dave Langstaff

A single detector is about 100 times less efficient in measuring a spectrum than an array of 100 detectors. Miniaturisation of the large amounts of circuitry needed for many detectors can only be achieved by integration on silicon. Aberystwyth is at the forefront of research and development of detector arrays fully integrated on silicon chips with all associated control and data storage electronics. Since 1985 we have developed an array with 384 detectors fully integrated on a silicon chip and we now have the know-how produce an even larger single-chip array.

Most modern applications of spectrometry require the highest quality data possible and therefore characterisation of the array and accurate deconvolution are essential. Detector development can only move forward efficiently when all parts of the measurement are considered on an equal footing - the multiplier, read-out electronics (array), data acquisition electronics, and data processing/deconvolution. A weakness in one of these is a potential source of lost opportunity. For about 30 years since the introduction of MCPs and array detectors, the problem of non-uniformity (always present in arrays of detectors) has remained unsolved until the Aberystwyth work. Previously, non-uniformity was corrected by for example moving a spectrum across the array so that every detector sampled every part of the spectrum, but this compromises the simplicity and efficiency of the array and does not recover the resolution of the incident spectrum.A new mathematical model of the measurement process developed at Aberystwyth has successfully enabled deconvolution of measured spectra in the presence of array non-uniformity and this model is now to be developed further to give an optimum deconvolution algorithm which will correct both non-uniformity and non-linearity simultaneously.

The structure of liquids and amorphous materials under extreme conditions 1: The influence of pressure

Supervisor: Dr Martin Wilding

There is indirect evidence to suggest that liquid structures may change abruptly with pressure. Such structural changes would have important consequences for the liquid transport properties; viscosity, diffusion and electrical conductivity. One influence of pressure is to change the liquid structure discontinuously over a narrow interval, or though a first order liquid-liquid transition: this is the phenomenon of polyamorphism.

The aim of this proposal is to employ the successful techniques already developed to investigate the structure of amorphous materials under pressure in order to explore the structural changes responsible for polyamorphism. Specifically, four questions will be addressed:

What are the structural changes that occur when amorphous materials are compressed?
Are changes in short-range order (coordination number) or mid-range ordering responsible for polyamorphic changes?
Are the structural changes a reflection of chemical or structural ordering?
Are changes in dynamics (floppy-rigid transitions) in the same pressure range related to polyamorphism?

The main experimental techniques will be neutron and X-ray diffraction. The diffraction measurements will be made in situ and will require the use of a high pressure cell (Paris-Edinburgh) to be used at large national neutron and synchrotron X-ray sources.

The structure of liquids and amorphous materials under extreme conditions 2: The influence of temperature

Supervisor: Dr Martin Wilding

The macroscopic properties of liquids, such as their viscosity and thermodynamic properties reflect their microscopic structure. As is well known the viscosity of a liquid will change as a function of temperature but the nature of this temperature dependence varies. Although in principle it is straightforward to link the structure of liquids and viscosity there is no proven link. This is because there are very few studies of liquid structure in situ and even fewer studies in the supercooled liquid regime, where transport properties such as viscosity change most rapidly.

Containerless levitation can be combined with synchrotron X-ray techniques to determine liquid structure and we have successfully applied small and wide angle X-ray scattering at the Synchrotron radiation Source, Daresbury (UK) to investigate, for the first time the transition in a Y2O3-Al2O3 liquid, this set of experiments has shown that there is an increase in small angle signal at the point of transition, indicating fluctuations in density and some apparent changes in wide angle signal.

Liquids lack long-range order and the periodicity that so characterises the crystalline state. While neutron diffraction is ideal, the scattered signal is weak and requires long counting times to achieve reasonable statistics. High energy X-rays are neutron-like in that they penetrate liquid samples and act more as a bulk probe, also they can be focused and masked to give a small spot size allowing diffraction data to be collected on a short time scale for samples such as levitated liquid spheres. High energy X-rays are further suited to studying oxide liquids because they are able to probe metal-metal correlations (the signal from neutron diffraction is dominated by the scatter from oxygen), that reflect differences in mid-range order, that is differences in the way that individual structural units are configured. High energy X-ray studies have been carried out successfully on stable levitated liquids this proposed experiment is the first time that the super cooled regime of Y2O3-Al2O3 has been probed.

