Optical thin films for high temperature gas sensing in advanced power plant applications Plasmon resonance of TiO 2 / Au at extreme temperatures Presented by Michael P. Buric Paper Authored by Paul Ohodnicki Co-Authors: Thomas Brown, John Baltrus, Benjamin Chorpening

2 Overview Sensors needs for energy production and energy efficiency Au / TiO 2, Au / YSZ, and other Au incorporated metal oxides Experimental results for Au / TiO 2 films at extreme temperatures Modeling of the plasmon resonance absorption peak of Au at extreme temperatures

3 Opportunities in existing coal-based plants

4 Range of ‘harsh-opportunities’ Table of Relevant Harsh Environments in Advanced Fossil Energy Technologies

5 Previous authors: Au nanoparticle incorporated oxides Au / YSZ Au / WO 3 Au / TiO 2 An optical absorption peak associated with the gold nanoparticles shifts in reducing versus oxidizing atmospheres.

6 Our previous work : Sol-gel deposited TiO 2 / Au Operation up to 800C Interesting optical response characteristics P. Ohodnicki et al., Journal of Applied Physics, 111, (2012).

7 Current work: sputter-deposited Au / TiO 2 films Multi-layered sputter deposition of Au and Ti Followed by a 950 o C oxidation step. Increasing Au layer thickness yields larger particles ~15nm Au Layer Thickness = 0nm Au Layer Thickness = 0.4nmAu Layer Thickness = 1nm

8 Optical constants for Au nanoparticles (no scattering) Interband electronic transitions significantly modify the optical constants of Au as compared to the damped free electron theory. Interband Transitions Dominate Free Electrons Dominate  ’’ Au  ’ Au

9 Sputter-deposited Au / TiO 2 film properties Upon exposure to a reducing environment at 700 o C, a shift of both the absorption and scattering peaks attributed to Au are observed Shift of LSPR absorption peak in changing gas atmospheres is well documented Shift in the peak associated with diffuse scattering is a new observation.

10 Sputter-deposited Au / TiO 2 film properties Increasing Au layer-thickness results in an increase in LSPR absorption and a shift to longer wavelengths Diffuse scattering associated with the LSPR resonance of Au shows a similar trend with increasing Au thickness. 0.4nm Au ~ 1nm 0nm Au ~ 1nm 0nm 0.4nm

11 Temperature dependence of optical constants Increased scattering of electrons with increasing temperatures results in an increased damping frequency for free carriers Thermal expansion of Au causes a decrease in free-carrier concentration and plasmon frequency with increasing temperature The thermo-optic coefficient of TiO 2 is assumed to be constant or decreasing with increasing temperature.

12 Custom setup for simulated optical sensing

13 Temperature dependence of sputtered films Evidence for an increased broadening and decreased peak height with increasing temperatures. Extinction spectra were fitted to a Gaussian function for >575nm as done by previous authors.

14 Conclusions TiO 2 / Au LSPR absorption and scattering shift with high temperature exposure to a reducing gas. BUT -The LSPR absorption peak broadens at high temperatures resulting in reduced sensitivity. Results are expected to be general for Au embedded in other metal oxide systems. Future Work More fundamental materials studies Fiber-optic device fabrication with high-temperature core materials Metal oxide matrix design for multi-gas sensing

15 Thank you! Our Recent Publications in this Area: 1)M. Buric et al, Proceedings of SPIE Optics and Photonics )P. Ohodnicki et al, Proceedings of SPIE Optics and Photonics )P. Ohodnicki et al., Journal of Applied Physics, 111, (2012). 4)P. Ohodnicki et al., Thin Solid Films, 520, (2012). 5)P. Ohodnicki et al., Under Review, Journal of Applied Physics, Questions or Comments? Please contact: Paul Ohodnicki or Michael Buric This work was funded by the Cross-Cutting Technologies Program at NETL and managed by Patricia Rawls (project manager) and Robert Romanosky (technology manager). This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

16 Sputter-deposited Au / TiO 2 films Extinction spectra measured using the elevated temperature system deviate from measurements performed with an integrating sphere Modifications to measured transmittance and reflectance spectra are observed with increasing temperatures

17 Previous authors: sensing mechanism in Au / YSZ system Defect chemistry linked with ratios of H 2 to O 2 partial pressure in Au / YSZ. Prior work suggest oxidation/reduction followed by charge transfer is occurring

18 Optical constants of Au nanoparticles (elevated T) The thermo-optic coefficient of TiO 2 strongly affects the shift in LSPR absorption peak Peak broadening is dictated by the increased damping frequency of Au

19 Theoretical modeling of LSPR at high temps A peak in absorption occurs if: Re[  ]~=-2  m Froelich Condition Localized surface plasmon resonance in noble metal nanoparticles is associated with the free electrons Quasi-static approximation : Particles much smaller than 2a2a

20 LSPR absorption peak sensitivity  = Damping coefficient related to the effective resistivity of Au N = Carrier density of free electrons of the Au nanoparticle  m = Dielectric constant of matrix phase

21 LSPR absorption peak sensitivity Peak sensitivity for transmission or absorption based sensing occurs on either side of the LSPR absorption peak maximum Theoretically predicted wavelength dependence is consistent with the literature

22 LSPR absorption peak sensitivity (elevated T) The wavelength of LSPR absorption peak sensitivity shifts with increasing temperature due to broadening and peak shifting The maximum sensitivity decreases with increasing temperature The sensitivity should also depend on the mechanism responsible for the solid-gas interaction