Laser techniques nanoparticle preparation

Another thin film technology based nanoparticle preparation route is gas condensation, in which metal vapor is cooled to high levels of supersaturation in an inert gas ambient [126-128]. In these experiments particles necessarily nucleate in the gas phase. In a promising extension of this technique a pulsed laser beam replaces the conventionally used thermal metal vapor source [120,121,129-134]. 

The approaches used for preparation of inorganic nanomaterials can be divided into two broad categories solution-phase colloidal synthesis and gas-phase synthesis. Metal and semiconductor nanoparticles are usually synthesized via solution-phase colloidal techniques,4,913 whereas high-temperature gas-phase processes like chemical vapor deposition (CVD), pulsed laser deposition (PLD), and vapor transfer are widely used for synthesis of high-quality semiconductor nanowires and carbon nanotubes.6,7 Such division reflects only the current research bias, as promising routes to metallic nanoparticles are also available based on vapor condensation14 and colloidal syntheses of high-quality semiconductor nanowires.15... 

The various methods of preparation employed to prepare nanoscale clusters include evaporation in inert-gas atmosphere, laser pyrolysis, sputtering techniques, mechanical grinding, plasma techniques and chemical methods (Hadjipanyas Siegel, 1994). In Table 3.5, we list typical materials prepared by inert-gas evaporation, sputtering and chemical methods. Nanoparticles of oxide materials can be prepared by the oxidation of fine metal particles, by spray techniques, by precipitation methods (involving the adjustment of reaction conditions, pH etc) or by the sol-gel method. Nanomaterials based on carbon nanotubes (see Chapter 1) have been prepared. For example, nanorods of metal carbides can be made by the reaction of volatile oxides or halides with the nanotubes (Dai et al., 1995). 

The same dependence between concentration of M nanoparticles (Ns) on a surface of a dielectric substrate and their catalytic activity has been also found out in the investigation of an amorphous films of M nanoparticles [117], prepared by laser electrodispersion technique and deposited on Si02 dielectric surface layer of thermally oxidized Si (see Chapter 15). It has been shown that in various reactions of chlorinated hydrocarbons catalyzed by so prepared nanostructured Cu film with growth Ns the value of Y increases firstl, reaches a maximum at Ns 4 x 1012 particles/cm2, and then quickly falls. 

All the metallic nanostructures deposited by laser electrodispersion on both types of silicon substrates were found to be exceedingly active in the above processes. The activity was orders of magnitude higher than that of typical supported catalysts prepared by the standard techniques. Such a high activity is presumably due not only to the small size and amorphous state of nanoparticles, but also to the influence exerted by the charge effects discussed above. 

Another specific feature of the catalytic behavior of the structures under study consists in that the chemical nature of a metal becomes a factor less important for catalysis as the surface nanoparticles density increases. This is well seen in Figure 15.14, which shows the results obtained in measurements of the activity of copper- and nickel-based catalysts in the reaction of carbon tetrachloride addition to olefins. Presented in this figure are the activities of catalysts prepared by laser electrodispersion and the conventional deposition techniques. Two important features are worth noting. First, the activity... 

RE-TM-based films. - The deposition techniques (sputtering or PLD - pulsed laser deposition) have been used successfully to prepare SmCo5 and Nd2Fel4B nanoparticles [38, 39]. After room temperature deposition, the nanoparticles are amorphous. The desired crystal structure and magnetic properties are obtained through application of a post-deposition annealing treatment. Due to the high reactivity of the materials, this constitutes a non trivial task. 

SWCNTs exhibit unique physical and chemical properties that make them very attractive candidates for the production of new materials. Carbon nanotubes are made by wrapping up single sheets of graphite, known as graphene, upon themselves to form hollow, straw-like structures. Traditionally, SWCNTs have been prepared by electric arc-discharge, laser ablation and chemical vapor deposition (CVD) methods these techniques produce significant quantities of impurities, such as amorphous and graphitic forms of carbon and encapsulated catalytic metal nanoparticles. 

Last years considerable efforts have been directed to preparation of metal nanoparticles having a desired diameter and shape. A number of production techniques has been reported such as wet chemical processes, (co-precipitation, complexation, sol-gel), physical vapor deposition, sputtering, and laser ablation methods [1]. The ultimate goal of each technique is fabrication of monodisperse stmctures with a predetermined size, shape and arrangement. 

Laser ablation of metal targets in liquids provides a rapid and simple method for preparation of stable metal nanoparticles. Advantages of this technique include its versatility with respect to metals or solvents, and the absence of chemical reagents or ions in the final preparation. The developed technique offers a good control over the particles formation process and an effective collection and conservation of fabricated materials. 

The changes of character of distribution on nanoparticles sizes take place depending on the nature of nanocomposites, dielectric penetration and polarity of liquid phase. Below characteristics of finely dispersed suspensions of metal/carbon nanocomposites are given. The distribution of nanoparticles in water, alcohol and water-alcohol suspensions prepared based on the above technique are determined with the help of laser ana-... 

Figure 9.2 is a schematic representation of CdSe QDs dispersed in poly(hexyl methacrylate) by in situ polymerization. The polymer with long alkyl branches is expected to prevent or reduce phase separation of the QDs from the polymer matrix during polymerization. This technique resulted in the preparation of a series of QD-based nanocomposite materials for which laser scanned confocal microscopy imaging revealed a nearly uniform dispersion of nanoparticles within the polymethacrylate matrix (Fig. 9.3). Notably, the resulting macroscopic QD-polymer composites appeared to be clear and uniformly colored.

Summary Crystalline Si nanoparticles with diameters between 2.5 and 8 nm were prepared by CO2 laser-induced decomposition of silane in a gas flow reactor. A small portion of the products created in the reaction zone was extracted dirough a nozzle into a high-vacuum apparatus to form a freely propagating molecular beam of clusters and nanoparticles. This technique enables us to select the Si particles according to their size, to deposit them on a suitable substrate, and to study their photoluminescence (PL) as a function of their size. In another experiment, the evolution of the PL was monitored as a function of the time the samples were exposed to air. With increasing oxidation time, the PL became more efficient and shifted to smaller wavelengths. In a final experiment, the Si nanoparticle samples were treated with HF to remove the oxide layer and to study the effect on the PL properties. All observations can be explained in terms of quantum confinement as the origin for the PL behavior. 

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