Thomas Maurer, Julien Proust, Jérémie Béal, Silvère Schuermans, Eddy Wijaya, Jean-Pierre Vilcot, Jérôme Plain, Rabah Boukherroub, Pierre-Michel Adam, Sabine Szunerits, Gaëtan Lévêque, Abdellatif Akjouj, Bahram Djafari-Rouhani,
Enhanced gold film-coupled graphene-based plasmonic nanosensor
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Motivated by previous work showing that the LSPR behavior of metallic nanostructures on the top of a thin metal thin film is exquisitely sensitive to the spacer distance of the film-nanoparticles [1], we seek to investigate in the present work the influence of a few-layered graphene spacer film. As mentioned in most introductions of reports about graphene, the great deal of efforts around graphene comes from its “unique” electronic properties. However, as underlined by A. K. Geim, graphene cannot be reduced only to its “unique electronic properties” and it should be reminded that it is the only one-atom-thick materials [2]. Moreover, graphene has been theoretically considered as an alternative coating for gold based SPR [3]. Here, we take advantage of graphene’s one-atom thickness in order to use it as the thinnest possible spacer between a gold film and gold nanoparticles. We investigate graphene-based SPR interface decorated with an ordered array of gold nanostructures. A commercial thermal release tape, which enables the mechanical transfer of a surface of 1 mm2 graphene sheet originally grown by chemical vapor deposition (CVD) on nickel-coated silicon substrate onto classical SPR interface (glass/5 nm Ti/ 50 nm Au). The graphene-modified SPR interface was in the following decorated with an ordered array of gold nanoparticles via electron beam lithography (EBL). On top of a chromium adhesion layer (d= 3 nm), gold nanoparticles of 50 nm in height, with a centre-to-centre distance of 300 nm and varying particle diameter (80 nm, 110 nm and 150 nm) were formed by EBL. We have shown [4] that the deposition of a few-layers of graphene between a gold thin film of 50 nm and an ordered array of gold nanoparticles displays two clear resonances: the planar LSPR mode of the gold nanostructures at λ2 ≈ 770 nm and in addition a low wavelength LSPR mode at λ 1 ≈ 510 nm (see Figure 1 left). This low wavelength mode is excited by the high energy branch of the gold film surface plasmon polaritrons, which is itself excited by the gold nanoparticle gratings. The importance in this low wavelength LSPR mode is linked to its low FMHW of 38 nm, which results in a Figure of Merit (FoM) of 3.8 at wavelength of 510 nm (see Figure 1 right). Fig. 1: (A) Extinction spectra of the graphene-modified SPR surface decorated with a gold nanoparticles array of particles with 150 nm in diameter for different refractive index n obtained by deposited drops of glycerol/water mixtures: 1.00 (black), 1.33 (grey), 1.37 (blue), 1.40 (magenta), 1.44 (red) 1.47 (green); (B) Shift of the low wavelength LSPR peak depending on the refractive index of the surrounding medium. This FoM ranks this interface among the highly sensitive LSPR sensors with plasmon band in the visible at 500 nm. Indeed, most of the interfaces with high FoM (4-16.5) take advantage of the fact that higher sensitivities are achieved with plasmon bands in the near-infrared of the spectrum (850-1200 nm) [5,6]. Finally, this study provides a further understanding of systems based on arrays of resonant metallic nanoparticles coupled to metallic films. From a practical point of view, it opens the way to the engineering in a controlled and predictable way the spectral properties of metallic nanoparticles-based systems to reinforce their applicability especially in for sensing applications. References: [1] J. J. Mock et al., Nano Lett., 8 (2008) 8, 2245-2252 [2] A. K. Geim, Science 324 (2009) 5934, 1530-1534 [3] L. Wu et al., Opt. Expr. 18 (2010) 14395 [4] T. Maurer et a., submitted [5] K. M. Mayer, J. H. Hafner, Chem. Rev. 111 (2011) 3828 [6] K. Lodewijkset al., Nano Lett. 12 (2010) 1655