Metal Nanoparticle Films Research Articles

Fluorophore-Induced Plasmonic Current Generation from Copper Nanoparticle Films.

We describe the process of generating a fluorophore-induced plasmonic current (FIPC) from copper nanoparticle films. Previous work and the literature have shown that excited near-field fluorophores are able to plasmonically couple with metal nanoparticle films (MNFs), inducing surface plasmons in the films. These induced surface plasmons are then in turn able to generate a directly measurable electrical current across the film. These generated currents have been quantified and detected in noble metal films, such as those made from Ag and Au, but due to the cost of such films, there has been a push to use lower cost materials for FIPC. Previous work has detailed the use of gold, silver, and aluminum films for these purposes, and in this paper, we will subsequently examine the ability of thermally deposited copper films to generate FIPC when in close proximity to excited near-field fluorophores. We report the effects of copper film thickness, the effects of light polarization and solution conductance, and the effects of metal-enhanced fluorescence (MEF) emission on the generation of plasmonic current.

Exfoliation Technology for Scalable Ligand-Free Core-Semishell Metal Nanoparticle Films.

Core-shell metallic nanoparticles (NPs) are considered promising materials for their multifunctional properties. However, traditionally synthesized NPs have crucial issues that their ligands interfere with the direct interaction between NPs and neighboring materials, and it is very difficult to form a uniform film without the mixture of a template. In this article, we report an unprecedented exfoliation technology for fabricating a scalable ligand-free core-semishell metal NP film based on the evaporation system through a self-assembled monolayer-assisted surface energy control combined with a deep ultraviolet surface treatment around the core NPs. Owing to fabrication merits, the properties of the core-semishell NPs can be easily modulated depending on the shell material; the ligand-free core-shell NPs are directly attached to the surface of a material by Scotch tape, allowing interfacial interactions. Therefore, the proposed technique presents a new scientific method for studying interfacial interactions with heterogeneous materials and can be universally applied in optoelectronic devices, biopatches, photocatalysts, and so on.

Modular Attachment of Nanoparticles on Microparticle Supports via Multifunctional Polymers.

Nanoparticles are key to a range of applications, due to the properties that emerge as a result of their small size. However, their size also presents challenges to their processing and use, especially in relation to their immobilization on solid supports without losing their favorable functionalities. Here, we present a multifunctional polymer-bridge-based approach to attach a range of presynthesized nanoparticles onto microparticle supports. We demonstrate the attachment of mixtures of different types of metal-oxide nanoparticles, as well as metal-oxide nanoparticles modified with standard wet chemistry approaches. We then show that our method can also create composite films of metal and metal-oxide nanoparticles by exploiting different chemistries simultaneously. We finally apply our approach to the synthesis of designer microswimmers with decoupled mechanisms of steering (magnetic) and propulsion (light) via asymmetric nanoparticle binding, aka Toposelective Nanoparticle Attachment. We envision that this ability to freely mix available nanoparticles to produce composite films will help bridge the fields of catalysis, nanochemistry, and active matter toward new materials and applications.

A nanogenerator based on metal nanoparticles and magnetic ionic gradients

Developing a new technique/method and/or mechanism for separating ionic charges is critical to the fabrication of a high-performance nanogenerator. Inspired by charge migration and/or separation under an electric bias, herein, we demonstrate a reverse process in which the ionic charge gradients within a metal nanoparticle film are used to output electrical energy. The metal nanoparticle core is ‘jammed’ within the film, while the counterion itself is mobile and magnetic; therefore, ionic gradients are created through an external magnetic field. The distribution gradient of magnetic counterions is subsequently transformed into an electric potential within the metal nanoparticle film and into an electron flow in the external circuit. The performance of the nanoparticle nanogenerator is also optimized, and the highest output open-circuit voltage reaches 0.55 V. Finally, we develop a continuum charge-transport model combining Poisson and Nernst‒Planck diffusion equations to simulate the production of electrical energy within metal nanoparticle films.

