Surface maps of organic molecules.

Surface maps of organic molecules

Imagine closing your eyes, then running your finger over a corrugated surface. Your finger would detect the surface's regular pattern of alternating ridges and grooves, and you could convert what your finger feels into an image in which, say, a white dot would correspond to each elevated point encountered by your finger and a black dot to any other point. By scanning the surface systematically, you would generate a black-and-white image of the surface. The atomic-force microscope applies a similar principle -- but on an atomic or molecular scale (SN: 4/19/86, p.244).

Recently, a group of researchers at Stanford University and the University of California at Santa Barbara used an atomic-force microscope to reveal parallel rows of polymer molecules, lying side by side only 0.5 nanometer apart in a one-molecule-thick layer on the surface of a glass slide. "These results demonstrate that atomic-force microscope images can be obtained for an organic system," the researchers report in the Jan. 1 SCIENCE. Previous experiments had involved graphite surfaces and inorganic materials such as sodium chloride.

The atomic-force microscope is a descendant of the scanning tunneling microscope (SN: 10/25/86, p.262). It operates by maintaining a minute but steady force between its needle-like diamond tip and the surface being scanned. The force involved is about a millionth of that applied by a phonograph needle on a record and is small enough not to disturb or damage any surface molecules significantly. Because no electric current need flow between surface and needle, as it must for the scanning tunneling microscope, the atomic-force microscope works equally well with conducting and insulating materials.

"Our [atomic-force microscope] is not yet, however, useful for routine imaging of biological materials," the researchers say. Further improvements are needed in diamond-tip preparation and cleaning and protection against accidental breakage of the needle's delicate support system.

Other researchers are looking at the possibility of using atomic-force microscopes not only for detecting surface atoms and molecules but also for understanding the nature of electromagnetic, chemical and frictional forces on a microscopic scale. For example, a sharp ferromagnetic needle in place of a diamond tip could pick up the fine details of magnetic fields above a magnetized nickel film or near a magnetic recording head.