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June 1, 2007

The long and short of nanowire

New findings on growing and removing molecular wires at different temperatures could be used in nanoscale electronics

Figure 1: Scanning tunneling microscopy images showing (a) a dangling bond site, (b) the growth of a molecular wire from that site, and (c) the wire after shortening.

A study looking at growing and erasing simple nanowires on a silicon surface has provided insight into their potential use in molecular electronics at differing temperatures. Molecular electronic components attract much attention; however, the key to using them successfully depends on being able to join them, using nanowires, forming circuits.

Recently, Md. Zakir Hossain and colleagues at the RIKEN Discovery Research Institute, Wako, succeeded in growing interconnected nanowires—an important step towards the development of nanocircuits. It is important that these circuits can withstand heat so that they are useful for electronics applications.

Hossain and colleagues investigated these simple nanowires using scanning tunnelling microscopy (STM) to reveal their thermal stability and to gain understanding of how best to form nanowires. The team used a technique known as ‘dangling bond initiated chain reaction’ to create their molecular wires. This technique uses a silicon surface coated with hydrogen, where the surface appears as rows of hydrogen and silicon (H-Si-Si-H) units.

By removing one hydrogen atom a radical is left behind on the silicon. This radical reacts with a building-block for the molecular wire that contains a carbon-carbon double bond. As a result, the building-block then possesses a free radical which further reacts to remove hydrogen from an adjacent silicon atom.

The end result is one molecule of building-block added to the surface and another silicon radical is generated, a so-called dangling bond. The dangling bond can then continue the cycle adding further molecules of building-block forming a molecular line, or wire (Fig. 1).

The dangling bond that remains at the end of the line can play an important role in the stability of the nanowire. Hossain observed the molecular lines’ growth with various building-blocks at different temperatures. At lower temperatures the lines grew, whilst at higher temperatures the dangling bond reaction worked in reverse effectively erasing the wire.

Therefore growth of a wire depends on the stability of the terminal dangling bond and the rate of the reverse reaction. These results imply that the scope to grow nanowires using this technique could be wider than anticipated. The properties and growth of wires could be engineered using different building-blocks and temperatures, and capping the terminal dangling bond of a wire could also help make them more stable.

The next step is to look at other building-blocks. “Using various molecular systems, we intend to fabricate complex molecular lines leading to functional molecular circuits,” says Hossain.

References

  1. Hossain, M. Z., Kato, H.S. & Kawai, M. Competing forward and reversed chain reactions in one-dimensional molecular line growth on the Si(100)-(2 x 1)-H surface. Journal of the American Chemical Society 129, 3328–3332 (2007). |  | (Link)