Catalyst-free, self-assembly, and controllable synthesis of graphene flake–carbon nanotube composites for high-performance field emission

Catalyst-free and self-assembled growth of graphene flakes (GFs) on carbon nanotube (CNT) arrays have been realized by using microwave plasma enhanced chemical vapor deposition. The shape of GFs was highly manipulated by adjusting the growth time, C concentration, and microwave power. We qualitatively discussed the nucleation and growth mechanism of GFs based on the growth parameter–GF shape studies. The field emission (FE) properties of graphene flake–carbon nanotube (GF–CNT) composites for different GF shapes were measured and found to be strongly influenced by the GF distribution. The optimal shape of GFs for FE had small scales, sharp edges, and sparse distribution on CNTs. The best FE properties with the optimal shape were observed with a low turn-on electric field of 0.73 V/μm and excellent stability, which are superior to those of the as-grown CNT arrays and GF–CNT composites covered by densely distributed GFs. We consider that the large aspect ratio of CNTs and the unique FE stability of GFs play a synergetic effect on the improved FE properties.