Figure 6a,b show the MoS2 flakes before and after PMMA transfer. Compared to the as-grown MoS2, the PMMA-transferred MoS2 presents remarkable resist residues although the sample has been cleaned with acetone for long time. It has been reported that some polymeric residues are still observed on the surface of the transferred graphene when the PMMA-mediated transfer method is applied28,29,30,31,32. The free radicals form during PMMA cleaning process and result in the difficulty of completing removing PMMA residuals28. Long and heavy molecular fragments can attach to these radical sites and interact with the adjacent polymer chain. Several techniques such as chloroform treatments29, plasma exposure30 and annealing under gaseous (H2 and Ar) or high vacuum conditions31,32 are used to remove polymer contaminants. Though these techniques achieved some degrees of success, they are far from satisfactory for obtaining clean surface because the process flows are complicated and suffer from process specific drawbacks33. The harsh environment required in these techniques has uncertain influence on monolayer MoS213, which is subject to further investigation. These polymeric residuals will degrade the performance of the electronic and optical devices based on MoS2. In contrast, Fig. 6c,d present the typical optical images of the as-grown and TRT transferred MoS2 flakes, respectively. There is no significant difference between the as-grown and the TRT transferred samples. The surfaces of the samples are clean, indicating the absence of observable resist residues. The Cu thin film introduced between MoS2 and TRT prevents the MoS2 from directly exposing to the glue. Compared to polymer, Cu thin film is relatively easier to be completely etched, which has been widely adopted for transferring graphene34. The EDX spectrum and XPS spectra in Figure S5 clearly show that no Cu residual exists after the etching process.
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