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You are here Research > Research Groups > Surface, Interface Physics & X-ray Spectroscopy > Research

Research

Our research interests all involve the application of x-ray spectroscopic techniques to the investigation of the electronic structure and magnetic behaviour of materials. These investigations use high-brightness synchrotron x-ray sources such as MAX-lab, the NSLS and the ALS. The techniques used are soft x-ray absorption spectroscopy (XAS), soft x-ray emission spectroscopy (XES) and soft x-ray photoemission spectroscopy (XPS) and x-ray magnetic circular dichroism (XMCD).

Polarisation dependent soft x-ray absorption spectroscopies can measure either x-ray natural linear dichroism (XNLD) for alternately linearly polarised light, or x-ray magnetic circular dichroism (XMCD) for alternately circularly polarised light.

The application of x-ray magnetic circular dichroism (XMCD), and the associated XMCD sum rules, to materials can probe on an element specific basis the orbital and spin components of the magnetic moment within a material.

The anisotropic electronic structure or anisotropic chemical bonding revealed via XNLD can give further insight into electronic structure, and not just act as a probe of molecular orientation on surfaces, though this is its most common usage and is properly termed NEXAFS.

These x-ray spectroscopic techniques reveal detailed information about the conduction band structure, the valence band structure, the core-level structure of the material and its magnetic behaviou respectively. Further, through the use of resonant soft x-ray emission spectroscopy (RXES) either the low-energy electronic excitations can be directly probed where this is known as resonant inelastic x-ray scattering (RIXS), or site-selective probes of the valence band via XES from differing chemical species can be exploited, e.g. in organic molecules; or lastly bandstructure effects are prominent. The latter two are more adequately described as RXES rather than RIXS though the quantum mechanics make no distinction. Polarisation dependent RXES of both pure and doped transition metal oxides and fluorides, as well as from organic molecular semiconductors, are one of my principal research interests. My other principal research interest is in the magnetic behaviour of atomic width nanowires arrays capped with noble metals.

You can also find a complete listing of the scientific publications that I have helped to contribute to the literature.

Templating of organic thin film growth: organic molecular semiconductors, nanomeshes and graphene nanoribbons

Controlled structured growth of organic materials, organic molecular semiconductor thin films as well as graphene in particular, is of importance for future devices where either interfaces or local intermolecular forces dominate in determining the structure and, as it turns out, the most useful device characteristics. This project will seek to perform measurements of the adsorption, chemical bonding and electronic structure of organic molecular semiconductor materials forming either heteroepitaxial organic thin films on inorganic semiconductor or metal surfaces, or in the formation of covalently bonded organic nanostructured networks on inorganic semiconductor or metal surfaces. Where possible real-time in-situ measurements will be made to probe intermolecular forces in thin films. Of particular interest is growth templating on stepped or terraced vicinal single crystal metal surfaces which may allow for useful regular nanoribbons of graphene to be formed by MOCVD. Associated density functional theory calculations of adsorption, electronic structure, and x-ray spectroscopy on these surfaces may play a significant part of this project.

Measuring adsorption, chemical bonding and electronic structure of organic molecular semiconductors or thin films requires ultra high vacuum chambers organic molecular beam deposition or MOCVD growth and XPS or UPS photoemission, all available in TCD. Other x-ray spectroscopic techniques are available at international synchrotron radiation facilities, while real- time in-situ measurements, as well as scanning probe measurements of these interfaces, surfaces and films to occur in collaborators laboratories. Research will be in collaboration with groups in Chemistry (G. Duesberg), Dublin City University (A. Cafolla), Aberystwyth, Wales (A. Evans) and Boston U. (K. Smith) with measurements at synchrotron radiation facilities such as MAXLAB in Sweden or National Synchroton Light Source, NY USA.

Poster Templated Growth Organics 2011 (pdf)

Resonant inelastic x-ray scattering - Probing local electronic structure and chemical bonding in transition metal compounds

To measure the symmetry dependent resonant x-ray emission spectroscopy or resonant inelastic x-ray scattering for a variety of structurally similar transition metal oxides and fluorides of formula unit MA2, where excitations at the anion K-edge exciting the anion A 1s electron allows us to probe the anion A 2p densities of states.

Defects can be created in transition metal oxide (TMO) thin films or bulk samples, e.g by high temperature annealing, or by oxidation or reduction of the surfaces in ultra high vacuum. The distribution of these defects can then be controlled by applying electric fields, where the migration of these defects is called electromigration. With defect density gradients established the resultant optical properties are also changed (electrocoloration). The conductivity of defect rich TMOs is significantly changed, and the local physical and local electronic structure surrounding these defects will be probed. Local physical structure can be probed in TCD using electron microscopies. Local electronic structure will be probed by synchrotron radiation based x-ray emission and x-ray absorption and resonant inelastic x-ray scattering (RIXS) spectroscopies at the Advanced Light Source, Lawrence Berkeley Laboratory, California or MAXLAB in Sweden, among others.

