Metallic Nanostructures Research Articles

Investigating SERS performance of controllable fabricated Ag periodic nanostructures with morphologies of nanoclaws and nanocones in detecting SARS-CoV-2 S protein

Surface-enhanced Raman scattering (SERS) spectroscopy plays an important role in enabling high-precision detection and non-invasive diagnostics. Periodic metallic nanostructures with strong electromagnetic fields and tunable localized surface plasmon resonance can achieve high SERS sensitivity, but fabricating such nanostructures remains a challenge. In this work, the anodic aluminum oxide (AAO) nanocavity template-assisted process of fabricating periodic nanostructures with modulating morphologies from Ag nanoclaws (AgNCWs) to Ag nanocones (AgNCEs) is proposed. The morphologies-related SERS performances of nanostructures are analyzed by using rhodamine 6 g (R6G) as a probe molecule and the optimized AgNCEs possess a limit of detection (LOD) of 1 × 10−9 M. A sandwich strategy is applied to detect SARS-CoV-2 S protein in phosphate-buffered saline (PBS) and pharyngeal swab solution (PSS). The ACE2-modified AgNCEs (ACE2/AgNCEs) structure is applied to bind to the SARS-CoV-2 S protein. The 4-MBA and SARS-CoV-2 S antibody functionalized Au nanoparticles (AuNPs) are used as nanotags. By forming the ACE2/AgNCEs-SARS-CoV-2 S protein-nanotags sandwich structure, characteristic peak intensities of 1580 cm−1 are used as the report signal, and the LOD for SARS-CoV-2 S protein in PBS and PSS solution are 10 fg/mL.

Anisotropic Fluorescence Surface Quenching of Quantum Dot-in-Rods on Silver Nanopatterned Film

Semiconductor quantum dot-in-rods (QDIRs) have served in the broad applications such as luminescent applications, sensor technology, quantum information processing, due to their polarized emission and polarized absorption characteristics. The localized surface plasmon resonance (LSPR) characteristics of the noble metal nanostructures can regulate the fluorescence intensity by controlling the radiation and non-radiation channels of fluorescent substances. However, the reports on the fluorescence surface quenching of QDIRs by plasmonic nanoparticles (NPs) are very rare. Herein, we propose a new method to realize QDIRs orientation, as well as the fluorescence intensity can be regulated. The effect of Ag nanopatterned film on the fluorescence surface quenching of QDIRs is discussed from both theoretical and experimental perspectives. Generally, the direct combination of Ag nanopatterned film and QDIRs results in 75% fluorescence surface quenching. Through FDTD simulation, we further analyzed the modulation of fluorescence intensity of the dipole source by the Ag nanopatterned film. This work will be useful for the development of QDIRs in the future.

Controllable Ultrathin Nickel Nanoislands With Dense Discrete Space Charge Regions: Steering Hole Extraction for High-Performance Underwater Multispectral Weak-Light Photodetection.

Undersea optical communication (UOC) is vital for ocean exploration and military applications. In the dim-light underwater environment, photodetectors must maximize photon utilization by minimizing optical losses and carrier recombination. This can be achieved by integrating ultrathin metal nanostructures with photocatalysts to form Schottky junctions, which enhance charge separation and injection while mitigating metal-induced light shading. The strategic design of discrete metal nanostructures providing numerous high-depth space charge regions (SCRs) without overlap offers a promising approach to optimize hole transport paths and further suppress recombination. Here, a facile phase-separation lithography technique is explored to fabricate tunable ultrathin Ni nanoislands atop n-Si, yielding high-performance photoelectrochemical photodetectors (PEC PDs) tailored for underwater weak-light environments. This results indicate that key determinant of hole extraction behavior is the relationship between the spacing distance of adjacent Ni nanostructures (ds) and twice the SCR radius (Ws). PEC PDs with optimized 8nm ultrathin Ni nanostructures featuring closely but non-overlapping SCRs, exhibit a 55-fold increase in photoresponsivity (2.2mAW-1) and a 128-fold enhancement in detection sensitivity (3.2 × 1011 Jones) at 0V over Ni film, revealing the exceptional stability. Furthermore, this approach enables effective detection across UV-vis-near infrared spectrum, supporting reliable multispectral UOC and underwater imaging capabilities.

Functional Immunoaffinity 3D Magnetic Core-Shell Nanometallic Structure for High-Efficiency Separation and Label-Free SERS Detection of Exosomes.

