Hybrid structure with VO 2 layer

The first structural configuration under consideration is that of the hybrid structure examined in Chapter 5, with a VO2 layer of 30 nm included above the polymer layer. This presumes a fabrication procedure following those in Chapter 5, followed by the deposition of a 30 nm thick VO2 layer. The result is a structure with a partially imbedded Au disc. For simplicity of design, VO2 is not present at the top of the Au discs, achievable by the deposition of an intermediate lift-off material before deposition of the VO2. Figure 7.2 demonstrates the schematic of this configuration.

Figure 7.2: (a) Schematic profile of the Hybrid nanostructure unit cell on Si substrate,

with layer thicknesses of 100 nm Ag, Polymer and Au, 30 nm VO2 layer. (b) Top-down

schematic of the structure unit cell, with 150 nm diameter Au disc, and 300 nm pitch.

Similar to the polymer in Chapter 5, an “irradiated” polymer refractive index is used

below the Au particle with the same diameter, to represent exposure to the layer as a result of the Electron Beam Lithography (EBL) process. A non-irradiated polymer

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refractive index is used elsewhere. The reflectance spectra of this structure in the low-temperature monoclinic semiconducting phase, and high-temperature rutile metallic phase of VO2 are shown in Figure 7.3.

Figure 7.3: Reflectance spectra of the structure with VO2 in the semiconducting and

metallic phase. The line for each spectrum is shown in the sRGB value colour.

Figure 7.3 demonstrates a negligible change in the reflectance upon phase transition below 550 nm. The peak noted at approximately 580 nm in the semiconducting phase notably blue-shifts to 565 nm, due to the decrease in the real, n component of the refractive index in the metallic phase. The peak also reduces in intensity, consistent with the increase in the imaginary, κ component of the refractive index. The dip at 650 nm also blue-shifts to 620 nm on phase change, due to the decrease in the n component. This is a trend noted throughout the chapter, upon phase change from the semiconducting to metallic phase of VO2. These changes in spectral shape are further demonstrated in Figure 7.4 (a).

The changes in spectral shape result in an overall reduction in the stimulation of the middle colour cone, responsible for green colours, and so the colour shifts towards

157 are more blue hue (the change of colour from one discernible tone to another) in the metallic phase.

Figure 7.4: (a) ΔR (=RS-RM) spectral dependence, where RS is the reflectance for the

semiconducting phase and RM is the reflectance for the metallic phase. (b) CIE colour-

map positions of the colour presented by the structure with VO2 transitioning from

semiconductor (black dot) to metallic (white dot) phase.

Figure 7.3 (b) illustrates the CIE colour-map positions of this structure as the VO2 changes phase. It is clear from this that the change in colour hue is small, particularly when compared to that presented by the thin film structure in Figure 6.17 (b), at smaller thickness of ITO. The positions in the CIE in Figure 6.18 (b), and change in position at phase change in this structure is consistent with that of positions noted in Figure 6.17 (b) for the thickest layers of ITO. This indicates that this structural configuration does not present any advantages in performance over that of the thin film design presented in Chapter 6.

Figure 7.5 reveals the (a,b) Electric field and (c,d) Poynting vector profiles for the structure under consideration. These profiles are taken at (a,c) 580 nm for the semiconducting phase of VO2, the peak position of reflectance mentioned earlier for this phase. The profiles (b,d) are measured at 565 nm, and are for the metallic phase of VO2, the peak position of this structure in this phase.

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Figure 7.5: (a,b) Electric field, and (c,d) Poynting vector profiles of the structure at (a,c)

580 nm for the semiconducting phase, and (b,d) 565 nm for the metallic phase of VO2.

Figure 7.5 (a,b) shows the electric field profiles at 580 nm and 656 nm, which is the peak position for each phase. A large electric field localisation is observed at the top corners of the disc in both, with no discernible changes in the field profiles at the disc-VO2 boundary. The largest field strength at both wavelengths is observed at

the top corners of the disc, with lesser “hotspots” also observed at the top and

bottom corners of the VO2 layer that contact the disc. As the largest E fields are observed a considerable distance from the Au-VO2 interfaces, the interaction between these components is relatively small, and so the LSPR of the disc is not significantly altered by the phase change of the VO2. The change in spectral shape is largely attributed to the VO2 thin film, and not the disc interaction with the film. In the Poynting vector profiles in Figure 7.5 (c,d), there largest magnitudes are also observed outside of the VO2 layer at the top disc corners, further demonstrating weak disc-VO2 interaction.

159 Figure 7.6 reveals the E and P profiles for the semiconducting and metallic phases at the same wavelength of 580 nm. From this, the impact of the phase change on the disc LSPR, and energy flux in the structure can be examined.

Figure 7.6: (a,b) Electric field, and (c,d) Poynting vector profiles of the structure at 580

nm for the (a,c) semiconducting, and (b,d) metallic phase of VO2.

Again, the largest E field magnitudes in the structure in both phases of VO2 are

located at the top corners of the disc in Figure 7.6 (a,b). However, there is a notable increase in the E field at the bottom corners of the disc, and along the wall that interfaces with the VO2 layer. Poynting vector magnitudes in these locations are also increased, indicating that this region has an increase in energy flux as a result of the increased absorption of the metallic phase of VO2. There is also an increase in P at the lower disc surface, indicating an increase in energy flow to this region of the structure, which may also cause a reduction in reflectance at this wavelength. There are several design parameters that can be improved to maximise the interaction between the disc and VO2, and subsequently increase the colour change as the VO2 switches phase. By reducing the dielectric layer thickness, the CIE

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positions noted in Figure 7.3 (b) may trend towards larger separation, as seen in Figure 6.17 (b) with thinner ITO layers in the thin film structure. Changing the shape of the Au particle may also serve to increase field enhancement presented by the particles to have a stronger interaction with the VO2, and improve colour change potential. Finally, imbedding the Au disc array into the VO2 layer may place the areas of high field enhancement in the layer, increasing the interaction between the particles and the VO2. The following sections investigates these parameter and geometry changes, first investigating thinner layers of dielectric below the VO2 layer, followed by an examination of Au nanoparticle shape. The incorporation of the nanoparticle into the VO2 layer is also explored, for both discs and nanoparticles of varying shapes.