Mathew, Shiny; (2020) Study of Doping Phenomena in Functional Materials. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The disruptive technologies that are currently emerging across the semi-conductor and electronics industries demand the need for a continual focus on decreasing the size of integrated circuits and renewable energy technology devices. To support this demand requires comprehensive research into understanding the functional properties of materials at an atomic level across a three-dimensional space. These functional properties of materials originate from atomic level properties such as structural, optical and electronic properties, all of which can be modified to optimise the functionality of a material. This is the reason why these atomic level properties were comprehensively studied and reported in this thesis. The introduction of atomic level impurities, via the phenomena of doping, has helped to modify the structural, optical and electronic properties of the materials investigated in this thesis. An insight into the effect and potential that high temperature solid-state doping can bring towards improving the functional properties of three materials namely titania-rutile (single crystal substrates), titanates (powders) and Magnesium Silicon Nitrides MgSiN2 (powders), were gained from the experiments and results reported in this thesis. NB: titanates studied include sodium and potassium hexatitanates, sodium trititanate and caesium titanate. Chapters 3-8 of this thesis were written with a specific focus on the spatial arrangement of dopant atoms (such as B, C, S and N) introduced into photocatalytic titania-rutile and the associated influence it has on bonding, diffusion behaviours as well as structural and electronic properties. The insight gained about these properties of titania-rutile are essential when working on an industrial scale to optimise the performance of renewable energy devices, or to at least match with that of fossil fuels. The choice of anionic dopant introduced into the titania-rutile can help to vary the structural or electronic properties in titania-rutile. Additionally, the unit cell structure that determines the surface and bulk structure of the titania-rutile single crystal substrate that was chosen was observed to also help modify the structural or electronic properties. While the carbon, sulphur and nitrogen anions were predominantly incorporated as surface dopants in titania-rutile, this was not the case with boron anions, which also showed results that were dependent on the orientation of the titania-rutile. Boron incorporation in (110)-titania-rutile led to the formation of a TiBO3 surface layer, approximately 120 nm thick as per XPS data. This TiBO3 layer, as per XRD data, is epitaxially arranged on the rutile (110) surface along the (108), (118) and (018) planes. While this layer was also seen on the rutile (100) surface, no XRD evidence of TiBO3 was found with the rutile (001) surface. As well as observing a shift in the XPS valence band onset, the emergence of new states and O2p orbital mixing was also observed upon anion incorporation into rutile. This study, reporting the structural and electronic effects observed as a result of doping, will be crucial when working with photocatalysts that are widely studied for the water splitting process, used to produce hydrogen, which is a ‘clean’ energy fuel. The main insight gained from chapter 9 is about making use of the structure of titanate materials (e.g. in open layered and tunnelled titanates) as a scaffold, to control the spatial distribution of any given dopant. This is particularly relevant when the material being investigated is in its powdered form, with no well-defined surface or a bulk. Chapter 9 was written with a specific focus on the effect of doping temperature on the location of the incorporated nitrogen dopant (aka. structural properties), electronic and optical properties in the open layered and tunnelled titanates. While these relationships are widely reported in the literature already, the challenge that this study addresses are about carrying out nitrogen doping at three different temperatures in the same system to ensure the same ammonia flow rate, which is a parameter that is often very challenging to reproduce. This ensures reproducibility of results and therefore reliability in the conclusions. Also, this study is also more comprehensive than that reported in literature the and discusses samples that are fully characterised. The preferential incorporation of nitrogen into the Ti-O-Ti bonds than the Na-O-Ti bonds was observed in the tunnelled titanates, Na2Ti6O13 and K2Ti6O13, and (not in the open layered titanates Na2Ti3O7 and Cs0.68Ti1.825O4), is potentially what led to the creation of Ti3+ defects as observed in their optical absorption spectrum. The resulting Ti 3d and N 1s states observed in the XPS valence band spectrum is potentially what caused the observed band gap narrowing. The modification of the electronic, optical and structural properties of the titanates using nitrogen doping can be used to optimise the functionality of titanates e.g. when it is used as photocatalysts or as battery materials. The main insight gained from chapter 10 is about exploring the alternative material that can replace the expensive aluminium nitride, which is known to be a promising substrate material with ideal thermal conductivity and minimal dissipation of heat. This was done by studying the change in structural properties including associated unit cell volume. The effect of the addition of varying amounts of aluminium as a dopant into the MgSiN2 structure, helped to find that the phase transformation from MgSiN2 to AlN-wurtzite structure is observed between 30% and 50% aluminium dopant introduction, as per XPS and XRD. While the doubling of the magnesium reactant mass led to a single phase MgSiN2, it can potentially affect particle size properties, as hinted by the XRD peak broadening observed. Increasing the aluminium content to above 50% led to unit cell volume contraction as Al3+ ions are smaller than the lattice Mg2+ ions they are substituting. These findings can help gain an understanding of the fundamental chemistry underpinning the development of cheaper, alternative materials for any applications.

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