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

Abstract

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 [Appl. Phys. Lett.2014105011107]. 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.

Widely Tunable Infrared Plasmonic Nanoantennas Using Directed Assembly

“…[9] An unpolarized white light source (DH-2000) was used to probe the suspension and the light was collected with bifurcated optical fiber (OceanOptics QBIF50-VIS-NIR) coupled into spectrophotometers (USB4000 (0.4-1 µm) and NIRQuest (1.1-1.6 µm)). [9] An unpolarized white light source (DH-2000) was used to probe the suspension and the light was collected with bifurcated optical fiber (OceanOptics QBIF50-VIS-NIR) coupled into spectrophotometers (USB4000 (0.4-1 µm) and NIRQuest (1.1-1.6 µm)).…”

“…A charge transfer plasmon (CTP) absorption peak, λ CTP , clearly emerges (λ CTP /fwhm = 4) at 1.2 ± 0.2 µm (red curve) from the welded chains. [9] By using relatively large laser fluences (≈mJ cm −2 ), [9] convection in the suspension was induced without damaging the nanorods, verified via TEM, enabling the entire suspension (≈4 mL) to be welded at a rate of trillion nanorods per minute (≈10 12 nanorods per mL). A typical TEM analysis consisted of ≈500 nanorods and was used to verify the number of nanorods in the chains and confirm welding.…”

“…The high quality factor (λ CTP /fwhm) CTP absorption peaks observed in Figure 2 are attributed to the CTP mode being insensitive to the relative orientation of the individual nanorods. [9] By using relatively large laser fluences (≈mJ cm −2 ), [9] convection in the suspension was induced without damaging the nanorods, verified via TEM, enabling the entire suspension (≈4 mL) to be welded at a rate of trillion nanorods per minute (≈10 12 nanorods per mL). The large laser fluence also led to a reduction in the chain length of the CTP nanoantennas, relative to the CCP chain length at the onset of welding, and is proposed to be a laser fluence induced instability, which is not well understood.…”

“…The experimental wavelength tunability of this approach was ≈0.4 µm at near infrared wavelengths. [9] Although this approach enabled high quality yields it is not practical for producing infrared nanoantennas due to the long assembly times and the large water absorption peaks which inhibit the ability to process and measure the suspensions at infrared wavelengths. [8] Further work refined this approach experimentally demonstrating discrete nanorod dimer nanoantennas with large and sharp absorption peaks, but used a slowly reacting aqueous based method.…”

“…[8] Further work refined this approach experimentally demonstrating discrete nanorod dimer nanoantennas with large and sharp absorption peaks, but used a slowly reacting aqueous based method. [9] Although this approach enabled high quality yields it is not practical for producing infrared nanoantennas due to the long assembly times and the large water absorption peaks which inhibit the ability to process and measure the suspensions at infrared wavelengths. The inability to experimentally direct and weld nanorod assemblies into higher order structures, while simultaneously producing high quality yields, has impeded progress into infrared wavelengths.…”