Optical Engineering of Metal Oxides
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Optical Engineering of Metal Oxides Jessica Bristow Department of Chemistry University of Bath E-mail: [email protected] Supervisors: Dr Aron Walsh, Professor Chris Bowen, Professor Frank Marken
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Optical Engineering of Metal Oxides. Jessica Bristow Department of Chemistry University of Bath E-mail: [email protected] Supervisors: Dr Aron Walsh, Professor Chris Bowen, Professor Frank Marken. A Band Gap. Conduction band. Band gap. e -. Valence band. - PowerPoint PPT Presentation
Transcript of Optical Engineering of Metal Oxides
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
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
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
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
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.
Al2O3
Fe + Ti impurities
WHY?Corundum (α-Al2O3)
(Source: Unithaigems)
Sapphire (α-Al2O3 + Fe,Ti)
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
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
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.
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.
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
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)
