Plasmomechanical Systems: Principles and Applications

Extreme confinement of electromagnetic waves and mechanical displacement fields to nanometer dimensions through plasmonic nanostructures offers unprecedented opportunities for greatly enhanced interaction strength, increased bandwidth, lower power consumption, chip-scale fabrication, and efficient actuation of mechanical systems at the nanoscale. Conversely, coupling mechanical oscillators to plasmonic nanostructures introduces mechanical degrees of freedom to otherwise static plasmonic structures thus giving rise to the generation of extremely large resonance shifts even for minor position changes. This nanoscale marriage of plasmonics and mechanics has led to the emergence of a new field of study called plasmomechanics that explores the fundamental principles underneath the coupling between light and plasmomechanical nanoresonators. In this review, both the fundamental concepts and applications of plasmomechanics as an emerging field of study are discussed. After an overview of the basic principles of plasmomechanics, the active tuning mechanisms of plasmonic nano-mechanical systems are extensively analyzed. Moreover, the recent developments on the practical implications of plasmomechanic systems for such applications as biosensing and infrared detection are highlighted. Finally, an outlook on the implications of the plasmomechanical nanosystems for development of point-of-care diagnostic devices that can help early and rapid detection of fatal diseases are forwarded.     

Emerging Plasmonic Assemblies Triggered by DNA for Biomedical Applications

Plasmonic gold nanocrystal represents plasmonic metal nanomaterials, and has a variety of unique and beneficial properties, such as optical signal enhancement, catalytic activity, and photothermal properties tuned by local temperature, which are useful in physical, chemical, and biological applications. In addition, the inherent properties of predictable programmability, sequence specificity, and structural plasticity provide DNA nanostructures with precise controllability, spatial addressability, and targeting recognition, serving as ideal ligands to link or position building blocks during the self-assembly process. Self-assembly is a common technique for the organization of prefabricated and discrete nanoparticle blocks for the construction of extremely sophisticated nanocomposites. To this end, the integration of DNA nanotechnology with Au nanomaterials, followed by assembly of DNA-functionalized Au nanomaterials can form novel functional Au nanomaterials that are difficult to obtain through conventional methods. Here, recent progress in DNA-assembled Au nanostructures of various shapes is summarized, and their functions are discussed. The fabrication strategies that employ DNA for the self-assembly of Au nanostructures, including dimers, tetramers, satellites, nanochains, and other nanostructures with more complex geometric configurations are first described. Then, the characteristic optical properties and applications of biosensing, bioimaging, drug delivery, and therapy are discussed. Finally, the remaining challenges and prospects are elucidated.     

Distinct Bimodal Roles of Aromatic Molecules in Controlling Gold Nanorod Growth for Biosensing

New aromatic molecule–seed particle interactions are examined and exploited to control and guide seed‐mediated gold nanorod (Au NR) growth. This new approach enables better understanding of how small molecules impact the synthesis of metallic nanostructures, catalyzing their use in various biomedical applications, such as plasmonic biosensing. Experimental studies and theoretical molecular simulations using a library of aromatic molecules, making use of the chemical versatility of the molecules with varied spatial arrangements of electron‐donating/withdrawing groups, charge, and Au‐binding propensity, are performed. Au NR growth is regulated by two principal mechanisms, producing either a red or blue shift in the longitudinal localized surface plasmon resonance (LLSPR) peaks. Aromatic molecules with high redox potentials produce an increase in NR aspect ratio and red shift of LLSPR peaks. In contrast, molecules that strongly bind gold surfaces result in blue shifts, demonstrating a strong correlation between their binding energy and blue shifts produced. Through enzymatic conversion of selected molecules, 4‐aminophenylphosphate to 4‐aminophenol, opposing growth mechanisms at opposite extremes of target concentration are obtained, and a chemical pathway for performing plasmonic enzyme‐linked immunosorbent assays is established. This unlocks new strategies for tailoring substrate design and enzymatic mechanisms for controlling plasmonic response to target molecules in biosensing applications.     

