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Revealing the electronic band structure of quasi-free trilayer graphene on SiC(0001)

C. Coletti1,2,*, S. Forti2, A. Principi3, K.V. Emtsev2, A.A. Zakharov4, K.M. Daniels5, B.K. Daas5, M.V.S. Chandrashekhar5, A.H. MacDonald6, M. Polini3, U. Starke2 1

Center for Nanotechnology Innovation @ NEST, Istituto Italiano di Tecnologia, Piazza San Silvestro 12, I-56127 Pisa, IT 2

Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, D-70569 Stuttgart, DE

3

NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, I-56126 Pisa, IT

4

MAX-lab, Lund University, Lund, S-22100, SE

5

University of South Carolina, 301 S. Main St, Columbia, SC 29208, USA

6

Department of Physics, University of Texas at Austin, Austin, Texas 78712, USA

*

[email protected]

ABSTRACT Recently, much attention has been devoted to trilayer graphene because it displays stacking and electric field dependent electronic properties well-suited for electronic and photonic applications [1-8]. Several theoretical studies have predicted the electronic dispersion of Bernal (ABA) and rhombohedral (ABC) stacked trilayers. However, a direct experimental visualization of a well-resolved band structure has not yet been reported. In this work, we obtain large area highly homogenous quasi-free trilayer graphene (TLG) on 6H-SiC(0001) and measure its electronic bands via angle resolved photoemission spectroscopy (ARPES). We demonstrate by low energy electron microscopy measurements that that trilayer domains on SiC extend over areas of tens of square micrometers. By fitting tight-binding bands to the experimental data we extract the interatomic hopping parameters for Bernal and rhombohedral stacked trilayers. For ABC stacks and in the presence of an electrostatic asymmetry, we detect the existence of a bandgap of about 120 meV. Notably our results suggest that on SiC substrates the occurrence of ABC-stacked TLG is significantly higher than in natural bulk graphite. Hence, growing TLG on SiC might be the answer to the challenge of controllably synthesizing ABC-stacked trilayer – an ideal material for the fabrication of a new class of gap-tunable devices. INTRODUCTION Trilayer graphene has two naturally stable allotropes characterized by either Bernal (ABA) or rhombohedral (ABC) stacking of the individual carbon layers. In ABA-stacking the atoms of the topmost layer obtain lateral positions exactly above those of the bottom layer (Fig. 1(a)). In an ABC-stacked trilayer each layer is laterally shifted with respect to the layer below by a third of the diagonal of the lattice unit cell (Fig. 1(b)). Several theoretical studies have predicted the

electronic dispersion of ABA- and ABC-stacked trilayers using tight-binding approaches [1-3,913]. The low-energy band structure of ABA TLG consists of a linearly dispersing (monolayerlike) band and bilayer-like quadratically dispersing bands (Fig. 1(c)) [1,3,11]. Quite differently, ABC trilayers have a single low-energy band with approximately cubic dispersion (Fig. 1(d)) [13,12]. A very intriguing distinction