Cerium-induced changes in the pi-band of graphene

Modifying or controlling the intrinsic properties of graphene, such as by controlling its band gap and carrying out spin injection of its p-electrons, have been a recent focus of research in graphene technology in order to promote the industrial applications of its superb properties. Here, we carried out photoemission spectroscopy experiments using synchrotron photons and showed several unique changes in the electronic and structural properties of graphene resulting from its adsorption of magnetic cerium (Ce) atoms. A band gap as large as E-g = 0.50 eV opened when the Ce-adsorbed graphene was cooled to 41 K after a brief annealing at a temperature T-a of 1200 degrees C. As the temperature of this sample was then increased to room temperature (RT), the size of the band gap decreased gradually to an Eg of 0.36 eV, indicative of a temperature-dependent structural and/or spin-ordering phase transition. We also observed the presence of two different stages of Ce-intercalation upon annealing the graphene with Ce adsorbed at RT: the Ce atoms first intercalated below graphene at a T-a of 530 degrees C and then below the buffer layer at a T-a of 1050 degrees C. We discuss the physical implications of these temperature-dependent features of the Ce-adsorbed graphene.11Nsciescopu

Band and bonding characteristics of N2 + ion-doped graphene

We report that the doping of energetic nitrogen cations (N2 +) on graphene effectively controls the local N-C bonding structures and the ??-band of graphene critically depending on ion energy Ek (100 eV ??? Ek ??? 500 eV) by using a combined study of photoemission spectroscopy and density functional theory (DFT) calculations. With increasing Ek, we find a phase transformation of the N-C bonding structures from a graphitic phase where nitrogen substitutes carbon to a pyridinic phase where nitrogen loses one of its bonding arms, with a critical energy Eck = 100 eV that separates the two phases. The N2 +-induced changes in the ??-band with varying Ek indicate an n-doping effect in the graphitic phase for Ek < Eck but a p-doping effect for the pyridinic graphene for Ek > Eck. We further show that one may control the electron charge density of graphene by two orders of magnitude by varying Ek of N2 + ions within the energy range adopted. Our DFT-based band calculations reproduce the distinct doping effects observed in the ??-band of the N2 +-doped graphene and provide an orbital origin of the different doping types. We thus demonstrate that the doping type and electron number density in the N2 + ion-doped SLG can be artificially fine-controlled by adjusting the kinetic energy of incoming N2 + ions.clos

Observation of Mg-induced structural and electronic properties of graphene

We report the formation of superstructures induced by Mg adatoms on a single layer graphene (SLG) formed on Ni(111) substrate, where a strong metallic parabolic band is found near the Fermi level at the ??-point of the Brillouin zone. Our valence band and core level data obtained by using synchrotron photons indicate that Mg adatoms intercalate initially to lift the SLG from the Ni substrate to produce a well-defined ??-band of SLG, and then the parabolic band appears upon adding extra Mg atoms on the Mg-intercalated SLG. Our scanning tunneling microscopy images from these systems show the presence of superstructures, a 2???3 ?? 2???3 phase for the intercalated Mg layer below the SLG and then a ???7 ?? ???7 phase for the Mg overlayer formed on the Mg-intercalated SLG. We discuss the physical implications of these superstructures and the associated parabolic band in terms of a possible graphene-based two-dimensional superconductivity.clos