Surface dynamics on atomic length and time scales: A new opportunity in experimental science

Lead Research Organisation: University of Cambridge
Department Name: Physics

Abstract

Our proposal is designed to open up the emerging field of nanoscale surface transport on ultra-short time scales (i.e. picoseconds to nanoseconds). The motion of single atoms and molecules on these time scales is both fundamentally and technologically important. Fields ranging from the development of new catalysts, to understanding how nanostructures can be built by self assembly, and even understanding the microscopic origins of friction and wear, will all benefit from the fundamental work we propose.Measuring surface transport on such fast time scales (which correspond to the typical length of time required for two atoms to pass each other), is difficult; current microscopy cannot achieve the necessary imaging rates. However, the applicants have recently developed a new instrument at the Cavendish Laboratory, Cambridge, which makes work in this challenging field possible. The recent demonstration of the capabilities of 'helium-3 spin-echo' represents the culmination of a very successful high-risk project. The present grant application is designed to realise the full potential of this apparatus, by providing vital support instrumentation, specialist staff and running costs for the next four years. These will complete a unique, world-leading facility, allowing the U.K to maintain its present, leading position in this important field.The programme will include a combination of measurement and instrumentation development. Measurements will address a wide range of systems, ranging from simple atoms and molecules on clean metals, to larger molecules and transport processes on nanostructured surfaces. We will obtain otherwise unavailable information, describing dynamics under the thermally activated conditions, where technologically important processes occur. The information made available will be important to disciplines ranging from understanding the processes involved in catalysis, to testing the validity of theoretical models of structure and bonding, and understanding the microscopic origins of friction.New equipment is needed to enable effective use of the new apparatus. Both the beam source and detection equipment will be improved, which will allow us to make measurements much more quickly and enable a much wider range of measurements. We will add the standard sample preparation equipment, which is currently missing from the apparatus, to increase the throughput and optimise the scientific return on the investment, as well as sample transfer equipment to allow rapid sample exchanges and ex-situ sample preparation.The unique scope of the 'spin-echo' instrument will form a focus for a series of new international collaborations, supporting several of the research council's current operating objectives. Several research groups, both in the UK and abroad have already expressed interest in co-operation, both experimentally and theoretically. These collaborations are likely to be particularly productive, given the diverse range of measurements that will be possible and the interdisciplinary nature of the field - linking fundamental physics with surface chemistry, materials science and even nanoscale engineering.
 
Description The award enabled the technique of helium spin echo spectroscopy for quasielastic helium scattering for studies of surface diffusion and dynamics to be opened up. The proof of principle machine that was already in existence prior to the award was developed into a fully operational, high throughput instrument, with a detector 500 times as sensitive as the existing one, and roughly 10000 times the sensitivity of a commercial mas spectrometer. A wide a range of systems were studied - these were used to evaluate the first principle 'Density functional theory' predictions of potential energy atom/molecule-surface and these were generally shown to be considerable more accurate than even the main exponents of the method would expect. The application of the technique was developed from simple single atom-surface systems to complex molecular adsorbates, with translational, vibrational, rotational and 'flapping' modes of motion being all adsorbed and quantified. 'Ballistic', 'Fick's law diffusion', jump, multiple jump and correlated diffusion mechanisms seen. Atom/molecule friction coefficients were determined for a wide range of systems. Real surprises were found. The exhaustively studied 'benchmark' CO-Pi(111) system has been commonly described in terms of strong pair-wise interactions, which we showed were almost entirely absent - showing the system is dominated by many body effects. Deep tunneling deuterium was found to tunnel only a factor of 2 or 3 slower than hydrogen - confounding the accepted wisdom that there should be orders of magnidute difference in rates. Rigid aromatic ring structures were found to diffuse on relatively flat surfaces (such as graphite) with extremely high frictions - with a mean free path much smaller than a typical atom-atom spacing, Charged species were found to have almost no lateral repulsion due to compensating charge movements in the substrate, whereas strong repulsions were round between neutral species on account of large dipole motions. Systems were found, where, in contradiction to all theoretical expectations, no single, only multiple jumps were observed. Molecles were observed to move with a high effective mass - and effect more usually seen for 'bandstructures' of electrons in solids. CO diffusion on Cu(100) was show to defy current rate theory predictions of its motion- and as yet not solution to this conundrum has been found. The technique was shown to give very accurate measurements of the pre-factors for activated surface processes - and hence a provided data that was almost uniquely capable of evaluating rate theory as applied to surfaces. Ultra low vibrations of surface 'soliton modes' on Au(111) were observed that are entirely inaccessible to any other technique. All in all the grant enabled the huge potential of the technique to be developed and demonstrated and set the scene for a whole new field of research on dynamics, rate theory, quantum and classical couple of systems to a heat bath and interatomic/molecular potentials on atomic length and time scales at surfaces
Exploitation Route A similar machine has already been built at Technion university in Israel - and the technique attracts every wider interest form other scientific groups. The new detector is being used in the proof of principle helium atom microscope that has been developed in the group and it is anticipated that this too will result in a whole new field of microscopy and research. One would have thought that the diffusion of relatively simple atomic and molecular species across a surface would be well understood- but to date a fair proportion of systems studied have defied conventional wisdom and as a result numerous 'paradigm setting' results have been obtained that inform in a very wide sense the scientific communities understanding of rate processes at surfaces. The reliability of density functional theory's application to the very demanding 'low electron density' environment at a surface has been extensively probed giving much greater confidence to its use in surface systems (such as catalysis and device growth) - yet also pointing out glaring errors and providing extremely accurate benchmark data with which can be used to evaluate the many schemes currently being developed for the addition of non-local forces to DFT calculations. The work has emphasized the central role of energy transfer rates between the adsorbate and the substrate heat bath, and between the translational and internal degress of freedom of diffusion species - factors which have been largely ignored in first principle simulations of atom scale processes. The proper inclusion of these will greatly enhance the accuracy of simulated diffusion and reaction rates. The work lays down the challenge to the theoretical community to be develop ways of calculating these rates - methods for doing so realistically are almost entirely absent at the current time.
Sectors Chemicals

Electronics

Energy

Environment

Manufacturing

including Industrial Biotechology