Building structures atom by atom, first to the molecular scale and then to the macro scale, is the defining characteristic of the emerging field of nanotechnology. Wiesendanger contends that a key nanotechnology tool is the scanning tunneling microscope (STM). In STM and related scanning-probe methods, he writes, a probe tip of atomic sharpness is brought within close proximity to the object under investigation until some physical signal can be measured that might originate from electronic, electrical, magnetic, optical, thermal, or other kinds of interactions between tip and sample. Point probing by a sharp tip allows one to receive local information about the physical, chemical, or biological state of a sample, which facilitates the investigation of site-specific properties.7
As Wiesendanger explains, to achieve high spatial resolution the distance between the probe tip and the sample is chosen to be smaller than the characteristic wavelength of the particular type of interaction acting between tip and sample. In the case of STM, that distance would be the electron wavelength, whereas for a scanning optical microscope it would be the optical wavelength. STM and related scanning-probe methods are therefore exceptional types of microscopes because they work without lenses, in contrast to optical and electron microscopes, and thus achieve superresolution.
He writes that, for strongly distance-dependent interactions, the dominant tip-sample interaction region can be as small as a few angstroms, thereby allowing the imaging of individual atoms and molecules on surfaces. Increasing the interaction strength between probe tip and sample in a controllable manner has become important for the fabrication of well-defined nanometer-scale structures. It has even become possible to synthesize artificial structures by sliding individual atoms and molecules on surfaces by means of the probe tip.
According to Wiesendanger, the controlled manipulation of matter at the scale of individual atoms and molecules may lead to new generations of nanoelectronic and mass storage devices. Scanning-probe instruments have also become powerful metrological devices that allow measurement of the distances between objects with extremely high accuracy. The precision of distance measurements offered by scanning-probe methods is also of great importance for a variety of sensor applications.
Scientists using scanning-probe microscopes and advanced optical methods are able to closely study single molecules. As Bai et al. note in their writings, the findings of these studies not only confirm the results expected from studies of bulk matter, but also provide substantially new information on the complexity of
Frontiers of Science/1997. Roland Wiesendanger, at <http://www.pnas.org/cgi/content/full/94/24/12749>.