Jingul Kim

Spin-induced band modifications of graphene through intercalation of magnetic iron atoms

Intercalation of magnetic iron atoms through graphene formed on the SiC(0001) surface is found to induce significant changes in the electronic properties of graphene due mainly to the Fe-induced asymmetries in charge as well as spin distribution. From our synchrotron-based photoelectron spectroscopy data together with ab initio calculations, we observe that the Fe-induced charge asymmetry results in the formation of a quasi-free-standing bilayer graphene while the spin asymmetry drives multiple spin-split bands. We find that Fe adatoms are best intercalated upon annealing at 600 °C, exhibiting split linear π-bands, characteristic of a bilayer graphene, but much diffused. Subsequent changes in the C 1s, Si 2p, and Fe 3p core levels are consistently described in terms of Fe-intercalation. Our calculations together with a spin-dependent tight binding model ascribe the diffuse nature of the π-bands to the multiple spin-split bands originated from the spin-injected carbon atoms residing only in the lower graphene layer.

Band gap engineering for single-layer graphene by using slow Li(+) ions.

In order to utilize the superb electronic properties of graphene in future electronic nano-devices, a dependable means of controlling the transport properties of its Dirac electrons has to be devised by forming a tunable band gap. We report on the ion-induced modification of the electronic properties of single-layer graphene (SLG) grown on a SiC(0001) substrate by doping low-energy (5 eV) Li(+) ions. We find the opening of a sizable and tunable band gap up to 0.85 eV, which depends on the Li(+) ion dose as well as the following thermal treatment, and is the largest band gap in the π-band of SLG by any means reported so far. Our Li 1s core-level data together with the valence band suggest that Li(+) ions do not intercalate below the topmost graphene layer, but cause a significant charge asymmetry between the carbon sublattices of SLG to drive the opening of the band gap. We thus provide a route to producing a tunable graphene band gap by doping Li(+) ions, which may play a pivotal role in the utilization of graphene in future graphene-based electronic nano-devices.

Cerium-induced changes in the π-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 π-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 Eg = 0.50 eV opened when the Ce-adsorbed graphene was cooled to 41 K after a brief annealing at a temperature Ta of 1200 °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 Ta of 530 °C and then below the buffer layer at a Ta of 1050 °C. We discuss the physical implications of these temperature-dependent features of the Ce-adsorbed graphene.

Band modification of graphene by using slow Cs + ions

We report new wide band gap engineering for graphene using slow Cs+ ions, which allows both fine-tuning and on–off switching capability of the band gap in a range suitable for most applications without modifying or deteriorating the relativistic nature of the Dirac fermions. The doping of Cs+ ions opens the band gap up to Eg = 0.68 eV, which can be closed completely by adding neutral Cs atoms, as observed in angle-resolved photoemission spectroscopy. The operating mechanism of this band gap engineering is understood by a simple capacitor model, which is fully supported by the density-functional theory calculations.

Emergence of Kondo Resonance in Graphene Intercalated with Cerium

The interaction between a magnetic impurity, such as cerium (Ce) atom, and surrounding electrons has been one of the core problems in understanding many-body interaction in solid and its relation to magnetism. Kondo effect, the formation of a new resonant ground state with quenched magnetic moment, provides a general framework to describe many-body interaction in the presence of magnetic impurity. In this Letter, a combined study of angle-resolved photoemission (ARPES) and dynamic mean-field theory (DMFT) on Ce-intercalated graphene shows that Ce-induced localized states near Fermi energy, EF, hybridized with the graphene π-band, exhibit gradual increase in spectral weight upon decreasing temperature. The observed temperature dependence follows the expectations from the Kondo picture in the weak coupling limit. Our results provide a novel insight how Kondo physics emerges in the sea of two-dimensional Dirac electrons.

Observation of variable hybridized-band gaps in Eu-intercalated graphene

We report europium (Eu)-induced changes in the π-band of graphene (G) formed on the 6H-SiC(0001) surface by a combined study of photoemission measurements and density functional theory (DFT) calculations. Our photoemission data reveal that Eu intercalates upon annealing at 120 °C into the region between the graphene and the buffer layer (BL) to form a G/Eu/BL system, where a band gap of 0.29 eV opens at room temperature. This band gap is found to increase further to 0.48 eV upon cooling down to 60 K. Our DFT calculations suggest that the increased band gap originates from the enhanced hybridization of the graphene π-band with the Eu 4f band due to the increased magnetic ordering upon cooling. These Eu atoms continue to intercalate further down below the BL to produce bilayer graphene (G/BL/Eu) upon annealing at 300 °C. The π-band stemming from the BL then exhibits another band gap of 0.37 eV, which appears to be due to the strong hybridization between the π-band of the BL and the Eu 4f band. The Eu-intercalated graphene thus illustrates an example of versatile band gaps formed under different thermal treatments, which may play a critical role for future applications in graphene-based electronics.

Modification of electronic properties of graphene by using low-energy K+ ions

Despite its superb electronic properties, the semi-metallic nature of graphene with no band gap (Eg) at the Dirac point has been a stumbling block for its industrial application. We report an improved means of producing a tunable band gap over other schemes by doping low energy (10 eV) potassium ions (K+) on single layer graphene formed on 6H-SiC(0001) surface, where the noble Dirac nature of the π-band remains almost unaltered. The changes in the π-band induced by K+ ions reveal that the band gap increases gradually with increasing dose (θ) of the ions up to Eg = 0.65 eV at θ = 1.10 monolayers, demonstrating the tunable character of the band gap. Our core level data for C 1s, Si 2p, and K 2p suggest that the K+-induced asymmetry in charge distribution among carbon atoms drives the opening of band gap, which is in sharp contrast with no band gap when neutral K atoms are adsorbed on graphene. This tunable K+-induced band gap in graphene illustrates its potential application in graphene-based nano-electronics.

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 π-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 π-band with varying Ek indicate an n-doping effect in the graphitic phase 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 π-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.

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 Γ-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 π-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.