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)