Nanowire growth seeds

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. 

The seeds are then added to a growth solution that consists of fresh metal salt, and a surfactant, cetyltrimethylammonium bromide, CTAB, that directs the growth of nanopartides into nanorods and nanowires. Growth is initiated by the addition... 

In early VLS whisker growth studies, there was no attempt to control the liquid alloy droplet size. Consequently, whisker diameters were large, ranging from 100 nm to 0.2 mm. The smallest whisker diameters that could be produced were limited by the minimum stable liquid droplet diameter ( 100 nm). In 2000, researchers began to apply size-selected nanometer-size colloidal metal particles as seeds to promote semiconductor nanowire growth. 

Fig. 3 (A) Schematic of the LCG model. (B) TEM image of Si nanowires produced via LCG. (From Ref. (C) SEM image of GaP nanowires, single seed particle composed mostly of Au is shown in the inset. (D) HRTEM image of GaAso.6Po.4 nanowire with (111) growth axis. (E) CdSe nanowire with 18 nm diameter (wurtzite crystal structure is indicated in the inset). (From Ref. l) (View this art in color at www.dekker.com.)...

FIG. 20-24 High -resolution TEM image of Si nanowires produced at 500 C and 24.1 MPa in supercritical hexane from gold seed crystals. Inset Electron diffraction pattern indexed for the <111> zone axis of Si indicates <110> growth direction. [Reprinted with permission from Lu et al. Nano Lett., 3(1), 93-99 (2003). Copyright 2003 American Chemical Society. ]... 

The synthesis of conductive metallic nanowires that bridged two microelectrodes separated by a gap of 12-16 im was demonstrated by the growth of a silver nanowire on a DNA template that bridged the gap (Fig. 12.26).92 Short thiolated nucleic acids (12 bases long) were attached to the microelectrodes, and these acted as sticky ends for the hybridization of /.-DNA that bridged the gap. The association of Ag+ to the phosphate groups of the template, followed by their reduction with hydroquinone under basic conditions, resulted in the formation of Ag° nanoclusters on the DNA template. The subsequent enlargement of the Ag° seeds by the catalytic reduction of Ag+ by hydroquinone, under acidic conditions, yielded continuous Ag nanowires with a width of ca. lOOnm. The nanowires revealed nonlinear I—V... 

While the formation of the nanoribbons can be explained in terms of a crystallization process that leads to these structures on solid surfaces, the formation of the nanocables and nanowires is not so straightforward. The origin of the cable-like structures thus became a topic of interest. They could either arise by a crystallization process when a homogeneous solution of 12 is dried on a surface, or these superstructures could be preformed in solution and act as seeds for the subsequent growth of cables. 

Seed-Mediated Growth Approach to the Synthesis of Inorganic Nanorods and Nanowires 287 9.2... 

Our seed-mediated growth approach has been successful for the synthesis of gold nanorods, with diameters of ca. 20 nm and aspect ratios from 2 to 20, in a controllable fashion (Figure 9.3). For silver, we have been able to make both short nanorods (aspect ratio 4) and long nanowires (aspect ratios 50-350) in the case of nanowires the level of control we have is more limited. 

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