Deuterium adsorption on (and desorption from) SiC(0001)-(3×3), (√3×√3)R30°, (6√3×6√3)R30° and quasi-free standing graphene obtained by hydrogen intercalation

International audienceWe present a comparative high-resolution electron energy-loss spectroscopy study on the interaction of atomic hydrogen and deuterium with various reconstructions of SiC(0 0 0 1). We first show that on both the (3 × 3) and reconstructions, deuterium atoms only bind to silicon atoms, thereby confirming the silicon-rich appellation of these reconstructions. Deuterium passivation of the (3 × 3) is only reversible when exposed to atomic deuterium at a surface temperature of 700 K since tri- and dideuterides, necessary precursors for silicon etching, are not stable. On the other hand, we show that the deuteration of the is always reversible because precursors to silicon etching are scarce on the surface. Then, we demonstrate that hydrogen (deuterium) adsorption at 300 K on both the (buffer-layer) and the quasi-free-standing graphene occurs on carbon atoms justifying their carbon-rich appellation. Comparison of the deuterium binding in the intercalation layer of quasi-free-standing graphene with the deuterated surface provides some indication on the bonding structure at the substrate intercalation layer. Finally, by measuring C-H (C-D) vibrational frequencies and hydrogen (deuterium) desorption temperatures we suggest that partial sp2-to-sp3 rehybridization occurs for the carbon atoms of the buffer-layer because of the corrugation related to covalent bonding to the SiC substrate. In contrast, on quasi-free-standing graphene hydrogen (deuterium) atoms adsorb similarly to what is observed on graphite, i.e. without preferential sticking related to the underlying SiC substrate

Charge density waves and surface Mott insulators for adlayer structures on semiconductors: extended Hubbard modeling

Motivated by the recent experimental evidence of commensurate surface charge density waves (CDW) in Pb/Ge(111) and Sn/Ge(111) sqrt{3}-adlayer structures, as well as by the insulating states found on K/Si(111):B and SiC(0001), we have investigated the role of electron-electron interactions, and also of electron-phonon coupling, on the narrow surface state band originating from the outer dangling bond orbitals of the surface. We model the sqrt{3} dangling bond lattice by an extended two-dimensional Hubbard model at half-filling on a triangular lattice. We include an on-site Hubbard repulsion U and a nearest-neighbor Coulomb interaction V, plus a long-ranged Coulomb tail. The electron-phonon interaction is treated in the deformation potential approximation. We have explored the phase diagram of this model including the possibility of commensurate 3x3 phases, using mainly the Hartree-Fock approximation. For U larger than the bandwidth we find a non-collinear antiferromagnetic SDW insulator, possibly corresponding to the situation on the SiC and K/Si surfaces. For U comparable or smaller, a rich phase diagram arises, with several phases involving combinations of charge and spin-density-waves (SDW), with or without a net magnetization. We find that insulating, or partly metallic 3x3 CDW phases can be stabilized by two different physical mechanisms. One is the inter-site repulsion V, that together with electron-phonon coupling can lower the energy of a charge modulation. The other is a novel magnetically-induced Fermi surface nesting, stabilizing a net cell magnetization of 1/3, plus a collinear SDW, plus an associated weak CDW. Comparison with available experimental evidence, and also with first-principle calculations is made.Comment: 11 pages, 9 figure

Magnetic Coupling and Single-Ion Anisotropy in Surface-Supported Mn-based Metal-Organic Networks

The electronic and magnetic properties of Mn coordinated to 1,2,4,5-tetracyanobenzene (TCNB) in the Mn-TCNB 2D metal-ligand networks have been investigated by combining scanning tunneling microscopy and X-ray magnetic circular dichroism (XMCD) performed at low temperature (3 K). When formed on Au(111) and Ag(111) substrates the Mn-TCNB networks display similar geometric structures. Magnetization curves reveal ferromagnetic (FM) coupling of the Mn sites with similar single-ion anisotropy energies, but different coupling constants. Low-temperature XMCD spectra show that the local environment of the Mn centers differs appreciably for the two substrates. Multiplet structure calculations were used to derive the corresponding ligand field parameters confirming an in-plane uniaxial anisotropy. The observed interatomic coupling is discussed in terms of superexchange as well as substrate-mediated magnetic interactions.Comment: J. Phys. Chem. C 201

Reversible hydrogenation of deuterium-intercalated quasi-free-standing graphene on SiC(0001)

Hydrogenation of deuterium-intercalated quasi-free-standing monolayer graphene on SiC(0001) is obtained and studied with low-energy electron diffraction and high-resolution electron energy loss spectroscopy. While the carbon honeycomb structure remains intact, it is shown that a significant band gap opens in the hydrogenated material. Vibrational spectroscopy evidences for hydrogen chemisorption on the quasi-free-standing graphene is provided and its thermal stability is studied

High-temperature desorption of C60 covalently bound to 6H-SiC(0001)-(3x3)

