Hysteresis and Spikes Observed in Quantum Hall Effect
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face so as to physically limit the number of metal atoms that could join together in each cluster. While measuring the properties of the clusters, the researchers found that, in one cluster, the circular dichroic effect exceeded 300 ppm in the yellow-green region. In another cluster, the effect exceeded 1000 ppm in the red and near-infrared. As reported in the researchers’ article in the March 30 issue of the Journal of Physical Chemistry, these optical measurements suggest that the clusters have a helical structure. Schaaff said, “Such effects had not previously been measured in metal-cluster compounds, and it’s kind of a shock that small metals might prefer to have a helical structure.” Using gel electrophoresis to separate the clusters by weight, Schaaff found that certain cluster sizes dominated, with 28-atom assemblies—slightly less than 1 nm across—being the most common. The chiral properties varied by the size of the cluster and, therefore, were only observed clearly when the clusters were separated by weight. Only clusters with 40 or fewer atoms displayed the intense optical properties. The optical effect changed direction as the researchers moved from one cluster size to the next, suggesting a direct correlation to the energies of the conduction electrons in the metal’s outer shell. Whetten, a professor in the School of Physics and School of Chemistry and Biochemistry, said, “Even though the optical absorption increases more or less monotonically here, the preferences for right- versus left-handed light changes direction from one band to another.” He said that the effect may be related to the high level of confinement created in the conduction electrons by formation of the small clusters, but research has not yet confirmed this. A helical geometrical pattern or “tiling” of the glutathione adsorption sites (gold-sulfur bonds) could also affect the circulation of the conduction electrons, he said.
Feedback-Controlled Lithography Enables Molecule Manipulation on Atomic Scale Researchers at the University of Illinois—Urbana-Champaign have tethered individual organic molecules at specific locations on silicon surfaces. Joseph Lyding, a professor of electrical and computer engineering and a researcher at the university’s Beckman Institute for Advanced Science and Technology, and his research team first passivated the silicon bonds with hydrogen. They then used an MRS BULLETIN/JUNE 2000
ultrahigh vacuum scanning tunneling microscope to break individual siliconhydrogen bonds and dislodge hydrogen atoms from selected sites. Graduate student Mark Hersam said, “By removing individual hydrogen atoms, we create holes in the clean silicon surface. Since these holes—or dangling bonds—serve as effective binding sites, molecules injected in the gas phase will spontaneously self-assemble into the predefined patterns.” A technique called feedback-controlled lithography gives the patterning process an atomic precision. “Feedback-controlled lithography works by actively monitoring the microscope feedback signal and the tunne
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