Events
Photochemistry on polarisable semi-conductors - where are we?
| Date: | Wednesday 13 February 2008 16:00 - 17:00 |
| Location: | Eng 305 |
S Dunn
Nanotechnology Centre, Cranfield University, Bedfordshire, MK43 0AL
E-mail: s.c.dunn@cranfield.ac.uk
The controlled deposition of metal nanostructures and the use of semi-conductors for efficient energy harvesting has been an area of research that has been gaining renewed interest. Ferroelectrics, or materials that can sustain a polarisation state but still behave as semi-conductors, have been used as catalysts in redox reactions for a number of years, and recent applications include their use as lithographic tools, where the domain orientation spatially controls the reduction of metal [1-4]. The deposition of metals onto a ferroelectric surface is due to photoexcited electron/hole (e-/h+) pairs. The energy bands are bent near the surface due to polarization of the material, creating a space charge region (SCR) just below the surface. This causes the photo-generated e-/h+ to migrate either to the surface or into the bulk of the film depending on the ferroelectric domain orientation [5]. In positive domains (c+) the energy bands bend downwards and e- migrate to the surface during irradiation; this allows metal reduction, i.e. the domain acts as a cathode. In negative domains (c-) the bands bend upwards, h+ migrate to the surface and so oxidation occurs, i.e. it acts as an anode.
In this work I will present a number of historical studies on the use of materials such as barium titanate in photochemistry as well as focus on some of the recent results from my laboratory. Some of these results focus on the investigation of the the photochemical reaction of metal salt solutions (Sn, Fe, Cr, Mn and Al) on the surface of PZT 30/70 [111] during super-band gap UV irradiation. Observations show that there is metal deposition on positive domains or photo-corrosion on negative domains. These results are shown to be dependent on the position of the reduction potential of the metal respect to the conduction and valence band of PZT. For metals with a reduction potential within the band gap we observed growth of nanostructured metal particles on the negative domains, such as with Ag and Sn. If the metal reduction potential is over the conduction band, the reduction reaction is not possible on positive domains. In those cases, corrosion of the negative domains can be induced by weakening of the Pb-O bond by photo-generated h+, as happens with AlCl3.
However, we show that for certain metals such as Fe, with reduction potential near the edge of the conduction band of the PZT, either reduction or photo-corrosion can happen. This effect can be explained due to the presence of a processing-dependent uncertainty in the location of the band edges at the surface of the PZT. The exact position of these edges is determined by the Femi level's pinning location, which in time is dependent on the surface states specific to each sample. Therefore, the location of the band edges is sensitive to the manufacturing process within an interval of energies. Our results show an upper limit of 0.36eV for this uncertainty region. The uncertainty in the position of the band edges can influence properties such as the width of the space-charge region and barriers for charge injection, which play an important role in devices built with PZT.
I will also discuss the nucleation mechanisms of photochemical growth and some aspects of surface states and defects that influence the photochemistry.
[1] J. L. Giocondi and G. S. Rohrer, "Spatially selective photochemical reduction of Silver on the surface of ferroelectric Barium Titanate," Chemistry of Materials, vol. 13, pp. 241-242, 2001.
[2] S. V. Kalinin and D. A. Bonnell, "Local potential and polarization screening on ferroelectric surfaces," Physical Review B, vol. 63, pp.
125411-125413, 2001.
[3] J. B. Lowekamp, G. S. Rohrer, P. A. Hotsenpiller, J. D. Bolt and W. E. Farneth, "Anisotropic photochemical reactivity of bulk TiO2 Crystals," J Phys Chem B, vol. 102, pp. 7323-7327, 1998.
[4] D. A. Bonnell and S. V. Kalinin, "Local polarization, charge compensation and chemical interactions on ferroelectric surfaces: A route towards new nanostructures," in Ferroelectric Thin Films X, 2002, pp. 317-328.
[5] P. M. Jones and S. Dunn, "Photo-reduction of silver salts on highly heterogeneous lead zirconate titanate," Nanotechnology, vol.
18, pp. 185702-185708, 2007.
Bio
Dr Steve Dunn, joined the Nanotechnology Centre at Cranfield University as a post-doctoral Research Officer in December 1998 following the completion of his PhD (Potentiometric Sensors) at the University of Cambridge. His first projects at Cranfield involved the development of tools and techniques for patterning and investigating surfaces at the nanoscale. In August 2001 he was promoted to the position of Lecturer, and then to Senior Lecturer in October 2006. In addition to the responsibilities as a Senior Lecturer he is also an Associate Dean for the faculty, and a mentoring Course Director. Dr Dunn has also been responsible for the growth of the MSc programme on Microsystems and Nanotechnology and runs a research group that focuses on Photochemistry and the applications of band-structure engineered materials to devices. The band structure may be engineered through size constraints, or in the case of ferroelectric materials through local poling. The materials of interest are semi-conductors such CdTe and CdSe, as well as the ferroelectric semi-conductors
PbZr(x)Ti(1-x)O3 and BaTiO¬¬3. Since the start of 2007 Dr Dunn's group have published 2 seminal papers on the surface reactions of
photoexcited3,4 ferroelectric semiconductors. The size constraints or domain orientation of the materials affects the band structure, thus making them interesting in devices such as LED, the photolysis of water or potentially photovoltaic's. Recently the work has taken the route of incorporation of nanostructured semi-conductors into LED's type structures , using the band gap of the semiconductors to produce nanostructured Ag particles and developing an understanding of the impact of domain orientation on photoactivity on ferroelectric materials , . The initial work investigating the nanoscale control of surfaces has been used as a springboard for further investigations into nanoscale surface control . This work consolidated an interest in the use of nanostructured materials in devices and led to the development of fruitful collaborations and publications on the development of metal nanostructures and new device architectures1, , . The work is now being undertaken by a research group of 2 PDRA, 5 PhD students and 2-3 MSc students. Dr Dunn has (co)authored over 35 peer reviewed journal papers, over 35 conference proceedings and is a named inventor on patents concerning the development of N-LED devices.