The desorption or fragmentation temperature of C-60 bound to Si-rich-(3 x 3) and (root 3 x root 3) R30 degrees. reconstructions of 6H-SiC(0001) is investigated using inverse photoemission spectroscopy (IPES) and LEED experiments. On SiC-(3 x 3), C-60 film is found desorbed after annealing at a high temperature of 1140 K, supporting covalent bonding. Meanwhile, the Si tetramers of the (3 x 3) nanostructured substrate are recovered, as can be inferred from the full reappearance of the Mott-Hubbard surface state in the IPE spectra. SiC-(3 x 3) behaves in a singular way among the other semiconducting substrates, which covalently bind to C-60. This remarkable feature is attributed to the low density of Si dangling bonds and to the highly corrugated character of this reconstruction

Oxygen 2s spectroscopy of tin oxides with synchrotron radiation-induced photoemission

Oxygen 2s spectroscopy can be especially useful in studies using synchrotron radiation (SR) where the deeper O 1s level frequently can not be excited. For tin oxides (SnO and SnO2), the weak emission from the O 2s levels has been previously found degenerate with the intense photoemission peak from Sn 4d levels around a binding energy (BE) of 26 eV. By working at the Cooper minimum of the interfering tin signal, a distinct peak near BE ≈22.5 eV could be unambiguously attributed to emission from O 2s levels in tin oxides considering photoionization cross-section arguments. The O 2s intensity could now be used for the quantitative evaluation of the near-surface oxygen-species concentration. We have used the O 2s in combination with the Sn 4d areas to study the variations of the [Sn]/[O] ratio as a function of the preparation of a SnO2 single crystal. Although the dominant formal valence state for tin determined from a Sn 4d lineshape analysis remains essentially Sn4+, we show that this [Sn]/[O] ratio can vary from simple to double. This can be explained by oxygen interstitials buried in a sub-surface region during ion bombardment, which segregate at the surface upon annealing

Interaction of C60_{60} with clean and hydrogenated SiC-(3×3) probed through the unoccupied electronic states

The effect of hydrogenation on the conduction bands of the Si-rich 6H-SiC(0001)-(3×3) reconstruction is studied using inverse photoemission spectroscopy in order to distinguish surface from bulk states. These results are exploited for the comparative study of the interaction of C60 adsorbed on (3×3) and on hydrogen-terminated (3×3). For the latter, as in the case of hydrogen-terminated Si, C60 is electronically decoupled from the substrate. Upon annealing a C60 thick film deposited on hydrogenated (3×3) up to 670 K, there is a hint for a possible hydrogen transfer to the C60 molecules. The initially physisorbed molecules then adopt covalent bonding with Si, forming the contact layer. Part of the substrate is already found uncovered at this temperature. By further annealing up to 860 K all H atoms have desorbed. Finally, at 1100 K the remaining covalently bound C60 have desorbed. Unexpectedly, the structural damages caused by H and C60 deposition and by the successive annealing steps do not prevent a final restoration of the initial (3×3) reconstruction at about 1100 K

Unoccupied surface state on the (√3 × √3) R30° of 6H-SiC(0001)

Applying k-resolved inverse photoemission (KRIPES) to the $\sqrt 3\times \sqrt3$ $\rm R30^\circ$-reconstructed 6H-SiC(0001) face, we have observed a sharp surface state U located at $1.10 \pm 0.05\;{\rm eV}$ above the Fermi level at the centre of the surface Brillouin zone. Its bandwidth of $0.34 \pm 0.05\;{\rm eV}$ is in good agreement with the 0.35 eV predicted by first-principle calculations based on a Si-adatom model. However, LDA calculations predict a half-filled {\mit\Sigma}_1 state and a metallic character for this reconstruction. Together with recent ARUPS data, our results reveal that the one-electron band {\mit\Sigma}_1 is split into two bands, giving a semiconducting surface with a reduced indirect bandgap around 2.0 eV at the $\bar K'$ point. Many-body correlation effects may give rise, in the limit of strong localization, to this bandgap opening

Solid-state graphitization mechanisms of silicon carbide 6H-SiC polar faces

International audienceDue to the higher vapour pressure of silicon, silicon carbide surfaces annealed at high temperature under vacuum tend to graphitize. The comparison of graphite formation on the silicon and carbon terminations of 6H-SiC reveals significant differences in the graphitization mechanisms involved. The conduction-band structure of these interfaces has been Ž. determined by angle-resolved inverse photoemission spectroscopy KRIPES. Although the graphite layers grown on the C face are essentially polycrystalline, a small fraction of the film keeps a preferred orientation, where the graphite lattice basis vectors are rotated by 308 with respect to the basis vectors of the SiC lattice as in the case of the Si face. This in-plane disorder is in contrast with the growth of graphite on the Si face that takes place on a ''passivated'' adatom-terminated surface, leading to single-crystalline, heteroepitaxial graphite growth. The observation of unshifted p) states indicates a very small interaction of the first graphite monolayer with the Si face. In contrast, KRIPES reveals that the first graphite layer is strongly bound to the C face. A rehybridization of the graphite p) states with occupied orbitals of the substrate is inferred from an observed increase in the density of states in the vicinity of the Fermi level.