Scientists at the Lawrence Berkeley National Laboratory (Berkeley Lab) have produced the first ever action movies starring individual water molecules on a metal surface. The ending was a surprise even to the producers.
These two STM images show molecules of water being adsorbed on a palladium surface at 40 Kelvins. In (A), two individual molecules or monomers approach one another and in (B) they collide to form a dimer.
This graphic shows the trajectory of the STM tip as it tracks a water molecule in its random hopping from one nearest neighbor lattice point to another across the crystal of a palladium surface. The image was produced at 52.4 Kelvins.
Working with a unique scanning tunneling microscope (STM), a team led by Miquel Salmeron, a physicist with Berkeley Labs Materials Sciences Division, cooled the surface of a single crystal of palladium, a good catalyst for reactions involving hydrogen and water, to a temperature of about 40 Kelvins (-233 degrees Celsius) in an ultrahigh vacuum. Water molecules were then introduced onto this surface and their motion was tracked with the STM. As expected from previous studies, single molecules migrated across the surface to aggregate into clusters of two (dimers), three (trimers), four (tetramers) five (pentamers) and six (hexamers). The surprise came when the scientists were able to watch the molecules as they moved.
Isolated water molecules moved by hopping from one lattice point (on the substrates crystal) to the nearest neighboring point whereupon if they collided with another water molecule they began to form clusters," says Salmeron. "The speed with which the molecules moved increased by four orders of magnitude when dimers were formed. The mobility of trimers and tetramers was also very high compared to the isolated molecules."
Lynn Yarris | EurekAlert!
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