Fig. 1. Design of thermo-responsive plasmonic system with VOC2O4 films. (a) Scheme of Au NPoM with VOC2O4 spacer and overcoated TiO2 film. The white dashed line indicates another layer of VOC2O4 coating. (b) Charge distribution profile of the Au NPoM with VOC2O4 spacer (14 nm). (c) Scattering spectrum of an Au NPoM with 14 nm VOC2O4 film in the gap. Inset is the dark field image of the Au NPoM.

Fig. 2. Synthesis of VOC2O4 precursors and physicochemical analysis. (a) Production kinetics of VOC2O4 monitored by UV-vis extinction spectra. (b) Change of the extinction intensity with reaction time. Inset is the picture of the VOC2O4 sol-gel after 10 h reaction. (c) XPS profiles of VOC2O4 films spin-coated on Si substrate. (d) XRD patterns of VOC2O4·xH2O powders at 20°C and 80°C. (e) Change of the conductivity of VOC2O4 films with one temperature cycle between 15°C and 80˚C. Inset is the electrode setup for the electric measurement. (f) Change of the conductivity of VOC2O4 films within the temperature range of 40°C and 80°C.

Fig. 3. Temperature-responsive Au NPoM with VOC2O4/TiO2 composite films. (a) 3D AFM profile of the Au NPoM with VOC2O4 medium overcoated with TiO2 films. (b) SEM image of the Au NPoMs in TiO2 film. (c) Change of scattering spectra of Au NPoM with temperature, and (d) the corresponding change of the plasmon resonances with one cycle of heating and cooling. (e) Reversible shift of plasmon resonances over several cycles of heating and cooling between 15°C and 50°C, and (f) the corresponding change of the resonance wavelength.

Fig. 4. Comparison of the RI results measured from the TGA and SPR methods. (a) Change of weight ratio with temperature measured via TGA. (b) Change of surface relative humidity (RH) with temperature measured via a humidimeter. (c) Simulated change of SPR with RI. (d) Comparison of the RI measured from the TGA and SPR approaches.

Fig. 5. Laser-directed tuning of the plasmon resonances of Au NPoM coated with VOC2O4/TiO2 composite films. (a) Change of scattering spectra of Au NPoM with laser on and off (3 mW). (b) Change of the light scattering signal (integrated over 550–650 nm) in response to the square wave modulation of the CW laser. (c) Response-time of light-induced plasmon switching. (d) Scattering spectra of Au NPoM at different irradiation powers. (e) Change of the resonance wavelength with laser power.

Fig. 6. Characterizations of VOC2O4 films. (a) EDS spectrum of VOC2O4 films on Si substrate. (b) Raman spectra of VOC2O4 powder and VOC2O4/TiO2 films. (c) Ellipsometry spectra of VOC2O4 films spin-coated on Au substrate. Insets are AFM images of Au films with and without VOC2O4 overcoating. (d) Change of VOC2O4 film thickness with temperature.

Fig. 7. Change of scattering spectra of Au NPs coated with VOC2O4/TiO2 composite films on Si substrate.

Fig. 8. Thermal switching performance of Au NPoM with VOC2O4/TiO2 composite films measured after 10 days. (a) Reversible shift of plasmon resonances over several cycles of heating and cooling between 15°C and 50°C, and (b) the corresponding change of resonance wavelength.

Fig. 9. Theoretical simulations of the change of (a) the SPR wavelength with RI at different gaps and (b) the surface temperature of Au NPoM with irradiation power. Laser wavelength: 532 nm.

Table 1. Comparison of This Work to Other Typical Tunable Nanoplasmonic Systems