Jacek Piechota

Stacking sequence dependence of graphene layers on SiC (0001−)—Experimental and theoretical investigation

Different stacking sequences of graphene are investigated using a combination of experimental and theoretical methods. High-resolution transmission electron microscopy (HRTEM) of the stacking sequence of several layers of graphene, formed on the C-terminated 4H-SiC (0001−) surface, was used to determine the stacking sequence and the interlayer distances. These data prove that the three metastable multilayer graphene configurations exist: AB, AA, and ABC. In accordance, those three cases were considered theoretically, using density functional theory (DFT) calculations comparing properties of graphene, both free-standing and positioned on the SiC (0001−) substrate. Total energies were calculated, the most stable structure was identified, and the electronic band structure was obtained. These results were compared with results obtained for a graphene single layer, having six or three H atoms attached to the carbon ring. It was found that sixfold symmetry leads to linear dispersion relations and threefold symm...

A comparative DFT study of electronic properties of 2H-, 4H- and 6H-SiC(0001) and SiC(000 \bar{1} ) clean surfaces: significance of the surface Stark effect

The electric field, uniform within a slab, emerging due to Fermi level pinning at both sides of the slab, is analyzed using DFT simulations of SiC surface slabs of different thicknesses. It is shown that for thicker slabs the field is nonuniform and this fact is related to the surface state charge. Using the electron density and potential profiles, it is proved that for high-precision simulations it is necessary to take into account a sufficient number of SiC layers. We show that the use of 12 diatomic layers leads to satisfactory results. It is also demonstrated that the change of the opposite side slab termination, both by different types of atoms or by their location, can be used to adjust the electric field within the slab, creating a tool for simulation of surface properties, depending on the doping in the bulk of the semiconductor. Using these simulations, it was found that, depending on the electric field, the energy of the surface states changes in a different way than the energy of the bulk states. This criterion can be used to distinguish Shockley and Tamm surface states. The electronic properties, i.e. energy and type of surface states of the three clean surfaces: 2H-, 4H-, 6H-SiC(0001) and SiC(), are analyzed and compared using field-dependent DFT simulations.Electric field, uniform within the slab, emerging due to Fermi level pinning at its both sides is analyzed using DFT simulations of the SiC surface slabs of different thickness. It is shown that for thicker slab the field is nonuniform and this fact is related to the surface state charge. Using the electron density and potential profiles it is proved that for high precision simulations it is necessary to take into account enough number of the Si-C layers. We show that using 12 diatomic layers leads to satisfactory results. It is also demonstrated that the change of the opposite side slab termination, both by different type of atoms or by their location, can be used to adjust electric field within the slab, creating a tool for simulation of surface properties, depending on the doping in the bulk of semiconductor. Using these simulations it was found that, depending on the electric field, the energy of the surface states changes in a different way than energy of the bulk states. This criterion can be used to distinguish Shockley and Tamm surface states. The electronic properties, i.e. energy and type of surface states of the three clean surfaces: 2H-, 4H-, 6H-SiC(0001), and SiC(

Adsorption of selected ions on the anatase TiO2 (101) surface: a density-functional study

000 \bar{1}

Density functional study of sulphur hexafluoride (SF6) and its hydrogen derivatives

) are analyzed and compared using field dependent DFT simulations.

Hydrogen intercalation of single and multiple layer graphene synthesized on Si-terminated SiC(0001) surface

Density-functional calculations of adsorption of the F− and OH− ions on the TiO2 anatase (101) surface have been performed. Changes to the electronic properties prior to and after adsorption have been investigated. The most profound change in the electronic structure of the anatase surface after adsorption is the diminishing of the band gap of the system. While for the pure (without adsorbents) surface the band gap is 1.77 eV wide, for the modified surface with the F− and OH− ions the band gap widths equal to 0.35 and 0.39 eV, respectively. That means, the excitation energy after adsorption is over four times smaller than in the case of pure system. Nevertheless, in all the cases studied the anatase surface maintains its semiconducting properties.

