Tunable optical and plasmonic response of Au nanoparticles embedded in Ta-doped ${\mathrm{TiO}}_2$ transparent conducting films

Localized surface plasmon resonances (LSPRs) are fascinating optical phenomena occurring in metal nanostructures, like gold nanoparticles (NPs). Plasmonic excitation can be tailored efficiently in the visible range by acting on size, shape, and NP surroundings, whereas carrier density is fixed, thus restricting the LSPR modulation. Transparent conductive oxides (TCOs), on the other hand, are gaining increasing interest for their transparency, charge carrier tunability, and plasmonic features in the infrared. The combination of these two materials into a metal-TCO nanocomposite can give access to unique electrical and optical characteristics, to be tailored in view of the desired optoelectronic application. In this paper, Au NPs and Ta-doped ${\mathrm{TiO}}_2$ TCO films have been merged with the aim to master the Au plasmon resonance by acting on the dielectric properties of the surrounding TCO. Morphology, structure, and electrical properties have been investigated as well to understand the optical response of the nanosystems. The role of the embedding geometry has been explored, revealing that the largest LSPR shift (550–760 nm) occurs when the NPs are sandwiched in the middle of the film and not at the bottom of the film (substrate-film interface). Ta doping in the TCO has been varied (5–10% at. and bare ${\mathrm{TiO}}_2$) to induce a permittivity change of the matrix. As a result, Au LSPR is clearly blueshifted when decreasing the dielectric permittivity at higher Ta content in the sandwich configuration. Despite the nonoptimal electrical performance caused by defectivity of the films, Au-$\mathrm{Ta}:\mathrm{Ti}{\mathrm{O}}_2$ multifunctional nanocomposites are promising candidates for their optical behavior as highly tunable plasmonic conductive metamaterials for advanced light management.

Figure 1

(a) Scanning electron microscopy (SEM) image (top view) of Au nanoparticles (NPs) deposited on Si. (b) Mean diameter and coverage for different underlayer substrates (with associated SEM images) extracted from statistical distributions. Cross-section SEM images (and schematical representations) of final (c) Au-bottom and (d) Au-sandwich configurations where Au NPs are embedded in $\mathrm{Ta}\left(5\%\right):\mathrm{Ti}{\mathrm{O}}_2$. The inset in (c) displays $\mathrm{Ta}\left(5\%\right):\mathrm{Ti}{\mathrm{O}}_2$ compact film for reference.

Figure 2

Resistivity ρ of Au-bottom and Au-sandwich configurations with Ta 5% (orange) and Ta 10% (blue) transparent conductive oxide (TCO) matrix compared with the reference $\mathrm{Ta}:\mathrm{Ti}{\mathrm{O}}_2$ film of same thickness without gold.

Figure 3

Direct transmittance spectra measured on each synthesis procedure step for an Au transparent conductive oxide (TCO) Ta 5% sandwich: as deposited (as dep) 35-nm-thick TCO layer (red); 35-nm-thick TCO with Au nanoparticles (NPs) on top after vacuum annealing (green); addition on top of the second 35-nm-thick TCO layer as deposited (blue); final vacuum annealed sandwich (light blue). Transmittance of Au NPs on glass (after air annealing) is shown as reference (black, dashed line).

Figure 4

Total transmittance spectra of (a) Au-bottom and (b) Au-sandwich configurations as a function of different Ta doping (5%, 10%, and bare ${\mathrm{TiO}}_2$). Transmittance of Au NPs on glass substrate are shown for reference (black, dashed line).

Figure 5

(a) Real (left, ${\varepsilon }_1$) and imaginary (right, ${\varepsilon }_2$) components of the dielectric functions of Au-free ${\mathrm{TiO}}_2$, transparent conductive oxide (TCO; 5%, 10% Ta) thin films extracted from ellipsometry. The insets show the expanded view of curves within 250–350 nm (${\varepsilon }_1$ and ${\varepsilon }_2$). (b) Real (left, ${\varepsilon }_1$) and imaginary (right, ${\varepsilon }_2$) components of the effective dielectric functions of Au-NP layer modeled in a bottom configuration with ${\mathrm{TiO}}_2$, TCOs (5%, 10% Ta) obtained by total transmittance fitting. (c) Real (left, ${\varepsilon }_1$) and imaginary (right, ${\varepsilon }_2$) components of the effective dielectric functions of Au NP layer modeled in a sandwich configuration with ${\mathrm{TiO}}_2$, TCOs (5%, 10% Ta) obtained by transmittance fitting. Effective optical constants of Au NPs deposited on glass are shown for reference (black, dashed line).

Figure 6

Plasmon resonance of Au nanoparticles (NPs; ${\lambda }_{\mathrm{LSPR}}$) taken from ${\varepsilon }_2$ in Figs. 5 and 5 as a function of the ${\varepsilon }_1$ of the surrounding medium [transparent conductive oxide (TCO) Ta 10%, TCO Ta 5%, ${\mathrm{TiO}}_2$, extracted at the localized surface plasmon resonance (LSPR) wavelength] embedded in two configurations (Au-bottom, black curve, and Au-sandwich, red curve) compared with the expected dependence based on the Frölich condition (blue curve).