  We investigated the growth of twisted graphene on graphene/silicon carbide
(SiC-G) templates by metal-free chemical vapor deposition (CVD) through a
sequential thermal (ST) process, which exploits the ultraclean surface of SiC-G
without exposing the surface to air before CVD. By conducting control
experiments with SiC-G templates exposed to air (AirE process), structural
analysis by atomic force microscopy revealed that the nucleation density of CVD
graphene (CVD-G) was significantly suppressed in the ST process under the same
growth condition. The nucleation behavior on SiC-G surfaces is observed to be
very sensitive to carbon source concentration and process temperature. The
nucleation on the ultraclean surface of SiC-G prepared by the ST process
requires higher partial pressure of carbon source compared with that on the
surface by the AirE process. Moreover, analysis of CVD-G growth over a wide
temperature range indicates that nucleation phenomena change dramatically with
a threshold temperature of 1300{\deg}C, possibly due to arising of etching
effects. The successful synthesis of twisted few-layer graphene (tFLG) was
affirmed by Raman spectroscopy, in which analysis of the G' band proves a high
ratio of twisted structure in CVD-G. These results demonstrate that metal-free
CVD utilizing ultraclean templates is an effective approach for the scalable
production of large-domain tFLG that is valuable for electronic applications.
