Using computer modelling to help design materials for optical applications Robert A Jackson Chemical & Forensic Sciences School of Physical & Geographical

Using computer modelling to help design materials for optical applications

Robert A JacksonChemical & Forensic Sciences

School of Physical & Geographical SciencesKeele University

[email protected] @robajackson

Emerging Analytical Professionals Conference, 8-10 May 2015 2

Plan for talk

1. A (short) introduction to materials modelling2. Optical materials and their applications3. How computer modelling is applied to optical materials4. Two recent applications5. Conclusions and ongoing work

See http://www.slideshare.net/robajackson

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Examples of materials of interest

Emerging Analytical Professionals Conference, 8-10 May 2015

UO2– nuclear fuel

LiNbO3– many optical applications

YAG– example of laser material


Computational Chemistry and Materials Modelling

Computational Chemistry • Calculate material structures and properties.

• Help explain and rationalise experimental data.

• Predict material structures and properties.


Introduction to materials modelling

• The modelling being described here is at the atomic level (quantum mechanics is not involved).– Materials are described in terms of the positions

(coordinates) of their atoms, and the forces acting between them.

– Interatomic forces are described using interatomic potentials.


Generating a starting model

The fundamental principle of atomistic simulation is to describe the forces acting between the ions and to minimise this energy through shifting atomic coordinates.

1) Input the unit cell information: unit cell size, atomic coordinates, space group.

2) Place charges on the ions and define interatomic potentials acting between them.

3) Interatomic potentials typically represent electron repulsion/van der Waals attraction.


Energy minimisation

• Given the unit cell of the structure, we can generate the crystal structure using space group symmetry.– We can then calculate the lattice energy by summing the

interatomic interactions.

• The structure is then adjusted systematically to get the lowest possible energy (structure prediction).– Lattice properties like dielectric constants can be calculated.– The method can be adapted for defects and dopants in the

crystal.


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• Model the LiNbO3 structure using energy minimisation.

• Calculate the energy involved in co-doping the crystal with pairs of ions (e.g. Fe3+, Cu+) at different sites, so the optimum sites can be determined.

• The resulting information is useful for designing new doped forms of LiNbO3 for specific applications.

Example of materialsmodelling:


Optical materials

• Materials that have interesting/useful properties in the solid state:

• e.g. YLF (Yttrium Lithium Fluoride, YLiF4), which behaves as a solid state laser when doped with rare earth ions, e.g. Nd3+ (0.4 -1.2 at %)

http://www.redoptronics.com/Nd-YLF-crystal.html

YLF in more detail

• The rare earth ions (e.g. Nd3+) substitute at the Y sites, so there is no need for charge compensation.

• For Nd-YLF, laser frequency is 1047 or 1053 nm depending on crystal morphology.


Figure taken from T E Littleford, PhD thesis(Keele University, 2014)


What information can computer modelling provide?

• If optical properties depend on dopants, where do they substitute in the lattice?– Not always obvious, e.g. M3+ ions in LiCaAlF6, where there

are 3 different cation sites.

• How is the crystal morphology (shape) changed?– Important if the crystals are used in devices.

• Can optical properties (e.g. transitions) be predicted?

Example of an application

• BaY2F8 can be used as a scintillator for detecting radiation when doped with rare earth ions, specifically Nd and Tb.

• In the diagram, the Ba2+ ions are green, and the Y3+ ions are orange.


http://www.slideshare.net/nnhsuk/fine-structure-in-df-and-f-f-transitions


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Details of the paper

• Experimental: samples were grown & characterised using XRD, photoluminescence (PL) and radioluminescence (RL). – PL measurements allowed identification of the main optical

active transitions of the RE dopant.– RL measurements proved that the material is a promising

material for scintillation detectors.

• Modelling: confirmed the dopants substitute at the Y3+ site, and identified the optical transitions observed.


Crystal field calculation of the optical transitions

• The RE ions are predicted to substitute at the Y sites, and relaxed coordinates of the RE ion and the nearest neighbour F ions are used as input for a crystal field calculation.

• Crystal field parameters Bkq are calculated, which can

then be used in two ways – (i) assignment of transitions in measured optical spectra, and (ii) direct calculation of predicted transitions.

16Emerging Analytical Professionals Conference, 8-10 May 2015

How good is the method?

• In the paper, measured and calculated transitions were compared, and a typical agreement of between 3-5% was observed:

transition Exp. /cm-1 Calc. /cm-1

5D4 7F4

17181 17724

18037 18041

5D4 7F5

18116 19111

19900 19364


Conclusions on this work

• Computer modelling, used in conjunction with experimental methods, can help characterise optical materials and suggest ones.– e.g. by calculating transitions with different dopants before

the sample preparation is carried out.

• Crystal field calculations are still ‘classical’, and ultimately we would like to use quantum methods. But usable software is still not available.


How is the shape of crystals affected by doping?

• YLF (YLiF4) has already been considered, and it was mentioned that laser frequency depends on crystal morphology.

• We have used modelling to predict changes in the morphology when YLF crystals are doped.– This can be done by calculating surface energies, and

predicting morphology from the most stable surfaces.

YLF Morphology


T E Littleford, R A Jackson, M S D Read: ‘An atomistic simulation study of the effects of dopants on the morphology of YLiF4’, Phys. Stat. Sol. C 10 (2), 156-159 (2013)

YLF morphology as affected by Ce dopants


Ce-YLFSurface energy approach

021 face appears, 111 disappears

Relative effect on surfaces• The (011) surface becomes less prominent with the (111) surface disappearing. • The 021 surface is stabilised by Ce dopants and appears in the defective morphology.



Conclusions on morphology study

• Changes in morphology can be predicted, and comparison with experimental results made where these are available.

• The next step is to look at how the optical behaviour of the dopant ions depend on location in the bulk or surface of the crystal.– This might explain dependence of laser frequency on

morphology.


Conclusions

• I have shown how computer modelling can be used to:– (i) interpret optical behaviour of materials, and potentially

help to design new ones.– (ii) predict the effect on crystal morphology of dopants,

with a view to extending this to looking at the effect on optical behaviour as well.

Acknowledgements

24Emerging Analytical Professionals Conference, 8-10 May 2015

Tom Littleford (PhD, Keele, 2014)Mark Read (AWE, then Birmingham)

Mário Valerio, Jomar Amaral (UFS, Brazil)

Documents

Using computer modelling to help design materials for optical applications Robert A Jackson Chemical & Forensic Sciences School of Physical & Geographical