Unusual Circularly Polarized Photocatalytic Activity in Nanogapped Gold–Silver Chiroplasmonic Nanostructures

Gold‐gap‐silver nanostructures (GGS NSs) with interior nanobridged gaps are enantioselectively fabricated. Guided by l/d ‐cysteine, the GGS‐L/D (L/D represents l/d ‐cysteine) NSs show reversed plasmon‐induced circular dichroism (CD) signals in the visible region. It is found that the nanogap plays a key role in the plasmonic CD of GGS NSs and the chiroptical response can be tailored by adjusting the amount of cysteine. The anisotropy factor of GGS‐L/D NSs with a 0.5 nm interior gap at 430 nm is as high as ≈0.01. The circularly polarized photocatalytic activity of GGS NSs is examined. It is shown that upon irradiation with left‐circularly polarized light, the catalytic efficiency of GGS‐L NSs is 73‐fold and 17‐fold higher than that of Au nanoparticles (NPs) and Au@Ag core–shell NPs, respectively. Upon irradiation with right‐circularly polarized light, the catalytic activity of GGS‐D NSs is about 71 times and 17 times higher than that of Au NPs and Au@Ag core–shell NPs, respectively. These unique chiral NSs with high plasmonic response can be applied to enantioselective catalysis.     

Large‐Area Fabrication of Periodic Arrays of Nanoholes in Metal Films and Their Application in Biosensing and Plasmonic‐Enhanced Photovoltaics

Plasmonics is a fast developing research area with a great potential for practical applications. However, the implementation of plasmonic devices requires low cost methodologies for the fabrication of organized metallic nanostructures that covers a relative large area (～1 cm²). Here the patterning of periodic arrays of nanoholes (PANHs) in gold films by using a combination of interference lithography, metal deposition, and lift off is reported. The setup allows the fabrication of periodic nanostructures with hole diameters ranging from 110 to 1000 nm, for 450 and 1800 nm of periodicity, respectively. The large areas plasmonic substrates consist of 2 cm × 2 cm gold films homogeneously covered by nanoholes and gold films patterned with a regular microarray of 200 μm diameter circular patches of PANHs. The microarray format is used for surface plasmon resonance (SPR) imaging and its potential for applications in multiplex biosensing is demonstrated. The gold films homogeneously covered by nanoholes are useful as electrodes in a thin layer organic photovoltaic. This is first example of a large area plasmonic solar cell with organized nanostructures. The fabrication approach reported here is a good candidate for the industrial‐scale production of metallic substrates for plasmonic applications in photovoltaics and biosensing.     

Selective Pd Deposition on Au Nanobipyramids and Pd Site‐Dependent Plasmonic Photocatalytic Activity

The synthesis of anisotropic metal nanostructures is strongly desired for exploring plasmon‐enabled applications. Herein, the preparation of anisotropic Au/SiO₂ and Au/SiO₂/Pd nanostructures is realized through selective silica coating on Au nanobipyramids. For silica coating at the ends of Au nanobipyramids, the amount of coated silica and the overall shape of the coated nanostructures exhibit a bell‐shaped dependence on the cationic surfactant concentration. For both end and side silica coating on Au nanobipyramids, the size of the silica component can be varied by changing the silica precursor amount. Silica can also be selectively deposited on the corners or facets of Au nanocubes, suggesting the generality of this method. The blockage of the predeposited silica component on Au nanobipyramids enables further selective Pd deposition. Suzuki coupling reactions carried out with the different bimetallic nanostructures functioning as plasmonic photocatalysts indicate that the plasmonic photocatalytic activity is dependent on the site of Pd nanoparticles on Au nanobipyramids. Taken together, these results suggest that plasmonic hot spots play an important role in hot‐electron‐driven plasmonic photocatalysis. This study opens up a promising route to the construction of anisotropic bimetallic nanostructures as well as to the design of bimetallic plasmonic‐catalytic nanostructures as efficient plasmonic photocatalysts.     

