76 research outputs found
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
Interaction of C 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
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
Unoccupied electronic states of 2D Si on Ag-√ 3-Si(111)
International audienceOptimizing substrate characterization to grow 2D Si layers on surfaces is a major issue towards the development of synthesis techniques of the promising silicene. We have used inverse photoemission spectroscopy (IPES) to study the electronic band structure of an ordered 2D Si layer on the √ 3× √ 3-Ag/Si(111) surface (√ 3-Ag). Exploiting the large upwards band bending of the √ 3-Ag substrate, we could investigate the evolution of the unoccupied surface and interface states in most of the Si band gap. In particular, the k-dispersion of the √ 3-Ag free-electron-like S1 surface state measured by IPES, is reported for the first time. Upon deposition of ∼1 ML Si on √ 3-Ag maintained at ∼ 200 • C, the interface undergoes a metal-insulator transition with the complete disappearance of the S1 state. The latter is replaced by a higher-lying state U0 with a minimum at 1.0 eV above EF. The origin of this new state is discussed in terms of various Si 2D structures including silicene
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