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Image A is the kind that would be obtained using a conventional STM equipped with a metal tip. What shows up in the image are the atoms (shown in blue) that have the highest energy electrons associated with them. Image B is a color- or energy-filtered image in which the researchers have suppressed the blue atoms and can now observe others that have electronic states at lower energy (shown in red). This silicon surface is actually a special case in which the 'red' atoms actually lie sort of beneath the blue ones. Via energy filtering, researchers can thus "see through" the blue atoms and selectively image the red ones!
Scanning tunneling microscopes yield atomic-scale landscapes of electronically conducting surfaces like metal. Researchers at the Colorado School of Mines have demonstrated a powerful new technique for filtering these images. Just as color filters make it easier to discern desired features in a photograph, "color-filtered STM" makes it easier to see desired atoms and chemical bonds on a surface. In this new technique, electrons of different energies are analogous to different colors. Only electrons in desired energy ranges are allowed to jump or "tunnel" to the STM tip, to build up images of the atoms or chemical bonds of interest.
[Support for the design and construction of the microscope used to obtain these images and for the research activities themselves was provided by a grant from the National Science Foundation's (NSF) Faculty Early Career Development (CAREER) program (DMR 99-85178, "Energy-Filtered Scanning Tunneling Microscopy: A Novel Pathway to Chemical Contrast Imaging on Single Molecules"). NSF also awarded a Small Grant for Exploratory Research (SGER) (DMR 00-81183, "Exploration of Ordering Mechanisms in the Nucleationless Self-Assembly of Quantum Dot Islands") to Percy Zahl, a postdoctoral student who work on the project.] Thumbnail">
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