Probing the Nanoworld: Near-Field Scanning Optical Microscopy from Topometrix

Penetrating ever farther into the submicron world, microscope designers have broken through the diffraction barrier that once stood between researchers and the mysterious depths of the nano world. Simultaneously acquired topographic and NSOM images of j-aggregates of pseudocyanine embedded in polyvinylsulfate. Courtesy of Paul Barbara and Dan Higgins, Univ. of Minnesota Researchers are now returning from this vast hidden frontier with images that are both startling in their beauty and compelling in their structural complexity. Truly, the visual delights of Earth have nothing that compares with the topography of AlO fibers embedded in an Al matrix, which is evocative of the most exquisite Italian marble, cast in three dimensions and bathed in the scintillating hues of lavender and deep purple. What macroscale artist could imagine in his or her most lucid dreams the brilliant lattice of boron doped with silicon? These near-field images reveal a reality that moves humans one more step from the center of creation, consigning us to a desert outpost in the architecture of the universe. It is the near-field scanning optical microscope that has been the conveyance of these rare treasures of sight. The NSOM, as it is called, has overcome the optical effects known as diffraction which tend to muddy the resolution of typical scanning optical microscopes. Bringing an optical fiber to within 10 nm (the near field) of the sample surface eliminates the normal effects of diffraction. The result is a clear, vibrant picture that resolves even individual molecules in three dimensions. Because this innovation is relatively new, it has the quality of a hothouse technology where everyone is trying to "grow" one in his or her own backyard, which is remarkable considering the sophistication of the technique. Caught up in the excitement of exploring this new device, many researchers are transforming their atomic force microscopes into near-field microscopes. Virtual cook books for transforming microscopes abound on the Internet, and for every successful conversion there are probably a dozen frustrated enthusiasts. Yet this needn't be the case. Topometrix, which is one of the preeminent manufacturers of microscopes in the United States, has developed near-field scanning capability that can be purchased right off the shelf. Such a quick and easy fix might go against the grain of many scientist who insist on developing their own technology, but while they tinker and fuss trying to get their microscopes to work, many of their colleagues are enjoying the benefits of near-field optics. According to Kenneth Goodson, professor of mechanical engineering at Stanford University, "Near-field optics are just beginning to exert an impact on high resolution thermal microscopy". Goodson has turned his Topometrix microscope toward semiconductor circuits and transistors in order to study temperature fields in high technology devices. His investigation of temperature distributions and thermal properties in microstructures is on the leading edge of a science that promises significant advances in the microtechnical industry. But as Goodson cautions, "The spatially varying optical properties and topography of semiconductor circuits make it difficult. This is a demanding application." In order to compensate for unusual topography such as that experienced by Professor Goodson, Topometrix has developed the latest in tuning-fork technology. This technology adjusts the position of the optical tip according to the peaks and valleys of the sample. An added benefit is that tuning-fork technology permits the viewing of soft samples such as DNA and polymers without the use of a feedback laser. More common uses of the NSOM, but still far from ordinary, involve its use in chemical spectroscopy. Paul Barbara, who is a professor of chemistry at the University of Minnesota, has been achieving consistent and reproducible results from the very begining. He says that his Topometrix near-field scanning microscope has had a tremendous impact on his work, allowing him to conflate optical and topographical experiments into one procedure. When asked about criticisms that NSOMs are difficult to use, he admitted that the procedures for using NSOMs are less routine than for other kinds of microscopes, but he also believed that people were using too much laser power. The image of scientists blasting their samples apart with a laser probe and all the while demanding more power is perhaps hitting closer to home than many researchers would care to admit. Yet another problem associated with NSOMs is variation in the quality of the aluminum-coated optical probe tips: some tips work and others don't. Topometrix has responded to this issue by purchasing an original patent from AT&T for pulling extremely reproducible optical fibers. The whole process, which takes place in a clean room, is conducted under very stringent conditions and yields optical tips that are consistent and effective. Those who are curious about applying near-field magnification to their own investigations can contact Topometrix at inquiry@topometrix.com. Author Brent Johnson can be reached at bjohnson@the-scientist.com

Probing the Nanoworld: Near-Field Scanning Optical Microscopy from Topometrix

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