▎ 摘　　要

Chemical vapor deposition (CVD) is an important method to synthesis graphene on a substrate. Recently, Cu became the most popular CVD substrate for graphene growth. Here, we combine electronic structure calculation, molecular dynamics simulation, and thermodynamics analysis to study the graphene growth process on Cu surfaces. As a fundamentally important but previously overlooked fact, we find that carbon atoms are thermodynamically unfavorable on a Cu surface under typical experimental conditions. The active species for graphene growth should thus mainly be CHx instead of atomic carbon. On the basis of this new picture, the nucleation behavior can be understood, which explains many experimental observations and also provides us a guide to improve graphene sample quality.