Metallic nanoparticles have been demonstrated to play a key role for the synthesis of nanowires by a vapor-liquid-solid growth process [146, 147]. Moreover, a metallic nanoparticle could catalyze two different materials to grow nanowire heterojunctions, such as silicon/carbon nanotube nanowire heterojunctions . Using the same tactic, several groups recently reported their work about the fabrication of semiconductor nanowires with segments of varying chemical-or dopant-composition superlattice nanowires. This greatly increases the versatility and power of these building blocks in the application of nanotechnology [12-14, 148-150]. All of this work focuses on semiconductors composed of modulated Group III-V [12, 14, 148], Group II-VI , and Group IV  elements. Gold nanoparticles have been used as catalysts in all this work.
In the synthesis of nanowire superlattices, a catalyst available to two or more different materials serves as the critical point for nucleation and directional growth of the nanowires. Meanwhile, the reactants are modulated during growth. Figure 9 is a schematic diagram of the process . The nanoparticle catalyst (yellow) nucleates and directs the nanowire (blue) growth with the catalyst remaining at the terminus of the nanowire (Fig. 9a). To create a single junction within the nanowire, the addition of the first reac-tant is stopped during growth, and then a second reactant is introduced for the remainder of the synthesis (Fig. 9b). Repeated modulation of the reactants during growth produces nanowire superlattices (Fig. 9c).
Nanowire superlattices of gallium arsenide (GaAs)/gall-ium phosphide (GaP) were reported by Gudiksen et al. . GaAs/GaP nanowire superlattices are an attractive system to explore for nanophotonic applications because GaAs is a direct-bandgap semiconductor, while GaP has an indirect gap. The GaAs/GaP nanowire superlattices were grown by laser-assisted catalytic growth using GaAs and GaP targets. Au nanoparticles were used as a catalyst to direct the growth. Solid targets of GaAs and GaP were ablated using either a pulsed ArF excimer or Nd:YAG lasers and growth was carried out at 700-850 °C in an argon flow. The number
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