Nanoparticle Printing for Microfluidic Applications: Bipolar Electrochemistry and Localized Raman Sensing Spots

The local integration of metal nanoparticle films on 3D-structured polydimethylsiloxane (PDMS)-based microfluidic devices is of high importance for applications including electronics, electrochemistry, electrocatalysis, and localized Raman sensing. Conventional processes to locally deposit and pattern metal nanoparticles require multiple steps and shadow masks, or access to cleanroom facilities, and therefore, are relatively imprecise, or time and cost-ineffective. As an alternative, we present an aerosol-based direct-write method, in which patterns of nanoparticles generated via spark ablation are locally printed with sub-mm size and precision inside of microfluidic structures without the use of lithography or other masking methods. As proof of principle, films of Pt or Ag nanoparticles were printed in the chambers of a multiplexed microfluidic device and successfully used for two different applications: Screening electrochemical activity in a high-throughput fashion, and localized sensing of chemicals via surface-enhanced Raman spectroscopy (SERS). The versatility of the approach will enable the generation of functional microfluidic devices for applications that include sensing, high-throughput screening platforms, and microreactors using catalytically driven chemical conversions.

Langmuir and Langmuir-Blodgett Films of Gold and Silver Nanoparticles.

Recently the focus of the Langmuir-Blodgett technique as a method of choice to transfer monolayers from the air/water interface onto solid substrates in a controllable fashion has been shifting toward purely hydrophobic gold and silver nanoparticles. The fundamental interactions between particles that become relevant in the absence of polar groups range from dispersive attractions from the metal cores and repulsions between ligand shells to weaker entropic factors. The layer evolution is explored, starting with interfacial self-assembly upon solution spreading and domain and circular island formation, which subsequently merge into a complete monolayer and finally form multilayers or macroscopic wrinkles. Moreover, structural properties such as the core:ligand size ratio are investigated in the context of dispersive forces, whereby the nanoparticles with small cores and long ligands tend not to aggregate sufficiently to produce continuous films, those with large cores and short ligands were found to aggregate irreversibly, and those in between the two extremes were concluded to be able to form highly organized crystalline films. Similarly, the characteristics of the spreading solution such as the concentration and the solvent type crucially influence the film crystallinity, with the deciding factor being the degree of affinity between the capping ligand and the solvent used for spreading. Finally, the most common strategies employed to enhance the mechanical stability of the metal nanoparticle films along with the recent attempts to functionalize the particles in attempts to improve their applicability in the industry are summarized and evaluated in relation to their future prospects. One of the objectives of this feature article is to elucidate the differences between hydrophobic metal nanoparticles and typical amphiphilic molecules that the majority of the literature in the field describes and to familiarize the reader with the knowledge required to design Langmuir-Blodgett nanoparticle systems as well as the strategies to improve existing ones.

Fluorophore-Induced Plasmonic Current Generation from Aluminum Nanoparticle Films.

In this paper we demonstrate fluorophore induced plasmonic current (FIPC) from aluminum nanoparticle films. It has been previously shown that near-field excited fluorophores are able to plasmonically couple with metal nanoparticle films (MNF's) and induce surface plasmons, which in turn leads to a direct measurable electrical current through the MNF. These currents have been detected and quantified in noble metal MNF's, however due to future envisioned cost considerations there has been a push to adapt FIPC for use with less expensive metals. Subsequently, we observe that plasmonic aluminum films are able to produce these current changes when in close proximity to excited fluorophores, and the magnitude of the current changes are respective to the magnitude of the extinction coefficients of the fluorophores themselves. These findings also further support recent literature reports showing the inverse relationship between metal-enhanced fluorescence (MEF) and FIPC.

Electron transport in discontinuous metal thin films

The structure and basic experimental electrical properties of vacuum evaporated discontinuous (island) metal thin films of discrete metal nanoparticles on insulating substrates are briefly reviewed. Then the widely accepted Neugebauer and Webb (N&W) electrostatically activated electron tunneling conduction model is covered (with enhancements) before the numerous discrepancies between this model and experimental observations are identified, e.g. minimal substrate bias effect, non-linear field distribution, anomalous AC effects, asymmetrical contact effects, and switching. A modified model, based on contact electron injection and extraction, and computer simulations are introduced which explain these discrepancies at a qualitative level. However, quantitative experimental verification of the model is not possible without stable, reproducible films of known structures. The paper concludes with a review of possible preparation techniques which could yield satisfactory samples, especially self-assembly of organically protected metal nanoparticles. One of these has already demonstrated electrostatically activated conduction.

