Strain-induced pseudo-magnetic fields and charging effects on CVD-grown graphene
Atomically resolved imaging and spectroscopic characteristics of graphene grown by chemical vapor deposition (CVD) on copper are investigated by means of scanning tunneling microscopy and spectroscopy (STM/STS). For CVD-grown graphene remaining on the copper substrate, the monolayer carbon structures exhibit ripples and appear strongly strained, with different regions exhibiting different lattice structures and electronic density of states (DOS). In particular, ridges appear along the boundaries of different lattice structures, which exhibit excess charging effects. Additionally, the large and non-uniform strain induces pseudo-magnetic field up to ~ 50 Tesla, as manifested by the integer and fractional pseudo-magnetic field quantum Hall effect (IQHE and FQHE) in the DOS of graphene. In contrast, for graphene transferred from copper to SiO2 substrates after the CVD growth, the average strain on the whole is reduced, so are the corresponding charging effects and pseudo-magnetic fields except for sample areas near topographical ridges. These findings suggest feasible "strain engineering" of the electronic states of graphene by proper design of the substrates and growth conditions.