Vapor-solid growth method

Nanowires are seen as a solution with which to improve the sensitivity, selectivity, stability and response time of metal-oxide gas sensors. Meier et al (2007) grew Sn02 nanowires of 100 nm in diameter by the vapor-solid growth method. For testing, they were deposited onto micromachined hotplates with a focused ion beam scanning electron microscope (FIB-SEM), as shown in Fig. 6.19. Due to their diameter being similar to the Debye length, a completely depleted conduction channel can be obtained. Maximum response to CO and NH3 occurred at about 260°C. 

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-... 

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. 

Many SiCNWs formation mechanisms such as vapor-solid mechanism (VS), vapor-liquid-solid mechanism (VLS) and oxide-assist growth (OAG) have been proposed. It is needed to clarify the formation mechanism of SiCNWs synthesized by thermal evaporation method for improvement of the product quantity, which leads to the sufficient amount of SiCNWs for the application researches. Various chemical reactions have been discussed but there are no experimental results about exhaust gas from the SiCNWs production system and no direct observation of the formation of SiCNWs. 

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. 

---