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Register a new userGrowth Structure and Work Function of Bilayer Graphene on Pd(111)
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Using in situ low-energy electron microscopy and density functional theory, we studied the growth structure and work function of bilayer graphene on Pd(111). Low-energy electron diffraction analysis established that the two graphene layers have multiple rotational orientations relative to each other and the substrate plane. We observed heterogeneous nucleation and simultaneous growth of multiple, faceted layers prior to the completion of second layer. We propose that the facetted shapes are due to the zigzag-terminated edges bounding graphene layers growing under the larger overlying layers. We also found that the work functions of bilayer graphene domains are higher than those of monolayer graphene, and depend sensitively on the orientations of both layers with respect to the substrate. Based on first-principles simulations, we attribute this behavior to oppositely oriented electrostatic dipoles at the graphene/Pd and graphene/graphene interfaces, whose strengths depend on the orientations of the two graphene layers.
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We determine the ground-state structure of a double vacancy in a hydrogen monolayer on the Pd(111) surface. We represent the double vacancy as a triple vacancy containing one additional hydrogen atom. The potential-energy surface for a hydrogen atom moving in the triple vacancy is obtained by density-functional theory, and the wave function of the fully quantum hydrogen atom is obtained by solving the Schrodinger equation. We find that an H atom in a divacancy defect experiences significant quantum effects, and that the ground-state wave function is centered at the hcp site rather than the fcc site normally occupied by H atoms on Pd(111). Our results agree well with scanning tunneling microscopy images.
Advances in hybrid organic/inorganic architectures for optoelectronics can be achieved by understanding how the atomic and electronic degrees of freedom cooperate or compete to yield the desired functional properties. Here we show how work-function changes are modulated by the structure of the organic components in model hybrid systems. We consider two cyano-quinodimethane derivatives (F4-TCNQ and F6-TCNNQ), which are strong electron-acceptor molecules, adsorbed on H-Si(111). From systematic structure searches employing range-separated hybrid HSE06 functional including many body van der Waals contributions, we predict that despite their similar composition, these molecules adsorb with significantly different densely-packed geometries in the first layer, due to strong intermolecular interaction. F6-TCNNQ shows a much stronger intralayer interaction (primarily due to van der Waals contributions) than F4-TCNQ in multilayered structures. The densely-packed geometries induce a large interface-charge rearrangement that result in a work-function increase of 1.11 and 1.76 eV for F4-TCNQ and F6-TCNNQ, respectively. Nuclear fluctuations at room temperature produce a wide distribution of work-function values, well modeled by a normal distribution with {sigma}=0.17 eV. We corroborate our findings with experimental evidence of pronounced island formation for F6-TCNNQ on H-Si(111) and with the agreement of trends between predicted and measured work-function changes.
Large-area bilayer graphene (BG) is grown epitaxially on Ru(0001) surface and characterized by low temperature scanning tunneling microscopy. The lattice of the bottom layer of BG is stretched by 1.2%, while strain is absent from the top layer. The lattice mismatch between the two layers leads to the formation of a moire pattern with a periodicity of ~21.5 nm and a mixture of AA- and AB-stacking. The root3 x root3 superstructure around atomic defects is attributed to the inter-valley scattering of the delocalized pi-electrons, demonstrating that the as-grown BG behaves like intrinsic free-standing graphene.
Ultrathin (111)-oriented polar iron oxide films were grown on a Pt(111) single crystal either by the reactive deposition of iron or oxidation of metallic iron monolayers. These films were characterized using low energy electron diffraction, scanning tunneling microscopy and conversion electron Mossbauer spectroscopy. The reactive deposition of Fe led to the island growth of Fe3O4, in which the electronic and magnetic properties of the bulk material were modulated by superparamagnetic size effects for thicknesses below 2 nm, revealing specific surface and interface features. In contrast, the oxide films with FeO stoichiometry, which could be stabilized as thick as 4 nm under special preparation conditions, had electronic and magnetic properties that were very different from their bulk counterpart, wustite. Unusual long range magnetic order appeared at room temperature for thicknesses between three and ten monolayers, the appearance of which requires severe structural modification from the rock-salt structure.
We report on the charge spill-out and work function of epitaxial few-layer graphene on 6H-SiC(0001). Experiments from high-resolution, energy-filtered X-ray photoelectron emission microscopy (XPEEM) are combined with ab initio Density Functional Theory calculations using a relaxed interface model. Work function values obtained from theory and experiments are in qualitative agreement, reproducing the previously observed trend of increasing work function with each additional graphene plane. Electrons transfer at the SiC/graphene interface through a buffer layer causes an interface dipole moment which is at the origin of the graphene work function modulation. The total charge transfer is independent of the number of graphene layers, and is consistent with the constant binding energy of the SiC component of the C 1s core-level measured by XPEEM. Charge leakage into vacuum depends on the number of graphene layers explaining why the experimental, layer-dependent C 1s-graphene core-level binding energy shift does not rigidly follow that of the work function. Thus, a combination of charge transfer at the SiC/graphene interface and charge spill-out into vacuum resolves the apparent discrepancy between the experimental work function and C1s binding energy.
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