科学者が語る科学の楽しみ

The Rise of Local Probe Methods

Looking back on the early days of scanning tunneling microscopy (STM), for the development of which Gerd Binnig and I were awarded the Nobel Prize for Physics in 1986, we find a few aspects common to many ventures into an unknown world. The original goal was to learn about the local structural, electronic, and growth properties of very thin insulating layers, in particular at tunnel junctions. The term "local" meant on the scale of the inhomogeneities of these properties, which were believed to be no larger than a few nanometers in size, a scale that was entirely inaccessible with existing techniques. Electron tunneling appeared to be a promising approach, provided it could be done locally. This led in a natural, non-premediated way to the local probe method now known as scanning tunneling microscopy. Electron tunneling already contained two of the four major technical elements of a local probe method: a strongly distant-dependent interaction and, inherently necessary, close proximity of probe and object. One tunneling electrode in the form of a sharp conducting tip would provide the third element, the local probe. Metal tips with a radius of curvature of about 20 nm to bring the resolution to the desired level were already in use as field emitters and in field ion microscopy. These three elements determine the resolution. The fourth and final element was the stable positioning of the probe with respect to the object with an accuracy better than the desired resolution and within the practical range of the interaction. We expected to achieve this in a vibration-protected environment with piezo drives made from commercially available material.
　
Although the development of STM appears straightforward in retrospective, it required nevertheless ideas and efforts; we had mistakes to correct and, in particular, had to deal with many unknowns. We were given the time and opportunity to do both. For instance, replacing the well defined field emission tip with a ground-metal tip simplified matters and this could - and did - help achieve atomic resolution since, unless specially prepared, most tips end up with an apex of one atom, thus becoming an atomic size probe. However, it was by no means clear that such a tip would be mechanically stable. Indeed, it was usually unstable in the early days of STM; nowadays there are nearly as many recipes for stable tips as there are scientists using them. The same goes for the piezo drives. With atomic resolution in sight, the tip position had to be controlled within a fraction of an angstrom, not just of a nanometer. Only the experiments themselves showed afterwards that the response of the piezo to an applied material was continuous and reasonably linear down to at least the picometer level. It is remarkable how often success is the reward for trying the unknown.
　
In the beginning, the anticipated resolution matched only that of scanning electron microscopy as far as structural properties were concerned. But STM offered something else. A local tunneling experiment, e.g. tunneling spectroscopy, contains a wealth of information and provides local electronic and chemical properties, in addition to the structural ones. This and the conceptual simplicity of the approach were sufficient incentive, and by the time we had finished our first successful experiment, resolution was nearly at an atomic level. This is reminiscent of electron microscopy, which at the beginning offered lower resolution than optical microscopy, was more complicated and even destroyed most of the samples in the imaging process. We might say that what is different constitutes progress, and not so much what is "better".
　
STM was not developed from one of the already existing local probe methods or ideas about them nor within the community of microscopists nor in other circles with the appropriate competence. No technologically new component or new material was necessary, no new physical insight was required nor did an additional theoretical basis have to be established, yet somehow the believe prevailed in those communities that "it" could not be done. Vacuum tunneling apparently crossed many a mind but was dismissed as infeasible. The topografiner came closest. Stylus profilometry did not go beyond carefully shaped, smoothed, and well defined sensing tips with a radius of curvature of about a micron and, therefore, stayed in the micron resolution range. Instead, applying a hammer blow to such a tip and taking a splinter of it, accompanied by a few novel ideas could have brought about atomic force microscopy (AFM) with nanometer resolution. In the case of STM, we heard many objections, including those citing the uncertainty principle, strangely enough, even after the STM had worked. We might learn from this that an occasional change in the field of interest can bring new opportunities for science as well as for scientists.
　
The initial reservations, if not to say skepticism, with which scanning tunneling microscopy was met ranged from healthy scientific caution to a competition-inspired defensiveness, which has yet to be overcome in some communities. new approaches, however, should not be regarded as competitors of existing ones; they complement them or replace them, and each one has its merits in its own time. Defensiveness does not prevent progress, at most it merely slows it down.
　
Nevertheless, what initially appeared to be rather exotic is now seen largely for what it is, namely a class of new methods, called local probe methods, for working on the nanometer scale. By "work" I mean observing a single, individually selected nano-object, measuring and understanding its properties, manipulating it, modifying it, and ultimately observing and controlling its possible functions and related processes. The local probes are so to speak the "finger tips" of the nanoworld and are now regarded as a key to the new coming period, the nanometer age. This period will affect technology and society no less than did today's microtechnology. A modest but extraordinary venture can indeed have big, extraordinary consequences.

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