Large spectral shift of plasmon resonances in Au–Cu alloy nanoparticles through anisotropy and interaction

Abstract

Nowadays, alloy nanoparticles (NPs) consisting of noble metals find much importance due to their enhanced localized surface plasmon resonance (SPR) in the visible range. Besides the compositional changes, the SPR of alloy NPs independently can be tailored by changing the shape and interparticle separation. Here, we report the effects of shape and interaction parameters (β and K, respectively) on the plasmonic properties of Au–Cu alloy NPs considering Au concentrations in the range of 0.0–1.0 using a modified effective medium theory. There is always a single SPR for the alloys’ NPs. A large shift of this SPR peak from visible to infrared regions is seen with decrease or increase in the values of β or K. A linear variation of peak shift with 1/β and an exponential decay variation of peak shift with 1/K are observed. Although there is a small increase in the values of slopes (in the peak shift vs. 1/β curves) with increase in concentration of Au in the Au–Cu alloys, there is almost no change in the fitting parameter, τ of the exponential decay function (in the curves for peak shift vs. 1/K) for different sizes of NPs and concentration of Au or Cu in the alloys. This establishes the universal nature of the exponential decay behaviour. These observations are consistent with the data for elemental (Au and Cu) NPs. The correlations between (i) shape parameter, β and the aspect ratio, AR and (ii) interaction parameter, K with the interparticle separation, s are established as in case of elemental NPs. The study thus, verifies the model further and establishes universality of the applicability of the model.