UC Merced

Plasmonic molecules are small assemblies of metal nanoparticles with definitive bond angles and gap sizes. Simulations predict plasmonic molecules support strong plasmonic coupling and have other interesting plasmonic effects, which make them attractive for numerous optical, sensing applications and investigative tools for fundamental plasmonic theories. Despite the promising simulation results, the syntheses of those plasmonic molecules are challenging because the current assembly approaches cannot precisely control the geometry of these assemblies, especially when the constituent nanoparticles have disparate sizes. Consequently, the plasmonic coupling within these assembled structures is often much lower than that predicted by simulations of ideal plasmonic molecules. To unleash the enormous potentials in plasmonic molecules, it is critical to organize different-sized nanoparticles with well-defined bond angles and gap sizes. This dissertation describes two new stepwise assembly approaches to form linear trimeric plasmonic molecules that consist of two large nanoparticles flanking a small nanoparticle in the center, which can serve as plasmonic lenses concentrating intense electric fields in the inter-particle gap. Both approaches use a DNA origami cage to encapsulate the DNA functionalized central particle. In the first approach, termed docking to DNA origami cage (D-DOC), the two DNA functionalized terminal nanoparticles bind to the openings of the cage via hybridization with capture strands. In the second approach, termed cage-constrained inter-particle hybridization (CCIPH), the terminal nanoparticles are connected to the central nanoparticle as their ligands hybridize with the ligands of the central nanoparticle exposed at the two cage openings. These two approaches have been used to align the centers of 10 nm, 30 nm, and 50 nm gold nanoparticles into plasmonic heterotrimeric molecules. Two symmetric timers and an asymmetric trimer are synthesized, and each assembly step is investigated. For all three trimers, structural analyzes are conducted by scanning electron microscopy (SEM) to assess the bond angles and gap distances. The plasmonic effects of two symmetric trimers are evaluated by UV-Vis absorption spectroscopy and Raman spectroscopy. In accordance with experimental data, extensive finite-difference time-domain (FDTD) simulations are performed. The bond angles and gap sizes of our assembled plasmonic molecules are precisely defined, and one of our model trimers shows strong plasmonic coupling and expected surface enhanced Raman scattering enhancement factor. The optimizations and future directions of these two assembly approaches are also discussed.