How an Atomic Force Microscope Works

An atomic force microscope, or AFM, is an instrument for mapping and measuring surface features of extremely small objects – from a carbon atom that is 0.25 nanometers (nm) or 2.5 Angstroms in diameter to a cross section of human hair (approximately 80 microns in diameter).

Basic Principles of an Atomic Force Microscope

The atomic force microscope is basically a miniaturized cantilever (a tiny beam is anchored at one end while another projects out into space like a diving board) with a tiny, pointed probe (with an extremely fine ceramic or semi-conductor tip which is measured on the scale of nanometers) underneath one end, much like the stylus on a lie detector or even a seismograph. Unlike a stylus which prints on paper or other medium, an atomic force microscope has several refinements which allow for atomic-level measurement of the attractive or repulsive forces between the "stylus" tip and a sample's surface.

As the tip is attracted or repelled by the sample's surface, the cantilever is deflected. The magnitude of deflection is measured by a laser which reflects at an oblique angle from the end of the probe. Plotting the laser deflection against the tip position on the sample surface creates a "map" of the hills and valleys of the surface. This provides a high resolution image of the sample's surface.

The atomic force microscope has two scanning modes. In contact mode, the atomic force microscope's probe touches the sample's surface. As the instrument drags the tip over the surface, a detection device measures the cantilever's vertical deflection and provides an indication of the local sample height – in effect, measuring the 'repulsion' forces between tip and sample. In non-contact mode, the atomic force microscope's probe does not touch the surface of the sample; it measures attractive forces between the tip and the surface to draw a topographic map of the surface.

Advantages and Disadvantages of the Atomic Force Microscope

An atomic force microscope has advantages over a scanning electron microscope (SEM). For one, an atomic force microscope can function in ambient air or a liquid environment, unlike an electron microscope which requires that all probes be undertaken in a vacuum. Given this, researchers have started testing atomic force microscope's suitability for use in studying living organisms at the nano-scale (e.g., scanning and studying biological macromolecules such as DNA and the like). For another, an atomic force microscope can plot a three-dimensional image; an SEM can only provide a two-dimensional image or projection of a sample.

On the other hand, a major downside of an atomic force microscope is the area it can scan and the image resolution that it can generate. An electron microscope can scan an area measured in millimeters; an atomic force microscope's scan covers micrometers (nanometers, in fact). From this perspective, it is easy to see that an electron microscope can scan a wider area faster than an atomic force microscope.

atomic force microscope is quite new and still has some bugs but it is currently being used for a wide range of study in the electronics, chemical and biological fields including such esoteric subjects as abrasion and adhesion, cleaning and corrosion, as well as a host of other applications.