Aluminum Nanocubes Have Sharp Corners

Angular momentum transfer from swift electrons to non-spherical nanoparticles within the dipolar approximation

In this work, we study the angular momentum transfer from a single swift electron to non-spherical metallic nanoparticles, specifically investigating spheroidal and polyhedral (Platonic Solids) shapes. While previous research has predominantly focused on spherical nanoparticles, our work expands the knowledge by exploring various geometries. Employing classical electrodynamics and the small particle limit, we calculate the angular momentum transfer by integrating the spectral density, ensuring causality through Fourier-transform analysis. Our findings demonstrate that prolate spheroidal nanoparticles exhibit a single blueshifted plasmonic resonance, compared to spherical nanoparticles of equivalent volume, resulting in lower angular momentum transfer. Conversely, oblate nanoparticles display two resonances - one blueshifted and one redshifted - resulting in a higher angular momentum transfer than their spherical counterparts. Additionally, Platonic Solids with fewer faces exhibit significant redshifts in plasmonic resonances, leading to higher angular momentum transfer due to edge effects. We also observe resonances and angular momentum transfers with similar characteristics in specific pairs of Platonic Solids, known as duals. These results highlight promising applications, particularly in electron tweezers technology.

Plasmon-enhanced fluorescence for biophotonics and bio-analytical applications

Fluorescence spectroscopy serves as an ultrasensitive sophisticated tool where background noises which serve as a major impediment to the detection of the desired signals can be safely avoided for detections down to the single-molecule levels. One such way of bypassing background noise is plasmon-enhanced fluorescence (PEF), where the interactions of fluorophores at the surface of metals or plasmonic nanoparticles are probed. The underlying condition is a significant spectral overlap between the localized surface plasmon resonance (LSPR) of the nanoparticle and the absorption or emission spectra of the fluorophore. The rationale being the coupling of the excited state of the fluorophore with the localized surface plasmon leads to an augmented emission, owing to local field enhancement. It is manifested in enhanced quantum yields concurrent with a decrease in fluorescence lifetimes, owing to an increase in radiative rate constants. This improvement in detection provided by PEF allows a significant scope of expansion in the domain of weakly emitting fluorophores which otherwise would have remained unperceivable. The concept of coupling of weak emitters with plasmons can bypass the problems of photobleaching, opening up avenues of imaging with significantly higher sensitivity and improved resolution. Furthermore, amplification of the emission signal by the coupling of free electrons of the metal nanoparticles with the electrons of the fluorophore provides ample opportunities for achieving lower detection limits that are involved in biological imaging and molecular sensing. One avenue that has attracted significant attraction in the last few years is the fast, label-free detection of bio-analytes under physiological conditions using plasmonic nanoparticles for point-of-care analysis. This review focusses on the applications of plasmonic nanomaterials in the field of biosensing, imaging with a brief introduction on the different aspects of LSPR and fabrication techniques.

Reduced-Dimensionality Al Nanocrystals: Nanowires, Nanobars, and Nanomoustaches

Aluminum nanocrystals created by catalyst-driven colloidal synthesis support excellent plasmonic properties, due to their high level of elemental purity, monocrystallinity, and controlled size and shape. Reduction in the rate of nanocrystal growth enables the synthesis of highly anisotropic Al nanowires, nanobars, and singly twinned "nanomoustaches". Electron energy loss spectroscopy was used to study the plasmonic properties of these nanocrystals, spanning the broad energy range needed to map their plasmonic modes. The coupling between these nanocrystals and other plasmonic metal nanostructures, specifically Ag nanocubes and Au films of controlled nanoscale thickness, was investigated. Al nanocrystals show excellent long-term stability under atmospheric conditions, providing a practical alternative to coinage metal-based nanowires in assembled nanoscale devices.

Tailoring the aluminum nanocrystal surface oxide for all-aluminum-based antenna-reactor plasmonic photocatalysts

Aluminum nanocrystals (AlNCs) are of increasing interest as sustainable, earth-abundant nanoparticles for visible wavelength plasmonics and as versatile nanoantennas for energy-efficient plasmonic photocatalysis. Here, we show that annealing AlNCs under various gases and thermal conditions induces substantial, systematic changes in their surface oxide, modifying crystalline phase, surface morphology, density, and defect type and concentration. Tailoring the surface oxide properties enables AlNCs to function as all-aluminum-based antenna-reactor plasmonic photocatalysts, with the modified surface oxides providing varying reactivities and selectivities for several chemical reactions.

Ultraviolet Resonant Nanogap Antennas with Rhodium Nanocube Dimers for Enhancing Protein Intrinsic Autofluorescence

Plasmonic optical nanoantennas offer compelling solutions for enhancing light-matter interactions at the nanoscale. However, until now, their focus has been mainly limited to the visible and near-infrared regions, overlooking the immense potential of the ultraviolet (UV) range, where molecules exhibit their strongest absorption. Here, we present the realization of UV resonant nanogap antennas constructed from paired rhodium nanocubes. Rhodium emerges as a robust alternative to aluminum, offering enhanced stability in wet environments and ensuring reliable performance in the UV range. Our results showcase the nanoantenna's ability to enhance the UV autofluorescence of label-free streptavidin and hemoglobin proteins. We achieve significant enhancements of the autofluorescence brightness per protein by up to 120-fold and reach zeptoliter detection volumes, enabling UV autofluorescence correlation spectroscopy (UV-FCS) at high concentrations of several tens of micromolar. We investigate the modulation of fluorescence photokinetic rates and report excellent agreement between the experimental results and numerical simulations. This work expands the applicability of plasmonic nanoantennas to the deep UV range, unlocking the investigation of label-free proteins at physiological concentrations.

