Threading plasmonic nanoparticle strings with light

Thermally Stable Oxide-Capsulated Metal Nanoparticles Structure for Strong Metal-Support Interaction via Ultrafast Laser Plasmonic Nanowelding

Strong metal-support interaction (SMSI) has drawn much attention in heterogeneous catalysts due to its stable and excellent catalytic efficiency. However, construction of high-performance oxide-capsulated metal nanostructures meets great challenge in materials thermodynamic compatibility. In this work, dynamically controlled formation of oxide-capsulated metal nanoparticles (NPs) structures is demonstrated by ultrafast laser plasmonic nanowelding. Under the strong localized electromagnetic field interaction, metal (Au) NPs are dragged by an optical force toward oxide NPs (TiO2). Intense energy is simultaneously injected into this heterojunction area, where TiO2 is precisely ablated. With the embedding of metal into oxide, optical force on Au gradually turned from attractive to repulsive due to the varied metal-dielectric environment. Meanwhile, local ablated oxides are redeposited on Au NP. Upon the whole coverage of metal NP, the implantation behavior of metal NP is stopped, resulting in a controlled metal-oxide eccentric structure with capsulated oxide layer thickness ≈0.72-1.30 nm. These oxide-capsulated metal NPs structures can preserve their configurations even after thermal annealing in air at 600 °C for 10 min. This ultrafast laser plasmonic nanowelding can also extend to oxide-capsulated metal nanostructure fabrication with broad materials combinations (e.g., Au/ZnO, Au/MgO, etc.), which shows great potential in designing/constructing nanoscale high-performance catalysts.

Welded Gold Nanoparticle Assemblies Defined Plasmonic Coupling

Nanoparticle assemblies with interparticle ohmic contacts are crucial for nanodevice fabrication. Despite tremendous progress in DNA-programmable nanoparticle assemblies, seamlessly welding discrete components into welded continuous three-dimensional (3D) configurations remains challenging. Here, we introduce a single-stranded DNA-encoded strategy to customize welded metal nanostructures with tunable morphologies and plasmonic properties. We demonstrate the precise welding of gold nanoparticle assemblies into continuous metal nanostructures with interparticle ohmic contacts through chemical welding in solution. We find that the welded gold nanoparticle assemblies show a consistent morphology with welded efficiency over 90%, such as the rod-like, triangular, and tetrahedral metal nanostructures. Next, we show the versatility of this strategy by welding gold nanoparticle assemblies of varied sizes and shapes. Furthermore, the experiment and simulation show that the welded gold nanoparticle assemblies exhibit defined plasmonic coupling. This single-stranded DNA encoded welding system may provide a new route for accurately building functional plasmonic nanomaterials and devices.

Unconventional Breathing Currents Far beyond the Quantum Tunneling Distances in Large-Gapped Nanoplasmonic Systems

Localized surface plasmon resonance (LSPR) in plasmonic nanoparticles propels the field of plasmo-electronics, holding promise for transformative optoelectronic devices through efficient light-to-current conversion. Plasmonic excitations strongly influence the charge distribution within nanoparticles, giving rise to electromagnetic fields that can significantly impact the macroscopic charge flows within the nanoparticle housing material. In this study, we present evidence of ultralow, unconventional breathing currents resulting from dynamic irradiance interactions between widely separated nanoparticles, extending far beyond conventional electron (quantum) tunneling distances. We develop an electric analogue model and derive an empirical expression to elucidate the generation of these unconventional breathing currents in cascaded nanoplasmonic systems under irradiance modulation. This technique and theoretical model have significant potential for applications requiring a deeper understanding of current dynamics, particularly on large nanostructured surfaces relevant to photocatalysis, energy harvesting, sensing, imaging, and the development of future photonic devices.

Assembly and Healing: Capacitive and Conductive Plasmonic Interfacing via a Unified and Clean Wet Chemistry Route

Solution-based nanoparticle assembly represents a highly promising way to build functional metastructures based on a wealth of synthetic nanomaterial building blocks with well-controlled morphology and crystallinity. In particular, the involvement of DNA molecular programming in these bottom-up processes gradually helps the ambitious goal of customizable chemical nanofabrication. However, a fundamental challenge is to realize strong interunit coupling in an assembly toward emerging functions and applications. Herein, we present a unified and clean strategy to address this critical issue based on a H2O2-redox-driven "assembly and healing" process. This facile solution route is able to realize both capacitively coupled and conductively bridged colloidal boundaries, simply switchable by the reaction temperature, toward bottom-up nanoplasmonic engineering. In particular, such a "green" process does not cause surface contamination of nanoparticles by exogenous active metal ions or strongly passivating ligands, which, if it occurs, could obscure the intrinsic properties of as-formed structures. Accordingly, previously raised questions regarding the activities of strongly coupled plasmonic structures are clarified. The reported process is adaptable to DNA nanotechnology, offering molecular programmability of interparticle charge conductance. This work represents a new generation of methods to make strongly coupled nanoassemblies, offering great opportunities for functional colloidal technology and even metal self-healing.

Three-dimensional building of anisotropic gold nanoparticles under confinement in submicron capsules

The low collision rate and contact time of gold nanoparticles (NPs) in solution afford a low welding probability, which hinders their welding structure, orientation, and dimension. Encapsulated anisotropic NPs, gold nanotriangles (AuNTs), were successfully assembled into a three-dimensional structure inside a permeable silica nanocapsule under light illumination to generate localized surface plasmon resonance (LSPR). AuNTs were trapped in the permeable silica nanocapsules and diffused in the nanospace because of copolymer release, which increased the contact probability of AuNTs and promoted the three-dimensional building of AuNTs. Electron energy loss mapping simulations revealed that the obtained three-dimensional AuNT structure exhibited spatially separated multiple LSPR modes with different energies of incident light, which are photophysically attractive beyond the facet-selective chemical growth of NPs, and postmodification for anchoring substances with site-selective attachment to the obtained structure will be applicable to expand the sensing design and class of substances for sensing.

