Professor Geoff Thornton

Nanoscience Group website.

Research Overview

Work in Chemistry and the London Centre for Nanotechnology focuses on the surface- and nano-science associated with technologically important metal oxides. The overall aim is to establish the relationship between the geometry, electronic structure and reactivity of these important materials at the atomic level. This understanding is used to guide strategies for surface modification to provide enhanced catalysts and gas sensors. Methods to pattern oxide surfaces are being evaluated as a route to the construction of nanoscale electronic circuits. The principal techniques used in this work are scanning tunneling and non-contact atomic force microscopies, which provide atomically resolved images of single crystal surfaces, and inelastic electron tunnelling spectroscopy. The latter technique is used to measure single molecule vibrational spectra. Ancillary techniques of photoemission, high resolution electron energy loss spectroscopy and LEED are also used, as well as the synchrotron radiation techniques of XPEEM and surface X-ray diffraction. These employ facilities at Grenoble and Berkeley as well as at the Diamond Light Source in Oxfordshire.

Examples of some recent highlights of our research are shown below.

Manipulating Single Atoms

O Bikondoa1, CL Pang, R Ithnin2, CA Muryn3, H Onishi4, G Thornton, Nature Materials 5 (2006) 189
1Continuing related research in Grenoble, 2Scientific collaborator in Kuala Lumpur, 3Research Section Leader in Manchester, 4Scientific collaborator in Kobe.

The ability of TiO2 to split water when exposed to light can be exploited in solar cells and self-cleaning windows. We used STM to shed light on this system. By applying electical pulses (3 V) from an STM tip, individual H-atoms were selectively desorbed (Fig 1). This allowed us to distinguish H-atoms from oxygen vacancies, which have a similar appearance in STM. Then, by exposing the H-free titania to water, individual vacancies were observed being transformed into H-atoms as water molecules dissociate in the vacancies.

Representation of an electrical pulcse from an STM tip

Fig 1. Representation of an electrical pulse from an STM tip.

Pd Nanowires

DS Humphrey, G Cabailh, CL Pang, CA Muryn1, SA Cavill2, H Marchetto2, A Potenza2, SS Dhesi2, G Thornton
1Research Section Leader in Manchester, 2Diamond Light Source staff.

In this project we use photoemission electron microscopy (PEEM) at the Diamond Light Source, and STM at UCL, to investigate Pd nanostructures deposited on TiO2; a system relevant to strong metal support interactions in catalysis. Pd forms two distinct types of nanoparticles: long rod-like 'wires' and pseudo-hexagonal flat-topped islands. X-ray PEEM shows definitively that the nanoparticles are composed of palladium and separated by bare substrate. Pd nanoparicles appear bright on a dark background. (Fig 2) The brightness indicates their Pd composition, whereas the dark background indicates no Pd is present on the surface of the substrate between the nanoparticles.

XPEEM image with a 10um field of view

Fig 2. XPEEM image with a 10 um field of view. Photon energy is set at 414.5 eV so that Pd features are highlighted.

Nanoscale modification of surfaces

CL Pang, O Bikondoa, DS Humphrey, AC Papageorgiou1, G Cabailh, R Ithnin, Q Chen2, CA Muryn, H Onishi, G Thornton, Nanotechnology 17(2006) 5397
1Now investigating novel TiO2 photocatalysts in Cambridge, 2Leading his own research group in Sussex

Electrical pulses, between 5-10 V, create circles of a ‘1×2 reconstruction’, 6-8 nm in diameter. As can be seen from fig. 3, the separation of rows in the pulsed areas have twice the separation of the rows elsewhere. Arrays of these reconstructions could be used as templates to direct the growth of organic molecules or metal nanoparticles which could ultimately allow nanostructures, like miniature circuits, to be built.

(25nm2) STM image of TiO2 (110).

Fig 3. (25nm2) STM image of TiO2 (110). One of the areas reconstructed by the pulses is circled.

