Author: Abhijeet Sureka and Venugopal Santhanam
The optical properties of metal nanoparticles have garnered researchers’ interests, since 1800s, owing to their strong extinction efficiency in the visible spectrum. With increasing interest in basic and applied nanosciences, the use of metal nanoparticles spans various fields like data storage, therapeutic purposes, biological sensors etc. This article describes a study, using numerical simulation, of the optical properties of nanoparticles as a function of their shape and size. The Discrete Dipole Approximation (DDA), wherein a target geometry is modelled as a finite array of polarizable elements, is employed for determining the optical properties of the nanoparticles. The results of the simulation provide a physical insight into the origins of the extinction peaks in the UV-Visible spectra. They also indicate that for gold and silver spheres, with sizes smaller than 20 nm, the significant contribution to scattering is from the dipolar term and the extinction efficiency varies linearly with size. The simulation results were compared with experimental spectra for platinum coated gold nanospheres and a homogeneous mixture of iron and platinum nanospheres, and served as a verification tool to understand the nature of the bimetallic nanoparticles. For triangular gold nanoprisms, the extinction efficiency proved to be largely dependent upon their aspect ratio and the pattern of multi-polar oscillations were verified by polarization plots. The results of this study can be helpful in designing appropriately shaped nanoparticles for in-vivo applications like photo-thermal cancer treatment and optical sensors.