Controlling the Surface of Aluminum Nanocrystals: From Aluminum Oxide to Aluminum Fluoride

Aluminum nanocrystals are emerging as a promising alternative to silver and gold for various applications ranging from plasmonic functionalities to photocatalysis and as energetic materials. Such nanocrystals often exhibit an inherent surface oxidation layer, as aluminum is highly reactive. Its controlled removal is challenging but required, as it can hinder the properties of the encaged metal. Herein, two wet-chemical colloidal approaches toward the surface coating of Al nanocrystals, which afford control over the surface chemistry of the nanocrystals and the oxide thickness, are presented. The first approach utilizes oleic acid as a surface ligand by its addition toward the end of the Al nanocrystals synthesis, and the second approach is the post-synthesis treatment of Al nanocrystals with NOBF4 , in a "wet" colloidal-based approach, which is found to etch and fluorinate the surface oxides. As surface chemistry is an important handle for controlling materials' properties, this research paves a path for manipulating Al nanocrystals while promoting their utilization in diverse applications.

Sensitivity of Localized Surface Plasmon Resonance and Acoustic Vibrations to Edge Rounding in Silver Nanocubes

Precise knowledge of the dependence of nano-object properties on their structural characteristics such as their size, shape, composition, or crystallinity, in turn, enables them to be finely characterized using appropriate techniques. Spectrophotometry and inelastic light scattering spectroscopy are noninvasive techniques that are proving highly robust and efficient for characterizing the optical response and vibrational properties of metal nano-objects. Here, we investigate the optical and vibrational properties of monodomain silver nanocubes synthesized by the chemical route, with edge length ranging from around 20 to 58 nm. The synthesized nanocrystals are not perfectly cubic and exhibit rounded edges and corners. This rounding was quantitatively taken into account by assimilating the shape of the nanocubes to superellipsoids. The effect of rounding on their optical response was clearly evidenced by localized surface plasmon resonance spectroscopy and supported by calculations based on the discrete dipole approximation method. The study of their acoustic vibrations by high-resolution low-frequency Raman scattering revealed a substructure of the T2g band, which was analyzed as a function of rounding. The measured frequencies are consistent with the existence of an anticrossing pattern of the two T2g branches. Such an avoided crossing in the T2g modes is clearly evidenced by calculating the vibrational frequencies of silver nanocubes using the Rayleigh-Ritz variational method that accounts for both their real size, shape, and cubic elasticity. These results show that it is possible to assess the rounding of nanocubes, including by means of ensemble spectroscopic measurements on well-calibrated particles.

Branched Aluminum Nanocrystals with Internal Hot Spots: Synthesis and Single-Particle Surface-Enhanced Raman Scattering

Owing to their unique and sustainable surface plasmonic properties, Al nanocrystals have attracted increasing attention for plasmonic-enhanced applications, including single-particle surface-enhanced Raman scattering (SERS). However, whether Al nanocrystals can achieve single-particle SERS is still unknown, mainly due to the synthetic difficulty of Al nanocrystals with internal gaps. Herein, we report a regrowth method for the synthesis of Al nanohexapods with tunable and uniform internal gaps for single-particle SERS with an enhancement factor of up to 1.79 × 10⁸. The uniform branches of the Al nanohexapods can be systematically tuned regarding their dimensions, terminated facets, and internal gaps. The Al nanohexapods generate hot spots concentrated in the internal gaps due to the strong plasmonic coupling between the branches. A single-particle SERS measurement of Al nanohexapods shows strong Raman signals with maximum enhancement factors comparable to that of Au counterparts. The large enhancement factor indicates that Al nanohexapods are good candidates for single-particle SERS.

Nanoparticle Imprint Lithography: From Nanoscale Metrology to Printable Metallic Grids

Large scale and low-cost nanopatterning of materials is of tremendous interest for optoelectronic devices. Nanoimprint lithography has emerged in recent years as a nanofabrication strategy that is high-throughput and has a resolution comparable to that of electron-beam lithography (EBL). It is enabled by pattern replication of an EBL master into polydimethylsiloxane (PDMS), that is then used to pattern a resist for further processing, or a sol-gel that could be calcinated into a solid material. Although the sol-gel chemistry offers a wide spectrum of material compositions, metals are still difficult to achieve. This gap could be bridged by using colloidal nanoparticles as resist, but deep understanding of the key parameters is still lacking. Here, we use supported metallic nanocubes as a model resist to gain fundamental insights into nanoparticle imprinting. We uncover the major role played by the surfactant layer trapped between nanocubes and substrate, and measure its thickness with subnanometer resolution by using gap plasmon spectroscopy as a metrology platform. This enables us to quantify the van der Waals (VDW) interactions responsible for the friction opposing the nanocube motion, and we find that these are almost in quantitative agreement with the Stokes drag acting on the nanocubes during nanoimprint, that is estimated with a simplified fluid mechanics model. These results reveal that a minimum thickness of surfactant is required, acting as a spacer layer mitigating van der Waals forces between nanocubes and the substrate. In the light of these findings we propose a general method for resist preparation to achieve optimal nanoparticle mobility and show the assembly of printable Ag and Au nanocube grids, that could enable the fabrication of low-cost transparent electrodes of high material quality upon nanocube epitaxy.

