Abstract
Surface-enhanced Raman spectroscopy (SERS) has attracted increasing interest for chemical and biochemical sensing. Several studies have shown that SERS intensities are significantly increased when an optical interference substrate composed of a dielectric spacer and a reflector is used as a supporting substrate. However, the origin of this additional enhancement has not been systematically studied. In this paper, high sensitivity SERS substrates composed of self-assembled core-satellite nanostructures and silica-coated silicon interference layers have been developed. Their SERS enhancement is shown to be a function of the thickness of silica spacer on a more reflective silicon substrate. Finite difference time domain modeling is presented to show that the SERS enhancement is due to a spacer contribution via a sign change of the reflection coefficients at the interfaces. The magnitude of the local-field enhancement is defined by the interference of light reflected from the silica-air and silica-silicon interfaces, which constructively added at the hot spots providing a possibility to maximize intensity in the nanogaps between the self-assembled nanoparticles by changing the thickness of silica layer. The core-satellite assemblies on a 135 nm silica-coated silicon substrate exhibit a SERS activity of approximately 13 times higher than the glass substrate.
Highlights
Surface-enhanced Raman spectroscopy (SERS) is a sensitive analytical tool that is capable of both detecting and identifying chemical and biological compounds at low concentrations[1,2,3,4]
Beyond the tailoring of plasmonic antennas themselves, there is a search for the change of their external environments that can further increase the amount of the field enhancement
We have demonstrated the phase control for SERS enhancement using a combinatory system of self-assembled AuNP core-satellite nanostructures and an optical interference substrate as a SERS substrate
Summary
Surface-enhanced Raman spectroscopy (SERS) is a sensitive analytical tool that is capable of both detecting and identifying chemical and biological compounds at low concentrations[1,2,3,4]. The excitation through the dielectric material (a “back-side” SERS) generates a larger SERS signal This is due to the enhancement of the electric field accounted for by the transmission and reflection coefficients and their sign change which are dependent on the propagation direction of light: from low-to-high refractive index or vice versa[16]. This is achieved by combining strong near-field-coupled core-satellite assemblies with an optical interference substrate (i.e., a silica layer on the top of a silicon substrate) that allows the modulation of the local field enhancement at the hotspot In this geometry, the hot spots have contribution of the “back side” excitation occurring for the reflected light at the inner silicon-silica interface[16]. SERS emitted light into a large solid angle from the near field patterns was collected using a high-NA optical detection system and the collection efficiency could be further increased following the relation between solid angle and NA, which is close to linear[22]
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