ANÁLISIS COMPARATIVO DE CAPACIDAD DE PROCESO LUEGO DE LA IMPLEMENTACIÓN DE LAS MEJORAS

Our ability to detect plasmon resonances of nanoparticles with ellipsometry is an incredibly exciting development, which opens up new opportunities for research. Primarily, it allows us to quickly and accurately characterize the anisotropy and in-plan ordering of AuNRs during the fabrication process. In the short term, film thickness dependence studies and polymer swelling studies could give insight into what drives orientation in nanocomposites, and what might be done to selectively orient the nanorods. Furthermore, annealing studies could give us insight into a measurement of thin film viscosity. As discussed previously, nanocomposites become more isotropic during short annealing times as the AuNRs can reorient once the polymer goes through the glass transition. If the rate of reorientation can

be accurately measured, than that rate should be related to the viscosity of the film.

Figure 63: A typical plot ofψ(λ) (red) andδ(λ) (green) for the dewetted gold nanoparticles used in this study

Additionally, our ability to analyze the optical properties of nanoparticles in situ opens up this method to characterize nanoparticle based processes such as molecular sensing or chemical reactions. Preliminary work in collaboration with the Borguet group out of Temple University has sought to use ellipsometry to test the efficacy of gold nanorods as hydrogen sensors. In these experiments, a gold film is sputtered onto a glass substrate, and then annealed such that the film has completely dewetted into nanoparticles. This film is then placed on the ellipsometer in a sealed chamber connected to a source of nitrogen and hydrogen gases. An example of the raw ψ(λ) and δ(λ) can be found in figure 7.2. The model used to fit this data was similar to the aforementioned nanocomposite procedure. The resulting real (0(λ)) and imaginary (”(λ)) can be seen in figure 64. The sample was exposed to an initial flow of nitrogen for 30 minutes to attain a baseline for the strength of the imaginary part of the dielectric constant (”). The sample was then exposed to hydrogen, purged with nitrogen, and finally exposed to air.

The strength of the absorption peak of ” is plotted as a function of time in Figure 65. It is clear from figure 65 that the optical properties of the gold nanoparticle undergo a subtle change upon the addition ofH2. Furthermore, whenH2 is removed via the removal of the

Figure 64: ”(λ) before (black) and after (red) exposure to H2

stage lid, the strength of” returns to it’s previous level. The proposed mechanism for this change in the strength of the peak in ” is that the optically excited plasmon resonance from the gold nanoparticle creates a high energy electron, to whichH2 can bind, generating

a layer of AuH. The presence of AuH causes the blue shift in ” upon the addition of

H2. While we are still performing studies to confirm the mechanism, the significance of

this result, again, comes from the technique. Other methods for measuring the optical properties are far less sensitive, and/or extremely difficult to perform. Ellipsometry, on the other hand, is a simple technique that is able to detect a sub monolayer of hydrogen on the surface of gold nanoparticles.

7.3. Conclusion

Throughout this report, I have shown the power of ellipsometry to study everything from polymer dynamics to nanoparticle plasmonics. Through examination of the thickness and cooling rate dependence of Tg we show that the two interfaces in supported polymer films

can influence the overall dynamics of a film over much longer length scales than those typ- ically associated with cooperative motion. Furthermore, we showed that the gradient of

Figure 65: ” intensity as a function of time as a film of dewetted gold nanoparticles are exposed to a first control of N2 (black), H2 (red), a N2 purge (green), and finally upon

exposure to air (blue)

dynamics separates as the interfacial dynamics diverge, generating, in some cases, the pres- ence of two distinct Tgs within a single thin polymer film. Furthermore, using nanoparticle

plasmonics as a probe to study substrate effects, or to attain a measure of thin film viscosity via nanorod reorientation, would allow us to answer many outstanding questions about the length scales of the glass transition.

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