Abstract
Optical waveguide lightmode spectroscopy (OWLS) is usually applied as a biosensor system to the sorption-desorption of proteins to waveguide surfaces. Here, we show that OWLS can be used to monitor the quality of oxide thin film materials and of coatings of pulsed laser deposition synthesized CdSe quantum dots (QDs) intended for solar energy applications. In addition to changes in data treatment and experimental procedure, oxide- or QD-coated waveguide sensors must be synthesized. We synthesized zinc stannate (Zn2SnO4) coated (Si,Ti)O2 waveguide sensors, and used OWLS to monitor the relative mass of the film over time. Films lost mass over time, though at different rates due to variation in fluid flow and its physical effect on removal of film material. The Pulsed Laser Deposition (PLD) technique was used to deposit CdSe QD coatings on waveguides. Sensors exposed to pH 2 solution lost mass over time in an expected, roughly exponential manner. Sensors at pH 10, in contrast, were stable over time. Results were confirmed with atomic force microscopy imaging. Limiting factors in the use of OWLS in this manner include limitations on the annealing temperature that maybe used to synthesize the oxide film, and limitations on the thickness of the film to be studied. Nevertheless, the technique overcomes a number of difficulties in monitoring the quality of thin films in-situ in liquid environments.
Highlights
With the development of thin film and nanoparticle synthesis techniques, the ability to characterize and monitor such materials over time has increased in importance
We show that Optical waveguide lightmode spectroscopy (OWLS) can be used to monitor the stability of thin film materials and of quantum dot layers over time
Remembering that a stable baseline has no effect upon the relative mass change measurements, on the zero point, we used a post-experiment baseline for which we know the end result
Summary
With the development of thin film and nanoparticle synthesis techniques, the ability to characterize and monitor such materials over time has increased in importance. In dye-sensitized solar cells or quantum dot sensitized solar cells [1,2,3], the substrate material (often an oxide semiconductor) or the quantum dots (QDs) can be subjected to harsh liquid environments. There are important questions about the long-term stability of substrate and QD materials in solutions that may reach 80 °C or more in field deployment [4]. This is especially true in the case of multicomponent oxides because one component may react or leach out of the material faster than the other, creating an altered residual solid lacking the optimal photoelectrochemical characteristics of the original material [5,6,7]. Sensor systems that allow monitoring of thin film and nanoparticulate materials in liquid environments are needed for the experimental assessment of long-term chemical stability of such solar cell materials
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