Plasmonic Nanoparticles with Supramolecular Recognition

Even after more than two decades of intense studies, the research on self‐assembly processes involving supramolecular interactions between nanoparticles (NPs) is continuously expanding. Plasmonic NPs have attracted particular attention due to strong optical, electrical, biological, and catalytic effects they are accompanied with. Surface plasmon resonance characteristics of plasmonic NPs and their assemblies enable fine‐tuning of these effects with unprecedented dynamic range. In turn, the uniquely high polarizability of plasmonic nanostructures and related optical effects exemplified by surface‐enhanced Raman scattering and red–blue color changes give rise to their application to biosensing. Since supramolecular interactions are ubiquitous in nature, scientists have found a spectrum of biomimetic properties of individual and assembled NPs that can be regulated by the layer of surface ligands coating all NPs. This paradigm has given rise to multiple studies from the design of molecular containers and enzyme‐like catalysts to chiroplasmonic assemblies. Computational and theoretical advances in plasmonic effects for geometrically complex structures have made possible the nanoscale engineering of NPs, assemblies, and supramolecular complexes with biomolecules. It is anticipated that further studies in this area will be expanded toward chiral catalysis, environmental monitoring, disease diagnosis, and therapy.     

One‐Dimensional Metal‐Oxide Nanostructures: Recent Developments in Synthesis,Characterization, and Applications

1D metal‐oxide nanostructures have attracted much attention because metal oxides are the most fascinating functional materials. The 1D morphologies can easily enhance the unique properties of the metal‐oxide nanostructures, which make them suitable for a wide variety of applications, including gas sensors, electrochromic devices, light‐emitting diodes, field emitters, supercapacitors, nanoelectronics, and nanogenerators. Therefore, much effort has been made to synthesize and characterize 1D metal‐oxide nanostructures in the forms of nanorods, nanowires, nanotubes, nanobelts, etc. Various physical and chemical deposition techniques and growth mechanisms are exploited and developed to control the morphology, identical shape, uniform size, perfect crystalline structure, defects, and homogenous stoichiometry of the 1D metal‐oxide nanostructures. Here a comprehensive review of recent developments in novel synthesis, exceptional characteristics, and prominent applications of one‐dimensional nanostructures of tungsten oxides, molybdenum oxides, tantalum oxides, vanadium oxides, niobium oxides, titanium oxides, nickel oxides, zinc oxides, bismuth oxides, and tin oxides is provided.     

Synthesis of Dumbbell‐Like Gold–Metal Sulfide Core–Shell Nanorods with Largely Enhanced Transverse Plasmon Resonance in Visible Region and Efficiently Improved Photocatalytic Activity

The metallic nanostructures with unique properties of tunable plasmon resonance and large field enhancement have been cooperated with semiconductor to construct hetero‐nanostructures for various applications. Herein, a general and facile approach to synthesize uniform dumbbell‐like gold–sulfide core–shell hetero‐nanostructures is reported. The transformation from Au nanorods (NRs) to dumbbell‐like Au NRs and coating of metal sulfide shells (including Bi₂S₃, CdS, Cu_xS, and ZnS) are achieved in a one‐pot reaction. Due to the reshaping of Au core and the deposition of sulfide shell, the plasmon resonances of Au NRs are highly enhanced, especially the about 2 times enhancement for the visible transverse plasmon resonance compared with the initial Au NRs. Owing to the highly enhanced visible light absorption and strong local electric field, we find the photocatalytic activity of dumbbell‐like Au–Bi₂S₃ NRs is largely enhanced compared with pure Bi₂S₃ and normal Au–Bi₂S₃ NRs by testing the photodegradation rate of Rhodamine B (RhB). Moreover, the second‐layer sulfide can be coated and the double‐shell Au–Bi₂S₃–CdS hetero‐nanostructures show further improved photodegradation rate, especially about 2 times than that of Degussa P25 TiO₂ (P25) ascribing to the optimum band arrangement and then the prolonged lifetime of photo‐generated carriers.     

