Nanometer-scale size dependent imaging of cetyl trimethyl ammonium bromide (CTAB) capped and uncapped gold nanoparticles by apertureless near-field optical microscopy

Near-infrared excitation/emission microscopy with lanthanide-based nanoparticles

Near-infrared optical imaging offers some advantages over conventional imaging, such as deeper tissue penetration, low or no autofluorescence, and reduced tissue scattering. Lanthanide-doped nanoparticles (LnNPs) have become a trend in the field of photoactive nanomaterials for optical imaging due to their unique optical features and because they can use NIR light as excitation and/or emission light. This review is focused on NaREF₄ NPs and offers an overview of the state-of-the-art investigation in their use as luminophores in optical microscopy, time-resolved imaging, and super-resolution nanoscopy based on, or applied to, LnNPs. Secondly, whenever LnNPs are combined with other nanomaterial or nanoparticle to afford nanohybrids, the characterization of their physical and chemical properties is of current interest. In this context, the latest trends in optical microscopy and their future perspectives are discussed.

Nanoscopy of Phase Separation in InxGa1-xN Alloys

Phase separations in ternary/multinary semiconductor alloys is a major challenge that limits optical and electronic internal device efficiency. We have found ubiquitous local phase separation in In1-xGaxN alloys that persists to nanoscale spatial extent by employing high-resolution nanoimaging technique. We lithographically patterned InN/sapphire substrates with nanolayers of In1-xGaxN down to few atomic layers thick that enabled us to calibrate the near-field infrared response of the semiconductor nanolayers as a function of composition and thickness. We also developed an advanced theoretical approach that considers the full geometry of the probe tip and all the sample and substrate layers. Combining experiment and theory, we identified and quantified phase separation in epitaxially grown individual nanoalloys. We found that the scale of the phase separation varies widely from particle to particle ranging from all Ga- to all In-rich regions and covering everything in between. We have found that between 20 and 25% of particles show some level of Ga-rich phase separation over the entire sample region, which is in qualitative agreement with the known phase diagram of In1-xGaxN system.

Nanorod Surface Plasmon Enhancement of Laser-Induced Ultrafast Demagnetization

Ultrafast laser-induced magnetization dynamics in ferromagnetic thin films were measured using a femtosecond Ti:sapphire laser in a pump-probe magneto-optic Kerr effect setup. The effect of plasmon resonance on the transient magnetization was investigated by drop-coating the ferromagnetic films with dimensionally-tuned gold nanorods supporting longitudinal surface plasmon resonance near the central wavelength of the pump laser. With ~4% nanorod areal coverage, we observe a >50% increase in demagnetization signal in nanorod-coated samples at pump fluences on the order of 0.1 mJ/cm² due to surface plasmon-mediated localized electric-field enhancement, an effect which becomes more significant at higher laser fluences. We were able to qualitatively reproduce the experimental observations using finite-difference time-domain simulations and mean-field theory. This dramatic enhancement of ultrafast laser-induced demagnetization points to possible applications of nanorod-coated thin films in heat-assisted magnetic recording.

Probing individual redox PEGylated gold nanoparticles by electrochemical--atomic force microscopy

Electrochemical-atomic force microscopy (AFM-SECM) was used to simultaneously probe the physical and electrochemical properties of individual ~20 nm sized gold nanoparticles functionalized by redox-labeled PEG chains. The redox PEGylated nanoparticles were assembled onto a gold electrode surface, forming a random nanoarray, and interrogated in situ by a combined AFM-SECM nanoelectrode probe. We show that, in this so-called mediator-tethered (Mt) mode, AFM-SECM affords the nanometer resolution required for resolving the position of individual nanoparticles and measuring their size, while simultaneously electrochemically directly contacting the redox-PEG chains they bear. The dual measurement of the size and current response of single nanoparticles uniquely allows the statistical distribution in grafting density of PEG on the nanoparticles to be determined and correlated to the nanoparticle diameter. Moreover, because of its high spatial resolution, Mt/AFM-SECM allows "visualizing" simultaneously but independently the PEG corona and the gold core of individual nanoparticles. Beyond demonstrating the achievement of single-nanoparticle resolution using an electrochemical microscopy technique, the results reported here also pave the way toward using Mt/AFM-SECM for imaging nano-objects bearing any kind of suitably redox-labeled (bio)macromolecules.

