In the beginning…
In 1986 by Binning et. al., developed the first Atomic Force Microscope shown above. Since then, the AFM has undergone continuous development in all areas. The first AFMs operated in contact mode. (See Binning et al., Physics Review Letters 1986, and Rugar and Hansma, Physics Today 1990). In contact mode, the tip is mounted onto the end of a flexible cantilever and raster scans the surface of the sample. The tip-surface interaction deflects the cantilever, which gives information about the surface topography. Samples can be analyzed in air, liquids or vacuum. Resolution in liquid and vacuum is increased because of the absence of strong capillary forces due to a thin liquid film on all samples in air. Unfortunately, biological samples are challenging to study in contact mode because they are generally soft, weakly bound to the surface, and damaged easily.
Efforts to image biological samples drove the development of non-contact mode in 1987. (See Martin et al., 1987). In non-contact mode, the cantilever oscillates close to its resonant frequency at a small distance (1-10 nm) above the surface. Long-range attractive forces induce changes in the amplitude, frequency and phase of the cantilever, while the cantilever itself maintains a constant separation while scanning. The significantly lower forces by non-contact mode than by contact mode, allow for even the softest samples can be imaged without damage.
The fourth generation prototype was developed in 1989. This particular sparked interest in the commercialization of AFM
The earliest micro-fabricated tips were developed in 1991. (Prater et al., 1991) In 1993, Tapping Mode® was first introduced. (See Zhong et al., 1993) In this mode, the cantilever oscillates at its resonant frequency, but unlike non-contact mode, the cantilever gently taps the surface during scanning, reducing damaging lateral forces.
Fluid tapping, a major development for biological samples, occurred in 1994 in the Hansma Lab. (See Hansma et al., 1994) In this first trial, the sample, which sat on a piezoelectric scanner, was oscillated up and down and tapped the tip on oscillation cycle. The amplitude of the piezoelectric was set manually at the beginning of the image, and the tapping force was held constant by a feedback loop. Modern implementations oscillate the tip and not the sample for finer and easier force control.
Smaller cantilevers were developed in 1996, allowing higher resolution and smaller scanning times by significantly reducing the moving mass and increasing the detection bandwidth. The Hansma group began developing a new generation of AFMs that would utilize these smaller, lighter cantilevers. For biological samples, these tips were as significant improvement. By reducing the mass of the cantilever, high resonance frequencies could be reached for fast imaging, while maintaining a low spring constant lever that does not damage the sample. Further developments lead to cantilevers on the order of 9-40 microns in length with resonant frequencies an order of magnitude higher than commercially available cantilevers. (See Schaeffer et al., 1997b)
The next prototype was a small cantilever AFM Version 5. This prototype used custom designed optics for the optical detection of the cantilever to minimize the spot size on the cantilever. This significantly reduced spot size allows for even smaller cantilevers to be used, resulting in higher speeds and lower forces.
*Adapted from http://hansmalab.physics.ucsb.edu/afmhistory.html