Physicochemical properties of ultrasmall colloidal silver nanoparticles: an experimental and computational approach

Silver nanoparticles (AgNPs) exhibit very interesting properties that have been employed in several kinds of applications spanning from antibacterial activity to plasmon-polaritons generation. Nanoparticle size strongly influences these applications. However, the characterization of ultrasmall AgNPs (particle diameter < 10 nm) encompassing different aspects such as average size, polydispersion, shape (anisotropy), concentration, and density remains a challenging task. To address these challenges, we combined TEM measurements with a computational framework based on Mie-Gans theory. This allowed us to describe the aforementioned AgNP features accurately. The synthesis of AgNPs in an aqueous medium involved the use of silver nitrate as a chemical precursor and sodium borohydride as a reducing agent, with polyvinylpyrrolidone acting as a stabilizing agent. Our outcomes showed that increasing the concentration of the precursor and reducing agent with a fixed 1 : 2 molar ratio tends to yield ultrasmall AgNPs with low to moderate polydispersion, a nearly spherical shape (low anisotropy), concentration in the nanomolar range and density close to silver bulk. Also, we established an analytical expression that correlates the extinction molar absorptivity to AgNP size considering the nanoparticle shape. Notably, the computational framework proved to be highly effective in extracting crucial information about the AgNPs from UV-vis spectroscopy data. In conclusion, our study sheds light on the unique properties of ultrasmall AgNPs and presents a comprehensive approach for properly characterizing these nanoparticles, paving the way for further advancements in their applications.

Influence of plasmons on the luminescence properties of solvatochromic merocyanine dyes with different solvatochromism

The effect of localized surface plasmon resonance (LSPR) of a system consisting of a highly dipolar merocyanine dye and a silver nanoparticle (NP) was studied experimentally and theoretically. A theoretical model for estimating the fluorescence quantum yield (φfl) using quantum chemical calculations of intramolecular and intermolecular electronic transition rate constants was developed. Calculations show that the main deactivation channels of the lowest excited singlet state of the studied merocyanines are internal conversion (kIC(S1 → S0)) and fluorescence (kr(S1 → S0)). The intersystem-crossing transition has a low probability due to the large energy difference between the singlet and triplet levels. In the presence of plasmonic NPs, the fluorescence quantum yield is increased by a factor of two according to both experiment and computations. The calculated values of φfl, when considering changes in kr(S1 → S0) and the energy-transfer rate constant (ktransfer) from the dye to the NP was also twice as large at distances of 6-8 nm between the NP and the dye molecule. We also found that the LSPR effect can be increased or decreased depending on the value of the dielectric constant (εm) of the environment.

Ultrafast Laser Plasmonic Fabrication of Nanocrystals by Molecule Modulation for Photoresponse Multifunctional Structures

Nanotechnology has attracted wide research attention in constructing functional devices, including integrated circuits, transparent electrodes, and flexible actuators. Bottom-up fabrication is an important approach for functional structure manufacture, however, the controllable fabrication of complex architectures for practical applications has long been a challenge. Here, a novel strategy of laser plasmonic fabrication based on glue molecule modulation is proposed that can assemble metal nanocrystals into interconnected pattern networks. The plasmonic response of nanocrystals is adjustable with molecule modulation, which is a benefit for the effective formation of laser-induced localized oscillating electrons. The further decomposition of molecules and the movement of nanocrystal surface atoms can achieve the coalescence of assembled nanocrystals. It demonstrates that complex architectures can be controllably constructed by molecule level modulation. Through molecule-assisted laser plasmonic fabrication, the functional nanocrystals with enhanced photothermal capacity can be used for information encryption and soft machinery. This work expands the knowledge of bottom-up fabrication and provides a method for designing functional nanocrystals for a wide range of applications.

Plasmonic Cyclic Au Nanosphere Hexamers

Rational design of plasmonic colloidal assemblies via bottom-up synthesis is challenging but would show unprecedented optical properties that strongly relate to the assembly's shape and spatial arrangement. Herein, the synthesis of plasmonic cyclic Au nanosphere hexamers (PCHs) is reported, wherein six Au nanospheres (Au NSs) are connected via thin metal ligaments. By tuning Au reduction, six dangling Au NSs are interconnected with a core hexagon nanoplate (NPL). Then, Pt atoms are selectively deposited on the edges of the spheres. After etching of the core, necklace-like nanostructures of Pt framework are obtained. Deposition of Au is followed, leading to PCHs in high yield (≈90%). Notably, PCHs exhibit the combinatorial plasmonic characteristics of individual Au NSs and the in-plane coupling of the six linked Au NSs. They yield highly uniform, reproducible, and polarization-independent single-particle surface-enhanced Raman scattering signals, which are attributed to the 2-dimensional isotropic alignment of the Au NSs. Those are applied to a SERS-based immunoassay as quantitative and qualitative single particle SERS nanoprobes. This assay shows a low limit-of-detection, down to 100 pm, which is orders of magnitude lower than those based on Au NSs, and one order of magnitude lower than an assay using analogous particles of smooth Au nanorings.

Nanoscale thermoplasmonic welding

Establishing direct, close contact between individual nano-objects is crucial to fabricating hierarchical and multifunctional nanostructures. Nanowelding is a technical prerequisite for successfully manufacturing such structures. In this paper, we review the nanoscale thermoplasmonic welding with a focus on its physical mechanisms, key influencing factor, and emerging applications. The basic mechanisms are firstly described from the photothermal conversion to self-limited heating physics. Key aspects related to the welding process including material scrutinization, nanoparticle geometric and spatial configuration, heating scheme and performance characterization are then discussed in terms of the distinctive properties of plasmonic welding. Based on the characteristics of high precision and flexible platform of thermoplasmonic welding, the potential applications are further highlighted from electronics and optics to additive manufacturing. Finally, the future challenges and prospects are outlined for future prospects of this dynamic field. This work summarizes these innovative concepts and works on thermoplasmonic welding, which is significant to establish a common link between nanoscale welding and additive manufacturing communities.

