Finite Difference Time Domain Calculation on Layer-by-Layer Assemblies of Close-Packed Gold Nanoparticles

Abstract

The finite difference time domain (FDTD) method was applied to obtain transmission spectra and local electric field distributions for layer-by-layer assemblies of close-packed gold nanospheres (NSs) and gold nanorods (NRs).The computational results were used to understand variations in the extinction spectra and in the surface enhanced Raman scattering (SERS) for the corresponding experimental samples.High local electric fields were found at small interstices among neighboring nanoparticles.In general, the sum of the local field intensities of the so-called ‘hot spots’ in a layer was greater in the NR assemblies than in the NS assemblies.Furthermore, high local electric fields, polarized along the light propagation direction, were found at the interlayers of the NR assemblies, while they were small in the NS assemblies.These results are in accord with the SERS results and can be explained in large part by the greater contact area in the NR assemblies than in the NS assemblies.However, the calculated local fields for the multilayer assemblies were limited to the top few layers while the interlayer couplings were significantly reduced at the lower layers, as the incident light was extinguished mostly at the top few layers.Such a layer-by-layer local field distribution was insufficient to explain the SERS intensity variation with the number of layers.Implications of the mismatch and possible limitations of the computations will be discussed.