Hybrid Au/Si Disk-Shaped Nanoresonators on Gold Film for Amplified SERS Chemical Sensing

Abstract

We present here the amplification of the surface-enhanced Raman scattering (SERS) signal of nanodisks on a gold film for SERS sensing of small molecules (thiophenol) with an excellent sensitivity. The enhancement is achieved by adding a silicon underlayer for the composition of the nanodisks. We experimentally investigated the sensitivity of the suggested Au/Si disk-shaped nanoresonators for chemical sensing by SERS. We achieved values of enhancement factors of 5 × 10 7 - 6 × 10 7 for thiophenol sensing. Moreover, we remarked that the enhancement factor (EF) values reached experimentally behave qualitatively as those evaluated with the E 4 model.

Keywords: SERS; gold; plasmonics; sensors; silicon.

Figure 1

Scheme of the Au/Si disk-shaped nanoresonator array on gold film. D and P correspond to the nanodisk (ND) diameter and the period between the nanodisks, respectively. P is identical along x-axis and y-axis. In the red zoom are indicated the thicknesses of Si and Au layers constituting the bilayer of a hybrid nanodisk. An adhesion layer of Ti (2 nm) is used between Si substrate and gold film.

Figure 2

Results of analytical model (red) and COMSOL computer simulations (blue) of the system shown in Figure 1, disk diameter $D=2a=130$ nm, period of square lattice $P=300$ nm, thicknesses of the upper gold plate and silicon interlayer are 20 nm, gold film has thickness of 40 nm. (a) Reflectance from surface-enhanced Raman scattering (SERS) substrates and (b) electric field enhancement factor (EF) averaged over the lateral side of the plasmon resonator EF = $\langle |E/E_0{|}^4\rangle$, where $E_0$ corresponds to the amplitude of the incident EM wave.

Figure 3

SEM picture of a hybrid Au/Si nanodisk array on gold film (scale bar = 300 nm). The nanodisk dimensions are 130 nm of diameter, 40 nm of total height, and 300 nm of periodicity.

Figure 4

(a) SERS spectra of thiophenol realized on 3 distinct SERS substrates among 10 for the excitation wavelength of 785 nm. In purple is represented the SERS spectrum of thiophenol (1 mM) obtained on a 40-nm gold film at the same excitation wavelength (an offset is applied to the purple spectrum to see all the SERS spectra). (b) Electric field mapping $|E/E_0|$ of a hybrid Au/Si nanodisk on gold film for an excitation wavelength of 785 nm (cross-sectional view). White circles correspond to the strong electric field zones accessible for thiophenol molecules.

Figure 5

Calculated extinction spectrum of the hybrid Au/Si disk-shaped nanoresonators. The red line matches to the excitation wavelength of 785 nm. The full red rectangle represents all the Raman wavelengths (${\lambda }_{Raman}$) corresponding to the associated Raman shifts (from 1000 to 1575 cm${}^{-1}$, see Table 1).