Daylight Photocatalysis Achieved on Carbon-Doped TiO 2
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RESEARCH/RESEARCHERS
Low-Temperature AFM Reveals Hidden Atom in Graphite Surface The existence of atoms was hotly debated by leading scientists only 100 years ago. While field ion microscopy was the first technique to image individual atoms, scanning tunneling electron microscopy (STM) is now routinely used to observe single atoms in real space on flat surfaces. However, a classic problem is the “missing” atoms in the surface of graphite; that is, the three atoms (α) in the hexagonal lattice bonded to subsurface atoms have less electron density at the Fermi level than the three atoms (β) lying above holes. Because STM only probes the mobile electrons that cause the electric conductivity—that is, the electrons with an energy close to the Fermi level—only β atoms appear in STM images. Although atomic force microscopy (AFM) should in principle be able to image all surface atoms, previous AFM images showed only a single protrusion per unit cell. Recently, however, S. Hembacher, F.J. Giessibl, and J. Mannhart of Universität Augsburg, Germany, and C.F. Quate of Stanford University have shown that the missing surface atoms in graphite are observable with an AFM capable of detecting very small repulsive forces between single atoms. As reported in the October 28, 2003 issue of the Proceedings of the National Academy of Sciences, the researchers combined STM and AFM at low temperature to simultaneously record the tunneling current and short-range forces to probe the charge density at the Fermi level and the total charge density, respectively, of graphite. Built on a 30-ton foundation to damp external vibrations and immersed in a liquid He bath cryostat, the combined STM-AFM operates in an ultrahigh vacuum with a sample temperature of 4.89 K. The frequency modulation force microscopy method makes possible combined STM-AFM experiments. The simultaneous measurement of current and force allows the research team to rule out double tip effects that probably have caused the sporadic previous observations of the full hexagon in STM or AFM experiments. The researchers had previously achieved subatomic resolution with this technique and imaged structures related to atomic orbitals. The researchers said that because the layers in graphite are only weakly coupled, the normal forces between the AFM tip and the sample must be kept extremely small in order to avoid distorting the graphite lattice. The tungsten tip was shaped by field emission and controlled collisions until topographic images with corrugation of only 20 pm were obtained. 4
Whereas an STM image recorded in constant-height mode shows only the β atoms, both types of atoms are imaged when the frequency-shift data are recorded simultaneously, that is, with the STMAFM. The researchers attribute the shortrange forces acting between the tip and the sample, indicated by the positive frequency shifts, to overlap of the 5s-, 5p-, and 6s-like orbitals of the tungsten tip with the sample orbitals. Giessibl and co-workers have demonstrated an additional capability of AFM to gather inf
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