Prof. D. Phil Woodruff FRS
Quantitive structure determination of molecular adsorbates on metals and semiconductors - specifically using energy scanned photoelectron diffraction (PhD).
Water dissociation on perfect TiO2(110)
D. A. Duncan, F. Allegretti and D. P. Woodruff Water does partially dissociate on the perfect TiO2(110) surface, Phys. Rev. B., 86 (2012), 045411
There has been a long-standing controversy as to whether water can dissociate on perfect areas of a TiO2(110) surface; most early theoretical work indicated this dissociation was facile, while experiments indicated little or no dissociation. More recently the consensus of most theoretical calculations is that no dissociation occurs. New results presented here, based on analysis of scanned-energy mode photoelectron diffraction data from the OH component of O 1s photoemission, show the coexistence of molecular water and OH species in both atop (OHt) and bridging (OHbr) sites. OHbr can arise from reaction with oxygen vacancy defect sites (Ovac), but OHt have only been predicted to arise from dissociation on the perfect areas of the surface. The relative concentrations of OHt and OHbr sites arising from these two dissociation mechanisms are found to be fully consistent with the initial concentration Ovac sites, while the associated Ti-O bondlengths of the OHt and OHbr species are found to be 1.85±0.08 Å and 1.94±0.07 Å, respectively.
Tartaric acid on Cu(110)
D. A. Duncan, W. Unterberger, D. C. Jackson, M. K. Knight, E. A. Kröger, K. A. Hogan, C. L. A. Lamont, T. J. Lerotholi and D. P. Woodruff Quantitative local structure determination of R,R-tartaric acid on Cu(110): Monotartrate and bitartrate phases, Surface Science, 606 (2012), 1435
The local adsorption site of the monotartrate and bitartrate species of R,R-tartaric acid deposited on Cu(110) have been determined by scanned-energy mode photoelectron diffraction (PhD). In the monotartrate phase the molecule is found to adsorb upright through the O atoms of the single deprotonated carboxylic acid (carboxylate) group, which are located in different off-atop sites with associated Cu-O bondlengths of 1.92 ± 0.08 Å and 1.93 ± 0.06 Å; the plane of the carboxylate group is tilted by 17 ± 6° off the surface normal. The bitartrate species adopts a ‘lying down’ orientation, bonding to the surface through all four O atoms of the two carboxylate groups, also in off-atop sites. Three slightly different models give comparably good fits to the PhD data, but only one of these is similar to that predicted by earlier density functional theory calculations. This model is found to have Cu-O bondlengths of 1.93 ± 0.08 Å and 1.95 ± 0.08 Å, while the planes of the carboxylate groups are tilted by 38 ± 6° from the surface normal.
Glycine on Cu(111)
D. A. Duncan, M. K. Bradley, W. Unterberger, D. Kreikemeyer Lorenzo, T. J. Lerotholi, J. Robinson, D. P. Woodruff, Deprotonated glycine on Cu(111): A quantitative structure determination by energy-scanned photoelectron diffraction, Journal of Physical Chemistry C, 116 (2012), 9985
The local adsorption site of the simple amino acid glycine on Cu(111) has been investigated quantitatively by O 1s and N 1s energy-scanned photoelectron diffraction (PhD). The nitrogen atom is found to adsorb in a near-atop site with a Cu-N bond length of 2.02±0.02 Å. However, there is some ambiguity in the adsorption sites occupied by the oxygen atoms, although at least one of these atoms occupies a near-atop site with a Cu-O bond length of 2.00 - 2.02 ± 0.02-0.07 Å. Density functional theory (DFT) calculations, conducted on simple models (a low-coverage (3x3) phase of non-interacting molecules and a higher-coverage ordered (4x4) structure) failed to clarify this ambiguity, although they did indicate that the energetic differences between different possible oxygen atom sites are small. However, the structural conclusions of the DFT calculations proved very sensitive to the use of different functionals.
