Spin Coating of PVA and PMMA Layer

For patterning on SiO₂/Si substrates and PET substrates, A4 PMMA (MW 495k from MicroChem) in anisole was used for spin coating. A1 PMMA (MW 495k from MicroChem) in anisole was used to pattern electrodes for the MoS₂ photodetector. PVA solution (5% w/v) was prepared by dissolving PVA (Mowiol 4–88, MW ∼ 31 000 from Sigma-Aldrich) in DI water. A layer of PVA was coated on the SiO₂/Si substrate for 30 s at 2000 rpm and heated for 10 min at 150 °C. A layer of PMMA was then coated on the SiO₂/Si substrate for 60 s at 4000 rpm and heated for 10 min at 150 °C. Because of the low glass transition temperature of PET substrates, heating time and temperature after coating PMMA and PVA on PET substrates were adjusted to 70 °C and 20 min.