Graphene on the C-terminated SiC (000 1ˉ\bar{1}) surface: An ab initio study

The atomic and electronic structures of a graphene layer on top of the $(2\times2)$ reconstruction of the SiC (000$\bar{1}$) surface are studied from ab initio calculations. At variance with the (0001) face, no C bufferlayer is found here. Si adatoms passivate the substrate surface so that the very first C layer presents a linear dispersion characteristic of graphene. A small graphene-substrate interaction remains in agreement with scanning tunneling experiments (F.Hiebel et al. {\it Phys. Rev. B} {\bf 78} 153412 (2008)). The stacking geometry has little influence on the interaction which explains the rotational disorder observed on this face.Comment: 4 pages, 3 figures, additional materia

Impact of local stacking on the graphene-impurity interaction: theory and experiments

We investigate the graphene-impurity interaction problem by combining experimental - scanning tunneling microscopy (STM) and spectroscopy (STS) - and theoretical - Anderson impurity model and density functional theory (DFT) calculations - techniques. We use graphene on the SiC(000-1)(2x2)_C reconstruction as a model system. The SiC substrate reconstruction is based on silicon adatoms. Graphene mainly interacts with the dangling bonds of these adatoms which act as impurities. Graphene grown on SiC(000-1)(2x2)_C shows domains with various orientations relative to the substrate so that very different local graphene/Si adatom stacking configurations can be probed on a given grain. The position and width of the adatom (impurity) state can be analyzed by STM/STS and related to its local environment owing to the high bias electronic transparency of graphene. The experimental results are compared to Anderson's model predictions and complemented by DFT calculations for some specific local environments. We conclude that the adatom resonance shows a smaller width and a larger shift toward the Dirac point for an adatom at the center of a graphene hexagon than for an adatom just on top of a C graphene atom.Comment: 13 pages, 6 figures, Accepted for publication in Phys. Rev.

Single 3dd transition metal atoms on multi-layer graphene systems: electronic configurations, bonding mechanisms and role of the substrate

The electronic configurations of Fe, Co, Ni, and Cu adatoms on graphene and graphite have been studied by x-ray magnetic circular dichroism and charge transfer multiplet theory. A delicate interplay between long-range interactions and local chemical bonding is found to influence the adatom equilibrium distance and magnetic moment. The results for Fe and Co are consistent with purely physisorbed species having, however, different 3$d$-shell occupancies on graphene and graphite ($d^{n+1}$ and $d^n$, respectively). On the other hand, for the late 3$d$ metals Ni and Cu a trend towards chemisorption is found, which strongly quenches the magnetic moment on both substrates.Comment: 7 pages, 4 figure

Comparative life cycle assessment of first- and second-generation ethanol from sugarcane in Brazil

ABSTRACT: Purpose: The use of bagasse and trash from sugarcane fields in ethanol production is supposed to increase the ethanol yield per hectare, to reduce the energy demand, greenhouse gas emissions, and other environmental impacts. In this article, different technological options of ethanol production are investigated and quantified looking at potential environmental impacts. The first generation ethanol from sugarcane is compared to stand-alone second-generation ethanol as well as an integrated first- and second-generation ethanol production. Methods: The method applied for this life cycle assessment follows the ISO standards 14040/44. The data used in this life cycle assessment is mainly derived from process simulation, literature, and primary data collection. Background data was taken from databases such as GaBi and ecoinvent. The life cycle impact assessment follows the default methods at midpoint level recommended by the International Reference Life Cycle Data System. The calculations were performed using the GaBi 7 life cycle assessment software. It is assumed that 50% of sugarcane trash is recovered and used for second-generation ethanol production, whereas the other 50% remain in the field to maintain soil fertility and to prevent soil erosion. In the case of first-generation ethanol, the same amount of trash is used for energy generation. Results and discussion: The results of the life cycle impact assessment show that, compared to first-generation ethanol, secondgeneration ethanol from sugarcane in Brazil allows significant reductions in all investigated impact categories except resource depletion. Resource depletion, however, is strongly influenced by the demand for ammonium phosphate which is needed for inoculum preparation. Integrated first- and second-generation ethanol production also allows reductions in most of the environmental impacts except for global warming, photochemical ozone depletion, and resource depletion. The yield of ethanol per hectare increases since bagasse and trash are used for the production of second-generation ethanol. Consequently, the results show that agricultural land occupation is reduced for integrated first- and second-generation ethanol by approximately 11%, whereas second-generation ethanol allows reduction of land use by approximately a factor of 30. Conclusions: The use of bagasse and trash for ethanol production allows both the reduction of several environmental impacts and land use, in particular, because impacts caused by sugarcane cultivation are avoided. For the integrated first- and second generation ethanol scenario, it is important to further reduce the total energy demand in order to achieve self-sufficiency for the plant energy and to avoid additional emissions from burning fossil fuels.info:eu-repo/semantics/publishedVersio

Interplay between edge states and simple bulk defects in graphene nanoribbons

We study the interplay between the edge states and a single impurity in a zigzag graphene nanoribbon. We use tight-binding exact diagonalization techniques, as well as density functional theory calculations to obtain the eigenvalue spectrum, the eigenfunctions, as well the dependence of the local density of states (LDOS) on energy and position. We note that roughly half of the unperturbed eigenstates in the spectrum of the finite-size ribbon hybridize with the impurity state, and the corresponding eigenvalues are shifted with respect to their unperturbed values. The maximum shift and hybridization occur for a state whose energy is inverse proportional to the impurity potential; this energy is that of the impurity peak in the DOS spectrum. We find that the interference between the impurity and the edge gives rise to peculiar modifications of the LDOS of the nanoribbon, in particular to oscillations of the edge LDOS. These effects depend on the size of the system, and decay with the distance between the edge and the impurity.Comment: 10 pages, 15 figures, revtex

Micro-Raman and micro-transmission imaging of epitaxial graphene grown on the Si and C faces of 6H-SiC

Micro-Raman and micro-transmission imaging experiments have been done on epitaxial graphene grown on the C- and Si-faces of on-axis 6H-SiC substrates. On the C-face it is shown that the SiC sublimation process results in the growth of long and isolated graphene ribbons (up to 600 μm) that are strain-relaxed and lightly p-type doped. In this case, combining the results of micro-Raman spectroscopy with micro-transmission measurements, we were able to ascertain that uniform monolayer ribbons were grown and found also Bernal stacked and misoriented bilayer ribbons. On the Si-face, the situation is completely different. A full graphene coverage of the SiC surface is achieved but anisotropic growth still occurs, because of the step-bunched SiC surface reconstruction. While in the middle of reconstructed terraces thin graphene stacks (up to 5 layers) are grown, thicker graphene stripes appear at step edges. In both the cases, the strong interaction between the graphene layers and the underlying SiC substrate induces a high compressive thermal strain and n-type doping