Core Shell Nanowire Heterostructures

One-dimensional core-shell nanowire heterostructures are another kind of nanocable with radial heterostructures. After high-temperature reaction processes, many semiconductors and metal nanowires are found to have a thin oxide shell, forming a core-shell nanowire. Such configurations are Si/SiOx [125], SiC/SiOx [126-128], Ge/ GeO2 [129], GaN/GaOx [130], and Zn/ZnO [131]. Other similar structures, such as Si3N4/Si/SiO2 [132], B/SiO2 [133], Ge/SiO2 [134], Ag/SiOx [135], CdS/SiO2 [136], Zr/ZrO2 [137], La(OH)3/Ni(OH)2 [138], NiFe/Cu [139], Au/Ag [140], and Cu2S/Au [141], have also been synthesized using chemical vapor deposition, laser ablation, electrodeposition, and redox deposition techniques. In addition to the inorganic core-shell nanowires, semiconductor/polymer core-shell nanowires, such as CdSe/poly(vinyl acetate), have been produced using low-temperature solution methods [142].

Very recently, controlled growth of core-shell and multishell heterostructures of silicon-germanium systems has been approached using a chemical vapor deposition system [143]. This work is based upon control of radial versus axis growth (Fig. 7). Metal nanoparticles could act as a catalyst to direct axis growth by a vapor-liquid-solid growth process (Fig. 7a). Axial growth proceeds when reactant activation and addition occurs at the catalyst and not on the nanowire surface (Fig. 7b). By altering conditions to favor radial growth on the nanowire surface, it is possible to drive conformal shell growth (Fig. 7c). In this work, the radial growth is turned on by moving the growth substrate downstream to favor uncatalyzed surface growth. Subsequent introduction of different reactants and/or dopants produces multiple shell structures (Fig. 7d). By this method, core-shell structures such as intrinsic silicon (i-Si)/boron doped (p-type) silicon, i-Si/SiOx/p-Si, i-Ge/p-Si, Si-Ge, Si-Ge-Si, and p-Si/i-Ge/SiOx/p-Ge core-multishell nanowire structures have been synthesized [143]. TEM images and elemental mapping show the Si-Ge core-shell structure with a sharp interface (Fig. 8a). Figure 8b shows that the Ge shell is fully crystallized. Figure 8c shows a cross-section elemental mapping of a Si-Ge-Si core-double-shell nanowire. Another interesting experiment in this work is to measure the I-V curve of p-Si/i-Ge/SiOx/p-Ge core-multishell nanowires, which can be used as field-effect transistors. The source, drain, and gate contacts were made by selective etching and metal deposition onto the inner i-Ge shell and outer p-Ge shell, respectively.

Another controllable example is the growth of CdSe/ CdS/ZnS core-shell nanorods [144]. Short CdSe nanorods (aspect ratios range from 2:1 to 10:1) were first grown by sol-gel processes [145] and then put into the precursor solution for the growth of CdS/ZnS graded shells at low temperature (160 °C) [144]. Interfacial segregation is used to

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