Researchers at Sandia National Laboratories in Livermore, Calif., recently snapped the first photos of nanometer-thick ice films taken by a scanning tunneling microscope (STM). By capturing in detail how water molecules deposited on a cold solid surface (platinum in this case) aggregate into an ice film, the scientists are hoping to enhance understanding of how water and solids interact. Such basic knowledge might aid the design of better fuel cells and water purification membranes and help to decipher the complex processes in the earth's atmosphere leading to rain and snowfall.
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Using an STM, Sandia physicists Norm Bartelt and Konrad Thürmer created a sequence of images that reveal how an ice film grows when a platinum surface—chilled to 140 kelvins (–208 degrees Fahrenheit or –133 Celsius)—is exposed to water vapor in a vacuum chamber. The images reveal that when an ice film is extremely thin, just a nanometer thick (40 millionths of an inch), the water molecules form small, tabular (flat) islands of crystalline ice on the platinum surface. Once it thickens to four or five nanometers, the ice islands link to form a continuous film of ice.
An STM functions by positioning a sharp, needlelike tip near a sample (in this case, the ultrathin ice) and then allowing a tiny electrical current to flow across the gap between the tip and the sample. To create images, scientists apply voltage to the tip of the STM, (a fine probe topped with a single atom) in such a way that a tiny tunneling current arises between the tip and the surface being probed. The tip slides over the surface like a finger reading braille, tracking the surface topography as it goes. The tip's recorded coordinates are then used to produce the images.
But an STM only works if the surface it is probing is conductive. Because ice is not, the researchers cut the STM's current to a fraction of the normal amount and applied a negative voltage to the ice film. This allowed the STM tip to strip electrons from the ice film, which provided enough of a current for the imaging to work properly.
"Using these exotic parameters made it possible to image for the first time ice films that are a few nanometers thick," Thürmer says. "This new insight can be used to test and improve theoretical models of water–solid interaction."