Microscopy, Scanning Probe

Influential Forebears

Almost all scanning probe microscopes today trace their lineage back to the STM, invented in 1981. The STM was the first instrument to use a small, solid probe, kept very close to a sample, to obtain nondestructive information about that sample that was radically better (in some capacity) than information obtainable from other techniques. There were, however, at least four predecessor technologies that contained some of the elements of a generic scanning probe microscope.

The first was the near-field scanning optical microscope (NSOM)—not actually demonstrated at optical wavelengths until the mid-1980s, but proposed in 1928 by E.H. Synge and demonstrated at microwave wavelengths in the early 1970s. In NSOM, light (or other forms of electromagnetic radiation) that is transmitted through a sample is collected through an aperture that is kept close to the sample and scanned over it.

Next came stylus profilometers, developed in the 1950s to measure surface roughness. In profilometry, a spring-loaded stylus is scraped along a surface and the deflection of the spring measures the roughness of the surface at a given point. Distinctions between profilometry and later probe microscopes, particularly the atomic force microscope, are fuzzy, but in general AFM is higher resolution and causes less damage to the surface.

The 1970s saw the development of scanning acoustic microscopy (SAM), essentially a near-field microscope in which ultrasonic vibrations are the imaging radiation. Acoustic microscopy had a significant impact on probe microscopy because SAM's inventor, Calvin Quate, became an important probe microscopist along with many of his former students and postdocs, such as Daniel Rugar and Kumar Wickramasinghe.

Finally, the most immediate predecessor to STM was the Topografiner, invented by Russell Young at the U.S. Bureau of Standards around 1968. The Topografiner contained all the elements of the STM, but never quite combined them successfully—for instance, it used field-emitted (rather than tunneling) electrons to form an image, resulting in lower resolution and more sample damage than the STM. Nevertheless, the Topografiner deeply influenced early probe microscopists such as Quate.

Scanning (Blank) Microscopy

What made STM foundational for probe microscopy (and nanoscience) is that it tied together an increasingly diverse group of researchers dedicated to building and using related instruments. By 1984, early STM builders had begun to realize that the STM was not unique—other instruments could be built in which a small solid probe gathered information about a sample, and in which the probe could be rastered to build up an image of the sample. Not surprisingly, the earliest variants of the STM built on the STM's predecessors. For instance, NSOMs started to appear in which the aperture was coated with metal to turn it into an STM tip—this allowed the microscope to feed back on the tunneling current as a way to keep the aperture close enough to the surface to capture near-field optical radiation.

...