Structural and topological features of graphene have been investigated widely in the (scanning) transmission electron microscope and include: grain structure [1], layer number and mis-stacking [2], dislocations [3] and out of plane buckling [4, 5]. Here we explore new insights offered by scanning electron diffraction (SED), including quantitative analysis of crystal orientation and local strain. SED involves scanning the electron beam across a specimen and recording a diffraction pattern at each point. This provides a four-dimensional (4d) dataset combining real and reciprocal space information with nanoscale spatial resolution [6]. SED can be performed over areas of a few square micrometres, and the rich 4d data can be analysed using a number of versatile schemes, as described below. This automated analysis enables numerous regions to be considered, an example of which is shown (Fig.1) from a graphene sample grown by chemical vapour deposition on copper [7].

‘Diffraction images’ can be formed by plotting the intensity of a particular sub-set of pixels in each diffraction pattern as a function of probe position to elucidate any variations in the diffraction condition. In Fig.1a, integration windows are selected around a particular set of first-order ((1,0)-type) and second-order ((1,1)-type) reflections to yield the ‘virtual’ dark field images in Fig.1b. These images reveal the local grain structure, in this case a grain in the lower right area of the map. They also show light/dark fringes associated with a small island (arrowed) as well as a fold (starred). This contrast can be attributed to deviation from perfect stacking between the island and the underlying graphene grain, or between layers in the fold. The contrast is understood in terms of variations in the interference condition for electrons scattered from atoms in each of the layers as their relative position varies spatially [5]. The most notable benefits of SED lie in further computational analysis. Orientation images can be produced by matching each diffraction pattern to a library of simulated patterns to automatically map the grain structure and determine the local orientation. All grains are then revealed, and the disorientation across grain boundaries can be determined (Fig. 1c). Strain and small orientation variations are also of considerable importance, and can be mapped with SED by comparing each pattern to an unstrained reference. Fig.1d shows up to 3% strain around the fold, as well as the rotation associated with the ~2º small angle grain boundary. Our approach can thus provide a comprehensive ‘crystal cartography’ of layered materials, paving the way for thorough understanding and exploitation of their unique structure and topology.

[1] Huang, P. et al., Nature, 2011, 496, 389-392

[2] Brown, L. et al., Nano Lett., 2012, 12, 1609-161

[3] Butz, B. et al., Nature 2014, 505, 533-537

[4] Yazyev, O. V., et al, Nature Nanotechnology, 2014, 9, 755-767

[5] Ovid’ko, I.A., Rev. Adv. Mater. Sci., 2012, 30, 201-224

[6] Moeck, P. et al., Cryst. Res. Technol., 2011, 46, 589-606

[7] Bae, S. et al., Nature Nanotechnology, 2010, 5, 574-578

We acknowledge funding from the EU Graphene flagship, the ERC (291522-3DIMAGE), ERC Hetero2D, the EC (312483-ESTEEM2), a Vice Chancellor’s award from the University of Cambridge, a Junior Research Fellowship at Clare College, a Royal Society University Research Fellowship, the Cambridge NanoDTC and GrapheneCDT.

Figures:

Figure 1: SED analysis of CVD graphene. (a) Example diffraction pattern, with integration windows (labelled 1-12) used to produce the ‘diffraction images’ in (b). (c) Orientation image coloured as an inverse pole, showing the structure of all grains in the mapped region and average disorientation across grain boundaries. (d) Strain maps (θ in radians) covering the grains labelled 0º and 2º in (c). A positive rotation is defined in the anti-clockwise sense. Scale bars = 300 nm.

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EMC Abstracts - https://emc-proceedings.com/abstract/structure-and-topology-of-chemical-vapour-deposited-graphene-by-scanning-electron-diffraction/