Download RAJ EAP talk - Rob Jackson`s Website

Using computer modelling to help design
materials for optical applications
Robert A Jackson
Chemical & Forensic Sciences
School of Physical & Geographical Sciences
Keele University
[email protected]
@robajackson
Plan for talk
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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/robajackson
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Examples of materials of interest
LiNbO3– many optical applications
YAG– example of laser material
UO2– nuclear fuel
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Computational Chemistry and Materials
Modelling
Computational Chemistry
• Calculate material structures
and properties.
• Help explain and rationalise
experimental data.
• Predict material structures
and properties.
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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.
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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.
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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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Example of materials
modelling:
• 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.
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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
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YLF in more detail
Figure taken from T E Littleford, PhD thesis
(Keele University, 2014)
• 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.
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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?
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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.
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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 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.
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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
5D
4

5D
4
 7F5
7F
4
Exp. /cm-1
Calc. /cm-1
17181
18037
18116
19900
17724
18041
19111
19364
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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.
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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.
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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)
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YLF morphology as affected by Ce dopants
Surface energy approach
Ce-YLF
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, 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.
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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.
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Acknowledgements
Tom Littleford (PhD, Keele, 2014)
Mark Read (AWE, then Birmingham)
Mário Valerio, Jomar Amaral (UFS, Brazil)
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