SiNWs nanowires

Ni thin films were used as catalysts by Jin et al. and Yan etal. to produce amorphous SiNW at 1200°C. No external supplies of Si were needed, but hydrogen gas was used. It was unclear whether the nanowires were amorphous silicon nanowires covered with silica layers, or silica nanowires. 

Morphology of Si-Based Nanowires. The overall diameters of SiNW produced so far range from 3 nm to 100 nm, and many methods have yielded a frequently occurring diameter of 15 nm when they are made between the temperature of 1100 and 1200°C. This size of nanowires, as speculated at the end of this chapter, may be determined by the selected growth temperature. 

Other morphologies have also been made and observed for SiNW. Although straight nanowires dominate, other forms such as string beans (strings of pearls), shells. 

Milligram quantity SiNW have been obtained, although no stand-alone materials have been generated for easy handling. In general, nanowires so produced are attached to the substrate, and removal has been difficult or nearly impossible due to the small quantities available. 

Correlation between Sizes of Catalysts and SiNW. A loose correlation exists between the nanowire diameter and the size of the catalysts. The size of... 

Improved morphology and compositions of SiNW can further enable many new applications. For example, SiO c nanowires usually possess the most intense blue PL 47,48 ]-ecent study, long wavelength PL from oxygen vacancies in Si02 has... 

SiNW can also be used as sensors. In this case, SiNW of a silicon core without or with a very thin Si02 sheath are preferred. Since oxygen in the air normally reacts with Si, thin Si nanowires will eventually become Si02 nanowires if no protective layers are in place. 

At 1100°C, SiNW dominated. Figure 10.8 shows SiNW grown at 1100°C for 5 s. Reaction time was also a factor for SAN formation. Reaction times of 30 min and a few seconds produced similar results, although longer times generally produced more SiNW. Long SiNW fully covered the Si wafers after a few hours. The nanowires were between 5 and 50 nm in diameter, and microns to millimeter long. 

The nanowires in Fig. 10.5, straight and coiled, are believed to be SiNW. Experiments were performed to verify that they were not carbon nanotubes or metal nanorods. One method to prove this was by using a substrate other than Si. In this case, AI2O3 wafers replaced Si as the substrate to avoid interference from the Si substrate for characterization. The SiNW grown on Si are shown in Fig. 10.11, and those grown on the AI2O3 wafer are shown in Fig. 10.12. Characterization of these nanowires yielded that they were SiNW. 

HRTEM of Si-Based Nanowires. High resolution TEM images of SiNW showed amorphous structures for all the nanowires inspected. Although it is difficult... 

Evidence for Tip Growth. The evidence for tip growth can be found from the images which directly show that nanowires are connected to metal catalysts. Figures 10.18 and 10.19 show two types of SiNW, one straight and one coiled. This clearly indicates that nanowires were grown from the tip. 

FIGURE 10.18. A single SiNW pointing into space with a nanoparticle catal yst on its tip. The nanowires are slightly coiled to maintain its straightness over a long distance. [Chem Comm 2005]—Reproduced by permission of The Royal Society of Chemistry, (ref 54)... 

Another possibility is that the growth rates of SiNW from catalysts were nonuniform across SiNW cross sections. This can be tme if the catalysts are close to melting, but not quite fully molten. At much higher temperatures, catalysts are all well molten and adopt a spherical shape. At near-melting temperatures, they may not be spherical, and therefore the SiNW produced under this condition may have nonsymmetric growth with respect to its nanowire axis, which will lead to the growth of coiled nanowires. 

HRTEM results showed that SiNW were made of amorphous materials. EDX of the nanowires showed both Si and O peaks, with the ratio ranged from SiOi s to Si02. The observation that SiNW were amorphous materials was further supported by the Raman measurements, in which very small amounts of crystalline Si was detected. Even the amorphous Si peak at 480 cm was undetectable due to the presence of high PL signal. On the basis of the information presented here, we concluded that the majority of the SiNW are made of amorphous silicon dioxide, with a certain amount of oxygen deficiencies. 

It is possible that the as-made nanowires contain some crystalline Si core, and subsequent reaction with oxygen in the air results in the formation of silica nanowires. Since hydrogen gas was used in the reaction, pure silicon nanowires were probably made first, followed by oxidation in air. In the following discussion, for simplicity reason we will assume that amorphous SiNW were made during catal)Tic reactions. 

Our experiments have shown that hydrogen is critical for growth. The required presence of both hydrogen and metal catalysts, and the virtual absence of silicon vapor suggest that totally new reaction paths assist in the growth of these nanowires under the conditions studied. If silicon does not come directly from the wafer substrate, then it is required to become airborne in some form, as in the CVD production of SiNW, thus enabling tip growth. 

Applications. We have shown here that bulk quantity SiNW can be made in the form of thin films. These films can be used as (1) templates for making other nanostructures because of the large surface area offered by those nanowires (2) PL devices and (3) nanoelectronic and chemical sensor devices or fuel cells because the silicon oxide nanowires may be passivated before they are oxidized. We are pursuing all these options in our lab. 

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]. 

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