computer modelling to help design materials for optical applications Robert A Jackson Chemical amp Forensic Sciences School of Physical amp Geographical Sciences Keele University rajacksonkeeleacuk ID: 320285
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Slide1
Using computer modelling to help design materials for optical applications
Robert A JacksonChemical & Forensic SciencesSchool of Physical & Geographical SciencesKeele University
r.a.jackson@keele.ac.uk
@robajacksonSlide2
Emerging Analytical Professionals Conference, 8-10 May 2015
2
Plan for talk
A (short) introduction to materials modelling
Optical materials and their applications
How computer modelling is applied to optical materials
Two recent applications
Conclusions and ongoing work
See http://www.slideshare.net/robajacksonSlide3
Examples of materials of interestEmerging Analytical Professionals Conference, 8-10 May 2015
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UO
2
– nuclear fuel
LiNbO3– many optical applicationsYAG– example of laser materialSlide4
Computational Chemistry and Materials Modelling
Computational Chemistry
Calculate
material structures and properties.
Help
explain and rationalise experimental data.Predict material structures and properties.4Emerging Analytical Professionals Conference, 8-10 May 2015Slide5
Introduction to materials modellingThe 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.Emerging Analytical Professionals Conference, 8-10 May 2015
5Slide6
Generating a starting modelThe fundamental
principle of atomistic simulation is to describe the forces acting between the ions and to minimise this energy through shifting atomic coordinates
.
Input the
unit cell information
: unit cell size, atomic coordinates, space group.Place charges on the ions and define interatomic potentials acting between them.Interatomic potentials typically represent electron repulsion/van der Waals attraction.
Emerging Analytical Professionals Conference, 8-10 May 2015
6Slide7
Energy minimisationGiven 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.
Emerging Analytical Professionals Conference, 8-10 May 2015
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Emerging Analytical Professionals Conference, 8-10 May 20158
Model the LiNbO
3 structure using energy minimisation.Calculate the
energy involved
in co-doping the crystal with pairs of ions (e.g. Fe
3+, 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:Slide9
Optical materialsMaterials that have interesting/useful properties in the solid state:
Emerging Analytical Professionals Conference, 8-10 May 20159
e.g.
YLF (
Y
ttrium 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.htmlSlide10
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.
Emerging Analytical Professionals Conference, 8-10 May 2015
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Figure taken from T E Littleford, PhD thesis(Keele University, 2014)Slide11
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?
Emerging Analytical Professionals Conference, 8-10 May 201511Slide12
Example of an applicationBaY2F8
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
.
Emerging Analytical Professionals Conference, 8-10 May 2015
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http://www.slideshare.net/nnhsuk/fine-structure-in-df-and-f-f-transitionsSlide13
Emerging Analytical Professionals Conference, 8-10 May 201513Slide14
Details of the paperExperimental: 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.
Emerging Analytical Professionals Conference, 8-10 May 2015
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Emerging Analytical Professionals Conference, 8-10 May 2015Crystal 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 B
kq 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.
15Slide16
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
5
D
4
7
F
4
17181
17724
18037
18041
5
D
4
7
F
5
18116
19111
19900
19364Slide17
Conclusions on this workComputer 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.
Emerging Analytical Professionals Conference, 8-10 May 201517Slide18
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.Emerging Analytical Professionals Conference, 8-10 May 2015
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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)Slide20
YLF morphology as affected by Ce dopants
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Ce-YLF
Surface energy approach
021 face appears, 111 disappearsSlide21
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.Emerging Analytical Professionals Conference, 8-10 May 201521Slide22
Conclusions on morphology studyChanges 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.Emerging Analytical Professionals Conference, 8-10 May 2015
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ConclusionsI 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.Emerging Analytical Professionals Conference, 8-10 May 2015
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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)