Promoting plasmonic photocatalysis with ligand-induced charge separation under interband excitation.

Plasmonic nanoparticles have been demonstrated to enhance photocatalysis due to their strong photon absorption and efficient hot-carrier generation. However, plasmonic photocatalysts suffer from a short lifetime of plasmon-generated hot carriers that decay through internal relaxation pathways before being harnessed for chemical reactions. Here, we demonstrate the enhanced photocatalytic reduction of gold ions on gold nanorods functionalized with polyvinylpyrrolidone. The catalytic activities of the reaction are quantified by in situ monitoring of the spectral evolution of single nanorods using a dark-field scattering microscope. We observe a 13-fold increase in the reduction rate with the excitation of d-sp interband transition compared to dark conditions, and a negligible increase in the reduction rate when excited with intraband transition. The hole scavenger only plays a minor role in the photocatalytic reduction reaction. We attribute the enhanced photocatalysis to an efficient charge separation at the gold-polyvinylpyrrolidone interface, where photogenerated d-band holes at gold transfer to the HOMO of polyvinylpyrrolidone, leading to the prolonged lifetime of the electrons that subsequently reduce gold ions to gold atoms. These results provide new insight into the design of plasmonic photocatalysts with capping ligands.

Comparison of three-dimensional motion of bacteria with and without wall accumulation.

A comparison of the movement characteristics between bacteria with and without wall accumulation could potentially elucidate the mechanisms of biofilm formation. However, authors of previous studies have mostly focused on the motion of bacteria that exhibit wall accumulation. Here, we applied digital holographic microscopy to compare the three-dimensional (3D) motions of two bacterial strains (Shewanella japonica UMDC19 and Shewanella sp. UMDC1): one exhibiting higher concentrations near the solid surfaces, and the other showing similar concentrations in near-wall and bulk regions. We found that the movement characteristics of the two strains are similar in the near-wall region but are distinct in the bulk region. Near the wall, both strains have small velocities and mostly perform subdiffusive motions. In the bulk, however, the bacteria exhibiting wall accumulation have significantly higher motility (including faster swimming speeds and longer movement trajectories) than the one showing no wall accumulation. Furthermore, we found that bacteria exhibiting wall accumulation slowly migrate from the bulk region to the near-wall region, and the hydrodynamic effect alone is insufficient to generate this migration speed. Future studies are required to test if the current findings apply to other bacterial species and strains.

Polarized evanescent waves reveal trochoidal dichroism.

Matter's sensitivity to light polarization is characterized by linear and circular polarization effects, corresponding to the system's anisotropy and handedness, respectively. Recent investigations into the near-field properties of evanescent waves have revealed polarization states with out-of-phase transverse and longitudinal oscillations, resulting in trochoidal, or cartwheeling, field motion. Here, we demonstrate matter's inherent sensitivity to the direction of the trochoidal field and name this property trochoidal dichroism. We observe trochoidal dichroism in the differential excitation of bonding and antibonding plasmon modes for a system composed of two coupled dipole scatterers. Trochoidal dichroism constitutes the observation of a geometric basis for polarization sensitivity that fundamentally differs from linear and circular dichroism. It could also be used to characterize molecular systems, such as certain light-harvesting antennas, with cartwheeling charge motion upon excitation.

Synthesis and Multipole Plasmon Resonances of Spherical Aluminum Nanoparticles.

In comparison to Au and Ag, the high plasma frequency of Al allows multipole plasmon resonances from the ultraviolet to visible (UV-vis) range to be achieved by its nanoparticles with much smaller sizes and even a spherical shape. Herein, we report the high-supersaturation growth of monodisperse spherical Al nanoparticles (Al NPs) from 84 to 200 nm and their distinctive size-dependent multipole plasmon resonance properties in the UV-vis range. Linear relationships between the particle diameter and resonance peak positions of the dipole, quadrupole, and octupole were observed experimentally and confirmed by finite-difference time-domain (FDTD) calculations. FDTD calculations further reveal the high scattering-to-extinction ratio of multipole modes for the particle diameters >100 nm. The extinction coefficients of spherical Al NPs with different diameters were also determined. The excellent matching between the experimental and simulated results in the present work not only offers a standard for the synthesis and characterization of high-quality Al NPs but also provides new insight into the multipole plasmonic properties of Al NPs for advanced optical and sensing applications.