A new type of photocathode driven by a laser

To meet the needs of high-frequency, miniaturized vacuum microwave devices and identify suitable cathode materials and laser systems, a new photocathode for microwave vacuum electronic devices has been studied. Based on our previous work, we have proposed a strategy to develop this new type of cathode. First, a tungsten sponge diffusion barrier layer was used as the evaporation source of the emission material, instead of the traditional nickel tube heating method. This serves as the emission material by heating a layer of the emissive material storage chamber to provide a controlled photoemissive layer for replacing the evaporation loss of the active material, thereby extending the life of the cathode and enabling recovery from poisoning and exposure to the atmosphere. Then, the photocathode with Os and Osnanoparticle films were prepared. This n-type photocathode demonstrated some features that were different from those of traditional photocathodes. To overcome the drawback of chemical methods used to clean the copper, we have proposed a strategy to develop a new method. A tungsten copper alloy substrate was heated by a high-frequency heating coil crucible fired at 1900 K. The copper in the tungsten copper alloy matrix was thereby heated and it evaporated rapidly. Then the copper was deposited onto a condenser. The results show that this technology leaves no residual on the substrate surface, occurs over a short time interval, and produces little environmental pollution. A 50-nm-diameter OS metal nanoparticle film was prepared on the surface of the photocathode substrate by using a DC magnetron sputtering metal target method, and a Cs$_3$Sb photocathode emission experimental diode with a titanium pump was prepared. The degree of vacuum in the tube can reach 5$\\times$10$^{-8}$ Pa. The emission performance of the Os film deposited on the substrate and the Os nanoparticle film photocathode was tested. In the experiment, the maximum laser power values for stable emission were found to be 0.416 and 0.193 W, respectively, and the photoemission current densities were 40.8 and 53.9 mA/cm$^2$, respectively. The quantum efficiencies were calculated to be 1.80$\\times$10$^{-3}$ and 5.13$\\times$10$^{-3}$, respectively. It can be inferred that the light absorption rate of the Os-nanoparticle-film-coated photocathode is a factor of 2.16 higher than that of the Os metal film, and the quantum efficiency is a factor of 2.85 greater. Analysis indicates that the main reason for improvement of the quantum efficiency of the Os nanoparticle film photocathode comes from the improvement of the light absorption rate.

Metal Particle Evolution Behavior during Metal Assisted Chemical Etching of Silicon

Due to its high practicability, metal assisted chemical etching (MACE) has become a popular silicon micro-nano structure preparation method. This paper explores the metal nanoparticle film dissolution and re-deposition evolution behavior in oxidizing HF solution during the MACE process. The results show Ag nanoparticles experience both oxidation dissolve and reduction deposit during MACE in HF–H2O2–H2O and HF–Fe(NO3)3–H2O corrosion solution. The Au nanoparticle can remain stable in the HF–Fe(NO3)3–H2O solution, providing a strategy to keep the patterns’ high etching accuracy of the silicon micro-nano structure. This work illuminates the reaction principle and provides a new insight to improving the etching accuracy, stability, and speed of the MACE technology.

Slot-Die Coating Operability Window for Nanoparticle Bed Deposition in a Microscale Selective Laser Sintering Tool

Abstract This work seeks to develop a fundamental understanding of slot-die coating as a nanoparticle bed deposition mechanism for a microscale selective laser sintering (μ-SLS) process. The specific requirements of the μ-SLS process to deposit uniform sub-5 μm metal nanoparticle films while enabling high throughput fabrication make the slot-die coating process a strong candidate for layer-by-layer deposition. The key challenges of a coating system are to enable uniform nanoparticle ink deposition in an intermittent layer-by-layer manner. Identifying the experimental parameters to achieve this using a slot-die coating process is difficult. Therefore, the main contribution of this study is to develop a framework to predict the wet film thickness and onset of coating defects by simulating the experimental conditions of the μ-SLS process. The single-layer deposition characteristics and the operational window for the slot-die coating setup have been investigated through experiments and two-dimensional computational fluid dynamics simulations. The effect of coating parameters such as inlet speed, coating speed, and coating gap on the wet film thickness has been analyzed. For inlet speeds higher than the coating speed, it was found that the meniscus was susceptible to high instabilities leading to coating defects. Additionally, the study outlines the conditions for which the stability of the menisci upstream and downstream of the slot-die coater can affect the uniformity and thickness range of the coating.

