▎ 摘　　要

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. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim