Ligand-Dependent Colloidal Stability Controls the Growth of Aluminum Nanocrystals

The precise size- and shape-controlled synthesis of monodisperse Al nanocrystals remains an open challenge, limiting their utility for numerous applications that would take advantage of their size and shape-dependent optical properties. Here we pursue a molecular-level understanding of the formation of Al nanocrystals by titanium(IV) isopropoxide-catalyzed decomposition of AlH₃ in Lewis base solvents. As determined by electron paramagnetic resonance spectroscopy of intermediates, the reaction begins with the formation of Ti³⁺-AlH₃ complexes. Proton nuclear magnetic resonance spectroscopy indicates isopropoxy ligands are removed from Ti by Al, producing aluminum(III) isopropoxide and low-valent Ti³⁺ catalysts. These Ti³⁺ species catalyze elimination of H₂ from AlH₃ inducing the polymerization of AlH₃ into colloidally unstable low-valent aluminum hydride clusters. These clusters coalesce and grow while expelling H₂ to form colloidally stable Al nanocrystals. The colloidal stability of the Al nanocrystals and their size is determined by the molecular structure and density of coordinating atoms in the reaction, which is controlled by choice of solvent composition.

Toroidal Dipole-Enhanced Third Harmonic Generation of Deep Ultraviolet Light Using Plasmonic Meta-atoms

The harmonic generation of light with plasmonic and all-dielectric nanostructures has gained much recent interest. This approach is especially promising for short wavelength (i.e., ultraviolet (UV)) generation, where conventional nonlinear crystals reach their limits both in transparency and in their ability to achieve phase-matching between the input and output fields. Here, we demonstrate that the third harmonic generation of deep UV light in an indium tin oxide (ITO) film can be substantially enhanced by a metasurface consisting of metallic toroidal meta-atoms covered with an alumina layer for protection against laser-induced damage. This approach combines the benefits of the large nonlinear susceptibility of ITO with the unique field enhancement properties of a toroidal metasurface. This ITO-meta-atom combination produces a third harmonic signal at a wavelength of 262 nm that is nominally five times larger than that of an ITO film patterned with a conventional hotspot-enhanced plasmonic dimer array. This result demonstrates the potential for toroidal meta-atoms as the active engineered element in a new generation of enhanced nonlinear optical materials and devices.

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.

Optical-Force-Dominated Directional Reshaping of Au Nanodisks in Al-Au Heterodimers

The optical reshaping of metallic nanostructures typically requires intense laser pulses to first approach or achieve melting, followed by surface-tension-dominated reshaping, transforming the original nanostructures into more spherical morphologies. Here, we report the directional optical reshaping of the Au nanodisk of an Al-Au heterodimer in the illuminated junction of an atomic force microscope (AFM). Both the heightening and the repositioning of the Au nanodisk component are induced, reducing the gap between the two nanodisks. There are three contributors to this process: the photothermal softening of the Au lattice, the optical force applied to the Au nanodisk by the Al nanodisk, and the optical force from the nearby AFM tip. The asymmetric reshaping of the heterodimer is observable structurally, through electron microscopic imaging, and through changes in the heterodimer optical response. This optical-force-directed shape manipulation may have potential applications in nanofabrication, optically induced nanomanufacturing, sensing, and quality control.

Quantifying hot carrier and thermal contributions in plasmonic photocatalysis

Photocatalysis based on optically active, “plasmonic” metal nanoparticles has emerged as a promising approach to facilitate light-driven chemical conversions under far milder conditions than thermal catalysis. However, an understanding of the relation between thermal and electronic excitations has been lacking. We report the substantial light-induced reduction of the thermal activation barrier for ammonia decomposition on a plasmonic photocatalyst. We introduce the concept of a light-dependent activation barrier to account for the effect of light illumination on electronic and thermal excitations in a single unified picture. This framework provides insight into the specific role of hot carriers in plasmon-mediated photochemistry, which is critically important for designing energy-efficient plasmonic photocatalysts.