Surface-Plasmon-Assisted Growth, Reshaping and Transformation of Nanomaterials

Excitation of surface plasmon resonance of metal nanostructures is a promising way to break the limit of optical diffraction and to achieve a great enhancement of the local electromagnetic field by the confinement of optical field at the nanoscale. Meanwhile, the relaxation of collective oscillation of electrons will promote the generation of hot carrier and localized thermal effects. The enhanced electromagnetic field, hot carriers and localized thermal effects play an important role in spectral enhancement, biomedicine and catalysis of chemical reactions. In this review, we focus on surface-plasmon-assisted nanomaterial reshaping, growth and transformation. Firstly, the mechanisms of surface-plasmon-modulated chemical reactions are discussed. This is followed by a discussion of recent advances on plasmon-assisted self-reshaping, growth and etching of plasmonic nanostructures. Then, we discuss plasmon-assisted growth/deposition of non-plasmonic nanostructures and transformation of luminescent nanocrystal. Finally, we present our views on the current status and perspectives on the future of the field. We believe that this review will promote the development of surface plasmon in the regulation of nanomaterials.

Plasmonic Superstructure Arrays Fabricated by Laser Near-Field Reduction for Wide-Range SERS Analysis of Fluorescent Materials

Surface-enhanced Raman scattering (SERS) enables trace-detection for biosensing and environmental monitoring. Optimized enhancement of SERS can be achieved when the energy of the localized surface plasmon resonance (LSPR) is close to the energy of the Raman excitation wavelength. The LSPR can be tuned using a plasmonic superstructure array with controlled periods. In this paper, we develop a new technique based on laser near-field reduction to fabricate a superstructure array, which provides distinct features in the formation of periodic structures with hollow nanoclusters and flexible control of the LSPR in fewer steps than current techniques. Fabrication involves irradiation of a continuous wave laser or femtosecond laser onto a monolayer of self-assembled silica microspheres to grow silver nanoparticles along the silica microsphere surfaces by laser near-field reduction. The LSPR of superstructure array can be flexibly tuned to match the Raman excitation wavelengths from the visible to the infrared regions using different diameters of silica microspheres. The unique nanostructure formed can contribute to an increase in the sensitivity of SERS sensing. The fabricated superstructure array thus offers superior characteristics for the quantitative analysis of fluorescent perfluorooctanoic acid with a wide detection range from 11 ppb to 400 ppm.

Exploring Synergistic Effect of Capillary Force and Electrostatic Attraction towards the SERS Sensitivity of D-SERS

Constructing well defined nanostructure is the primary step realizing the SERS detection with high sensitivity. The solid substrate prepared by liquid-liquid interface self-assembly has been demonstrated with the controllable and...

Ultrafast, Non-Equilibrium and Transient Heating and Sintering of Nanocrystals for Nanoscale Metal Printing

The carrier excitation, relaxation, energy transport, and conversion processes during light-nanocrystal (NC) interactions have been intensively investigated for applications in optoelectronics, photocatalysis, and photovoltaics. However, there are limited studies on the non-equilibrium heating under relatively high laser excitation that leads to NCs sintering. Here, the authors use femtosecond laser two-pulse correlation and in-situ optical transmission probing to investigate the non-equilibrium heating of NCs and transient sintering dynamics. First, a two-pulse correlation study reveals that the sintering rate strongly increases when the two heating laser pulses are temporally separated by <10 ps. Second, the sintering rate is found to increase nonlinearly with laser fluence when heating with ≈700 fs laser pulses. By three-temperature modeling, the NC sintering mechanism mediated by electron induced ligand transformation is suggested. The ultrafast and non-equilibrium process facilitates sintering in dry (spin-coated) and wet (solvent suspended) environments. The nonlinear dependence of sintering rate on laser fluence is exploited to print sub-diffraction-limited features in NC suspension. The smallest feature printed is ≈200 nm, which is ≈¼ of the laser wavelength. These findings provide a new perspective toward nanomanufacturing development based on probing and engineering ultrafast transport phenomena in functional NCs.

Atomic-level ablation of Au@Ag NRs using ultrafast laser excitation

Metallic nanorods (NRs) are an important class of materials with widespread applications because of their appealing tunable plasmon resonances, high photothermal conversion efficiency, and chemical stability. It is essential to control the shape and atomic structures of metallic NRs for practical applications. Laser processing of metallic NRs relying on light-matter interactions provides many opportunities. However, the atomic-level fabrication of NRs remains a challenge, and the understanding of laser-induced ablation is still limited. Here, we proposed the atomic-level ablation of Au@Ag NRs using ultrafast laser excitation, which suggests that the near-field effect plays a key role in comparison with thermal evaporation. Through ultrafast laser pulse excitation, abundant atomic steps are fabricated in Au@Ag NRs, which can enhance the surface activity. We suggest that this study highlights the role of the laser near-field effect and also provides a facile strategy to tailor the external shape of metallic NRs at the atomic level, opening a pathway to design metallic NRs for energy and environmental applications.

Nanowelding in Whole-Lifetime Bottom-Up Manufacturing: From Assembly to Service

The continuous miniaturization of microelectronics is pushing the transformation of nanomanufacturing modes from top-down to bottom-up. Bottom-up manufacturing is essentially the way of assembling nanostructures from atoms, clusters, quantum dots, etc. The assembly process relies on nanowelding which also existed in the synthesis process of nanostructures, construction and repair of nanonetworks, interconnects, integrated circuits, and nanodevices. First, many kinds of novel nanomaterials and nanostructures from 0D to 1D, and even 2D are synthesized by nanowelding. Second, the connection of nanostructures and interfaces between metal/semiconductor-metal/semiconductor is realized through low-temperature heat-assisted nanowelding, mechanical-assisted nanowelding, or cold welding. Finally, 2D and 3D interconnects, flexible transparent electrodes, integrated circuits, and nanodevices are constructed, functioned, or self-healed by nanowelding. All of the three nanomanufacturing stages follow the rule of "oriented attachment" mechanisms. Thus, the whole-lifetime bottom-up manufacturing process from the synthesis and connection of nanostructures to the construction and service of nanodevices can be organically integrated by nanowelding. The authors hope this review can bring some new perspective in future semiconductor industrialization development in the expansion of multi-material systems, technology pathway for the refined design, controlled synthesis and in situ characterization of complex nanostructures, and the strategies to develop and repair novel nanodevices in service.

