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Surfaces, Microstructure and Fracture Group

PhD Projects for 2022 Entry

Application and Funding

Information about the general application procedure for MPhil and PhD research projects is available here on the departmental website.  All our projects listed below are available to individuals applying for any DTP or Trust studentships, as well as other scholarships and awards.  Specific funding may be available for particular projects.

We welcome informal enquiries about graduate research with us, prior to formal applications - since graduate research is very much about finding a good match between student and project.

Use this link to email Andrew Jardine to arrange an informal in-person meeting or a video-call.

 

Project List

The following list (see full details below) represent projects for entry this year.  We are also happy to discuss tailored projects, to suit individual research interests.

* Projects marked with an asterisk may also be suitable for 1-year MPhil research projects

 

Novel modes of transport within nanostructured arrays (Prof J Ellis & Prof AP Jardine)

Development of fast-moving nanoscale systems present important prospects for rapid processing of materials, energy or information [1]. A fundamental step in these processes is the control and confinement of the motion of individual atoms and molecules. Nanostructured surface geometries provide templates which can be used to explore the fundamental physics and chemistry underlying such confinement. The aim of this project will be to study the relationships between diffusion, surface structure and adsorption mechanism, in nano-patterned systems.

Initially the project focus will be on the motion of atoms and molecules on stepped and faceted templates.  We then expect to move to technologically produced mesh structures, including h-BN nanomesh [2,3] and honeycomb assembled anthraquinone [4].  These open up the opportunity to study intermolecular interactions in the out-of-equilibrium conditions made possible by the confining geometry.  The project will mainly involve atom diffraction and HeSE atom-interference measurements to follow nanostructure and dynamics, which will be analysed using molecular dynamics simulations [5].  The project may involve complimentary real space STM experiments and has the potential to link with collaborative measurements using neutron techniques to study confinement effects in 3D geometries, enabling surface contributions to be analysed.

  1. “Controlling Motion at the Nanoscale: Rise of the Molecular Machines”, JM Abendroth, OS Bushuyev, PS Weiss and CJ Barrett, ACS Nano 9(8), 7746–7768 (2015).  https://doi.org/10.1021/acsnano.5b03367
  2. “Nano boron nitride flatland”, A Pakdel, Y Bandoa and D Golberg, Chem. Soc. Rev. 43, 934-959 (2014). https://doi.org/10.1039/C3CS60260E
  3. “Adsorption of Azobenzene on Hexagonal Boron Nitride Nanomesh Supported by Rh(111)”, Á. Szitás et. al, J. Phys. Chem. C 124(26), 14182–14194 (2020). https://doi.org/10.1021/acs.jpcc.0c01725
  4. “A Homomolecular Porous Network at a Cu(111) Surface”, G Pawin, KL Wong, K-Y Kwon, L Bartels, Science 313, 5789 (2006). https://doi.org/10.1126/science.1129309
  5. “Helium-3 spin-echo: Principles and application to dynamics at surfaces”, AP Jardine, H Hedgeland, G Alexandrowicz, W Allison, and J. Ellis, Prog. Surf. Sci. 84, 323–379 (2009). https://doi.org/10.1016/j.progsurf.2009.07.001

 

Lensing and Twisting Neutral Atoms for Microscopy (Prof AP Jardine)

The new technique of ‘scanning helium atom microscopy’ (SHeM), co-developed in our group over recent years, has wide and interdisciplinary applications, ranging from light sensitive molecules, to delicate polymers making up MEMS structures and even biological structures [1,2]. The overarching aim of this project is to improve the resolution of SHeM technique from the current 100’s of nanometres range to 10’s of nanometres – this huge step change in performance will dramatically increase the range of applications, which the candidate will be able to exploit.

SHeM uses neutral atoms, which while providing unique benefits, are challenging to manipulate.  High resolution focussing will be achieved using Fresnel zone plates, fabricated using electron-beam lithography methods.  The first phase of the project will involve developing zone plate components, installing and testing performance, then exploiting the performance enhancement in a range of applications.  It will likely involve working closely with our commercial partner, Ionoptika Ltd.

