Structure and Dynamics
Dr J. M. Cole
[Experimental and Theoretical Projects available:- marked (E) or (T)]
This area of research comprises establishing structure / property relationships in materials of interest to the
optics and optoelectronics community, the aim being to use the knowledge derived from these relationships
to 'structurally engineer' new materials for their desired application.
A wide variety of structural techniques are used to realise this goal: X-ray and neutron diffraction,
anomalous X-ray scattering, EXAFS, XANES, solid-state NMR, FTIR and inelastic neutron scattering.
Most of these experiments are carried out at synchrotron and neutron central facilities in France, the UK and
the USA.
Computational work is also undertaken to complement some of the experimental areas of study. In some
cases, the student may have the option to combine experimental and theoretical work. In other cases, the
development of theory is crucial to our experimental progress. As such, several of our projects listed are
entirely computational.
Enquiries for MPhil/PhD studentships in any of the following topics (experimental or theoretical) are
equally welcomed from physics or chemistry students. The wholly computational projects may also suit
mathematics or computer science students.
(E) The Structure and Dynamics of Phase-transitions in materials with potential as optical and
magnetic data storage:- X-ray diffraction studies (PhD) - Largely experimental work. Minor option of
complementary theory work if desired.
X-ray diffraction will be employed to study the nature of the structural deformation, at the atomic level,
caused by phase-transitions in photonic and magnetic materials that have potential application in data
storage media. The structure and dynamics of these phase-transitions will be probed by variable temperature
measurements, and subsequent data analysis employing Einstein and Einstein-Debye oscillator models. Such
analysis will afford the derivation of energetic parameters, such as specific heat capacity, directly from the
X-ray crystal structures obtained. Depending on the diffraction results, phonon-dispersion measurements via
neutron spectroscopy may also feature as part of this work. There will be an option to undertake
complementary infra-red spectroscopy experiments in some of these materials if desired.
(E) The Structure and Dynamics of Phase-transitions in photonic and magnetic materials:- variable
Temperature and Pressure Infra-red Spectroscopy (MPhil or PhD) - Largely experimental work. Minor
option of complementary theory work if desired.
Infra-red spectroscopy experiments will be conducted on a range of materials that exhibit interesting optical
and opto-electronic phenomena. The samples will be probed under variable pressure and temperature
conditions. Corresponding pressure temperature (P-T) phase-diagrams of these materials will be constructed
from the experiments, from which we will identify phase-transitions, and undertake quantitative analysis of
proportionality relationships between structure and thermal and hydrostatic effects. There will be an option
to undertake related crystallography experiments of some of these materials if desired. Additionally, there
will be an additional option to gain experience in complementary theoretical calculations.
(T) Predicting the world’s best organometallic non-linear optical materials (PhD) - data-mining,
numerical analysis algorithms, statistical analysis.
The Cambridge Crystallographic Data Centre (CCDC) houses the world’s repository of all published
organometallic crystal structures. In this project, we shall probe this repository database to search for the
structural parameters that are known to be optimal for non-linear optical performance. The structures found
to have the most optimal characteristics will be short-listed for structural calculations (semi-empirical and
density functional theory). The results from this work will yield a final short-list of the structures that are
predicted to be the world’s top 100 organometallic non-linear optical materials.
The data-mining process will involve extensive programming, primarily using C++ (and embracing the
CCDC’s own C++ toolkit), and the use of numerical analysis algorithms, derived from discrete mathematics,
to identify the non-linear optical active structural fragments that will be used to search the database.
Statistical correlation methods will also be employed to facilitate the short-listing of suitable materials. The
project will involve direct collaboration with the CCDC, including work on-site which will provide valuable
experience with industry. This work follows on from a very successful analogous project aimed at predicting
the world’s top 100 organic non-linear optical materials using the CCDC database.
(T with E option) Charge-density studies of non-linear optical materials (MPhil or PhD) –
computational but complementary experimental option possible
This work will involve the analysis of high-resolution X-ray diffraction data. The student may perform their
own X-ray diffraction experiments or use previously acquired experimental data as their starting point. The
primary objective of this work is to be able to relate the molecular structure of these materials to their nonlinear optical properties.
The data analysis will employ spherical harmonic descriptions of atomic orbitals, within a quantum
mechanical formalism, and accompanying topological analysis of the electron and charge density.