Plasmonic Janus‐Composite Photocatalyst Comprising Au and C–TiO2 for Enhanced Aerobic Oxidation over a Broad Visible‐Light Range

Asymmetric Janus nanostructures containing a gold nanocage (NC) and a carbon–titania hybrid nanocrystal (AuNC/(C–TiO₂)) are prepared using a novel and facile microemulsion‐based approach that involves the assistance of ethanol. The localized surface plasmon resonance of the Au NC with a hollow interior and porous walls induce broadband visible‐light harvesting in the Janus AuNC/(C–TiO₂). An acetone evolution rate of 6.3 μmol h^?1 g^?1 is obtained when the Janus nanostructure is used for the photocatalytic aerobic oxidation of iso‐propanol under visible light (λ = 480–910 nm); the rate is 3.2 times the value of that obtained with C–TiO₂, and in photo‐electrochemical investigations an approximately fivefold enhancement is obtained. Moreover, when compared with the core–shell structure (AuNC@(C–TiO₂) and a gold–carbon–titania system where Au sphere nanoparticles act as light‐harvesting antenna, Janus AuNC/(C–TiO₂) exhibit superior plasmonic enhancement. Electromagnetic field simulation and electron paramagnetic resonance results suggest that the plasmon–photon coupling effect is dramatically amplified at the interface between the Au NC and C–TiO₂, leading to enhanced generation of energetic hot electrons for photocatalysis.     

Ultrasensitive Plasmonic Response of Bimetallic Au/Pd Nanostructures to Hydrogen

Hydrogen detection is crucial for the safety of all hydrogen‐related applications. Compared to electrical hydrogen sensors, which usually suffer from possible electric sparks, optical hydrogen sensors offer advantages of remote and contact‐free readout and therefore the avoidance of spark generation. Herein, bimetallic Au/Pd nanostructure monolayers that exhibit ultrasensitive plasmonic response to hydrogen are reported. Bimetallic Au/Pd nanostructures with continuous and discontinuous Pd shells are prepared. The plasmonic response to hydrogen is monitored by measuring the extinction spectra of the ensemble Au/Pd nanostructures deposited on glass slides. Introduction of hydrogen induces red plasmon shifts, which become larger for the nanostructures with thicker Pd shells. For the nanostructures with continuous Pd shell, the plasmon shift can reach 56 nm at the hydrogen volume concentration below the explosion limit. The plasmon resonance wavelength displays an excellent linear dependence on the hydrogen volume concentration below 1%. The detection limit in the experiments reaches 0.2%. The nanostructures with discontinuous Pd shell show smaller plasmon shifts than those with continuous Pd shell. The extinction measurements on the ensemble nanostructures supported on transparent substrates and the unprecedentedly large plasmon shifts and sensitivity make the results very promising for the development of practical optical hydrogen sensors.     

“Ship‐in‐a‐Bottle” Growth of Noble Metal Nanostructures

Noble metal nanostructures are grown inside hollow mesoporous silica microspheres using “ship‐in‐a‐bottle” growth. Small Au seeds are first introduced into the interior of the hollow microspheres. Au nanorods with synthetically tunable longitudinal plasmon wavelengths and Au nanospheres are obtained through seed‐mediated growth within the microspheres. The encapsulated Au nanocrystals are further coated with Pd or Pt shells. The microsphere‐encapsulated bimetallic core/shell nanostructures can function as catalysts. They exhibit high catalytic performance and their stability is superior to that of the corresponding unencapsulated core/shell nanostructures in the catalytic oxidation of o‐phenylenediamine with hydrogen peroxide. Therefore, these hollow microsphere‐encapsulated metal nanostructures are promising as recoverable and efficient catalysts for various liquid‐phase catalytic reactions.     