Acoustic Vibrations of Al Nanocrystals: Size, Shape, and Crystallinity Revealed by Single-Particle Transient Extinction Spectroscopy.

Acoustic vibrations in plasmonic nanoparticles, monitored by an all-optical means, have attracted significant increasing interest because they provide unique insight into the mechanical properties of these metallic nanostructures. Al nanostructures are a recently emerging alternative to noble metal nanoparticles, because their broad wavelength tunability and high natural abundance make them ideal for many potential applications. Here, we investigate the acoustic vibrations of individual Al nanocrystals using a combination of electron microscopy and single-particle transient extinction spectroscopy, made possible with a low-pulse energy, high sensitivity, and probe-wavelength-tunable, single-particle transient extinction microscope. For chemically synthesized, faceted Al nanocrystals, the observed vibration frequency scales with the inverse particle diameter. In contrast, triangularly shaped Al nanocrystals support two distinct frequencies, corresponding to their in- and out-of-plane breathing modes. Unlike ensemble measurements, which measure average properties, measuring the damping time of the acoustic vibrations for individual particles enables us to investigate variations of the quality factor on the particle-to-particle level. Surprisingly, we find a large variation in quality factors even for nanocrystals of similar size and shape. This observed heterogeneity appears to result from substantially varying degrees of nanoparticle crystallinity even for chemically synthesized nanocrystals.

Nanoelectrode-emitter spectral overlap amplifies surface enhanced electrogenerated chemiluminescence.

Electrogenerated chemiluminescence (ECL) is a promising technique for low concentration molecular detection. To improve the detection limit, plasmonic nanoparticles have been proposed as signal boosting antennas to amplify ECL. Previous ensemble studies have hinted that spectral overlap between the nanoparticle antenna and the ECL emitter may play a role in signal enhancement. Ensemble spectroscopy, however, cannot resolve heterogeneities arising from colloidal nanoparticle size and shape distributions, leading to an incomplete picture of the impact of spectral overlap. Here, we isolate the effect of nanoparticle-emitter spectral overlap for a model ECL system, coreaction of tris(2,2'-bipyridyl)dichlororuthenium(ii) hexahydrate and tripropylamine, at the single-particle level while minimizing other factors influencing ECL intensities. We found a 10-fold enhancement of ECL among 952 gold nanoparticles. This signal enhancement is attributed exclusively to spectral overlap between the nanoparticle and the emitter. Our study provides new mechanistic insight into plasmonic enhancement of ECL, creating opportunities for low concentration ECL sensing.

Gold Nanotetrapods with Unique Topological Structure and Ultranarrow Plasmonic Band as Multifunctional Therapeutic Agents.

Owing to their excellent surface plasmonic properties, Au nanobranches have drawn increasing attention in various bioapplications, such as contrast agents for photoacoustic imaging, nanomedicines for photothermal therapy, and carriers for drug delivery. The monodispersity and plasmonic bandwidth of Au nanobranches are of great importance for the efficacy of those bioapplications. However, it is still a challenge to accurately synthesize size- and shape-controlled Au nanobranches. Here we report a facile seed-mediated growth method to synthesize monodisperse Au nanotetrapods (NTPs) with tunable and ultranarrow plasmonic bands. The NTPs have a novel D2d symmetry with four arms elongated in four ⟨110⟩ directions. The growth mechanism of NTPs relies on the delicate kinetic control of deposition and diffusion rates of adatoms. Upon laser irradiation, the PEGylated NTPs possess remarkable photothermal conversion efficiencies and photoacoustic imaging properties. The NTPs can be applied as a multifunctional theranostic agent for both photoacoustic imaging and image-guided photothermal therapy.

Active Far-Field Control of the Thermal Near-Field via Plasmon Hybridization.

The ability to control and manipulate temperature at nanoscale dimensions has the potential to impact applications including heat-assisted magnetic recording, photothermal therapies, and temperature-driven reactivity. One challenge with controlling temperature at nanometer dimensions is the need to mitigate heat diffusion, such that the temperature only changes in well-defined nanoscopic regions of the sample. Here we demonstrate the ability to use far-field laser excitation to actively shape the thermal near-field in individual gold nanorod heterodimers by resonantly pumping either the in-phase or out-of-phase hybridized dipole plasmon modes. Using single-particle photothermal heterodyne imaging, we demonstrate localization bias in the photothermal intensity due to preferential heating of one of the nanorods within the pair. Theoretical modeling and numerical simulation make explicit how the resulting photothermal images encode wavelength-dependent temperature biases between each nanorod within a heterodimer, demonstrating the ability to actively manage the thermal near-field by simply tuning the color of incident light.

