Single Au Nanorods Research Articles

Multiphoton photoelectron emission microscopy of single Au nanorods: combined experimental and theoretical study of rod morphology and dielectric environment on localized surface plasmon resonances

Multiphoton photoelectron emission from individual Au nanorods deposited on indium tin oxide (ITO) substrates is studied via scanning photoionization microscopy, based on femtosecond laser excitation at frequencies near the rod longitudinal surface plasmon resonance (LSPR). The observed resonances in photoemission correlate strongly with plasmon resonances measured in dark field microscopy (DFM), thus establishing a novel scheme for wavelength-resolved study of plasmons in isolated metallic nanoparticles based on highly sensitive electron counting methods. In this work, we explore experimental and theoretical effects of (i) morphology and (ii) aspect ratio (AR) for longitudinal plasmon resonance behavior in Au nanorods. A quasilinear dependence between LSPR and aspect ratio (AR) is experimentally determined [Δλ≈ +100(10) nm/AR unit] for Au nanorods on ITO, in excellent agreement with the first principles value from finite element computer modeling [Δλ = +108(5) nm/AR unit]. Interestingly, however, LSPR values for larger vs. smaller diameter rods (w≈ 20 nm and 10 nm) are systematically red-shifted [ΔE≈-0.03(1) eV; Δλ≈ +15(5) nm at λ≈ 800 nm], indicating that electromagnetic retardation effects must also be considered for highest accuracy in LSPR position. To augment these results, the influence of the dielectric environment on the rod LSPR has been explored both experimentally and numerically. In particular, detailed finite-element simulations for ITO supported Au nanorods are found to yield plasmon resonances in near quantitative agreement (ΔE≈±0.04 eV) with experiment, with residual differences arising from uncertainty in the refractive index of the ITO thin film. Furthermore, the results indicate that plasmon resonance predictions based on infinitely thick ITO substrates are reliable to a few meV for film thicknesses larger than approximately twice the rod width.

Shape-Templated Growth of Au@Cu Nanoparticles

We report the formation of copper nanoparticles with various morphologies and low polydispersity, using Au nanoparticles as templates. This seeded growth strategy is based on the reduction of Cu2+ with hydrazine in water at low temperature. Additionally, the use of poly(acrylic acid) as capping agent allows synthesis under aerobic conditions. The dimensions of the resulting Au@Cu nanoparticles can be readily tuned through either the dimensions of the Au cores or the Cu/Au molar ratio. Although Au and Cu show a significant lattice mismatch, epitaxial growth of Cu onto single crystal Au nanorods was confirmed through high-resolution electron microscopy and electron diffraction analysis. The effects of core morphology on the optical properties of the core–shell nanoparticles were analyzed by vis-NIR spectroscopy and were found to agree with simulations based on the boundary element method. This work contributes to understand the strong effect of interband transitions on the optical response of Au@Cu and to confirm the importance of tuning the localized surface plasmon resonance away from the interband transitions.

The Role of Bromide Ions in Seeding Growth of Au Nanorods

We report our findings on the important role of bromide ions in the seeding growth process of Au nanorods. The seed-mediated process constitutes a well-developed method for synthesizing gold nanorods in high yield, which is facilitated by a micelle-forming surfactant, cetyltrimethylammonium bromide (CTA-Br). Despite the tremendous work in recent years, the growth mechanism of Au nanorods has not been fully understood. Contrary to the widely accepted mechanism of CTA(+) micelle-templated growth of Au nanorods, we have identified the critical role of bromide ions in the seeding growth of Au nanorods. We found that even when the micelle-forming agent (CTA(+)) concentration is below its critical micelle concentration (cmc), bromide ions added in the form of NaBr can successfully effect the growth of Au nanorods in good yield. By controlling the concentration of externally added bromide ions, the rod shape and dimensions of the resulting Au nanoparticles can be readily controlled in the presence of only a minimum amount of CTABr (as a steric stabilizer for nanorods). High-resolution TEM studies show that the as-formed nanorods are perfectly single crystalline, instead of penta-twinned ones, and are bound by {111} and {100} facets with a [110] direction as the elongation direction. A mechanism is proposed to account for the seeding growth of single crystalline Au nanorods. Overall, this work explicitly demonstrates that Br(-) indeed serves as an important shape-directing agent for gold nanorod formation in the seed-mediated process.

Nanoscale characterization of single Au nanorods by confocal microscopy

We analyzed the scattering patterns of individual Au nanorods detected by means of confocal interference scattering microscopy in combination with a higher order laser mode. We placed the Au nanorods at the interface between two dielectric media and examined the influence of different interfaces on the signal amplitude, the signal-to-noise ratio as well as on the precision of topological measurements. Approaching the index matching regime allows for topological measurements with high accuracy minimizing the acquisition time. We were also able to track the position and the orientation of particles embedded in water even when they were not thoroughly sticking to the glass surface. These results underscore the potential of the presented technique for applications in life sciences.

Tuning the Morphology of Gold Nanocrystals by Switching the Growth of {110} Facets from Restriction to Preference

Single crystalline Au nanorods (Au NRs), synthesized via seed-mediated growth, show unique surface structures. Apart from the oft-observed {100} and {111} facets, unexpectedly, unstable {110} facets dominate in such nanorods due to {110} restricted growth. Unique properties have been suggested for the nanorods. One novel property, we believe, is that the the high-energy {110} endows the nanorod with a high reactivity, thus making the growth to more stable morphologies possible. Herein, by switching the growth to the {110} preference, we successfully obtained thermodynamically more stable morphologies (arrow-headed gold nanorods and gold nano-octahedra) with a high quality and yield. A blockade of selective underpotential deposition of silver is suggested to be responsible for the switching.

Rapid Formation of Novel Au Core–Ag Shell Nanostructures by a Microwave-polyol Method

Abstract Au core–Ag shell bimetallic nanoparticles, denoted as Au@Ag, have been prepared by a microwave(MW)-polyol method through the two-step reduction of AuCl4− and Ag+ ions in the presence of poly(vinylpyrrolidone) (PVP) as a capping reagent. When single crystal polygonal Au nanoplates and nanorods were used as seeds, a mixture of various Au@Ag core–shell nanostructures was prepared after MW heating for only 2 min. Among them, small amounts of novel polygonal and rod types of Au@Ag nanostructures, in which morphologies of Au core were conserved, were produced.