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COMMENTARY How the doors to the nanoworld were opened

CHRISTOPH GERBER AND HANS PETER LANG

are at the National Competence Center for Research in Nanoscale Science, Institute of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland.

e-mail: [email protected]

The invention of the scanning tunnelling microscope 25 years ago, followed by the arrival of the atomic force microscope ﬁ ve years later, were crucial events in the history of nanoscience and nanotechnology. As the recent International Conference on Nanoscience and Technology in Basel made clear, scanning probe microscopes based on these discoveries are still having a tremendous impact on many areas of research.

n March 1981 a new type of accepted, so the experiment would not microscope made its debut. Unlike give any new insight; the other report traditional microscopes, however, the described the work as “extraordinary” scanning tunnelling microscope (STM) and a “technical jewel”, but this referee Idid not use lenses. Instead, a sharp tip said that whether such technological work was moved close enough to a conductive should be published in this particular surface for the electron wavefunctions of physics journal was an editorial decision. the atoms in the tip to overlap with the Eventually the results were published in wavefunctions of the surface atoms. When another leading journal, Applied Physics a voltage was applied, electrons started to Letters, in January 19821. ‘tunnel’ through the vacuum gap, causing In terms of science, the real a current to fl ow from the foremost atom breakthrough for the STM came in 1983 of the tip into the surface. Quantum with the experimental observation of one tunnelling had been studied theoretically of the most intriguing phenomena in before, but had never been demonstrated surface science at that time: atom-by-atom so elegantly as in these experiments at the AARHUS INANO, BESENBACHER, F. imaging of the 7×7 surface reconstruction IBM Zurich Research Laboratory in Si(111) (ref. 2). Th e STM images allowed in Switzerland. Figure 1 Keep it clean. Molybdenum disulphide the now widely accepted model of the Moreover, the tunnelling current nanocrystals are used as catalysts to remove reconstruction to be worked out from the 3 depended exponentially on the distance sulphur impurities at oil reﬁ neries. Reducing models of the time . For the fi rst time it was between the tip and the surface: changing harmful sulphur emissions from the combustion of possible to get up close and personal with this distance by just an atomic diameter transport fuel is a major environmental challenge. individual atoms on surfaces in a three- changed the current by a factor of a 6 dimensional representation. Recent STM studies of MoS2 nanocrystals thousand. Th erefore, by scanning the tip — which are triangular — on gold surfaces have It took the small but steadily growing over a sample in two dimensions and clariﬁ ed how these catalysts work and lead to community another two years to verify keeping the tunnelling current constant, improvements in their performance. the initial results obtained in Zurich, and it was possible to image surfaces on the it was only at a workshop at Oberlech in atomic scale. the Austrian Alps in 1985 that researchers Th e initial results were written up in a started to realize the potential of the new manuscript entitled “Tunnelling through declined by the editors based on the method. Devices such as the atomic force a controllable vacuum gap”, which was following referee reports: one referee said microscope (AFM) have their roots in submitted to a leading physics journal that the exponential dependence of the this meeting, and during the last night of in June 1981. However, the paper was tunnelling current on distance was well the workshop the Alps were buzzing with

