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.
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.
Abstract:Laser ablation has several advantages over the chemical synthesis of nanoparticles due to its simplicity and because it is a faster and cleaner process. In this paper, we use femtosecond laser ablation to generate highly concentrated silver colloidal nanoparticle solutions. Those high concentrations usually lead to agglomeration of the nanoparticles, rendering the solution nearly useless. We employ two different organic stabilizers (hexadecyltrimethylammonium bromide, CTAB, and polyvinylpyrrolidone, PVP) to avoid this problem and study their effect on the nanoparticle size distribution, structural characteristics, and the solution concentration.
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