基于局域表面等离子体增强的NaYF4：Yb/Er荧光上转换及其应用

阐述了局域表面等离子体共振增强荧光上转换的相关机制,并以此为基础总结了三种调节机制和四种上转换/金属复合材料结构。具有明显增强效果的上转换/金属结构复合材料大致分为四种:掺入Au和Ag纳米颗粒的稀土掺杂基质;core/shell结构;稀土掺杂的NaYF₄靠近金属颗粒或金属纳米线所形成的gap结构;周期性金属阵列结构。最后介绍了它们在生物医学和光电子器件领域的应用进展。     

Chirality on Hierarchical Self‐Assembly of Au@AuAg Yolk–Shell Nanorods into Core–Satellite Superstructures for Biosensing in Human Cells

Here, Au@AgAu yolk–shell nanorods (YSNRs) with tunable plasmonic circular dichroism (PCD) responses are synthesized, for the first time, using chiral d ‐ or l ‐penicillamine. The concentration of chiral molecules and the nanogap size play key roles in the PCD signal of the YSNRs. Importantly, the PCD response could be regulated by changing the aspect ratio of gold nanorods, and the largest anisotropy factor (g‐factor) is 0.009. Remarkably, the YSNRs are used as new chiroptical building blocks together with Au nanoparticles (NPs) to build YSNR@NP core–satellite (CS) assemblies, which display much stronger PCD intensity and an increased g‐factor of 0.021. The theoretical calculation data demonstrate that the significant PCD activity of nanoassemblies is originated from chiral structures and the intrinsic chirality of the building blocks. Noticeably, the developed CS superstructures with photothermal effects are utilized to quantitatively detect zinc ion in living cell lines. This study reports an emerging class of chiral inorganic nanostructures with important future applications, including enantioselective separation, asymmetric catalysis, and biomedical sensing.     

Plasmon‐Enhanced Fluorescence‐Based Core–Shell Gold Nanorods as a Near‐IR Fluorescent Turn‐On Sensor for the Highly Sensitive Detection of Pyrophosphate in Aqueous Solution

Developing plasmon‐enhanced fluorescence (PEF) technology for identifying important biological molecules has a profound impact on biosensing and bioimaging. However, exploration of PEF for biological application is still at a very early stage. Herein, novel PEF‐based core–shell nanostructures as a near‐infrared fluorescent turn‐on sensor for highly sensitive and selective detection of pyrophosphate (PPi) in aqueous solution are proposed. This nanostructure gold nanorod (AuNR)@SiO₂@meso‐tetra(4‐carboxyphenyl) porphyrin (TCPP) contains a gold nanorod core with an aspect ratio of 2.3, a silica shell, and TCPP molecules covalently immobilized onto the shell surface. The silica shell is employed a rigid spacer for precisely tuning the distance between AuNR and TCPP and an optimum fluorescence enhancement is obtained. Due to the quenching effect of Cu²⁺, the copper porphyrin (TCPP‐Cu²⁺) results in a weak fluorescence. In the presence of PPi, the strong affinity between Cu²⁺ and PPi can promote the disassembly of the turn‐off state of TCPP‐Cu²⁺ complexes, and therefore the fluorescence can be readily restored. By virtue of the amplified fluorescence signal imparted by PEF, this nanosensor obtains a detection limit of 820 × 10^?9m of PPi with a good selectivity over several anions, including phosphate. Additionally, the potential applicability of this sensor in cell imaging is successfully demonstrated.     

Structural Color Boosted Electrochromic Devices: Strategies and Applications

Electrochromic devices (ECDs) have attracted substantial attention on account of their unique characteristics and prosperous applications. However, their monotonous color modification remains a momentous challenge that significantly restricts their application scope. Integrating structural colors that arise from the interaction between light and periodic micro/nanostructures with electrochromic materials (ECMs) turns out to be an effective approach to enhancing the comprehensive performance of ECDs more than the coloration effectiveness. This review provides an overview on the recent burgeoning development of structural color ECDs regarding strategies and applications. The performance parameters of ECDs and physical principles regarding the generation of structural colors are introduced. Then, strategies that introduce structural colors into ECDs are discussed according to the category of chromogenic micro/nanostructures, namely, gratings, (single-layer and multilayer) thin films, photonic crystals, plasmonic metasurfaces, and composite structures. The current state-of-the-art research activities of structural color ECDs for display application including segmented and pixelated devices are also presented. Additionally, the future perspective of structural color ECDs with respect of major challenges and potential solutions is proposed.     

