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Probing the atomic structure of mirror coatings using transmission electron microscopy

Stuart Reid, Stuart Reid, Riccardo BassiriRiccardo Bassiri1 1 , Konstantin B. Borisenko, Konstantin B. Borisenko2 2 , David J. H. , David J. H. CockayneCockayne2 2 , Keith Evans, Keith Evans11, James Hough, James Hough11, Ian MacLaren, Ian MacLaren11, Iain Martin, Iain Martin11, , Sheila RowanSheila Rowan1 1

11 SUPA, University of Glasgow, SUPA, University of Glasgow, 2 2 Department of Materials, University of Oxford Department of Materials, University of Oxford

GWADW, Kyoto, Japan – May 2010GWADW, Kyoto, Japan – May 2010

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Coating development critical for the futureCoating development critical for the future

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Frequency [Hz]

3rd generationS

train

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Coating thermal Noise

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• The test mass mirror coatings are estimated to be a significant source of thermal noise in future ground-based GW detectors

• Thermal noise is proportional to the mechanical loss(internal friction) of the material

• Considerable research is being conducted into understanding the material properties of these coatings (see previous and following talks)

– The focus here is on the atomic structure and how this affects the material properties - and in particular the mechanical loss

IntroductionIntroduction

What is causing the What is causing the mechanical loss on an mechanical loss on an

atomic level?atomic level?

GEO600 test-mass

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• Useful for probing atomic structure and chemistry

• Allows us to characterise atomic structure– Imaging– Diffraction – Spectroscopy

Transmission electron microscopyTransmission electron microscopy

Transmission Electron MicroscopeTecnai T-20

X-Rays

Direct beam

Diffracted

beam

Ener

gy lo

ss

elec

tron

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Electronbeam

Sample

Interactions of electron beam with sample

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Image of multilayer coating, (bright- silica, dark - tantala)

Amorphous diffraction pattern of 300oC tantala

Crystalline diffraction patternof 800oC tantala

Crystalline diffraction patternof 800oC tantala

• Compare TEM results to mechanical loss Compare TEM results to mechanical loss

measurementsmeasurements• The 800The 800oC sample has high loss peak at 80-

90K probably due to crystallisation• To probe the properties of the amorphous To probe the properties of the amorphous

samples we need Reduced Radial Density samples we need Reduced Radial Density

FunctionsFunctionsMechanical loss measurements for heat treated tantala (see previous talk by Matt Abernathy

Initial interesting results: (from Ta2O5 samples heat-treated at a range of temperatures)

Transmission electron microscopyTransmission electron microscopy

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Reduced radial density functionsReduced radial density functions

• The reduced density function is a Fourier transform of the information gained from the intensity profile [D. J. H. Cockayne, Annu.Rev.Mater.Res, 37:159-87, (2007)]

Intensity profile Reduced density functionTantala diffraction pattern

• Silica and tantala are amorphous materials– They do not have long range order– They do have short range order

• We can probe this short range order with reduced density functions– RDFs give a statistical representation of where atoms sit with regards to a central atom

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Reduced density functionsReduced density functions

RDFs of heat-treated Ta2O5

• Three Ta2O5 coatings were measured– Each one was heat-treated at a different temperature (300, 400 & 600oC)

– RDFs show differences in local atomic structure as heat treatment temperature rises

– From comparison to the structure of crystalline Ta2O5 we can deduce that the first peak arises from Ta - O bonds and second peak from Ta - Ta bonds

– Both first and second peaks become more defined and difference in heights between them decrease as heat treatment temperature rises implying that:

– Material is becoming more ordered– There is an increase in Ta - Ta bonding

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Modelling the atomic structureModelling the atomic structure

• Why? – If we accurately interpret the RDF

– What do the peaks mean? – What bond types correspond to each peak?

• We can then: – Investigate the atomic structure

– Average distances between atoms

– Co-ordination numbers

– Bond types

– Bond angles

– Probe the material properties– Relationship to mechanical loss

– Optical properties?

– Other material properties?

Reduced density function

Energy optimised Ta2O5 atomic model(blue - Ta, red - O)

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• Reverse Monte Carlo refinements comparing model to experimental RDFsReverse Monte Carlo refinements comparing model to experimental RDFs

• Constrains the model Constrains the model further further

• Ensure atoms are sitting in Ensure atoms are sitting in a physically stable positiona physically stable position

• Gives a greater degree of Gives a greater degree of accuracyaccuracy

Initial constraints from the Initial constraints from the crystalline structure of Tacrystalline structure of Ta22OO55

– Atom typesAtom types

– Bond lengthsBond lengths

– Bond typesBond types

– Bond distributionsBond distributions

RMC refinementRMC refinement

Do experimental and theoretical RDFs agree?

Do experimental and theoretical RDFs agree?

ConstraintsConstraints

Final structureFinal structure

Energy optimisationEnergy optimisation

Modelling the atomic structureModelling the atomic structure

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Modelling the atomic structureModelling the atomic structure

Refinement process

RDF comparison before RMC (using initial boundary conditions model)

RDF comparison after RMC (using RMC + energy optimised model)

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Modelling the atomic structureModelling the atomic structure• Reverse Monte Carlo modelling was carried out on the 400oC heat

treated tantala coating

– Results from this model show an average Ta to Ta bond length of 3.19Å and Ta to O bond length of 1.99Å

– Co-ordination number for Ta = 6.53, O=2.09

– Ring structure of Ta and O bonds remains partially intact from the crystalline phase

Energy optimised Ta2O5 atomic model(Blue - Ta ,Red - O)

Crystalline model of Ta2O5

(Aleshina et al., Cryst. Rep. 47, 2002)

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Modelling the atomic structureModelling the atomic structure

Partial RDF of 400oC model

RDFs of heat-treated Ta2O5

– Allows a greater understanding of the relative distances from one atom to another

– Shows exactly what the peaks in the initial RDF mean

– Initial assumptions on comparing the peaks to the crystal phase are accurate

– Ta - O bonds dominate first peak

– Ta - Ta bonds dominate second peak

• Partial RDF data:

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Modelling the atomic structureModelling the atomic structure

Bond type distribution

Bond angle distribution of 400oC model

Bond angle type distribution

– Atomic modelling makes understanding bond structures in the sample easier

– Bond types

– Bond angle distributions

• Bond types and angles:

• Provides an excellent way to compare the changes in the atomic structure

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Future workFuture work• Near future – Compare results from each of

the three heat-treated atomic models

– Start modelling Ti doped and low water Ta2O5 samples (similar process)

Ti doped tantala RDFs

RDFs of heat-treated Ta2O5• Future– Investigate ways of getting

material properties from models

– X-ray scattering measurements for single element ‘RDF’ analysis of Ti doped samples

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What is causing the mechanical loss on an atomic level?What is causing the mechanical loss on an atomic level?

ConclusionConclusion

•Significant progress towards answering this question-Now have well developed techniques in order to probe the atomic structure

•For Ta2O5 coatings heat-treated to 300, 400, 600 and 800oC: -Samples heat-treated to 300, 400, 600oC are amorphous, the 800oC sample has crystallised possibly causing the high mechanical loss peak at low temperatures

-Preliminary results from the 300, 400 and 600oC show an increase in local ordering and number of Ta - Ta bonds as heat treatment temperature increases

-Atomic modelling provides an accurate way to fully understand the RDF and investigate bond types and distributions

•Combining microscopic techniques together with mechanical loss measurements will allow us to gain a better understanding of how these mirror coatings perform and help produce low mechanical loss coatings

•The same techniques will be applied to other mirror coatings that have varying material properties