tincluster
Image: Martin Jarrold/Northwestern University

When it comes to melting, metals have always acted like chocolates: the smaller the piece, the less heat it takes. The reason is that smaller clusters have more atoms exposed on their surface, and so less heat is needed to break those bonds than if the atoms are embedded in a crystal. But physical chemists have now found an exception to that rule. In nanoscale clusters of tin--those having between 15 and 30 atoms--the melting point actually rises to at least 50 degrees Kelvin higher than in bulk tin. Their results appear in this week's issue of Physical Review Letters.

Martin Jarrold and graduate student Alexandre Shvartsburg of Northwestern University needed to develop a special technique to measure these melting points. "You can't stick a thermometer in these things," Jarrold explains. They first blasted a tin rod with a laser pulse to evaporate ionized atoms, which cooled into nanoscale clusters; a mass spectrometer counted the atoms in each. Next came an ion mobility measurement, conducted by timing the clusters as they traveled through an electric field in a temperature-controlled drift chamber filled with helium. This test indicates a cluster's shape: cigar-shaped clusters hit more helium atoms and take more time through the chamber than do spherical ones of the same mass.

At melting point, the researchers expected the normally cigar-shaped clusters to become spherical--but they never got there. Jarrold and Shvartsburg raised the temperature to their heater's maximum, 550 K, to no effect. For now, they cannot explain the behavior but suggest that it has to do with the unsual, elongated shapes adopted by small clusters of tin, silicon and germanium. (above)