Atomistic Simulations of the Crystalline-to-Amorphous Transformation of γ-Al2O3 Nanoparticles: Delicate Interplay between Lattice Distortions, Stresses, and Space Charges

The size-dependent phase stability of γ-Al₂O₃ was studied by large-scale molecular dynamics simulations over a wide temperature range from 300 to 900 K. For the γ-Al₂O₃ crystal, a bulk transformation to α-Al₂O₃ by an FCC-to-HCP transition of the O sublattice is still kinetically hindered at 900 K. However, local distortions of the FCC O-sublattice by the formation of quasi-octahedral Al local coordination spheres become thermally activated, as driven by the partial covalency of the Al-O bond. On the contrary, spherical γ-Al₂O₃ nanoparticles (NPs) (with sizes of 6 and 10 nm) undergo a crystalline-to-amorphous transformation at 900 K, which starts at the reconstructed surface and propagates into the core through collective displacements of anions and cations, resulting in the formation of 7- and 8-fold local coordination spheres of Al. In parallel, the reconstructed Al-enriched surface is separated from the stoichiometric core by a diffuse Al-depleted transition region. This compositional heterogeneity creates an imbalance of charges inside the NP, which induces a net attractive Coulombic force that is strong enough to reverse the initial stress state in the NP core from compressive to tensile. These findings disclose the delicate interplay between lattice distortions, stresses, and space-charge regions in oxide nanosystems. A fundamental explanation for the reported expansion of metal-oxide NPs with decreasing size is provided, which has significant implications for, e.g., heterogeneous catalysis, NP sintering, and additive manufacturing of NP-reinforced metal matrix composites.

Photothermal Nanomaterials: A Powerful Light-to-Heat Converter

All forms of energy follow the law of conservation of energy, by which they can be neither created nor destroyed. Light-to-heat conversion as a traditional yet constantly evolving means of converting light into thermal energy has been of enduring appeal to researchers and the public. With the continuous development of advanced nanotechnologies, a variety of photothermal nanomaterials have been endowed with excellent light harvesting and photothermal conversion capabilities for exploring fascinating and prospective applications. Herein we review the latest progresses on photothermal nanomaterials, with a focus on their underlying mechanisms as powerful light-to-heat converters. We present an extensive catalogue of nanostructured photothermal materials, including metallic/semiconductor structures, carbon materials, organic polymers, and two-dimensional materials. The proper material selection and rational structural design for improving the photothermal performance are then discussed. We also provide a representative overview of the latest techniques for probing photothermally generated heat at the nanoscale. We finally review the recent significant developments of photothermal applications and give a brief outlook on the current challenges and future directions of photothermal nanomaterials.

A comparative study of iron nanoflower and nanocube in terms of antibacterial properties

It is known that heavy metal containing nanomaterials can easily prevent the formation of microbial cultures. The emergence of new generation epidemic diseases in the last 2 years has increased the importance of both personal and environmental hygiene. For this reason, in addition to preventing the spread of diseases, studies on alternative disinfectant substances are also carried out. In this study, the antibacterial activity of nanoflower and nanocube, which are easily synthesized and nanoparticle species containing iron, were compared. The antioxidant abilities of new synthesized NF@FeO(OH) and NC@α-Fe2O3 were tested by DPPH scavenging activity assay. The highest DPPH inhibition was achieved with NC@α-Fe2O3 as 71.30% at 200 mg/L. NF@FeO(OH) and NC@α-Fe2O3 demonstrated excellent DNA cleavage ability. The antimicrobial capabilities of NF@FeO(OH) and NC@α-Fe2O3 were analyzed with micro dilution procedure. In 500 mg/L, the antimicrobial activity was 100%. In addition to these, the biofilm inhibition of NF@FeO(OH) and NC@α-Fe2O3 were investigated against S. aureus and P. aeruginosa and it was found that they showed significant antibiofilm inhibition. It is suggested that additional studies can be continued to be developed and used as an antibacterial according to the results of the nanoparticles after various toxicological test systems.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13204-023-02822-5.

Extreme enhancement of optical force via the acoustic graphene plasmon mode

We have investigated the effect of enhanced optical force via the acoustic graphene plasmon (AGP) cavities with the ultra-small mode volumes. The AGP mode can generate stronger field confinement and higher momentum, which could provide giant optical force, and has no polarization preference for the optical source. We have demonstrated that the trapping potential and force applied on polystyrene nanoparticle in the AGP cavities are as high as -13.6 × 10² k_BT/mW and 2.5 nN/mW, respectively. The effect of radius of rounded corners and gap distance of AGP cavities on the optical force has been studied. Compared with an ideal nanocube, nanocube with rounded corners is more in line with the actual situation of the device. These results show that the larger radius of nanocube rounded corners, the smaller trapping potential and force provided by AGP cavities. Our results pave a new idea for the investigation of optical field and optical force via acoustic plasmon mode.

Facet Tunability of Aluminum Nanocrystals

Aluminum nanocrystals (Al NCs) with a well-defined size and shape combine unique plasmonic properties with high earth abundance, potentially ideal for applications where sustainability and cost are important factors. It has recently been shown that single-crystal Al {100} nanocubes can be synthesized by the decomposition of AlH₃ with Tebbe's reagent, a titanium(IV) catalyst with two cyclopentadienyl ligands. By systematically modifying the catalyst molecular structure, control of the NC growth morphology is observed spectroscopically, as the catalyst stabilizes the {100} NC facets. By varying the catalyst concentration, Al NC faceted growth is tunable from {100} faceted nanocubes to {111} faceted octahedra. This study provides direct insight into the role of catalyst molecular structure in controlling Al NC morphology.

Coupled plasmons in aluminum nanoparticle superclusters

Metallic nanoparticles can self-assemble into highly ordered superclusters for potential applications in optics and catalysis. Here, using first-principles quantum mechanical calculations, we investigate plasmon coupling in superclusters made of aluminum nanoparticles. More specifically, we study/compare the plasmon coupling in close-pack FCC (face-centered-cubic) and non-close-pack BCC (body-centered-cubic) superclusters. We demonstrate that the optical properties of these clusters can be fine-tuned with respect to the packing arrangement. As a key result of this work, plasmon coupling is found to be enhanced (diminished) in FCC (BCC) superclusters due to constructive (destructive) plasmon coupling. Our quantum calculations would help in the design of Al-based superclusters beneficial for plasmonics applications.