Photo-Responsive Self-Assembly of Plasmonic Magnetic Janus Nanoparticles

Stimuli-responsive self-assembly of nanoparticles is a versatile approach for the bottom-up fabrication of adaptive and functional nanomaterials. For this purpose, anisotropic building blocks are of particular importance due to the unique shapes and structures that can be obtained upon self-assembly. Here, we demonstrate the photo-responsive self-assembly of plasmonic magnetic "dumbbell" Janus nanoparticles (Au-Fe₃O₄) via the host-guest interaction of the supramolecular host cyclodextrin and the molecular photoswitch arylazopyrazole. We developed efficient ligand exchange procedures that enable the introduction of functional ligands, respectively, to the surface of the gold or magnetite core of the dumbbell. Our results indicate that distinct nanoparticle superstructures arise in aqueous solutions if nanoparticle aggregation is crosslinker-induced or self-induced and that the reversible formation and fragmentation of the superstructures can be modulated with light.

Ultrafast Laser-Induced Atomic Structure Transformation of Au Nanoparticles with Improved Surface Activity

Metallic nanoparticles (NPs) play a significant role in nanocatalytic systems, which are important for clean energy conversion, storage, and utilization. Laser fabrication of metallic NPs relying on light-matter interactions provides many opportunities. It is essential to study the atomic structure transformation of nonactive monocrystalline metallic NPs for practical applications. The high-density stacking faults were fabricated in monocrystalline Au NPs through tuning the ultrafast laser-induced relaxation dynamics, and the thermal and dynamic stress effects on the atomic structure transformation were revealed. The atomic structure transformation mainly arises from the thermal effect, and the dynamic stress distribution induced by local energy deposition gives rise to the generation of stacking faults. Au NPs with abundant stacking faults show enhanced surface activity owing to their low coordination number. We suggest that this work expands the knowledge of laser-metallic nanomaterial interactions and provides a method for designing metallic NPs for a wide range of applications.

Surface-enhanced Raman spectroscopy for bioanalysis and diagnosis

In recent years, bioanalytical surface-enhanced Raman spectroscopy (SERS) has blossomed into a fast-growing research area. Owing to its high sensitivity and outstanding multiplexing ability, SERS is an effective analytical technique that has excellent potential in bioanalysis and diagnosis, as demonstrated by its increasing applications in vivo. SERS allows the rapid detection of molecular species based on direct and indirect strategies. Because it benefits from the tunable surface properties of nanostructures, it finds a broad range of applications with clinical relevance, such as biological sensing, drug delivery and live cell imaging assays. Of particular interest are early-stage-cancer detection and the fast detection of pathogens. Here, we present a comprehensive survey of SERS-based assays, from basic considerations to bioanalytical applications. Our main focus is on SERS-based pathogen detection methods as point-of-care solutions for early bacterial infection detection and chronic disease diagnosis. Additionally, various promising in vivo applications of SERS are surveyed. Furthermore, we provide a brief outlook of recent endeavours and we discuss future prospects and limitations for SERS, as a reliable approach for rapid and sensitive bioanalysis and diagnosis.

Quantitative Ultrafast Electron-Temperature Dynamics in Photo-Excited Au Nanoparticles

The femtosecond evolution of the electronic temperature of laser-excited gold nanoparticles is measured, by means of ultrafast time-resolved photoemission spectroscopy induced by extreme-ultraviolet radiation pulses. The temperature of the electron gas is deduced by recording and fitting high-resolution photo emission spectra around the Fermi edge of gold nanoparticles providing a direct, unambiguous picture of the ultrafast electron-gas dynamics. These results will be instrumental to the refinement of existing models of femtosecond processes in laterally-confined and bulk condensed-matter systems, and for understanding more deeply the role of hot electrons in technological applications.

Tailoring Broad-Band-Absorbed Thermoplasmonic 1D Nanochains for Smart Windows with Adaptive Solar Modulation

Controlling solar transmission through windows promises to reduce building energy consumption. A new smart window for adaptive solar modulation is presented in this work proposing the combination of the photothermal one-dimensional (1D) Au nanochains and thermochromic hydrogel. In this adaptive solar modulation system, the Au nanochains act as photoresponsive nanoheaters to stimulate the optical switching of the thermochromic hydrogel. By carefully adjusting the electrostatic interactions between nanoparticles, different chain morphologies and plateau-like broad-band absorption in the NIR region are achieved. Such broad-band-absorbed 1D nanochains possess excellent thermoplasmonic effect and enable the solar modulation with compelling features of improved NIR light shielding, high initial visible transmittance, and fast response speed. The designed smart window based on 1D Au nanochains is capable of shielding 94.1% of the solar irradiation from 300 to 2500 nm and permitting 71.2% of visible light before the optical switching for indoor visual comfort. In addition, outdoor cooling tests in model house under continuous natural solar irradiation reveal the remarkable passive cooling performance up to ∼7.8 °C for the smart window based on 1D Au nanochains, showing its potential in the practical application of building energy saving.

Activatable NIR-II Plasmonic Nanotheranostics for Efficient Photoacoustic Imaging and Photothermal Cancer Therapy

Precise manipulation of optical properties through the structure-evolution of plasmonic nanoparticles is of great interest in biomedical fields including bioimaging and phototherapy. However, previous success has been limited to fixed assembled structures or visible-NIR-I absorption. Here, an activatable NIR-II plasmonic theranostics system based on silica-encapsulated self-assembled gold nanochains (AuNCs@SiO2 ) for accurate tumor diagnosis and effective treatment is reported. This transformable chain structure breaks through the traditional molecular imaging window, whose absorption can be redshifted from the visible to the NIR-II region owing to the fusion between adjacent gold nanoparticles in the restricted local space of AuNCs@SiO2 triggered by the high H2 O2 level in the tumor microenvironment (TME), leading to the generation of a new string-like structure with strong NIR-II absorption, which is further confirmed by finite-difference-time-domain (FDTD) simulation. With the TME-activated characteristics, AuNCs@SiO2 exhibits excellent properties for photoacoustic imaging and a high photothermal conversion efficiency of 82.2% at 1064 nm leading to severe cell death and remarkable tumor growth inhibition in vivo. These prominent intelligent TME-responsive features of AuNCs@SiO2 may open up a new avenue to explore optical regulated nano-platform for intelligent, accurate, and noninvasive theranostics in NIR-II window.