Metal Nanoparticle Film Deposition by Femtosecond Laser Ablation at Atmospheric Pressure.

Nanoparticle gold films were deposited using femtosecond laser ablation in argon at atmospheric pressure in an arrangement where a flat Au target was irradiated through a transparent substrate in close proximity. Spatially extended films were made by rastering the target and substrate assembly together in the laser beam. Fast imaging clearly showed pronounced narrowing of the ablation plume, which can be understood in terms of laser induced multiphoton ionisation and heating of the gas near the ablation site. Deposition was possible for target-substrate separation up to 2 mm. The equivalent thickness of the nanoparticle film was controlled in the range 0.4–28 nm by changing the target-substrate separation and the shot-to-shot spacing of ablation spot raster. The mean Feret diameter varied in the range 14–40 nm depending on the deposition conditions, and all the films showed a surface plasmon resonance at about 525 nm, which was nearly independent of the equivalent thickness. The technique can readily be applied to other materials for the fabrication of nanoparticulate films at atmospheric pressure.

Silver nanoparticle films by flowing gas atmospheric pulsed laser deposition and application to surface‐enhanced Raman spectroscopy

For the fast uptake into industrial applications, the further development of robust methods of nanomaterials, which are inexpensive and simultaneously technologically feasible, is one of the major key factors. A newly introduced atmospheric pulsed laser deposition method, based on a flowing gas approach, was used for plasmonic metal nanoparticle (NP) film of silver. Contrary to vacuum, in this method, the ambient air restricts expansion of the ablation plume within 1 to 3 mm above the target surface. These sets constrain on the formation of NP film close to the ablation spot. For deposition on a widely spaced surface, ablation material was entrained in a flow of argon, supplied at ~32 ms−1, and effectively delivered to the substrate at ~20 ms−1. The films produced were crystalline and particulate in nature, showing spectral plasmonic feature of surface plasmon resonance in the visible region. The film was directly tested in surface-enhanced Raman spectroscopy for chemical detection of crystal violet; the film with large particulates and aggregated crystallites was well-performed, showing enhanced Raman signals and detection sensitivity. Certainly, flowing gas atmospheric pulsed laser deposition seems a fast alternative to vacuum-pulsed laser deposition but needs further investigations to bring it in the industry for applications in sensor, catalysis, solar cell, and coating technology.

Enhanced Broadband and Harmonic Upconversion from Coupled Semiconductor and Metal Nanoparticle Films

The enhancement of nonlinear optical properties of hybrid nanoparticle films by coupling between metallic and plasmonic semiconducting nanoparticles is shown. A heterostructure comprising CuS and A...

Plasmonic Electricity II: The Effect of Particle Size, Solvent Permittivity, Applied Voltage, and Temperature on Fluorophore-Induced Plasmonic Current

In a recent paper, our laboratory has shown that fluorophores in close proximity to a non-continuous metal nanoparticle film can induce a detectable electrical current in the film. This current was...

Plasmonic Electricity: Fluorophore-Induced Plasmonic Current

Fluorophore-induced plasmonic current is generated when an excited fluorophore in close proximity to a metal nanoparticle film nonradiatively transfers energy to the metal, resulting in an electrical current across the film. Although a growing literature reports the use of surface plasmons for fluorescence enhancement as well as plasmons for current generation, little has been published hitherto regarding the electrical current generation via the fluorophore excitation of plasmons. Our “plasmon to current” technique utilizes electron transport between closely spaced metal nanoparticles, generating a measurable electrical signal upon excitation of a proximal fluorophore. This induced electrical signal is found to be strongly dependent on the magnitude of the fluorophore extinction coefficient. In other words the electrical signal contains photophysical information pertaining to the fluorophore, potentially leading to the direct detection of fluorescence without the need for traditional detectors such as ph...

Biomaterial-based possibilities for managing peri-implantitis.