A Dual Catalyst Strategy for Controlling Aluminum Nanocrystal Growth

The synthesis of Al nanocrystals (Al NCs) is a rapidly expanding field, but there are few strategies for size and morphology control. Here we introduce a dual catalyst approach for the synthesis of Al NCs to control both NC size and shape. By using one catalyst that nucleates growth more rapidly than a second catalyst whose ligands affect NC morphology during growth, one can obtain both size and shape control of the resulting Al NCs. The combination of the two catalysts (1) titanium isopropoxide (TIP), for rapid nucleation, and (2) Tebbe's reagent, for specific facet-promoting growth, yields {100}-faceted Al NCs with tunable diameters between 35 and 65 nm. This dual-catalyst strategy could dramatically expand the possible outcomes for Al NC growth, opening the door to new controlled morphologies and a deeper understanding of earth-abundant plasmonic nanocrystal synthesis.

Thermoplasmonics in Solar Energy Conversion: Materials, Nanostructured Designs, and Applications

The indispensable requirement for sustainable development of human society has forced almost all countries to seek highly efficient and cost-effective ways to harvest and convert solar energy. Though continuous progress has advanced, it remains a daunting challenge to achieve full-spectrum solar absorption and maximize the conversion efficiency of sunlight. Recently, thermoplasmonics has emerged as a promising solution, which involves several beneficial effects including enhanced light absorption and scattering, generation and relaxation of hot carriers, as well as localized/collective heating, offering tremendous opportunities for optimized energy conversion. Besides, all these functionalities can be tailored via elaborated designs of materials and nanostructures. Here, first the fundamental physics governing thermoplasmonics is presented and then the strategies for both material selection and nanostructured designs toward more efficient energy conversion are summarized. Based on this, recent progress in thermoplasmonic applications including solar evaporation, photothermal chemistry, and thermophotovoltaic is reviewed. Finally, the corresponding challenges and prospects are discussed.

Nanocube Epitaxy for the Realization of Printable Monocrystalline Nanophotonic Surfaces

Plasmonic nanoparticles of the highest quality can be obtained via colloidal synthesis at low-cost. Despite the strong potential for integration in nanophotonic devices, the geometry of colloidal plasmonic nanoparticles is mostly limited to that of platonic solids. This is in stark contrast to nanostructures obtained by top-down methods that offer unlimited capability for plasmon resonance engineering, but present poor material quality and have doubtful perspectives for scalability. Here, an approach that combines the best of the two worlds by transforming assemblies of single-crystal gold nanocube building blocks into continuous monocrystalline plasmonic nanostructures with an arbitrary shape, via epitaxy in solution at near ambient temperature, is introduced. Nanocube dimers are used as a nanoreactor model system to investigate the mechanism in operando, revealing competitive redox processes of oxidative etching at the nanocube corners and simultaneous heterogeneous nucleation at their surface, that ensure filling of the sub-nanometer gap in a self-limited manner. Applying this procedure to nanocube arrays assembled in a patterned poly(dimethylsiloxane) (PDMS) substrate, it is able to obtain printable monocrystalline nanoantenna arrays that can be swiftly integrated in devices. This may lead to the implementation of low-cost nanophotonic surfaces of the highest quality in industrial products.

Correlating structure, morphology and properties of metal nanostructures by combining single-particle optical spectroscopy and electron microscopy

The nanoscale morphology of metal nanostructures directly defines their optical, catalytic and electronic properties and even small morphological changes can cause significant property variations. On the one hand, this dependence allows for precisely tuning and exploring properties by shape engineering; next to advanced synthesis protocols, post-synthesis modification through tailored laser modification has become an emerging tool to do so. On the other hand, with this interconnection also comes the quest for detailed structure-property correlation and understanding of laser-induced reshaping processes on the individual nanostructure level beyond ensemble averages. With the development of single-particle (ultrafast) optical spectroscopy techniques and advanced electron microscopy such understanding can in principle be gained at the femtosecond temporal and atomic spatial scale, respectively. However, accessing both on the same individual nanostructure is far from straightforward as it requires the combination of optical spectroscopy and electron microscopy. In this Minireview, we highlight key studies from recent years that performed such correlative measurements on the same individual metal nanostructure either in a consecutive ex situ manner or in situ inside the electron microscope. We demonstrate that such a detailed correlation is critical for revealing the full picture of the structure-property relationship and the physics behind light-induced nanostructure modifications. We put emphasis on the advantages and disadvantages of each methodology as well as on the unique information that one can gain only by correlative studies performed on the same individual nanostructure and end with an outlook on possible further development of this field in the near future.

Localized surface plasmon resonances in a hybrid structure consisting of a mono-layered Al sheet and Ti3C2F MXene

MXenes are a novel class of two-dimensional materials that exhibit unique light-matter interactions. In this work, using quantum-mechanical simulations based on the time dependent density functional theory, we investigate the electronic and optical properties of a hybrid structure consisting of a mono-layered aluminum (Al) sheet and Ti₃C₂F MXene. As a key result of this work, we reveal that the coupling of a mono-layered Al sheet on top of Ti₃C₂F MXene causes interlayer charge transfer accompanied by strong signatures of localized surface plasmon resonances (LSPRs) in the visible region of the electromagnetic spectrum. Our theoretical findings demonstrate a promising strategy to generate LSPRs in MXene-based heterostructures.

