With atomic-scale tools, you can write your name in molecular letters billionths of a meter wide. Now scientists have put these tiny tweezers to better use, to test the strength of bonds between individual atoms. Their technique, reported in tomorrow's issue of Science, could be used to study chemical reactions at the level of individual atoms.
In scanning tunneling microscopy (STM), a metal tip, sharpened to the thickness of a few atoms, is held at a fixed voltage and passed over a material to be imaged. Electrons jump from the tip to the material more easily where the atoms conduct better, so the ebb and flow of current can be used to paint a picture of the hills and valleys formed by surface atoms. The atoms are held together by bonds that push and pull like springs. So theorists have predicted that if the tip were at just the right voltage, it could start the atoms vibrating on the "springs." The vibrating atoms would conduct a little better and increase the current flow. If the tip and surface were held still, the extra flow might be detectable.
So Wilson Ho, a physicist at Cornell University, and two students bolted their STM to a table that sat on shock absorbers and cooled the tip down to 8 degrees above absolute zero to quell the electrical noise generated by heat. They stuck five different kinds of molecules on a copper surface and imaged them; none, however, would sit still. "We kept saying, 'It has to work, it has to work!' " says Ho. Finally they tried acetylene--a pair of carbon atoms bookended by two hydrogen atoms--which seemed to stick to the substrate better, and had success. At a particular voltage, they saw an extra trickle of current, due to the stretching and contracting of the carbon-hydrogen bond.
"It's very good work," says Richard Colton, a physical chemist at the Naval Research Lab in Washington, D.C. Because the voltage tells how strong the bond is, researchers can now try placing two different atoms next to each other and measuring how they affect each other's bonds, he says. That could be useful, for instance, for designing new semiconductors or understanding how catalysts speed up chemical reactions.