Simultaneous, controllable synthesis of large-surface-area graphene and graphene quantum dots (GQDs) typically relies on multi-step, chemically intensive routes. Here we report a one-step, chemical-free approach based on low-pressure hydrogen-assisted thermal decomposition of silicon carbide (SiC), without supplying any external carbon precursor, which yields graphene forests (GFs) decorated with GQDs in a single heating process. Notably, the morphology is dominantly controlled by the hydrogen f