Ultrafast Electron Dynamics in Single Aluminum Nanostructures.

Aluminum nanostructures are a promising alternative material to noble metal nanostructures for several photonic and catalytic applications, but their ultrafast electron dynamics remain elusive. Here, we combine single-particle transient extinction spectroscopy and parameter-free first-principles calculations to investigate the non-equilibrium carrier dynamics in aluminum nanostructures. Unlike gold nanostructures, we find the sub-picosecond optical response of lithographically fabricated aluminum nanodisks to be more sensitive to the lattice temperature than the electron temperature. We assign the rise in the transient transmission to electron-phonon coupling with a pump-power-independent lifetime of 500 ± 100 fs and theoretically confirm this strong electron-phonon coupling behavior. We also measure electron-phonon lifetimes in chemically synthesized aluminum nanocrystals and find them to be even longer (1.0 ± 0.1 ps) than for the nanodisks. We also observe a rise and decay in the transient transmissions with amplitudes that scale with the surface-to-volume ratio of the aluminum nanodisks, implying a possible hot carrier trapping and detrapping at the native oxide shell-metal core interface.

Anti-Stokes Emission from Hot Carriers in Gold Nanorods.

The origin of light emission from plasmonic nanoparticles has been strongly debated lately. It is present as the background of surface-enhanced Raman scattering and, despite the low yield, has been used for novel sensing and imaging applications because of its photostability. Although the role of surface plasmons as an enhancing antenna is widely accepted, the main controversy regarding the mechanism of the emission is its assignment to either radiative recombination of hot carriers (photoluminescence) or electronic Raman scattering (inelastic light scattering). We have previously interpreted the Stokes-shifted emission from gold nanorods as the Purcell effect enhanced radiative recombination of hot carriers. Here we specifically focused on the anti-Stokes emission from single gold nanorods of varying aspect ratios with excitation wavelengths below and above the interband transition threshold while still employing continuous wave lasers. Analysis of the intensity ratios between Stokes and anti-Stokes emission yields temperatures that can only be interpreted as originating from the excited electron distribution and not a thermally equilibrated phonon population despite not using pulsed laser excitation. Consistent with this result as well as previous emission studies using ultrafast lasers, the power-dependence of the upconverted emission is nonlinear and gives the average number of participating photons as a function of emission wavelength. Our findings thus show that hot carriers and photoluminescence play a major role in the upconverted emission.

Hot Holes Assist Plasmonic Nanoelectrode Dissolution.

Strong light-absorbing properties allow plasmonic metal nanoparticles to serve as antennas for other catalysts to function as photocatalysts. To achieve plasmonic photocatalysis, the hot charge carriers created when light is absorbed must be harnessed before they decay through internal relaxation pathways. We demonstrate the role of photogenerated hot holes in the oxidative dissolution of individual gold nanorods with millisecond time resolution while tuning charge-carrier density and photon energy using snapshot hyperspectral imaging. We show that light-induced hot charge carriers enhance the rate of gold oxidation and subsequent electrodissolution. Importantly, we distinguish how hot holes generated from interband transitions versus hot holes around the Fermi level contribute to photooxidative dissolution. The results provide new insights into hot-hole-driven processes with relevance to photocatalysis while emphasizing the need for statistical descriptions of nonequilibrium processes on innately heterogeneous nanoparticle supports.

Exploiting Evanescent Field Polarization for Giant Chiroptical Modulation from Achiral Gold Half-Rings.

For applications seeking to realize on-chip polarization-discriminating nanoantennas, efficient energy conversion from surface waves to far-field radiation is desirable. However, the response of individual nanoantennas to the particular polarization states achievable in surface waves, such as evanescent fields, has not yet been thoroughly investigated. Here, we report the giant modulation of visible light scattering from achiral gold half-rings when switching between evanescent surface wave excitation produced from the total internal reflection of left-handed and right-handed circularly polarized light. The effect is driven by a differing relative phase between the in-plane transverse and longitudinal field oscillations of the evanescent wave depending on the incident light handedness. Because longitudinal field oscillations are not found in free-space excitation, this presents a fundamentally different mechanism for chiroptical responses as traditional mechanisms for circular dichroism only account for purely transversal field oscillations. Although the half-ring scattering modulation is dependent on the wave-vector orientation, an orientation invariant response is also realized in planar chiral nanoantennas composed of 8 half-rings in a rotationally symmetric arrangement, with up to 50% scattering modulation observed at 725 nm. Although both structures are found to produce scattering modulation when switching the handedness of free-space light, the distinct polarization properties of evanescent fields are shown to be strictly required to observe giant scattering modulation. These results ultimately deepen our understanding of the range of possible chiroptical effects in light-matter interactions.