The second phase of the project is to develop more sophisticated zone plate structures which can create ‘vortex beam’ atom microprobes [4].  Such beams carry intrinsic orbital angular momentum, so will result in the capability for performing an entirely new class of atom-scattering experiment. In the context of SHeM, such beams have potential to generate unique forms of contrast from, for example, chiral surfaces.

The project will benefit from ongoing collaborations with European and Australian physicists in this area and the project may involve some work in those laboratories.

  1. “Unlocking new contrast in a scanning helium microscope”, M Barr et. al., Nature Communications 7:10189 (2016).  https://doi.org/10.1038/ncomms10189
  2. “Taxonomy through the lens of neutral helium microscopy”, TA Myles, SD Eder, MG Barr, A Fahy, J Martens and PC Dastoor, Scientific Reports 9:2148 (2019). https://doi.org/10.1038/s41598-018-36373-5
  3. “Focusing of a neutral helium beam…”, SD Eder, T Reisinger, MM Greve, G Bracco, and B Holst, New J. Phys. 14, 073014 (2012). https://doi.org/10.1088/1367-2630/14/7/073014
  4. “Vortex beams of atoms and molecules”, A Luski et. al., Science 373, 1105–1109 (2021). https://doi.org/10.1126/science.abj2451

 

3D Microscopy with Helium Atoms (Prof AP Jardine)

Scanning helium microscopy (SHeM) is a unique new form of microscopy capable of imaging samples on the micro- and nanoscale [1,2]. The technique offers unique benefits and can deliver novel information that underpins many emerging technologies and devices. The approach has been pioneered in Cambridge and can be applied to almost any material, without any form of sample preparation.  The method is particularly useful for imaging delicate materials and insulators, as there is no possibility of the measurement process causing damage, since the probe particles are uncharged and have extremely low energies.

The core objective of this project is to dramatically extend the technique to imaging samples in three-dimensions. The resulting technique of ‘3D-SHeM’ will enable the complete topographic structure of materials and technological devices to be accurately measured, while still leveraging all the established advantages of imaging with neutral atoms.  Creating 3D helium images will involve sampling the helium distribution from multiple directions, as the atoms scatter from each point on the sample. We have developed a reconstruction method using simulations and demonstrated that such data can be used to produce accurate 3D surface maps [3], as confirmed by initial experiments.

The project will involve making a complete realisation of 3D-SHeM by building on the helium microscopy research platform recently assembled in Cambridge.  Data will be acquired on naturally and synthetically textured materials, 3D structured mesoscale and nanoscale devices, as well as insulating materials such as polymers. Reconstruction and precision will be assessed and further developed.  The project will combine experiment and analysis, instrument development, as well as computational modelling.  It will also involve reaching out to the relevant research communities, as well as working with collaborators and our commercial partner, Ionoptika Ltd.

  1. “Unlocking new contrast in a scanning helium microscope”, M Barr et. al., Nature Communications 7:10189 (2016).  https://doi.org/10.1038/ncomms10189
  2. “Multiple scattering in scanning helium microscopy”, SM Lambrick et al., Appl. Phys. Lett. 116, 061601 (2020). https://doi.org/10.1063/1.5143950
  3. “True-to-size surface mapping with neutral helium atoms”, SM Lambrick et al., Physical Review A 103, 053315 (2021). https://doi.org/10.1103/PhysRevA.103.053315

 

Experimental and Theoretical Investigations of Rate Theory Using He Spin-Echo (Prof J Ellis)

Why, how, and how fast things change are some of the most basic and important questions of science underlying most aspects of science and technology, be it the dephasing of qubits, the kinetics of the processes that limit the development of the ‘hydrogen economy’, the reactions that underpin the chemical industry or the self-assembly processes which underpin nanotechnology, microelectronics and biology.