Vacuum Ultraviolet Light-Generating Metasurface

Vacuum ultraviolet (VUV) light has important applications in many fields, ranging from device fabrication to photochemistry, from environmental remediation to microscopy and spectroscopy. Methods to produce coherent VUV light frequently utilize high harmonic generation in media such as rare gases or atomic vapors; nonlinear optical crystals that support second harmonic generation into the VUV are quite rare. Here, we demonstrate an all-dielectric metasurface designed for the nonlinear optical generation of VUV light. Consisting of an array of zinc oxide nanoresonators, the device exhibits a magnetic dipole resonance at a wavelength of 394 nm. When excited with ultrafast laser pulses at this wavelength, the second harmonic at 197 nm is readily generated. Manipulation of the metasurface design enables control over the radiation pattern. This work has the potential to open the door toward simple and compact VUV sources for new applications.

Routes to Potentially Safer T1 Magnetic Resonance Imaging Contrast in a Compact Plasmonic Nanoparticle with Enhanced Fluorescence

Engineering a compact, near-infrared plasmonic nanostructure with integrated image-enhancing agents for combined imaging and therapy is an important nanomedical challenge. Recently, we showed that Au@SiO₂@Au nanomatryoshkas (NM) are a highly promising nanostructure for hosting either T₁ MRI or fluorescent contrast agents with a photothermal therapeutic response in a compact geometry. Here, we show that a near-infrared-resonant NM can provide simultaneous contrast enhancement for both T₁ magnetic resonance imaging (MRI) and fluorescence optical imaging (FOI) by encapsulating both types of contrast agents in the internal silica layer between the Au core and shell. We also show that this method of T₁ enhancement is even more effective for Fe(III), a potentially safer contrast agent compared to Gd(III). Fe-NM-based contrast agents are found to have relaxivities 2× greater than those found in the widely used gadolinium chelate, Gd(III) DOTA, providing a practical alternative that would eliminate Gd(III) patient exposure entirely. This dual-modality nanostructure can enable not only tissue visualization with MRI but also fluorescence-based nanoparticle tracking for quantifying nanoparticle distributions in vivo, in addition to a near-infrared photothermal therapeutic response.

Atomic Scale Photodetection Enabled by a Memristive Junction

The optical control of atomic relocations in a metallic quantum point contact is of great interest because it addresses the fundamental limit of "CMOS scaling". Here, by developing a platform for combined electronics and photonics on the atomic scale, we demonstrate an optically controlled electronic switch based on the relocation of atoms. It is shown through experiments and simulations how the interplay between electrical, optical, and light-induced thermal forces can reversibly relocate a few atoms and enable atomic photodetection with a digital electronic response, a high resistance extinction ratio (70 dB), and a low OFF-state current (10 pA) at room temperature. Additionally, the device introduced here displays an optically induced pinched hysteretic current (optical memristor). The photodetector has been tested in an experiment with real optical data at 0.5 Gbit/s, from which an eye diagram visualizing millions of detection cycles could be produced. This demonstrates the durability of the realized atomic scale devices and establishes them as alternatives to traditional photodetectors.

Wavelength-Dependent Optical Force Imaging of Bimetallic Al-Au Heterodimers

Many important applications of nanometer-scale metallic complexes arise from the light-induced, near-field interactions between their component structures. Here we examine the near-field interactions in bimetallic Al-Au plasmonic nanodisk heterodimers, where the coupling between the primitive plasmons of nanostructures composed of two different metals is studied. Understanding the correlations between nanoparticle morphology and near-field optical properties, particularly for nanostructures composed of two different metals, requires spectrally resolved near-field spatial information. An ideal tool for such investigations is the recently developed photoinduced force microscopy, where the electromagnetic forces between an optically excited plasmonic nanostructure and an adjacent scanning nanoscale tip are measured. Using this approach, we visualize the wavelength-dependent near-field interactions in these bimetallic heterodimers. This system provides a prime example of the diabatic, antenna-reactor picture of plasmon coupling where for a given wavelength the more resonant primitive "driving" plasmon induces a response, the "forced" plasmon, in the off-resonant component. We critically examine spectrally resolved tip-nanostructure forces, comparing experiment with theory, for tips and nanoscale structures of realistic dimensions relative to frequently used approximations for tip geometries. The contrasting effects of dielectric versus metallic tips on acquired spectral force profiles are also examined.

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.

