  By mechanically distorting a crystal lattice it is possible to engineer the
electronic and optical properties of a material. In graphene, one of the major
effects of such a distortion is an energy shift of the Dirac point, often
described as a scalar potential. We demonstrate how such a scalar potential can
be generated systematically over an entire electronic device and how the
resulting changes in the graphene work function can be detected in transport
experiments. Combined with Raman spectroscopy, we obtain a characteristic
scalar potential consistent with recent theoretical estimates. This direct
evidence for a scalar potential on a macroscopic scale due to deterministically
generated strain in graphene paves the way for engineering the optical and
electronic properties of graphene and similar materials by using external
strain.