Electrostatic properties of the molecules will be deduced from this data analysis procedure. Complementary
density-functional theory and wavefunction fitting procedures will also be performed.
The subject compounds for this work have been predicted to be in the world’s top 100 best organic nonlinear optical materials, as previously determined by our group. This project will complement on-going
experimental non-linear optical experiments where the lead candidates for industrial application are
currently being tested.
(T with E option) Charge-density studies of pharmaceutical compounds (MPhil or PhD) –
computational but complementary experimental option possible
This work will involve the analysis of high-resolution X-ray diffraction data collected by a Pharmaceutical
company that is based in Cambridge, through which this graduate project will be in direct collaboration. The
primary objective of this work is to be able to relate the structure of these materials to their pharmaceutical
function.
The data analysis will employ spherical harmonic descriptions of atomic orbitals, within a quantum
mechanical formalism, and accompanying topological analysis of the electron and charge density.
Electrostatic properties of the molecules will be deduced from this data analysis procedure. Complementary
density-functional theory and wavefunction fitting procedures will also be performed.
(E and/or T) Structure and Dynamics of Technologically- and biologically-important Optical
Switching via X-ray diffraction studies (PhD) - Largely experimental. Minor option of related theoretical
component is available.
There are three PhD projects available in this area: (1,2) investigating short-lived photo-active species via
time-resolved X-ray diffraction and spectroscopy; where materials have potential as (1) building blocks for
LEDs or molecular wires; (2) biologically-important optical switching. (3) studying metastable photo-active
compounds that are of industrial interest in the area of optical data storage.
The group is world leading in the development of a new crystallographic technique which reveals the threedimensional crystal structure of a material in its photo-excited form. This light-induced structure may be
very short-lived (ms-ps is typical for light-emitting diode materials) or metastable (trapped eternally in a
light-activated state pending suitable temperature or pressure conditions). In the former case, such
ephemeral structures are captured using a combination of time-resolved X-ray diffraction and optical
spectroscopy techniques. In the latter case, low temperature X-ray crystallography experiments are
employed to reveal the photo-induced structure.
To be able to observe directly the structural difference between the ground-state and photo-excited state of a
material, in three-dimensions, is key to determining which is the photo-active fragment(s) of a material, and
knowing this enables one to chemically tweak the active part of the compound such that the resulting
physical properties improves. This leads to the ultimate goal of being able to ‘structurally engineer’ a
material to best meet the needs of a given solid-state photonic application.
(T) Developing new crystallographic refinement code for time-resolved photo-crystallography
experiments (PhD) - computer programming.
Crystal structure refinements of photocrystallography experiments (see relevant project above) are
problematic where the light-induced lifetime is particularly short (ns-ps). This is because one is measuring
very small intensity differences for each Bragg reflection of the compound, for the two-part experiment:
light(on) - light(off). Therefore, rather than conduct data analysis using conventional means, i.e. refining the
ground state and photo-excited state structure separately and then deducing the structural differences, one
will obtain a much more sensitive measure of the light-induced structural change sought by employing
computer software that takes this difference, at source, via using so-called response-ratios, and thence
performing a non-linear least-squares fit analysis. Developing and testing this software on existing photocrystallography data is the primary focus of this project.
(T) Advancing modelling of EXAFS data using data-mining computational techniques (PhD) computational data-mining / programming work; option of some experimental EXAFS studies if desired
Multiple-scattering is a secondary effect frequently observed in data obtained from X-ray absorption
experiments. It is commonly an unwanted contaminant in the data and yet efforts are seldomly made to
remove such scattering effects from the desired underlying structural data and this limits the data
interpretation. This project aims to overcome this limitation by employing a data-mining approach to build
up a vast library of multiple-scattering signatures and then use this library to pin-point and extract multiplescattering effects from existing Extended X-ray Absorption Fine Structure (EXAFS) data. A comparison of
the multiple-scattering corrected and uncorrected structural refinements of this data will test the library that
has been developed. It is the hope that, once proven, this approach will then be implemented in EXAFS
refinement packages world-wide in order to make global this significant improvement in EXAFS data
analysis. This project therefore stands to contribute directly to the fundamental advancement of structural
analysis.
------------------------------------------------------------------------------------------------------For more information on any of these projects, please contact Jacqui Cole by e-mail and/or see the group
website: www-smf.phy.cam.ac.uk/index.html