Probing the atomic structure of mirror coatings using transmission electron microscopy

1. Probing the atomic structure of mirror coatings using transmission electron microscopy • Stuart Reid, Riccardo Bassiri1 , Konstantin B. Borisenko2 , David J. H. Cockayne2 , Keith Evans1, James Hough1, Ian MacLaren1, Iain Martin1, Sheila Rowan1 • 1 SUPA, University of Glasgow, 2 Department of Materials, University of Oxford • GWADW, Kyoto, Japan – May 2010 1

2. -19 10 -20 10 1st generation Strain [Hz-1/2] -21 10 2nd generation -22 10 3rd generation -23 10 -24 10 -25 10 1 10 100 1000 10000 Frequency [Hz] Coating thermalNoise Coating development critical for the future 2

3. Introduction • 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 GEO600 test-mass What is causing the mechanical loss on an atomic level?

4. Transmission electron microscopy Electron beam X-Rays Sample Diffracted beam Energy loss electrons Direct beam • Useful for probing atomic structure and chemistry • Allows us to characterise atomic structure • Imaging • Diffraction • Spectroscopy Transmission Electron Microscope Tecnai T-20 Interactions of electron beam with sample

5. Transmission electron microscopy Initial interesting results: (from Ta2O5 samples heat-treated at a range of temperatures) Image of multilayer coating, (bright- silica, dark - tantala) Amorphous diffraction pattern of 300oC tantala Crystalline diffraction pattern of 800oC tantala • Compare TEM results to mechanical loss measurements • The 800oC sample has high loss peak at 80- 90K probably due to crystallisation • To probe the properties of the amorphous samples we need Reduced Radial Density Functions Mechanical loss measurements for heat treated tantala (see previous talk by Matt Abernathy

6. Reduced radial density functions • 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 Tantala diffraction pattern Intensity profile Reduced density function • 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)]

7. Reduced density functions • Three Ta2O5 coatings were measured • Each one was heat-treated at a different temperature (300, 400 & 600oC) RDFs of heat-treated Ta2O5 • 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

8. Modelling 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)

9. Modelling the atomic structure • Initial constraints from the crystalline structure of Ta2O5 • Atom types • Bond lengths • Bond types • Bond distributions • Reverse Monte Carlo refinements comparing model to experimental RDFs • Constrains the model further • Ensure atoms are sitting in a physically stable position • Gives a greater degree of accuracy RMC refinement Energy optimisation Constraints Do experimental and theoretical RDFs agree? Final structure

10. Modelling the atomic structure RDF comparison before RMC (using initial boundary conditions model) Refinement process RDF comparison after RMC (using RMC + energy optimised model)

11. Modelling the atomic structure • 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 • Reverse Monte Carlo modelling was carried out on the 400oC heat treated tantala coating Crystalline model of Ta2O5 (Aleshina et al., Cryst. Rep. 47, 2002) Energy optimised Ta2O5 atomic model (Blue - Ta ,Red - O)

12. Modelling the atomic structure • Partial RDF data: • 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 RDFs of heat-treated Ta2O5 Partial RDF of 400oC model

13. Modelling the atomic structure • Bond types and angles: • Atomic modelling makes understanding bond structures in the sample easier • Bond types • Bond angle distributions Bond type distribution Bond angle type distribution • Provides an excellent way to compare the changes in the atomic structure Bond angle distribution of 400oC model

14. Future 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) 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 Ti doped tantala RDFs

15. Conclusion What is causing the mechanical loss on an atomic level? • 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