Ab initio predictions of structural and elastic properties of struvite: contribution to urinary stone research

Density functional study has been performed for a group of compounds derived from sulphur hexafluoride (SF6) by consecutively substituting fluorine with hydrogen. SF6 is widely used as the insulating gas in the electrical industry and is recognised as one of the greenhouse gases with extraordinary global warming potential. The aim of the present study is to look for potential industrial alternatives to SF6 as well as to examine mechanisms that can contribute to its faster atmospheric decay. The ground state geometries, binding energies, vibrational spectra, charge distributions, dipole moments, as well as thermodynamic properties for the series of the SF6 − n H n (n = 0,…,6) molecules have been obtained and discussed. For comparison, computational results for the SCl6 molecule have also been included in the present study.

Experimental and theoretical investigation of graphene layers on SiC(0001¯) in different stacking arrangements

Ab initio density functional theory simulations were used to investigate the influence of hydrogen intercalation on the electronic properties of single and multiple graphene layers deposited on the SiC(0001) surface (Si-face). It is shown that single carbon layer, known as a buffer layer, covalently bound to the SiC substrate, is liberated after hydrogen intercalation, showing characteristic Dirac cones in the band structure. This is in agreement with the results of angle resolved photoelectron spectroscopy measurements of hydrogen intercalation of SiC-graphene samples. In contrast to that hydrogen intercalation has limited impact on the multiple sheet graphene, deposited on Si-terminated SiC surface. The covalently bound buffer layer is liberated attaining its graphene like structure and dispersion relation typical for multilayer graphene. Nevertheless, before and after intercalation, the four layer graphene preserved the following dispersion relations in the vicinity of K point: linear for (AAAA) stacking, direct parabolic for Bernal (ABAB) stacking and “wizard hat” parabolic for rhombohedral (ABCA) stacking.

Structural defects in epitaxial graphene layers synthesized on C-terminated 4H-SiC (0001¯) surface—Transmission electron microscopy and density functional theory studies

In the present work, we carried out density functional calculations of struvite – the main component of the so-called infectious urinary stones – to study its structural and elastic properties. Using a local density approximation and a generalised gradient approximation, we calculated the equilibrium structural parameters and elastic constants C ijkl . At present, there is no experimental data for these elastic constants C ijkl for comparison. Besides the elastic constants, we also present the calculated macroscopic mechanical parameters, namely the bulk modulus (K), the shear modulus (G) and Youngs modulus (E). The values of these moduli are found to be in good agreement with available experimental data. Our results imply that the mechanical stability of struvite is limited by the shear modulus, G. The study also explores the energy-band structure to understand the obtained values of the elastic constants.

First principles study of Al(100) twisted interfaces

High-resolution transmission electron microscopy was used to investigate graphene layers formed on the C-terminated 4H-SiC(0001¯) surface in different arrangements, including various stacking sequences and spatial layer separation. Various stacking types such as ABAB and ABCA configurations were identified. The density functional theory (DFT) calculations of the graphene in various configurations were performed showing the following dispersion relations: AAAA—linear, ABBBA—close to linear, and ABAB—hyperbolic (strongly nonlinear). An increase of the interlayer separation of ABAB and ABCA systems leads to gradually increased linear dispersion, typical for AAAA stacking. It is shown, however, that for this transition to occur, a separation of the adjacent layers by about 5 A is necessary, which is not likely to occur in the graphene layer grown on the SiC(0001¯) surface. DFT calculations employing rotation of the adjacent AB planes of bilayer graphene by either 27.7 or 32.2 arc deg demonstrate similar linea...

Calculations of ZnO properties using the Heyd-Scuseria-Ernzerhof screened hybrid density functional

The principal structural defects in graphene multilayers synthesized on the carbon-terminated face of a 4H-SiC (0001¯) substrate were investigated using the high-resolution transmission electron microscopy. The analyzed systems include a wide variety of defected structures such as edge dislocations, rotational multilayers, and grain boundaries. It was shown that graphene layers are composed of grains of the size of several nanometres or larger; they differ in a relative rotation by large angles, close to 30°. The structure of graphene multilayers results from the synthesis on a SiC (0001¯) surface, which proceeds via intensive nucleation of new graphene layers that coalesce under various angles creating an immense orientational disorder. Structural defects are associated with a built-in strain resulting from a lattice mismatch between the SiC substrate and the graphene layers. The density functional theory data show that the high-angular disorder of AB stacked bi-layers is not restoring the hexagonal symm...