Strength and plasticity in small volumes (Prof DJ Dunstan)
This is an on-going EPSRC-supported research programme in collaboration with the Materials Department in which we are establishing why all materials are stronger when plasticity is confined to small volumes, as in nanoindentation, or when micron-sized components are stressed. New experiments have been designed to measure the stress-strain curves for thin foils and wires, and more experimental data is needed. There is opportunity also for students to work on theoretical developments, on exploitation, and on related outreach.
Intrinsic spin and charge carrier dynamics in organic semiconductors (Dr AJ Drew)
The importance and performance of organic electronic devices have increased significantly in the last two decades, and are increasingly showing great promise for new applications. A novel class of techniques makes use of a spin probe to measure the motion of charge carriers and their spin relaxation. Muon spin relaxation (muSR) has the unique feature that in organic materials it can both generate an excitation by chemical reaction with the system and also act as a sensitive probe of the dynamics of this excitation. Thus far, the application of muSR to investigating charge carrier transport in organic materials has been mostly limited to conducting polymers and has not been widely applied to small molecular systems. Furthermore, there has been little work on using muons to measure intrinsic electron spin dynamics. This project aims to develop the muSR technique in these areas and will involve combining the expertise in muon science with materials science, chemistry, physics and molecular electronics. This multi-disciplinary approach guaranties an unrivalled chance of unravelling the intricacies of intrinsic molecular charge and spin dynamics, which is critical to future applications of these already technologically relevant materials. Considerable travel will be associated with this project, as the main experimental work is carried out at world-leading research facilities spread across Europe and beyond.
Organic magnetoresistance (Dr WP Gillin)
It has recently been demonstrated that magnetic fields can have dramatic effects upon the current in and efficiency of organic light emitting diodes and organic photovoltaic cells. This is due to the fact that electron spin is vitally important in all of these devices and a magnetic field acts to perturb spin interaction processes. The study of this phenomena has led us to an improved understanding of the operation of these devices and in particular to the role that excited states have on charge transport. This project will involve performing a detailed study of the effect of magnetic fields on a number of OLEDs. The project will involve growing devices using our state-of-the-art facilities and then characterising these devices under various magnetic fields.
Organic Lasers (Dr WP Gillin)
There has been a long standing interest in trying to produce solid state lasers from organic materials. Organic molecules have been used for many years in solutions to produce very efficient lasers but in the solid state there are a number of processes that make continuous wave (CW) performance appear very difficult. Therefore despite the rapid development of organic light emitting diodes in the last 10 years solid state lasers made out of the same materials are generally limited to fast pulsed operation. We have recently developed new materials that have demonstrated gain at the important telecommunications wavelength of 1.5µm. These materials provide a new route to achieving laser operation from a solid state organic material. This project will involve characterising the energy transfer processes in these materials using time resolved optical spectroscopy and if the results look promising making an attempt to form a simple optical cavity to look for evidence of laser action.
Nanoscale magnetic materials (Dr M Baxendale)
Ferromagnetism in low-dimensional systems is of great interest from the fundamental and technological points of view. This experimental project involves synthesis and characterisation of 1D, nanometre-scale ferromagnetic wires. The wires will be made by a chemical vapour deposition method and characterisation will be mainly by transmission electron microscopy (TEM) in the NanoVision centre. TEM can resolve features on the scale 1-10 nm. TEM training will be with an experienced microscopist. The aim of the TEM characterisation is to resolve the magnetic domain structure; this information will tell us something about ferromagnetic behaviour on the nanoscale.
Charge Transfer (Dr A Misquitta)
The hydrogen-bond remains one of the most versatile and often-seen types of intermolecular bonds in biological systems: Base-pairs are bonded by hydrogen bonds in a zipper-like manner; water forms hydrogen bonds to give ice its open hexagonal structure. A characteristic of these bonds is charge-transfer which can be understood as the delocalisation or sharing of electronic charge between the bonded molecules. This sharing leads to stabilization and consequently to the strength of the bond. Charge-transfer poses a considerable problem for interaction models (the simple potentials used to model large proteins) and quantum mechanical methods like density functional theory. The more we understand about charge-transfer the better. In this project we will use state-of-the-art perturbation theory methods to investigate charge transfer in a variety of situations and novel charge-density analysis methods to see the extent of this transfer in complex systems.
Day-to-day: Review of literature on charge-transfer complexes; review of basic ideas in intermolecular perturbation theory; review of current literature on charge-transfer; get hands-on experience with using the CamCASP program to calculate and analyse intermolecular interactions; identify a set of systems that we will perform calculations on; calculate charge-transfer energies for these systems; understand how charge-density-decomposition methods work; use these to develop an understanding of the charge flow in the system; compare results with literature; make well-defined statements about the charge-transfer process and, possibly, on the many-body aspects of this process.
Ref: A. J. Misquitta, J. Comput. Theor. Chem. (2013) (on arXiv at http://arxiv.org/abs/1308.1231)
Pre-requisites: (not all essential, but will need to be acquired) The Linux shell, basic Python scripting, basic Fortran90, Quantum Mechanics.
Anomalous van der Waals interactions in low-dimensional systems (Dr A Misquitta)
The van der Waals, or dispersion, interaction is one of the fundamental intermolecular interactions. It is usually attractive and is responsible for the existance of gases, liquids and many solids. It is a special case of the Casimir force that is usually defined as an entropic force originating from zero-point fluctuations of the electromagnetic field. The dispersion interaction is usually modelled as the sum of pair-wise Cab_6/Rab^6 interactions  but this leads to qualitatively incorrect results for low-dimensional systems like pairs of 1-D semi-metallic wires . Recently we have also shown that trimers of 1-D wires exhibit even more deviations from this simple picture . This is due to the existance of long wavelength fluctuations in these systems. In this project we will attempt to extend these results to more complex and realistic 1-D systems. We will derive both the conventional and anomalous van der Waals models for these systems using state-of-the-art electronic structure methods. We will attempt to answer questions like:
Organo-Erbium(III) Infra-red emitters for telecommunications based on sensitized Erbium tretrachloropicolinate. (Dr I Hernandez)
Since their discovery, Erbium-doped fiber amplifiers have become widely used for telecommunications. However, amplification over small distances is limited due to the relatively low erbium absorption cross-section and concentration which must be used. Organic hosts are a means to overcome this problem: the organic binding groups or nearby molecules acting as sensitizers^ result in much higher absorption and the encapsulated ions are be kept separated allowing higher concentrations. However, ligands and coordinating solvent molecules usually contain C-H and O-H bonds that can cause vibrational quenching of luminescence.
The proposed study consists of the study of fully chlorinated of fluorinated Erbium(III) and Chromium(III) complexes based on picolinic acid (pyridine-2-carboxylic acid) derivatives. Transition-metal ion picolinates provide coloured materials meaning strong absorption, (for instance, Cr(pic)_3 , which is used as a chromium complement for athletes, is vividly red-violet coloured). We will explore the mechanisms for the energy absorbed in these centres to be transferred to the IR emitting molecule.
Some simple chemistry will be employed to produce de complexes from the picolinic acids and the corresponding metal chloride or acetate. The materials will be characterized and processed. Their optical properties will be studied in the spectroscopy laboratory. This implies the use of spectroscopic techniques such as optical absorption, photoluminescence and time resolved photoluminescence.