Extraordinary Strain Hardening from Dislocation Loops in Defect-Free Al Nanocubes

The complex interaction of crystalline defects leads to strain hardening in bulk metals. Metals with high stacking fault energy (SFE), such as aluminum, tend to have low strain hardening rates due to an inability to form stacking faults and deformation twins. Here, we use in situ scanning electron microscopy (SEM) mechanical compressions to find that colloidally synthesized defect-free 114 nm Al nanocubes combine a high linear strain hardening rate of 4.1 GPa with a high strength of 1.1 GPa. These nanocubes have a 3 nm self-passivating oxide layer that has a large influence on mechanical behavior and the accumulation of dislocation structures. Postcompression transmission electron microcopy (TEM) imaging reveals stable prismatic dislocation loops and the absence of stacking faults. MD simulations relate the formation of dislocation loops and strain hardening to the surface oxide. These results indicate that slight modifications to surface and interfacial properties can induce enormous changes to mechanical properties in high SFE metals.

Al@TiO2 Core-Shell Nanoparticles for Plasmonic Photocatalysis

Plasmon-induced photocatalysis is a topic of rapidly increasing interest, due to its potential for substantially lowering reaction barriers and temperatures and for increasing the selectivity of chemical reactions. Of particular interest for plasmonic photocatalysis are antenna-reactor nanoparticles and nanostructures, which combine the strong light-coupling of plasmonic nanostructures with reactors that enhance chemical specificity. Here, we introduce Al@TiO₂ core-shell nanoparticles, combining earth-abundant Al nanocrystalline cores with TiO₂ layers of tunable thickness. We show that these nanoparticles are active photocatalysts for the hot electron-mediated H₂ dissociation reaction as well as for hot hole-mediated methanol dehydration. The wavelength dependence of the reaction rates suggests that the photocatalytic mechanism is plasmonic hot carrier generation with subsequent transfer of the hot carriers into the TiO₂ layer. The Al@TiO₂ antenna-reactor provides an earth-abundant solution for the future design of visible-light-driven plasmonic photocatalysts.

Thermo-photo catalysis: a whole greater than the sum of its parts

Thermo-photo catalysis, which is the catalysis with the participation of both thermal and photo energies, not only reduces the large energy consumption of thermal catalysis but also addresses the low efficiency of photocatalysis. As a whole greater than the sum of its parts, thermo-photo catalysis has been proven as an effective and promising technology to drive chemical reactions. In this review, we first clarify the definition (beyond photo-thermal catalysis and plasmonic catalysis), classification, and principles of thermo-photo catalysis and then reveal its superiority over individual thermal catalysis and photocatalysis. After elucidating the design principles and strategies toward highly efficient thermo-photo catalytic systems, an ample discussion on the synergetic effects of thermal and photo energies is provided from two perspectives, namely, the promotion of photocatalysis by thermal energy and the promotion of thermal catalysis by photo energy. Subsequently, state-of-the-art techniques applied to explore thermo-photo catalytic mechanisms are reviewed, followed by a summary on the broad applications of thermo-photo catalysis and its energy management toward industrialization. In the end, current challenges and potential research directions related to thermo-photo catalysis are outlined.

Assembly mechanism of surface-functionalized nanocubes

Faceted nanoparticles can be used as building blocks to assemble nanomaterials with exceptional optical and catalytic properties. Recent studies have shown that surface functionalization of such nanoparticles with organic molecules, polymer chains, or DNA can be used to control the separation distance and orientation of particles within their assemblies. In this study, we computationally investigate the mechanism of assembly of nanocubes grafted with short-chain molecules. Our approach involves computing the interaction free energy landscape of a pair of such nanocubes via Monte Carlo simulations and using the Dijkstra algorithm to determine the minimum free energy pathway connecting key states in the landscape. We find that the assembly pathway of nanocubes is very rugged involving multiple energy barriers and metastable states. Analysis of nanocube configurations along the pathway reveals that the assembly mechanism is dominated by sliding motion of nanocubes relative to each other punctuated by their local dissociation at grafting points involving lineal separation and rolling motions. The height of energy barriers between metastable states depends on factors such as the interaction strength and surface roughness of the nanocubes and the steric repulsion from the grafts. These results imply that the observed assembly configuration of nanocubes depends not only on their globally stable minimum free energy state but also on the assembly pathway leading to this state. The free energy landscapes and assembly pathways presented in this study along with the proposed guidelines for engineering such pathways should be useful to researchers aiming to achieve uniform nanostructures from self-assembly of faceted nanoparticles.

Free Vibrations of Anisotropic Nano-Objects with Rounded or Sharp Corners

An extension of the Rayleigh-Ritz variational method to objects with superquadric and superellipsoid shapes and cylinders with cross-sections delimited by a superellipse is presented. It enables the quick calculation of the frequencies and displacements for shapes commonly observed in nano-objects. Original smooth shape variations between objects with plane, convex, and concave faces are presented. The validity of frequently used isotropic approximations for experimentally relevant vibrations is discussed. This extension is expected to facilitate the assignment of features observed with vibrational spectroscopies, in particular in the case of single-nanoparticle measurements.

