The moveable "hot spots" effect in an Au nanoparticles-Au plate coupled system

Surface-enhanced Raman spectroscopy (SERS) is mainly contributed by "hot spots". Due to the huge electromagnetic enhancement, "hot spots" have wide applications in surface analysis and surface catalysis. The in-depth research on the "hot spots" effect is conducive to understanding SERS enhancement mechanisms and designing substrates with high enhancement. At present, the investigation on the "hot spots" effect is mainly based on theoretical simulation and simple experimental models. However, little attention has been paid to the SERS substrates with practical applications. The main reason is that it is difficult to construct a suitable coupled model with great uniformity and sensitivity, which led to the lack of comparability of SERS intensities from different spots or substrates. In this work, Au nanoparticle mono-/bi-layer films coupled with Au single-crystal plate systems were constructed to investigate the distribution and transformation of "hot spots" dependent on the excitation wavelength by a single or dual probe-modified strategy, in which one or two types of molecules with distinct characteristic peaks were modified in different enhanced gaps. The results demonstrated that the wavelength that drove the transformation of the coupling mode from the "particle-particle" mode to the "particle-surface" mode was around 638 nm in the Au nanoparticle monolayer film (Au MLF) covered Au plate system. As the second naked Au MLF was transferred onto the first Au MLF, "hot spots" were transferred to the "particle-particle" gap between the upper and lower Au MLFs with a 638 nm laser as the excitation line. This work offers a novel avenue to investigate the "hot spots" effect in the complex multidimensional nanostructures, which is beneficial for the development of theoretical research and practical applications of SERS.