Nanowire formation

This chapter deals with the selective preparation, TEM/EXAFS/XPS characterization and catalysis of mono- and bimetallic nanowires and nanoparticles highly ordered in silica FSM-16, organosilica HMM-1 and mesoporous silica thin films. The mechanism of nanowire formation is discussed with the specific surface-mediated reactions of metal precursors in the restraint of nanoscale void space of mesoporous silica templates. The unique catalytic performances of nanowires and particles occluded in mesoporous cavities are also reviewed in terms of their shape and size dependency in catalysis as well as their unique electronic and magnetic properties for the device application. 

Figure 15.13 Pictorial representation of proposed mechanism for Pt nanowire formation in mesoporous silica templates such as FSM-16 and HMM-1 under UV-irradiation of H2PtCl6/ FSM-16 and H2PtCl6/HMM-l in the presence of methanol and water vapor.

Figure 15.18 Proposed structures of (a) Pt nanorod wires in silica FSM-16 and (b) Pt nanonecklace wires in organosilica such as HMM-1 (Et-HMM) and HMM-p (Ph-HMM), which are used as the mesoporous templates for nanowire formation.

Alumina nanotubes have been prepared by the anodic oxidation of aluminum [41] the resulting tubes have one-dimensional channels with uniform diameters of 5nm and lengths of 50-100 nm. An alumina membrane with a highly ordered nanohole array in 50-100 nm diameter has also been synthesized by long-period anodization thus these local alumina nanotubes have been tried as a template for metal nanowire formation. 

Bhattacharrya, S., and Saha, S. K., Nanowire formation in a polymeric film. Appl. Phys. Lett. 76, 3896 (2000). 

Pore filling by Sol-Gel technique and oxide nanowire formation. 

Pore filling Nanowire formation using vapor route. 

A couple of years later, Buhro and coworkers observed polycrystalline InP, InAs, and GaAs nanowire formations in hydrocarbon solvents from organo-metallic precursors at relatively low temperatures of 200°C— well below the Au Si eutectic temperature. The nanowires had diameters ranging from 10 to ISOnm and their formation was attributed to a VLS-like growth mechanism. The organometallic precursors... 

Kim, D.H. et al.. Solvent vapor-induced nanowire formation in poly(3-hexylth-iophene) thin films, Macromol. Rapid Commun. 26, 834—839, 2005. 

Even though the nanowire electrical conductance depends exclusively on its geometrical dimensions, and does not depend either on the type of metal or on temperature, the dynamic nanowire formation itself is different for different metals. In particular, the intensity of nanowire formation and the duration of the process are very strongly influenced by the type of metals used. The nanowires forming intensity is measured with statistical calculations of characteristic G =f(t). The result of the calculations is a histogram. 

In this work, ac electrochemical deposition has been applied to nanowire formation in PAA. The advantage of the ac electrochemical deposition is that the membrane remains on the A1 substrate and that the barrier layer does not avoid the deposition. In this case, the fabrication of ordered and metal-filled porous alumina structures is not limited by the thickness and size of the PAA layer. These structures may be used in high temperature technological operations. 

Figure 14.11. Schematic representation of the DNA-pectin templated silver nanowire formation between two interdigitated electrodes.

Bhatt, A. I., Mechler, A, Martin, L. L., and Bond, A. M. [2007]. Synthesis of Ag and Au nanostructures in an ionic liquid Thermodynamic and kinetic effects underlying nanoparticle, cluster and nanowire formation./ Mater. Chem., 17, pp. 2241-2250. 

Nanostructuring by this technique, therefore, affords several control parameters crucial for nanowire formation that can be controlled accurately, viz. bias voltage, electrode tip diameter, voltage leads separation (which determines the field value), substrate material (should be robust to heat treatment), film material (lower melting points lead to agglomeration of material and require lower field for organization) and film thickness (which controls diameter and separation of nanoparticles). 

Mesoporous (normal 5-50 nm Ag, Au DP p-Si, dendrite growth. Si nanowire formation Peng and Zhu (2004)... 

Shi J, Xu F, Zhou P, Yang J, Yang Z, Chen D-S, Yin Y, Chen D, Ma Z-Q (2013b) Refined nano-textured surface coupled with SiNx layer on the improved photovoltaic properties of multicrystalline silicon solar cells. Solid-State Electron 85 23-27 Shiu S-C, Hung S-C, Syu H-J, Lin C-F (2011) Fabrication of silicon nanostructured thin film and its transfer from bulk wafers onto alien substrates. J Electrochem Soc 158 D95-D98 Smith ZR, Smith RL, Collins SD (2013) Mechanism of nanowire formation in metal assisted chemical etching. Electrochim Acta 92 139-147... 

Sloan J, Wright DM, Bailey S, Brown G, York APE, Coleman KS, Green MLH, Hutchison JL, Woo H-G. Capillarity and silver nanowire formation observed in single walled carbon nanotubes. Chem Commun 1999 699-700. 

The role of HEBM on nanotube and nanowire formation could be divided into three effects ... 

Huang MH, Choudrey A, Yang P. Ag nanowire formation within mesoporous silica. Chem Commun 2000 1063—4. 

Horn R.G., IsraelachvUi J.N. Direct measurement of structural forces between two surfaces in a nonpolar hquid. J. Chem. Phys. 1981 75 1400-1411 Hough D.B., White L.R. The calculation of Hamaker constants from Lifshitz theory with applications to wetting phenomena. Adv. Colloid Interface Sci. 1980 14 2-14 Huang M.H., Choudrey A., Yang P. Ag nanowire formation within mesoporous silica. Chem. Com-mun. 2000 1063-1064... 

The vapor-hquid-solid (VLS) mechanism first proposed by Wagner and EUis [10] is simpUstic in nature, yet subtly complex. The VLS model is most readily grasped pictorially a schematic representation of nanowire formation by to the VLS mechanism is shown in Figure 3.1. The key feature here is a hquid catalyst that is able to absorb material from the surrounding vapor. If the catalyst is not hquid, then material in the vapor will not be absorbed and the nanowire formation will be prohibited. Under the appropriate conditions of temperature and vapor pressure, the liquid will absorb material from the vapor phase, introduced either in elemental form or as a molecular precursor. 

In the chemical vapor deposition (CVD) process, a molecular precursor is used [11-13]. Here, as the catalyst absorbs an increasing amount of material it eventually becomes supersaturated, at which point material is deposited at the catalyst-substrate interface (Figure 3.1b), thereby establishing nanowire formation (Figure 3.1c). Extended formation of the nanowire can be maintained as long as a sufficient quantity of reactant is present, and the temperature of the catalyst is maintained above the melting point. If either of these conditions is not met, nanowire formation will cease. Based on the above description of the VLS mechanism, it is clear that the catalyst dictates whether or not nanowire formation will occur. 

Figure 3.1 A schematic representation of the vapor-liquid-solid (VLS) growth mechanism for nanowire formation.

In the case of ex situ application of an elemental catalyst, a number of physical vapor deposition techniques can be utilized, including sputtering and molecular beam epitaxy. In this way, a few to tens of monolayers of the catalyst metal are deposited onto the substrate, which is then inserted into the CVD reactor. For both of these choices the rationale for depositing a few monolayers of the catalyst is twofold. First, an extremely low coverage of the catalyst material will facilitate islanding, which is critical to nanowire formation. Second, the amount of catalyst on the substrate will dictate the size of the metal islands, which in turn will dictate the diameter of the nanowires. The primary drawback to these two approaches is that the size distribution of the islands caimot be readily controlled. 

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