  The observation of novel physical phenomena such as Hofstadter's butterfly,
topological currents and unconventional superconductivity in graphene have been
enabled by the replacement of SiO$_2$ with hexagonal Boron Nitride (hBN) as a
substrate and by the ability to form superlattices in graphene/hBN
heterostructures. These devices are commonly made by etching the graphene into
a Hall-bar shape with metal contacts. The deposition of metal electrodes, the
design and specific configuration of contacts can have profound effects on the
electronic properties of the devices possibly even affecting the alignment of
graphene/hBN superlattices. In this work we probe the strain configuration of
graphene on hBN contacted with two types of metal contacts, two-dimensional
(2D) top-contacts and one-dimensional (1D) edge-contacts. We show that
top-contacts induce strain in the graphene layer along two opposing leads,
leading to a complex strain pattern across the device channel. Edge-contacts,
on the contrary, do not show such strain pattern. A finite-elements modelling
simulation is used to confirm that the observed strain pattern is generated by
the mechanical action of the metal contacts clamped to the graphene. Thermal
annealing is shown to reduce the overall doping whilst increasing the overall
strain, indicating and increased interaction between graphene and hBN.
Surprisingly, we find that the two contacts configurations lead to different
twist-angles in graphene/hBN superlattices, which converge to the same value
after thermal annealing. This observation confirms the self-locking mechanism
of graphene/hBN superlattices also in the presence of strain gradients. Our
experiments may have profound implications in the development of future
electronic devices based on heterostructures and provide a new mechanism to
induce complex strain patterns in 2D materials.
