Plasmon Energy Transfer Driven by Electrochemical Tuning of Methylene Blue on Single Gold Nanorods.

Plasmonic photocatalysis has attracted interest for its potential to generate energy-efficient reactions, but ultrafast internal conversion limits efficient plasmon-based chemistry. Resonance energy transfer (RET) to surface adsorbates offers a way to outcompete internal conversion pathways and also eliminate the need for sacrificial counter-reactions. Herein, we demonstrate RET between methylene blue (MB) and gold nanorods (AuNRs) using in situ single-particle spectroelectrochemistry. During electrochemically driven reversible redox reactions between MB and leucomethylene blue (LMB), we show that the homogeneous line width is broadened when spectral overlap between AuNR scattering and absorption of MB is maximized, indicating RET. Additionally, electrochemical oxidative oligomerization of MB allowed additional dipole coupling to generate RET at lower energies. Time-dependent density functional theory-based simulated absorption provided theoretical insight into the optical properties, as MB molecules were electrochemically oligomerized. Our findings show a mechanism for driving efficient plasmon-assisted processes by RET through the change in the chemical states of surface adsorbates.

Active Control of Energy Transfer in Plasmonic Nanorod-Polyaniline Hybrids.

The hybridization of plasmonic energy and charge donors with polymeric acceptors is a possible means to overcome fast internal relaxation that limits potential photocatalytic applications for plasmonic nanomaterials. Polyaniline (PANI) readily hybridizes onto gold nanorods (AuNRs) and has been used for the sensitive monitoring of local refractive index changes. Here, we use single-particle spectroscopy to quantify a previously unreported plasmon damping mechanism in AuNR-PANI hybrids while actively tuning the PANI chemical structure. By eliminating contributions from heterogeneous line width broadening and refractive index changes, we identify efficient resonance energy transfer (RET) between AuNRs and PANI. We find that RET dominates the optical response in our AuNR-PANI hybrids during the dynamic tuning of the spectral overlap of the AuNR donor and PANI acceptor. Harnessing RET between plasmonic nanomaterials and an affordable and processable polymer such as PANI offers an alternate mechanism toward efficient photocatalysis with plasmonic nanoparticle antennas.

d-Band Hole Dynamics in Gold Nanoparticles Measured with Time-Resolved Emission Upconversion Microscopy.

The performance of photocatalysts and photovoltaic devices can be enhanced by energetic charge carriers produced from plasmon decay, and the lifetime of these energetic carriers greatly affects overall efficiencies. Although hot electron lifetimes in plasmonic gold nanoparticles have been investigated, hot hole lifetimes have not been as thoroughly studied in plasmonic systems. Here, we demonstrate time-resolved emission upconversion microscopy and use it to resolve the lifetime and energy-dependent cooling of d-band holes formed in gold nanoparticles by plasmon excitation and by following plasmon decay into interband and then intraband electron-hole pairs.

Single-Particle Photoluminescence and Dark-Field Scattering during Charge Density Tuning.

Single-particle spectroelectrochemistry provides optical insight into understanding physical and chemical changes occurring on the nanoscale. While changes in dark-field scattering during electrochemical charging are well understood, changes to the photoluminescence of plasmonic nanoparticles under similar conditions are less studied. Here, we use correlated single-particle photoluminescence and dark-field scattering to compare their plasmon modulation at applied potentials. We find that changes in the emission of a single gold nanorod during charge density tuning of intraband photoluminescence can be attributed to changes in the Purcell factor and absorption cross section. Finally, modulation of interband photoluminescence provides an additional constructive observable, giving promise for establishing dual channel sensing in spectroelectrochemical measurements.

Mechanism for plasmon-generated solvated electrons.

Solvated electrons are powerful reducing agents capable of driving some of the most energetically expensive reduction reactions. Their generation under mild and sustainable conditions remains challenging though. Using near-ultraviolet irradiation under low-intensity one-photon conditions coupled with electrochemical and optical detection, we show that the yield of solvated electrons in water is increased more than 10 times for nanoparticle-decorated electrodes compared to smooth silver electrodes. Based on the simulations of electric fields and hot carrier distributions, we determine that hot electrons generated by plasmons are injected into water to form solvated electrons. Both yield enhancement and hot carrier production spectrally follow the plasmonic near-field. The ability to enhance solvated electron yields in a controlled manner by tailoring nanoparticle plasmons opens up a promising strategy for exploiting solvated electrons in chemical reactions.

