Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346

Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

This review focuses on the synthesis, protection, functionalization, and application of magnetic nanoparticles, as well as the magnetic properties of nanostructured systems. Substantial progress in the size and shape control of magnetic nanoparticles has been made by developing methods such as co-precipitation, thermal decomposition and/or reduction, micelle synthesis, and hydrothermal synthesis. A major challenge still is protection against corrosion, and therefore suitable protection strategies will be emphasized, for example, surfactant/polymer coating, silica coating and carbon coating of magnetic nanoparticles or embedding them in a matrix/support. Properly protected magnetic nanoparticles can be used as building blocks for the fabrication of various functional systems, and their application in catalysis and biotechnology will be briefly reviewed. Finally, some future trends and perspectives in these research areas will be outlined.

Magnetic nanoparticles: Synthesis, protection, functionalization, and application

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

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

A dynamic model combining molecular kinetic theory and classical nucleation theory was used to simulate the heterogeneous vapor-to-crystal phase transition in a closed vacuum chamber at low substrate temperature. The governing equations for the motes of molecules adsorbed to the substrate, the pressure in the chamber, and the number and mass of the crystals formed were simultaneously integrated. The model showed that the heterogeneous phase transition changed from one controlled by crystal growth to one controlled by heterogeneous nucleation as the condensation coefficient decreased. On the basis of the observation and the assumption for the adherence between the crystals and the substrate surface to be incomplete, the substrate temperature and the crystal surface temperature, respectively, were raised by 0.4 K and by 1 K at the onset of nucleation in the model computation. The changes in vapor pressure with elapsed time in the computed results agreed reasonably well with the experimental ones.

Simulation of a transient heterogeneous phase transition

Preparation of Functional Fine Particles by Low Pressure Spray Pyrolysis

Particles floating in plasma, similar to those in a gas phase, are influenced by Brownian motion, inertial motion due to the flow, thermophoresis, diffusiophoresis, and settling motion due to external forces such as gravity and electrical force. However, in contrast to the gas phase, particles move in a space in which there are pressures of 1–10−4 Torr, a temperature gradient, and an electrical field, so the conditions in which the theories of particle motion are applied are different from those in the gas phase. Because the theoretical equations of particle motion are the same as described in the previous chapter, this chapter will give the basic equations without any explanation.

Particle Deposition in Plasma

The behavior of aerosol particles of sub-micron diameter undergoing Brownian coagulation, Brownian diffusion and gravitational settling was studied by numerically solving the equation of population balance and by experimentally observing the change of aerosol properties with time. In calculation, two dimensionless parameters, which are determined by the initial properties of aerosols and vessel dimension in which aerosols are suspended, were introduced to evaluate the magnitude of influence of Brownian coagulation and Brownian diffusion on gravitational settling. The results of numerical calculation were indicated by graphical representation, which is usable for quantitative estimation of the magnitude of these two effects. These computational results were found to be in good agreement with the experimental results obtained by an ultramicroscopic technique of particle size analysis.

https://www.jstage.jst.go.jp/article/jcej1968/8/2/8_2_137/_pdf

Effect of brownian coagulation and brownian diffusion on gravitational settling of polydisperse aerosols

A model for determination of the electrical resistivity of isotropic conductive adhesives and insulator/conductor composites in general is introduced by considering the role of contact resistance between conductive adhesive constituents Conductive behaviour is considered to depend on the creation of conducting contacts between metallic filler particles instead of increasing in the concentration of metal particles This paper resolves several previously unsolved issues, including the effect of the volume fraction of filler particles and particle size on the development of conductivity; the observed variation in percolation threshold; and the possibility of occurence of two percolation thresholds In addition, the predicted pressure dependence of conductive behaviour is fully supported by the experimental observations The model also predicts that the percolation threshold can be reduced by pressure

Kikuo Okuyama

Papers

Simulation of a transient heterogeneous phase transition

Preparation of Functional Fine Particles by Low Pressure Spray Pyrolysis

Particle Deposition in Plasma

Effect of brownian coagulation and brownian diffusion on gravitational settling of polydisperse aerosols

Electrically isotropic conductive adhesives: an effective contact resistance model for conductivity development