Nanowire oriented, synthesis

Recently, the VLS growth method has been extended beyond the gas-phase reaction to synthesis of Si nanowires in Si-containing solvent (Holmes et al, 2000). In this case 2.5-nm Au nanocrystals were dispersed in supercritical hexane with a silicon precursor (e.g., diphenylsilane) under a pressure of 200-270 bar at 500°C, at which temperature the diphenylsilane decomposes to Si atoms. The Au nanocrystals serve as seeds for the Si nanowire growth, because they form an alloy with Si, which is in equilibrium with pure Si. It is suggested that the Si atoms would dissolve in the Au crystals until the saturation point is reached then they are expelled from the particle to form a nanowire with a diameter similar to the catalyst particle. This method has an advantage over the laser-ablated Si nanowire in that the nanowire diameter can be well controlled by the Au particle size, whereas liquid metal droplets produced by the laser ablation process tend to exhibit a much broader size distribution. With this approach, highly crystalline Si nanowires with diameters ranging from 4 nm to 5 nm have been produced by Holmes et al. (2000). The crystal orientation of these Si nanowires can be controlled by the reaction pressure. 

Synthesis forms a vital aspect of the science of nanomaterials. In this context, chemical methods have proved to be more effective and versatile than physical methods and have therefore, been employed widely to synthesize a variety of nanomaterials, including zero-dimensional nanocrystals, one-dimensional nanowircs and nanotubes as well as two-dimensional nanofilms and nanowalls. Chemical synthesis of inorganic nanomaterials has been pursued vigorously in the last few years and in this article we provide a perspective on the present status of the subject. The article includes a discussion of nanocrystals and nanowires of metals, oxides, chalcogenides and pnictides. In addition, inorganic nanotubes and nanowalls have been reviewed. Some aspects of core-shell particles, oriented attachment and the use of liquid-liquid interfaces are also presented. 

Fundamental aspects of vapor-liquid-solid (VLS) semiconductor nanowire growth are presented here. The synthesis of VLS semiconductor has been extended to different reaction media and pathways from the early chemical vapor deposition (CVD) approach, including solution-liquid-solid (SLS) and supercritical fluid-liquid-solid (SFLS), laser-catalyzed growth, and vapor-liquid-solid-epitaxy. The properties of nanowires grown by these VLS embodiments are compared. In this entry, VLS growth of nanowire heterostructures and oriented and hyperbranched arrays is examined. In addition, surface passivation and functionalization are assessed, and the importance of these techniques in the progress toward VLS produced nanowire devices is detailed. 

During the past few years, a significant effort has been directed towards the synthesis of low-dimensional Mn02 nanostructures with controlled morphologies. For instance, a-, P-, y-, 6-, k-, and E-Mn02 polymorphs were synthesized in the shape of nanorods [141-145], nanowires [144, 146-148], nanofibers [149, 150], nanoneedles [151, 152], and nanotubes [153, 154]. Lately, the attention of a considerable number of chemists and materials scientists has also been oriented towards the self-assembly of 1-D nanostructured manganese dioxides into 2- and 3-D ordered microstructures [144, 155-158]. 

A variety of solution methods such as seed-assisted growth, template-based synthesis, polyol method, solvothermal method and oriented attachment have also been developed for the synthesis of one-dimensional nanostructures. Here we will present various examples of the nanowires including metals, oxides, chalcogenides and pnictides with different synthetic methods. 

Fig. 6.9 Synthesis of Au nanoparticles and nanowires exploiting highly oriented pyrolitic graphite (HOPG) step edges (Reprinted from Ref. [115] with the permission of Wiley)...

Oriented attachment of nanocrystals can be used to make one-dimensional and other complex nanostructures. Thus, nanotubes and nanowires of II-VI semiconductors have been synthesized using surfactants [566]. The nanorods or nanotubes of CdS and other materials produced in this manner actually consist of nanocrystals. The synthesis of Ti02 nanowires from nanoparticles has been reported [567]. 

Fig. 6.10 (a) TEM and HRTEM images of Pt-Ag alloy nanowires formed by oriented coalescence of Pt-Ag crystallites along the [111] direction. Modified with permission from ref. [37]. (b, top) Scheme showing the reversed micelle formed by OM in the synthesis of Fe-Pt-aUoy nanowires and (b, bottom) TEM and HRTEM images of Fe-Pt-alloy nanowires. Modified with permission from ref. [32]... 

In selected cases, it is desired to pattern the nanoparticles into nanoarrays rather than to arrange them into a continuous film. Polleux et al. reported the synthesis of titania nanoparticles and their arrangement in one step by combining the benzyl alcohol route with micellar nanolithography [196]. TiClj dissolved in benzyl alcohol in the presence of a diblock copolymer was aged and dip coated, upon which different titania nanoparticle patterns formed, ranging from quasi-hexagonally ordered nanodots to curved and parallel oriented nanowires. 

Ionic liquids (substituted imidazolium salts) were utilized in the electrochemical synthesis of PEEKTT films with specific structural or morphological features randomly oriented nanofibers and particles in submicrometersized domains with EMI-bis(pentafluoroethylsulfonyl)imide, the structure of a solid polymer actuator with PEDOT layer on the surface of a solid nitrile rubber-BMI/BH-mixture, single strand nanowires with BMI-hexafluorophosphate, and codeposited polypyrrole/PEEXDT in BMI-BTI. ... 

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