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Optical Engineering of Metal Oxides. Jessica Bristow Department of Chemistry University of Bath E-mail: j.bristow@bath.ac.uk Supervisors: Dr Aron Walsh, Professor Chris Bowen, Professor Frank Marken. A Band Gap. Conduction band. Band gap. e -. Valence band.
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Optical Engineering of Metal Oxides • Jessica Bristow • Department of Chemistry • University of Bath • E-mail: j.bristow@bath.ac.uk • Supervisors: DrAron Walsh, Professor Chris Bowen, Professor Frank Marken
A Band Gap Conduction band Band gap e- Valence band Oxides are stable, abundant materials: ZnO (3.4 eV) , Al2O3 (9.25 eV), MgO (7.8 eV)
Light Absorption and Emission Photovoltaics (PV) “light to electricity” (Pixomar image) Light emitting diodes (LED) “electricity to light” AIM: Control λto tune optical properties
Maximum theoretical efficiency Shockley–Queisser limit for solar cells under AM1.5 illumination Most metal oxides Peter L M, Phil. Trans. R. Soc. A 2011; 369 : 1840-1856
Sensitise with 3d Metals Band gap engineering Conduction band Applications: LED Phosphors Intermediate band PV Transition metal impurities Tuning optical Properties by doping λ1 λ2 λ3 Valence band Predicted maximum PV efficiency for intermediate gap device: 63% Luque, A. and Marti, A., Phys. Rev. Lett. 1997, 78, 5014–5017.
Al2O3 Fe + Ti impurities (Source: Unithaigems) Sapphire (α-Al2O3 + Fe,Ti) Corundum (α-Al2O3) WHY?
Materials modelling The principle is to model materials and resolve their properties: INPUT OUTPUT Methods employed: Ionic potentials Electronic structure techniques Atom coordinates and identities Electronic and material properties
Blue Sapphire Mechanism of colour: Born-ionic potential results TiIII+ FeIII TiIV+ FeII III/III cations are the ground state configuration II/IV configuration represents a meta-stable state Only stable Tri-cluster in sapphire: TiIII-(TiIV-FeII) Jessica K. Bristow, Stephen C. Parker, C. Richard A. Catlow, Scott M. Woodley and Aron Walsh, ChemCommun., 2013, 49, 5259.
Density Functional Theory (with hybrid exchange-correlation) Electronic structure results The spin density confirms the self-consistent solution to the III/III ground state, even when starting from a IV/II initial configuration. The III/III configuration is shown to be the ground state with spherical (d5) spin density on Fe and a single electron (d1) on Ti. J. K. Bristow et al, Defect theory of Ti and Fe impurities and aggregates in alpha-Al2O3, To be submitted.
Interatomic potential calculations 4 cores on local iMac Primary code: GULP (General Utility Lattice Program) Electronic structure calculations 64 and 128 core jobs (for defective supercells) on Aquila 12 – 96 hours (dependent on level of theory and optimisation) Primary code: VASP (Vienna ab-initio Simulation Package) k-point parallelised version available: potential 256 and 512 core jobs on HECToR Future codes: FHI-AIMS and GPU accelerated Quantum Espresso Computational Requirements G. Kresse and J. Hafner., Phys. Rev. B, 1994, 49:14251.
Conclusion • From this work we propose: • A new ground state for neighbouring Fe/Ti pairs (III/III) • The FeII/TiIV pairs represent a metastable state with a limited life time • The tri-cluster [TiIII-(TiIV-FeII)] may be present in sapphire and aid the stability of the FeII/TiIV pairs
Acknowledgements • EPSRC (CSCT DTC) • University's HPC service(Aquila) • MCC HPC service (HECToR) • Supervisor: Dr Aron Walsh • Dr DavideTiana & Walsh Group • Professor Steve Parker • Additional supervisors: Professors Frank Marken and Professor Chris Bowen (MechEng)