Methanol oxidation on Ru(0001)
D. A. Duncan, W. Unterberger, D. Kreikemeyer Lorenzo and D. P. Woodruff, Does methanol produce a stable methoxy species on Ru(0001) at low temperatures?, Surface Science, 606 (2012), 1298
Soft X-ray photoelectron spectroscopy (SXPS) and energy-scanned photoelectron diffraction (PhD) have been used to study the surface species produced by exposure of Ru(0001) to methanol at ~ 150 K. SXPS shows a single surface species is formed at sub-monolayer coverages with an O 1 s peak binding energy of ~ 532.6 eV, 2.8 eV greater than that of chemisorbed atomic oxygen. O 1 s PhD data from this species shows no significant modulations, in contrast to simulated PhD spectra from a methoxy species occupying a three-fold coordinated hollow site, as predicted by earlier density functional theory calculations, or atop or bridging sites. By contrast, PhD data from the O 1 s of the atomic oxygen species in the Ru(0001)(2 × 1)-O phase are consistent with the oxygen atoms occupying ‘hcp’ hollow sites (above second-layer Ru atoms) at a Ru-O bondlength of 2.01 ± 0.02 Å, essentially identical to previous structure determinations of this phase. O 1 s PhD recorded at normal emission from adsorbed CO are also consistent with the known CO atop adsorption species. We conclude that the methanol-derived surface molecular species is not methoxy in a well-defined local site on the surface, but is consistent with clusters of intact methanol identified in a recent infrared spectroscopy investigation.
Particle Swarm Optimisation and its application to PhD
D.A. Duncan, J.I.J. Choi and D. P. Woodruff, Global search algorithms in surface structure determination using photoelectron diffraction, Surface Science, 606 (2012), 278
Three different algorithms to effect global searches of the variable-parameter hyperspace are compared for application to the determination of surface structure using the technique of scanned-energy mode photoelectron diffraction (PhD). Specifically, a new method not previously used in any surface science methods, the swarm-intelligence-based particle swarm optimisation (PSO) method, is presented and its results compared with implementations of fast simulated annealing (FSA) and a genetic algorithm (GA). These three techniques have been applied to experimental data from three adsorption structures that had previously been solved by standard trial-and-error methods, namely H2O on TiO2(110), SO2 on Ni(111) and CN on Cu(111). The performance of the three algorithms is compared to the results of a purely random sampling of the structural parameter hyperspace. For all three adsorbate systems, the PSO out-performs the other techniques as a fitting routine, although for two of the three systems studied the advantage relative to the GA and random sampling approaches is modest. The implementation of FSA failed to achieve acceptable fits in these tests.
Hydroxyl species on rutile titanium dioxide
W. Unterberger, T. J. Lerotholi, E.A. Kröger, M. J. Knight, D. A. Duncan, D. Kreikemeyer Lorenzo, K. A. Hogan, D. C. Jackson, R. Wlodarczyk, M. Sierka, J. Sauer and D. P. Woodruff, Local hydroxyl adsorption geomerty on TiO2(110), Physical Review B, 84 (2011), 115461
The local structure of the hydroxyl species on the rutile TiO2(110) surface has been determined both experimentally and computationally. The experimental study exploited chemical state-specific O 1s scanned-energy mode photoelectron diffraction from a surface exposed to atomic hydrogen, while density functional theory calculations were used to provide complementary information. As expected on the basis of previous studies, the bridging O atoms of the clean surface are hydroxylated, but this causes surprisingly small changes in the surrounding surface relaxation. Experiment and theory are in good agreement regarding the magnitude of these atomic movements. Specifically, the Ti-OOH surface bond is significantly longer (by 0.10-0.15 Å) than that of Ti-Obridging bonds on the clean surface.
Substrate Effect on Bondlength
D. Kreikemeyer Lorenzo, W. Unterberger, D. A. Duncan, M.K. Bradley, T.J. Lerotholi, J. Robinson and D.P. Woodruff, Face-Dependent Bond Lengths in Molecular Chemisorption: The Formate Species on Cu(111) and Cu(110), Physical Review Letters, 107 (2011), 046102
Many previous structural studies of molecular adsorbates on metal surfaces indicate that the local coordination and bonding is closely similar to that in organometallic compounds, implying that the metallic substrate has no significant influence. Here we show that such an influence is detectable for one model system, namely, the formate species, HCOO, adsorbed on the atomically rough and smooth (110) and (111) surfaces of Cu, leading to a statistcally significant difference (0.09±0.06 Å) in the Cu-O chemisorption bond length. The effect is reproduced in density functional theory calculations.
Uracil on Cu(110)
D.A. Duncan, W. Unterberger, D. Kreikemeyer-Lorenzo and D.P. Woodruff, Uracil on Cu(110): A quantitative structure determination by energy-scanned photoelectron diffraction, Journal of Chemical Physics 135 (2011), 014704
The local adsorption site of the nucleobase uracil on Cu(110) has been determined quantitatively by energy-scanned photoelectron diffraction (PhD). Qualitative inspection of the O 1s and N 1s soft x-ray photoelectron spectra, PhD moudlation spectra and O K-edge near edge x-ray absorption fine structure indicate that uracil bonds to the surface through its nitrogen and oxygen constituent atoms, each in near atop sites, with the molecular plane essetially perpendicular to the surface and aligned along the close packed [1-10] azimuth. Multiple scattering simulations of the PhD spectra confirm and refine this geometry. The Cu-N bondlength is 1.96±0.04Å, while the Cu-O bondlengths of the two inequivalent O atoms are 1.93±0.04Å and 1.96±0.04Å, respectively. The molecule is twisted out of the [1-10] direction by 11±5°.
