Quantum-sized metallic particles

Composites containing nanometer-sized metal particles of a controllable and uniform size in an insulating ceramic matrix are very interesting materials for use as heterogeneous catalysts and for magnetic and electronic applications. They show quantum size effects, particularly the size-induced metal-insulator transition (SIMIT) [1],... 

Synthesis and Optical Properties of Quantum-Size Metal Sulfide Particles in Aqueous Solution 109... 

Cao, G., L. K. Rabenberg, C. M. Nunn, and T. E. Mallouk. 1991. Formation of quantum-size semiconductor particles in a layered metal phosphonate host lattice. Chem. Mater. 3 149-156. 

We note that zeolites have also been used as hosts for a number of other intriguing "nanocomposites", for example in the field of encapsulated quantum-size semiconductor particles such as Se, CdS, CdSe, PbS, and GaP.3h32,33,34,35 phe encapsulation of metals such as Bi, Hg, Sn and Ga in zeolites has been described by Bogolomov.36 These studies demonstrate the enormous versatility of zeolite host systems for studies and control of structural/electronic relationships. 

Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

The rapid development of nanotechnology has revolutionized scientific developments in recent decades [1]. The synthesis, characterization, and application of functionalized nanoparticles are currently a very active field of research [2], Due to the size limitation of metal nanoparticles, they show very unique properties, which are called nano-size effect or quantum-size effect , which is different from those of both bulk metals and metal atoms. Such specific properties are usually dominated by the atoms located on the surface. In nanoparticles systems, the number of atoms located on the surface of the particles increases tremendously with decreasing of the particle diameter [3]. 

The emission of the metal particles may thus originate from a band-to-band transition in the metal particle, which occurs at about 516 nm for gold [60, 119]. As stated above, the nature of the interaction of the dendrimer (PAMAM) host is still uncertain, there could be very strong electrostatic interactions that may play a part in the enhancement of the metal particles quantum efficiency for emission. However, one would expect that this enhancement would result in slightly distorted emission spectra, different from what was observed for the gold dendrimer nanocomposite. Further work is necessary to completely characterize the manner in which the dendrimer encapsulation enhances the emission of the metal nanoparticles. With further synthetic work in preparation of different size nanoparticles (in other words elongated and nonspherical shape particles, including nanorods) it may be possible to develop the accurate description of a... 

Practical metal catalysts frequently consist of small metal particles on an oxide support. Suitable model systems can be prepared by growing small metal aggregates onto single crystal oxide films, a technique whereby the role of the particle size or of the support material may be studied. [37] A quite remarkable example of the variation of the catalytic activity with particle size has recently been found for finely dispersed Au on a Ti02 support, which was revealed to be highly reactive for combustion reactions. [38] On the basis of STM experiments it was concluded that this phenomenon has to be attributed to a quantum size effect determined by the thickness of the gold layers. 

There are families of metal cluster compounds (Fig. 6.40) containing metal clusters surrounded by ligands (Lewis Green, 1982). In small cluster compounds, the electrons are paired, but in large clusters there will be closely spaced electronic levels, as in metal particles. In such clusters, quantum size effects would be expected. Benfield et al (1982) have found intrinsic paramagnetism in H20sio(CO)24 below 70K as expected of an osmium particle of approximate diameter of 10 A the excess paramagnetism increases with cluster size in osmium compounds (Johnson et al, 1985). 

The formula (11) in view of relations for /ie and /ih describes above-mentioned basic features of size effects in semiconductor crystal. It is important that as against metals, semiconductors show appreciable quantum dimensional effects at the sizes of particles from 3 to lOnm (depending on electronic structure of the semiconductor and sizes of AE0) [20]. Such nanoparticles are usually formed at synthesis of nanocomposite films. 

Thus, the size effects for catalytic reactions of metal atom clusters in a gas phase are manifested only in very small, essentially quantum clusters, which are in essence nonmetal particles. Another situation takes place in films, containing a set of nanoparticles immobilized at a surface or inside of a dielectric matrix. In this case the influence of M nanoparticle size on catalytic activity and structure of products formed is observed for considerably larger already classical particles of sizes from 2 ( 150 atoms) to 20-30 nm ( 105 atoms) [113, 114]. It is necessary to note that catalytic properties of M nanoparticles in composite systems are determined substantially by their interaction with a matrix, which depends on the size of particles. 

As it was mentioned above, even if at least one of the metal particle sizes is of nanometer range, the electron energy quantization could affect the physical properties of the nanogranular metal. The influence of the quantum-size effects (QSE) on the electronic transport in granular metals is especially pronounced in the vicinity of the percolation threshold [85] and it is the main subject of the present section. 

The use of a bulk-like dielectric constant, such as those in Equations (2.334)-(2.336), neglects the specific contribution given by the surface to the dielectric response of the metal specimen. For metal particles, such a contribution is often introduced in the model by considering the surface as an additional source of scattering for the metal conduction electrons, which consequently affects the relaxation time r [69], Experiments indicate that the precise chemical nature of the surface also plays a role [70], The presence of a surface affects the nonlocal part of the metal response as well, giving rise to surface-assisted excitations of electron-hole pairs. The consequences of these excitations appear to be important for short molecule-metal distances [71], It is worth remarking that, when the size of the metal particle becomes very small (2-3 nm), the electron behaviour is affected by the confinement, and the metal response deviates from that of the bulk (quantum size effects) [70],... 

For the same particles, the volume plasmon is located at very high energies (6-9 eV). The surface obviously plays a very important role for the observation of the surface plasmon resonance because it alters the boundary conditions for the polarizability of the metal and therefore shifts the resonance to optical frequencies. In this sense, the surface plasmon absorption is a small particle (or thin layer) effect but is definitely not a quantum size effect [14]. 

The relevance to small particles and indeed massive surfaces now becomes clear, because the preponderance of low CN atoms increases as particle size goes down, and this may turn out to be the most important factor in determining reactivity14 — more important than quantum size effects (i.e. the metal —> nonmetal transition), surface mobility or any of the other properties that are characteristic of very small assemblies of atoms (see Section 3.4). It becomes possible to imagine that activity in catalytic oxidations is solely due... 

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