  There are a number of theoretical proposals based on strain engineering of
graphene and other two-dimensional materials, however purely mechanical control
of strain fields in these systems has remained a major challenge. The two
approaches mostly used so far either couple the electrical and mechanical
properties of the system simultaneously or introduce some unwanted disturbances
due to the substrate. Here, we report on silicon micro-machined comb-drive
actuators to controllably and reproducibly induce strain in a suspended
graphene sheet, in an entirely mechanical way. We use spatially resolved
confocal Raman spectroscopy to quantify the induced strain, and we show that
different strain fields can be obtained by engineering the clamping geometry,
including tunable strain gradients of up to 1.4 %/$\mu$m. Our approach also
allows for multiple axis straining and is equally applicable to other
two-dimensional materials, opening the door to an investigating their
mechanical and electromechanical properties. Our measurements also clearly
identify defects at the edges of a graphene sheet as being weak spots
responsible for its mechanical failure.