Plasmon-Enhanced Photothermal and Optomechanical Deformations of a Gold Nanoparticle

Plasmon-enhanced photothermal and optomechanical effects on deforming and reshaping a gold nanoparticle (NP) are studied theoretically. A previous paper (Wang and Ding, ACS Nano 13, 32-37, 2019) has shown that a spherical gold nanoparticle (NP) irradiated by a tightly focused laser beam can be deformed into an elongated nanorod (NR) and even chopped in half (a dimer). The mechanism is supposed to be caused by photothermal heating for softening NP associated with optical traction for follow-up deformation. In this paper, our study focuses on deformation induced by Maxwell's stress provided by a linearly polarized Gaussian beam upon the surface of a thermal-softened NP/NR. We use an elastic model to numerically calculate deformation according to optical traction and a viscoelastic model to theoretically estimate the following creep (elongation) as temperature nears the melting point. Our results indicate that a stretching traction at the two ends of the NP/NR causes elongation and a pinching traction at the middle causes a dent. Hence, a bigger NP can be elongated and then cut into two pieces (a dimer) at the dent due to the optomechanical effect. As the continuous heating process induces premelting of NPs, a quasi-liquid layer is formed first and then an outer liquid layer is induced due to reduction of surface energy, which was predicted by previous works of molecular dynamics simulation. Subsequently, we use the Young-Laplace model to investigate the surface tension effect on the following deformation. This study may provide an insight into utilizing the photothermal effect associated with optomechanical manipulation to tailor gold nanostructures.

Attomolar Sensing Based on Liquid Interface-Assisted Surface-Enhanced Raman Scattering in Microfluidic Chip by Femtosecond Laser Processing

Surface-enhanced Raman scattering (SERS) is a multidisciplinary trace analysis technique based on plasmonic effects. The development of SERS microfluidic chips has been exploited extensively in recent times impacting on applications in diverse fields. However, despite much progress, the excitation of label-free molecules is extremely challenging when analyte concentrations are lower than 1 nM because of the blinking SERS effect. In this paper, a novel analytical strategy which can achieve detection limits at an attomolar level is proposed. This performance improvement is due to the use of a glass microfluidic chip that features an analyte air-solution interface which forms on the SERS substrate in the microfluidic channel, whereby the analyte molecules aggregate locally at the interface during the measurement, hence the term liquid interface-assisted SERS (LI-SERS). The microfluidic chips are fabricated using hybrid femtosecond (fs) laser processing consisting of fs laser-assisted chemical etching, selective metallization, and metal surface nanostructuring. The novel LI-SERS technique can achieve an analytical enhancement factor of 1.5 × 10¹⁴, providing a detection limit below 10^-17 M (<10 aM). The mechanism for the extraordinary enhancement afforded by LI-SERS is attributed to Marangoni convection induced by the photothermal effect.

Laser-Generated Supranano Liquid Metal as Efficient Electron Mediator in Hybrid Perovskite Solar Cells

Creating colloids of liquid metal with tailored dimensions has been of technical significance in nano-electronics while a challenge remains for generating supranano (<10 nm) liquid metal to unravel the mystery of their unconventional functionalities. Present study pioneers the technology of pulsed laser irradiation in liquid from a solid target to liquid, and yields liquid ternary nano-alloys that are laborious to obtain via wet-chemistry synthesis. Herein, the significant role of the supranano liquid metal on mediating the electrons at the grain boundaries of perovskite films, which are of significance to influence the carriers recombination and hysteresis in perovskite solar cells, is revealed. Such embedding of supranano liquid metal in perovskite films leads to a cesium-based ternary perovskite solar cell with stabilized power output of 21.32% at maximum power point tracing. This study can pave a new way of synthesizing multinary supranano alloys for advanced optoelectronic applications.

Modeling broadband cloaking using 3D nano-assembled plasmonic meta-structures

The concept of "cloaking" an object is a very attractive one, especially in the visible (VIS) and near infra-red (NIR) regions of the electromagnetic spectrum, as that would reduce the visibility of an object to the eye. One possible route to achieving this goal is by leveraging the plasmonic property of metallic nanoparticles (NPs). We model and simulate light in the VIS and NIR scattered by a core of a homogeneous medium, covered by plasmonic cloak that is a spherical shell composed of gold nanoparticles (AuNPs). To consider realistic, scalable, and robust plasmonic cloaks that are comparable, or larger, in size to the wavelength, we introduce a multiscale simulation platform. This model uses the multiple scattering theory of Foldy and Lax to model interactions of light with AuNPs combined with the method of fundamental solutions to model interactions with the core. Numerical results of our simulations for the scattering cross-sections of core-shell composite indicate significant scattering suppression of up to 50% over a substantial portion of the desired spectral range (400 - 600 nm) for cores as large as 900 nm in diameter by a suitable combination of AuNP sizes and filling fractions of AuNPs in the shell.

Resonant Optical Antennas with Atomic-Sized Tips and Tunable Gaps Achieved by Mechanical Actuation and Electrical Control

Enhanced electromagnetic fields in nanometer gaps of plasmonic structures increase the optical interaction with matter, including Raman scattering and optical absorption. Quantum electron tunneling across sub-1 nm gaps, however, lowers these effects again. Understanding these phenomena requires controlled variation of gap sizes. Mechanically actuated plasmonic antennas enable repeatable tuning of gap sizes from the weak-coupling over the quantum-electron-tunneling to the direct-electrical-contact regime. Gap sizes are controlled electrically via leads that only weakly disturb plasmonic modes. Conductance signals show a near-continuous transition from electron tunneling to metallic contact. As the antenna's absorption cross-section is reduced, thermal expansion effects are negligible, in contrast to conventional break-junctions. Optical scattering spectra reveal first continuous red shifts for decreasing gap sizes and then blue shifts below gaps of 0.3 nm. The approach provides pathways to study opto- and electromolecular processes at the limit of plasmonic sensing.