The foundations of ‘rate theory’ were laid in the 1930s with the development of transition state theory (TST), but whilst TST is useful for ‘back of the envelope’ calculations it is only quantitatively accurate in relatively unusual circumstances.  Over the last few decades, major progress has been made in first principles calculations of  the energetics of atomic scale processes to a point where the oversimplifications of rate theories such as TST have become the limiting factor. A major problem has been that rate measurements  are typically limited to timescales slower than micro to milli seconds, yet the process themselves involve atomic motion on the picosecond time scale. The errors introduced by the time scale extrapolation needed to evaluate rate theories were so large that an experimental test of rate theories was all but impossible. ‘Spin Echo’ techniques applied to the scattering of beams of helium atoms or neutrons from moving target atoms are however uniquely capable of probing atomic length and time scale motion [1] and provide a wealth of information that enables rate theory to be evaluated and developed in a way that has not be previously possible.

A major goal of the Helium Spin Echo (HeSE) project in Cambridge is to exploit HeSE to provide benchmark measurements and to use them to help develop a coherent picture of how you actually ‘do’ rate theory in real situations, from both classical and quantum perspectives.

Our current work has a series of strands which either individually or in combination can be pursued in a Ph.D. All these projects involve a combination of experimental work, data analysis and then extensive numerical/theoretical modelling both of classical and quantum processes, and provide an excellent training in a wide range of both experimental and theoretical techniques. Over recent years a whole host of unexplained phenomena have been found in ‘pilot’ HeSE experiments, and now a systematic study of each strand is needed.

  1. “Helium-3 spin-echo: Principles and application to dynamics at surfaces”, AP Jardine, H Hedgeland, G Alexandrowicz, W Allison, and J Ellis, Prog. Surf. Sci. 84(11-12), 323-379 (2009).  https://doi.org/10.1016/j.progsurf.2009.07.001

 

Atomic / Nanoscale friction (Prof J Ellis)

Macroscopic ‘kinetic’ or ‘dynamic’ friction is dominated by energy transfer as one surface ‘rubs’ against another. If friction were just due to ‘stick/slip’ of asperities as is commonly envisaged, mechanical systems would degrade very rapidly: the wear would be so great that a train would fall apart after a few miles. This process has its direct analogue of kinetic friction on the molecular scale: as a molecule moves across a surface, surface phonons are created just as a boat leaves a wake in water, dissipating energy as they are formed.

On the nanoscale,  friction dominates over inertia and controls the motion of any ‘nano machine’ [1] be it man made or biological. To maintain thermal equilibrium, this friction induced dissipation must be matched on average by energy gain from the ‘heat bath’, so measurements of atomic scale friction also give the rate of energy exchange to and from the whatever the heat bath is in a particular context. On the atomic scale an energy barrier must be overcome for a process to start, so this rate of energy exchange with the heat bath becomes a key factor in rate theory.

An understanding of atomic scale friction is therefore important on atomic, nano, and macroscopic scales [2]. HeSE allows us to  ‘watch’ atomic motion enabling us to probe the forces that produce it, including atomic scale friction [1].

This project involves taking both existing and new HeSE measurements of atomic scale kinetic friction, and looking theoretically and computationally this friction may be predicted and its role on a wide range of length and time scales evaluated. More specifically, one aspect of this project is to study the atomic scale friction of small, lubricant-related molecules adsorbed on surfaces, through the effect of friction of diffusional motion. Different sizes of aromatic hydrocarbons will be explored, which will provide simple, size-adjustable mobile species. Initial work will use a graphitic substrate, upon which benzene motion has already been successfully studied [1]. These systems are particularly interesting as the large hydrocarbon limits represent well defined frictional geometries. Later, we expect measurements to be extended to other surfaces, including single crystal graphite and metals. The project will involve state of the art helium spin-echo measurements, plus collaborative neutron scattering work and will suit a keen experimentalist.

