Controlling the shapes and assemblages of graphene.

C arbon, a vital element in our daily lives, is still fascinating us at the nanoscale. In PNAS, Geng et al. (1) manage to synthesize ordered arrays of atom-thick hexagonal microplatelets consisting of sp hybridized carbon. Since the discovery of C60 (Buckminsterfullerene; an icosahedral carbon cage molecules with diameter of approximately 0.7 nm) (2), carbon nanoscience emerged, and it was witnessed that nanoscale carbon materials possessed different physicochemical properties compared with the well known bulk allotropes of carbon: graphite and diamond. During the past 20 y, other novel carbon materials have been synthesized and intensively studied: carbon nanotubes (3–5) and graphene (6, 7). Graphene consists of an atom-thick sp hybridized carbon sheet, in which each carbon atom is bonded to three carbon atoms, thus forming a hexagonal framework with bond lengths of 1.42 Å. This 2D hexagonal sheet has shown outstanding properties compared with other forms of carbon. Graphene exhibits a room-temperature quantum Hall effect (8), it is a semimetal, and it has an extremely high room temperature carrier mobility (at least two orders of magnitude greater than that of silicon) (9). Graphene could also exhibit extremely high thermal conductivity values ranging from (2.50 ± 0.44)×10 to (5.30 ± 0.48)×10 W/m·K (10, 11). From the mechanical standpoint, graphene is very robust and exhibits a Young modulus of approximately 1 TPa (12). It is noteworthy that the aforementioned properties of graphene systems strongly depend on, for example, the degree of crystallinity (e.g., domain size of crystalline domains and types/number of defects), edge morphology (atomically smooth edges or rough edges), and number of stacking layers (e.g., bilayer, trilayer). Individual graphene sheets were first isolated by using a repeated peeling “scotch-tape” method (6). However, alternative methods involving the thermal decomposition of SiC (13), and chemical vapor deposition (CVD) of CH4 on Ni (14) and Cu (15) films, have been used to synthesize large area graphene sheets. During CVD growth, the carbon precursor (e.g., CH4) decomposes primarily on metallic crystalline imperfections that act as nucleation sites, and these are also responsible for diffusing carbon on the metal surface. During growth, carbon atoms keep precipitating on the growing domains (Fig. 1A), and these graphene domains eventually meet (Fig. 1A) and form a uniform single layer exhibiting grain boundaries containing pentagons and heptagons (16). Parameters such as substrate type (Cu, Pt, Ni), the carrier gas flow (Ar, H2), and the carbon source (e.g., CH4) are crucial to control the number of stacking layers and the morphologies of graphene domains. Geng et al. (1) use a modified CVD approach that is able to produce symmetric arrays of single-layered hexagonal graphene microplatelets by using liquid Cu as a substrate, instead of solid Cu. In particular, the authors used high temperatures (1,020–1,080 °C) to convert a 25-μm-thick Cu foil (i.e., solid) into liquid droplets and observed that these hexagonal platelets were mainly monolayers and tended to arrange symmetrically on the droplet upon specific flow rates and CVD time. For example, as time increased, the interplatelet distance decreased and the platelet arrays became more ordered, with an almost perfect edge-to-edge alignment (Fig. 1B). The authors claim that translations and rotations of the growing graphene platelets (Fig. 1C) occur because of the presence of the molten Cu surface, responsible for achieving the self-assembly of the hexagonal platelets (Fig. 1 B and D). The authors propose a mechanism based on the fast precipitation of carbon atoms and the quick assembly of the growing hexagonal platelets on molten droplets. However, the mechanism is still far from clear and further experiments are required to fully understand the process. For example, it may be possible to have the coexistence of solid and liquid Cu phases, and this coexistence would cause instabilities that could trigger nucleation and growth of the graphene platelets. Alternatively, it might be possible that a very fast cooling rate results in the self-fracture of the graphene precipitated on the molten Cu, and the observed patterns are related to the cooling conditions and the crack propagation (17). The method Geng et al. (1) describe appears to be a single-step CVD method able to control the growth of hexagonal graphene micropatterns. In this context, it is important to mention that other authors have used other top-down routes to produce graphene arrays that include the patterning of regular patterned arrays using e-beam lithography in conjunction with of oxygen plasma etching (18). In addition, Fig. 1. (A and B) SEM images in which dark and bright parts represent the graphene platelets and the Cu surface, respectively. Changing the synthesis temperature and CH4 flow rates (A) shows an average plate size of approximately 50 μm in diameter, approaching a more perfect packing. [Reproduced from Geng, et al. (1)]. (B) Nearly perfect 2D lattice of graphene platelets obtained by Geng et al. (1). [Reproduced fromGeng, et al. (1)]. (C) Molecularmodel of a hexagonal graphene platelet and (D) molecular model of an ordered array of hexagonal graphene platelets (courtesy of F. López-Urías).