Tumor exosomes, known as maternal cell messengers, play an important role in cancer occurrence, proliferation, metastasis, immune escape, drug resistance, and other processes and are an entry point for cancer research. However, there is still a lack of an efficient detection technology for exosomes. In this study, the ultrahigh sensitivity SERS nanoprobe with a three dimensional (3D) magnetic core/Au nanocolumn/Au nanoparticles shell strongly coupling multistage structure (Fe3O4@NR-NPs) was constructed by crystal growth of nanocrystals in the confined space of a central radiating single particle mesoporous molecular sieve channel and strong coupling secondary growth of gold particles. The exosomes were confined onto the "hot spot" of plasmonic nanoparticles and rapidly enriched by CD63 antibody functional-Fe3O4@NR-NPs to achieve high sensitivity detection, with the limit of detection of 1 × 103 particles/mL (S/N = 3). The spectral data set of different exosomes is applied to train for multivariate classification of cell types and to estimate how the normal exosome data resemble cancer cell exosomes by principal component analysis (PCA). Finally, this detection method has also been successfully employed for the detection of exosomes in complex samples; this proves that the proposed SERS-based method is a promising tool for clinical cancer screening.

Bonding Interaction Within Concentric Structural Layers in Gold Superatoms. The Concentric Bond.

Ligand-protected gold clusters display a rich structural diversity, featuring remarkable structures such as Au25(SR)18, Au55(PPh3)12Cl6, and CuAu144(SR)603+, involving a central core composed of consecutive layers. The respective Au@Au12, Au@Au12@Au42, and Cu@Au12@Au42@Au60cores with concentric structural layers enable a variable bonding/antibonding character between the electronic shells ascribed to each layer. Here, we rationalize the bonding within concentric structural layers in order to gain a further understanding of the related bonding patterns in such species. The proposed bonding concept differs from the classical situation in adjacent atoms, now being considered between concentric shells and, thus, coined as the concentric bond. From this approach, the bonding/antibonding character of each concentric shell is evaluated, and its contribution to the overall bonding is discussed. The concentric bond enables building a clear picture of the bonding acting in the overall cluster under the superatom concept. Such an approach expands the understanding of multi-layered cluster cores and is useful to further rationalize the bonding situation in metallic nanostructures.

Symmetry Breaking in Inorganic Nanostructures: Chirality vs. Optical Activity or Structural vs. Electronic Effects

AbstractThis essay presents the viewpoint of the author on the topic of chirality and optical activity in nanostructures. It particularly focuses on the interaction of chiral molecules with plasmonic and excitonic nanocrystals and on induction of circular dichroism in such achiral nanocrystals. It discusses recent developments in the shape symmetry breaking of achiral metal nanostructures using photochemical processes induced by asymmetric localized plasmonic hot spots excited through circularly polarized light illumination. Finally, it addresses symmetry breaking in intrinsically chiral inorganic nanocrystals using chiral ligands during their formation, leading to 100 % enantiomeric excess in the nanocrystals of TbPO4⋅H2O. These nanocrystals exhibit an interesting nucleation mechanism, which leads to very high chiral amplification (secondary nucleation).

Nanoimprinting and backside ultraviolet lithography for fabricating metal nanostructures with higher aspect ratio

This paper introduces an innovative approach to increasing the aspect ratio of metal nanostructures fabricated using nanoimprint lithography (NIL). Although conventional NIL and metal lift-off processes can fabricate metal nanostructures, the achievable aspect ratio is often limited by the inherent constraints of NIL. In this study, we demonstrate that for an ultraviolet (UV) transparent substrate, metal nanostructures patterned via NIL can serve as a photomask. A negative-tone photoresist (PR) layer was then deposited on top of the patterned metal nanostructures. By illuminating the substrate from the backside with UV light and subsequently developing the PR, PR structures complementary and self-aligned to the metal layer were obtained. This enabled a second round of metal deposition and lift-off, thereby increasing the height of the metal structures and enhancing the aspect ratio. Experimentally, we demonstrated that this method can improve the aspect ratio from less than 1.0 to as high as 2.1. This paper also addresses the further developments and potential applications of this technique.

Improving Energy Density in Lithium-Ion Batteries Using Nanomaterials

Lithium-ion batteries are pivotal in modern energy storage, powering everything from consumer electronics to electric vehicles. Despite their advantages, current materials limit their energy density, particularly for electric vehicles. This paper explores the potential of nanomaterials to enhance lithium-ion battery performance, focusing on anodes, cathodes, and electrolytes. The study discusses the limitations of traditional graphite anodes and highlights nanostructured silicon and metal oxides as alternatives to significantly increase capacity and reduce volume expansion. For cathodes, it examines how nanomaterials like S-TiO2 yolk-shell structures and high-voltage cathode materials can improve energy density and stability. The role of nanostructured solid electrolytes in enhancing ionic conductivity and overall battery performance is also analyzed. The paper addresses the challenges of using nanomaterials, such as higher costs and the need for scalable production methods. It suggests future research directions, including optimizing nanostructures and developing cost-effective manufacturing processes. The findings underscore the potential of nanotechnology to overcome current limitations and significantly boost the energy density of lithium-ion batteries.

Electrochemical Nanoimprinting of Colored Infrared Reflective Patterns for Radiative Heating

AbstractMetallic nanostructures hold immense promise for radiative heating technology because of their distinctive capability to simultaneously achieve high mid‐infrared reflectance and generate vibrant colors through structural or plasmonic effects. However, fabricating metallic nanostructures remains challenging using traditional top–down techniques. Here, the solid‐state superionic stamping (S4) process is demonstrated as a scalable and economical tool to electrochemically imprint metallic grid nano‐patterns onto flexible substrates. With precise control over grid width and length at the nanoscale, the S4 approach enables the fabrication of nano‐patterned grid coatings exhibiting a spectrum of colors. These coatings also show high mid‐infrared reflectance exceeding 80% at 9.5 µm, leading to radiative heating effects of 5.9 and 3.1 °C compared to bare skin and cotton, respectively. This work introduces a viable strategy for creating colored infrared‐reflective patterns, paving the way for diverse thermal management applications.

Developments in Localized Surface Plasmon Resonance

AbstractLocalized surface plasmon resonance (LSPR) is a nanoscale phenomenon associated with noble metal nanostructures that has long been studied and has gained considerable interest in recent years. These resonances produce sharp spectral absorption and scattering peaks, along with strong electromagnetic near-field enhancements. Over the past decade, advancements in the fabrication of noble metal nanostructures have propelled significant developments in various scientific and technological aspects of LSPR. One notable application is the detection of molecular interactions near the nanoparticle surface, observable through shifts in the LSPR spectral peak. This document provides an overview of this sensing strategy. Given the broad and expanding scope of this topic, it is impossible to cover every aspect comprehensively in this review. However, we aim to outline major research efforts within the field and review a diverse array of relevant literature. We will provide a detailed summary of the physical principles underlying LSPR sensing and address some existing inconsistencies in the nomenclature used. Our discussion will primarily focus on LSPR sensors that employ metal nanoparticles, rather than on those utilizing extended, fabricated structures. We will concentrate on sensors where LSPR acts as the primary mode of signal transduction, excluding hybrid strategies like those combining LSPR with fluorescence. Additionally, our examination of biological LSPR sensors will largely pertain to label-free detection methods, rather than those that use metal nanoparticles as labels or as means to enhance the efficacy of a label. In the subsequent section of this review, we delve into the analytical theory underpinning LSPR, exploring its physical origins and its dependency on the material properties of noble metals and the surrounding refractive index. We will discuss the behavior of both spherical and spheroidal particles and elaborate on how the LSPR response varies with particle aspect ratio. Further, we detail the fundamentals of nanoparticle-based LSPR sensing. This includes an exploration of single-particle and ensemble measurements and a comparative analysis of scattering, absorption, and extinction phenomena. The discussion will extend to how these principles are applied in practical sensing scenarios, highlighting the key experimental approaches and measurement techniques.

Distinguishing Inner and Outer-Sphere Hot Electron Transfer in Au/p-GaN Photocathodes.

Exploring nonequilibrium hot carriers from plasmonic metal nanostructures is a dynamic field in optoelectronics, with applications including photochemical reactions for solar fuel generation. The hot carrier injection mechanism and the reaction rate are highly impacted by the metal/molecule interaction. However, determining the primary type of reaction and thus the injection mechanism of hot carriers has remained elusive. In this work, we reveal an electron injection mechanism deviating from a purely outer-sphere process for the reduction of ferricyanide redox molecule in a gold/p-type gallium nitride (Au/p-GaN) photocathode system. Combining our experimental approach with ab initio simulations, we discovered that an efficient inner-sphere transfer of low-energy electrons leads to an enhancement in the photocathode device performance in the interband regime. These findings provide important mechanistic insights, showing our methodology as a powerful tool for analyzing and engineering hot-carrier-driven processes in plasmonic photocatalytic systems and optoelectronic devices.

