Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

Coinage metals, such as Au, Ag, and Cu, have been important materials throughout history.1 While in ancient cultures they were admired primarily for their ability to reflect light, their applications have become far more sophisticated with our increased understanding and control of the atomic world. Today, these metals are widely used in electronics, catalysis, and as structural materials, but when they are fashioned into structures with nanometer-sized dimensions, they also become enablers for a completely different set of applications that involve light. These new applications go far beyond merely reflecting light, and have renewed our interest in maneuvering the interactions between metals and light in a field known as plasmonics.2–6

In plasmonics, the metal nanostructures can serve as antennas to convert light into localized electric fields (E-fields) or as waveguides to route light to desired locations with nanometer precision. These applications are made possible through a strong interaction between incident light and free electrons in the nanostructures. With a tight control over the nanostructures in terms of size and shape, light can be effectively manipulated and controlled with unprecedented accuracy.3,7 While many new technologies stand to be realized from plasmonics, with notable examples including superlenses,8 invisible cloaks,9 and quantum computing,10,11 conventional technologies like microprocessors and photovoltaic devices could also be made significantly faster and more efficient with the integration of plasmonic nanostructures.12–15 Of the metals, Ag has probably played the most important role in the development of plasmonics, and its unique properties make it well-suited for most of the next-generation plasmonic technologies.16–18

1.1. What is Plasmonics?
Plasmonics is related to the localization, guiding, and manipulation of electromagnetic waves beyond the diffraction limit and down to the nanometer length scale.4,6 The key component of plasmonics is a metal, because it supports surface plasmon polariton modes (indicated as surface plasmons or SPs throughout this review), which are electromagnetic waves coupled to the collective oscillations of free electrons in the metal.

While there are a rich variety of plasmonic metal nanostructures, they can be differentiated based on the plasmonic modes they support: localized surface plasmons (LSPs) or propagating surface plasmons (PSPs).5,19 In LSPs, the time-varying electric field associated with the light (Eo) exerts a force on the gas of negatively charged electrons in the conduction band of the metal and drives them to oscillate collectively. At a certain excitation frequency (w), this oscillation will be in resonance with the incident light, resulting in a strong oscillation of the surface electrons, commonly known as a localized surface plasmon resonance (LSPR) mode.20 This phenomenon is illustrated in Figure 1A. Structures that support LSPRs experience a uniform Eo when excited by light as their dimensions are much smaller than the wavelength of the light.



Figure 1

Schematic illustration of the two types of plasmonic nanostructures discussed in this article as excited by the electric field (Eo) of incident light with wavevector (k). In (A) the nanostructure is smaller than the wavelength of light and the free electrons ...



In contrast, PSPs are supported by structures that have at least one dimension that approaches the excitation wavelength, as shown in Figure 1B.4 In this case, the Eo is not uniform across the structure and other effects must be considered. In such a structure, like a nanowire for example, SPs propagate back and forth between the ends of the structure. This can be described as a Fabry-Perot resonator with resonance condition l=nλsp, where l is the length of the nanowire, n is an integer, and λsp is the wavelength of the PSP mode.21,22 Reflection from the ends of the structure must also be considered, which can change the phase and resonant length. Propagation lengths can be in the tens of micrometers (for nanowires) and the PSP waves can be manipulated by controlling the geometrical parameters of the structure.23

/pdf/controlling-the-synthesis-and-assembly-of-silver-1ktlgomyt6.pdf

Controlling the synthesis and assembly of silver nanostructures for plasmonic applications

Surface modification of inorganic nanoparticles for development of organic–inorganic nanocomposites—A review

Thermal Conductivity of Polymer-Based Composites: Fundamentals and Applications

https://www.cell.com/trends/biotechnology/pdf/S0167-7799(10)00122-8.pdf

Nanosilver as a new generation of nanoproduct in biomedical applications

Mechanisms of PVP in the preparation of silver nanoparticles

Preparation of silver nanoparticles by chemical reduction method

Noble metallic nanomaterials are of particular interest today because of their applications in many areas. The selected topic is one of the most active regions in the metallic nanomaterials. First, common synthetic methods of silver nanomaterials, e.g., chemical reduction, photochemical method, ultrasonic-assisted reduction, electrochemical method, template, irradiating reduction and biochemical method, are introduced. The advantages and drawbacks of these methods are also reviewed. Secondly, we mainly discuss the formation of silver nanoparticles in microemulsion. Besides an introduction to the concept, type and formation of microemulsion, we focus especially on the influencing factors of silver nanoparticle formation. Effects of the type of continuous phase, molar ratio of water to surfactant (W), precursor concentration and reductant type and concentration on the particle formation are summarized and evaluated. Furthermore, current and possible developing trends of the synthetic technology of silver nanoparticles are also prospected.

Synthesis of silver nanoparticles—Effects of concerned parameters in water/oil microemulsion

Silver nanoparticles are considered to apply a silver paste for electrode because of their high conductivity. However, the dispersion of silver nanoparticles in electronically conductive adhesives (ECAs) restricts them used as conductive fillers. A simple method had enabled the synthesis of silver nanoparticles by reducing silver nitrate with ethanol in the presence of poly(N-vinylpyrrolidone) (PVP). Reaction conditions, such as silver nitrate concentration, PVP concentration, reaction time, and reaction temperature, had been studied. Fine dispersion and narrow size distribution of silver nanoparticles were obtained. They were added to ECAs by re-dispersing them in ethanol while it was used as the diluent to adjust the volatility of ECAs, preventing them from the aggregation and increasing the chance to fill the gaps between silver flakes. This proposed process offers the possibility to effectively use these synthesized silver nanoparticles for improving the conductivity of ECAs.

Jianguo Chen

Papers

Mechanisms of PVP in the preparation of silver nanoparticles

Preparation of silver nanoparticles by chemical reduction method

Synthesis of silver nanoparticles—Effects of concerned parameters in water/oil microemulsion

Synthesis and electrical properties of uniform silver nanoparticles for electronic applications

Investigation on the efficiency and mechanism of Cd(II) and Pb(II) removal from aqueous solutions using MgO nanoparticles.