Plasmonic Nanoneedle Arrays with Enhanced Hot Electron Photodetection for Near-IR Imaging

Hot electron photodetection based on metallic nanostructures is attracting significant attention due to its potential to overcome the limitation of the traditional semiconductor bandgap. To enable efficient hot electron photodetection for practical applications, it is necessary to achieve broadband and perfect light absorption within extremely thin plasmonic nanostructures using cost-effective fabrication techniques. In this study, an ultrahigh optical absorption (up to 97.3% in average across the spectral range of 1200−2400 nm) is demonstrated in the ultrathin plasmonic nanoneedle arrays (NNs) with thickness of 10 nm, based on an all-wet metal-assisted chemical etching process. The efficient hot electron generation, transport, and injection at the nanoscale apex of the nanoneedles facilitate the photodetector to achieve a record low noise equivalent power (NEP) of 4.4 × 10⁻¹² W Hz^−0.5 at the wavelength of 1300 nm. The hot-electron generation and injection process are elucidated through a transport model based on a Monte Carlo approach, which quantitatively matches the experimental data. The photodetector is further integrated into a light imaging system, as a demonstration of the exceptional imaging capabilities at the near-IR regime. The study presents a lithography-free, scalable, and cost-effective approach to enhance hot electron photodetection, with promising prospects for future imaging systems.     

Advanced Catalysts Derived from Composition‐Segregated Platinum–Nickel Nanostructures: New Opportunities and Challenges

Composition segregation, resulting from the rearrangement of atom positions and different enrichment behaviors of different atoms in alloys, has been linked to their enhanced performances in catalytic applications due to the strong electronic effect and largely improved number of available active sites. Hence, composition‐segregated metallic nanostructures have been actively pursued to prepare better‐performing nanocatalysts. Moreover, they also act as an emerging platform to develop unusual nanostructures with desirable functionalities. An overview about the recent advances in preparing unusual nanostructures with desirable functionalities such as highly open 3D structures (concave, frame, porous, etc.) and composites with suitable interfaces (metal–metal, metal–oxide, metal–sulfide, metal–boride, metal–organic, metal–hydroxide interfaces, etc.) based on composition‐segregated metallic nanostructures which can boost heterogeneous catalytic reactions with superior performances is provided here. The different strategies developed so far for the synthesis of composition‐segregated metallic nanostructures are also discussed. Finally, the challenges of the composition‐segregated nanostructure and their functionalized materials are discussed, as well as some perspectives are highlighted on the fine regulation and multifunctionalities of nanostructures, which provide a powerful material foundation for the potential electrocatalysis, organic catalysis, and energy conversion of multicomponent metal nanostructures.     

Topographically Flat Substrates with Embedded Nanoplasmonic Devices for Biosensing

The ability to precisely control the topography, roughness, and chemical properties of metallic nanostructures is crucial for applications in plasmonics, nanofluidics, electronics, and biosensing. Here a simple method to produce embedded nanoplasmonic devices that can generate tunable plasmonic fields on ultraflat surfaces is demonstrated. Using a template‐stripping technique, isolated metallic nanodisks and wires are embedded in optical epoxy, which is capped with a thin silica overlayer using atomic layer deposition. The top silica surface is topographically flat and laterally homogeneous, providing a uniform, high‐quality biocompatible substrate, while the nanoplasmonic architecture hidden underneath creates a tunable plasmonic landscape for optical imaging and sensing. The localized surface plasmon resonance of gold nanodisks embedded underneath flat silica films is used for real‐time kinetic sensing of the formation of a supported lipid bilayer and subsequent receptor‐ligand binding. Gold nanodisks can also be embedded in elastomeric materials, which can be peeled off the substrate to create flexible plasmonic membranes that conform to non‐planar surfaces.     

纳米电子学

纳米电子学是纳米技术的重要科学基础,纳米电子学将成为21世纪信息时代的科学核心,将使未来社会产生重大变革。本文介绍了纳米电子学的基本概念、纳米电子学的产生、纳米电子学的研究内容、纳米器件的基本结构及工作机理、纳米结构的微加工技术、纳米结构的检测与表征、纳米电子学的应用及发展对策。