Superstructural defects and superlattice domains in stacked graphene
Introduction
Since the discovery of graphene by May in 1969 [1], [2] and recent resurgence in interest in graphene particularly when electrically isolated [3], there has been a rapidly growing interest in graphene-based multilayer superstructures and particularly in their electronic properties [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. For example, graphene deposited on hexagonal boron nitride (hBN) is reported to behave like a semiconductor [9], [10], [11], [12], and few-layer graphene (FLG) has electronic and optical properties that are reported to depend on stacking order [13], [14], [15], [16], number of layers [17], [18], [19] and twist angles [20], [21], [22], [23], [24], [25]. Pristine single-layer graphene (SLG) and Bernal (AB)-stacked FLG have a zero bandgap and thus behave like a semimetal. In the twisted bi-layer graphene (BLG), however, the electronic band structure reportedly changes due to changes in the interlayer coupling, such as rotation angle dependent Van Hove singularities [20], [21], [22], [23], [24], [25]. Also, the AB-stacked BLG and rhombohedral tri-layer graphene (TLG) are reported to have a bandgap under a perpendicular electric field because the inversion symmetry is broken [15], [16], [26], [27], making them possible candidates for optoelectronic and nanoelectronic applications. The electronic properties of the AA-stacked BLG significantly change in local areas where there are twists as small as 0.1°, as are often generated during their manufacture and are referred to as ‘subtle disorders’ [28], [29]. However, the effect of such minute twists is disregarded in interpreting mesoscopic electrical or optical data, which shows the properties averaged over a large area of the sample [28].
Structural characterization of few-layer graphene has been reported by several methods, such as scanning tunneling microscopy (STM) and spectroscopy (STS) [30], [31], transmission electron microscopy (TEM) [13], [14], [32], [33], [34], [35], [36] and Raman spectroscopy [25], [29], [37]. However, these methods have been used to elucidate either atomic structures in nanometer-range areas, or average stacking structures at the micrometer level. High-resolution characterization of stacking registry and its variation over a large area in a superstructure has, to the best of our knowledge, not yet been reported, and is the focus of the work reported here.
The stacking-induced superlattices of ‘artificially stacked’ BLG, TLG and SLG/hBN heterostructures in real space were obtained from the Moiré pattern (an interference pattern between two mismatched grids) which enabled mapping of twist angle variations between stacked layers at a resolution less than degree. Dark-field transmission electron microscopy (DF-TEM) was performed to visualize Moiré patterns and determine twist angles of graphene sheets. Such measured twist angles were confirmed using scanning electron diffraction in the scanning TEM mode (SED-STEM). Also, atomic-resolution TEM (AR-TEM) image was acquired at grain boundaries with in-plane rotation over 4° using an image Cs corrected TEM operating at a low accelerating voltage of 80 kV.
Section snippets
Graphene and hBN sample preparation
Graphene films were synthesized using chemical vapor deposition (CVD) on 25-μm-thick copper foil (99.8% Alfa Aesar, Ward Hill, MA) [38]. Grain size was controlled by controlling the ratio of H2/CH4, as described in a previous report [39]. After graphene synthesis, the supporting Cu foil was etched away with a Na2S2O8 solution (concentration of 0.1 mg Na2S2O8/1 mL water) overnight. BLG and TLG were fabricated by consecutively direct transferring SLG onto a Quantifoil TEM grid with an array of
Direct structural mapping of stacked BLGs
Fig. 1a shows a SEM image of the large-area BLG fabricated by sequential transfer of monolayer graphene onto a Quantifoil TEM grid, and having random interlayer rotation angles (θinter) in different regions (Fig. S1) [25]. The BLG consists of two SLGs, one with a relatively large grain size (several μm) and the other with a small grain size (a few hundred nm) (Fig. S2). Electron beam contrast differences can be used to clearly identify voids, SLGs and BLGs. DF-TEM images in Fig. 1b and c and an
Conclusion
The comprehensive imaging techniques reported here not only provide the tools to precisely characterize layered 2D structural defects in rotated BLG, but also can be used to investigate multilayer heterostructures composed of 2D crystals, such as graphene, boron nitride and dichalcogenides or others, on all relevant length scales, by analyzing the superlattices. We observed discrete nanosized superlattice domains, inevitably created in artificially stacked BLG and demonstrated their formation
Acknowledgements
This work was supported by Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2012M3A7B4049807). This work was supported by IBS-R019-D1 and IBS-R004-G3 and Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0029714).
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