Mechanisms of plasmon-enhanced femtosecond laser nanoablation of silicon

We perform plasmon-enhanced femtosecond laser ablation of silicon using gold nanorods to produce sub-diffraction limit features. While the observed hole shape seems inconsistent with calculated field distribution, we show that using a carrier diffusion-based model, both shape and depth of the nanoholes can be reliably explained. The laser energy is first deposited into electron-hole pairs that are created in the nanostructure's enhanced near-field. Those carriers then diffuse and transfer their energy to the silicon lattice, producing ablation. Increased importance of the carrier diffusion process is shown to arise from the extreme localization of the deposited energy around the nanostructure, due to the plasmonic effect. The characteristic shape of holes is revealed as a striking signature of the screened charge carriers-phonon coupling that is shown to channel the heat transfer to the lattice and control ablation.

Role of in-plane polarizability of the tip in scattering near-field microscopy of a plasmonic nanoparticle

We report that a pyramid-shaped scanning probe microscopy tip has non-zero polarizability along the in-plane direction (perpendicular to the tip axis, z) at visible frequency. The in-plane polarizability enables the scattering-type scanning near-field optical microscopy (s-SNOM) to measure the in-plane field component around a plasmon-resonant nanoparticle. Because of the non-zero in-plane polarizability, the cross-polarized s-SNOM images may contain contributions from the in-plane field component of an out-of-plane plasmon mode as well as the out-of-plane field component of an in-plane mode. By comparing a scattering model and experimental s-SNOM images, we estimate the polarization anisotropies of pyramid-shaped Si-tips and metal-coated Si-tips.

Characterization of localized surface plasmon resonance transducers produced from Au(25) nanoparticle multilayers

This article reports the preparation of gold plasmonic transducers using a nanoparticle self-assembly/heating method and the characterization of the films using scattering-type scanning near-field optical microscopy (s-SNOM). Nanoparticle-polymer multilayer films were prepared by the layer-by-layer assembly on glass slides by alternating exposures to monodisperse Au(25) nanoparticles and ionic polymer linkers. Thermal evaporation of organic matters from the nanoparticle-polymer multilayer films at 600 °C allowed the nanoparticles to coalescence and form nanostructured films. Characterization of the nanostructured films generated from Au(25) nanoparticles using atomic force microscopy (AFM) showed that the films have rounded, small, island-like morphologies (d: 30-50 nm) with a pit in the center of many islands. However, further characterizations with s-SNOM revealed that the produced nanoislands contain a single gold cluster in a pit surrounded by donut-shaped dielectric species. Formation of such a structure is thought to be resulted from the embedding of gold clusters under the reorganized polysiloxane binder coatings and glass surfaces during heat treatment of the Au(25) nanoparticle multilayer films. The nanostructured films displayed strong surface plasmon resonance bands in UV-vis spectra with a peak absorbance occurring at ~545-550 nm. The optical sensing capability of the films was examined using D-glucose-functionalized gold island films with the interaction of Concanavalin A (ConA). The result showed that the adsorption of ConA on island films causes a large change in the LSPR band intensity.

Imaging three-dimensional rotational diffusion of plasmon resonant gold nanorods using polarization-sensitive optical coherence tomography

We demonstrate depth-resolved viscosity measurements within a single object using polarized optical scattering from ensembles of freely tumbling plasmon resonant gold nanorods (GNRs) monitored with polarization-sensitive optical coherence tomography. The rotational diffusion coefficient of the GNRs is shown to correlate with viscosity in molecular fluids according to the Stokes-Einstein relation. The plasmon resonant and highly anisotropic properties of GNRs are favorable for microrheological studies of nanoscale properties.

Effects of symmetry breaking and conductive contact on the plasmon coupling in gold nanorod dimers

We have explored the consequences of symmetry breaking on the coupled surface plasmon resonances in individual dimers of gold nanorods using single-particle dark-field scattering spectroscopy and numerical simulations. Pairs of chemically grown nanorods can exhibit wide variation in sizes, gap distances, and relative orientation angles. The combination of single-particle spectroscopy and theoretical analysis allowed us to discern the effects of specific asymmetry-inducing parameters one at a time. The dominant influence of symmetry breaking occurred for longitudinal resonances in strongly coupled nanorods in linear end-to-end configurations. In particular, we found that the normally dark antibonding dimer mode becomes visible when the sizes of the two nanorods are different. In addition, we observed a conductively coupled plasmon mode that was red-shifted by at least 250 nm from the bonding plasmon mode for the corresponding nontouching geometry. Gaining detailed insight into how symmetry breaking influences coupled surface plasmon resonances of individual nanorod dimers is an important step toward the general understanding of the optical properties of assemblies of chemically synthesized nanorods with unavoidable irregularities in size and orientation.