Structure and properties of surfaces, especially studies with synchrotron radiation.
Phil Woodruff could be described as a founder member of the Physics Department at Warwick, although he arrived in 1965 as a fresh young PhD student. Fast-forward 21 years and he found himself to be an established Professor in the Department, having been a member of the Department for all the intervening period - and ever since. However, with several periods of sabbatical leave in the USA and Germany, together with a lot of international collaborations, many based on the use of synchrotron radiation facilities in Wisconsin, New York, California, Paris, Berlin and Grenoble (as well as Cheshire!), there were those who sometimes questioned whether he was ever in the Department.
In some ways this became more true than ever in October 1998 when he took up a five-year EPSRC Senior Research Fellowship concurrent with his Professorship, allowing him to concentrate almost 100% of his time on research. The award of the Fellowship coincided with a need to take over primary responsibility for a long-standing collaboration with the Fritz Haber Institute of the Max Planck Society in Berlin, and for the next 13 years he spent roughly one week each month in this institute, with some of his PhD students and post-docs being based there. While formally retired from his full-time professorship in September 2011, he finds himself unable to give up his interest in physics and remains research-active in a role that the University sometimes describes as Emeritus and sometimes as a 'recognised researcher'.
His research interests all fall within the general area of surface science - trying to understanding the structural, electronic and chemical properties of well-characterised solid surfaces and interfaces (mainly, but not exclusively, of metals) at the atomistic level. This has been achieved through a mixture of Warwick-based (and Berlin-based) experiments and the use of large-scale facilities, especially synchrotron radiation but also including the medium energy ion scattering facility at Daresbury Laboratory in Cheshire until EPSRC pulled the plug on its funding. New beamlines at the UK's newest synchrotron radiation source, Diamond, are now making exciting new experiments possible. In particular, beamline I09 uniquely allows us to perform both PhD and NIXSW experiments (see below) on the same sample, while I07 provides opportunities for structure determination using surface X-ray diffraction.
Phil Woodruff's research programme covers a wide range of techniques and includes work on geometrical structure, electronic structure, and reactivity of predominantly metal surfaces. A particularly strong theme, however, is the development and application of quantitative structural methods to studies of adsorbate structures, especially of molecular species (including reaction intermediates) on transition and noble metal surfaces.
Much of the important technique development has been (and is) in synchrotron radiation methods. The two main techniques used in this part of the programme are (backscattering) photoelectron diffraction (PhD) and normal incidence X-ray standing wavefield absorption ( NIXSW). The photoelectron diffraction technique was developed in collaboration with Prof. Alex Bradshaw at the Fritz Haber Institute (FHI) in Berlin using the BESSY storage ring in Berlin. As a result of Alex Bradshaw's transfer to Scientific Director of the Institute for Plasma Physics in Munich in 1999, Phil spent about one week each month in Berlin at the FHI managing this project and its associated collaborations within a 'Sonderforschungsbereich' funded by the German research council (Deutsche Forschungsgemeinschaft) with PhD students in both Warwick and Berlin. The formal Berlin connection came to an end in December 2011, but new opportunities at the UK's Diamond synchrotron radiation source arenow providing even better facilities than BESSY for PhD and the ESRF in Grenoble for NIXSW.
The photoelectron diffraction programme has been particularly fruitful in the study of molecular species containing C, N and O atoms and has led to quantitative structural analyses of intermediates such as the methoxy, formate and acetate species and to increasing understanding of the structural aspects of simple hydrocarbon species and even more complex molecules such as glycine (the simplest amino acid) and methyl pyridine with Ni and Cu surfaces. The availability of new third-generation soft X-ray synchrotron radiation sources offers important new opportunities for a 'chemical-shift' variant of this technique (the figure shows the structure determined from such an experiment on coadsorbed PF3, PF2 and PF), and this work continued on the BESSY II source in Berlin, and most recently on the UK's Diamond Light Source. Latterly in Berlin we devoted increasing attention to the generally ill-understood problem of oxide surface structure.
NIXSW is a modification of the more general XSW method developed by the Warwick group which is particularly applicable to metal surfaces, and has been used to study atomic (e.g. Cl, S, O, Na, Rb) and molecular (e.g. methyl thiolate and longer-chain alkyl thiolates) on Cu, Ni and Al surfaces. All the early development was conducted at the Daresbury SRS facility in Cheshire, but subsequently some of this work transferred to the third-generation ESRF X-ray source in Grenoble in France, and now to Diamond in the UK, where it has proved possible to exploit chemical state sensitivity and extend studies even to light atoms like C, N and O. The focus of this work is currently on the study of larger pi-bonded molecules adsorbed on coinage metal surfaces, model systems for organic semiconductor/metal interfaces, in collaboration with the group of Giovanni Costantini in our Chemistry Department.
One further structural probe that was integrated into the programme is MEIS - medium energy ion scattering - an experimental technique to probe both surface structure and the structure of near-surface buried interfaces. This project used the UK national MEIS facility, also based at Daresbury Laboratory near Warrington, until EPSRC in its wisdom chose to close the facility on the basis of entirely ill-informed advice that the information given by MEIS 'could be obtained in other ways'. However, the instrument is now being recommissioned at the University of Huddersfield where we have high hopes that it will once again become a valuable facility.
At Warwick scanning tunnelling microscopy (STM) is also being used to study structural aspects of adsorption and reactions at surfaces with a particular emphasis on the role of adsorbate-induced restructuring of surfaces. Laboratory-based structural methods supporting this programme include X-ray photoelectron diffraction (XPD) and low energy electron diffraction (LEED).
A quite separate activity has been the study of the surface electronic structure of metals and metal films, especially those of interest as magnetic materials, using angle-resolved ultraviolet photoemission and momentum(k)-resolved inverse photoemission (ARUPS & KRIPES). Early work focussed on the KRIPES studies at Warwick and included investigations of Mn and Rh layers on Cu(100). Subsequently the emphasis of this work shifted to angle-resolved photoemission studies of the occupied states, and especially of quantum well states in the metallic overlayers, making use of synchrotron radiation in the vacuum ultra-violet at the SRS and at the NSLS in Brookhaven, New York. This involved a very fruitful collaboration with the group of Petar Pervan and Milorad Milun in the Institute of Physics in Zagreb. For this work, too, a new state-of-the-art beamline at Diamond is opening up new opportunities.
A further important strand to the research in the last few years has been a collaboration with Dr Reinhard Maurer in the Chemistry Department on the application of Density Functional Theory calculations to gain further insight into some of the adsorbate structures we have been solving experimentally. Opportunities exist for PhD studies by students with suitable research studentship fundings; currently there is no available funding for new postdoctoral assistantships.