Optical gas sensing responses in transparent conducting oxides with large free carrier density

Abstract

Inherent advantages of optical-based sensing devices motivate a need for materials with useful optical responses that can be utilized as thin film functional sensor layers. Transparent conducting metal oxides with large electrical conductivities as typified by Al-doped ZnO (AZO) display attractive properties for high temperature optical gas sensing through strong optical transduction of responses conventionally monitored through changes in measured electrical resistivity. An enhanced optical sensing response in the near-infrared and ultraviolet/visible wavelength ranges is demonstrated experimentally and linked with characteristic modifications to the dielectric constant due to a relatively high concentration of free charge carriers. The impact of light scattering on the magnitude and wavelength dependence of the sensing response is also discussed highlighting the potential for tuning the optical sensing response by controlling the surface roughness of a continuous film or the average particle size of a nanoparticle-based film. The physics underpinning the optical sensing response for AZO films on planar substrates yields significant insight into the measured sensing response for optical fiber-based evanescent wave absorption spectroscopy sensors employing an AZO sensing layer. The physics of optical gas sensing discussed here provides a pathway towards development of sensing materials for extreme temperature optical gas sensing applications. As one example, preliminary results are presented for a Nb-doped TiO2 film with sufficient stability and relatively large sensing responses at sensing temperatures greater than 500 °C.

Optical and Chemi-Resistive Sensing in Extreme Environments: La-Doped SrTiO3 Films for Hydrogen Sensing at High Temperatures

“…The errors between simulated spectra and measured spectra were minimized by using a standard Levenberg−Marquardt algorithm with adjustable parameters including σ ω=0 , τ, d, and a number of additional parameters that are associated with the interband electronic transitions modeled using a single Tauc−Lorentz oscillator. 22,23,30,31 The empirically derived optical constants of the La-doped STO film obtained in this way, n and k, are presented in Figure 1a along with results of the experimental trans- mittance spectra measurements and the theoretically modeled spectrum showing excellent agreement in Figure 1b.…”

“…22,23 The near-IR sensing response for the films in these experiments is expected to follow a similar mechanism. Many studies have focused on explaining the defect structure of SrTiO 3 and La-doped SrTiO 3 depending on processing conditions, temperature, doping composition, and gas atmosphere.…”

High temperature fiber-optic evanescent wave hydrogen sensors using La-doped SrTiO3 for SOFC applications

“…[8,9] La acts as an Asite donor impurity at high temperature under reducing conditions, increasing the free carrier concentration. [8,[10][11][12] The increase in free carrier concentration has been shown to result in a nearinfrared (NIR) optical response in planar Ladoped SrTiO 3 films, [7] similar to previously studied Aldoped ZnO [5,6] and Nbdoped TiO 2 films. [5] …”

“…[2,3] More recently, several classes of opticalfiber sensors and associated functional sensor materials have been demonstrated in which the sensing layer showed a sufficiently large response that eliminates the need for fiberBragg grating based interrogation approaches previously utilized and which allows for potentially improved temperature stability of the device platform. [4][5][6][7] However, even for these previous works an instability of the selected functional sensor layer [5,6] and/or the silicabased optical fiber transmission in the visible range was observed, spurring additional research to demonstrate a device that is potentially suitable for H 2 sensing in SOFC and other power generation applications.…”

High temperature, harsh environment sensors for advanced power generation systems

“…The responses could be successfully modeled based upon standard optical constant models for transparent conducting oxides in both planar film and optical fiber sensing tests, with the near-infrared response directly linked with the electrical conductivity of the films and the band-edge response attributed to the well-known Burstein-Moss effect [39,[42][43]. A particularly interesting observation for this class of materials was the ability to tailor the magnitude and even the sign of the optical response through roughening of the film surface to impart a significant degree of attenuation due to light scattering rather than absorption [42]. However, potential instabilities of materials systems noted in the previous section would be expected to limit their relevance for the most aggressive high temperature conditions of interest.…”