Plasmonic photothermal catalysis for solar-to-fuel conversion: current status and prospects

Solar-to-fuel conversion through photocatalytic processes is regarded as promising technology with the potential to reduce reliance on dwindling reserves of fossil fuels and to support the sustainable development of our society. However, conventional semiconductor-based photocatalytic systems suffer from unsatisfactory reaction efficiencies due to limited light harvesting abilities. Recent pioneering work from several groups, including ours, has demonstrated that visible and infrared light can be utilized by plasmonic catalysts not only to induce local heating but also to generate energetic hot carriers for initiating surface catalytic reactions and/or modulating the reaction pathways, resulting in synergistically promoted solar-to-fuel conversion efficiencies. In this perspective, we focus primarily on plasmon-mediated catalysis for thermodynamically uphill reactions converting CO2 and/or H2O into value-added products. We first introduce two types of mechanism and their applications by which reactions on plasmonic nanostructures can be initiated: either by photo-induced hot carriers (plasmonic photocatalysis) or by light-excited phonons (photothermal catalysis). Then, we emphasize examples where the hot carriers and phonon modes act in concert to contribute to the reaction (plasmonic photothermal catalysis), with special attention given to the design concepts and reaction mechanisms of the catalysts. We discuss challenges and future opportunities relating to plasmonic photothermal processes, aiming to promote an understanding of underlying mechanisms and provide guidelines for the rational design and construction of plasmonic catalysts for highly efficient solar-to-fuel conversion.

Manipulation of hot electron flow on plasmonic nanodiodes fabricated by nanosphere lithography

Energy conversion to generate hot electrons through the excitation of localized surface plasmon resonance (LSPR) in metallic nanostructures is an emerging strategy in photovoltaics and photocatalytic devices. Important factors for surface plasmon and hot electron generation are the size, shape, and materials of plasmonic metal nanostructures, which affect LSPR excitation, absorbance, and hot electron collection. Here, we fabricated the ordered structure of metal-semiconductor plasmonic nanodiodes using nanosphere lithography and reactive ion etching. Two types of hole-shaped plasmonic nanostructures with the hole diameter of 280 and 115 nm were fabricated on Au/TiO₂Schottky diodes. We show that hot electron flow can be manipulated by changing the size of plasmonic nanostructures on the Schottky diode. We show that the short-circuit photocurrent changes and the incident photon-to-electron conversion efficiency results exhibit the peak shift depending on the structures. These phenomena are explicitly observed with finite difference time domain simulations. The capability of tuning the morphology of plasmonic nanostructure on the Schottky diode can give rise to new possibilities in controlling hot electron generation and developing novel hot-electron-based energy conversion devices.

Nanocrystallites, adsorption, surface tension, and Wulff rule

Chemisorption on the surface of metal nanocrystallites (NCs) sometimes induces their reshaping. This interesting phenomenon was observed experimentally in various systems. Related theoretical studies imply that it can be described using the Wulff rule with the surface tension dependent on the coverage of the NC facets by adsorbate. There is, however, no agreement as to how the surface tension should be calculated in this case. Relying on the laws of statistical physics, I clarify the situation in this area in general and also in the framework of the mean-field approximation in three situations: (i) with adsorption-desorption equilibrium, (ii) with a fixed amount of adsorbate at a NC, and (iii) with a fixed amount of adsorbate at facets of a NC. Under these conditions, the surface tension is shown to be described by the same expressions.

Exploiting Colloidal Metamaterials for Achieving Unnatural Optical Refractions

The scaling down of meta-atoms or metamolecules (collectively denoted as metaunits) is a long-lasting issue from the time when the concept of metamaterials was first suggested. According to the effective medium theory, which is the foundational concept of metamaterials, the structural sizes of meta-units should be much smaller than the working wavelengths (e.g., << 1/5 wavelength). At relatively low frequency regimes (e.g., microwave and terahertz), the conventional monolithic lithography can readily address the materialization of metamaterials. However, it is still challenging to fabricate optical metamaterials (metamaterials working at optical frequencies such as the visible and near-infrared regimes) through the lithographic approaches. This serves as the rationale for using colloidal self-assembly as a strategy for the realization of optical metamaterials. Colloidal self-assembly can address various critical issues associated with the materialization of optical metamaterials, such as achieving nanogaps over a large area, increasing true 3D structural complexities, and cost-effective processing, which all are difficult to attain through monolithic lithography. Nevertheless, colloidal self-assembly is still a toolset underutilized by optical engineers. Here, the design principle of the colloidally self-assembled optical metamaterials exhibiting unnatural refractions, the practical challenge of relevant experiments, and the future opportunities are critically reviewed.

Morphology-Dependent Reactivity of a Plasmonic Photocatalyst

The shape of a plasmonic nanoparticle strongly controls its light-matter interaction, which in turn affects how specific morphologies may be used in applications such as sensing, photodetection, and active pixel displays. Here, we show that particle shape also controls plasmonic photocatalytic activity. Three different Al nanocrystal morphologies, octopods, nanocubes, and nanocrystals, all with very similar plasmon resonance frequencies, were used as photocatalysts for the H₂ dissociation reaction. We observe widely varying reaction rates for the three different morphologies. Octopods show a 10 times higher reaction rate than nanocrystals and a 5 times higher rate than nanocubes, with lower apparent activation energies than either nanocubes or nanocrystals by 45% and 49%, respectively. A theoretical model of hot electron direct transfer from photoexcited Al nanoparticles to H₂ molecules is consistent with this observed morphological dependence. This research strongly suggests that nanoparticle geometry, in addition to plasmon resonance energy, is a critical factor in plasmonic photocatalyst design.

