Plasmon-Induced Electron Transfer between Gold Nanorods and a Carbon Thin Film

Plasmonic nanostructures have been demonstrated as emergent photocatalysts because of their efficient photon absorption and their ability to produce hot carriers. However, the plasmon-generated hot carriers decay through ultrafast relaxation pathways, resulting in a short lifetime that impedes the exploitation of hot carriers for chemical reactions. Charge separation at the heterojunction of the hybrid nanostructures can counteract the ultrafast decay to extend the carrier lifetime. Here, we fabricate hybrid nanostructures composed of gold nanorods and a carbon thin film and demonstrate efficient charge transfer between these two materials. Using single-particle dark-field scattering spectroscopy, we observe a broadening of the longitudinal plasmon for gold nanorods on a carbon film compared to those on a glass substrate. We attribute this plasmon damping to the electron transfer from gold nanorods to the carbon film and exclude the contribution from plasmon-induced resonance energy transfer. The electron transfer efficiencies are calculated as 52.8 ± 4.8 and 57.4 ± 4.0% for carbon films with thicknesses of 10 and 25 nm, respectively. This work demonstrates efficient charge separation at the gold–carbon film interface, which can extend the lifetime of hot carriers to promote plasmonic photocatalysts

Enhancing the Sensitivity of Single-Particle Photothermal Imaging with Thermotropic Liquid Crystals

Individual molecules and nanoparticles can be imaged based on their absorption using photothermal microscopy. This technique relies on the heating-induced changes in the refractive index of the surrounding medium. Here, we demonstrate an order of magnitude larger enhancement of the signal-to-noise ratio in photothermal imaging of 20 nm gold nanoparticles when using a thermotropic liquid crystal (5CB). We show quantitatively that this increase is due to the large change in the thermo-optical properties of 5CB mainly along the nematic director. Enhancing the sensitivity is important for the further development of absorption-based single-molecule spectroscopy techniques

Mechanistic Study of Bleach-Imaged Plasmon Propagation (BlIPP)

Bleach-imaged plasmon propagation, BlIPP, is a far-field microscopy technique developed to characterize the propagation length of surface plasmon polaritons in metallic waveguides. To correctly extract the propagation length from the measured photobleach intensity, it is necessary to understand the mechanism by which dye photobleaching occurs. In particular, 1- vs 2-photon bleaching reactions yield different propagation lengths based on a kinetic model for BlIPP. Because a number of studies have reported on the importance of 2-photon processes for dye photobleaching, we investigate here the nature of the photobleaching step in BlIPP. We are able to demonstrate that only 1-photon absorption is relevant for typical BlIPP conditions as tested here for a thin film of indocyanine green fluorescent dye molecules coated over gold nanowires and excited at a wavelength of 785 nm. These results are obtained by directly measuring the excitation intensity dependence of the photobleaching rate constant of the dye in the presence of the metallic waveguide

Active Modulation of Nanorod Plasmons

Confining visible light to nanoscale dimensions has become possible with surface plasmons. Many plasmonic elements have already been realized. Nanorods, for example, function as efficient optical antennas. However, active control of the plasmonic response remains a roadblock for building optical analogues of electronic circuits. We present a new approach to modulate the polarized scattering intensities of individual gold nanorods by 100% using liquid crystals with applied voltages as low as 4 V. This novel effect is based on the transition from a homogeneous to a twisted nematic phase of the liquid crystal covering the nanorods. With our method it will be possible to actively control optical antennas as well as other plasmonic elements

Active Modulation of Nanorod Plasmons

Confining visible light to nanoscale dimensions has become possible with surface plasmons. Many plasmonic elements have already been realized. Nanorods, for example, function as efficient optical antennas. However, active control of the plasmonic response remains a roadblock for building optical analogues of electronic circuits. We present a new approach to modulate the polarized scattering intensities of individual gold nanorods by 100% using liquid crystals with applied voltages as low as 4 V. This novel effect is based on the transition from a homogeneous to a twisted nematic phase of the liquid crystal covering the nanorods. With our method it will be possible to actively control optical antennas as well as other plasmonic elements

