Fig. 4

a The extinction efficiency of Au nanospheres with different diameters, reproduced with permission from [32]. Copyright (2018) IOP Publishing Ltd. b) SERS spectra of pyridine adsorbed on Au nanoparticles with different sizes: 16 to 160 nm (from a–f). c The normalized SERS intensity of pyridine as a function of the Au nanoparticle size, reproduced with permission from [33]. Copyright (2008) John Wiley and Sons, Ltd. d The top view of 3D-FDTD simulation from a 2 × 2 array of 55 nm Au@SiO₂ nanoparticles showing the distribution of electromagnetic field enhancement as a function of shell thickness (the color gradient represents the strength of the electromagnetic field, and the white dash circles indicate the Au@SiO₂ nanoparticles. e SHINERS spectra of pyridine adsorbed on a smooth Au surface coated with Au@SiO₂ with different silica shell thicknesses. f The dependence of the normalized intensity of pyridine on the shell thickness, reproduced with permission from [21]. Copyright (2010) Macmillan Publishers Limited