Processing graphene devices

Six processing steps for graphene devices: annealing away PMMA residue, surface and substitutional chemical doping, ALD oxide on pristine and PTCA-functionalised graphene, ionic-liquid gating, one-dimensional edge contacts to hBN-encapsulated graphene, and the block-copolymer nanomesh.

1. Clean: anneal: PMMA physisorbed on graphene decomposes in two steps of weight loss on annealing and cannot be removed entirely at a temperature graphene survives (Lin et al. 2012); residue stays on graphene but almost disappears from hBN and MoS₂ (Park et al. 2015); hBN/graphene/hBN stacks are annealed at 350 °C in ultra-high vacuum for 1 h (Xu et al. 2018). 2. Dope: Atoms swapped into the lattice (substitutional doping) or molecules on the surface that exchange charge with it (surface charge transfer); stable p-type doping is well established, n-type harder because most donors are unstable (Liu, Wei & Hu 2025); NO₂ is a strong acceptor (Wehling et al. 2008). 3. Gate: ALD oxide: On pristine graphene ALD oxide grows only on edges and defect sites; after perylene tetracarboxylic acid (PTCA) functionalisation the coating is uniform and ultrathin, a route to thin high-κ gate dielectrics (Wang, Tabakman & Dai 2008). 4. Gate: ionic liquid: Electric double-layer transistors tune the carrier density of graphene continuously to beyond 10¹⁴ cm⁻² (Ye et al. 2011); the Fermi-level shift is set by the quantum capacitance in series with the double layer (George et al. 2018); the ions’ Coulomb scattering limits the mobility (Petach et al. 2017). 5. Edge contacts: Only the one-dimensional edge of the graphene is metallised, which outperforms surface contacts (Wang et al. 2013); steps as Xu et al. (2018) give them: a PC/PDMS stamp picks up the top hBN and the graphene, lands them on the bottom hBN, the stack is annealed, etched and contacted with Cr/Au. 6. Nanomesh: Block-copolymer lithography with P(S-b-MMA): periodicities of about 39 and 27 nm and necks down to about 5 nm; on–off ratio more than 100 at necks of ≈ 7 nm (Bai et al. 2010).

Starting materials: Transferred CVD or exfoliated graphene, hBN flakes. Product: Clean, doped, gated, contacted and patterned graphene devices.

Lin et al., “Graphene annealing: how clean can it be?”, Nano Lett. 2012 (doi:10.1021/nl203733r); Liu, Wei & Hu, “A review on n-type chemical doping of graphene films: preparation, characterization and applications”, Nanoscale 2025 (doi:10.1039/d5nr02633d); Wang, Tabakman & Dai, “Atomic layer deposition of metal oxides on pristine and functionalized graphene”, J. Am. Chem. Soc. 2008 (doi:10.1021/ja8023059); Ye et al., “Accessing the transport properties of graphene and its multilayers at high carrier density”, PNAS 2011 (doi:10.1073/pnas.1018388108); Wang et al., “One-dimensional electrical contact to a two-dimensional material”, Science 2013 (doi:10.1126/science.1244358); Bai et al., “Graphene nanomesh”, Nat. Nanotechnol. 2010 (doi:10.1038/nnano.2010.8); Park et al., “Effective characterization of polymer residues on two-dimensional materials by Raman spectroscopy”, Nanotechnology 2015 (doi:10.1088/0957-4484/26/48/485701); Xu et al., “Spin inversion in graphene spin valves by gate-tunable magnetic proximity effect at one-dimensional contacts”, Nat. Commun. 2018 (doi:10.1038/s41467-018-05358-3); Wehling et al., “Molecular doping of graphene”, Nano Lett. 2008 (doi:10.1021/nl072364w); George et al., “Probing the electric double-layer capacitance in a Keggin-type polyoxometalate ionic liquid gated graphene transistor”, Phys. Chem. Chem. Phys. 2018 (doi:10.1039/c8cp02307g); Petach et al., “Disorder from the bulk ionic liquid in electric double layer transistors”, ACS Nano 2017 (doi:10.1021/acsnano.7b03864).

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