Rise of the Charge Transfer Plasmon: Programmable Concatenation of Conductively Linked Gold Nanorod Dimers

The ability to tune the resonant frequency in plasmonic nanostructures is fundamental to developing novel optical properties and ensuing materials. Recent theoretical insights have shown through the conductive concatenation of plasmonic nanoparticles that the effective depolarization factor of the nanostructure, and subsequent charge transfer plasmon (CTP) resonance, can be intricately controlled [Fontana, J.; Ratna, B. R. Appl. Phys. Lett. 2014, 105, 011107 ]. However, translating these charge transfer properties from proof-of-principle experiments to high-quality, macroscale quantities for material applications remains challenging. Here, we experimentally demonstrate by using an electrostatic-based molecular assembly approach how to controllably concatenate gold nanorods end-to-end into discrete dimers, preventing unwanted longer structures and forming a capacitively coupled plasmon (CCP) resonance along the long axis of the dimer. Irradiating these suspensions with femtosecond laser pulses at the CCP dimer resonance wavelength selectively welds only the CCP dimers together, bridging the nanorods with gold nanojunctions and producing large, high-quality yields of welded dimers. Macroscale (∼1012 dimers) absorbance measurements reveal a CTP resonance arising from these welded dimers with absorbance peak magnitudes as large as 0.5 and full-width-at-half-maximum of 274 nm. We show by controlling the aspect ratio of the welded dimers that the CTP absorbance wavelength can differ significantly (∼20%) from a single nanorod with a similar aspect ratio, demonstrating the ability to modulate the effective depolarization factor of the dimer structures and resulting CTP resonance. We also carried out three-dimensional finite element simulations showing less than a 5% shift in the CTP absorbance wavelength as a function of the contact point connecting the nanorods and relative orientation, in agreement with our experiments.