Double localized surface plasmon‒enhanced-second-harmonic generation behavior of equilateral triangular aluminum nanoprisms

This paper presents second-harmonic generation (SHG) behaviors for different-sized equilateral triangular aluminum nanoprisms. These behaviors were observed under the double localized surface plasmon (LSP) resonance condition. The SHG conversion efficiency was substantially enhanced when the excitation and SHG wavelengths were simultaneously tuned to the LSP-resonance wavelengths. The fundamental and higher-order LSP modes could be used to enhance the SHG conversion efficiency. The nonlinear light‒matter interaction associated with the double LSP resonance was numerically analyzed while focusing on the spatial overlap of the near-field distributions between the excitation and SHG wavelengths. The double LSP resonance enhanced SHG behaviors have been realized by using several elegantly designed plasmonic metal nanostructures consisting of multi-elements. Our present study demonstrates this double LSP resonance enhanced SHG behavior from a single, simple-shape triangular aluminum nanoprism.

Near-field probing of surface plasmon polariton formation in imprinted gold nanoisland arrays

Large-area imprinting stamps with nanometer-scale features are a rapidly developing area of research in plasmonics. In integrated photonic structures, surface plasmon (SPs) and surface plasmon polaritons (SPPs) are tuned by selecting both the appropriate wavelength and the angle of incidence of the excitation light. The resulting exponential fields can be studied by an optical technique such as scanning near-field optical microscopy (SNOM). Here, we report on the application of the aperture-type SNOM technique to characterize, at nanoscopic and microscopic scales, the formation of the SPPs and the beat pattern formed with the superposition of SPs and the effective component of the probing light formed in discrete metallic nanostructures of Au fabricated on imprinting stamps. We discuss a model to describe the beat pattern in terms of this superposition and demonstrate that the dominant SPs have a transverse nature. Our experiments are supported by modeling the optical response and near-field in gold nanostructures using the simulation tool Tidy3D. Our results provide a straightforward way to investigate and characterize SPPs at the nanostructure level.

Advances in Nanostructured Metallic Materials—A Pathway to Future Innovations

The development of civilization has always been deeply intertwined with advancements in metallurgy [...]

Long-range enhancement for fluorescence and Raman spectroscopy using Ag nanoislands protected with column-structured silica overlayer

We demonstrate long-range enhancement of fluorescence and Raman scattering using a dense random array of Ag nanoislands (AgNIs) coated with column-structured silica (CSS) overlayer of over 100 nm thickness, namely, remote plasmonic-like enhancement (RPE). The CSS layer provides physical and chemical protection, reducing the impact between analyte molecules and metal nanostructures. RPE plates are fabricated with high productivity using sputtering and chemical immersion in gold(I)/halide solution. The RPE plate significantly enhances Raman scattering and fluorescence, even without proximity between analyte molecules and metal nanostructures. The maximum enhancement factors are 107-fold for Raman scattering and 102-fold for fluorescence. RPE is successfully applied to enhance fluorescence biosensing of intracellular signalling dynamics in HeLa cells and Raman histological imaging of oesophagus tissues. Our findings present an interesting deviation from the conventional near-field enhancement theory, as they cannot be readily explained within its framework. However, based on the phenomenological aspects we have demonstrated, the observed enhancement is likely associated with the remote resonant coupling between the localised surface plasmon of AgNIs and the molecular transition dipole of the analyte, facilitated through the CSS structure. Although further investigation is warranted to fully understand the underlying mechanisms, the RPE plate offers practical advantages, such as high productivity and biocompatibility, making it a valuable tool for biosensing and biomolecular analysis in chemistry, biology, and medicine. We anticipate that RPE will advance as a versatile analytical tool for enhanced biosensing using Raman and fluorescence analysis in various biological contexts.

Growth of zinc oxide nanowires by a hot water deposition method.

Recently, various methods have been developed for synthesizing zinc oxide (ZnO) nanostructures, including physical and chemical vapor deposition, as well as wet chemistry. These common methods require either high temperature, high vacuum, or toxic chemicals. In this study, we report the growth of zinc oxide ZnO nanowires by a new hot water deposition (HWD) method on various types of substrates, including copper plates, foams, and meshes, as well as on indium tin oxide (ITO)-coated glasses (ITO/glass). HWD is derived from the hot water treatment (HWT) method, which involves immersing piece(s) of metal and substrate(s) in hot deionized water and does not require any additives or catalysts. Metal acts as the source of metal oxide molecules that migrate in water and deposit on the substrate surface to form metal oxide nanostructures (MONSTRs). The morphological and crystallographic analyses of the source-metals and substrates revealed the presence of uniformly crystalline ZnO nanorods after the HWD. In addition, the growth mechanism of ZnO nanowires using HWD is discussed. This process is simple, inexpensive, low temperature, scalable, and eco-friendly. Moreover, HWD can be used to deposit a large variety of MONSTRs on almost any type of substrate material or geometry.

