Physics News 866, June 4, 2008

ON VERY THIN ICE For the first time, scientists have obtained pictures of ice only a few nanometers thick in the act of forming bulk ice at the coldest of temperatures. The new images, showing ice sheets about 50,000 times thinner than a human hair, add to our knowledge of water, that remarkable molecular substance that is common on Earth, and found in significant amounts in other places around the solar system. Usually we think of ice as forming from tiny six-sided crystals. The six-fold symmetry of classic snowflakes comes from the way water molecules fit together. But at very low temperatures, close to absolute zero -- the coldest conceivable state of matter -- water molecules don’t behave the way they do at warmer temperatures. Like people first waking from a sound sleep, water molecules in this chilly environment can’t move very well. If you spray them onto a platinum surface they tend to stay where they land. If you keep adding water, the molecules form a solid called amorphous ice, with all of the molecules sticking together wherever they can. Because of the extreme cold, the molecules don’t have enough energy to line up to form a crystalline array. Raise the temperature a bit, just above 120 K, and now the molecules have a chance to creep around enough to start assembling a proper crystal. But these little blobs still aren’t in the familiar hexagonal shape. Instead the water molecules form a cubic crystal structure. To form common ice, with its hexagonal structure, a little bit more warmth is required - a still-frosty 160 K. Konrad Thürmer (kthuermer@gmail.com) and Norman C. Bartelt, two scientists at Sandia National Laboratory in Livermore, California, were interested in exploring the early formation of ice as part of their research into the initial stages of the growth of crystal films. The device they used to make the ice pictures is a scanning tunneling microscope (STM), whose tip is scanned across the face of the ice sheet, can be used to form an image of the ice. The images show ice crystallites much smaller than previously seen (see figures at http://www.aip.org/png/2008/303.htm). Earlier attempts to image very thin ice sheets with an STM failed because it is difficult to conduct electricity through ice. But this time the scientists got just enough electricity to flow across the gap from tip to sample - partly by cranking up the voltage and partly by making a stepping-stone path for the electricity to follow through the ice. What did they find? When the ice film was really thin, only about 1 nm thick on average, the water molecules formed into little islands of ice. When the thickness reached 4 or 5 nanometers, the ice started to form larger joined chunks. They believe that when one tiny crystallite adding molecules on a downward going slope meets with a crystallite with an upward-going slope, the two structures start to pivot around each other. This accounts for the somewhat corkscrew appearance of the patchwork quilt of merging ice sheets. (Physical Review B, May 2008)