Peri‐implantitis is an inflammatory disease of hard and soft tissues around osseointegrated implants, followed by a progressive damage of alveolar bone. Oral microorganisms can adhere to all types of surfaces by the production of multiple adhesive factors. Inherent properties of materials will influence not only the number of microorganisms, but also their profile and adhesion force onto the material surface. In this perspective, strategies to reduce the adhesion of pathogenic microorganisms on dental implants and their components should be investigated in modern rehabilitation concepts in implant dentistry. To date, several metallic nanoparticle films have been developed to reduce the growth of pathogenic bacteria. However, the main drawback in these approaches is the potential toxicity and accumulative effect of the metals over time. In view of biological issues and in attempt to prevent and/or treat peri‐implantitis, biomaterials as carriers of antimicrobial substances have attracted special attention for application as coatings on dental implant devices. This review will focus on biomaterial‐based possibilities to prevent and/or treat peri‐implantitis by describing concepts and dental implant components suitable for engagement in preventing and treating this disease. Additionally, we raise important criteria referring to the geometric parameters of dental implants and their components, which can directly affect peri‐implant tissue conditions. Finally, we overview currently available biomaterial systems that can be used in the field of oral implantology.

Various pulsed laser deposition methods for preparation of silver-sensitised glass and paper substrates for surface-enhanced Raman spectroscopy

Silver nanoparticle films on glass and paper were prepared using vacuum pulsed laser deposition, and various methods of atmospheric pulsed laser deposition, where ablated material forms a nanoparticle aerosol near the target and is delivered in a gas flow to the substrate. The performance of the films for surface-enhanced Raman spectroscopy (SERS) was investigated using a 10–4 M aqueous solution of rhodamine 6G organic dye. The SERS sensitivity was quantified in terms of the apparent enhancement factor and the signal-to-noise ratio. For the films made by supersonic atmospheric pulsed laser deposition, the apparent enhancement factor was found to be 15,000, which is 150 times higher than the value for a commercial silver nanoparticle paper-based substrate. This study demonstrates the utility of atmospheric pulsed laser deposition for the fabrication of noble metal nanoparticle films, and offers new approaches to tailoring the particle morphology for high SERS performance.

Fluorophore-Induced Plasmonic Current

Fluorophore-induced plasmonic current is generated when a near-field excited fluorophore in proximity to a metal nanoparticle film induces plasmon resonance in the metal, subsequently generating electrical current, only if certain conditions are met. This highly novel technology allows for the detection of fluorescent molecules without the need for traditional detectors such as photomultiplier tubes, avalanche photodiodes, and linear arrays. In this paper, we describe our recent photophysical experiments which describe how plasmonic current can be generated. For example, we discuss the excitation polarization, fluorophore oscillator strength, nanoparticle spacing, and solution dielectric. In addition, we show that the background current from common matrices, such as urine, blood, and faeces contributes little to background plasmonic current in plasmonic immunoassays, which is in stark contrast to traditional fluorescence assays that reply on optical based detection.

Surface-enhanced Raman scattering effect of composite structure with gold nano-cubes and gold film separated by polymethylmethacrylate film

The composite structure of metal nanoparticle and metal film can be used as a surface-enhanced Raman scattering (SERS) substrate to significantly enhance the Raman signal of adsorbed molecules due to the strong coupling between local surface plasmons and propagating surface plasmons. An SERS substrate of the composite structure with gold nano-cubes and gold film separated by polymethylmethacrylate (PMMA) film is proposed. The optimum thickness of PMMA is 15 nm obtained by numerical simulation through using finite element method. The composite structure of PMMA spacer with a thickness of 14 nm is prepared experimentally. Using R6G as the Raman probe molecules and He-Ne laser with a wavelength of 633 nm as an excitation source, the SERS effect of the composite structure and single gold nano-cubes are studied. It is found that the composite structure can make the probe molecules produce a stronger Raman signal than the single structure. Furthermore, the SERS spectra of R6G molecules on the composite structure under the condition of aqueous solution of gold nano-cubes with different concentrations are studied. The results show that when the concentration of gold nano-cubes’ aqueous solution is 5.625 <inline-formula><tex-math id="M2">\begin{document}${\text{μ}}{\rm g/mL}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20190054-ue-wan-revised_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20190054-ue-wan-revised_M2.png"/></alternatives></inline-formula>, the SERS signal of the R6G molecules on the composite structure is strongest. The lowest concentration of R6G molecules which can be detected is about 10<sup>–11</sup> mol/L.