Au@CdSe heteroepitaxial nanorods: An example of metal nanorods fully covered by a semiconductor shell with strong photo-induced interfacial charge transfer effects.

Synthesis of metal@semiconductor heteroepitaxial nanorods fully covered by a semiconductor shell remains challenging due to the large lattice mismatch between the two components. Here, we prepared Au@CdSe heteroepitaxial nanorods by employing pre-growth of Ag2Se as an intermediate layer that favored the formation of a complete CdSe shell via a cation-exchange process. The optical properties of these hybrid nanostructures can be tailored by changing the shell thickness with thicker shells resulting in a redshift of the longitudinal surface plasmon resonance. The resonance energy, intensity, and linewidth of the longitudinal surface plasmon resonance were measured by single-particle dark-field scattering spectroscopy, confirming significant electron transfer from the Au nanorod to the CdSe shell. In addition, we also studied the dependence of the catalytic reactivity on shell thickness in photocatalysis of methylene blue under UV illumination. These studies revealed that a thinner shell thickness resulted in higher photocatalytic activity.

Polycrystallinity of Lithographically Fabricated Plasmonic Nanostructures Dominates Their Acoustic Vibrational Damping.

The study of acoustic vibrations in nanoparticles provides unique and unparalleled insight into their mechanical properties. Electron-beam lithography of nanostructures allows precise manipulation of their acoustic vibration frequencies through control of nanoscale morphology. However, the dissipation of acoustic vibrations in this important class of nanostructures has not yet been examined. Here we report, using single-particle ultrafast transient extinction spectroscopy, the intrinsic damping dynamics in lithographically fabricated plasmonic nanostructures. We find that in stark contrast to chemically synthesized, monocrystalline nanoparticles, acoustic energy dissipation in lithographically fabricated nanostructures is solely dominated by intrinsic damping. A quality factor of Q = 11.3 ± 2.5 is observed for all 147 nanostructures, regardless of size, geometry, frequency, surface adhesion, and mode. This result indicates that the complex Young's modulus of this material is independent of frequency with its imaginary component being approximately 11 times smaller than its real part. Substrate-mediated acoustic vibration damping is strongly suppressed, despite strong binding between the glass substrate and Au nanostructures. We anticipate that these results, characterizing the optomechanical properties of lithographically fabricated metal nanostructures, will help inform their design for applications such as photoacoustic imaging agents, high-frequency resonators, and ultrafast optical switches.

Using Particle Lithography to Tailor the Architecture of Au Nanoparticle Plasmonic Nanoring Arrays.

The facile assembly of metal nanostructured arrays is a fundamental step in the design of plasmon enhanced chemical sensing and solar cell architectures. Here we have investigated methods of creating controlled formations of two-dimensional periodic arrays comprised of 20 nm Au nanoparticles (NPs) on a hydrophilic polymer surface using particle lithography. To direct the assembly process, capillary force and NP concentration both play critical roles on the resulting nanostructured arrays. As such, tuning these experimental parameters can directly be used to modify the nature of the nanostructures formed. To explore this, two different concentrations of Au NP solutions (∼7 × 1011 or 4 × 1012 NPs/mL) were used in conjunction with a fixed concentration of polystyrene microspheres (PS MS, ∼6 × 109 PS MS/mL). Assembly at a relative humidity (RH) of 45% with the higher concentration resulted in the formation of well-defined Au nanorings of ca. 23 nm in height and 881 nm in diameter with a pitch of 2.5 µm. Assembly at 65% RH with the lower concentration of NPs resulted in Au nanodonut arrays comprised of isolated single Au NPs. To explore the extent of coupling in the well-defined structures, dark field scattering spectra were collected and showed a broad localized surface plasmon resonance (LSPR) peak with a shoulder, which full-wave electrodynamics modeling (finite-difference time domain (FDTD) method) attributed to be a result of pronounced particle-particle coupling along the circumference of the nanoring array.

