Nanowires

Semiconductor nanowires are nanoscale building blocks that could, through bottom-up assembly, enable diverse applications, principally in nanoelectronics, photonics and bio/chemical sensor field [52]. In comparison to spherical NPs, nanowires offer additional degrees of freedom in self-assembly due to their inherent shape anisotropy. Furthermore, the ability to synthesize particles with different 

Solution-Liquid-Solid (SLS) growth of semiconductor nanowires by Wang etal. (2006). The synthesis proceeds by a solution-based catalysed growth mechanism in which nanometer-scale metallic droplets catalyse the decomposition of metallo-organic precursors and crystalline nanowire growth. 

Solution-based synthetic strategies for one-dimessional nano-structures by Wang and Li (2006). Recent progress in the solution-based routes to prepare onedimensional nano-structures are reviewed. The role of crystal structure in the determination of the growth behaviour of the nano-crystals is underlined. 

Solution-grown zinc oxide nanowires by Greene etal. (2006). Strategies for growing ZnO nanowires from zinc salts in aqueous and organic solvents are reviewed. 

One-dimensional colloidal gold and silver nano-structures by Murphy et al. (2006). Recent advances in the synthesis of metallic nanorods and nanowires are reviewed. The increasing relevance of the bottom-up chemical synthesis is underlined. Physical properties and potential applications are described with emphasis on silver and gold. 

Noble metal-metal oxide dumbbell-shaped NPs have been synthesized based on seed-mediated growth. Metal oxides are grown over the pre-synthesized noble metal seeds by the thermal decomposition of the metal carbonyl followed by oxidation in air. They show enhanced catalytic activity towards CO oxidation in comparison with their counterparts [94]. Heterostructured Cu2S-ln2S3 with various shapes and compositions can be obtained by a high-temperature precursor-injection method wherein Cuj is used as the catalyst for the nucleation and growth of In Sj NPs [95]. 

There has been eonsiderable interest in the synthesis, characterization and properties of nanowires of various inorganic materials [2, 99, 100]. Nanowires have been prepared using vapour phase methods such as vapour-liquid-solid (VLS) growth, vapour-solid 

FIGURE 13.9 (a) Schematic representation of VLS nanowire growth mechanism including 

A variety of solution methods such as seed-assisted growth, template-based synthesis, polyol method, solvothermal method and oriented attachment have also been developed for the synthesis of one-dimensional nanostructures. Here we will present various examples of the nanowires including metals, oxides, chalcogenides and pnictides with different synthetic methods. 

FIGURE 13.11 TEM image of gold nanorods with aspect ratio 25 obtained by solution-based reduction method making use of nanoparticle seeds (From Ref. 106, Adv. Mater, IS (2003) 414. 2003 Wiley-VCH Verlag GmbH Co. K GaA). 

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Although all real surfaces have steps, they are not usually labelled as vicinal unless they are purposely misoriented in order to create a regular array of steps. Vicinal surfaces have unique properties, which make them useful for many types of experiments. For example, steps are often more chemically reactive than terraces, so that vicinal surfaces provide a means for investigating reactions at step edges. Also, it is possible to grow nanowires by deposition of a metal onto a surface of another metal in such a way that the deposited metal diflfiises to and attaches at the step edges [3]. 

Himpsel F J, Jung T and Ortega J E 1997 Nanowires on stepped metal surfaces Surf. Rev. Lett. 4 371... 

Fig. 2. The TEM images of starch-capped CdSe nanoparticles at 0.5 1 precursor molar ratio showing (A) nanowires with interspersed spherical particles and (B) a fraction of the interspersed spherical particles at low magnification.

Tang, Z. Kotov, N. A. and Giersig, M. (2002). Spontaneous Organization of Single CdTe Nanopartides into Luminescent Nanowires. Science, 297, 237-240. 

Yang, Q. Tang, K. Wang, C. Qian, Y. and Zhang, S. (2002). PVA-Assisted Synthesis and Characterization of CdSe and CdTe Nanowires.. Phys. Chem. B, 106, 9227-9230. 

Orthorhombic 81283 (bismuthinite) via a solventiess method can yield either high aspect ratio (>100) nanowires at about 225 °C (Fig. 20.3 a) or lower aspect ratio ( 7) (plus sulfur which seems to change the growth kinetics) at around 160 °C. Interestingly, it is possible to fabricate a polymeric matrix from these wires and rods at higher temperatures, 250°C, as shown in Fig. 20.3b [6]. 

We found recently that the viscosity (//vac) of the colloidal thiolate precursor is a key parameter in controlling the shape of the nanoproducts in the solventless method [8]. Uniform nanowires, rods, or spheres could be made from the corresponding precursors that came from the solutions with different viscosities. The viscosity is a measure of the polymerization of the metal-thiolate complexes. Accordingly, the precursor with the highest viscosity produces nanowires (Fig. 20.5 a), and with decreases in the viscosity, the product morphology changes to rods (Fig. 20.5b) and then spheres (Fig. 20.5c). 

Fig. 20.5 CU2S nanowires (a), rods (b) and spheres (c) produced at 155°C for 120 min with Cu-thiolate precursors of different viscosity values (a) vac = 93.5 mPa/s ...

In general, nanotechnology MBBs are distinguished for their unique properties. They include, for example, graphite, fullerene molecules made of various numbers of carbon atoms (C60, C70, C76, C240, etc.), carbon nanotubes, nanowires, nanocrystals, amino acids, and diamondoids [97]. All these molecular building blocks are candidates for various applications in nanotechnology. 

Li Q, Walter EC, van der Veer WE, Murray BJ, Newberg JT, Bohannan EW, Switzer JA, Hemminger JC, Penner RM (2005) Molybdenum disulfide nanowires and nanoribbons by electrochemical/chemical synthesis. J Phys Chem B 109 3169-3182 Tenne R, Homyonfer M, Feldman Y (1998) Nanoparticles of layered compounds with hollow cage structures (inorganic fuUerene-like structures). Chem Mater 10 3225-3238 Shibahara T (1993) Syntheses of sulphur-bridged molybdenum and tungsten coordination compounds. Coord Chem Rev 123 73-147... 

Templated and Free-Standing Nanowires and other Forms... 

Non-aqueous electrolytes have been used for the preparation of CdS, CdSe, and CdTe nanowire arrays by dc electrodeposition in porous AAO templates of various pore diameters [155, 156]. For instance, CdSe NW arrays with uniform wurtzite crystal structure were fabricated from a non-aqueous DMSO solution containing CdCl2 and elemental Se. The NWs were shown to be of uniform length (2-15 p.m) and diameter (about 20 nm). The c-axis, [00.2], of the grown... 

In the first step, amorphous MoO , nanowires were electrodeposited by ESED according to the reaction... 

Fig. 4.16 Schematic representation of the E/C method for synthesizing polycrystalline M0S2 nanowires and nanoiibbons on graphite surfaces. (Reproduced in gray scale with permission from [171]. Copyright 2009, American Chemical Society)...

According to absorption spectroscopy, these nanoribbons were composed of 3R-M0S2. The E/C synthesis of M0S2 wires and ribbons was size selective control over the wire or ribbon size was provided by control of the MoO , nanowire dimensions, which in turn were controlled using the electrodeposition potential and time in the first step of the synthesis. 

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