Scanning probe microscopy

Scanning probe microscopy

Among the most powerful microscopes are scanning probe microscopes. These microscopes measure surface features of an object by moving a sharp probe over the object's surface. The scanning tunneling microscope was invented in 1980. It can achieve magnification of 100 million times, and it allows scientists to view atoms on the surface of a solid. The scanning tunneling microscope has a needle like probe with a point so sharp that often there is only a one atom at its tip.

The probe is lowered toward the specimen surface until its electron cloud just touches that of the surface atoms. If a small voltage is applied between the tip and the specimen, electrons flow through a narrow channel in the electron clouds. This tunneling current, as it is called, is extraordinarily sensitive to distance and will decrease about a thousandfold if the probe is moved away from the surface by a distance equivalent to the diameter of an atom.

The arrangement of atoms on the specimen surface is determined by moving the probe tip back and forth over the surface while keeping the probe at a constant height above the specimen. As the tip moves up and down while following the surface contours, its motion is recorded and analyzed by a computer to create an accurate three dimensional image of the surface atoms.

The surface map can be displayed on a computer screen or plotted on paper. The resolution is so great that individual atoms are observed easily. Even more exciting is that the microscope can examine objects when they are immersed in water. Therefore it can be used to study biological molecules such as DNA. The microscope's inventors, Gerd Binning and Heinrich Rohrer, shared the 1986 Nobel prize in physics for their work, together with Ernst Ruska, the designer of the first transmission electron microscope.

More recently, a second type of scanning probe microscope has been developed. The atomic force microscope moves a sharp probe over the specimen surface while keeping the distance between the probe tip and the surface constant. It does this by exerting a very small amount of force on the tip, just enough to maintain a constant distance but not enough force to damage the surface.

The vertical motion of the tip usually is followed by measuring the deflection of laser beam that strikes the lever holding the probe. Unlike the scanning tunneling microscope, the atomic force microscope can be used to study surfaces that do not conduct electricity well. The atomic force microscope has been used to study the interaction of proteins, to follow the behavior of living bacteria and other cells, and to visualize membrane proteins such as aquaporins.

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