Optical Engineering of Metal Oxides
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Optical Engineering of Metal Oxides
Jessica BristowDepartment of Chemistry
University of Bath E-mail: [email protected]
Supervisors: Dr Aron Walsh, Professor Chris Bowen, Professor Frank Marken
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A Band Gap
Conduction band
Valence band
Oxides are stable, abundant materials:
ZnO (3.4 eV) , Al2O3 (9.25 eV), MgO (7.8 eV)
e-Band gap
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Light Absorption and Emission
Photovoltaics (PV) “light to electricity”
(Pixomar image)
Light emitting diodes (LED) “electricity to light”
AIM: Control λ to tune optical properties
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Maximum theoretical efficiency
Peter L M, Phil. Trans. R. Soc. A 2011; 369 : 1840-1856
Shockley–Queisser limit for solar
cells under AM1.5 illumination
Most metal oxides
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Sensitise with 3d Metals
Band gap engineering
Conduction band
Valence band
Transitionmetal impurities
λ3λ2λ1
Tuning optical Properties by doping
Applications:
LED PhosphorsIntermediate band PV
Predicted maximum PV efficiencyfor intermediate gap device: 63%
Luque, A. and Marti, A., Phys. Rev. Lett. 1997, 78, 5014–5017.
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Al2O3
Fe + Ti impurities
WHY?Corundum (α-Al2O3)
(Source: Unithaigems)
Sapphire (α-Al2O3 + Fe,Ti)
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Materials modelling
The principle is to model materials and resolve their properties:
INPUT OUTPUT
Methods employed:
1. Ionic potentials
2. Electronic structure techniques
Atom coordinates
and identities
Electronic and material
properties
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Born-ionic potential results
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, Chem Commun., 2013, 49, 5259.
TiIII + FeIII TiIV + FeII
Blue Sapphire
Mechanism of colour:
III/III cations are the ground state configuration
II/IV configuration represents a meta-stable state
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Electronic structure results
Theory Input Output Relative Energy (eV) Spin density
HSE 06 TiIV + FeII TiIII + FeIII 0.00
HSE 06 TiIII + FeIII TiIII + FeIII 0.82
Density Functional Theory (with hybrid exchange-correlation)
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.
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Computational Requirements
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
G. Kresse and J. Hafner., Phys. Rev. B, 1994, 49:14251.
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ConclusionFrom 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
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Acknowledgements
EPSRC (CSCT DTC)University's HPC service (Aquila)MCC HPC service (HECToR)Supervisor: Dr Aron Walsh Dr Davide Tiana & Walsh GroupProfessor Steve ParkerAdditional supervisors: Professors Frank Marken and Professor Chris Bowen (Mech Eng)