Epitaxial graphene on step bunching of a 6H-SiC(0001) substrate: Aromatic ring pattern and Van Hove singularities

Abstract

We report scanning tunneling microscopy and spectroscopy investigation of graphene nanoribbons grown on an array of bunched steps of a 6H-SiC(0001) substrate. Our STM images of a GNR on a step terrace feature a (√3x√3)R30° pattern of aromatic rings which define our armchair nanoribbons. This is in agreement to a simulation based on density functional theory. As another signature of the one-dimensional electronic structure, in the corresponding STS spectra we find well developed, sharp Van Hove singularities.In the ongoing efforts to investigate the properties of graphene and related structures, graphene nanoribbons (GNRs) have attracted considerable attention. To a big part, this is due to the opening of an electronic band gap caused by the confinement of the electronic states in one dimension. The boundary conditions imposed by the edges, however, give rise to more phenomena than that. In this article, we address two of them, the formation of aromatic rings of delocalized π-electrons in the real space electron distribution and the appearance of Van Hove singularities (VHSs) in the local density of states (LDOS).Composed of a network of sp 2 hybridized carbon atoms, graphene can be thought of as a prototypic aromatic crystal. While three of the four valence electrons of each carbon atom are engaged in localized σ-bonds, the remaining one can form a π-bond with any of the three neighboring atoms. As long as nothing breaks the symmetry of the graphene crystal, all three options for the π-bond co-exist at the same time. When confined in narrow ribbons, however, considerations based on Clar's theory of the aromatic sextet have shown that certain π-bond configurations are preferred over others [1,2]. Depending on the edge configuration and the width of the ribbon, a distinct (√3x√3)R30° pattern of aromatic rings can form. As the states of the π-electrons lie close to the Fermi energy, this superstructure is detectable in STM measurements and has been observed near graphene edges [3]. Another effect of the one dimensional confinement of the electrons in GNRs is the appearance of divergences in the density of states (DOS) as a function of the energy, the so-called VHSs. They appear when a new sub-band in the electronic structure enters the bias window. Even though predicted theoretically, VHSs remain difficult to measure and when they are observed they are often suppressed by edge disorder [4,5].Contrary to other graphene growth methods, graphitization of SiC does not require a graphene transfer since SiC represents itself a suitable substrate for high power, high frequency, and opto-electronic devices [6]. The structure and electronic properties of graphene grown on SiC, however, strongly depend on the presence of steps on the substrate [6]. These steps, concentrated or bunched in narrow regions separated by larger atomically flat (0001) terraces, are known to be detrimental for the mobility of graphene. On the other hand, since the widths of the terraces are homogeneous to a high degree [7], they have proven to be us...

Flower-Shaped Domains and Wrinkles in Trilayer Epitaxial Graphene on Silicon Carbide

“…STS probed the electronic properties of the surface proportional to the local density of states (LDOS)38. Each spectrum was averaged over 20 STS spectra and taken at different cross locations on the flower-shape (spectra 3 and 4), flower middle (spectra 7) and on the flat graphene trilayer (spectra 1, 2, 5 and 6).…”

“…In addition to their specific structural features, the flower-shaped graphene domains exhibited a singular electronic structure, as demonstrated by the STS measurements, shown in Figure 3(b) . STS probed the electronic properties of the surface proportional to the local density of states (LDOS) 38 . Each spectrum was averaged over 20 STS spectra and taken at different cross locations on the flower-shape (spectra 3 and 4), flower middle (spectra 7) and on the flat graphene trilayer (spectra 1, 2, 5 and 6).…”

Atomic and electronic structure of trilayer graphene/SiC(0001): Evidence of Strong Dependence on Stacking Sequence and charge transfer

“…STM/STS measurements were carried out using an Omicron ultra-high vacuum low temperature scanning tunnelling microscope (UHV-LT-STM) 37 . The STM tips used in this study were electrochemically-etched polycrystalline tungsten tips, flashed by Joule heating under UHV to temperatures up to white light emission followed by field emission.…”

Step bunching with both directions of the current: Vicinal W(110) surfaces versus atomistic-scale model

“…The observation that monoatomic steps on the vicinal surfaces of Si(111) break their initial equidistant distribution under the influence of an electric field to form wide terraces almost free of steps [1], led this system to become the architype for a class of surface instabilities -the bunching of straight steps. This surface in particular is also the most studied both experimentally and theoretically, although step bunching (SB) has been observed in other experimental systems as well [2][3][4][5][6][7][8][9][10][11][12][13][14]. However, the step bunching behaviour on Si(111) is unique in part due to the scale of the bunches produced by electromigration alone.…”