Abstract
A facile transfer process for transition metal dichalcogenide WS2 flakes is reported and the effect of the underlying substrate on the flake properties is investigated using Raman spectroscopy. The flakes are transferred from their growth substrate using polymethyl methacrylate (PMMA) and a wet etch to allow the user to transfer the flakes to a final substrate using a microscope and micromanipulator combined with semi-transparent Kapton tape. The substrates used range from insulators such as industry standard high-k dielectric HfO2 and “green polymer” parylene-C, to conducting chemical vapor deposition (CVD) grown graphene. Raman spectroscopy is used first to confirm the material quality of the transferred flakes to the substrates and subsequently to analyze and separate the effects arising from material transfer from those arising from interactions with the substrate. We observe changes in the Raman spectra associated with the interactions between the substrates in the flakes. These interactions affect both in-plane and out-of-plane modes in different ways depending on their sources, for example strain or surface charge. These changes vary with final substrate, with the strongest effects being observed for WS2 transferred onto graphene and HfO2, demonstrating the importance of understanding substrate interaction for fabrication of future devices.
Highlights
Two dimensional transition metal dichalcogenide (TMD) materials are an important addition to the two-dimensional material family as they fill in the gap between conducting graphene and insulating boron nitride
30 minutes and again, using a glass slide, is removed from the water and allowed to float on top of the glass slide. (e) A glass slide with double-sided Kapton tape is placed and pressed over the floating polymethyl methacrylate (PMMA) to secure it to the tape. (f) The glass slide with the PMMA is aligned over a target substrate secured to a separate glass slide and has two Kapton tape pillars that attach the two slides together upon contact. (g) The desired flake during alignment is visible under the optical microscope through the glass, tape, and PMMA and is outlined in red
The previously reported aspects are the etching of SiO232,37,45,46 to remove the flakes from the growth substrate and the use of micromanipulation of flakes on PMMA under a microscope[44,47,48,49,50]
Summary
Two dimensional transition metal dichalcogenide (TMD) materials are an important addition to the two-dimensional material family as they fill in the gap between conducting graphene and insulating boron nitride. Monolayer tungsten disulfide (WS2) in particular, with a direct band gap of 2.1 eV1, is interesting due to its predicted strong spin orbit coupling in the single layer[2,3] and potential for use in optoelectronics with a measured photoluminescence that is stronger than MoS24,5. Each of these applications will require the use of different substrates that have the potential to enhance or degrade the native properties of the WS2 flakes. In MoS2 and WSe214, the choice of substrate has been shown to have a significant impact on the Raman and properties of the material, further motivating our study of these effects on WS2
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