Molecular nanowire junctions

Other important experimental techniques utilized in the molecular transport studies include the mercury-drop junctions52,53 which have been used to study the transport through alkanethiols, and nanorods (a metallic nanowire-S AM-metallic nanowire junction)54 which have been used to measure 16-mercaptohexadecanoic acid. 

Single-walled carbon nanotubes (SWNTs) had been considered for the crossbar components of the defect-tolerant molecular computers but they have been found to be too difficult to handle due to their insolubility and their tendency to form bundles or ropes. Instead, metallic nanowires have become the materials of choice used in the construction of the crossbar devices, with ultrahigh-density lattices and circuits being built, having groups of nanowires 8 nm in diameter and 16 nm apart in layers perpendicular to each other to create nanowire junction densities of 1011 per cm2.52 The process does not depend on self-assembly but rather on molecular beam epitaxy. 

Mbindyo, J.K.N., Template synthesis of metal nanowires containing monolayer molecular junctions, J. Am. Chem. Soc., 124, 4020,2002. 

Yoon HP, Maitani MM, Cabarcos OM, Cai L, Mayer TS, Allara DL (2010) Crossed-nanowire molecular Junctions a new multi-spectroscopy platform for conduction-structure correlations. Nano Lett 10 2897-2902... 

Mbindyo Jeremiah, K. N., Mallouk Thomas, E., Mattzela James, B., Kratochvilova, I., Razavi, B., Jackson Thomas, N. Mayer Theresa, S. Template synthesis of metal nanowires containing monolayer molecular junctions. J. Am. Chem. Soc. 124, 4020-6 (2002). 

While the discussion in this chapter has focused on molecular layers on single crystal silicon surfaces, the attachment chemistries discussed here could easily be applied to functionalize silicon nanowires or nanoparticles. Silicon nanowires have been shown to exhibit interesting electrical transport characteristics and have been used to fabricate nanoscale pn junctions [95], field effect transistors [96] and biochemical sensors [97-100]. However, all these interesting phenomena have been reported on oxidized silicon nanowires. It is likely that better control over the surface properties, as could be achieved by employing some of the chemistry discussed here, could significantly improve the performance of these nanowire-based devices. From another perspective, silicon nanowires could prove extremely... 

A method to form metal-SAM-metal nanowires with a diameter < 40 nm was developed by Mallouk and coworkers [51, 76]. The nanowires were produced by electrodeposition of Au or Pd into the nanopores of a polycarbonate membrane. A SAM was formed at the end of the wire and a second metal contact (Au, Ag or Pd) was deposited on top of this. The polycarbonate was subsequently dissolved in dichloromethane, which released a large quantity (1011 cm-2) of nanowires that could be aligned individually between pairs of lithographically fabricated metal electrodes. A schematic illustration of the nanowire molecular junctions is shown in Fig. 10.14. 

Nanocrystal and cluster science is the study of the chemical synthesis and physical properties of individual nanocrystals and nanotubes. It seeks to understand the evolution of molecular properties into solid state properties with increasing size. Methods include so-called bottom-up chemical synthesis of nanocrystals, nanowires, and very large species, as well as physical molecular beam approaches. Advanced physical characterization of single nano-objects by local probe methods and optics is critical here. The area is intrinsically interdisciplinary at the junction of physics, chemistry, and materials science. Outstanding chemical research in nanocrystal science is often found in a wide variety of science and engineering departments. 

The electron- and ion-beam irradiation of crossing single-waUed CNTs induces structural defects, which lead to the formation of stable junctions of various geometries between their surfaces (Krasheninnikov et al. 2003, Terrones et al. 2002). The selective EB irradiation of individual CNTs can be used for welding of tubes to a metal surface that improves electric contact between the tube and the surface (Ando et al. 2004, Krasheninnikov et al. 2002). The influence of electron and ion beams on bundles of single-layer and multilayer CNTs modifies their structure with formation of molecular junctions between the tubes and between individual layers in the multilayer tubes (Krasheninnikov and Nordlund 2004). This prcxtess is acccmpanied, as a rule, by the distortion of the tube surface due to the formation of numerous defects in the carbon lattice (see Figure 18.11). The irradiation of multilayer CNTs with ion beams cmi lead to their transformation to amorphous carbon nanowires and diamond nanorods (Sun et al. 2005, Wang et al. 2004). 

The work by Kruger et al. was initiated by an investigation into the influence of mechanical force on the thiolate - gold interaction which was relevant to mechanical break junctions. In this review we focus on the aspects of this work which relate to mechanochemistry, and not to the mechanical strength of gold nanowires. Their research applied first principles molecular dynamics simulations to investigate the abstraction of an ethylthiolate molecule from an Au(211) substrate, thus investigating the response of the substrate - molecule interaction to an external force applied to the surface normal (Fig. 7). 

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