skip to main content


Title: Structure and morphology of low mechanical loss TiO 2 -doped Ta 2 O 5

The exceptional stability required from high finesse optical cavities and high precision interferometers is fundamentally limited by Brownian motion noise in the interference coatings of the cavity mirrors. In amorphous oxide coatings these thermally driven fluctuations are dominant in the high index layer compared to those in the low index SiO2layer in the stack. We present a systematic study of the evolution of the structural and optical properties of ion beam sputtered TiO2-doped Ta2O5films with annealing temperature. We show that low mechanical loss in TiO2-doped Ta2O5with a Ti cation ratio = 0.27 is associated with a material that consists of a homogeneous titanium-tantalum-oxygen mixture containing a low density of nanometer sized Ar-filled voids. When the Ti cation ratio is 0.53, phase separation occurs leading to increased mechanical loss. These results suggest that amorphous mixed oxides with low mechanical loss could be identified by considering the thermodynamics of ternary phase formation.

 
more » « less
Award ID(s):
1708010 1708175 1707866
NSF-PAR ID:
10169020
Author(s) / Creator(s):
; ; ; ; ; ;
Publisher / Repository:
Optical Society of America
Date Published:
Journal Name:
Optical Materials Express
Volume:
10
Issue:
7
ISSN:
2159-3930
Page Range / eLocation ID:
Article No. 1687
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract We report on the development and extensive characterization of co-sputtered tantala–zirconia (Ta 2 O 5 -ZrO 2 ) thin films, with the goal to decrease coating Brownian noise in present and future gravitational-wave detectors. We tested a variety of sputtering processes of different energies and deposition rates, and we considered the effect of different values of cation ratio η = Zr/(Zr + Ta) and of post-deposition heat treatment temperature T a on the optical and mechanical properties of the films. Co-sputtered zirconia proved to be an efficient way to frustrate crystallization in tantala thin films, allowing for a substantial increase of the maximum annealing temperature and hence for a decrease of coating mechanical loss φ c . The lowest average coating loss was observed for an ion-beam sputtered sample with η = 0.485 ± 0.004 annealed at 800 °C, yielding φ ¯ c = 1.8 × 1 0 − 4 rad. All coating samples showed cracks after annealing. Although in principle our measurements are sensitive to such defects, we found no evidence that our results were affected. The issue could be solved, at least for ion-beam sputtered coatings, by decreasing heating and cooling rates down to 7 °C h −1 . While we observed as little optical absorption as in the coatings of current gravitational-wave interferometers (0.5 parts per million), further development will be needed to decrease light scattering and avoid the formation of defects upon annealing. 
    more » « less
  2. In this work, we explore inverse designed reconfigurable digital metamaterial structures based on phase change material Sb2Se3for efficient and compact integrated nanophotonics. An exemplary design of a 1 × 2 optical switch consisting of a 3 µm x 3 µm pixelated domain is demonstrated. We show that: (i) direct optimization of a domain containing only Si and Sb2Se3pixels does not lead to a high extinction ratio between output ports in the amorphous state, which is owed to the small index contrast between Si and Sb2Se3in such a state. As a result, (ii) topology optimization, e.g., the addition of air pixels, is required to provide an initial asymmetry that aids the amorphous state's response. Furthermore, (iii) the combination of low loss and high refractive index change in Sb2Se3, which is unique among all phase change materials in the telecommunications 1550 nm band, translates into an excellent projected performance; the optimized device structure exhibits a low insertion loss (∼1.5 dB) and high extinction ratio (>18 dB) for both phase states.

     
    more » « less
  3. Anatase TiO2is a promising anode material for lithium‐ion batteries (LIBs) owing to its low cost and stability. However, the intrinsically kinetic limits seriously hindered its lithium‐ion storage capability. Here we present that anatase TiO2with rich oxygen vacancies can enhance its lithium‐ion storage performance. We synthesize anatase TiO2with well‐retained hierarchical structure by annealing the H2Ti5O11·3H2O yolk‐shell spheres precursor in nitrogen atmosphere. EPR and XPS data evidence that the oxygen‐deficient environment could generate abundant oxygen vacancies in the as‐derived anatase TiO2, which leads to improved electron conductivity and reduced charge‐transfer resistance. The rich oxygen vacancies and high structural integrity of the hierarchical yolk‐shell spheres enable the as‐derived anatase TiO2yolk‐shell spheres with a high specific capacity of 280 mAh g−1at 100 mA g−1and 71% of capacity retention after 5000 cycles at 2 A g−1.

     
    more » « less
  4. Abstract

    Titanium dioxide (TiO2) is a promising electrode material for reversible lithium storage. However, the poor electronic conductivity, sluggish diffusivity, and intrinsic kinetics limit hinder its fast lithium storage capability. Here we present that the oxygen‐deficient TiO2hierarchical spheres can address the issues for high capacity, long‐term lithium‐ion battery anode. First‐principles calculations show that introducing oxygen vacancies to anatase TiO2can reduce the bandgap, thus improving the electronic conductivity and further the lithium storage properties of TiO2. By annealing TiO2/H2Ti5O11⋅3H2O hierarchical spheres precursor in nitrogen, accompanying with the phase transfer process, the growth of TiO2crystallites is restricted due to the generation of residual carbon species, resulting in a well maintained hierarchical spherical structure. Rich oxygen vacancies are generated in the oxygen‐deficient environment and evidenced by EPR, XPS, and UV‐Vis spectra, which enable the TiO2hierarchical spheres reduced bandgap. The oxygen vacancies in the as‐obtained TiO2hierarchical spheres together with the high structural integrity of the hierarchical spheres gives rise to superior lithium storage properties including a high specific capacity of 282 mAh g−1at 200 mA g−1, and long‐term cycling stability with a capacity retention of 85.2 % at 4 A g−1over 10000 cycles.

     
    more » « less
  5. One key objective in electrocatalysis is to design selective catalysts, particularly in cases where the desired products require thermodynamically unfavorable pathways. Electrochemical synthesis of hydrogen peroxide (H 2 O 2 ) via the two-electron water oxidation reaction (2e − WOR) requires a +0.54 V higher potential than four-electron O 2 evolution. So far, best-performing electrocatalysts require considerable overpotentials before reaching peak faradaic efficiency. We present Mn-alloyed TiO 2 coatings prepared by atomic layer deposition (ALD) and annealing as a stable and selective electrocatalyst for 2e − WOR. Faradaic efficiency of >90% at < 150 mV overpotentials was achieved for H 2 O 2 production, accumulating 2.97 mM H 2 O 2 after 8 hours. Nanoscale mixing of Mn 2 O 3 and TiO 2 resulted in a partially filled, highly conductive Mn 3+ intermediate band (IB) within the TiO 2 mid-gap to transport charge across the (Ti,Mn)O x coating. This IB energetically matched that of H 2 O 2 -producing surface intermediates, turning a wide bandgap oxide into a selective electrocatalyst capable of operating in the dark. However, the high selectivity is limited to the low overpotential regime, which limits the system to low current densities and requires further research into increasing turn-over frequency per active site. 
    more » « less