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Keywords:

  • doping;
  • freestanding graphene;
  • Raman scattering;
  • strain

We report a detailed spatially resolved Raman study on a set of eight pristine freestanding graphene monolayers. While we find, as previously reported, that freestanding graphene is quasi-undoped, our study also reveals that non-negligible built-in strain occurs in these samples. The level of built-in strain varies significantly from one sample to another and can be estimated with accuracy from the correlation of the frequencies of the G and 2D Raman modes. Sample-dependent compressive and tensile strains as high as 0.1% are reported.