Colloidal systems optical properties

Metal nanoparticles have extraordinary size-dependent optical properties, not present in the bulk metal and have, consequently, been the subject of intense research during the past decade or so.27 Attention has recently focused on functionalising colloidal nanoparticles with molecular recognition components for potential sensing applications.28,29,30 We have prepared a new amido-disulfide functionalised zinc metalloporphyrin (8) which was self-assembled on to gold nanoparticles to produce a novel anion-selective optical sensing system (9) (Scheme 3).4... 

As discussed in Chapters 1-7, diffusion, Brownian motion, sedimentation, electrophoresis, osmosis, rheology, mechanics, interfacial energetics, and optical and electrical properties are among the general physical properties and phenomena that are primarily important in colloidal systems [12,13,26,57,58], Chemical reactivity and adsorption often play important, if not dominant, roles. Any physical chemical feature may ultimately govern a specific industrial process and determine final product characteristics, and any colloidal dispersions involved may be deemed either desirable or undesirable based on their unique physical chemical properties. Chapters 9-16 will provide some examples. 

The first part of the chapter reviews progress in the synthesis of monodisperse semiconductor NCs and gives a basic introduction to their specific physical properties. In conformity with the literature, the term monodisperse is used here to describe colloidal samples, in which the standard deviation of the particle diameter does not exceed 5%. Throughout the text we will restrict ourselves to the description of binary II-VI (CdSe, CdS, CdTe, ZnSe, etc.), III-V (InP, InAs), and IV-VI (PbS, PbSe, PbTe) semiconductor NCs. These systems exhibit optical properties that can be varied in the visible part of the spectrum, the near UV or near IR by changing the NC size and/or composition. 

It becomes obvious why surface effects were first observed and studied in the colloidal state. In this case one deals with particles that are so small that the number of atoms or molecules in the surface represents a substantial fraction of all the material that makes up the colloidal system. Colloidal particles are defined as having at least one dimension in the range of 1 micron to 1 nm. In modern chemistry however, particles in the nanometer range are given a special name, nanoparticles. The reason is that in this range several completely unpredictable phenomena start to appear [234], The reason is obvious these particles behave largely quantum-mechanically and therefore completely differently from ordinary colloidal particles. In many instances unusual mechanical, acoustical, electrical and optical properties are associated with nanomaterials, which have also been mentioned in connection with controversial issues such as room-temperature superconductivity and cold fusion. 

New routes to high quality colloidal NCs of varying compositions and shapes appear in the literature almost on a weekly basis. The most successful of these approaches to crystalline semiconductor NCs with excellent optical properties share the same basic premise of pyrolysis of metal-organic precursors in solutions of hot coordinating solvents. In contrast, the syntheses of noble metal NCs typically rely on the reduction of metal salts in the presence of citrate ions (e.g. An or Ag), in two-phase systems (e.g. An or Ag), or under high-temperature conditions in the presence of stabilizing... 

Most of the electrochemical phenomena occur in size regimes that are very small. The effects of size on diffusion kinetics, electrical double layer at the interface, elementary act of charge transfer and phase formation have recently been reviewed by Petrri and Tsirlina [12]. Mulvaney has given an excellent account of the double layers, optical and electrochemical properties associated with metal colloids [11]. Special emphasis has been given to the stability and charge transfer phenomenon in metal colloid systems. Willner has reviewed the area of nanoparticle-based functionalization of surfaces and their applications [6-8]. This chapter is devoted to electrochemistry with nanoparticles. One of the essential requirements for electrochemical studies is that the material should exhibit good conductivity. 

In Fig. 6, we illustrate some different ways that the core-shell topology could be varied for silica and gold. So far we have considered the two normal core-shell structures. We now focus on the third example the assembly of Au Si02 nanoparticles onto spherical polystyrene latex colloids. The resulting spheres are also essentially different to continuous metal shells grown on colloid templates, which have been reported by Halas and colleagues [17] and by van Blaaderen and coworkers [18]. Such continuous shells display optical properties associated with resonances of the whole shell, and are therefore extremely sensitive to both core size and shell thickness, while in the system presented here... 

An additional system prepared by colloidal chemistry which continues to attract considerable interest is that of QRs that exhibit electronic and optical properties that differ from those of QDs. For example, due to their cylindrical symmetry, QRs have a linearly polarized emission, as demonstrated by fluorescence measurements on single rods [41], leading to polarized lasing [14]. The powerful combination of optical and turmeling spectroscopy discussed here was also applied to investigations of the length- and diameter-dependence of the electronic level stmcture of CdSe QRs [40]. 

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