▎ 摘　　要

The growth of high quality graphene layers by chemical vapor deposition (CVD) has been found to strongly depend on growth conditions with results varying greatly from one laboratory to another for nominally identical conditions. We report the results of a systematic investigation of the role of hydrogen and oxidizing impurities present in the gas feedstock during the growth and cooling stages in low-pressure CVD. First, we show that for a partial pressure of oxidizing impurities below 1 ppb, hydrogen is not required for graphene growth from methane. Second, we demonstrate that purified hydrogen does not etch graphene films at typical growth temperatures. Third, a flow of purified hydrogen during cooling counterbalances graphene etching by oxygen, thus protecting the films. Films grown under high purity conditions (low level of oxidizing impurities) exhibit a higher bilayer and multilayer coverage; Surprisingly some of these bi- and multilayer graphene islands are twisted with respect to the first graphene layer as revealed by hyperspectral Raman imaging. Overall, this growth behavior suggests a competitive action between film growth from the carbon precursors and etching by the oxidative species. Our results provide new fundamental insights on the graphene CVD growth, highlighting the important yet indirect role of hydrogen and its major influence on controlling the action of oxidizing impurities on nucleation and etching during the growth process.