Fig. 4. Full extinction spectra (a), LSPR spectra (b) and diffraction spectra (c) of Au, Au–Ag, Au–Cu and Au–Ag–Cu NPs array obtained by colloidal monolayer with periodic length of 500 nm on quartz sheet; (d) value of LSPR and diffraction peaks of Au, Au–Ag, Au–Cu and Au–Ag–Cu NPs array.