Gold Nanocylinders on Gold Film as a Multi-spectral SERS Substrate

Abstract

The surface enhanced Raman scattering (SERS) efficiency of gold nanocylinders deposited on gold thin film is studied. Exploiting the specific plasmonic properties of such substrates, we determine the influence of the nanocylinder diameter and the film thickness on the SERS signal at three different excitation wavelengths (532, 638 and 785 nm). We demonstrate that the highest signal is reached for the highest diameter of 250 nm due to coupling between the nanocylinders and for the lowest thickness (20 nm) as the excited plasmon is created at the interface between the gold and glass substrate. Moreover, even if we show that the highest SERS efficiency is obtained for an excitation wavelength of 638 nm, a large SERS signal can be obtained at all excitation wavelengths and on a wide spectral range. We demonstrate that it can be related with the nature of the plasmon (propagative plasmon excited through the nanocylinder grating) and with its angular dependence (tuning of the plasmon position with the excitation angle). Such an effect allows the excitation of plasmon on nearly the whole visible range, and paves the way to multispectral SERS substrates.

Keywords: SERS; multi-spectral substrate; optimization; plasmon.

Figure 1

(a) Scheme of gold nanocylinders deposited on a gold thin film (D: nanocylinder diameter, h: nanocylinder height, P: grating period, d: film thickness); (b) SEM image of nanocylinders with a 250 nm diameter on a gold film (scale bar: 2 μm).

Figure 2

(a) Extinction spectra for nanocylinders with a diameter of 250 nm and for different gold film thicknesses (black spectrum: 20 nm, red spectrum: 30 nm, blue spectrum: 40 nm, green spectrum: 50 nm); (b) surface enhanced Raman scattering (SERS) spectra of MBA on gold nanocylinders with a diameter of 250 nm and a film thickness of 20 nm. Excitation wavelength: 638 nm.

Figure 3

Evolution of the SERS intensities of the band located at 1580 cm⁻¹ depending on the nanocylinder diameter and for the four film thicknesses: (a) 20, (b) 30, (c) 40, and (d) 50 nm. Excitation wavelength: 638 nm. Points size includes the error bars.

Figure 4

Evolution of the SERS intensities of the band located at 1580 cm⁻¹ depending on the nanocylinder diameter and for the four film thicknesses: (a) 20, (b) 30, (c) 40, and (d) 50 nm. Excitation wavelength: 785 nm. Points size includes the error bars.

Figure 5

Evolution of the SERS intensities of the band located at 1580 cm⁻¹ depending on the nanocylinder diameter and for the four film thicknesses: (a) 20, (b) 30, (c) 40, and (d) 50 nm. Excitation wavelength: 532 nm. Points size includes the error bars.

Figure 6

Evolution of the SERS intensities of the band located at 1580 cm⁻¹ depending on the nanocylinder diameter and for the four film thicknesses: (a) 20, (b) 30, (c) 40, and (d) 50 nm. The SERS intensities have been normalized by the silicon Raman signal. Green triangles: excitation wavelength of 532 nm; Red squares: excitation wavelength of 638 nm; Brown circles: excitation wavelength of 785 nm.

Figure 7

(a) Extinction spectra for our plasmonic substrate (nanocylinder diameter: 140 nm and thickness: 30 nm) depending on the incident angle (from 0 to 50°, step of 2.5°). (b) Evolution of the position of the different plasmon bands depending on the incident angle (each symbol corresponds to the position of one band on the spectra shown in (a) and each color shows the evolution of one plasmon mode). The green, red and brown rectangles correspond to the spectral ranges between the excitation wavelength and the 1580 cm⁻¹ Raman band (532–581, 638–710, and 785–896 nm respectively).