Fermi Level Local Density Of States Research Articles

Analysis of the adsorbates and determination of the adsorption sites on Si(1 1 1)-7×7 after a large amount of Ar exposure at room temperature

The surface structure of Si(1 1 1)-7×7 after exposure to a large dose of Ar at room temperature has been investigated by scanning tunneling microscopy and the adsorbates on the surface have been analyzed. It was found that Ar does not adsorb on the Si(1 1 1)-7×7 surface. The adsorption is dominated by the impurities, most likely to be H 2O, in the Ar gas. The impurities dissociatively adsorb on the Si(1 1 1)-7×7 surface and appear as dark spots in both empty and filled state images due to the diminution of Fermi-level local density of states over the adsorption sites. The chemisorption is site selective and the ratio of reacted center sites vs. reacted corner sites is about 2:1, indicating that center-adatoms react with H 2O twice as fast as corner-adatoms. After annealing the substrate to 660 °C, this ratio keeps invariant while the total number of dark sites is significantly decreased. The remaining dark spots on the Si(1 1 1)-7×7 surface after annealing at 660 °C are likely to be surface vacancies caused by the desorption of SiO.

A Detailed NMR-Based Model for CO on Pt Catalysts in an Electrochemical Environment: Shifts, Relaxation, Back-Bonding, and the Fermi-Level Local Density of States

13C NMR shift and spin−lattice relaxation measurements have been used to investigate 13CO (ex MeOH) on fuel cell grade Pt electrodes (having average particle diameters of 2, 2.5, and 8.8 nm) in an electrochemical environment from 80 to 293 K at 8.47 and 14.1 T. The temperature dependence of the 13C spin−lattice relaxation rate, 1/T1, shows a Korringa relationship which is independent of magnetic field, for all three samples. However, the peak positions and the corresponding T1T values depend on particle size, with those of the 8.8 nm sample approaching values found for unsupported polycrystalline platinum black in an electrochemical environment (J. B. Day et al., J. Am. Chem. Soc. 1996, 118, 13046−13050). The 13C T1 is single exponential, independent of particle size and temperature, in contrast to previous results obtained on oxide-supported Pt−CO systems in a “dry” environment, in which relaxation was nonexponential at low temperatures, but exponential at high temperatures, suggesting strongly a quantum...

Local metal to non-metal transition on oxygen-covered platinum particles from 195Pt nuclear magnetic resonance

The nuclear magnetic resonance (NMR) parameters (shift, relaxation time) in a metal are determined by the Fermi-level local density of states (LDOS), and the nuclear spin-lattice relaxation rate is proportional to the temperature (Korringa relation). In the 195Pt NMR spectrum of platinum catalysts, signals from surface atoms and from atoms deeper in the particles can be distinguished; furthermore the NMR parameters are sensitive to the chemisorption of gases (oxygen in the present case). Using the Korringa relation as the criterion, we find that at moderate oxygen coverage (O/surface Pt ratios of 0.16 and 0.40) part of the surface atoms are non-metallic (have zero LDOS), while at zero or at saturation coverage (O/surface Pt=0.75) all atoms show metallic behaviour. In addition, some possible relations between NMR-derived variations of the LDOS and chemisorption properties are indicated.

Theory of single-atom imaging in the scanning tunneling microscope.

The tunneling-current density is computed in the vacuum region between two planar metal electrodes, on each of which is an adsorbed atom. Scanning of one atom (taken as the tip) past the other (the sample) permits plotting of tip displacement versus lateral separation for constant current. The calculation shows the extent to which scanning-tunneling-microscope images of an individual atom are visible. It confirms that for low bias, the microscope images the Fermi-level local density of states of the sample at the position of the tip.