  1. “Measurement of single-molecule frictional dissipation in a prototypical nanoscale system”, H Hedgeland, P Fouquet, AP Jardine, G Alexandrowicz, W Allison and J Ellis, Nature Physics 5, 561 (2009). https://doi.org/10.1038/nphys1335
  2. There is a special section in  J.Phys: Condens Matter Volume 20, #35 , (2008) is devoted to  friction across the length scales. https://doi.org/10.1088/0953-8984/20/35/350301

 

Dynamics in 2D Materials (Prof AP Jardine)

2D materials currently represent some of the most active and exciting areas of research in the field of condensed matter. Since the isolation of graphene from bulk graphite in 2004, a point widely regarded as the ‘birth’ of the field, the immense potential of 2D materials has been widely recognised and there have been huge levels of investment. As well as exhibiting remarkable fundamental properties [1], 2D materials hold great promise for addressing global challenges: examples include new semiconductor devices to reduce the huge power requirements of computing, especially with the challenges of ‘big-data’; revolutionary photonic devices and photodetectors; and compact bio-sensors with the capability for rapid diagnosis. The diversity in the overarching field of 2D materials has also been realised, and much attention is now being given to “2D materials beyond graphene” [2,3] – such as hexagonal boron nitride (hBN), transitional metal dichalcogenides (TMDs) and other 2D compounds, and Van der Waals bonded layered materials.

In order to exploit their potential, it is crucial to understand the characteristics of 2D materials fully, and an vast array of experimental characterisation techniques have been established, complimented by theoretical modelling methods.  In contrast, their lattice vibrations, or phonons remain relatively poorly characterised due to difficulties in applying the available tools to monolayer materials.  Phonon dynamics are fundamental material characteristics and are intrinsically coupled its electronic response; for example electron-phonon coupling, energy dissipation and dephasing are all crucial in semiconductor and quantum computing applications, while phonon structure and scattering controls thermal conductivity and hence underpins a variety of heat dissipation applications.  Generally speaking, current research is highly reliant on theoretical models, with very limited experimental validation.

The Helium Spin-Echo (HeSE) technique, developed uniquely available in Cambridge, offers a unique approach to phonon characterisation in 2D materials, through its ability to probe the Brillouin zone with ultra-high energy resolution (micro-eV range) [3].  The aim of this project will be to apply the method to make the first such characterisations of the vibrational properties of a range of “beyond graphene” materials.  A particular focus on mode lifetimes and the effect of atomic scale defects in the material structure is envisaged.  The project will likely involve a substantial amount of theory and simulation to interpret the measurements, and may include collaboration with European collaborators in order to access complimentary helium time-of-flight techniques.

  1. “An outlook into the flat land of 2D materials beyond graphene: synthesis, properties and device applications”, A McCreary, O Kazakova, D Jariwala and ZY Al Balushi, 2D Materials 8, 013001 (2021). https://doi.org/10.1088/2053-1583/abc13d
  2. “Recent Advances in Two-Dimensional Materials beyond Graphene”, GR Bhimanapati et al., ACS Nano 9(12), 11509–11539 (2015). https://doi.org/10.1021/acsnano.5b05556
  3. “The world of two-dimensional carbides and nitrides (MXenes)” A VahidMohammadi, J Rosen, Y Gogotsi, Science 372, eabf1581 (2021). https://doi.org/10.1126/science.abf1581
  4. “Helium-3 spin-echo: Principles and application to dynamics at surfaces”, AP Jardine, H Hedgeland, G Alexandrowicz, W Allison, and J. Ellis, Prog. Surf. Sci. 84, 323–379 (2009). https://doi.org/10.1016/j.progsurf.2009.07.001

 

2D Materials in Helium Microscopy (Prof AP Jardine)

Scanning helium microscopy (SHeM) is a unique new form of microscopy capable of imaging samples on the micro- and nanoscale [1,2].  The approach has been co-developed in Cambridge and can be applied to almost any material, without any form of sample preparation.  The method is particularly useful for imaging delicate materials and insulators, as there is no possibility of the measurement process causing damage, since the probe particles are uncharged and have extremely low energies.

We have recently demonstrated that the technique can simultaneously be used to perform atomic level characterisation with exclusive surface sensitivity, using atom diffraction techniques.  The method opens up the possibility of performing atomically resolved measurements on 2D materials, over large surface areas – which opens up a huge range of possibilities for characterising 2D material quality.  Such 2D materials represent a huge growth area in condensed matter physics, and so the unique characterisation method available with SHeM has great potential.

