  Integrating layered two-dimensional (2D) materials into 3D heterostructures
offers opportunities for novel material functionalities and applications in
electronics and photonics. In order to build the highest quality
heterostructures, it is crucial to preserve the cleanliness and morphology of
2D material surfaces that come in contact with polymers such as PDMS during
transfer. Here we report that substantial residues and up to ~0.22% compressive
strain can be present in monolayer MoS$_{2}$ flakes transferred using PDMS. We
show that a UV-ozone pre-cleaning of the PDMS surface before exfoliation
significantly reduces organic residues on transferred MoS$_{2}$ flakes. An
additional 200$^{\circ}$C vacuum anneal after transfer efficiently removes
interfacial bubbles and wrinkles as well as accumulated strain, thereby
restoring the surface morphology of transferred flakes to their native state.
Our recipe is important for building clean heterostructures of 2D materials and
increasing the reproducibility and reliability of devices based on them.
