Nanowires substrate

Earlier observations by Cesario et al. [60] of a decay in fluorescence for arrays of Au nanoparticles spaced above a Ag film by a Si02 layer of increasing thickness, were interpreted as due to the finite vertical extent of the evanescent fields associated with a surface plasmon. In this model the coupling results in an enhanced interaction between individual localized plasmons at individual nanostructures [61] and thus an enhancement in the radiative efficiency increasing the spacer layer thickness moves the nanowires out of the evanescent field of the surface plasmon. A possible physical mechanism for the experimentally observed decay involves nonradiative decay of the excited states. The aluminum oxide deposited in these experiments was likely to be nonstoichio-metric, and defects in the oxide could act as recombination centers. Thicker oxides would result in higher areal densities of defects, and decay in fluorescence. A definitive assignment of the mechanism for the observed fall off of fluorescence would require determination of the complex dielectric function of our oxides (as deposited onto an Ag film), and inclusion in the field-square calculations. Finally it should be noted that at very small thicknesses quenching of the fluorescence is expected [38,62] consistent with observations of an optimum nanowire-substrate spacer thickness. 

LaLonde, A., Norton, M., Zhang, D., Gangadean, D., Alkhateeb, A., Padmanabhan, R. and Mcllroy, D. (2006) A rapid method for growth of metal nanoparticles on nanowire substrates. 

Porous Membranes of Nanoparticies from Templating Against AAO Membranes Using LB Technique. AAO-porous substrate has broad applications in making metal and semiconductor nanowires, aligned mesostructured nanorods, inorganic nanotubes. 

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

Figure 10.17. (a) Schematic diagram of the nanowire dye-sensitized solar cell. Light is incident through the bottom electrode, (b) SEM cross section of a solution-fabricated ZnO nanowire array on fluorine-doped tin oxide. The wires are in direct contact with the substrate. Scale bar, 5 pm. Reproduced from Ref. 41, Copyright 2005, with permission from the Nature Publishing Group. 

McAlpine, M. C. Ahmad, H. Wang, D. Heath, J. R. 2007. Highly ordered nanowire arrays on plastic substrates for ultrasensitive flexible chemical sensors. Nature Mater. 5 379-384. 

Figure 11.11. Integration of nanowire photonics with silicon electronics. Schematic illustrating fabrication of hybrid structures. A silicon-on-insulator (SOI) substrate is patterned by standard electron-beam or photolithography followed by reactive ion etching. Emissive NWs are then aligned onto the patterned SOI substrate to form photonic sources. [Reprinted with permission from Ref. 59. Copyright 2005 Wiley-VCH Verlag.]...

NW-TFT device performance does not change significantly upon slightly flexing the plastic substrate (Fig. 11.14d).67 The use of free-standing nanostructures for flexible electronics has also been demonstrated by various groups using nanotubes,68 nanowires,63 and nano- or micro-ribbons.69,70... 

McAlpine, M. C. Friedman, R. S. Lieber, C. M. 2005. High-performance nanowire electronics and photonics and nanoscale patterning on flexible plastic substrates. Proc. IEEE 93 1357-1363. 

The Russell group has applied the template synthesis approach to nanoporous films generated from UV-treated PS-fo-PMMA copolymers [43, 147,233,235,241], which were pre-aligned perpendicular to the substrate by an electric field. Through direct current electrodeposition, they fabricated high-density vertical arrays of ferromagnetic cobalt nanowires (Fig. 10a) [43]. Through subsequent work, they also demonstrated the successful replication... 

Measurements of mobility in PS suffer from the fact that the number of free charge carriers is usually small and very sensitive to illumination, temperature and PS surface condition. Hall measurements of meso PS formed on a highly doped substrate (1018 cm3, bulk electron mobility 310 cm2 V-1 s-1) indicated an electron mobility of 30 cm2 V 1 s 1 and a free electron density of about 1013 cm-3 [Si2]. Values reported for effective mobility of electron and hole space charges in micro PS are about five orders of magnitude smaller (10-3 to 10 4 cm2 V 1 s ) [PelO]. The latter values are much smaller than expected from theoretical investigations of square silicon nanowires [Sa9]. For in-depth information about carrier mobility in PS see [Si6]. 

Most of the aforementioned methods use gas-phase feedstock, including CVD via the VLS mechanism in the presence of metal catalysts, evaporation at high temperatures without the use of metal catalysts, or laser vaporization in the presence of metal catalysts. Solution-liquid-solid methods have been explored in the presence of metal catalysts and under supercritical conditions. These two mechanisms can result in either tip or root growth, meaning that the catalysts can be either suspended in space at the tips of the growing nanowires, or anchored at the surface of the substrate, depending on the strength of interactions between the nanoparticles and the substrate. 

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. 

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. 

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