Aluminum Nanorods

Al nanocrystals can be synthesized by high-temperature decomposition of triisobutyl aluminum, creating a mixture of nanoparticle geometries with a significant fraction (∼15%) being single-crystalline Al nanorods. The Al nanorods are elongated along their ⟨110⟩ direction, and generally exhibit hexagonal cross sections consisting of two adjacent {111} facets separated by {100} facets on opposite sides. Dark-field scattering spectroscopy of individual Al nanorods reveals that rods of varying aspect ratios all possess transverse quadrupolar and octupolar modes in the visible (2-3 eV) and ultraviolet (3-5 eV) regimes. Theoretical modeling indicates that the longitudinal resonances of these nanorods span the near- and mid-infrared regions of the spectrum. This work introduces a new class of anisotropic metal nanocrystals composed of single-crystalline Al, opening the door to highly modifiable plasmonic nanorods from Earth-abundant metals.

Transition-Metal Decorated Aluminum Nanocrystals

Recently, aluminum has been established as an earth-abundant alternative to gold and silver for plasmonic applications. Particularly, aluminum nanocrystals have shown to be promising plasmonic photocatalysts, especially when coupled with catalytic metals or oxides into "antenna-reactor" heterostructures. Here, a simple polyol synthesis is presented as a flexible route to produce aluminum nanocrystals decorated with eight varieties of size-tunable transition-metal nanoparticle islands, many of which have precedence as heterogeneous catalysts. High-resolution and three-dimensional structural analysis using scanning transmission electron microscopy and electron tomography shows that abundant nanoparticle island decoration in the catalytically relevant few-nanometer size range can be achieved, with many islands spaced closely to their neighbors. When coupled with the Al nanocrystal plasmonic antenna, these small decorating islands will experience increased light absorption and strong hot-spot generation. This combination makes transition-metal decorated aluminum nanocrystals a promising material platform to develop plasmonic photocatalysis, surface-enhanced spectroscopies, and quantum plasmonics.

Two-Dimensional Active Tuning of an Aluminum Plasmonic Array for Full-Spectrum Response

Color pixels composed of plasmonic nanostructures provide a highly promising approach for new display technologies, capable of vivid, robust coloration and incorporating the use of low-cost plasmonic materials, such as aluminum. Here we report a plasmonic device that can be tuned continuously across the entire visible spectrum, based on integrating a square array of aluminum nanostructures into an elastomeric substrate. By stretching the substrate in either of its two dimensions, the period and therefore the scattering color can be modified to the blue or the red of the at-rest structure, spanning the entire visible spectrum. The unique two-dimensional design of this structure enables active mechanical color tuning, under gentle elastic modulation with no more than 35% strain. We also demonstrate active image switching with this structure. This design strategy has the potential to open the door for next-generation flexible photonic devices for a wide variety of visible-light applications.

Quantifying Remote Heating from Propagating Surface Plasmon Polaritons

We report a method to electrically detect heating from excitation of propagating surface plasmon polaritons (SPP). The coupling between SPP and a continuous wave laser beam is realized through lithographically defined gratings in the electrodes of thin film gold "bow tie" nanodevices. The propagating SPPs allow remote coupling of optical energy into a nanowire constriction. Heating of the constriction is detectable through changes in the device conductance and contains contributions from both thermal diffusion of heat generated at the grating and heat generated locally at the constriction by plasmon dissipation. We quantify these contributions through computational modeling and demonstrate that the propagation of SPPs provides the dominant contribution. Coupling optical energy into the constriction via propagating SPPs in this geometry produces an inferred temperature rise of the constriction a factor of 60 smaller than would take place if optical energy were introduced via directly illuminating the constriction. The grating approach provides a path for remote excitation of nanoconstrictions using SPPs for measurements that usually require direct laser illumination, such as surface-enhanced Raman spectroscopy.

Nanogapped Au Antennas for Ultrasensitive Surface-Enhanced Infrared Absorption Spectroscopy

Surface-enhanced infrared absorption (SEIRA) spectroscopy has outstanding potential in chemical detection as a complement to surface-enhanced Raman spectroscopy (SERS), yet it has historically lagged well behind SERS in detection sensitivity. Here we report a new ultrasensitive infrared antenna designed to bring SEIRA spectroscopy into the few-molecule detection range. Our antenna consists of a bowtie-shaped Au structure with a sub-3 nm gap, positioned to create a cavity above a reflective substrate. This three-dimensional geometry tightly confines incident mid-infrared radiation into its ultrasmall junction, yielding a hot spot with a theoretical SEIRA enhancement factor of more than 10⁷, which can be designed to span the range of frequencies useful for SEIRA. We quantitatively evaluated the IR detection limit of this antenna design using mixed monolayers of 4-nitrothiophenol (4-NTP) and 4-methoxythiolphenol (4-MTP). The optimized antenna structure allows the detection of as few as ∼500 molecules of 4-NTP and ∼600 molecules of 4-MTP with a standard commercial FTIR spectrometer. This strategy offers a new platform for analyzing the IR vibrations of minute quantities of molecules and lends insight into the ultimate limit of single-molecule SEIRA detection.

