Seeing graphene: a microscopy guide

Six ways to look at graphene in one figure: optical contrast on SiO₂/Si, AFM heights and bubbles on hBN, SEM of CVD domains on copper, the TEM lattice and its diffraction, the STM moiré of graphene on hBN and LEED of epitaxial graphene on SiC.

1. Optical contrast: Colours computed from thin-film interference with n = 2.6 − 1.3i and 0.34 nm per layer; on oxidised silicon a single layer becomes visible (Blake et al. 2007). 2. AFM: Each layer adds graphite’s interlayer spacing, 0.335 nm; tapping mode can add up to about 1 nm (Nemes-Incze et al. 2008); round bubbles of trapped contamination keep an aspect ratio h/R ≈ 0.11 (Khestanova et al. 2016). 3. SEM: Hexagonal domains on commercial Cu foil turned every which way and nucleating in rows (Robertson & Warner 2011; Yang et al. 2012); regions with more layers are darker (Liu et al. 2021; Kondrashov et al. 2021). 4. TEM: Lattice images of the honeycomb, and electron diffraction with spots at d = a√3/2 and a/2 for a = 2.46 Å, the periodicities of 2.13 and 1.23 Å (Meyer et al. 2007). 5. STM: The moiré of graphene on hBN, whose wavelength follows the twist and hBN’s 1.8 % larger lattice (Yankowitz et al. 2012). 6. LEED: On SiC(0001): SiC (1 × 1) spots, graphene (1 × 1) spots turned 30° from them and the (6√3 × 6√3)R30° spots of the coincidence cell (Riedl et al. 2009).

Starting materials: Exfoliated graphene on SiO₂/Si and hBN, CVD graphene on Cu, graphene on SiC(0001). Product: Where the flake is, how thick, its domains, its lattice and its stacking.

Blake et al., “Making graphene visible”, Appl. Phys. Lett. 2007 (doi:10.1063/1.2768624); Nemes-Incze et al., “Anomalies in thickness measurements of graphene and few layer graphite crystals by tapping mode atomic force microscopy”, Carbon 2008 (doi:10.1016/j.carbon.2008.06.022); Meyer et al., “The structure of suspended graphene sheets”, Nature 2007 (doi:10.1038/nature05545); Yankowitz et al., “Emergence of superlattice Dirac points in graphene on hexagonal boron nitride”, Nat. Phys. 2012 (doi:10.1038/nphys2272); Riedl et al., “Quasi-free-standing epitaxial graphene on SiC obtained by hydrogen intercalation”, Phys. Rev. Lett. 2009 (doi:10.1103/PhysRevLett.103.246804); Khestanova et al., “Universal shape and pressure inside bubbles appearing in van der Waals heterostructures”, Nat. Commun. 2016 (doi:10.1038/ncomms12587); Robertson & Warner, “Hexagonal single crystal domains of few-layer graphene on copper foils”, Nano Lett. 2011 (doi:10.1021/nl104142k); Yang et al., “A facile method to observe graphene growth on copper foil”, Nanotechnology 2012 (doi:10.1088/0957-4484/23/47/475705); Liu et al., “Sequential growth and twisted stacking of chemical-vapor-deposited graphene”, Nanoscale Adv. 2021 (doi:10.1039/d0na00982b); Kondrashov et al., “Preparation of copper surface for the synthesis of single-layer graphene”, Nanomaterials 2021 (doi:10.3390/nano11051071).

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