More ways to make graphene

Six more routes to graphene: hot-wall against cold-wall CVD, plasma-enhanced CVD of vertical graphene, wafer-scale single crystals on Ge(110), nanoribbons by unzipping nanotubes, six ways to reduce graphene oxide, and graphite intercalation stages for exfoliation.

1. Hot wall or cold wall: Hot-wall CVD heats Cu foil to about 1000 °C in a quartz tube furnace and can take a few hours; resistive-heating cold-wall CVD heats only the foil and made graphene of comparable quality 100 times faster and at 99 % lower cost, a run taking about 20 min against typically more than 70 min (Bointon et al. 2015). 2. Plasma CVD: In microwave PECVD on Si(100), higher temperatures and plasma powers promoted vertical graphene and lowering them gave planar graphene (Meškinis et al. 2022); plasma power sets how much vertical graphene branches, temperature the thickness of each flake and time its height (Ma et al. 2019). 3. Single crystal on Ge(110): The twofold symmetry of hydrogen-terminated Ge(110) aligns many seeds in one direction and they merge into single-crystal monolayer graphene; the weak interaction allows etch-free dry transfer and reuse of the Ge (Lee et al. 2014). 4. Unzipping nanotubes: A solution-based oxidative process cuts the side walls of multiwalled nanotubes lengthwise into nanoribbons in nearly 100 % yield; the ribbons are highly water-soluble and chemical reduction restores their conductivity (Kosynkin et al. 2009). 5. Reducing GO: Chemical and thermal reduction are the most used at industrial scale; chemically reduced rGO typically reaches O/C 0.1–0.2 and 10–100 S/cm; plasma reduction at room temperature removes about as much oxygen as thermal reduction at 1000 °C (Li et al. 2024). 6. Intercalation stages: A stage-n compound has one intercalant layer in every n-th interlayer; low-stage (≤ 2) compounds are key to high-yield exfoliation (Zhang et al. 2023); stage-2 LiC₁₂ exfoliated to bilayer graphene with 78 % selectivity (Zhu et al. 2024).

Starting materials: Methane and Cu foil, H-terminated Ge(110) on Si, multiwalled nanotubes, graphene oxide, graphite. Product: CVD films, vertical graphene, single crystals, nanoribbons, rGO and exfoliated flakes.

Bointon, Barnes, Russo & Craciun, “High quality monolayer graphene synthesized by resistive heating cold wall chemical vapor deposition”, Adv. Mater. 2015 (doi:10.1002/adma.201501600); Meškinis et al., “The direct growth of planar and vertical graphene on Si(100) via microwave plasma chemical vapor deposition: synthesis conditions effects”, RSC Adv. 2022 (doi:10.1039/d2ra02370a); Lee et al., “Wafer-scale growth of single-crystal monolayer graphene on reusable hydrogen-terminated germanium”, Science 2014 (doi:10.1126/science.1252268); Kosynkin et al., “Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons”, Nature 2009 (doi:10.1038/nature07872); Li, Han, Qiu & Qian, “Plasma-assisted preparation of reduced graphene oxide and its applications in energy storage”, Nanomaterials 2024 (doi:10.3390/nano14231922); Zhang et al., “Producing bilayer graphene oxide via wedge ion-assisted anodic exfoliation: implications for energy and electronics”, ACS Appl. Nano Mater. 2023 (doi:10.1021/acsanm.3c03284); Ma et al., “Experimental investigation on vertically oriented graphene grown in a plasma-enhanced chemical vapor deposition process”, ACS Appl. Mater. Interfaces 2019 (doi:10.1021/acsami.9b00896); Zhu et al., “Scalable synthesis of bilayer graphene at ambient temperature”, J. Am. Chem. Soc. 2024 (doi:10.1021/jacs.4c00975); Mitra & Katiyar, “A review on flash sintering of graphene oxide films: mechanism and recent advances in the field”, Chem. Asian J. 2023 (doi:10.1002/asia.202300369); Xu et al., “Ultrafast epitaxial growth of metre-sized single-crystal graphene on industrial Cu foil”, Sci. Bull. 2017 (doi:10.1016/j.scib.2017.07.005).

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