The aim of this project is to further develop SHeM methods for 2D material characterisation and to apply them to a range of materials.  The project will involve some instrument development, but will generally focus on experimental application and development of the method and analysis.

  1. “Unlocking new contrast in a scanning helium microscope”, M Barr et. al., Nature Communications 7:10189 (2016).  https://doi.org/10.1038/ncomms10189
  2. “Multiple scattering in scanning helium microscopy”, SM Lambrick et al., Appl. Phys. Lett. 116, 061601 (2020). https://doi.org/10.1063/1.5143950
  3. “An outlook into the flat land of 2D materials beyond graphene: synthesis, properties and device applications”, A McCreary, O Kazakova, D Jariwala and ZY Al Balushi, 2D Materials 8, 013001 (2021). https://doi.org/10.1088/2053-1583/abc13d

 

The Quantum to Classical Transition: Hydrogen and light species motion (Prof J Ellis)

It is widely assumed that the motion of atoms and molecules can be described by classical mechanics, and indeed, one of the key results of the Cavendish spin echo project so far has been to show that broadly, and rather surprisingly, this is true. It is generally understood however, but widely ignored, that hydrogen dynamics at thermal energies must be treated quantum mechanically, and very considerable theoretical work is being focused on this issue, not least because of the potential of the ‘hydrogen economy’ (the use of hydrogen as opposed to natural gas or electricity as a means of transporting and storing energy) and the importance of hydrogen atom dynamics in biological systems. Every aspect, however, of the hydrogen economy is currently limited by slow process rates, giving inefficient fuel cells/hydrogen production cells, slow diffusion in and out of storage media and slow rates for ‘direct use’ of hydrogen in chemical reactions, and thus a robust and computationally efficient method for applying rate theory to hydrogen dynamics is needed to guide attempts to understand, use, control and improve hydrogen based processes.

We have performed a series of investigations of hydrogen diffusion on close packed metal surfaces and now wish to extend these to other surfaces,  probing the use of the actual quantum states of the hydrogen in rate theory calculations.

We have also found cases where the motion of light molecules is significantly modified by quantum effects: methane moves with an effective mass ~3x its real mass, and CO has been found to move with an effective mass around 200x its real mass under certain circumstances. Both of these molecules are of technological relevance and this project would also involve experiments on these and other light molecules to probe how widespread and important quantum effects are in light molecule motion.

 

Quantum propagation in connection with a heat bath (Prof J Ellis)

The quantum mechanical version of friction is seen in the ‘quantum propagation in connection with a heat bath’ (QPCH) problem which is of considerable and wide topical interest. This covers issues such as the loss of coherence of qbits due to their connection with the  environment and quantum diffusion/tunnelling of, for example, hydrogen as it transfers energy to and from the rest of the system.  Essentially the QPCH problem is central to the behaviour of any quantum system that is not strictly isolated from the rest of the world.

In a field where there is relatively little reliable data, HeSE provides detailed information on the ‘incoherent’ tunnelling of adsorbed hydrogen, i.e. tunnelling which is slow compared to the rate of energy exchange between the hydrogen and the substrate-  an excellent prototypical QPCH system which is complex enough to be comparable with many systems of ‘real world’ interest, but simple enough to offer the prospect of being tractable theoretically.

In this project, which has a strong theoretical element, real data will be used to evaluate techniques for modelling QPCH problems which are fundamental to understanding hydrogen dynamics both at surfaces and in 3D biological/technological situations.

 

Identification of key ‘degrees’ of freedom for large dynamical systems (Prof J Ellis)

A molecule composed of N atoms has 3N degrees of freedom, so, for example, for a benzene molecule you would in principle have to take account of 36 coordinates if you wished to model its motion. This is almost impossible in a realistic way, not least because one does not have accurate ‘interatomic’ potentials and it is not even obvious that the interactions within a molecule can be expressed as the sum of pairwise potentials.

