Applied Science and Convergence Technology 2018; 27(5): 90-94
Published online September 30, 2018
Copyright © The Korean Vacuum Society.
Jinwoong Hwanga, Ji-Eun Leea, Minhee Kanga, Byeong-Gyu Parkb, Jonathan Denlingerc, Sung-Kwan Moc, and Choongyu Hwanga,*
aDepartment of Physics, Pusan National University, Busan 46241, Korea, bPohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 37673, Korea, cAdvanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
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The electron band structure of manganese-adsorbed graphene on an SiC(0001) substrate has been studied using angle-resolved photoemission spectroscopy. Upon introducing manganese atoms, the conduction band of graphene, that is observed in pristine graphene indicating intrinsic electron-doping by the substrate, completely disappears and the valence band maximum is observed at 0.4 eV below Fermi energy. At the same time, the slope of the valence band decreases by the presence of manganese atoms, approaching the electron band structure calculated using the local density approximation method. The former provides experimental evidence of the formation of nearly free-standing graphene on an SiC substrate, concomitant with a metal-to-insulator transition. The latter suggests that its electronic correlations are efficiently screened, suggesting that the dielectric property of the substrate is modified by manganese atoms and indicating that electronic correlations in grpahene can also be tuned by foreign atoms. These results pave the way for promising device application using graphene that is semiconducting and charge neutral.
Keywords: Graphene, SiC, Manganese, Free-standing, Metal-to-insulator transition, Angle-resolved photoemission spectroscopy
Graphene, consisting of a single atomic layer of carbon atoms arranged in a honeycomb lattice, has attracted a lot of research interest due to its two-dimensional nature of Dirac particles realized in a condensed matter system [1,2]. Moreover, graphene is expected to have great potential for fabrication of electronic devices with unique functionalities [3,4]. However, in practice, its gapless semi-metallic nature  and inevitable substrate effects, such as hybridization and charge transfer [6–9], make it difficult to utilize graphene as a building block for device applications, while preserving the characteristic properties of free-standing graphene. Therefore, decoupling of graphene from the substrate, especially aligning Dirac energy,
The pursuit for decoupled and gapped graphene has led to exploring the introduction of foreign atoms. Intercalation of foreign atoms breaks the bonding between graphene and the substrate, or compensates for the work function difference between them, leading to transfer of electrons to the substrate that shifts
Graphene epitaxially grown on an SiC substrate is a promising candidate for this purpose. The energy gap at
In this study, angle-resolved photoemission spectroscopy (ARPES) has been utilized to investigate graphene on an SiC substrate. The presence of manganese atoms in the system not only shifts
Graphene samples were epitaxially grown on a 6
Figures 1(a) and 1(b) show ARPES intensity maps of as-grown graphene taken parallel and perpendicular to the ΓK direction of the graphene unit cell, respectively, as denoted by red lines in the insets. The ARPES intensity maps show the characteristic conical energy-momentum dispersion of graphene with two almost parallel dispersions of strong and weak spectral intensity denoted by blue and red arrows, respectively. The pseudospin of quasiparticles in graphene results in a photoemission matrix element effect in which photoelectron intensity is strongly suppressed along the ΓK direction , such that one of the branches of the conical dispersion is not observed as shown in Fig. 1(a). The strong and weak spectral intensities correspond to single- and double-layer graphene, respectively. Since photoelectron intensity is closely related to the scattering area of a sample, the strong spectral intensity indicates that single-layer graphene is dominant in the sample. These spectral features are also illustrated in the energy distribution curves (EDCs) shown in Figs. 1(e) and 1(f).
In the measured ARPES intensity map for as-grown graphene,
The introduction of manganese atoms into the system makes three notable changes in the electron band structure of graphene as shown in Figs. 1(c) and 1(d). A non-dispersive band emerges at 2.8 eV below
The electronic structures observed in graphene on an SiC substrate without and with manganese atoms and their geometric structures are summarized in Fig. 2. The metal-to-insulator transition concomitant with the shift of
Detailed analysis of the electron band structure of graphene supports that the substrate is modified by the presence of manganese atoms. Figure 3(a) shows the graphene π band of as-grown graphene (red curves) and graphene with manganese atoms (blue curves) obtained using a Lorentzian fit to the momentum distribution curves (MDCs) for the ARPES intensity maps shown in Figs. 1(a) and 1(c), respectively. To study the change in the dispersion induced by manganese atoms, the dispersions are shown with respect to
The slope of the graphene π band is strongly influenced by dielectric environment . When graphene is placed on a metallic substrate, electron-electron interaction is screened by the metallic background from the substrate. Such screening results in the graphene π band convergence towards the LDA band, which describes the case of efficiently screened electron-electron interaction [6,29]. The energy-momentum dispersion of graphene with manganese atoms is in good agreement with the LDA band as shown in Fig. 3(a). This agreement indicates the possibility that the interface of SiC underneath graphene becomes metallic, leading to enhanced screening of electron-electron interaction in graphene compared to that for as-grown graphene. A plausible origin of this screening formation of silicon-manganese bonds, i.e., manganese silicide. Such silicide may not be well ordered, possibly resulting in random site potentials, which is exhibited as the enlarged MDC width shown in Fig. 3(b) (the peak observed at ~0.4 eV below
The interaction of graphene with foreign atoms can induce an interesting spin state in the system. The hybridization between the graphene π band and the cobalt 3
Electronic properties of graphene on an SiC(0001) substrate in the presence of manganese atoms have been investigated using the ARPES technique. The introduction of manganese atoms in the system allows graphene to approach its charge neutrality and to under go a metal-to-insulator transition. In addition, the electron-electron interaction in graphene is sufficiently screened, which is confirmed by the graphene π band agreement with the LDA band, suggesting the possibility that manganese atoms form a metallic silicide layer on top of the SiC substrate. These findings provide not only the experimental evidence of gapped nearly free-standing graphene on an SiC substrate, but also a promising route towards the application of graphene which is semiconducting and charge neutral to electronic devices.
This work was supported by a 2-Year Research Grant from the Pusan National University.