Evaluating size effects on the thermal conductivity and electron-phonon scattering rates of copper thin films for experimental validation of Matthiessen’s rule

As metallic nanostructures shrink towards the size of the electronic mean free path, thermal conductivity decreases due to increased electronic scattering rates. Matthiessen’s rule is commonly applied to assess changes in electron scattering rates, although this rule has not been validated experimentally at typical operating temperatures for most of the electronic systems (e.g., near room temperature). In this study, we experimentally evaluate the validity of Matthiessen’s rule in determining the thermal conductivity of thin metal films by measuring the in-plane thermal conductivity and electronic scattering rates of copper (Cu) films with varying thicknesses (27 nm — 5 µm), microstructures, and grain boundary segregation. Comparing total electron scattering rates measured with infrared ellipsometry to infrared ultrafast pump-probe measurements, we find that the electron-phonon coupling factor is independent of film thickness, whereas the total electronic scattering rate increases with decreasing film thickness. Our findings provide experimental validation of Matthiessen’s rule for electron transport in thin metal films at room temperature and also introduce an approach to discern critical heat transfer processes in thin metal interconnects, which holds significance for the advancement of future CMOS technology.

Plasmon-Enhanced Multiphoton Polymer Crosslinking for Selective Modification of Plasmonic Hotspots.

A novel approach to selectively modify narrow subareas of metallic nanostructures adjacent to plasmonic hotspots, where strong electromagnetic field amplification occurs upon localized surface plasmon (LSP) excitation, is reported. In contrast to surface plasmon-triggered polymerization, it relies on plasmonically enhanced multiphoton crosslinking (MPC) of polymer chains carrying photoactive moieties. When they are contacted with metallic nanostructures and irradiated with a femtosecond near-infrared beam resonantly coupled with LSPs, the enhanced field intensity locally exceeds the threshold and initiates MPC only at plasmonic hotspots. This concept is demonstrated by using gold nanoparticle arrays coated with two specifically designed polymers. Local MPC of a poly(N,N-dimethylacrylamide)-based copolymer with an anthraquinone crosslinker is shown via atomic force microscopy. Additionally, MPC is tested with a thermoresponsive poly(N-isopropylacrylamide)-based terpolymer. The reversible thermally induced collapse and swelling of the MPC-formed hydrogel at specific nanoparticle locations are confirmed by polarization-resolved localized surface plasmon resonance (LSPR) spectroscopy. These hybrid metallic/hydrogel materials can be further postmodified, offering attractive characteristics for future spectroscopic/bioanalytical applications.

Nanoscale Wet Etching of Molybdenum Interconnects with Organic Solutions.

Molybdenum (Mo) has emerged as a promising material for advanced semiconductor devices, especially in the design and fabrication of interconnects requiring sub-10nm metal nanostructures. However, current wet etching methods for Mo using aqueous solutions struggle to achieve smooth etching profiles at such scales. To address this problem, we explore wet chemical etching of patterned Mo nanowires (NWs) using an organic solution: ceric ammonium nitrate (CAN) dissolved in acetonitrile (ACN). In this study, we demonstrate two distinct etching pathways by controlling the reaction temperature: i) digital cyclic scheme at room temperature, with a self-limiting Mo recess per cycle of ≈1.6nm, and ii) direct etching at elevated temperature (60°C), with a time-controlled Mo recess of ≈2nm min-1. These methods not only offer a highly controllable nanoscale Mo etching but also ensure smooth and uniform etching profiles independent of the crystal grain orientation of the metal.

Ultrabroadband Photodetection with MoS2 and Gold Nanorod Using Gap-Mode Plasmon Effect.

Plasmon resonance using metal nanostructures enables the realization of high-performance optoelectronic devices via field enhancements in the vicinity of the metal nanostructure. This study proposes an ultrabroadband MoS2 photodetector based on the gap-mode plasmon of gold nanorods. The use of MoS2 as a gap spacer for the gap-mode plasmon effect and as a channel material for the photodetector is demonstrated. The proposed photodetector demonstrates superior performance, a high photoresponsivity of 2.8 A/W at a near-infrared wavelength of 1100 nm, and a fast response time of 17 μs. In addition to the gap-mode plasmon effect of the gold nanorod enhancing the photoresponsivity from 1.13 to 8.7 A/W, the lateral surface plasmon resonance of the gold nanorods enhances the absorption of the longitudinal mode of the gold nanorods (near-infrared to infrared range). Further, the gap-mode plasmon effect of the gold nanorods enhances the absorption of the transverse mode of the gold nanorods (visible range), thus realizing ultrabroadband photodetection. Therefore, the proposed device design strategy contributes significantly to overcoming the trade-off between photoresponsivity and response time.