Surface-enhanced Raman spectra and higher order scattering in Bi2O3-Ag nanoplasmonic eutectic composite

Generation of high electromagnetic field near metallic nanostructures upon excitation of localized surface plasmon resonance (LSPR) leads to an enhanced signal, i.a. Raman scattering, of adjacent nanosystems. Surface-enhanced... [ view full abstract ]

Generation of high electromagnetic field near metallic nanostructures upon excitation of localized surface plasmon resonance (LSPR) leads to an enhanced signal, i.a. Raman scattering, of adjacent nanosystems. Surface-enhanced Raman scattering (SERS) is a powerful tool in system characterization with sensitivity reaching down to a single molecule. As there are examples of application of Raman scattering in optical devices, e.g. amplifiers/lasers, SERS is a promising technique to be utilized in that field due to its tunability. However, most of the SERS-based studies involve either nanoparticle-modified solid surfaces or molecules attached to metallic nanostructures, which are challenging to incorporate in optical instruments. Therefore, there is an interest in development of novel volumetric SERS-active materials.

In this work, the first example of the SERS effect in a bulk plasmonic material is presented. Bi₂O₃‑Ag nanocomposite is obtained by micro-pulling-down from a eutectic mixture. In the structure, Bi₂O₃ grains with metallic microprecipitates in-between are observed. Raman microscopy confirms that two different phases of Bi₂O₃ occur in the composite: the grains consist of α‑Bi₂O₃, while γ‑Bi₂O₃ is incorporated in the precipitates.

Spectra of γ-Bi₂O₃ from the nanocomposite exhibit different features than for the bulk γ-Bi₂O₃. Not only there is a huge intensity increase of the bands, but also their positions shift (Figure 1). The most prominent change is visible for the band of the symmetric stretching vibrations of BiO₄ tetrahedra (825 cm^-1). Additionally, overtones of the bands appear. Intensity of the 825 cm^‑1 band displays resonant dependence on the excitation wavelength (not seen in bulk), which overlaps with the LSPR peak originating from silver nanoparticles (AgNPs; Figure 2). Control of the SERS effect efficiency in the composite is achieved through preparation conditions.

Acknowlegdements: This project was supported financially by the TEAM programme of the Foundation for Polish Science (No. TEAM/2016-3/29), co-financed by the European Union under the European Regional Development Fund.

Figure 1. a: Raman spectrum, bulk γ-Bi₂O₃, b: SERS spectrum, γ-Bi₂O₃ in Bi₂O₃-Ag. Differences are due to plasmonic AgNPs in the nanocomposite (λ_exc=532 nm).

Figure 2. Extinction spectrum of Bi₂O₃-Ag (blue) and intensities of the band at 825 cm^‑1 (red) exhibiting resonant behaviour.