Surface-enhanced Raman spectroscopy: benefits, trade-offs and future developments

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique with sensitivity down to the single molecule level that provides fine molecular fingerprints, allowing for direct identification of target analytes. Extensive theoretical and experimental research, together with continuous development of nanotechnology, has significantly broadened the scope of SERS and made it a hot research field in chemistry, physics, materials, biomedicine, and so on. However, SERS has not been developed into a routine analytical technique, and continuous efforts have been made to address the problems preventing its real-world application. The present minireview focuses on analyzing current and potential strategies to tackle problems and realize the SERS performance necessary for translation to practical applications.

Gold Nanoparticle Self-Aggregation on Surface with 1,6-Hexanedithiol Functionalization

Here we study the morphology and the optical properties of assemblies made of small (17 nm) gold nanoparticles (AuNPs) directly on silicon wafers coated with (3-aminopropyl)trimethoxysilane (APTES). We employed aliphatic 1,6-hexanedithiol (HDT) molecules to cross-link AuNPs during a two-stage precipitation procedure. The first immersion of the wafer in AuNP colloidal solution led mainly to the attachment of single particles with few inclusions of dimers and small aggregates. After the functionalization of precipitated NPs with HDT and after the second immersion in the colloidal solution of AuNP, we detected a sharp rise in the number of aggregates compared to single AuNPs and their dimers. The lateral size of the aggregates was about 100 nm, while some of them were larger than 1μm. We propose that the uncompensated dipole moment of the small aggregates appeared after the first precipitation and acts further as the driving force accelerating their further growth on the surface during the second precipitation. By having such inhomogeneous surface coating, the X-ray reciprocal space maps and modulation polarimetry showed well-distinguished signals from the single AuNPs and their dimers. From these observations, we concluded that the contribution from aggregated AuNPs does not hamper the detection and investigation of plasmonic effects for AuNP dimers. Meantime, using unpolarized and polarized light spectroscopy, the difference in the optical signals between the dimers, being formed because of self-aggregation and the one being cross-linked by means of HDT, was not detected.

Probing Cancer Metastasis at a Single-Cell Level with a Raman-Functionalized Anionic Probe

Cancer metastasis is the primary reason for cancer-related deaths, yet there is no technique capable of detecting it due to cancer pathogenesis. Current cancer diagnosis methods evaluate tumor samples as a whole/pooled sample process loses heterogeneous information in the metastasis state. Hence, it is not suitable for metastatic cancer detection. In order to gain complete information on metastasis, it is desirable to develop a nondestructive detection method that can evaluate metastatic cells with sensitivity down to single-cell resolution. Here we demonstrated self-functionalized anionic quantum probes for in vitro metastatic cancer detection at a single-cell concentration. We achieved this by incorporating a nondestructive SERS ability within the generated probes by integrating anionic surface species and NIR plasmon resonance. To the best of our knowledge, this was the first time that metastatic cancer cells were detected through their neoplastic transformations. With reliable diagnostic information at the single-cell sensitivity in an in vitro state, we successfully discriminated against cancer malignancy states.

Present and Future of Surface-Enhanced Raman Scattering

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

Photothermal structural modification of porous gold nanoshells via pulsed-laser irradiation: effects of laser wavelengths and surface conditions

We demonstrate the detailed effects of laser wavelengths and nanoparticle surface conditions, as well as laser fluences, in the structural modification of porous gold nanoshells induced by picosecond pulse irradiation.

Ag Ion Soldering: An Emerging Tool for Sub-nanomeric Plasmon Coupling and Beyond

Self-assembly represents probably the most flexible way to construct metastructured materials and devices from a wealth of colloidal building blocks with synthetically controllable sizes, shapes, and elemental compositions. In principle, surface capping is unavoidable during the synthesis of nanomaterials with well-defined geometry and stability. The ligand layer also endows inorganic building blocks with molecular recognition ability responsible for their assembly into desired structures. In the case of plasmonic nanounits, precise positioning of them in a nanomolecule or an ordered nanoarray provides a chance to shape their electrodynamic behaviors and thereby assists experimental demonstration of modern nanoplasmonics toward practical uses. Despite previous achievements in bottom-up nanofabrication, a big challenge exists toward strong coupling and facile charge transfer between adjacent nanounits in an assembly. This difficulty has impeded a functional development of plasmonic nanoassemblies. The weakened interparticle coupling originates from the electrostatic and steric barriers of ionic/molecular adsorbates to guarantee a good colloidal stability. Such a dilemma is rooted in fundamental colloidal science, which lacks an effective solution. During the past several years, a chemical tool termed Ag ion soldering (AIS) has been developed to overcome the above situation toward functional colloidal nanotechnology. In particular, a dimeric assembly of plasmonic nanoparticles has been taken as an ideal model to study plasmonic coupling and interparticle charge transfer. This Account starts with a demonstration of the chemical mechanism of AIS, followed by a verification of its workability in various self-assembly systems. A further use of AIS to realize postsynthetic coupling of DNA-directed nanoparticle clusters evidences its compatibility with DNA nanotechnology. Benefiting from the sub-nanometer interparticle gap achieved by AIS, a conductive pathway is established between two nanoparticles in an assembly. Accordingly, light-driven charge transfer between the conductively bridged plasmonic units is realized with highly tunable resonance frequencies. These situations have been demonstrated by thermal/photothermal sintering of silica-isolated nanoparticle dimers as well as gap-specific electroless gold/silver deposition. The regioselective silver deposition is then combined with galvanic replacement to obtain catalytically active nanofoci (plasmonic nanogaps). The resulting structures are useful for real time and on-site Raman spectroscopic tracking of chemical reactions in the plasmonic hotspots (nanogaps) as well as for study of plasmon-mediated/field-enhanced catalysis. The Account is concluded by a deeper insight into the chemical mechanism of AIS and its adaption to conformation-rich structures. Finally, AIS-enabled functional pursuits are suggested for self-assembled materials with strongly coupled and easily reshapable physicochemical properties.

