Probing the Plasmon Coupling, Quantum Yield, and Effects of Tip Geometry of Gold Nanoparticle Using Analytical Models and FDTD Simulation

In this paper, we investigated the plasmon coupling, quantum yield (QY) and effect of tip geometry of gold nanoparticles (AuNPs) using finite-difference time-domain (FDTD) simulation. Standalone AuNPs spectra have been obtained using the most reliable electromagnetic theory such as Mie theory for spherical nanoparticle such as gold nanosphere (AuNS) and Mie–Gans theory for ellipsoidal AuNPs such as gold nanorod (AuNR). We reported the scattering cross section for standalone and coupling particle using the modified dielectric constant, where the size effect of small particles and the retardation effect of large particles have been taken into consideration for both the analytical calculation and FDTD simulation. Plasmon coupling effect of AuNS dimer of 80-nm diameter and AuNR dimer (side-by-side geometry) of aspect ratio 3.8 has been investigated using FDTD simulation. We also quantified the QY in terms of cluster to monomer ratio for AuNS and AuNR, which is analogous to the ratio of acceptor and donor chromophore in biological systems. The QY of monomer, dimer, and trimer of 40-nm diameter AuNS has been perceived using FDTD simulation, integrating the whole spectrum over 400–1000 nm wavelength regime. Additionally, effect of tip geometry of AuNPs has been investigated and significant field enhancement due to the variation in shapes such as AuNS, AuNR, gold dumbbell, and gold bipyramid has also been reported. The novelty of the paper lies in the presentation of a systematic study of the plasmon coupling of different sizes and shapes of AuNP, quantification of the QY, and probing the effect of tip geometry for a single material such as AuNP using the FDTD simulation. Moreover, we believe that our in-depth analysis of AuNP laid the foundation to determine the scattering cross-section, QY, and near-field enhancement for more complex structures and geometries of other novel materials. Conclusively, the investigation results demonstrate that plasmonic nanoparticle can be used as a molecular probe for bioimaging, sensing, cell signaling, and biological therapeutic intervention.