VLS nanowire

It is clear from the VLS nanowire growth mechanism that the positions of the nanowires can be controlled by the initial positions of the Au clusters or Au thin films. By creating desired patterns of Au using the lithographic technique it is possible to grow ZnO nanowires of the same designed pattern since they grow... 

For CVD-VLS nanowire growth, colloidal particles are first size-selected in solution and then drop cast or spun cast onto the substrate prior to nanowire deposition. Nanowire diameter control is very important, as the optical, electronic, and mechanical properties of nanowire depend on the nanowire diameter. The histograms shown in Fig. 5B illustrate that the nanowire diameter... 

FIGURE 13.9 (a) Schematic representation of VLS nanowire growth mechanism including... 

Most of the aforementioned methods use gas-phase feedstock, including CVD via the VLS mechanism in the presence of metal catalysts, evaporation at high temperatures without the use of metal catalysts, or laser vaporization in the presence of metal catalysts. Solution-liquid-solid methods have been explored in the presence of metal catalysts and under supercritical conditions. These two mechanisms can result in either tip or root growth, meaning that the catalysts can be either suspended in space at the tips of the growing nanowires, or anchored at the surface of the substrate, depending on the strength of interactions between the nanoparticles and the substrate. 

Recently, a laser ablation-condensation technique was used to produce nanometer-sized catalyst clusters to grow nanowires by the VLS method. A schematic of the laser ablation apparatus used by Morales and Lieber (1998) to produce silicon nanowires is shown in Fig. 11. The target consists of silicon and the catalyst material (e.g., Sii AFeA), and a pulsed laser is used to produce nanometer-sized catalyst clusters within a reaction chamber at 1200°C. The ablated materials are carried by an argon gas flow, and the... 

Fig. 10. Schematic illustrating the growth of silicon nanowires by the VLS mechanism.

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. 

Figure 6.61. The original schematic used to describe vapor-liquid-solid (VLS) growth of semiconductor nanowires. Reproduced with permission from Wagner, R. S. Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. Copyright 1964 American Institute of Physics.

Gao, P. X., and Wang, Z. L. (2004). Substrate atomic-termination-induced anisotropic growth of ZnO nanowires/nanorods by the VLS process. J. Phys. Chem. B 108 7534-7537. 

Si nanowires were first produced using the classical metal catalyst VLS approach [21, 22, 46]. Laser ablation of a metal-containing Si target produces metal/metal silicide nanoparticles that act as the critical catalyst needed for the nucleation of SiNWs. The wires grow further by dissolution of silicon in the metallic nano-cap and concurrent Si segregation from the cap. In a typical experiment, an excimer laser is used to ablate the target placed in an evacuated quartz tube filled with an inert gas, e.g. argon [22]. 

---