Nanowires vapor-liquid-solid growth

The VLS technique has probably attracted most attention. Essentially this is an extension of that used by Wagner and Ellis to grow the original Si whiskers, which were themselves nanowires. Vapor-liquid-solid growth of... 

Figure 10.2. Vapor-liquid-solid growth of semiconductor nanowires.

Vapor phase growth is commonly used to produce nanowires. Starting with the simple evaporation technique in an appropriate atmosphere to produce elemental or oxide nanowires, vapor-liquid-solid, vapor-solid and other processes are made use of. 

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.

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. 

Westwater, J. Gosain, D.P. Tomiya, S. Usui, S. Ruda, H. Growth of silicon nanowires via gold/ silane vapor-liquid-solid reaction. J. Vac. Sci. Technol. B 1997, 15 (3), 554. 

Wu, Y. Yang, P. Direct observation of vapor-liquid-solid nanowire growth. J. Am. Chem. Soc. 2001, 123 (13), 3165-3166. 

An Au-catalysed chemical vapour transport and condensation (CVTC) process was used to produce ZnO nanorods and nanowires on Si02 and Si substrates [58], ZnO nanorods with a wide band gap (3.37eV) are regarded as promising candidates for the fabrication of nanoelectronic devices. In this work, EDX spectra of the tip and the body of ZnO nanorods were captured which indicates that Au-Zn alloyed droplets were present at the tips of the fabricated nanorods pointing to a nanorod growth via a vapor-liquid-solid (VLS) mechanism. 

Ma and Bando reported a systemic investigation of the growth of boron carbide nanowires via a vapor-liquid-solid mechanism. A template- and catalyst-free carbothennal route of nanowire production has been employed with boron powder, boron oxide, and carbon black, mixed in a 2 1 1 ratio, as the precursor. The component mixture was subjected to 1650°C temperature for 2 h under an argon flow inside a high-frequency induction furnace and resulted in boron carbide nanowires of diameters ranging from 50 to 200 nm (Figure 20.17b). 

Figure 3.1 A schematic representation of the vapor-liquid-solid (VLS) growth mechanism for nanowire formation.

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. 

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