September 1993 DISCOVER

Transcription

1 T H E F L A T F A C E P H OTO G R A P H S September 1993 DISCOVER BY O F M I C H A E L T E C H N O L O G Y W. DAV I D S O N 1

2 Welcome to the technology of flatland. Researchers around the world are discovering ways to create thin films of chemicals, some only a few atoms thick, that can do many things no other substances can. Some are semiconductors that can be made into computer chips. Others have special magnetic properties that let them act as memory storage systems. Others are superconductors-they carry electric cureent without any resistance. And some thin films can make a knife edge as hard as a diamond. To make a thin film, engineers fire a high-temperature beam of molecules (900 degrees Fahrenheit) into a vacuum chamber. The beam hits the surface of a wafer made out of silicon, silicon compounds, lanthanum aluminate, or another similiar material. If the conditions are right, the molecules from the beam organize themselves into a crystal. Getting the conditions right is no small task. IN THE MYSTERIOUS NEW WORLD OF MOLECULAR ARCHITECTURE, YOU ABSOLUTELY CANNOT BE TOO THIN. First, the wafer s suface has to be made of a perfect crystal similar to the one molecules will form on top of it. (Since the structure of the crystal on the wafer pulls the falling molecules into its own order, a hexagonal crystal on the surface could not produce a thin film with a square crystal.) Next, the molecules have to spread evenly over the surface in a thin, perfect layer. And finally, the newly formed thin film has to cool down without last-minute crumpling. Perfect thin films are as smooth and featureless as well-made mirrors. Imperfect films (like all the films in these photographs, taken at a magnification of 100 to 1,500 at the National High Magnetic Field Laboratory) are of no practical use, but they give engineers crucial hints for future efforts, such as which materials are the most likely to produce successful results and which techniques for deposting the chemicals on the surface work best. And flawed films give all of us a beautiful look at a strange two-deminsional world. mountains of buckyballs Researchers have discovered that several dozen-carbon atoms can form a single molecule shaped like a soccer ball. Dubbed buckminsterfullerenes, or buckyballs, they may turn out to be one of the most versatile substances ever to come out of the lab. They may, for example, be better than silicon for manufacturing semiconductors. Here engineers have coated silver with a layer of buckminsterfullerenes to examine the proper ties of buckyball thin film. If this one were smooth, it would be perfect, but its surface is flawed. September 1993 DISCOVER 2 2

3 a t o m i c j e w e l s The triangles in this picture are miniature diamonds, engineers created them by sowing carbon atoms on a surface of silicon carbide, where they grew into interlocking tetrahedrons (pyramids with a triangle for a base). Diamond films may soon be turned into unbreakable coatings and computer chips that can work at high temperatures. In a perfect diamond coating, the tetrahedrons would be the same size and would line up with one another, forming a smooth layer. Here the crystals have grown at different rates. i m a g i n a r y s t a i r c a s e s A compound called lanthanum aluminate (a combination of lanthanum, aluminum, and oxygen) makes a good foundation for superconducting thin films. A perfect sample is crystal clear, while flaws create patterns such as the staircase shown here. The light and dark bands aren t created by shadows; they actually reflect the light differently, much the way prisms do. If a superconductor is built on such an imperfect surface, it can handle only a low level of electric current. This film, which was formed at high temperature, cooled too quickly. Since the molecules didn t have enough time to settle into their most comfortable arrangement making a perfect crystal-flaws became locked into the structure. September 1993 DISCOVER 3

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5 m i c r o s c a p e s Thin films can trick the eye. This is a stack of alternating nickel oxide and iron oxide layers known as a superlattice. By combining materials with different magnetic properties. Researchers can come up with new and unexpected properties in superlattices, such as enormously intense magnetic fields that may turn out to be excellent for computer memory storage. But in this picture the superlattice itself, a few dozen atoms thick, is invisible. What looks like an angled photograph of miniature cliffs and ridges is actually a view of cracks in the surface wafer from directly overhead. They were formed when researchers chiseled the wafer from a larger piece of magnesium oxide. m a g n e t i c c h a m p a g n e If the surface wafer doesn t match the crystal structure of the molecules falling onto it, the molecules will bond with each other instead, which is what happened here. Molecules of nickel oxide have collected on a surface of indium phosphide, but instead of forming a smooth sheet. They ve bunched up into bubbles. The researchers who made this sample are trying to develop new kinds of magnetic films for storing data in computers. September 1993 DISCOVER 5

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