Size-dependent properties

Obtaining high-quality nanocry stalline samples is the most important task faced by experimentalists working in tire field of nanoscience. In tire ideal sample, every cluster is crystalline, witli a specific size and shape, and all clusters are identical. Wlrile such unifonnity can be expected from a molecular sample, nanocrystal samples rarely attain tliis level of perfection more typically, tliey consist of a collection of clusters witli a distribution of sizes, shapes and stmctures. In order to evaluate size-dependent properties quantitatively, it is important tliat tire variations between different clusters in a nanocrystal sample be minimized, or, at tire very least, tliat tire range and nature of tire variations be well understood. 

The environment (e.g. the substrate) of the nanoparticles is a critical experimental parameter, which should be inert with respect to the nanoparticles. In the case of gold the native Si02 covered Si(l 0 0) seems to be an environment without any influence on the valence band of Au nanoparticles. The chemical and catalytic properties which are probably strongly correlated with the electronic structures of different systems, give another possibility to use and check the size dependent properties of nanoparticles. 

Our approach is similar to that employed in research of free cluster ions in the gas phase, where various measurements are conducted on the cluster which is mass selected out of the size-distributed clusters generated by laser sputtering. Based on the chemical compositions of the isolated MFCs, we discuss the determining factors of core size in connection with the formation processes. Some core-size dependent properties of the MFCs are also presented. 

Shah KVPM, Gedanken A (1998) Sonochemical preparation and size-dependent properties of nanostructured CoFe204 particles. Chem Mater 10 3445-3450... 

Pejova B, Grozdanov I, Nesheva D, Petrova A (2008) Size-dependent properties of sono-chemically synthesized three-dimensional arrays of close-packed semiconducting AgBiS2 quantum dots. Chem Mater 20(7) 2551-2565... 

In the ideal collision free environment of a molecular beam, the properties of a metal cluster can be considered to be truely isolated from cluster-substrate effects. Therefore, spectroscopic methods that can selectively extract information from metal cluster beams hold great promise for illuminating diverse size dependent properties of aggregates of metal atoms in their equilibrium configuration (23). 

Studies of useful size-dependent properties of nanomaterials are only possible when they are prepared and isolated in a monodisperse form. The synthesis, therefore, should address the need for a great degree of control over the structure, size, and also the composition of the particles. The design of successful synthetic strategies has enabled continuous exploration and exploitation of the unusual properties of nanomaterials that differ both from the single atom (molecule) and the bulk. This also suggests that the intended use of the nanomaterials will dictate the method that can be conveniently applied to obtain them. 

Silicon microstructures can be categorized according to the dimensionality of the confinement. Most PL studies deal with silicon structures confined in three dimensions such dot-like structures are designated zero-dimensional (OD). An overview of size-dependent properties of silicon spheres is given in Table 6.1. Standard methods of generating such microstructures are gas-phase synthesis [Di3, Li7, Scl2], plasma CVD [Ru2, Col, Ta8] or conventional chemical synthesis [Mal5]. 

As a result of the particle size-dependent properties the accumulation mode particles having highest penetration efficiencies and lowest deposition rates tend to enter indoors most efficiently and remain suspended there, thus substantially contributing to indoor exposures. Another implication is that the particle size distribution indoors differs significantly from that outdoors, even in the absence of indoor sources. Finally particle infiltration varies from home to home, resulting in higher variability across homes in indoor particle concentrations compared to outdoor concentrations. 

Another reason for the size-dependent properties in granular metals is not manifestation of quantum effects, but the rise of the surface state fraction. If one considers a metal granule as a sphere with diameter D and thickness of the surface layer hs, it is clear that the interface portion depends on the granule size and its relative contents are described by the following equation ... 

The size-dependent properties of nanoparticles differ greatly from the corresponding bulk materials. An example is the size quantization phenomenon commonly observed in II-VI and III-V inorganic semiconductor nanocrystals.6 During the intermediate transition towards that of the bulk metal (usually between 2 and 20 nm), localization of electrons and holes in a confined volume causes an increase in its effective optical band gap as the size of the nanoparticle decreases, observed as a blue shift in its optical spectrum. Bms predicted that there should also be a dependence on the redox potential for these same classes of quantum dots.7 Bard and coworkers showed this experimentally and have reported on the direct observation between the... 