A key aspect of modelling a complex system is, therefore, deciding which degrees of freedom need to be modelled accurately and which can be handled in a more phenomenological way. What factors determine this choice? How do you know what is significant and what is not? In additional to the obvious 2 lateral centre of mass coordinates for a benzene molecule (which typically adsorbs ‘flat’ on a surface) do you have to include the vibrational motion perpendicular to the surface? The rotation about the molecular axis? The frustrated rotations about the other two axes? Internal vibrations and if so, which? One is looking for the ‘minimum set’ that can be used to model the motion, at least as far as it, or its effects, can be observed (i.e. ‘as far as it matters’). This project is not just about being able to make simulations tractable, but about identifying what controls the dynamics of larger molecules.

This question is prompted by HeSE data: it is commonly found that considering centre of mass motion alone cannot explain observations of either the rate, or the jump distribution [1,2]. More coordinates are need to quantify both the jump attempt rate, and the way jumps develop if they cover more than one site to site spacing.

This is a combined experimental and theoretical project, and involves extensive modelling, using a range of real systems to establish the rules of the ‘how free degrees of freedom can we get away with’ game.

  1. “Mass Transport in Surface Diffusion of van der Waals Bonded Systems: Boosted by Rotations?”, H Hedgeland, M Sacchi, P Singh, AJ McIntosh, AP Jardine, G Alexandrowicz, DJ Ward, SJ Jenkins, W Allison and J Ellis, J. Phys. Chem. Lett., 7(23), 4819-4824, (2016).  https://doi.org/10.1021/acs.jpclett.6b02024
  2. “Observation of microscopic CO dynamics on Cu(001) using 3He spin-echo spectroscopy.”, G Alexandrowicz, AP Jardine, P Fouquet, S Dworski, W Allison, and J Ellis, Phys. Rev. Lett. 93(15), 156103 (2004). https://doi.org/10.1103/PhysRevLett.93.156103

 

The effect on motion of the spectrum of ‘noise’ experienced by a dynamical classical system (Prof J Ellis)

Considerable success has been had in modelling thermal motion in terms of the Langevin equation in which the average interactions of the moving species with its environment are represented by a potential energy surface experienced by the moving species, a friction term proportional to velocity is included to account for energy transfer to the heat bath, and a ‘noise’ term – random impulses – models energy transfer from the heat bath. In the HeSE context this translates to the diffusion of a species across a surface, where the substate is also the heat bath. The Langevin approach assumes ‘white noise’ – equal power density in all frequency ranges, but in real systems the noise is anything but white. Surface phonons have an upper limit in frequency and one can see from the Debye model that one is not expecting uniform power density below this.

We have preliminary HeSE data that seems to show clear evidence that ‘coloured’ noise is indeed important in understanding the motion of light species (in this case lithium adatoms diffusing on Cu(111) [1,2]), and now more investigations are needed, both theoretical and experimental to see how widespread this effect is, how it may be modelled and to what degree, and when, it needs to be considered when modelling thermally activated motion.

  1. “A Study of spin-echo lineshapes in helium atom scattering from adsorbates”, David Ward, Ph.D Thesis, University of Cambridge 2013.
  2. “Numerical Simulations and Analytical Results Exploring the use of the Generalized Langevin Equation for Simulations of Atomic Scale Surface Dynamics”, Jeremy Wilkinson, 2021

 

Latest news

Review Article in Nature Reviews Physics

12 October 2021

A short review article, Probing surface motion above ambient temperature with helium spin-echo spectroscopy , has just been published in Nature Reviews Physics by Dr Nadav Avidor.

SHeM commercialisation project launched with Ionoptika Ltd.

1 September 2021

We are pleased to announce the start of a Knowledge Transfer Partnership with Ionoptika Ltd. to commercialise our scanning helium microscope (SHeM) technology. For more information, contact the project lead, David Ward .

Undergraduate Summer School and Research Programme 2021

28 June 2021

Welcome to the 46 students who will be joining us for our undergraduate summer school and 8-10 week research projects in surface nano-physics and atom-surface scattering.