Nanoparticle Optical Properties: Size Dependence of a Single Gold Spherical Nanoparticle

The optical properties of a spherical gold nanoparticle have become our interest due to its remarkable characteristics when interacting with the light. In this work, the dependence of surface plasmon resonance on the size of the gold nanosphere has been studied with varying diameter from 20 to 100 nm. The optical spectra of the single isolated gold nanoparticles are simulated by employing the versatile MNPBEM toolbox. The wavelength corresponding to the maximum extinction, absorption as well as scattering redshifts (shift to longer wavelengths) were observed as the nanoparticle size increased. The rate of change of scattering (Δsca) and absorption (Δabs) relative to the extinction is calculated to correlate these two properties. For the nanoparticle with size less than 70 nm, Δabs is larger than Δsca, whereas for the diameter of nanoparticles above 70 nm, they show that Δsca is larger than Δabs. Due to this, it is found that absorption efficiency initially increases with nanoparticle size, which eventually reaches a maximum at a size of 70 nm and then begins to decrease with further increase in the nanoparticle diameter. The observed volcano trend of absorption efficiency is the result of electromagnetic retardation and radiative scattering. It can be concluded that the intermediate-size nanoparticle has the highest absorption efficiency with an optimal size of 70 nm for gold nanoparticles.