Aluminum Nanocrystals: A Sustainable Substrate for Quantitative SERS-Based DNA Detection

Since its discovery in the 1970s, surface-enhanced Raman scattering (SERS) has been primarily associated with substrates composed of nanostructured noble metals. Here we investigate chemically synthesized nanocrystal aggregates of aluminum, an inexpensive, highly abundant, and sustainable metal, as SERS substrates. Al nanocrystal aggregates are capable of substantial near-infrared SERS enhancements, similar to Au nanoparticles. The intrinsic nanoscale surface oxide of Al nanocrystals supports molecule-substrate interactions that differ dramatically from noble metal substrates. The preferential affinity of the single-stranded DNA (ssDNA) phosphate backbone for the Al oxide surface preserves both the spectral features and nucleic acid cross sections relative to conventional Raman spectroscopy, enabling quantitative ssDNA detection and analysis.

Oblique Colloidal Lithography for the Fabrication of Nonconcentric Features

Herein, we describe the development of oblique colloidal lithography (OCL) and establish a systematic patterning strategy for creating libraries of nanosized nonconcentric plasmonic structures. This strategy combines OCL, capillary force lithography, and several wet and ion etching steps. Hexagonal arrays of nonconcentric gold features were created on glass substrates with highly controllable geometric parameters. The size, geometry, and eccentricity of the gold features could be independently tuned by controlling the experimental conditions. Gaps within surface elements could be shrunk to as small as 30 nm, while the total patterned area was about l cm². The goal was to devise a method that offers a high degree of control over the resolution and morphology of asymmetric structures without the need to resort to electron beam lithography. This technique also enabled the development of numerous surface patterns through the stepwise fabrication of separate elements. Complex features, including dots-surrounded nonconcentric targets, nonconcentric hexagram-disks, and nonconcentric annular aperture arrays, were demonstrated, and their optical properties were characterized. Indeed, spectroscopic studies and FDTD simulations demonstrated that Fano resonances could readily be generated by the nonconcentric gold features. Consequently, our patterning strategy should enable the high-throughput investigation of plasmonic coupling and Fano resonances as a function of the physical parameters of the elements within the nanopattern array.

How To Identify Plasmons from the Optical Response of Nanostructures

A promising trend in plasmonics involves shrinking the size of plasmon-supporting structures down to a few nanometers, thus enabling control over light-matter interaction at extreme-subwavelength scales. In this limit, quantum mechanical effects, such as nonlocal screening and size quantization, strongly affect the plasmonic response, rendering it substantially different from classical predictions. For very small clusters and molecules, collective plasmonic modes are hard to distinguish from other excitations such as single-electron transitions. Using rigorous quantum mechanical computational techniques for a wide variety of physical systems, we describe how an optical resonance of a nanostructure can be classified as either plasmonic or nonplasmonic. More precisely, we define a universal metric for such classification, the generalized plasmonicity index (GPI), which can be straightforwardly implemented in any computational electronic-structure method or classical electromagnetic approach to discriminate plasmons from single-particle excitations and photonic modes. Using the GPI, we investigate the plasmonicity of optical resonances in a wide range of systems including: the emergence of plasmonic behavior in small jellium spheres as the size and the number of electrons increase; atomic-scale metallic clusters as a function of the number of atoms; and nanostructured graphene as a function of size and doping down to the molecular plasmons in polycyclic aromatic hydrocarbons. Our study provides a rigorous foundation for the further development of ultrasmall nanostructures based on molecular plasmonics.

