  Graphene was deposited on a transparent and flexible substrate and tensile
strain up to ~0.8% was loaded by stretching the substrate in one direction.
Raman spectra of strained graphene show significant redshifts of 2D and G band
(-27.8 cm^-1 and -14.2 cm^-1per 1% strain, respectively), because of the
elongation of the carbon-carbon bonds. This indicates that uniaxial strain has
been successfully applied on graphene. We also proposed that by applying
uniaxial strain on graphene, tunable bandgap at K point can be realized. First
principle calculations predicted a bandgap opening of ~300 meV for graphene
under 1% uniaxial tensile strain. The strained graphene provides an alternative
way to experimentally tune the bandgap of graphene, which would be more
efficient and more controllable than other methods that are used to open
bandgap in graphene. Moreover, our results suggest that the flexible substrate
is ready for such strain process and Raman spectroscopy can be used as an
ultra-sensitive method to determine the strain.