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crazy new ideas for using such microscopes in liquids, at low temperatures, in high DNA, proteins, peptides and antibodies) in many diff erent areas of science and magnetic fi elds and so on), and also for is detected by changes in surface stress technology. People were eager to get back chemical and biological applications when in a way that could have advantages over to their labs and start working right away, the tip is suitably modifi ed. Its ability to standard biomolecular techniques. In although some almost missed the bus to the investigate surfaces with unprecedented general, the possibilities off ered by coating airport the next morning because they were resolution introduced a wealth of related the individual cantilevers in an array with still caught up in the discussions. A year techniques (see Fig. 2). For instance, layers to which only particular types of later, in 1986, Gerd Binnig and Heinrich local electric charges on the tip or surface biomolecules can attach are enormous11. Rohrer shared the Nobel Prize in Physics lead to electrostatic forces that allow Indeed, even the sky is not the limit “for their design of the scanning tunneling the distribution of electric charge on a for AFM technology. Th e Rosetta mission microscope”. A signifi cant number of the surface to be visualized (electrostatic force to comet 67P launched by the European protagonists in that meeting are still going microscopy). Similarly, magnetic forces Space Agency in 2004 includes an AFM in strong in the fi eld today. can be imaged if the tip is coated with a its MIDAS (Micro-Imaging Dust Analysis In the years that followed the magnetic material, such as cobalt, that System) instrument. Th e goal of this excitement of the mid-1980s, Don Eigler has been magnetized along the tip axis mission, which is expected to touch down and co-workers at the IBM Research Lab in (magnetic force microscopy). on 67P in 2014, is to analyse particle size Almaden, California, used an STM to write Information on force gradients can distributions in cometary material. Th e next the IBM logo with 35 xenon atoms on a be obtained if the cantilever is forced to NASA mission to Mars is also expected to nickel surface4, and later moved iron atoms oscillate. Depending on the amplitude include an AFM for similar studies. on a copper surface to make a quantum of the oscillation we can operate the corral5. Some researchers discussed the AFM in ‘tapping’ mode or perform OUTLOOK possibility of using such structures for dynamic force microscopy to provide quantum teleportation, whereas others have true atomic resolution8, which, in turn, With the emergence of scanning probe tackled important environmental problems. allows us to perform force spectroscopy microscopy (SPM) and related techniques Flemming Besenbacher and co-workers on specifi c sites. Material properties, in the 1980s, the door to the nanoworld was at the University of Aarhus in Denmark, especially diff erences in elasticity, can be pushed wide open. Today these methods are for instance, have recently used STMs to locally discerned using ultrasonic force still making a tremendous impact on many understand how molybdenum disulphide microscopy. But this is only the tip of the disciplines that range from fundamental nanoclusters act as catalysts to remove iceberg (Fig. 2). physics and chemistry through information potentially harmful sulphur compounds In addition to imaging surfaces, the technology, quantum computing, from crude oil6 (Fig. 1). Th ese are just two AFM can also be used to modify surfaces spintronics and molecular electronics, and of many examples that demonstrate the and manipulate individual atoms and all the way to life sciences. Indeed, some enormous potential of the STM. molecules. By depositing, removing or 4,000 AFM-related papers were published modifying material from the tip and/or last year alone, bringing the total to 22,000 MAY THE FORCE BE WITH YOU sample, it is possible to write and read since it was invented, and the STM has information to and from the surface. inspired a total of 14,000 papers. Th ere are Th is, however, is just half of the story. Th e Meanwhile, measuring forces as a function also at least 500 patents related to the various encore came with the development of the of the tip–sample separation (that is, forms of scanning probe microscopes. AFM 20 years ago7. Th is work has now been measuring force–distance curves) allows Commercialization of the technology cited more than 4,700 times, which makes us to draw conclusions regarding the started in earnest at the end of the 1980s, it the second-most-cited paper published in material characteristics of surfaces and and approximately 10,000 commercial Physical Review Letters in the last 25 years. their chemical properties. It is also possible systems have been sold so far to customers As with the STM, the AFM relies on a sharp to tether one end of a molecule (such as in areas as diverse as fundamental research, tip that is scanned over a surface. Th is tip is DNA) to a surface, and the other end to the the car industry and even the fashion part of a cantilever that can measure forces AFM tip, and investigate the mechanical industry. Th ere are also a signifi cant down to the lower piconewton range. In a and elastic properties of the molecule by number of home-built systems in operation. sense the AFM resembles a record player — stretching it (force–distance spectroscopy Today some 30–40 companies are involved the forces between the surface and the tip and single-molecule spectroscopy). in manufacturing SPM and related cause the cantilever to bend in the vertical Magnetic resonance force microscopy instruments, with an annual worldwide direction, and by measuring this defl ection, — the mechanical detection of electron or turnover of $250–300 million. Moreover, it is possible to produce an image of the nuclear spins — has shown great promise the market of SPMs is predicted to double surface with atomic resolution. Th e forces, and may fi nally lead to a way of locally over the next fi ve years. which can be attractive or repulsive, depend mapping the chemical structure of a surface9. Th e state-of-the-art in nano was on on the nature of the interaction between Moreover, the development of chemical force display in August when more than 1,600 the tip and the surface being investigated. microscopy10 has shown that the cantilever researchers gathered in Basel to mark Examples include chemical forces, van der itself is a very sensitive tool for observing the STM and AFM anniversaries at the Waals forces, electrostatic forces, capillary chemical reactions and processes11. International Conference on Nanoscience forces or friction forces. Another broad area of application is and Technology (www.icnt2006.ch). Th e Since 1986, the AFM has proved its biological and chemical sensing. In this breadth and scope of modern nanoscience suitability in various applications. First approach the absorption of molecules onto and its technological potential was designed as an instrument to image the the cantilever allows them to be detected underlined by the 40 diff erent topics covered surfaces of non-conductive materials because they change the mass, and hence at the conference12. Th ere were also sessions with high lateral and vertical resolution, the resonance frequency, of the cantilever. on the impact of new nanomaterials on the the technique has been adapted to work However, in physiological environments, environment and human health, and the in various environments (for example, the absorption of biomolecules (such as teaching of nanoscience at universities.

