Nanowire junctions

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. 

---

Landman, U., Luedtke, W.D., Salisbury, B.E. and Whetten, R.L. (1996) Reversible Manipulations of Room Temperature Mechanical and Quantum Transport Properties in Nanowire Junctions. Physical Review Letters, 77, 1362-1365. 

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. 

Alam MM, Wang J, Guo YY, Lee SP, Tseng HR (2005) Electrolyte-gated transistors based on conducting polymer nanowire junction arrays. J Phys Chem B 109 12777-12784... 

Figure 2.20 SEM images of (a) ZnO nanorods grown using gold as a catalyst, (b) ZnO nanowIres and nanonalls grown following the cylindrical shape of a substrate, (c) hierarchical ZnO nanowIre junction arrays. (Courtesy of Dr X. Y. Kong.)...

Atomic force microscopy of palladium nanowires showing the (a) open break junction before exposure to hydrogen and (b) closed break junction after exposure to hydrogen. 

A break junction can also be created by passing a current (0.5-1.0 V) through an Au nanowire (<20 nm diameter) defined by electron-beam lithography and shadow evaporation. Such electromigrated break junctions (EMBJs) have yielded reproducible 1-3 nm gaps between electrodes [48-50]. 

Zhou X-S, Wei Y-M, Liu L, Chen Z-B, Tang J, Mao B-W (2008) Extending the capability of STM break junction for conductance measurement of atomic-size nanowires an electrochemical strategy. J Am Chem Soc 130 13228-13230... 

Figure 11.6. Crossed nanowire p-n diode, (a) A typical SEM image of a crossed NW p-n diode, (b) Current-voltage (I-V) relation of the crossed p-n diode. Linear or nearly linear I-V behavior of the p-type and n-type NWs indicates good contact between NWs and metal electrodes. I-V curves across the junction show clear current rectification, (c) An SEM image of an NW p-n diode array, (d) I-V behavior for a 4(p) x l(n) multiple junction array. [Adapted from Ref. 57.]... 

Figure 11.9. NW LED. (a) Crossed InP nanowire LED. (top) Three-dimensional (3D) plot of light intensity of the electroluminescence from a crossed NW LED. Light is only observed around the crossing region, (bottom) 3D atomic force microscope image of a crossed NW LED. (inset) Photoluminescence image of a crossed NW junction, (b-c) Multicolor nanoLED array, (b) Schematic of a tricolor nanoLED array assembled by crossing one n-GaN, n-CdS, and n-CdS NW with a p-Si NW. The array was obtained by fluidic assembly and photolithography with ca. 5- xm separation between NW emitters, (c) Normalized EL spectra obtained from the three elements. [Reprinted with permission from Ref. 59. Copyright 2005 Wiley-VCH Verlag.]...

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

The polycarbonate membranes are stretch-oriented during fabrication in order to improve their mechanical properties. If the membrane is subsequently heated above its glass-transition temperature ( 150°C), the polymer chains relax to their unstretched conformation and the membrane shrinks. This shrinking of the membrane around the Au nanowires in the pores causes the junction between the nanowire and the pore wall to be sealed. This is illustrated in Fig. 5, which shows voltammograms for tri-methylaminomethylferrocene (TMAFc+) before (Fig. 5A) and after (Fig. 

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... 

Fig. 9. An electron microscope zooms in on an eight-by-eight nanowire grid [62]. Molecules between the grid junctions act as switches.

Nanocrystals and nanowires are utilized in a new generation of solar collectors (a nanometer is one billionth of a meter). In conventional solar cells, at the P-N junction one photon splits one electron from its "hole companion" as it travels to the electron-capturing electrode. If solar collectors are made of semiconducting nanocrystals that disperse the light, according to TU Delft s professor Laurens Siebbeles, an avalanche effect results and one photon can release two or three electrons, because this effect maximizes photon absorption while minimizing electron-hole recombination. This effect of the photon-scattering nanoparticles substantially increases cell efficiency. 

---