Spectral Response of Plasmonic Gold Nanoparticles to Capacitive Charging: Morphology Effects

We report a study of the shape-dependent spectral response of the gold nanoparticle surface plasmon resonance at various electron densities to provide mechanistic insight into the role of capacitive charging, a topic of some debate. We demonstrate a morphology-dependent spectral response for gold nanoparticles due to capacitive charging using single-particle spectroscopy in an inert electrochemical environment. A decrease in plasmon energy and increase in spectral width for gold nanospheres and nanorods was observed as the electron density was tuned through a potential window of -0.3 to 0.1 V. The combined observations could not be explained by existing theories. A new quantum theory for charging based on the random phase approximation was developed. Additionally, the redox reaction of gold oxide formation was probed using single-particle plasmon voltammetry to reproduce the reduction peak from the bulk cyclic voltammetry. These results deepen our understanding of the relationship between optical and electronic properties in plasmonic nanoparticles and provide insight toward their potential applications in directed electrocatalysis.

Balancing Near-Field Enhancement, Absorption, and Scattering for Effective Antenna-Reactor Plasmonic Photocatalysis

Efficient photocatalysis requires multifunctional materials that absorb photons and generate energetic charge carriers at catalytic active sites to facilitate a desired chemical reaction. Antenna-reactor complexes are an emerging multifunctional photocatalytic structure where the strong, localized near field of the plasmonic metal nanoparticle (e.g., Ag) is coupled to the catalytic properties of the nonplasmonic metal nanoparticle (e.g., Pt) to enable chemical transformations. With an eye toward sustainable solar driven photocatalysis, we investigate how the structure of antenna-reactor complexes governs their photocatalytic activity in the light-limited regime, where all photons need to be effectively utilized. By synthesizing core@shell/satellite (Ag@SiO₂/Pt) antenna-reactor complexes with varying Ag nanoparticle diameters and performing photocatalytic CO oxidation, we observed plasmon-enhanced photocatalysis only for antenna-reactor complexes with antenna components of intermediate sizes (25 and 50 nm). Optimal photocatalytic performance was shown to be determined by a balance between maximized local field enhancements at the catalytically active Pt surface, minimized collective scattering of photons out of the catalyst bed by the complexes, and minimal light absorption in the Ag nanoparticle antenna. These results elucidate the critical aspects of local field enhancement, light scattering, and absorption in plasmonic photocatalyst design, especially under light-limited illumination conditions.

Manipulating Coherent Plasmon-Exciton Interaction in a Single Silver Nanorod on Monolayer WSe2

Strong coupling between plasmons and excitons in nanocavities can result in the formation of hybrid plexcitonic states. Understanding the dispersion relation of plexcitons is important both for fundamental quantum science and for applications including optoelectronics and nonlinear optics devices. The conventional approach, based on statistics over different nanocavities, suffers from large inhomogeneities from the samples, owing to the nonuniformity of nanocavities and the lack of control over the locations and orientations of the excitons. Here we report the first measurement of the dispersion relationship of plexcitons in an individual nanocavity. Using a single silver nanorod as a Fabry-Pérot nanocavity, we realize strong coupling of plasmon in single nanocavity with excitons in a single atomic layer of tungsten diselenide. The plexciton dispersion is measured by in situ redshifting the plasmon energy via successive deposition of a dielectric layer. Room-temperature formation of plexcitons with Rabi splittings as large as 49.5 meV is observed. The realization of strong plasmon-exciton coupling by in situ tuning of the plasmon provides a novel route for the manipulation of excitons in semiconductors.

Hot Hole Photoelectrochemistry on Au@SiO2@Au Nanoparticles

There is currently a worldwide need to develop efficient photocatalytic materials that can reduce the high-energy cost of common industrial chemical processes. One possible solution focuses on metallic nanoparticles (NPs) that can act as efficient absorbers of light due to their surface plasmon resonance. Recent work indicates that small NPs, when photoexcited, may allow for efficient electron or hole transfer necessary for photocatalysis. Here we investigate the mechanisms behind hot hole carrier dynamics by studying the photodriven oxidation of citrate ions on Au@SiO₂@Au core-shell NPs. We find that charge transfer to adsorbed molecules is most efficient at higher photon energies but still present with lower plasmon energy. On the basis of these experimental results, we develop a simple theoretical model for the probability of hot carrier-adsorbate interactions across the NP surface. These results provide a foundation for understanding charge transfer in plasmonic photocatalytic materials, which could allow for further design and optimization of photocatalytic processes.