Scanning Probe Microscopy
The idea that a small solid probe, kept very close to a surface, might collect data
unavailable to other forms of microscopy is surprisingly old. Theoretical descriptions of
scanning probe microscopes have existed since the 1920s, and experimental
implementations since the late 1960s. However, modern probe microscopy dates from
the invention of the scanning tunneling microscope in 1981. Although the STM and the
atomic force microscope have been by far the most successful probe microscopes, the
past quarter century has seen an astonishing proliferation of other variations on the basic
idea. Collectively, the scanning probe microscopes have enabled collection of almost any
kind of data at the nanometer scale. Increasingly, they are also used to build tiny
structures – making them the most diverse and powerful family of techniques in
nanoscience.
Influential Forebears
Almost all scanning probe microscopes today trace their lineage back to the scanning
tunneling microscope, invented in 1981. The STM was the first instrument to use a
small, solid probe, kept very close to a sample, to obtain non-destructive 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 – 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, 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 US 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.
Soon, though, probe microscopists started to develop more novel instruments in
which the probe was tailored to gather an astonishing array of information: scanning
thermal microscopy, scanning electrochemical microscopy, scanning ion conductance
microscopy, magnetic force microscopy, scanning capacitance microscopy, etc. Some of
these kept the probe close to the surface using tunneling current, some used atomic forces
(as in AFM), some used more exotic feedback mechanisms. Some research groups, such
as Kumar Wickramasinghe’s at IBM, became famous for repeatedly inventing new probe
microscopes.
The Swiss Army Knife of Nanotechnology
By 1999, the proliferation of these instruments was causing a nomenclature problem, as
different groups gave the same instrument different names (e.g. NSOM is known as
SNOM in Europe). The International Union of Pure and Applied Chemistry set up a
commission to standardize the naming of such “SXMs”. One important vector for this
proliferation has been the commercialization of probe microscopy. Microscope
manufacturers have developed generic “controllers” that scan the probe, generate an
image, and analyze data. From there, they either sell a variety of add-ons that adapt the
controller to different variants of probe microscopy, or they sell the controller to research
groups that develop variants for themselves.
The great achievement of probe microscopy has not been the remarkable imaging
resolution of these instruments – other kinds of microscopes can achieve ultrahigh
resolution, and many probe microscopists have rather poor resolution. Instead, probe
microscopy has demonstrated that the nanoscale is an information-rich environment, in
which almost any material characteristic can be made visible. Visibility, in turn, means
control – either directly through manipulation by the probe, or indirectly by using
information from the probe to steer nanoscale processing techniques.
SEE ALSO: Atomic Force Microscopy, International SPM Image Competition, Scanning
Tunneling Microscopy
BIBLIOGRAPHY: Gernot Friedbacher and Harald Fuchs, “Classification of Scanning
Probe Microscopies (Technical Report),” Pure and Applied Chemistry (71/7, 1999);
Cyrus C.M. Mody, “How Probe Microscopists Became Nanotechnologists,” in
Discovering the Nanoscale (IOS Press, 2004); H. Kumar Wickramasinge, ed., Scanned
Probe Microscopy (American Institute of Physics, 1991).
Cyrus C.M. Mody
Rice University