In Situ Observation of Coulomb Fission of Individual Plasmonic Nanoparticles

Reshaping plasmonic nanoparticles with laser pulses has been extensively researched as a tool for tuning their properties. However, in the absence of direct observations of the processes involved, important mechanistic details have remained elusive. Here, we present an in situ electron microscopy study of one such process that involves Coulomb fission of plasmonic nanoparticles under femtosecond laser irradiation. We observe that gold nanoparticles encapsulated in a silica shell fission by emitting progeny droplets comprised of about 10-500 atoms, with ejection preferentially occurring along the laser polarization direction. Under continued irradiation, the emitted droplets coalesce into a second core within the silica shell, and the system evolves into a dual-core particle. Our findings are consistent with a mechanism in which electrons are preferentially emitted from the gold core along the laser polarization direction. The resulting anisotropic charge distribution in the silica shell then determines the direction in which progeny droplets are ejected. In addition to yielding insights into the mechanism of Coulomb fission in plasmonic nanoparticles, our experiments point toward a facile method for forming surfaces decorated with aligned dual-gold-core silica shell particles.

Plasmonic Oligomers with Tunable Conductive Nanojunctions

Engineering plasmonic hot spots is essential for applications of plasmonic nanoparticles. A particularly appealing route is to weld plasmonic nanoparticles together to form more complex structures sustaining plasmons with symmetries targeted to given applications. However, control of the welding and subsequent hot spot characteristics is still challenging. Herein, we demonstrate an original method that connects gold particles to their neighbors by another metal of choice. We first assemble gold bipyramids in a tip-to-tip configuration, yielding short chains of variable length, and grow metallic junctions in a second step. We follow the chain formation and the deposition of the second metal (i.e., silver or palladium) via UV/vis spectroscopy, and we map the plasmonic properties using electron energy loss spectroscopy. The formation of silver bridges leads to a huge red shift of the longitudinal plasmon modes into the mid-infrared region, while the addition of palladium results in a red shift accompanied by significant plasmon damping.

Tandem organic solar cells containing plasmonic nanospheres and nanostars for enhancement in short circuit current density

In this paper, we propose double junction tandem organic solar cells with PTB7:PC₇₀BM and PDPPSDTPS:PC₆₀BM as the polymeric active materials to cover the wide solar spectrum from 300 nm to 1150 nm. We present novel designs and finite-difference time-domain (FDTD) simulation results of plasmonic double junction tandem OSCs in which Ag nanospheres are present over the top surface of the OSC and Ag nanostars are present in the bottom subcell which substantially enhance the absorption, short circuit current density, and efficiency of the OSC as compared to the reference tandem OSCs that do not contain any nanoparticles. Different geometries of the plasmonic nanoparticles such as nanospheres and nanostars were used in the top subcell and the bottom subcell, respectively, so that the absorption in the different spectral regimes - corresponding to the bandgaps of the active layers in the two subcells (PTB7:PC₇₀BM in the top subcell and LBG:PC₆₀BM in the bottom subcell) - could be enhanced. The thickness of the bottom subcell active layer as well as the geometries of the plasmonic nanoparticles were optimized such that the short circuit current densities in the two subcells could be matched in the tandem OSC. An overall enhancement of 26% in the short circuit current density was achieved in a tandem OSC containing the optimized Ag nanospheres over the top surface and Ag nanostars inside the bottom subcell active layer. The presence of plasmonic nanoparticles along with the wide spectrum absorption band of the active materials in the tandem OSC leads to a typical power conversion efficiency of ∼ 15.4%, which is higher than that of a reference tandem organic solar cell (12.25%) that does not contain any nanoparticles.

Laser additive nano-manufacturing under ambient conditions

Additive manufacturing at the macroscale has become a hot topic of research in recent years. It has been used by engineers for rapid prototyping and low-volume production. The development of such technologies at the nanoscale, or additive nanomanufacturing, will provide a future path for new nanotechnology applications. In this review article, we introduce several available toolboxes that can be potentially used for additive nanomanufacturing. We especially focus on laser-based additive nanomanufacturing under ambient conditions.

Hybrid Plasmonic-Ferroelectric Architectures for Lasing and SHG Processes at the Nanoscale

Coherent light sources providing sub-wavelength confined modes are in ever more demand to face new challenges in a variety of disciplines. Scalability and cost-effective production of these systems are also highly desired. The use of ferroelectrics in functional optical platforms, on which plasmonic arrangements can be formed, is revealed as a simple and powerful method to develop coherent light sources with improved and novel functionalities at the nanoscale. Two types of sources with sub-diffraction spatial confinement and improved performances are presented: i) plasmon-assisted solid-state nanolasers based on the interaction between metallic nanostructures and optically active rare earth doped ferroelectric crystals and ii) nonlinear radiation sources based on quadratic frequency mixing processes that are enhanced by means of localized surface plasmon (LSP) resonances. The mechanisms responsible for the intensification of the radiation-matter interaction processes by LSP resonances are discussed in each case. The challenges, potential applications, and future perspectives of the field are highlighted.

Generating Ultrabroadband Deep-UV Radiation and Sub-10 nm Gap by Hybrid-Morphology Gold Antennas

We experimentally investigate the interaction between hybrid-morphology gold optical antennas and a few-cycle Ti:sapphire laser up to ablative intensities, demonstrating rich nonlinear plasmonic effects and promising applications in coherent frequency upconversion and nanofabrication technology. The two-dimensional array of hybrid antennas consists of elliptical apertures combined with bowties in its minor axis. The plasmonic resonance frequency of the bowties is red-shifted with respect to the laser central frequency and thus mainly enhances the third harmonic spectrum at long wavelengths. The gold film between two neighboring elliptical apertures forms an hourglass-shaped structure, which acts as a "plasmonic lens" and thus strongly reinforces surface currents into a small area. This enhanced surface current produces a rotating magnetic field that deeply penetrates into the substrate. At resonant frequency, the magnetic field is further intensified by the bowties. The resonant frequency of the hourglass is blueshifted with respect to the laser central frequency. Consequently, it spectacularly extends the third harmonic spectrum toward short wavelengths. The resultant third harmonic signal ranges from 230 to 300 nm, much broader than the emission from a sapphire crystal. In addition, the concentration of surface current within the neck of the hourglass antenna results in a structural modification through laser ablation, producing sub-10 nm sharp metallic gaps. Moreover, after laser illumination the optical field hotspots are imprinted around the antennas, allowing us to confirm the subwavelength enhancement of the electric near-field intensity.

