Scanned Probe Microscopy: Past, present, and future

In the past decade a new family of image-forming devices has been developed, machines that do not use lenses and are collectively called scanned probe microscopes (SPM). The SPM family evolved from the scanning tunneling microscope (STM) developed by Binnig and Rohrer in the early 1980s. The tunneling microscope and subsequent probe microscopes, such as the atomic force microscope (AFM), are based on the precise positioning and scanning of a probe within nanometer distances of a surface. Sub-nanometer precision is accomplished using piezoelectric ceramics that change shape with applied electrical potential allowing probes to be moved laterally with less than 0.1-nm resolution and vertically with less than 0.01-nm resolution. This method of positioning has been routinely used with SPM over the past 10 years, during which time many different probes have been developed. These probes measure signals from a variety of physical phenomena such as electron tunneling, atomic force, electrical conductivity, temperature gradients, light absorption, ion currents, and magnetic properties. A significant difference between SPM and conventional light and electron microscopes is that the probes can operate in a wide range of environments including pressures that range from ultrahigh vacuum to ambient pressure, temperatures that range from liquid helium to hundreds of degrees Kelvin, and physical states that include immersion in hydrophobic liquids such as oil and hydrophilic liquids such biological buffers. The probes are usually scanned in either a constant signal mode or in a constant height mode. Signals are amplified and can be used to control the probe's vertical position. The signal is recorded digitally and displayed on a computer screen and thus can be manipulated by image-processing tools to generate topographic maps of the surface. The references at the end of this article cite several of the major reviews of probe microscopy.