Abstract
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 MoS2 transferred using PDMS. We show that a UV-ozone pre-cleaning of the PDMS surface before exfoliation significantly reduces organic residues on transferred MoS2 flakes. An additional 200 ◦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.
Highlights
Two-dimensional (2D) materials like graphene, hexagonal boron nitride, transition metal dichalcogenides (TMDCs), etc., have gained immense attention in recent years due to the wealth of novel fundamental properties and fascinating physical phenomena exhibited by them [1, 2]
Occurring MoS2 crystals purchased from SPI Supplies were exfoliated on commercial PDMS films (Gel-Film® PF-40-X4 sold by Gel-Pak) using a blue tape (BT-150E-KL)
We have demonstrated that PDMS residues as well as compressive strain can be present in 2D materials exfoliated onto and transferred from PDMS stamps
Summary
Two-dimensional (2D) materials like graphene, hexagonal boron nitride (hBN), transition metal dichalcogenides (TMDCs), etc., have gained immense attention in recent years due to the wealth of novel fundamental properties and fascinating physical phenomena exhibited by them [1, 2]. Among the many available techniques for stacking 2D materials [11,12,13,14,15,16], one approach that has become common recently is to mechanically exfoliate bulk 2D crystals onto a stamp made of a viscoelastic material, such as poly-dimethylsiloxane (PDMS), bring them in contact with a desired substrate and slowly detach the stamp to leave the 2D flakes behind [17, 18] This procedure is very versatile, deterministic and fairly simple to perform, not much attention has been paid to the surface cleanliness of the flakes transferred this way
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