Shining Light on Aluminum Nanoparticle Synthesis

ConspectusAluminum in its nanostructured form is generating increasing interest because of its light-harvesting properties, achieved by excitation of its localized surface plasmon resonance. Compared to traditional plasmonic materials, the coinage metals Au and Ag, Al is far more earth-abundant and, therefore, more suitable for large-area applications or where cost may be an important factor. Its optical properties are far more flexible than either Au or Ag, supporting plasmon resonances that range from UV wavelengths, through the visible regime, and into the infrared region of the spectrum. However, the chemical synthesis of Al nanocrystals (NCs) of controlled size and shape has historically lagged far behind that of Au and Ag. This is partially due to the high reactivity of Al precursors, which react readily with O₂, H₂O, and many reagents used in traditional NC syntheses. The first chemical synthesis of Al NCs was demonstrated by Haber and Buhro in 1998, decomposing AlH₃ using titanium isopropoxide (TIP), with a number of subsequent reports refining this protocol. The role of a catalyst in Al NC synthesis is, we believe, unique to this synthetic approach. In 2015, the first synthesis of size controlled Al NCs was published by our group. Since then, we have significantly advanced Al NC synthesis, postsynthetic modifications, and applications of Al nanoparticles (NPs)-NCs with additional surface modifications-in chemical sensing and photocatalysis. Colloidal Al synthesis has its unique challenges, differing markedly from the far more familiar Au and Ag syntheses, which currently appears to present a de facto barrier to broader research activity in this field.The goal of this Account is to highlight developments in controlled synthesis of Al NCs and applications of Al NPs over the last five years. We outline techniques for successful Al NC synthesis and address some of the problems that may be encountered in this synthesis. A mechanistic understanding of AlH₃ decomposition using TIP has been developed, while new directions have been discovered for synthetic control. Facet-binding ligands, alternate Al precursors, new titanium-based reduction catalysts, even solvent composition have all been shown to control reaction products while also opening doors to future developments. A variety of postsynthetic modifications to the Al NC native oxide surface, including polymer, MOF, and transition metal island coatings have been demonstrated for applications in molecular sensing and photocatalysis. In this Account, we hope to convey that Al synthesis is more accessible than generally perceived and to encourage new synthetic development based on underlying mechanisms controlling size and shape. High selectivity in particle faceting and twinning, implementation of seeded growth principles for monodisperse samples, and the demonstration of new, practical applications of Al nanoparticles remain primary challenges in the field. As Al nanoparticle synthesis is refined and new applications emerge, colloidal Al will become an accessible and low-cost plasmonic nanomaterial complementary to Au and Ag.

Thinking outside the shell: novel sensors designed from plasmon-enhanced fluorescent concentric nanoparticles

The alteration of photophysical properties of fluorophores in the vicinity of a metallic nanostructure, a phenomenon termed plasmon- or metal-enhanced fluorescence (MEF), has been investigated extensively and used in a variety of proof-of-concept demonstrations over the years. A particularly active area of development in this regard has been the design of nanostructures where fluorophore and metallic core are held in a stable geometry that imparts improved luminosity and photostability to a plethora of organic fluorophores. This minireview presents an overview of MEF-based concentric core-shell sensors developed in the past few years. These architectures expand the range of applications of nanoparticles (NPs) beyond the uses possible with fluorescent molecules. Design aspects that are being described include the influence of the nanocomposite structure on MEF, notably the dependence of fluorescence intensity and lifetime on the distance to the plasmonic core. The chemical composition of nanocomposites as a design feature is also discussed, taking as an example the use of non-noble plasmonic metals such as indium as core materials to enhance multiple fluorophores throughout the UV-Vis range and tune the sensitivity of halide-sensing fluorophores operating on the principle of collisional quenching. Finally, the paper describes how various solid substrates can be functionalized with MEF-based nanosensors to bestow them with intense and photostable pH-sensitive properties for use in fields such as medical therapy and diagnostics, dentistry, biochemistry and microfluidics.

Aluminum Nanocrystals Grow into Distinct Branched Aluminum Nanowire Morphologies

Plasmonic nanowires (NWs) have generated great interest in their applications in nanophotonics and nanotechnology. Here we report the synthesis of Al nanocrystals (NCs) with controlled morphologies that range from nanospheres to branched NW and NW bundles. This is accomplished by catalyzing the pyrolysis of triisobutyl aluminum (TIBA) with Tebbe's reagent, a titanium(III) catalyst with two cyclopentadienyl ligands. The ratio of TIBA to Tebbe's reagent is critical in determining the morphology of the resulting Al NC. The branched Al NWs grow in their ⟨100⟩ directions and are formed by oriented attachment of isotropic Al NCs on their {100} facets. Branched NWs are strongly absorptive from the UV to the mid-IR, with longitudinal dipolar, higher-order, and transverse plasmons, all contributing to their broadband response. This rapid Al NW synthesis enables the expanded use of Al for plasmonic and nanophotonic applications in the ultraviolet, visible, and infrared regions of the spectrum.

Controlling Optically Driven Atomic Migration Using Crystal-Facet Control in Plasmonic Nanocavities

Plasmonic nanoconstructs are widely exploited to confine light for applications ranging from quantum emitters to medical imaging and biosensing. However, accessing extreme near-field confinement using the surfaces of metallic nanoparticles often induces permanent structural changes from light, even at low intensities. Here, we report a robust and simple technique to exploit crystal facets and their atomic boundaries to prevent the hopping of atoms along and between facet planes. Avoiding X-ray or electron microscopy techniques that perturb these atomic restructurings, we use elastic and inelastic light scattering to resolve the influence of crystal habit. A clear increase in stability is found for {100} facets with steep inter-facet angles, compared to multiple atomic steps and shallow facet curvature on spherical nanoparticles. Avoiding atomic hopping allows Raman scattering on molecules with low Raman cross-section while circumventing effects of charging and adatom binding, even over long measurement times. These nanoconstructs allow the optical probing of dynamic reconstruction in nanoscale surface science, photocatalysis, and molecular electronics.

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.