Photoluminescence of Gold Nanorods: Purcell Effect Enhanced Emission from Hot Carriers.

We demonstrate, experimentally and theoretically, that the photon emission from gold nanorods can be viewed as a Purcell effect enhanced radiative recombination of hot carriers. By correlating the single-particle photoluminescence spectra and quantum yields of gold nanorods measured for five different excitation wavelengths and varied excitation powers, we illustrate the effects of hot carrier distributions evolving through interband and intraband transitions and the photonic density of states on the nanorod photoluminescence. Our model, using only one fixed input parameter, describes quantitatively both emission from interband recombination and the main photoluminescence peak coinciding with the longitudinal surface plasmon resonance.

Imaging and Spectroscopy of Single Metal Nanostructure Absorption.

The highly tunable optical properties of metal nanoparticles make them an ideal building block in any application that requires control over light, heat, or electrons on the nanoscale. Because of their size, metal nanoparticles both absorb and scatter light efficiently. Consequently, improving their performance often involves shifting the balance between absorption and scattering to promote desirable features of their optical properties. Scattering by single metal nanoparticles is commonly characterized using dark-field scattering spectroscopy, but routine methods to characterize pure absorption over a broad wavelength range are much more complex. This article reviews work from our lab using photothermal imaging in combination with dark-field scattering and electron microscopy to separate radiative and nonradiative properties of single nanoparticles and their assemblies. We present both initial work using different laser wavelengths to explore pure absorption free from scattering contributions based on the heat released into the environment as well as the development of photothermal spectroscopy over a broad wavelength range, making it possible to resolve details that are otherwise hidden in ensemble measurements that most of the time also do not separate radiative and nonradiative properties.

Correlated Absorption and Scattering Spectroscopy of Individual Platinum-Decorated Gold Nanorods Reveals Strong Excitation Enhancement in the Nonplasmonic Metal.

Bimetallic nanocatalysts have the potential to surmount current limitations in industrial catalysis if their electronic and optical properties can be effectively controlled. However, improving the performance of bimetallic photocatalysts requires a functional understanding of how the intricacies of their morphology and composition dictate every element of their optical response. In this work, we examine Au and Pt-decorated Au nanorods on a single-particle level to ascertain how Pt influences the plasmon resonance of the bimetallic nanostructure. We correlated scattering, photoluminescence, and pure absorption of individual nanostructures separately to expose the impact of Pt on each component. We found that the scattering and absorption spectra of uncoated Au nanorods followed expected trends in peak intensity and shape and were accurately reproduced by finite difference time domain simulations. In contrast, the scattering and absorption spectra of single Pt-decorated Au nanorods exhibited red-shifted, broad features and large deviations in line shape from particle to particle. Simulations using an idealized geometry confirmed that Pt damps the plasmon resonance of individual Au nanorods and that spectral changes after Pt deposition were a consequence of coupling between Au and Pt in the hybrid nanostructure. Simulations also revealed that the Au nanorod acts as an antenna and enhances absorption in the Pt islands. Furthermore, comparing photoluminescence spectra from Au and Pt-decorated Au nanorods illustrated that emission was significantly reduced in the presence of Pt. The reduction in photoluminescence intensity indicates that Pt lowers the number of hot carriers in the Au nanorod available for radiative recombination through either direct production of hot carriers in Pt following enhanced absorption or charge transfer from Au to Pt. Overall, these results confirm that the Pt island morphology and distribution on the nanorod surface contribute to the optical response of individual hybrid nanostructures and that the damping observed in ensemble measurements originates not only from structural heterogeneity but also because of significant damping in single nanostructures.

Vibrational coupling in plasmonic molecules.

Plasmon hybridization theory, inspired by molecular orbital theory, has been extremely successful in describing the near-field coupling in clusters of plasmonic nanoparticles, also known as plasmonic molecules. However, the vibrational modes of plasmonic molecules have been virtually unexplored. By designing precisely configured plasmonic molecules of varying complexity and probing them at the individual plasmonic molecule level, intramolecular coupling of acoustic modes, mediated by the underlying substrate, is observed. The strength of this coupling can be manipulated through the configuration of the plasmonic molecules. Surprisingly, classical continuum elastic theory fails to account for the experimental trends, which are well described by a simple coupled oscillator picture that assumes the vibrational coupling is mediated by coherent phonons with low energies. These findings provide a route to the systematic optical control of the gigahertz response of metallic nanostructures, opening the door to new optomechanical device strategies.