Low-Dimensional, Reduced Phases of Ultrathin TiO2

AC Papageorgiou1, CL Pang, Q Chen2, G Thornton, ACS Nano 1 (2007) 409
1Now investigating novel TiO2 photocatalysts in Cambridge, 2Leading his own research group in Sussex

Reduction of an ultrathin film of TiO2 grown on Ni(110) results in the formation of crystallographic shear planes, which induce “half-height” steps in the TiO2 surface. The scanning tunneling microscopy image shown here reveals four intersecting facets. Since these are essentially one-dimensional structures, they could be used as templates for the assembly of molecules or nanoparticles.

ACS Nano cover article (1, 409, 2007)

Scanning Probe Microscope Facilities

The group currently uses three scanning probe microscopes; an Omicron UHV Low Temperature STM (LT-STM), which can operate down to 4K; an Omicron UHV Variable Temperature STM (VT-STM) which operates in the range 50-1100K; and an Omicron UHV Atomic Force Microscope / Scanning Tunneling Microscope (AFM/STM), which is capable of atomic resolution on insulating samples in its Non-contact AFM mode.

Photo of LT-STM
Photo of VT-STM
Photo of AFM/STM

Selected References

  1. Imaging water dissociation on TiO2(110), I.M. Brookes, C.A. Muryn, and G. Thornton, Phys. Rev. Lett., 87, 266103 (2001).
  2. Impact of defects on the surface chemistry of ZnO(0001)-O, R. Lindsay, E. Michelangeli, B.G. Daniels, A. Gutierrez-Sosa, G. Thornton, A. Baraldi, R. Larciprete and S. Lizzit, J. Am. Chem. Soc., 124, 7117-7122 (2002).
  3. Imaging in-situ Cleaved MgO(100) with non-contact atomic force microscopy, T. V. Ashworth, C. L. Pang, P. L. Wincott, D. J. Vaughan, G. Thornton, App. Surf. Sci., 210, 2-5 (2003)
  4. Single Molecule Vibrational Spectroscopy of N2on Cu(110), L. Leung, C.A. Muryn and G. Thornton, Surf. Sci., 566-568, 671-675 (2004).
  5. Revisiting the surface structure of TiO2(110): A quantitative low energy electron diffraction study, R. Lindsay, A. Wander, A.Ernst, B. Montanari, G. Thornton, and N.M. Harrison, Phys. Rev. Lett., 94, 246102 (2005).
  6. Direct visualization of defect-mediated dissociation of water on TiO2(110), O. Bikondoa, C.L. Pang, R. Ithnin, C.A. Muryn , H. Onishi, G. Thornton, Nature Materials 189-192 (2006).
  7. Visualization of complex-anion site conversion on a metal oxide surface, A.J. Limb, O. Bikondoa, C.A. Muryn, G. Thornton, Angew. Chem.-Int. Ed. 46 549-552 (2007)
  8. The geometric structure of TiO2(011)(2x1), X. Torrelles, G. Cabailh, R. Lindsay, O. Bikondoa, J. Roy, J. Zegenhagen, G. Teobaldi, W.A. Hofer, G. Thornton, Phys. Rev. Lett., 101 185501 (2008).
  9. Self-Assembled Metallic Nanowires on a Dielectric Support: Pd on Rutile TiO2(110), D.S. Humphrey, G. Cabailh, C.L. Pang, C.A. Muryn, S.A. Cavill, H. Marchetto, A. Potenza, S.S. Dhesi, G. Thornton, Nano Lett., 9 155–159 (2009).
  10. Charge traps and their effect on the surface chemistry of TiO2(110), A.C. Papageorgiou, N.S. Beglitis, C.L. Pang, G. Teobaldi, G. Cabailh, Q. Chen, A.J. Fisher, W.A. Hofer, G. Thornton. Proc. Nat. Acad. Sci. 10 2391-2396 (2010).
  11. Oxygen vacancy origin of the surface band-gap state of TiO2(110), C. M. Yim, C. L. Pang, G. Thornton, Phys. Rev. Lett. 104 036806 (2010)