Interface quality, one of critical factors, governs the properties of few-layer graphene, which exhibit exclusive electrical and mechanical properties owing to the strong interlayer coupling. However, fabrication of interface-clean few-layer graphene still remains challenging. Conventional stacking techniques often introduce interfacial contamination, such as amorphous carbon and residual impurities, which critically deteriorate the interlayer coupling and compromise the uniformity and reliability of graphene-based devices. Here, we present an active oxygen treatment (AOT) strategy to effectively remove surface impurities and amorphous carbon on graphene before stacking, yielding wafer-scale few-layer graphene with clean interface and controlled layer numbers. As-fabricated few-layer graphene exhibits excellent structural integrity (>95%), flatness (R_a ~ 2.3 nm) and uniformity. The suspended few-layer graphene shows superior mechanical stability due to the clean interface, which remains stable under repeated thermal shocks up to 1200 K, significantly outperforming counterparts assembled via conventional methods. Thermal light emitters based on the suspended few-layer graphene demonstrates strong visible-to-near-infrared emission, with lattice temperature reaches ~900 K and a working lifetime of ~70 min. This work highlights the potential of AOT in advancing the optoelectronic applications of graphene through precise interface engineering.

Bilayer graphene provides a versatile platform for exploring a variety of intriguing phenomena and shows much promise for applications in electronics, optoelectronics, etc. Controlled growth of large-area bilayer graphene is therefore highly desired yet still suffers from a slow growth rate and poor layer uniformity. Meanwhile, graphene wrinkles, including folds and ripples, form during cooling due to the thermal contraction mismatch between graphene and the metal substrates, and have been far from suppressed or eliminated, especially in bilayer graphene, which would greatly degrade the extraordinary properties of graphene. Here we report the ultrafast growth of wafer-scale fold-free bilayer graphene by chemical vapor deposition. Through well-tuning the alloy thickness and strain regulation of the single-crystal CuNi(111)/sapphire, the full coverage of a 2-inch fold-free bilayer graphene wafer via mainly isothermal segregation has been achieved as fast as 30 s. The tensile-strained CuNi(111) film reduces the thermal contraction mismatch and suppresses the formation of graphene folds during cooling, which is directly observed through in situ optical microscopy. The ultraflat bilayer graphene exhibits wafer-scale uniformity in electrical performance and enhanced mechanical property comparable to the exfoliated ones. Our results offer a promising route for large-scale production of bilayer graphene and enable its various applications.