Spatially Broadband Coupled-Surface Plasmon Wave Assisted Transmission Effect in Azo-Dye-Doped Liquid Crystal Cell

Active tuning on a plasmonic structure is discussed in this report. We examined the transient transmission effects of an azo-dye-doped liquid crystal cell on a metallic surface grating. The transition between isotropic and nematic phases in liquid crystal generated micro-domains was shown to induce the dynamic scattering of light from a He-Ne laser, thereby allowing transmission through a non-transparent aluminum film overlaying a dielectric grating. Various grating pitches were tested in terms of transmission effects. The patterned gratings include stripe ones and circular forms. Our results indicate that surface plasmon polariton waves are involved in the transmission process. We also demonstrated how momentum diagrams of gratings and Surface Plasmon Polariton (SPP) modes combined with Mie scattering effects could explain the broadband coupling phenomenon. This noteworthy transition process could be applied to the development of spatially broadband surface plasmon polariton coupling devices.

Quantum Leap from Gold and Silver to Aluminum Nanoplasmonics for Enhanced Biomedical Applications

Nanotechnology has been used in many biosensing and medical applications, in the form of noble metal (gold and silver) nanoparticles and nanostructured substrates. However, the translational clinical and industrial applications still need improvements of the efficiency, selectivity, cost, toxicity, reproducibility, and morphological control at the nanoscale level. In this review, we highlight the recent progress that has been made in the replacement of expensive gold and silver metals with the less expensive aluminum. In addition to low cost, other advantages of the aluminum plasmonic nanostructures include a broad spectral range from deep UV to near IR, providing additional signal enhancement and treatment mechanisms. New synergistic treatments of bacterial infections, cancer, and coronaviruses are envisioned. Coupling with gain media and quantum optical effects improve the performance of the aluminum nanostructures beyond gold and silver.

Site-Selective Nanoreactor Deposition on Photocatalytic Al Nanocubes

Photoactivation of catalytic materials through plasmon-coupled energy transfer has created new possibilities for expanding the scope of light-driven heterogeneous catalysis. Here we present a nanoengineered plasmonic photocatalyst consisting of catalytic Pd islands preferentially grown on vertices of Al nanocubes. The regioselective Pd deposition on Al nanocubes does not rely on complex surface ligands, in contrast to site-specific transition-metal deposition on gold nanoparticles. We show that the strong local field enhancement on the sharp nanocube vertices provides a mechanism for efficient coupling of the plasmonic Al antenna to adjacent Pd nanoparticles. A substantial increase in photocatalytic H₂ dissociation on Pd-bound Al nanocubes relative to pristine Al nanocubes can be observed, incentivizing further engineering of heterometallic antenna-reactor photocatalysts. Controlled growth of catalytic materials on plasmonic hot spots can result in more efficient use of the localized surface plasmon energy for photocatalysis, while minimizing the amount and cost of precious transition-metal catalysts.

Towards advanced wound regeneration

Wound management is a major contributor towards the economic burden placed upon the national health service (NHS), serving as an important target for the development of advanced therapeutic interventions. The economic expenditure of wound care for the NHS exceeds £5 billion per annum, thus presenting a significant opportunity for the introduction of alternative treatments in regards to their approach in tackling the ever increasing prevalence of wound management associated problems. As most wounds typically fall under the acute or chronic category, it is therefore necessary to design a therapeutic intervention capable of effectively resolving the pathologies associated with each problem. Such an intervention should be of increased economic viability and therapeutic effectiveness when compared to standardized treatments, thus helping to alleviate the financial burden imposed upon the NHS. The purpose of this review is to critically analyse the various aspects associated with wound management, detailing the fundamental concepts of dermal regeneration, whilst also providing an evaluation of the different materials and methods that can be utilised to achieve maximal wound regeneration. The primary aspects of this review revolve around the three concepts of antibacterial methodology, enhancement of dermal regeneration and the utilisation of a carrier medium to facilitate the regenerative process. Each aspect is explored, conveying its justifications as a target for dermal regeneration, whilst offering various solutions towards the fulfilment of a therapeutic design that is both effective and financially feasible.

Localized surface plasmon resonances of a metal nanoring

Using the linear combination of atomic orbitals real-time-propagation rt-TDDFT technique and transition contribution maps, we study the optical and plasmonic features of a metal nanoring made up of sodium atoms.

Decoration of plasmonic Mg nanoparticles by partial galvanic replacement

Plasmonic structures have attracted much interest in science and engineering disciplines, exploring a myriad of potential applications owing to their strong light-matter interactions. Recently, the plasmonic concentration of energy in subwavelength volumes has been used to initiate chemical reactions, for instance by combining plasmonic materials with catalytic metals. In this work, we demonstrate that plasmonic nanoparticles of earth-abundant Mg can undergo galvanic replacement in a nonaqueous solvent to produce decorated structures. This method yields bimetallic architectures where partially oxidized 200-300 nm Mg nanoplates and nanorods support many smaller Au, Ag, Pd, or Fe nanoparticles, with potential for a stepwise process introducing multiple decoration compositions on a single Mg particle. We investigated this mechanism by electron-beam imaging and local composition mapping with energy-dispersive X-ray spectroscopy as well as, at the ensemble level, by inductively coupled plasma mass spectrometry. High-resolution scanning transmission electron microscopy further supported the bimetallic nature of the particles and provided details of the interface geometry, which includes a Mg oxide separation layer between Mg and the other metal. Depending on the composition of the metallic decorations, strong plasmonic optical signals characteristic of plasmon resonances were observed in the bulk with ultraviolet-visible spectrometry and at the single particle level with darkfield scattering. These novel bimetallic and multimetallic designs open up an exciting array of applications where one or multiple plasmonic structures could interact in the near-field of earth-abundant Mg and couple with catalytic nanoparticles for applications in sensing and plasmon-assisted catalysis.