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YORKTOWN HEIGHTS, N.Y. April 24, 2003 — Electrons. Camera. Action.
Over the course of a few minutes, billions of bismuth atoms coalesce into a submicroscopic crystalline pattern captured in brilliant digital video. Welcome to real-time molecular cinema, the product of one man’s 20-year quest to see actual atoms in action.
Rudolf “Ruud” Tromp, IBM‘s manager of molecular assemblies and devices, may be the D.W. Griffith of thin films. His clips of crystal growth are helping IBM understand how molecules can be directed to assemble into useful structures for future generations of devices such as flat panel displays.
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In addition to flexible computer screens, applications for the kind of organic thin film transistors Tromp’s group is working on include inexpensive logic circuits for smart cards and radio-frequency identity tags, or large-area sensor arrays and other large-scale integrated circuits.
While organic circuitry is already making its way into displays based on OLED (organic light emitting diodes), it may be years before anyone can buy a screen based on organic crystal transistors.Tromp joined IBM in 1983, directly after completing his Ph.D. in physics at the FOM Institute in his native Netherlands. His work since then on the structure and growth of semiconductor surfaces was recently honored with the America Physical Society’s Davisson-Germer Prize, the highest accolade in surface science.
In that field, “there’s Ruud Tromp and then there’s everybody else,” said Norm Bartelt, a staff scientist at Sandia National Laboratories. Bartelt, a materials physicist, said Tromp’s molecular movies have allowed theorists to “see equations come to life.”
“I never imagined we could see and know things about the dynamics of surface science in such detail,” he said. “Ruud opened a new window” on how molecular structures form.
On a recent rainy afternoon at IBM’s T.J. Watson Research Center, Tromp recorded how bismuth, a heavy metal with unusual physical properties, orders itself on a silicon surface under varying conditions. He designed and built the imaging tool that made the movie clip — a low energy electron microscope, or LEEM.
IBM scientists have won Nobel Prizes for inventing other imaging tools such as the scanning tunneling and atomic force microscopes. Both have played leading roles in the advancement of nanoscience. But the very first LEEM was invented by Ernst Bauer in 1962 at the University of Clausthal in Germany.
After seeing Bauer’s device in the early ’80s, Tromp, who had an undergraduate background in engineering, decided to build his own with the help of colleague Marc Reuter. Over the last two decades he has built several generations of LEEMs, some of which IBM has sold or donated to university research programs.
One of Tromp’s LEEMs found its way to Max Lagally’s lab at the University of Wisconsin. Lagally first met Tromp in 1981. “Ruud stood out among a crowd of very gifted people,” said Lagally, a pioneer in self-assembling materials and founder of nPoint Inc., a maker of nanopositioning systems used in nanotech research and manufacturing.
Lagally also noted that Tromp has done brilliant work quietly and humbly. “He’s less self-aggrandizing than most, and works very interactively with others on joint projects.”
Today, Tromp’s group is using his latest LEEM to precisely measure how crystals of organic pentacene molecules grow. Pentacene, a molecular chain of five benzene rings, is considered one of the best organic semiconductors.
Tromp and colleagues have learned to grow pentacene crystals as large as 100 microns, large enough to serve as the pixels in an organic thin film transistor display. Thin films of organic transistor materials could, for example, enable display screens that could roll up or be applied to curved surfaces.
Tromp said he was attracted to the LEEM nearly 20 years ago because so many other people were gravitating toward scanning tunneling and atomic force microscopy. “I wanted to work in a field that wasn’t so crowded,” he said. His success has spawned a growing community of LEEM-based research.
His latest LEEM, which took about three years to design and build, starts with a component that generates a stream of high-energy electrons and steers them through a lens toward the target sample. Normally such high-energy electrons would destroy delicate organic molecules and make it impossible to film their growth into crystals. So, at the very last second before the high-speed electrons hit the target surface, they are dramatically slowed down.
According to Tromp, the electrons gently kiss the surface and bounce off it without penetrating more than one or two molecular layers into it.
The reflected signal of low-energy electrons is then accelerated again and strikes a detector that translates it into a visual image that can resolve structures as small as 5 nanometers. The tool can also use ultraviolet light rather than high-energy electrons to do photo-electron emission microscopy, or PEEM.
In the final analysis, Lagally said, what Ruud Tromp has done is to help the world not just see atoms at work, but also understand their behavior. “Once you can see films and crystals growing, you can model them and quantify their behavior,” he said. “That’s extremely important.”