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on large-scale circuitry at densities that span the projections of the semiconductor industry roadmap for the period 2020–2030 were also presented (J. Heath, Caltech). Th ere has also been remarkable progress in the development of high-speed STM, AFM and scanning near-fi eld optical microscopy. By using device resonances in the fast scanning direction, speeds of up to several tens of thousands of scan lines per second can be achieved (M. Miles, Univ. Bristol; J. W. M. Frenken, Univ. Leiden). Multiprobe microscopy is another area where progress has been rapid. STMs with up to four independent tips have been used to measure the electrical conductivity of nanostructures, and an AFM with four conductive probes has also been built (M. Aono, NIMS, Tsukuba). So what does the future hold? Th e top-down approach that dominates in nanotechnology today is already, to a certain extent, meeting the bottom-up approach of self-assembly and self- organization that has been so successful in the natural world, to create a new generation of materials, devices and systems that will spectacularly outperform those that we have today in information technology, medicine, environmental CH. GERBER CH. technologies, the energy industry and beyond. But for this to happen we need to learn more about how atoms and Figure 2 World of possibility. The AFM (centre) has inspired a variety of other scanning probe techniques. molecules behave on the nanoscale, and Originally the AFM was used to image the topography of surfaces, but by modifying the tip it is possible to we will therefore continue to rely on the measure other quantities (for example, electric and magnetic properties, chemical potentials, friction and so STM, the AFM and their many off spring. on), and also to perform various types of spectroscopy and analysis. REFERENCES 1. Binnig, G., Rohrer, H., Gerber, Ch. & Weibel, E. App. Phys. Lett. 40, 178–180 (1982). Here we present just a few of the Basel). And for the fi rst time it was shown 2. Binnig, G., Rohrer, H., Gerber, Ch. & Weibel, E. Phys. Rev. Lett. scientifi c highlights from this memorable that friction could be switched on and off 50, 120–123 (1983). 3. Takayanagi, K., Tanishiro, Y., Takahashi, S. & Takahashi, M. meeting. Spin-polarized STM has been on the atomic scale by modulating the Surf. Sci. 164, 367–392 (1985). used to probe nanomagnetic states for normal force between the tip and surface 4. Eigler, D. M. & Schweizer, E. K. Nature 344, 524–526 (1990). the fi rst time, with clear atomic-scale spin (A. Socoliuc and E. Meyer, Univ Basel; see 5. Manoharan, H. C., Lutz, C. P. & Eigler, D. M. Nature 403, contrast being seen on NiO(001) surfaces. also ref. 13, and p20 of this issue14). 512–515 (2000). 6. Lauritsen, J. V. & Besenbacher, F. Adv. Catal. 50, 97–143 (2006). Th ese results could have applications in In other work, carbon and boron nitride 7. Binnig, G., Quate, C. F. & Gerber, Ch. Phys. Rev. Lett. 56, nanomagnetism (R. Wiesendanger, Univ. nanotubes have been integrated in higher- 930–933 (1986). Hamburg). In nano-optics the control order nanoelectromechanical systems such 8. Giessibl, F. J. Science 267, 68–71 (1995). 9. Rugar, D., Budakian, R., Mamin, H. J. & Chui, B. W. Nature 430, of light propagation on length scales as motors, high-frequency resonators, 329–332 (2004). of a few nanometres by large periodic and sensors (A. Zettl, Univ. California, 10. Lee, D. et al. App. Phys. Lett. 84, 1558–1560 (2004). assemblies of identical nanostructures was Berkeley), and self-organization has been 11. Lang, H. P., Hegner, M., Meyer, E. & Gerber, Ch. Nanotechnol. reported, which could have applications used to make well-defi ned functional 3, R29–R36 (2002). 12. Venema L. Nature 442, 994–995 (2006). in new optical approaches to information molecular and supramolecular structures 13. Socoliuc, A. et al. Science 313, 207–210 (2006). technology (B. Hecht and H.-J. Eisler, Univ. (J. M. Lehn, Univ. Strasbourg). New results 14. Frenken, J. Nature Nanotech. 1, 20–21 (2006).

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