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Better Probes of Molecular Structure Now Possible with MRFMs and Micro-STMs

Researchers at IBM's Almaden Re- search Center have developed a new technique for three-dimensional imaging of atoms and molecules, which can see below the surface of materials and distinguish between different types of atoms, and may eventually enable scientists to see a single electron or nucleus. First proposed by University of Washington medical physicist John Sidles four years ago, the technique was described during a Monday morning session of the March Meeting in San Jose, which also featured recent advances in scanning tunneling microscopy (STM) and atomic force microscopy (AFM), including the world's smallest STM ever constructed.

Called magnetic resonance force microscopy (MRFM), the new technique combines aspects of atomic force microscopy and magnetic resonance imaging to allow three-dimensional imaging at microscopic scales. According to IBM's Dan Rugar, three-dimensional imaging would have tremendous medical implications for controlling diseases and understanding DNA and protein structures. Technological applications would be equally revolutionary. "Understanding what's happening on the atomic scale is very important to enable us to make smaller, better, and faster electronic components," he said.

Like traditional magnetic resonance imaging (MRI) instruments, the MRFM uses an rf coil to excite magnetic resonance in a sample. However, it relies on a cantilever to measure the magnetic force between a magnetized iron tip and nuclei in a sample. The force oscillates as the spin of the nuclei is flipped back and forth by the rf field, which in turn causes the cantilever to vibrate. Rugar's team is now developing sharper magnetic tips and softer, thinner cantilevers made of single crystal silicon to further improve force detection sensitivity.

By cooling the MRFM and its sample as low as 6K in combination with new low-energy-dissipation cantilevers, Rugar and his team have increased the device's sensitivity tenfold, thanks to reduced thermal vibrations in the cantilever and increased spin polarization at the low temperatures. This has allowed them to detect forces as small as 10-16 newtons (less than a trillionth of a gram) and to see resonance from as few as 1 million phosphorous dopant atoms in silicon. According to Rugar, the low-temperature MRFM is "an important milestone along the way to our ultimate goal of using the MRFM to detect the spin from single electrons and nuclei."

Micro STMs. Scientists at Cornell University have developed the world's smallest scanning tunneling microscopes -- driven by motors only 200 microns wide - that have been built on a single silicon chip, using modifications of conventional very large scale integration (VLSI) techniques. A larger version measures two-by-two millimeters and provides a means to scan macroscopic specimens to test the functional characteristics of these new micromachines.

Called a micromechanical scanning tunneling microscope (MEM STM), the device has a silicon tip with three actuators that provide the force to move the tip in three dimensions. A conventional STM is about the size of a human thumb and uses piezoelectric motors to scan a tip across the surface, generating an atomic image of the material's surface. The smaller the STM, the faster it can scan a tip. "As these devices are scaled down, we can make them scan much faster, on the order of a thousand to a million cycles per second," said Noel MacDonald, a Cornell professor of electrical engineering who invented the device.

Smaller STMs also means that thousands of them will fit on a single silicon chip in a massively parallel fashion, which allows for high-speed patterning on a nanometer scale. With thousands of tips, the microsystem could possibly pattern circuits at production speeds. MacDonald believes that these micro-STMs could also be used for manufacturing massively parallel micro-robots for information storage and molecular-scale robotics, enabling researchers to probe DNA and other molecular-scale structures.

However, he insists that this accomplishment is just the beginning. "These first devices represent a first but major step toward massively parallel microinstrumentation," he said. "But we still have a long, long way to go." He envisions large arrays of such STMs with each tip storing millions of bits of information, a technology that could also be used for microsensors. In fact, the Cornell Research Foundation is applying for patents on the technology and the university has formed a partnership with Ithaca's TMS Technologies, Inc. to license technology in microelectronic processing techniques based on MacDonald's work.

Atomic Force Microscopy. On Tuesday, Frank Ohnesorge of IBM's physics group in Munich, Germany, presented the first experimental demonstration that the AFM can indeed provide true atomic resolution according to Ohnesorge, this is now possible because extremely low imaging forces as small as 10-11N can be achieved, so that the imaging process is not perturbed by elastic deformations of the sample or the tip. At the same session, a new use of the AFM was described that has allowed researchers to follow the condensation and evaporation of water on a solid surface of mica.

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Editor: Barrett H. Ripin