When you're on a quest, it's important to get a sign that you're on the right path. Dan Rugar has spent 13 years trying to create the ultimate microscope, one that would reveal the 3-D atomic makeup of molecules below the surface. It would give scientists unprecedented understanding of the body's proteins, facilitate drug breakthroughs, and help IBM engineers make advances in microelectronics. In January 2004, Rugar got his sign, detecting the magnetic signal from a single electron in a piece of quartz. That makes his magnetic resonance force microscope about 10 million times more sensitive than a medical MRI. The ultimate goal -- detecting a single nucleus -- is still a ways off: A nucleus's magnetic force is at least 650 times smaller than an electron's. "We're not going to get there next week or next year," Rugar says. "But now we know it can really be done."
Manager of nanoscale studies
IBM
San Jose, California
Read Dan's original entry:
Dan Rugar detected the faint magnetic signal from a single electron buried inside a solid sample -- a breakthrough in nanoscale magnetic resonance imaging (MRI) and a major milestone toward creating a microscope that makes three-dimensional atom-scale images of molecules. Such a tool would have a major impact on studying materials for which a detailed understanding of the atomic structure is essential: from proteins and pharmaceuticals to integrated circuits and industrial catalysts. Knowing the exact location of specific atoms within nanoelectronic structures would improve their manufacture and performance. Directly imaging proteins' atomic structures would aid greatly in developing new drugs.
Rugar's innovation is a new type of microscope -- a Magnetic Resonance Force Microscope -- that combines features of MRI with Atomic Force Microscope. While medical MRI requires at least 1 trillion atoms for each image pixel, Rugar and colleagues demonstrated a method for detecting the much fainter signal from individual electrons. The device works by measuring a tiny magnetic force between a nanometer-size magnetic tip and the electron. This force is familiar to anyone who has played with magnets: Two bar magnets will either attract or repel one another depending on their orientations. In MRFM, one magnet is the magnetic tip and the other magnet is the electron. The challenge is that the magnetic force from a single electron is extremely small. Two things are needed: a very sensitive force detector and a way to distinguish the magnetic force from other atom-scale forces. Our force detectors are silicon microcantilevers, which look like miniature diving boards -- only 85 micrometers long (roughly the diameter of a human hair) and only 0.1 micrometers thick (1,000 times thinner than a human hair). The cantilever's extreme thinness makes it very flexible so it can respond to tiny forces. To distinguish the faint magnetic force, we vibrate the cantilever about 5,000 times a second and apply an external high-frequency magnetic field that causes the electron spin to flip up and down repeatedly as the cantilever vibrates above it. This creates alternating attractive and repulsive forces that alters the cantilever vibration in a detectable way.
To do this work we had to pioneer many experimental techniques and had to develop an understanding of how to manipulate and detect the spin without causing too much disturbance to it during the measurement process. Among the new techniques we developed: a) ultrasensitive force detection techniques, including operation at 1.6Kelvins (1.6 degrees above absolute zero) b) ultrasensitive silicon cantilevers that are only 100 nm thick and can detection attonewton forces (less than a millionth of a trillionth of a pound) c) nanometer-size but powerful magnetic tips that we mounted on the cantilever to generate intense magnetic field gradients d) methods to manipulate the spins e) methods to reduce disturbance of the spin during the measurement process
Dan Rugar's team successfully detected the spin of a single electron and imaged it at a resolution of 25 nanometers. Combined atom-scale sensitivity with sub-surface vision and sensitivity some 10 million times that of today's medical MRIs with a resolution about 40 times better than the best conventional MRI-based microscopes is truly a is a major milestone in microscopy. The American Institute of Physics hailed the MRFM as this year's top physics story; Rugar is one of 20 Research Leaders to win "2004 Scientific American 50" honors. Throughout history, the ability to see more clearly has enabled important new discoveries and insights. The MRFM should ultimately stimulate significant advancements in biology and microelectronics. A protein's structure determines its biological activity. But today's methods for determining biological structure (e.g., x-ray crystallography) have not worked for most proteins.Biomolecular interactions central to many medical and pharmacological processes remain hidden and unknown. Likewise, the location of dopant atoms and defects determines the electrical properties of microelectronic structures, especially as they shrink toward atomic dimensions. Without a way to see beneath surfaces, it is very difficult to design and study the relevant properties of nanoscale structures required for future circuits.
Our goals for 2005 are to increase the system's sensitivity and to eliminate noise. Our next milestone is detecting a single nuclear spin, which is 650 times fainter than a single electron spin and can reveal molecular structures. Further improvements will increase the speed and scope of this revolutionary nanoscale imaging technique. Applying MRFM for direct imaging of protein structures directly would be particularly far-reaching. A protein's biological activity is determined by its intricately folded atomic configuration. But scientists must now use indirect methods, such as the scattering of x-rays by crystallized proteins or computer simulations.
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