invited talk given at the eap conference, 2015
TRANSCRIPT
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Using computer modelling to help design materials for optical applications
Robert A JacksonChemical & Forensic Sciences
School of Physical & Geographical SciencesKeele University
[email protected] @robajackson
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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
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Examples of materials of interest
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UO2– nuclear fuel
LiNbO3– many optical applications
YAG– example of laser material
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Computational Chemistry and Materials Modelling
Computational Chemistry • Calculate material structuresand properties.
• Help explain and rationaliseexperimental data.
• Predict material structuresand properties.
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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.
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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.
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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.
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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:
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Optical materials
• Materials that have interesting/useful properties inthe solid state:
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• 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
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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.
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Figure taken from T E Littleford, PhD thesis(Keele University, 2014)
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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?
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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.
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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.
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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
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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.
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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.
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YLF Morphology
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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)
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YLF morphology as affected by Ce dopants
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Ce-YLFSurface energy approach
021 face appears, 111 disappears
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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.
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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.
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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.
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Acknowledgements
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Tom Littleford (PhD, Keele, 2014)
Mark Read (AWE, then Birmingham)
Mário Valerio, Jomar Amaral (UFS, Brazil)