We explain the robust p-type doping observed for quasi-free standing graphene on hexagonal silicon carbide by the spontaneous polarization of the substrate. This mechanism is based on a bulk property of SiC, unavoidable for any hexagonal polytype of the material and independent of any details of the interface formation. We show that sign and magnitude of the polarization are in perfect agreement with the doping level observed in the graphene layer. With this mechanism, models based on hypothetical acceptor-type defects as they are discussed so far are obsolete. The n-type doping of epitaxial graphene is explained conventionally by donor-like states associated with the buffer layer and its interface to the substrate which overcompensate the polarization doping.

The doping of quasi-freestanding graphene (QFG) on H-terminated, Si-face 6H-, 4H-, and 3C-SiC is studied by angle-resolved photoelectron spectroscopy close to the Dirac point. Using semi-insulating as well as n-type doped substrates we shed light on the contributions to the charge carrier density in QFG caused by (i) the spontaneous polarization of the substrate, and (ii) the band alignment between the substrate and the graphene layer. In this way we provide quantitative support for the previously suggested model of polarization doping of graphene on SiC (Ristein et al 2012 Phys. Rev. Lett. 108 246104).

The doping of graphene to tune its electronic structure is essential for its further use in carbon based electronics. Adapting strategies from classical silicon based semiconductor technology, we use the incorporation of heteroatoms in the 2D graphene network as a straightforward way to achieve this goal. Here, we report on the synthesis of boron-doped graphene on Ni (111) calculations. Furthermore, our calculations suggest that doping with boron leads to graphene preferentially adsorbed in the top-fcc geometry, since the boron atoms in the graphene lattice are then adsorbed at substrate fcc-hollow sites. The smaller adsorption distance of boron compared to carbon leads to a bending of the graphene sheet in the vicinity of the boron atoms. By comparing calculations of doped and undoped graphene on Ni(111), as well as the respective free-standing cases, we are able to distinguish between the effects that doping and adsorption have on the band structure of graphene. Both, doping and bonding to the surface, result in opposing shifts on the graphene bands.

Bilayer graphene is a highly promising material for electronic and optoelectronic applications since it is supporting massive Dirac fermions with a tuneable band gap. However, no consistent picture of the gap's effect on the optical and transport behavior has emerged so far, and it has been proposed that the insulating nature of the gap could be compromised by unavoidable structural defects, by topological in-gap states, or that the electronic structure could be altogether changed by many-body effects. Here we directly follow the excited carriers in bilayer graphene on a femtosecond time scale, using ultrafast time-and angle-resolved photoemission. We find a behavior consistent with a single-particle band gap. Compared to monolayer graphene, the existence of this band gap leads to an increased carrier lifetime in the minimum of the lowest conduction band. This is in sharp contrast to the second sub-state of the conduction band, in which the excited electrons decay through fast, phonon-assisted inter-band transitions.The lack of a band gap is the most important obstacle to using graphene in electronic devices but this can be elegantly solved in bilayer graphene (BLG) when an asymmetry between the layers is induced by a transverse electric field [1][2][3]. The promising properties of the thereby induced massive Dirac particles have been intensively explored for the development of semiconducting devices with tuneable band gaps [4-6] and for efficient photodetectors extending to the THz regime [7][8][9][10]. However, the quantitative transport properties of BLG are inconsistent with the simple scenario of a small band gap semiconductor [5] and different hypotheses for this have been given, including broken-symmetry ground states [11] or topological edge state effects [12]. Recently, a study of BLG by angle-resolved photoemission spectroscopy (ARPES) has revealed the presence of electronic states throughout the gap, arising from intrinsic AA-stacked domains [13], something that might be expected to short-circuit the gap of BLG.While static ARPES results provide crucial information about the spectral function of BLG, time-resolved spectroscopy is needed to complement the transport studies. Time-and angle-resolved photoemission (TR-ARPES) experiments near the Dirac cone have only recently become technically feasible [14,15] due to the high photon energies needed to reach theK point in the Brillouin zone. Here we report TR-ARPES measurements carried out using a Ti:sapphire amplified laser system with a repetition rate of 1 kHz. This provided ultrafast infrared pulses with a wavelength of 785 nm, a full width at half-maximum (FWHM) duration of 30 fs, and an energy per pulse of 12 mJ. A part of the laser energy was applied for high harmonic generation of extreme ultraviolet pulses in a pulsed jet of argon gas. A time-preserving monochromator was used to single-out the 13th harmonic with a photon energy of 21 eV. The remaining laser energy was utilized to drive an optical parametric amplifier (HE-Topas), which can provide ...

The work function and electronic structure of epitaxial graphene as well as of quasi-freestanding graphene multilayer samples were studied by Kelvin probe and angle resolved photoelectron spectroscopy. The work function converges towards the value of graphite as the number of layers is increased. Thereby, n-type doped epitaxial graphene layers have a work function lower than graphite and p-type doped quasi-freestanding graphene layers exhibit a work function higher than graphite. We explain the behaviour by the filling of the p-bands due to substrate interactions.