Extreme nanophotonics from ultrathin metallic gaps

Ultrathin dielectric gaps between metals can trap plasmonic optical modes with surprisingly low loss and with volumes below 1 nm³. We review the origin and subtle properties of these modes, and show how they can be well accounted for by simple models. Particularly important is the mixing between radiating antennas and confined nanogap modes, which is extremely sensitive to precise nanogeometry, right down to the single-atom level. Coupling nanogap plasmons to electronic and vibronic transitions yields a host of phenomena including single-molecule strong coupling and molecular optomechanics, opening access to atomic-scale chemistry and materials science, as well as quantum metamaterials. Ultimate low-energy devices such as robust bottom-up assembled single-atom switches are thus in prospect.

Dopamine-Mediated Assembly of Citrate-Capped Plasmonic Nanoparticles into Stable Core-Shell Nanoworms for Intracellular Applications

Plasmonic nanochains, derived from the one-dimensional assembly of individual plasmonic nanoparticles (NPs), remain infrequently explored in biological investigations due to their limited colloidal stability, ineffective cellular uptake, and susceptibility to intracellular disassembly. We report the synthesis of polydopamine (PDA)-coated plasmonic "nanoworms" (NWs) by sonicating citrate-capped gold (Cit-Au) NPs in a concentrated dopamine (DA) solution under alkaline conditions. DA mediates the assembly of Cit-Au NPs into Au NWs within 1 min, and subsequent self-polymerization of DA for 60 min enables the growth of an outer conformal PDA shell that imparts stability to the inner Au NW structure in solution, yielding "core-shell" Au@PDA NWs with predominantly 4-5 Au cores per worm. Our method supports the preparation of monometallic Au@PDA NWs with different core sizes and bimetallic PDA-coated NWs with Au and silver cores. The protonated primary amine and catechol groups of DA, with their ability to interact with Cit anions via hydrogen bonding and electrostatic attraction, are critical to assembly. When compared to unassembled PDA-coated Au NPs, our Au@PDA NWs scatter visible light and absorb near-infrared light more intensely and enter HeLa cancer cells more abundantly. Au@PDA NWs cross the cell membrane as intact entities primarily via macropinocytosis, mostly retain their inner NW structure and outer PDA shell inside the cell for 24 h, and do not induce noticeable cytotoxicity. We showcase three intracellular applications of Au@PDA NWs, including label-free dark-field scattering cell imaging, delivery of water-insoluble cargos without pronounced localization in acidic compartments, and photothermal killing of cancer cells.

Toward Plasmonic Tunnel Gaps for Nanoscale Photoemission Currents by On-Chip Laser Ablation

We demonstrate that prestructured metal nanogaps can be shaped on-chip to below 10 nm by femtosecond laser ablation. We explore the plasmonic properties and the nonlinear photocurrent characteristics of the formed tunnel junctions. The photocurrent can be tuned from multiphoton absorption toward the laser-induced strong-field tunneling regime in the nanogaps. We demonstrate that a unipolar ballistic electron current is achieved by designing the plasmonic junctions to be asymmetric, which allows ultrafast electronics on the nanometer scale.

Continuous-wave laser-induced welding and giant photoluminescence enhancement of Au nanospheres

Photoluminescence (PL) of Au nanoparticles is appealing for various biological applications, owing to their unique advantages. However, widespread applications are still limited by their extremely low quantum yield. Here, we report on the giant PL enhancement of aggregated Au nanospheres by continuous-wave (CW) laser irradiation. Our studies show that the laser-induced PL enhancement is influenced by the wavelength and power density of irradiation laser, as well as the size of Au nanospheres. The averaged intensity of Au nanospheres after irradiation by 405 nm CW laser at power density of 6 MW/cm² is 75 times that of the as-prepared sample, where the highest enhancement of 150 folds is obtained. The giant PL enhancement is attributed to laser-induced photothermal welding and reshaping of adjacent Au nanospheres, which will dramatically enhance the incidence light field in the crevices around the welding areas by surface plasmon resonance. These studies not only declare that Au nanospheres are expected to find many new applications in PL-based biosensing and bioiamging, but also suggest that CW laser can be used as a versatile tool to weld and reshape the Au nanospheres in order to build up functionalized electronic and optoelectronic devices.

Photothermal-Assisted Optical Stretching of Gold Nanoparticles

The synergy of photothermal energy and optical forces generated by tightly focused laser beams can be used to transform the shape of gold nanoparticles. Here, the combination of these two effects is demonstrated to be an effective way of elongating gold nanoparticles (Au NPs), massively tuning their plasmonic properties. The photothermal effect of the laser increases the temperature of Au NPs above the melting point, and optical forces deform the molten Au NPs. As a result, the shape of Au NPs transforms from nanospheres into nanorods or dimers, depending on the power and time of irradiation as well as the surface energy of the substrate. This process is reversible by using high laser power to transform nanorods back to nanospheres due to capillary dewetting. Such light-induced transformations of nanostructures not only provide a facile way to tune plasmon resonances but also shed light on how the synergistic effect of photothermal energy and optical forces works on plasmonic nanoparticles.

Nanosecond-Laser-Based Charge Transfer Plasmon Engineering of Solution-Assembled Nanodimers

The ability to re-engineer self-assembled functional structures with nanometer accuracy through solution-processing techniques represents a big challenge in nanotechnology. Herein we demonstrate that Ag⁺-soldered nanodimers with a steric confinement coating of silica can be harnessed to realize an in-solution nanosecond laser reshaping to form interparticle conductive pathway with finely controlled conductance. The high structural purity of the nanodimers, the rigid silica coating, and the uniform (but still adjustable) sub-1-nm interparticle gap together determine the success of the laser reshaping process. This method is applicable to DNA-assembled nanodimers, and thus promises DNA-based programming toward higher structural complexity. The resulting structures exhibit highly tunable charge transfer plasmons at visible and near-infrared frequencies dictated by the fluence of the laser pulses. Our work provides an in-solution, rapid, and nonperturbative route to realize charge transfer plasmonic coupling along prescribed paths defined by self-assembly, conferring great opportunities for functional metamaterials in the context of chemical, biological, and nanophotonic applications. The ability to continuously control a subnm interparticle gap and the nanomeric width of a conductive junction also provides a platform to investigate modern plasmonic theories involving quantum and nonlocal effects.