Plasmon Emission Quantum Yield of Single Gold Nanorods as a Function of Aspect Ratio

We report on the one-photon photoluminescence of gold nanorods with different aspect ratios. We measured photoluminescence and scattering spectra from 82 gold nanorods using single-particle spectroscopy. We found that the emission and scattering spectra closely resemble each other independent of the nanorod aspect ratio. We assign the photoluminescence to the radiative decay of the longitudinal surface plasmon generated after fast interconversion from excited electron–hole pairs that were initially created by 532 nm excitation. The emission intensity was converted to the quantum yield and was found to approximately exponentially decrease as the energy difference between the excitation and emission wavelength increased for gold nanorods with plasmon resonances between 600 and 800 nm. We compare this plasmon emission to its molecular analogue, fluorescence

Single-Particle Plasmon Voltammetry (spPV) for Detecting Anion Adsorption

Nanoparticle and thin film surface plasmons are highly sensitive to electrochemically induced dielectric changes. We exploited this sensitivity to detect reversible electrochemical potential-driven anion adsorption by developing single-particle plasmon voltammetry (spPV) using plasmonic nanoparticles. spPV was used to detect sulfate electroadsorption to individual Au nanoparticles. By comparing both semiconducting and metallic thin film substrates with Au nanoparticle monomers and dimers, we demonstrated that using Au film substrates improved the signal in detecting sulfate electroadsorption and desorption through adsorbate modulated thin film conductance. Using single-particle surface plasmon spectroscopic techniques, we constructed spPV to sense sulfate, acetate, and perchlorate adsorption on coupled Au nanoparticles. spPV extends dynamic spectroelectrochemical sensing to the single-nanoparticle level using both individual plasmon resonance modes and total scattering intensity fluctuations

One-Dimensional Coupling of Gold Nanoparticle Plasmons in Self-Assembled Ring Superstructures

Plasmon coupling in ordered metal nanoparticle assemblies leads to tunable collective surface plasmon resonances that strongly depend on the interparticle distance. Here we report on the surface plasmon scattering of polystyrene-functionalized 40 nm gold nanoparticles self-assembled into close-packed rings. Using single particle dark-field scattering spectroscopy, we observed strong near-field coupling between neighboring nanoparticles, which results in red-shifted multipolar plasmon modes highly polarized along the ring circumference. Correlated optical spectroscopy and scanning electron microscopy of individual rings with different diameters revealed that the plasmon coupling is independent of ring curvature and mostly insensitive to the local nanoparticle arrangement. Our results further suggest that a one-dimensional gold nanoparticle assembly yields long-range collective plasmonic properties similar to those of metallic nanowires

Plasmonic Nanoparticles−Liquid Crystal Composites

We report on the plasmonic properties of 6 nm gold nanoparticles that form highly stable solutions in the nematic liquid crystal 4-cyano-4-n-pentylbiphenyl (5CB). The nanoparticles were covalently functionalized with 4-sulfanylphenyl-4-[4-(octyloxy)phenyl]benzoate, which resembles the structure of the 5CB molecules. The solubility of these nanoparticles in 5CB was significantly higher than that of conventional alkanethiol-terminated nanoparticles. An 8 nm shift of the surface plasmon resonance was observed when the gold nanoparticles were dissolved in the nematic phase of 5CB, as compared to the isotropic solution in methylene chloride. Good agreement of the experimental surface plasmon resonance shift with Mie calculations using an adjusted dielectric function for a reduced electron mean free path in small nanoparticles confirmed that the gold nanoparticles are solvated by the liquid crystal molecules. The stability of this composite was verified by repeated temperature cycling between the isotropic and nematic phases. We also investigated the nematic-to-isotropic phase transition temperature and the threshold voltage for the Freedericksz transition in gold-nanoparticle-doped and undoped liquid crystal devices