Abstract
The argon content of titanium dioxide doped tantalum pentoxide thin films was quantified in a spatially resolved way using high angle annular dark field (HAADF) images and DualEELS (a form of electron energy loss spectroscopy (EELS) that takes two spectra in quick succession from the highs and low-loss region). Films annealed at 300 °C, 400 °C, and 600 °C were investigated to see if there was a relationship between annealing temperature and bubble formation. It was shown using HAADF imaging that argon is present in most of these films and that bubbles of argon start to form after annealing at 400 °C and coarsen after annealing at 600 °C. A semi-empirical standard was created for the quantification using argon data from the EELS Atlas and experimental data scaled using a Hartree Slater cross-section. The density and pressure of argon within the bubbles were calculated for 35 bubbles in the 600 °C sample. The bubbles had a mean diameter, density, and pressure of 22 Å, 870 kg/m3 and 400 MPa, respectively. The pressure was calculated using the Van der Waals equation. The bubbles may affect the properties of the films, which are used as optical coatings for mirrors in gravitational wave detectors. This spatially resolved quantification technique can be readily applied to other small noble gas bubbles in a range of materials.
Highlights
In recent years, gravitational waves from merging binary black holes [1, 2, 3, 4, 5] and binary neutron stars [6] have been detected by the Advanced LIGO [7] and Advanced Virgo [8] gravitational wave detectors
In both cases lower-density regions are seen, all a few nm in diameter. They are mostly rounded in appearance, suggesting that they are approximately spherical voids or bubbles. They are clearly larger and more prominent in the sample annealed at the highest temperature
Scanning transmission electron microscopy has been used to study the behaviour of argon in argon ion-beam assisted deposited thin films of Ta2O5 − TiO2 mixed oxides of the type used for multilayer mirrors in the Advanced LIGO gravitational wave detectors
Summary
Gravitational waves from merging binary black holes [1, 2, 3, 4, 5] and binary neutron stars [6] have been detected by the Advanced LIGO [7] and Advanced Virgo [8] gravitational wave detectors. These detectors are modified Michelson interferometers that measure changes, induced by gravitational waves, in the relative separation of mirrors located at the end of km-scale perpendicular arms. Understanding the atomic and nano structure of the mirror coatings is the first step in reducing their mechanical loss
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