Control and characterization of individual grains and grain boundaries in graphene grown by chemical vapour deposition

Abstract

The strong interest in graphene has motivated the scalable production of high-quality graphene and graphene devices. As the large-scale graphene films synthesized so far are typically polycrystalline, it is important to characterize and control grain boundaries, generally believed to degrade graphene quality. Here we study single-crystal graphene grains synthesized by ambient chemical vapour deposition on polycrystalline Cu, and show how individual boundaries between coalescing grains affect graphene's electronic properties. The graphene grains show no definite epitaxial relationship with the Cu substrate, and can cross Cu grain boundaries. The edges of these grains are found to be predominantly parallel to zigzag directions. We show that grain boundaries give a significant Raman 'D' peak, impede electrical transport, and induce prominent weak localization indicative of intervalley scattering in graphene. Finally, we demonstrate an approach using pre-patterned growth seeds to control graphene nucleation, opening a route towards scalable fabrication of single-crystal graphene devices without grain boundaries.

Structure of graphene and its disorders: a review

“…Pronounced D peak intensities were also observed at the grain edges, consistent with previous Raman studies [94,95]. The edge orientations of graphene grains can be determined by comparing their TEM images in real space with the corresponding selected area electron diffraction (SAED) patterns (see the supplementary information in Ref [78]. ).…”

“…As shown in Figure 6(b), Raman mapping of the D peak intensity can provide a clear identification of the locations of grain boundaries between two coalesced grains, which cannot be visualized by simply using SEM or atomic force microscopy (AFM). The few isolated spots displaying relatively large D peak intensities are suggested to be the nucleation centers where the CVD-growth of graphene is initiated [78]. Pronounced D peak intensities were also observed at the grain edges, consistent with previous Raman studies [94,95].…”

“…Figure 6(d) shows a large-scan-area STM topography image of a graphene grain near a hexagonal corner, and the atomic-resolution STM image in Figure 6(e) is taken from the area marked by a black square in Figure 6(d). By comparing the grain edges (white dashed lines) in Figure 6(d) with the crystal directions (zigzag versus armchair) in Figure 6(e), it is confirmed that the edges of CVD derived hexagonal grains are mostly bounded by zigzag terminations.

10.1080/14686996.2018.1494493-F0006Figure 6.(a) SEM image of an annealed Cu foil taken out of the furnace after graphene growth [78]. (b) Intensity map of D band for two coalesced graphene grains [78].

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“…Zigzag and armchair are two main types of edges along the crystallographic directions in graphene. In recent years, extensive studies were carried out to investigate the relative stability of these two edge orientations and the edge orientation-dependent physics for mechanically exfoliated monolayer graphene flakes [72–74], epitaxial graphene islands [75–77], CVD derived graphene grains [78], and graphene nanoribbons (GNRs) [79–81]. …”

“…These GBs can impede electron transport [92] and degrade the mechanical properties of the resulting films [93]. Figure 6(a) indicates the formation of isolated grains and merged grains at the early stage of ambient CVD growth of graphene on Cu substrates [78]. As shown in Figure 6(b), Raman mapping of the D peak intensity can provide a clear identification of the locations of grain boundaries between two coalesced grains, which cannot be visualized by simply using SEM or atomic force microscopy (AFM).…”