We have hitherto discussed in the earlier sections, electronic structure and properties, chemical reactivity and self-assembly of nanocrystals, particularly those of metals. Hie discussion should suffice to illustrate how size if a crucial factor in deciding the chemistry in the nano regime. These size dependent properties form the basis of nanoscience, where the properties are exploited for possible applications. 

A polymer/monomer (polymer/repeat-unit or polymer/macrocycle) switch may become of practical importance where a polymer decorated with certain groups has specific size-dependent properties that the monomeric units do not have. The modulation of the conversion between polymeric and monomeric (or macrocyclic) states would also result in the modulation of these properties. Moreover, such size switches, represented by polymerization/depolymerization processes that operate under the control of external events, are examples of environmentally-friendly recyclable polymers (reduction of waste treatment). As well, if the polymer has low solubility and the polymer/monomer switch can work in spite of this, then it becomes possible to reversibly generate a precipitating (solid) polymeric material from a liquid solution of monomer. 

A commonly used size dependent property is the equivalent spherical diameter. The equivalent spherical diameter is the diameter of a sphere with the same volume as the particle. For a cube this sphere would have a diameter 1.24 times the edge length of the cube. Another common equivalent spherical diameter is the Stokes diameter. The Stokes diameter is the diameter of a sphere that has the same terminal settling velocity as an irregular particle. (Note Settling has to be under laminar flow [i.e., Re3molds number less than 0.2] in both cases and the density of both the particle and the sphere are assumed to be the same). 

Specific size-dependent properties cover magnetic, mechanic, electronic, optical, thermodynamic, and thermal... 

Transition metal nanoparticles have attracted great attention due to their unique size-dependent properties and applications in diverse areas, including magnetic storage materials, catalysis, sensors and drug delivery. Depositions of various Pt-containing alloys are summarized in Table 2. Particularly, chemically synthesized transition metal alloy... 

Derived diameters are determined by measuring size-dependent properties of particles and relating them to single linear dimensions. The most widely used of these are the equivalent spherical diameters. Thus, a unit cube has the same volume as a sphere of diameter 1.24 units hence this is the derived volume diameter. The diameter therefore depends upon the measured property. Consider a cube of side 1 cm its volume V = 1 cm and its superficial surface area S = 6 cm, is the diameter of a sphere having the same volume as the cube and d is the diameter of a sphere having the same surface area. 

In making a decision on which particle sizing technique to us, the analyst must consider the purpose of the analysis. What is generally required is not the size of the particles, but the value of some property that is size dependent. In such circumstances it is important whenever possible to measure the size dependent property, rather than to measure the size by some other method and then deduce the required property. For example, in determining the size of boiler (fly) ash with a view to predicting atmospheric pollution the terminal velocities of the particles should be measured in measuring the size of catalyst particles, the surface area should be measured, since this is the property that determines the reactivity. The cost of the equipment as well as the ease and the speed with which the analysis can be carried out have then to be considered. The final criteria are that the method shall measure the appropriate property of the particles, with accuracy sufficient for the particular application at an acceptable cost, in a time that will allow the result to be used. 

Abstract Immobilized metallic and bimetallic complexes and clusters on oxide or zeolite supports made from well-defined molecular organometaUic precursors have drawn wide attention because of their novel size-dependent properties and their potential applications for catalysis. It is speculated that nearly molecular supported catalysts may combine the high activity and selectivity of homogenous catalysts with the ease of separation and robustness of operation of heterogeneous catalysts. This chapter is a review of the synthesis and physical characterization of metaUic and bimetallic complexes and clusters supported on metal oxides and zeohtes prepared from organometaUic precursors of well-defined molecularity and stoichiometry. 

Because of the importance of size-dependent property changes to the materials sciences, size-property relationships have been studied in detail for some systems. For example, for semiconductors, size effects become important when the particle diameter is close to the Bohr diameter of excitons in the bulk phase. Generally, semiconductor size quantization effects (relevant for naturally occurring metal sulfides, for example) appear when particles are less than 10 nm in diameter (Vogel and Urban 1997). 

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