invited talk given at the eap conference, 2015

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 modelling

2. Optical materials and their applications

3. How computer modelling is applied to optical materials

4. Two recent applications

5. Conclusions and ongoing work

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

Examples of materials of interest


UO2– nuclear fuel

LiNbO3– many optical applications

YAG– example of laser material

Computational Chemistry and Materials Modelling

Computational Chemistry • Calculate material structuresand properties.

• Help explain and rationaliseexperimental data.

• Predict material structuresand properties.

4Emerging Analytical Professionals Conference, 8-10 May 2015

Introduction to materials modelling

• The modelling being described here is at the atomiclevel (quantum mechanics is not involved).

– Materials are described in terms of the positions(coordinates) of their atoms, and the forces actingbetween them.

– Interatomic forces are described using interatomicpotentials.


Generating a starting modelThe fundamental principle of atomistic simulation is to describe the forcesacting between the ions and to minimise this energy through shifting atomiccoordinates.

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

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

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


Energy minimisation

• Given the unit cell of the structure, we can generate thecrystal 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 thelowest possible energy (structure prediction).– Lattice properties like dielectric constants can be calculated.

– The method can be adapted for defects and dopants in thecrystal.


Emerging Analytical Professionals Conference, 8-10 May 2015

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• Model the LiNbO3 structureusing energy minimisation.

• Calculate the energy involvedin co-doping the crystal withpairs of ions (e.g. Fe3+, Cu+) atdifferent sites, so the optimumsites can be determined.

• The resulting information isuseful for designing newdoped forms of LiNbO3 forspecific applications.

Example of materialsmodelling:

Optical materials

• Materials that have interesting/useful properties inthe solid state:


• e.g. YLF (Yttrium Lithium Fluoride,YLiF4), which behaves as a solidstate laser when doped with rareearth 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 Ysites, so there is noneed for chargecompensation.

• For Nd-YLF, laserfrequency is 1047 or1053 nm depending oncrystal 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 dothey substitute in the lattice?

– Not always obvious, e.g. M3+ ions in LiCaAlF6, where thereare 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 ascintillator for detectingradiation when dopedwith rare earth ions,specifically Nd and Tb.

• In the diagram, the Ba2+

ions are green, and theY3+ 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 & characterisedusing XRD, photoluminescence (PL) andradioluminescence (RL).– PL measurements allowed identification of the main optical

active transitions of the RE dopant.

– RL measurements proved that the material is a promisingmaterial 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 thenearest neighbour F ions are used as input for acrystal field calculation.

• Crystal field parameters Bkq are calculated, which can

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

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How good is the method?

• In the paper, measured and calculated transitions werecompared, and a typical agreement of between 3-5% wasobserved:

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 withexperimental methods, can help characterise opticalmaterials and suggest ones.– e.g. by calculating transitions with different dopants before

the sample preparation is carried out.

• Crystal field calculations are still ‘classical’, andultimately 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 wasmentioned that laser frequency depends on crystalmorphology.

• We have used modelling to predict changes in themorphology when YLF crystals are doped.

– This can be done by calculating surface energies, andpredicting 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, andcomparison with experimental results made wherethese are available.

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

morphology.


Conclusions

• I have shown how computer modelling can be usedto:

– (i) interpret optical behaviour of materials, and potentiallyhelp to design new ones.

– (ii) predict the effect on crystal morphology of dopants,with a view to extending this to looking at the effect onoptical behaviour as well.


Acknowledgements


Tom Littleford (PhD, Keele, 2014)

Mark Read (AWE, then Birmingham)

Mário Valerio, Jomar Amaral (UFS, Brazil)