The aim of this proposal is investigate the structure of oxide liquids in the stable and metastable regime. Here too, four questions will be addressed:

What is the distinction between the stable and metastable (supercooled) liquid structure?
Are these changes in short-range order (coordination number) or mid-range ordering?
Again, and related to pressure-induced changes, are these structural changes a reflection of chemical or structural ordering?
Are there changes in dynamics?

Contrast variation techniques for nano-composites

Supervisor: Dr Rudi Winter

Nano-structured materials consist of very small particles in a continuous matrix. The small size of the particles and the resulting large fraction of material located in internal surfaces and interfaces can result in dramatic changes of the properties of these materials. Nano-materials are frequently used as catalysts or for their size-dependent tunable optical properties. Because the interfaces are so dominant, it is necessary to develop techniques to probe specifically the interface structure if nano-materials with predictable function are to be designed. Contrast variation enables us to highlight atoms located in a particular phase in a complex, heterogeneous material. We will use a combination of anomalous small-angle x-ray scattering (ASAXS), a scattering technique that uses a resonant effect near the absorption edge of an element, and cross-polarisation (CP) nuclear magnetic resonance (NMR), where atoms in specific environments are selectively probed, to establish the interface structure at all levels from individual atoms up to the level of the grains that make up the embedded particulate phase. ASAXS experiments require an intense tunable x-ray source. Therefore, these experiments will be carried out at facilities such as the SRS at Daresbury, the new Diamond synchrotron, Bessy in Berlin and the ESRF at Grenoble, while we can do the CP NMR experiments in the Materials Lab here at Aber.

Initially, the study will be concerned with a nano-scale furnace refractory (alumina-zirconia silicate ceramic) but will be widened to other materials as we get more experienced with the novel techniques. Some background can be found here.

In-situ diffraction study of thermal shock and corrosion

Supervisor: Dr Rudi Winter

Using a new x-ray diffractometer to be installed this summer, we will build a specialist sample cell for real-time studies of the response of materials to thermal shock. Such effects can include phase transitions, amorphisation (or even local melting), formation of dislocations and cracks due to local strain build-up. The heat will be supplied by a strong infrared laser to maximise thermal gradients and heating rates. This kind of research feeds into the development of better materials for thermal barriers such as those used in melting furnaces or in turbine engines. Such materials are used both as bulk components and as thin protective coatings. To simulate the latter, we will do experiments on films produced by sol-gel dip coating.

In a second stage, it is planned to extend the sample cell to include variable reactive atmospheres. This will allow us to investigate the reaction kinetics of corrosion reactions.

The high-temperature diffraction experiments link well with our exisiting laser-heated nuclear magnetic resonance (NMR) work, and it is envisaged that both techniques are used in parallel on this project. Some background can be found here.

Modelling of two-dimensional flowing foams

Supervisor: Professor Simon Cox

Foams are familiar from everyday experience, but they are also used industrially. Examples include the flotation process for the separation of metal ores from rock and enhanced oil recovery from porous rocks. Research in this area is directed towards modelling the static structure of aqueous foams, and then exploring their dynamic properties as complex fluids.

This project involves the bubble-scale numerical simulation of the flow of foams. Possible areas of application include the sedimentation of non-spherical objects through a foam, and flow through pipes containing constrictions and corners, as appear in discrete microfluidics. Algorithms need to be developed to make simulations with many bubbles tractable. See http://users.aber.ac.uk/sxc/foam.html for further details.

Experiments in foam rheology: discrete microfluidics

Supervisor: Professor Simon Cox

The project will use our experimental apparatus for the study of foam flows to understand bubble motion in narrow channels with twists and turns. The project will measure pressure drops and bubble deformation; by varying the channel geometry and the chemical formulation of the foam this will allow us to understand the interplay of forces as a function of these parameters. See http://users.aber.ac.uk/sxc/foam.html for further details.