950 pmma a4

1L- and 2L-WS₂ were mechanically exfoliated from bulk WS₂ crystals (2D Semiconductors Inc.) onto the Si wafer with a 90-nm-thick oxide. The number of layers was confirmed by using a combination of photoluminescence and Raman spectroscopy (55 (link)). Monolayer graphene film was grown on copper (Cu) foils at atmospheric pressure using CVD (56 ). The CVD-grown graphene used in this work was polycrystalline with an average grain size of ~15 μm. The heterostructures were assembled by depositing CVD-grown graphene on top of 1L- and 2L-WS₂ using an alternative poly(methyl methacrylate) (PMMA) transfer process to minimize aqueous solution at the graphene-WS₂ interface (57 (link), 58 ). The CVD-grown graphene on Cu foil was spin-coated (3000 rpm) with 950PMMA-A4 (MicroChem). PMMA-coated graphene was adhered to a polymer frame with a hole at the center and suspended by Cu etching. Residual etchant was diluted with deionized water. The PMMA/graphene membrane was gently blown dry with nitrogen and then brought into contact with WS₂ layers followed by baking at around 350 K for 5 min to promote adhesion between graphene and WS₂. Finally, the heterostructure was annealed at 420 K in vacuum of 10⁻⁵ torr for 2 hours followed by natural cooling. The orientation of WS₂ and graphene layers is not aligned in any particular way in the momentum space.

Fabrication of 3D microscale structures in PI began with spin coating a thin sacrificial layer of poly(methyl methacrylate) (∼200 nm in thickness; 950 PMMA A4, MicroChem, USA), followed by spin coating a layer of PI (∼3 to 8 μm in thickness; paa1002, Furunte) on a silicon wafer. Electron beam evaporation allowed deposition of a thin layer of copper (∼100 nm in thickness) or Si (50 nm)/Ni (50 nm). Photolithography and wet etching defined desired patterns for the metal (Cu or Ni) layer, and inductively coupled plasma (ICP) etching defined patterns for the PI layer or bilayer of Si/PI. For 2D precursor structures in PI/Si/Ni, the Ni layer was removed by wet etching. Immersion in acetone to dissolve the underlying PMMA layer yielded released 2D precursor structures that were then retrieved by a PDMS stamp and transferred to a water-soluble tape (PVA; Aquasol Co., USA). The remaining process of the 3D assembly followed similar procedures of 3D microscale structures in SU-8 described above. A schematic illustration of the fabrication process appears in fig. S30.

Raman characterization was carried on graphene transferred onto the SiO₂/Si wafer by the wet-transfer method. The N-SLG/Cu was spin-coated with a thin poly(methyl methacrylate) (PMMA) layer (950 PMMA A4, 4% in anisole; MicroChem Corp.). After the coating, the PMMA/N-SLG/Cu was annealed at 60°C for 30 min to remove the solvent. Then, the PMMA/N-SLG/Cu was floated on a 20% Na₂S₂O₈ solution to remove the Cu foil. Following this, the floating film was transferred to deionized water to rinse the residual etchant and was scooped by SiO₂/Si wafer. To increase the bonding of PMMA/N-SLG, the sample was annealed at 150°C and then 190°C for 10 min. Last, PMMA was removed by acetone leaving N-SLG on the SiO₂/Si wafer for Raman characterization.
Single-point data collection and mapping were performed using Renishaw micro-Raman spectroscope equipped with a blue laser (λ_L = 457 nm, E_L = 2.71 eV) and a green laser (λ_L = 532 nm, E_L = 2.33 eV). Analysis of the Raman data was carried out using MATLAB. For calculation of the D and the G peak height, the background was subtracted from the Raman data using the least-squares curve fitting tool (lsqnonlin).