The irradiation of gold nanorod colloids with a femtosecond laser can be tuned to induce controlled nanorod reshaping, yielding colloids with exceptionally narrow localized surface plasmon resonance bands. The process relies on a regime characterized by a gentle multishot reduction of the aspect ratio, whereas the rod shape and volume are barely affected. Successful reshaping can only occur within a narrow window of the heat dissipation rate: Low cooling rates lead to drastic morphological changes, and fast cooling has nearly no effect. Hence, a delicate balance must be achieved between irradiation fluence and surface density of the surfactant on the nanorods. This perfection process is appealing because it provides a simple, fast, reproducible, and scalable route toward gold nanorods with an optical response of exceptional quality, near the theoretical limit.

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Directed assembly of gold nanorods through the use of dithiolated molecular linkers is one of the most efficient methodologies for the morphologically controlled tip-to-tip assembly of this type of anisotropic nanocrystals. However, in a direct analogy to molecular polymerization synthesis, this process is characterized by difficulties in chain-growth control over nanoparticle oligomers. In particular, it is nearly impossible to favor the formation of one type of oligomer, making the methodology hard to use for actual applications in nanoplasmonics. We propose here a light-controlled synthetic procedure that allows obtaining selected plasmonic oligomers in high yield and with reaction times in the scale of minutes by irradiation with low fluence near-infrared (NIR) femtosecond laser pulses. Selective inhibition of the formation of gold nanorod n-mers (trimers) with a longitudinal localized surface plasmon in resonance with a 800 nm Ti:sapphire laser, allowed efficient trapping of the (n – 1)-mers (dimers) by hot spot mediated photothermal decomposition of the interparticle molecular linkers. Laser irradiation at higher energies produced near-field enhancement at the interparticle gaps, which is large enough to melt gold nanorod tips, offering a new pathway toward tip-to-tip welding of gold nanorod oligomers with a plasmonic response at the NIR. Thorough optical and electron microscopy characterization indicates that plasmonic oligomers can be selectively trapped and welded, which has been analyzed in terms of a model that predicts with reasonable accuracy the relative concentrations of the main plasmonic species.

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The geometrical configuration of Au:Ag nanoparticle heterodimers can be optimized to produce prominent Fano resonance (FR) signals, strong enough to be detectable by standard spectroscopic techniques. The highest intensity of the FR in these bimetallic dimers is reached when the size ratio of the nanoparticles (R Au :R Ag ) is close to 3:1. Moreover, the FR can be induced only by the longitudinal component of the dimer surface plasmon resonance (SPR). Finally, it is found that the sensitivity of the dimer FR signal to the variation in the refractive index of the surrounding medium (n m ) is 2 times larger than that of the associated SPR for the region where n m < 1.5, which opens up the possibility of utilizing the FR band for more efficient optical sensing applications.

Fano resonances (FR) in strongly coupled systems like a metallic dimer arise due to the coupling between the spectrally localized surface plasmon resonance (SPR) of a noble metal nanoparticle and the continuum of interband transitions of the other. Since its discovery in Au-Ag dimers, several plasmonic structures have been proposed as candidates for obtaining Fano resonances. However, most of them either are difficult to synthesize or do not generate FR signal of adequate intensity. In this paper we demonstrate that simple Au@Ag core-shell nanoparticles with typical shell thickness below 5.0 nm, which can be synthesized through a common citrate reduction method, have a Fano resonance easily detectable in the far-field.

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In this work we have studied the elongation of silver nanoparticles irradiated with 40 MeV Bromine ions by means of in situ optical measurements, transmission electron microscopy and molecular dynamics simulations. The localized surface plasmon resonance of silver nanoparticles has a strong dependence on the particle shape and size, which allowed us to obtain the geometrical parameters with remarkable accuracy by means of a fit of the optical spectra. Optical results have been compared with transmission electron microscopy images and molecular dynamics simulations and the agreement is excellent in both cases. An important advantage of in situ measurements is that they yield an extremely detailed information of the full elongation kinetics. Final nanoparticle elongation depends on a complex competition between single-ion deformation, Ostwald ripening and dissolution. Building and validating theoretical models with the data reported in this work should be easier than with the information previously available, due to the unprecedented level of kinetic details obtained from the in situ measurements.