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.
Fabrication of ordered nanoparticle assemblies over extended areas and volumes is still a major challenge in nanomaterials research.[1] The current limitations in the production of such ordered assemblies dramatically hinder the application of nanoparticles in fields such as negative refractive index metamaterials or information technologies. In the particular case of metal nanocrystal assemblies, [2] nanoscale organization of readily accessible spherical gold nanoparticles [3,4] has been manipulated to produce a diverse range of topologies [5] with interesting optical and electrical properties. [6,7] However, the use of isotropic nanoparticles strongly limits potential applications that require the formation of lattices with vectorial properties. A recent report [8] demonstrated the formation of 3D gold nanorod (NR) superstructures from liquid-crystalline phases [9][10][11][12] with a limited degree of control over the dimensionality and directionality of the assembly. A major advance is demonstrated herein through the use of a gemini surfactant.[13] Replacement of cetyltrimethylammonium bromide (CTAB) by this unconventional surfactant during nanorod synthesis leads to production of monodisperse NRs that can undergo directional self-assembly into highly ordered 2D and 3D standing superlattices with anisotropic optical properties.The synthesis of highly monodisperse gold NRs is especially appealing because of their strong, polarizationdependent suface-plasmon-based optical properties, [14] which render their assemblies ideal candidates for the preparation of optically anisotropic lattices that allow manipulation of light in the nanoscale.[15] Nowadays, tuning of the longitudinal and transverse localized plasmon resonances of gold NRs by synthetic manipulation is a mature field of research. In particular, the seeded growth method in aqueous solution, [16] based on the reduction by a weak reducing agent of a gold salt on premade small seeds in the presence of CTAB and silver ions provides sufficient flexibility to synthesize nanorods (CTAB-NRs) with diverse sizes and shapes.[17] Besides being a shape-inducing agent, CTAB efficiently prevents aggregation through dynamic adsorption onto the gold NRs surface in a bilayer fashion.[18] This nanoparticle shielding of CTAB in water, together with the intense capillary forces generated at the solvent-air interfaces in aqueous solution and the typical Brownian motion of nanoparticles, brings colloidal stability face-to-face with controlled self-assembly of NRs in water and demands a rational search for new and simple strategies.To date, the construction of assemblies of standing gold NRs has mostly relied on postsynthesis surface functionalization with thiol and silane capping agents [19,20] and subsequent transfer into organic solvents. However, the degree of order in the self-assembly has still been limited to 2D sub-micrometer areas. Among the different capping agents that have been proposed for the preparation of metal nanoparticle arrays in organic solvents, thiol-func...
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The encapsulation processes of sodium dodecyl sulfate (SDS) or sodium perfluorooctanoate (SPFO) monomers into the cavity of /9-Cyclodextrin (9-CD) and its effect in the micellization process of the surfactant itself have been analyzed by measuring the speed of sound, u, at 298.15 K (a) as a function of [surfactant] in the presence of various constant concentrations of /3-CD and (b) as a function of [/9-CD] at different surfactant constant concentrations both in the premicellar and micellar regions. The predominant complex formed (/3-CD:surfactant) in both cases has a stoichiometry of 1:1 and the association constants K have been determined from speed of sound measurements by using a semiempirical model proposed by us previously. The apparent critical micellar concentration, cmc*, is found to increase upon the addition of cyclodextrin, for both systems. However, the concentration of free surfactant available for the micellization process in the postmicellar region when the cyclodextrin is present, [surf]f, remains constant in the case of SDS + 9-CD and presents an overall increase in the case of SPFO + /9-CD.
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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