Metal catalyzed chemical vapor deposition

A film is deposited in a conventional chemical vapor deposition (CVD) process when the gaseous reactants are presented with a large hot support surface. Supported growth of whiskers occurs also when the gaseous reactants are presented with discrete hot metal catalyst particles located on the surface of a suitable substrate. Unsupported whisker growth occurs when hot metal catalyst particles are freely interspersed with the gaseous reactants in the vapor phase. The most common mechanism for whisker growth is a vapor-liquid-solid transformation, and the most versatile VLS process is a metal particle catalyzed chemical vapor deposition. 

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The evolution of modern vapor phase processes starts with metal catalyzed chemical vapor deposition and ends with laser vaporization (see Table I). Most vapor phase processes require metal particle catalysts some proceed without the addition of metal particles. The growth temperatures range from 100 to 4000°C. The length of silicon nanowires is <10 nm [74] that of carbon nanotubes is <300 jm [76] but they can be potentially endless [81]. 

Metal catalyzed chemical vapor deposition has become the most versatile and therefore most important whisker growth process. This process and other vapor phase processes facilitate the formation of uniform and reproducible products for demanding applications, where they offer new premium electrical, magnetic, dielectric and near theoretical mechanical properties [1-2]. 

Except for initially producing a sheath/core SiOz/Si nanowhisker, the laser ablation process parallels the metal catalyzed chemical vapor deposition process (Chapter 2.2.3). In this process, the Si that is desired is generated by chemical vapor deposition and dissolved in molten metal droplets, e.g., Au or Fe. The molten alloy droplets, e.g., SIAu, which result in this process sequence, give rise to the grovrth of single crystal Si micro-whiskers by a similar overall VLS phase transformation. 

Until 1987, the only route to short carbon fibers was a metal catalyzed chemical vapor deposition. Since then, a novel process has become available [19] that facilitates the growth of discontinuous carbon fibers from mesopitch by a continuous liquid phase centrifuge process. Pitch may be considered to consist of a complex mixture of polycyclic aromatic hydrocarbons. It is a semisolid at room temperature but, depending on the composition, it melts above 100°C. Pitch has two phases, a high melting anisotropic, and a low melting isotropic, phase. The anisotropic phase, called mesopitch, is preferred for this process. 

VLS siiicon carbide whiskers with diameters ranging from <3 to 11 jm, and lengths ranging from 5 jm to <10 cm are readily obtained by metal catalyzed chemical vapor deposition. This process facilitates an exacting control over whisker shape and dimension in a batch or a continuous process. The diameter of the catalyst determines that of the whisker. Less well-defined whiskers have been obtained by vapor deposition, chemical mixing and carbothermal processes with diameters ranging from <3 pim to >30 nm. 

Short (or discontinuous) fibers are best prepared in a batch process, e.g., in a small cylindrical reaction chamber. The value of the technology, however, lies in its capability to facilitate the growth of continuous (potentially endless) fibers with a recently discovered automatic self-regulating growth mechanism [2], Finally, the diameter of the laser focus determines the diameter of fibers grown by laser assisted chemical vapor deposition, just as the diameter of the metal particles determines the diameter of the whiskers grown by metal catalyzed chemical vapor deposition. 

The structure property relationships observed in the Si-N system [17], are reminiscent of those observed for silicon nitride whiskers grown by metal catalyzed chemical vapor deposition (Chapter 2.2). The difficulty in obtaining binary (i.e., boron or silicon nitride) fibers with exactly required stoichiometry seems to have so far precluded the production of single crystal fibers by this route. 

Metal particle catalyzed chemical vapor deposition is the most versatile VLS process (Table II) yielding a wide range of single crystal whiskers and nanowhiskers [1-2] [5-6], short amorphous or polycrystalline fibers [1] [7], and nanotubes [8]. Laser ablation of selected metal alloys is a recent VLS process used for the synthesis of semiconductor nanowire [74]. Metal particle catalyzed carbothermal reduction, another VLS process, yields single crystal whiskers [9-10]. Metal catalyzed arc discharge [11], metal particle catalyzed laser ablation [12], and metal particle catalyzed plasma arc discharge [13] yield nanotubes by a VLS mechanism. 

