Jakub Sołtys

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(

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

000 \bar{1}

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

) are analyzed and compared using field dependent DFT simulations.

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

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.

Role of structure of C-terminated 4 H -SiC( 000 1 ¯ ) surface in growth of graphene layers: Transmission electron microscopy and density functional theory studies

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...

Electronic properties on GaN(0001) surface – ab initio investigation

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...