Growth of twisted bilayer graphene through two-stage chemical vapor deposition.

We investigate growth of twisted bilayer graphene through a two-stage chemical vapor deposition (CVD). Exploiting the synergetic nucleation and growth dynamics involving carbon sources from the residual carbon impurities in Cu bulk and gaseous CHx, sub-millimeter-sized single crystalline graphene grains with multiple merged adlayer grains formed underneath are grown on Cu substrate. The distribution of the twist angles is investigated through a computer algorithm utilizing spectral features from micro-Raman mapping. Besides the more thermodynamically stable AB-stacking (AB-BLG) or large angle (> 15°) decoupled bilayer graphene (DC-BLG) configurations, there are some bilayer regions that contain specific twist angles (3~8°, 8~13°, and 11-15°) (termed as TBLG). The statistics show no TBLG formation for BLG with single nucleation center. The formation probability of TBLG is strongly dependent on the relative orientation of merging adlayer grains. Significant defects are found at the grain boundaries formed in AB-DC merging event without creating TBLG domain. The areal fraction of TBLG increases as H2/CH4 ratio increases. The growth mechanism of TBLG is discussed in light of the interactions between the second layer grains with consideration of strain generation during merging of adlayers.