Catalytic Furan decomposition on Pd(111)
M.K. Bradley, D.A. Duncan, J. Robinson and D.P. Woodruff, The structure of furan reaction products on Pd(111), Physical Chemistry Chemical Physics 13 (2011), pg 7975-7984
Previous experimental studies of the interaction of molecular furan, C4H4O, with Pd(111) have led to the conclusion that partial dissociation leads to two coadsorbed reaction products, CO and a C3H3 species. Using density functional theory (DFT), a range of possible molecular conformation and adsorption sites of the C3H3 species have been explored and the lowest energy structures, and associated C 1s photoelectron core-level binding energy shifts (CLSs), have been determined. Comparison of these CLS values with published experimental measurements allows one possible conformation to be rejected. New simulations of the C 1s scanned-energy mode photoelectron diffraction (PhD) spectra for several of lowest-energy structures found in DFT are compared with the results of an earlier experimental study. The lowest energy structure found in DFT is not consistent with the PhD data, suggesting that energy barriers to achieve the associated conformation cannot be overcome in the dissociation process. Through consideration of the results of both methods, the most probable surface structures are discussed.
Cytosine on Cu(110)
D.C. Jackson, D.A. Duncan, W. Unterberger, T.J. Lerotholi, D. Kreikemeyer Lorenzo, M.K. Bradley, and D.P. Woodruff, Structure of Cytosine on Cu(110): A Scanned-Energy Mode Photoelectron Diffraction Study, Journal of Physical Chemistry C 114 (2010), pg 15454-15463
The local adsorption structure formed by the interaction of cytosine with a Cu(110) surface, following adsorption at ~300 K and annealing to ~420 K, has been investigated by O 1s and N 1s scanned-energy mode photoelectron diffraction (PhD). Initial characterization by soft-X-ray photoelectron spectroscopy and O K-edge near-edge X-ray absorption fine structure (NEXAFS) indicates that chemisorption leads to dehydrogenation of the NH species in the molecular ring and indicates that the molecular plane is perpendicular to the surface and close to the [1-10] azimuth. The PhD results confirm this orientation and show that the molecule bonds to the surface through the deprotonated N atom and the O atom, both of which occupy off-atop sites. The associated Cu-N and Cu-O bondlengths are 1.94(+0.07/-0.03) Å… and 1.94(+0.06/-0.04) Å…, respectively. Density functional theory calculations provide further support for this local structure and provide information on the possible role of intermolecular interaction. Possible structural models accounting for previously observed long-range ordered phases with glide symmetry are discussed in the light of these results.
Methanol on Cu(110)
D. Kreikemeyer Lorenzo, M.K. Bradley, W. Unterberger, D.A. Duncan, T.J. Lerotholi, J. Robinson, and D.P. Woodruff, The structure of methoxy species on Cu(110): A combined photoelectron diffraction and density functional theory determination, Surface Science 605 (2011), 193-205
The local structure of the methoxy species on Cu(110) has been investigated experimentally using chemical-state specific O 1s scanned-energy mode photoelectron diffraction (PhD), and also by density functional theory (DFT) calculations. The PhD data show a clear preference for adsorption with the O bonding atoms in short-bridge sites, though the best fit of multiple-scattering simulations to the experimental data is achieved with two slightly different short-bridge geometries. The DFT calculations also show that not only are the short-bridge sites energetically favoured in isolation, but that coordination to pairs of Cu adatoms has a similar energy. A structure consistent with both the PhD data and the DFT calculations is proposed for the previously-observed (5 × 2)pg ordered phase, based on methoxy species in short-bridge sites on pairs of Cu adatoms and on the underlying surface. Simulated scanning tunnelling microscopy images agree well with those observed experimentally, while the model is also shown to be consistent with the qualitative behaviour seen in early X-ray photoelectron diffraction (XPD) forward-scattering experiments
Partial oxidation of methanol to formaldehyde over Cu(110) is one of the most studied catalytic reactions in surface science, yet the local site of the reaction intermediate, methoxy, remains unknown. Using a combination of experimental scanned-energy mode photoelectron diffraction, and density functional theory, a consistent structural solution is presented in which all methoxy species occupy twofold coordinated "short-bridge" adsorption sites. The results are consistent with previously-published scanning tunnelling microscopy images and theoretical calculations of the reaction mechanism.
