Atomic force microscope

The atomic force microscope (AFM) or scanning force microscope (SFM) is a very high-resolution type of scanning probe microscopy, with demonstrated resolution of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. The precursor to the AFM, the scanning tunneling microscope, was developed by Gerd Binnig and Heinrich Rohrer in the early 1980s, a development that earned them the Nobel Prize for Physics in 1986. Binnig, Quate and Gerber invented the first AFM in 1986. The AFM is one of the foremost tools for imaging, measuring and manipulating matter at the nanoscale. The information is gathered by "feeling" the surface with a mechanical probe. Piezoelectric elements that facilitate tiny but accurate and precise movements on (electronic) command enable the very precise scanning. Electron micrograph of a used AFM cantilever image width ~100 micrometers (top) and ~30 micrometers (bottom)The AFM consists of a cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law. Depending on the situation, forces that are measured in AFM include mechanical contact force, van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces (see magnetic force microscope, MFM), Casimir forces, solvation forces, etc. As well as force, additional quantities may simultaneously be measured through the use of specialised types of probe (see scanning thermal microscopy, photothermal microspectroscopy, etc.). Typically, the deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes. Other methods that are used include optical interferometry, capacitive sensing or piezoresistive AFM cantilevers. These cantilevers are fabricated with piezoresistive elements that act as a strain gauge. Using a Wheatstone bridge, strain in the AFM cantilever due to deflection can be measured, but this method is not as sensitive as laser deflection or interferometry.

Further information

1. R. Wiesendanger, Scanning Probe Microscopy and Spectroscopy, Cambridge Universtiy Press, Cambridge (1994)
2. D. Sarid, Scanning Force Microscopy, Oxford Series in Optical and Imaging Sciences, Oxford University Press, New York (1991)
3. R. Dagani, Individual Surface Atoms Identified, Chemical & Engineering News, 5 March 2007, page 13. Published by American Chemical Society
4. Q. Zhong, D. Innis, K. Kjoller, V. B. Elings, Surf. Sci. Lett. 290, L688 (1993)
5. J. Morris, A. R. Kirby, A. P. Gunning, Atomic Force Microscopy for Biologists. (Book) (December 1999) Imperial College Press
6. P. Hinterdorfer, Y. F. Dufrêne, Nature Methods, 3, 5 (2006)
7. Суслов А. А., Чижик С. А. Сканирующие зондовые микроскопы (обзор) // Материалы, Технологии, Инструменты — Т.2 (1997), № 3, С. 78-89
8. Article Atomic force microscope from Wikipedia, the Free Enciclopedia. Available under the license Creative Commons Attribution-Share Alike.

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