Vapor-liquid-solid method

M. K. Sunkara, S. Sharma, R. Miranda, G. Lian, and E. C. Dickey, Bulk synthesis of silicon nanowires using a low-temperature vapor-liquid-solid method, Appl. Phys. Lett. 19, 1546-1548... 

The most important nanomaterial synthesis methods include nanolithography techniques, template-directed syntheses, vapor-phase methods, vapor-liquid-solid (VLS) methods, solution-liquid-solid (SLS) approaches, sol-gel processes, micelle, vapor deposition, solvothermal methods, and pyrolysis methods [1, 2]. For many of these procedures, the control of size and shape, the flexibility in the materials that can be synthesized, and the potential for scaling up, are the main limitations. In general, the understanding of the growth mechanism of any as-... 

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. 

For their rich potential in various applications described in the previous section, the synthesis and assembly of various ZnO micro and nanostructures have been extensively explored using both gas-phase and solution-based approaches. The most commonly used gas-phase growth approaches for synthesizing ZnO structures at the nanometer and micrometer scale include physical vapor deposition (40, 41), pulsed laser deposition (42), chemical vapor deposition (43), metal-organic chemical vapor deposition (44), vapor-liquid-solid epitaxial mechanisms (24, 28, 29, 45), and epitaxial electrodeposition (46). In solution-based synthesis approaches, growth methods such as hydrothermal decomposition processes (47, 48) and homogeneous precipitation of ZnO in aqueous solutions (49-51) were pursued. 

Fig. 17.12 Temperature dependence of the optical phonon frequency for (a) bulk Si, (b) a 50-mn-diameter Si NW grown by the vapor-liquid-solid (VLS) method [1] and (c) a 50-nm-diameter Si NW formed through Ag-catalyzed electrochemical etching (EE) [63]. (d) Slopes of the optical phonon frequency versus temperature for individual Si NWs of both types with diameters greater than 30 nm. They match the same value for bulk Si within experimental error (With permission from reference [62]. Copyright (2009) by the American Physical Society)...

The formation of silicon carbide whiskers can occur through one of three general methods vapor-solid (VS), chemical vapor deposition (CVD), vapor-liquid-solid (VLS). The processes vary widely in the raw... 

Vapor-Liquid-Solid Process In the VLS process, silicon- and carbon-rich vapors (usually CH4, SiO, or SiCU) react at 1400 °C to SiC on a liquid alloy [Eq. (9)]. Microscopic particles of an alloy are first distributed on a substrate (graphite) and then exposed to silicon- and carbon-rich vapors. The presence of a liquid catalyst, such as a transition metal or usually an iron alloy, distinguishes this method from all other whisker preparation methods. 

A method combining laser ablation cluster formation and vapor-liquid-solid (VLS) growth was recently developed for the synthesis of single crystal semiconductor siiicon and germanium nanowhiskers [74], Specificaily, iaser abiation was used to prepare dusters of moiten metal catalyst particles with a nanometer diameter. The droplet diameter defines the diameter of the resulting nanowhiskers. Buik quantities of uniform silicon and germanium nanowhiskers with diameters from 6 to 20 and from 3 to 9 nanometers, respectiveiy, and lengths from 10 to 300 nanometers were obtained. 

---