Vanadyl Phthalocyanine on Au(111)
D.A. Duncan, W. Unterberger, K.A. Hogan, T.J. Lerotholi, C.L.A. Lamont and D.P. Woodruff, A photoelectron diffraction investigation of vanadyl phthalocyanine on Au(1 1 1), Surface Science 604 (2010), pg. 47-53
Scanned-energy mode photoelectron diffraction using the O 1s and V 2p emission perpendicular to the surface has been used to investigate the orientation and internal conformation of vanadyl phthalocyanine (VOPc) adsorbed on Au(1 1 1). The results confirm earlier indications from scanning tunnelling microscopy that the VO vanadyl bond points out of, and not into, the surface. The VO bondlength is 1.60 ± 0.04 Å, not significantly different from its value in bulk crystalline VOPc. However, the V atom in the adsorbed molecule is almost coplanar with the surrounding N atoms and is thus pulled down into the approximately planar region defined by the N and C atoms by 0.52 (+0.14/−0.10) Å, relative to its location in crystalline VOPc. This change must be attributed to the bonding interaction between the molecule and the underlying metal surface.
Cysteine Chirally Etching Au
D. A. MacLaren, J. Johnston, D. A. Duncan, H. Marchetto, S. S. Dhesi, N. Gadegaard and M. Kadodwala, Asymmetric photoelectron transmission through chirally-sculpted, polycrystalline gold, Physical Chemistry Chemical Physics 11 (2009), pg. 8413-8416
Strong circular dichroism is observed in core-level photoelectron transmission through a chirally-etched polycrystalline Au surface, consisten with a chiral dependence on the electron's orbital angular momentum
Sub-picosecond fluorescence of amino-cyano-stilbenes in methanol
N.P. Ernsting, J. Breffke, D.Y. Vorobyev, D.A. Duncan, and I. Pfeffer, Sub-picosecond fluorescence evolution of amino-cyano-stilbenes in methanol: polar solvation obeys continuum theory without evidence of twisting, Physical Chemistry Chemical Physics 10 (2008), pg. 2043-2049
The spectral evolution of fluorescence from 4-(dimethylamino)-4-cyanostilbene (DCS) in methanol, and of two derivatives bearing either the anilino (ACS) or the julolidino (JCS) moiety, was measured by optical Kerr-gating with a time resolution of 0.35 ps. A special thin Glan polariser in the Kerr shutter allows high contrast without unnecessarily increasing the group delay dispersion. The emission band may thus be gated and observed even with highly fluorescent samples. The spectral dynamics consists of a continuous red-shift and narrowing with similar relaxation behavior throughout, i.e. between these two observables and the three compounds. This suggests that polar solvation is the common cause for spectral relaxation after 0.2 ps. The continuum model describes the dynamic Stokes shift quantitatively. Contrary to previous reports we do not find a temporary isosbestic point in the fluorescence of JCS, nor is there evidence for a dependence on anilino substituents. The crystal structures of DCS and JCS are provided
PhD in Physics, University of Warwick, 2008 - Present
Surface, Interface, and Thin Film Group,Deparment of Physics, University of Warwick
Chemistry - Physics Postgraduate Seminar Organiser, 2011
Postgraduate Seminar Organiser, 2009-2010, Department of Physics
Social Secretary, 2009-2010, Department of Physics
Condensed Matter Physics (upper floors) SSLC representative, 2009-2010, Department of Physics
MSci in Chemical Physics (2.1), University of Glasgow, 2003 - 2008
MSci Project, Solid State Physics Group, Department of Physics, University of Glasgow
Year in Industry Project, Ultrafast Spectroscopy Group, Institute for Chemistry, Humboldt University, Berlin
International Surface Science Conference 18, April 2011, University of Warwick
Presented poster on Nucleobases on Cu(110): A local geometry determination by energy scanned photoelectron diffraction
European Conference on Surface Crystallography and Dynamics 10, Sept 2010, University of Reading
Presented talk on The local structure of cytosine on Cu(110)
Presented poster on The local structure of biologically related molecules on Cu surfaces (see left)
International Vacuum Congress 18, Aug 2010, Beijing
Presented a talk on The local structure of biologically related molecules on Cu surfaces
Condensed Matter and Material Physics Conference, Dec 2009, University of Warwick
Presented a talk on Structure determination of vanadyl phthalocyanine (VOPc) adsorbed on Au(111) (see left)
First Joint BER II and BESSY II User's Meeting, Nov 2009, BESSY, Berlin
Presented a poster on A photoelectron diffraction investigation of vanadyl phthalocyanine on Au(111) (see left)