Close-Packed Two-Dimensional Silver Nanoparticle Arrays: Quadrupolar and Dipolar Surface Plasmon Resonance Coupling

Silver nanoparticles (NPs) ranging in size from 40 to 100nm were prepared in high yield by using an improved seed-mediated method. The homogeneous AgNPs were used as building blocks for 2D assembled AgNP arrays by using an oil/water interface. A close-packed 2D array of AgNPs was fabricated by using packing molecules (3-mercaptopropyltrimethoxysilane) to control the interparticle spacing. The homogeneous 2D AgNP array exhibited a strong quadrupolar cooperative plasmon mode resonance and a dipolar red-shift relative to individual AgNPs suspended in solution. A well-arranged 2D AgNP array was embedded in polydimethylsiloxane film and, with biaxial stretching to control the interparticle distance, concomitant variations of the quadrupolar and dipolar couplings were observed. As the interparticle distance increased, the intensity of the quadrupolar cooperative plasmon mode resonance decreased and dipolar coupling completely disappeared. The local electric field of the 2D AgNP array was calculated by using finite difference time domain simulation and qualitatively showed agreement with the experimental measurements.