Physical and chemical properties of bimetallic surfaces

The surface science of graphene: Metal interfaces, CVD synthesis, nanoribbons, chemical modifications, and defects

The formation of a surface metal-metal bond can produce large perturbations in the electronic, chemical, and catalytic properties of a metal. Recent studies indicate that charge transfer is an important component in surface metal-metal bonds that involve dissimilar elements. The larger the charge transfer, the stronger the cohesive energy of the bimetallic bond. On a surface, the formation of a heteronuclear metal-metal bond induces a flow of electron density toward the element with the larger fraction of empty states in its valence band. This behavior is completely contrary to that observed in bulk alloys, indicating that the nature of a heteronuclear metal-metal bond depends strongly on the structural geometry of the bimetallic system.

http://science.sciencemag.org/content/sci/257/5072/897.full.pdf

The Nature of the Metal-Metal Bond in Bimetallic Surfaces

Graphene, a single layer of carbon atoms in a honeycomb lattice, offers a number of fundamentally superior qualities that make it a promising material for a wide range of applications, particularly in electronic devices. Its unique form factor and exceptional physical properties have the potential to enable an entirely new generation of technologies beyond the limits of conventional materials. The extraordinarily high carrier mobility and saturation velocity can enable a fast switching speed for radio-frequency analog circuits. Unadulterated graphene is a semi-metal, incapable of a true off-state, which typically precludes its applications in digital logic electronics without bandgap engineering. The versatility of graphene-based devices goes beyond conventional transistor circuits and includes flexible and transparent electronics, optoelectronics, sensors, electromechanical systems, and energy technologies. Many challenges remain before this relatively new material becomes commercially viable, but laboratory prototypes have already shown the numerous advantages and novel functionality that graphene provides.

Graphene: An Emerging Electronic Material

Carbide and Nitride Overlayers on Early Transition Metal Surfaces: Preparation, Characterization, and Reactivities.

The valence-band structure of single crystals of titanium carbide (TiC) and titanium nitride (TiN) have been studied with angle-integrated photoelectron spectroscopy using ${\mathrm{He}}_{\mathrm{I}}$, ${\mathrm{He}}_{\mathrm{II}}$, and synchrotron radiation for excitation. The results are compared with the partial densities of states derived from an augmented-plane-wave band-structure calculation. Peak positions and widths agree fairly well, but the observed intensity modulations with photon energy cannot be interpreted by such a comparison.

Valence-band structure of TiC and TiN

Experimental studies of the diamond (111) surface are presented. While no intrinsic surface states have been found on the 1×1 surface, our new results in ultraviolet photoemission spectroscopy (15 eV?hν?25 eV) show evidence of a band of surface states on the 2×2/2×1 reconstructed surface. The band of surface states (with an integrated emission on the order of one filled state per surface atom) is centered at 2.5 eV below the Fermi level (1.1 eV below the valence‐band maximum). The band width is 1.8 eV FWHM. In the case of the 2×2/2×1 surface structure, the downwards band‐bending is greater by 0.75 eV and the electron affinity is greater by 1.4 eV over the respective values for the 1×1 surface. These new studies necessitate a quantitative revision of our band‐bending model [J. Vac. Sci. Technol. 17, 1087 (1980)], although no change in basic concept is required. Preliminary photon‐stimulated desorption (PSD) results over the photon energy range hν = 18–35 eV find hydrogen on the 1×1 surface. This surface hy...

Formation of surface states on the (111) surface of diamond

Photoemission experiments have been performed to investigate the electronic structure of Cu adlayers on a Pt(111) surface. The evolution of the $3d$ states as a function of Cu coverage is observed at the Cooper minimum of the $\mathrm{Pt}5d$ emission ($h\ensuremath{\nu}=150$ eV), where the $\mathrm{Cu}3d$ emission is enhanced. For Cu coverages up to \ensuremath{\sim}0.6 of the first monolayer (ML), the $3d$ states give a symmetrical and resonancelike peak with a maximum at -2.6 to -2.7 eV with respect to the Fermi level ${E}_{F}$, and a full width at half maximum of 1.4 to 0.5 eV. Between \ensuremath{\sim}0.6 and \ensuremath{\sim}1 ML, there is an increase in $\mathrm{Cu}3d$ emission around -3.5 eV, suggesting the onset of two-dimensional delocalization. The delocalization of the $3d$ states increases with increasing Cu coverage. Beyond \ensuremath{\sim}1 ML, Cu-Cu bonding states appear below -4 eV as well. The Cu adlayers studied, from submonolayer coverages up to \ensuremath{\sim}1.5 ML, show an interesting absence of Cu-derived emission near ${E}_{F}$, in contrast with the flat emission at the corresponding energies shown by pure Cu. For small (\ensuremath{\le}0.6 ML) submonolayer coverage, the $\mathrm{Cu}2{p}_{\frac{3}{2}}$ core level, excited by $\mathrm{Mg}K\ensuremath{\alpha}$ radiation, is observed to be shifted (upward) by -0.60 to -0.65 eV relative to the bulk $\mathrm{Cu}2{p}_{\frac{3}{2}}$ core level. By 3 ML, the $\mathrm{Cu}2{p}_{\frac{3}{2}}$ binding energy is almost identical with that of a pure Cu sample. The adsorption of Cu "removes" the $\mathrm{Pt}4{f}_{\frac{7}{2}}$ surface core level, which then becomes bulklike. The adsorption of Cu also results in a work-function decrease. However, the $\mathrm{Pt}4{f}_{\frac{7}{2}}$ core-level data do not permit the work-function decrease to be explained by simplistic arguments based on charge transfer.

M. L. Shek

Papers

Valence-band structure of TiC and TiN

Formation of surface states on the (111) surface of diamond

Photoemission study of the adsorption of Cu on Pt(111)

The temperature dependence of the interaction of oxygen with Pd(111); A study by photoemission and auger spectroscopy

Photoemission spectra and thermal desorption characteristics of two states of oxygen on Pd