Poster RXES Oxides Fluorides 2011 (pdf)


Ongoing research themes

  1. Novel element-selective symmetry, polarisation and state resolved investigations of chemical bonding in rutile metal oxide and fluoride systems

    • The electronic structure of a class of crystalline solids will be investigated through a novel application of polarisation dependent synchrotron radiation based resonant soft x-ray emission spectroscopy and x-ray absorption spectroscopies to obtain element specific, symmetry and state selective measurements of the occupied partial density of states or occupied molecular orbitals of these solids. Systematic investigations of the chemical bonding in these rutile systems can thus be carried out and compared to electronic bandstructure calculations. Opportunities then exist to examine the bonding within these systems to alternative transition metals substitutionally doped onto the cation sites in these rutile systems.
  2. X-ray magnetic dichroism of nanoscale magnetism in atomic wires protected by capping layers

    • Atomic wires of cobalt, possessing unusual magnetic properties, have been successfully grown on platinum single crystal surfaces but, to be useful, such nanowires must be capped by ultra-thin films to protect them from contamination. The interfacial region formed by capping will affect the properties of these nanoscale magnetic structures; for certain capping layers and thicknesses enhanced Curie temperatures are expected. X-ray magnetic circular dichroism spectromicroscopy will be used to probe the magnetisation of these atomic wires on an element and electronic orbital specific basis, to complement and extend new non-linear magneto-optic studies of the same advanced materials.
      In collaboration with Prof. John McGilp of the Surface Physics Group.
  3. Electronic structure of magnetic semiconductor materials: element-specific soft x-ray spectroscopies

    The project on electronic structure of magnetic semiconductors as measured by x-ray absorption and emission spectroscopy is already underway, but interested students are still invited to apply.
    • Ferromagnetic semiconductor devices are the future basis of magnetic semiconductor devices are the future basis of "spintronics". Some of the most promising candidate materials for room temperature ferromagnetic semiconductor are magnetically doped wide band gap transition metal oxides and nitrides. Specific examples are Co-doped ZnO or SnO2 or Mn-doped GaN. The electronic structure of these materials will be studied by resonant soft x-ray emission at the transition metal 2p edge and oxygen (or nitrogen) 1s edges. The resulting resonant inelastic x-ray scattering spectrum and its energy dependence will be studied to obtain information about the local electronic structure of the dopant transition metal cations within these oxide (or nitride) systems. These absorption and emission spectra will be modelled through use of atomic multiplet structure packages. The element specific spin and orbital moments will also be studied through x-ray magnetic circular dichroism measurements. All these measurements take place at synchrotron radiation facilities.
      The ultimate goal of the project is to correlate the spectral information with both the electronic structure and with the magnetic properties of these materials.

      Download this paper on Co-doped ZnO

  4. Synchrotron X-ray Spectroscopic Investigations of Electronic Structure in Organic Semiconductors

    The project on organic molecular semiconductors has started some time ago but interested students are still invited to apply.
    • Organic molecular semiconductors are of increasing technological importance. Ultrathin pure films of organic molecular semiconductors can be created by organic molecular beam deposition in ultrahigh vacuum environments. The electronic structure of selected organic molecular semiconductors will be studied using synchrotron based soft x-ray emission spectroscopy. This probes the valence band or highest occupied molecular orbitals of the material and combined with excitation on resonance can provide information which is unavailable through the standard tools of x-ray photoemission spectroscopy. This project focuses on combining this information obtained from x-ray emission spectroscopy with that obtained from photo-emission spectroscopy in a multi-technique multi-theme investigation of the metal-phthalocyanine family of organic semiconductors. The experiments will take place at synchrotron radiation facilities throughout Europe and in the USA. Much of this investigation will be coordinated with that of  Prof. McGovern of the Surface Physics Group.

      (Download this paper on copper phthalocyanine)


Current Funding

  • Postgraduate funding:

    Postgraduate funding is available through The Irish Research Council (formerly IRCSET) Postgraduate Research Scholarship Scheme. Such postgraduate funding is at a rate of €16,000 per annum.
    (The next round of applications have opened and are closing in mid February.)

    Visit the Irish Research Council website for details on post graduate funding, this is open to all EU citizens.

    TCD Scholarships are also available as partial fellowships.

    Further details on funding and on postgraduate fees may be found on the School of Physics website.

  • Beamtime funding:
    Funding for experimental beamtime at the synchrotron facilities is often obtained separately through the facilities themselves and is currently funded under the CALIPSO FP7 Integrating Activity.

Previous Funding

  • Project funding:
    The first and second projects have just been approved for funding (March 2007) through the 2007 Research Frontiers Programme of Science Foundation Ireland.
    Recruitment for these projects has begun but positions were filled in August 2007.
    The third and fourth projects have previously received funding through the Basic Research Award scheme of Science Foundation Ireland, and also through the Nanomaterials and Nanoscience, PRTLI initiative in the ami Nasr Institute for Advanced Materials.

    Funding has also been received from IRCSET with Intel as an industrial co-sponsor as well as the School of Physics.

    However, all positions created by this programmatic funding are currently filled

Collaborators

These projects will be carried out in collaboration with the following groups within Trinity:

  • Surface Physics Group of Prof. I. T. McGovern.
  • Surface Physics Group of Prof. J. McGilp.
  • Laser and Plasma Applications Group of Prof. J. G. Lunney.
  • Magnetism and Spin Electronics Group of Prof. J. M. D. Coey.
  • Electronic Structure Theory Group of Dr. Charles Patterson.

And other collaborators within Ireland:

I also collaborate with the following international groups: