Localized Surface Plasmonic Resonance and Sensing Properties of Ag-MgF2 Composite Nanotriangles

Regular nanotriangular (NT) patterns and thin films composed of a mixture of Ag and MgF2 with different composition ratios are prepared by combining nanosphere lithography and electron beam codeposition. The plasmonic property of Ag-MgF2 composites with a high Ag composition C-Ag (>= 90 at. %) are shown to be a function of C-Ag as well as the size of the NTs, whereas for samples with low C-Ag (<90 at. %), a nearly constant localized surface plasmon resonance (LSPR) peak appears in all samples, regardless of C-Ag, NT size, or thin film, which is confirmed to be because of Ag nanoparticles formed during the deposition. Thus, the LSPR property of the composite NTs can be tuned by C-Ag when C-Ag >= 90 at. %. The resulting LSPR sensor at C-Ag = 90 at. % with 500 nm diameter polystyrene nanosphere monolayer can achieve a sensitivity of 696 RIU/nm, as compared to 312 RIU/nm for the same NTs with pure Ag. This significantly improved sensitivity is due to the modified dispersion relationship of the dielectric constant by the metal-dielectric composite and will play an important role in future plasmonic material design and applications.