Edge structure of epitaxial graphene islands

The study of graphene, a single sheet of carbon with a honeycomb lattice, has the potential to lead to future carbonbased nanoelectronics.1,2 However, many experimental graphene studies are still in their infancy due to numerous challenges in the synthesis of graphene. One of the most promising methods of graphene synthesis is the graphitization of SiC substrates.1,3 Graphene grown epitaxially by this method has been shown to yield the physics of single-layer graphene, such as a linear band dispersion and chirality characteristic of massless Dirac fermions.4 The growth mechanisms for graphene formation on the SiC substrate are not fully understood, however, presenting challenges for the development of large-scale integrated graphene electronics. Therefore, detailed studies of the initial stages of graphene growth are vital. Recent work has shown that the growth of epitaxial graphene is predominantly from step edges for 6HSiC 0001 annealed in vacuum.5 In this work we show via scanning tunneling microscopy STM that a low density of epitaxial graphene islands also form on the surface. Graphene islands are of extreme scientific interest as their small sizes are suspected to yield novel electronic properties such as quantum confinement.6,7 In this initial work we emphasize structure and electronic properties, but note that the simple observation of island formation opens the possibility to derive surface kinetic parameters from detailed measurement of island sizes and areal densities.8 In order to study graphene islands, we needed to produce a sample with a small coverage of single-layer graphene, roughly 10% coverage of layer 1 graphene we use layer 0 to denote the insulating buffer layer and layer 1 to refer to the first layer with ostensibly graphene electronic structure . 9 To achieve such a coverage, we resistively heated a hydrogenetched 6H-SiC 0001 sample to 1200 °C very quickly 40 °C /s and held it at this temperature for 30 s in ultrahigh vacuum UHV . This temperature has been shown to be on the cusp of epitaxial graphene formation.10–12 The heating process was repeated five times to induce more growth on the surface. All measurements were performed at room temperature in a custom-built STM system, described in detail elsewhere.13 Iridium probe tips, used in tunneling measurements, were cleaned with field evaporation. Differential conductance, dI /dV, was measured with a lock-in amplifier, using a root-mean-square voltage modulation of 20 mV, at a frequency of 1.4 kHz. Under the above growth conditions, on large layer 0 terraces, there appear a very small number of layer 1 graphene islands, typically 10 nm in width. One such island is shown in Fig. 1 a . This island is approximately 11 nm wide and 0.3 nm high, the upper limit found previously for layer 1 to layer 0 step edges.9 Over a total scanned area of 10 m2, only eight islands were identified. Clearly, the growth is dominated by graphene formation at SiC step edges, consistent with prior observations.5 However, the observation of a low density of large and isolated islands provides evidence for a diffusing carbon species of relatively high surface mobility, and perhaps for a large critical nucleus, as observed for C deposition on Ru.14 The inset of Fig. 1 a displays the SiC 0001 -6 3 6 3R30° reconstruction layer 0 . Red arrows of the inset indicate the SiC 0001 -1 1 lattice vector directions, since the arrows are aligned with the “tetramers” white features that form a SiC 0001 -6 6 periodicity.9 Figure 1 b illustrates that the edges of the graphene island are aligned with the underlying SiC reconstruction, as also observed for layer 1 graphene terraces.15 This is highlighted in Fig. 1 b by overlaying two blue arrows, 120° apart and comparing to the SiC latticevector directions indicated in the inset of Fig. 1 a . Atomic-resolution STM images of graphene islands can be used to determine whether graphene step edges are terminated in the “armchair” or “zigzag” configuration. This is illustrated in Fig. 1 c , where blue arrows are drawn to correspond to armchair edges and a yellow arrow indicates a zigzag edge. The data shown is from the middle of the island because electron scattering from the island edges significantly distorts the lattice imaging otherwise.16 Combining the fact that a zigzag edge and an armchair edge are 30° apart and all the edges of the graphene islands are 120° or 60° apart, we conclude that a graphene island must be entirely surrounded by either zigzag or armchair step edges. By comparing the arrows of Fig. 1 c and the island edges of Fig. 1 b , it is easy to see that the blue armchair arrows accurately align with the island edges. Therefore, this graphene island is bounded entirely by armchair edges. All eight islands found on this surface were verified to have armchair edges three by direct atomically resolved STM imaging and five by edge directions relative to the SiC lattice . This observation is at odds with a simple bondbreaking argument that would predict zigzag edges 4.1 broken bonds per nm to have a lower energy than armchair PHYSICAL REVIEW B 81, 245408 2010