Metal particle catalyzed chemical vapor deposition Liquid metal and/or liquid metal alloy (Au-Si, Fe-FejC, other) droplets catalyze whisker growth o Si, SiC and other whiskers o Short SiC and SijN fibers o SiC nanowhiskers and batts o Short graphite and carbon fibers o Short carbon TiOj microcoils o Carbon nanotubes (Ugh yields)... 

Metal particle catalyzed chemical vapor deposition Solid metal particles, vapor species does not dissolve o Amorphous Si, SiC, Ge fibers o Polycrystalline Si, SiC fibers o Amorphous carbon fibers ... 

Laser assisted chemical vapor deposition is an evolutionary extension of the metal particle catalyzed chemical vapor deposition, wherein a hot laser focus takes the place of a hot solid or liquid metal particle catalyst (Figure 1). (Conventional chemical vapor deposition has no "hot spot" capable of preferentially focusing the vapor phase deposition. 

CNTs can be obtained by electric arc discharge, laser ablation, high-pressure carbon monoxide, and catalyzed chemical vapor deposition. The multiple synthesis methods for CNTs are outside the scope of this chapter and had been extensively reviewed elsewhere [26-28]. The first three methods produce a large amount of by-products such as graphitic debris, metallic NPs, and fullerenes. On the contrary, CNTs obtained by chemical vapor deposition are highly crystalline and have low defect densities, although a minimal amount of amorphous content and NPs is present [27]. 

Cobalt complexes among other metals such as iron and nickel are known to catalyze the growth of carbon nanotube (CNT) by chemical vapor deposition (CVD) [79-81]. Thanks to the thermal stability of the hyperbranched polyyne backbone, spin-coated films of the organometalhc polymer were successfully probed to function as catalyst and arrays of CNT bundles could be prepared (Figure 2.4). This preliminary result already suggests potential application in the field of paUemable tailor-made catalysts. 

Verplanck et al. [62] made superhydrophobic silicon (Si) nanofiber surfaces of vertically aligned posts, shown in Fig. 10a, using a process based on chemical vapor deposition of silicon catalyzed by the metal particles. First a thin (4 nm) layer... 

Figure Metal particle catalyzed and laser assisted chemical vapor deposition. Left Chemical vapor deposition causes the formation of a film or coating on a hot surface. Center and right Metal catalyzed and laser assisted chemical vapor deposition causes the formation of a potentially continuous fiber with a diameter corresponding to the hot metal catalyst particle or laser focus respectively. Redrawn from F. T. Wallenberger, P. C. Nordine and M. Boman, Inorganic fibers and microstructures directly from the vapor phase, Composites Science Technology, 5,193-222 (1994).

Mass transfer in metal catalyzed and in laser assisted CVD processes is driven by highly localized temperature gradients. The relatively small area of either a hot molten metal particle or of a hot laser focus affords whiskers [4] or continuous fibers, respectively [2] [18-19]. The transfer of an equal mass from the vapor to the solid phase in a conventional chemical vapor deposition results in a thin coating over the relatively large area of a hot surface, i.e., that of a flat complex shaped composites part. 

Table IV. Metal catalyzed versus laser assisted chemical vapor deposition...

Metal Particle Catalyzed CVD Laser Chemical Vapor Deposition... 

One of the most interesting arylcopper(I) compounds is mesitylcopper(I), which is a pentamer in the solid state and a tetramer in the presence of S-ligands. It has recently been shown to split dioxygen, and is highlighted here because it is prepared from CuCl. Hexafluoroacetylacetonatocopper(I)-alkyne complexes (prepared from CuCl) have been shown to be useful in the chemical vapor deposition of Cu metal. ( )-l-TrimethylsUyl-l-alkenes have been prepared in high yields by the CuCl-catalyzed decomposition of a-trimethylsilyldiazoalkanes. Finally, Indian chemists have shown that 1,2-diketones are produced in good yield when Na(RCO)Fe(CO)4 is treated with CuCl. ... 

The adsorption of chemical compounds on nanoparticles of synthesized composites not only leads to electrical sensor effects, but also can result in new catalytic processes caused by the surface properties of such nanoparticles [61]. The important catalytic properties of PPX-metal composites prepared via cry-ochemical vapor deposition synthesis were discovered while studying the isomerization of 3,4-dichlorobutene into trans- and cw-l,4-dichlorobutene catalyzed by Pd nanoparticles of Pd-PPX [64] and the reaction C-Cl bonds metathesis in the mixture of n-decane with CCI4 catalyzed by Cu nanoparticles of Cu-PPX [76]. 

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