Nonlinear effects in single-particle photothermal imaging.

Although photothermal imaging was originally designed to detect individual molecules that do not emit or small nanoparticles that do not scatter, the technique is now being applied to image and spectroscopically characterize larger and more sophisticated nanoparticle structures that scatter light strongly. Extending photothermal measurements into this regime, however, requires revisiting fundamental assumptions made in the interpretation of the signal. Herein, we present a theoretical analysis of the wavelength-resolved photothermal image and its extension to the large particle scattering regime, where we find the photothermal signal to inherit a nonlinear dependence upon pump intensity, together with a contraction of the full-width-at-half-maximum of its point spread function. We further analyze theoretically the extent to which photothermal spectra can be interpreted as an absorption spectrum measure, with deviations between the two becoming more prominent with increasing pump intensities. Companion experiments on individual 10, 20, and 100 nm radius gold nanoparticles evidence the predicted nonlinear pump power dependence and image contraction, verifying the theory and demonstrating new aspects of photothermal imaging relevant to a broader class of targets.

Tuning Electrogenerated Chemiluminescence Intensity Enhancement Using Hexagonal Lattice Arrays of Gold Nanodisks.

Electrogenerated chemiluminescence (ECL) microscopy shows promise as a technique for mapping chemical reactions on single nanoparticles. The technique's spatial resolution is limited by the quantum yield of the emission and the diffusive nature of the ECL process. To improve signal intensity, ECL dyes have been coupled with plasmonic nanoparticles, which act as nanoantennas. Here, we characterize the optical properties of hexagonal arrays of gold nanodisks and how they impact the enhancement of ECL from the coreaction of tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate and tripropylamine. We find that varying the lattice spacing results in a 23-fold enhancement of ECL intensity because of increased dye-array near-field coupling as modeled using finite element method simulations.

Plasmon Energy Transfer in Hybrid Nanoantennas.

Plasmonic metal nanoparticles exhibit large dipole moments upon photoexcitation and have the potential to induce electronic transitions in nearby materials, but fast internal relaxation has to date limited the spatial range and efficiency of plasmonic mediated processes. In this work, we use photo-electrochemistry to synthesize hybrid nanoantennas comprised of plasmonic nanoparticles with photoconductive polymer coatings. We demonstrate that the formation of the conductive polymer is selective to the nanoparticles and that polymerization is enhanced by photoexcitation. In situ spectroscopy and simulations support a mechanism in which up to 50% efficiency of nonradiative energy transfer is achieved. These hybrid nanoantennas combine the unmatched light-harvesting properties of a plasmonic antenna with the similarly unmatched device processability of a polymer shell.

Quantitative Analysis of Nanorod Aggregation and Morphology from Scanning Electron Micrographs Using SEMseg.

General methods to achieve better physical insight about nanoparticle aggregation and assembly are needed because of the potential role of aggregation in a wide range of materials, environmental, and biological outcomes. Scanning electron microscopy (SEM) is fast and affordable compared to transmission electron microscopy, but SEM micrographs lack contrast and resolution due to lower beam energy, topographic contrast, edge effects, and charging. We present a new segmentation algorithm called SEMseg that is robust to the challenges inherent in SEM micrograph analysis and demonstrate its utility for analyzing gold (Au) nanorod aggregates. SEMseg not only supports nanoparticle size analysis for dispersed nanoparticles, but also discriminates between nanoparticles within an aggregate. We compare our algorithm to those incorporated into the commonly used software ImageJ and demonstrate improved segmentation of aggregate structures. New physical insight about aggregation is demonstrated by the introduction of an order parameter describing side-by-side structure in nanoparticle aggregates. We also present the segmentation and fitting algorithms included in SEMseg within a user-friendly graphical user interface. The resulting code is provided with an open-source interface to provide quantitative image processing tools for researchers to characterize both dispersed nanoparticles and nanoparticle assemblies in SEM micrographs with high throughput.