  The recent emergence of strain gradient engineering directly affects the
nanomechanics, optoelectronics and thermal transport fields in 2D materials.
More specifically, large suspended graphene under very high stress represents
the quintessence for nanomechanical mass detection through unique molecular
reactions. Different techniques have been used to induce strain in 2D
materials, for instance by applying tip indentation, pressure or substrate
bending on a graphene membrane. Nevertheless, an efficient way to control the
strain of a structure is to engineer the system geometry as shown in everyday
life in architecture and acoustics. Similarly, we studied the concentration of
strain in artificial nanoconstrictions (~100 nm) in a suspended epitaxial
bilayer graphene membrane with different geometries and lengths ranging from 10
to 40 micrometer. We carefully isolated the strain signature from micro-Raman
measurements and extracted information on a scale below the laser spot size by
analyzing the broadened shape of our Raman peaks, up to 100 cm-1. We
potentially measured a strong strain concentration in a nanoconstriction up to
5percent, which is 20 times larger than the native epitaxial graphene strain.
Moreover, with a bilayer graphene, our configuration naturally enhanced the
native asymmetric strain between the upper and lower graphene layers. In
contrast to previous results, we can achieve any kind of complex strain tensor
in graphene thanks to our structural approach. This method completes the
previous strain-induced techniques and opens up new perspectives for bilayer
graphene and 2D heterostructures based devices.
