Since the single-layer graphene was successfully exfoliated from natural graphite in 2004, graphene has attracted widespread attention with its unique physical and chemical properties. However, the competition between the quality and output of graphene has always been the main problem for its large-scale application. For example, the mechanical peeling method can guarantee the high quality of graphene, but it is difficult to use in industrial production. The reduced graphene oxide method is currently the main choice for the industrial production of graphene, but the chemical/thermal reduction process inevitably causes defects in graphene. Therefore, if a technology can overcome the competitive issues of graphene quality and yield, then it will be the most commercially valuable synthetic method to prepare graphene. Recent studies have shown that peeling off two-dimensional materials with metal films may be the key to obtaining two-dimensional materials with controllable quality and yield.
Jae-Hyun Lee of Ajou University in South Korea, Seok-Kyun Son of Mokpo University in South Korea, Sung Ho Cho of Samsung Display in South Korea and others have proposed a layered industrial exfoliation technology (LEE) for graphene, which can not only obtain up to Large-size graphene in millimeters, and selective thickness control. The metal film evaporated on the graphite will cause tensile stress, so that the exfoliation of the metal film induces the exfoliation of graphene, and the number of exfoliated graphene layers can be adjusted by using different metal films. The author performed detailed spectroscopy and electron transmission measurement analysis on the obtained graphene, and proved that the proposed exfoliation technology simultaneously guaranteed the quality and yield of graphene. This layered industrial peeling can pave the way for the development of large-scale manufacturing processes for graphene and other two-dimensional materials in the electronics and optoelectronic fields. The research was published in "Science Advances" as a paper entitled "Layer-engineered large-area exfoliation of graphene".
In order to exfoliate large-area graphene with a specific number of layers, the author directly thermally evaporates and deposits the metal film on the pre-cracked bulk graphite flakes. The metal film can be used as a stress source for exfoliating graphene, where the tensile stress comes from the lattice constant difference between the graphite film and the metal film. The external bending moment will produce cracks at the boundary of the graphite sheet under the action of tensile stress, and then cause large-area peeling of graphite due to residual tension. The peeling depth depends on the difference between the binding energy of graphite/metal (γmetal-Gr) and the interlayer binding energy of graphite itself (γGr-Gr). The greater the difference, the deeper the peeling depth. The single-layer graphene peeled off by the author on the Au film can exhibit a lateral size of one millimeter without physical defects. It is reported that this is currently the largest single-layer graphene mechanically exfoliated.