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Nanoscience

Small talk

Is rebranding research as 'nanoscience' just jumping on the bandwagon? A recent conference in Basel proves that the name does at least attract researchers from different disciplines to mingle for mutual inspiration.

While most of Switzerland was caught up in the festivities on 1 August that commemorate the birth of the nation, 1,400 scientists gathered in Basel at ICN+T 2006Footnote 1 to discuss the latest developments in nanoscience and technology. They were also there to celebrate a pivotal event in the history of Swiss science: the invention of the scanning tunnelling microscope (STM) at the IBM labs in Zurich more than 25 years ago.

The sharp needle of the STM traces the roughness of surfaces with such high resolution that individual atoms become visible. By opening up a view of the world at nanometre scale, the now-ubiquitous STM catalysed the emergence of a highly diversified field of research — known variously as nanoscience or nano-technology. Exactly where its boundaries lie is notoriously unclear. The researchers who came to Basel fall roughly into two groups: those designing new materials and devices from the bottom up, using nano-metre-scale structures as building-blocks; and those creating new tools to investigate matter — mainly biological structures — at the nanometre scale.

Nano is the new micro

Conventional silicon microelectronics will, sooner rather than later, reach a point at which standard structures cannot be scaled down any further. Researchers hope that by then new-style bottom-up devices will be ripe for commercial use. Designing structures at the nanometre scale allows the quantum-mechanical properties of matter to be exploited and devices with unprecedented functions to be built. Indeed, the 'qubits' that are the basic units of quantum computing were recurring stars of the Basel conference — most frequently in their incarnation as single electrons confined within the islands of semiconductor material known as quantum dots. Although it is unlikely that the next generation of computer chips will be based on quantum logic, significant progress is being made in constructing practical qubits to allow logic operations in a small circuit (Lieven Vandersypen, Delft Univ. Technology)1.

Devices and electronic materials based on the properties of tailor-made molecules are also being designed. In various theoretical proposals and experimental realizations, single molecules sandwiched between two electrical contacts work as miniature electronic devices — as rectifiers, for example, which admit current in only one direction. The leap from constructing a prototype single-molecule device to integrating it into a larger-scale electronic circuit with reliable characteristics is clearly a big one. Here, the principle of 'self-assembly' is often exploited, in which anchor groups are chemically attached to molecules, forcing them to grab hold of surfaces and form an organized layer.

An innovative self-assembly approach reported at the conference involved the creation of a two-dimensional network in which organic molecules form electrical links between metal nanoparticles (Laetitia Bernard, Univ. Basel)2. First, the nanoparticles order themselves in a neat array, and are electrically isolated from each other by an insulating layer. After immersing this array in a solution of conducting organic molecules, its electrical resistance falls by several orders of magnitude, a drop ascribed to the organic molecules forming conducting links between the particles (Fig. 1). The organic molecules can be discarded, returning the system to an insulating state, and the high and low values for resis-tance can be reproduced over many cycles, indicating that a highly ordered network of molecular junctions is formed each time. The researchers claim that their network is a robust platform that can be used to experiment with other tailor-made molecules and nanoparticles and so create more complex nanoscale electronic circuits.

Figure 1: Forging links.
figure1

A two-dimensional array of nanoparticles (gold) with insulating spikes becomes electrically linked up after immersion in a solution of conducting organic molecules (red). It is thought that each pair of neighbouring particles is connected by approximately one molecule. The linking process is completely reversible and can be repeated many times. (Figure courtesy of Michel Calame.)

Taking the bioroute

Molecular electronics is an area where physicists have forged fruitful collaborations with chemists. Other physicists have crossed over to the life sciences, where there are many challenging problems to tackle at the single-molecule scale. Among the topics discussed at the conference were sensitive detectors for biological molecules in solution (Scott Manalis, MIT); what kind of tool can unfold single strands of RNA (Cees Dekker, Delft Univ. Technology); and whether a microscope can be constructed to determine the chemical structure of a complex three-dimensional molecule (Dan Rugar, IBM Almaden Research Center).

A basic tool now widely used for imaging biological structures is a cousin of the STM called the atomic force microscope (AFM). Instead of a sharp metallic needle, the AFM uses a thin vibrating strip of material — a cantilever — to feel the contours of a surface, potentially at nanometre resolution. The imaging process is based on the sensitive dependence of the cantilever's resonance frequency on interaction forces with the surface.

In a variation on this principle, microscale or nanoscale resonators have been found useful as stand-alone devices for sensing molecules or small particles and detecting their mass. At the ultimate detection limit, a nanoscale resonating strip cooled to cryogenic temperatures can weigh clusters of atoms down to a mass resolution of 7 zeptograms (7 × 10−21 g) (Michael Roukes, Caltech)3. That is equivalent to just 30 atoms of xenon. At the other end of the spectrum, micro- and nanoelectromechanical sensing systems that are cheap, portable, easy to use and have an integrated read-out are being developed for large-scale scientific or commercial applications, such as detecting chemical substances or monitoring biochemical reactions (Anja Boisen, Tech. Univ. Denmark)4.

Creative buzz

Many of the early STM enthusiasts were present in Basel, including Heinrich Rohrer and Gerd Binnig, the IBM researchers who shared the 1986 Nobel Prize in Physics for the technique's invention. Among the anecdotes was the story that the first seminal paper reporting the new technique was rejected by a leading physics journal: hindsight is indeed a wonderful thing. Nowadays, STM and AFM are used in a huge variety of studies, including the effect of green tea on health by imaging cell structure, detecting acoustic bursts in butterfly cocoons and even as one source of ideas for a new form of science-inspired dance, as entertainingly demonstrated in a plenary talk (Jim Gimzewski, UCLA).

Indeed, a remarkable, almost contagious, creative activity in nanoscience was evident at ICN+T 2006. A point emphasized time and again was the importance of doing things that seem crazy at first glance. As one contributor put it at the end of his presentation, “I wonder if this will work, but I am going to try it anyway.”

Notes

  1. 1.

    *International Conference on Nanoscience and Technology, Basel, Switzerland, 31 July–4 August 2006; http://www.icnt2006.ch

References

  1. 1

    Koppens, F. H. L. et al. Nature 442, 766–771 (2006).

  2. 2

    Liao, J., Bernard, L., Langer, M., Schönenberger, C. & Calame, M. Adv. Mater. (in the press).

  3. 3

    Yang, Y. T., Callegari, C., Feng, X. L., Ekinci, K. L. & Roukes, M. Nano Lett. 6, 583–586 (2006).

  4. 4

    Dohn, S., Hansen, O. & Boisen, A. Appl. Phys. Lett. 88, 264104 (2006).

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