Review: Layer-Number Controllable Preparation of High-Quality Graphene for Wide Applications

Graphene, a well-known two-dimensional (2D) material, has sparked broad enthusiasm in both scientific and industrial communities in these years, due to its exceptional electrical, thermal, mechanical, and versatile properties. However, many properties and applications of graphene are layer-number dependent. The preparation of high-quality graphene with controlled layer numbers is full of challenge, since the layer number varies much with the synthesis routes and relevant experimental conditions. Hence, there is an urgent need to improve the layer-number controllability of graphene preparation. Generally, graphene can be prepared by two complementary approaches: 'top-down' and 'bottom-up'. Since they have their own advantages, the recent advances in the layer-number tunable preparation of high-quality graphene are separately studied from the two aspects in this review, especially those dedicated to single parameter. Some effective strategies are discussed in detail, mainly including 1) supercritical-CO2 assisted sonication, electrochemical exfoliation of graphite intercalation compounds, and layer-by-layer thinning with plasma or laser, for 'top-down' graphene; 2) chemical vapor deposition (CVD) on dual-metal substrate, ion-implantation CVD, layer-by-layer CVD, plasma-enhanced CVD, layered-double-hydroxides template-assisted CVD; and 3) graphite-enclosure assisted epitaxial growth and pulsed-magnetron-sputtering assisted physical vapor deposition for 'bottom-up' graphene on various substrates. In addition, the respective advantages of graphene with different layer numbers in properties and applications are also presented. Finally, the contribution concludes with some important perspectives on the remained challenges and future perspectives.