Paraffin-enabled graphene transfer

Abstract

The performance and reliability of large-area graphene grown by chemical vapor deposition are often limited by the presence of wrinkles and the transfer-process-induced polymer residue. Here, we report a transfer approach using paraffin as a support layer, whose thermal properties, low chemical reactivity and non-covalent affinity to graphene enable transfer of wrinkle-reduced and clean large-area graphene. The paraffin-transferred graphene has smooth morphology and high electrical reliability with uniform sheet resistance with ~1% deviation over a centimeter-scale area. Electronic devices fabricated on such smooth graphene exhibit electrical performance approaching that of intrinsic graphene with small Dirac points and high carrier mobility (hole mobility = 14,215 cm² V^-1 s^-1; electron mobility = 7438 cm² V^-1 s^-1), without the need of further annealing treatment. The paraffin-enabled transfer process could open realms for the development of high-performance ubiquitous electronics based on large-area two-dimensional materials.

Fig. 1

Our paraffin-enabled graphene transfer method. a Schematics showing the process of paraffin-assisted graphene transfer. b Schematics showing the effect of paraffin’s thermal expansion on graphene wrinkle. c Photographs of a typical paraffin-supported graphene film floated on water at different temperatures as indicated confirming that the paraffin layer is still in solid state at ~40 °C

Fig. 2

Electrical properties comparison of graphene transferred with PMMA and paraffin support layers. a Room-temperature hall mobility versus carrier concentration of graphene films transferred with different support layers as indicated. Each data were extracted from a 15 × 15 mm² graphene transferred on Si/SiO₂ substrate using Hall measurement in the presence of 2800 Gauss magnetic field. b Sheet resistance distribution of graphene films transferred with different support layers, obtained via Hall measurement. c, d Spatial sheet resistance maps of a graphene film transferred with c PMMA and d paraffin support layers. Eight hundred sheet resistance values were measured using four-point probe measurement over a graphene area of 40 × 20 mm², with 1 mm step size in both x- and y-directions. The paraffin-transferred graphene film exhibits much lower and homogenous sheet resistance than that of PMMA. In figure a, b, blue-color round-shaped symbols represent paraffin-transferred graphene, and red-color square-shaped symbols represent PMMA-transferred graphene

Fig. 3

Back-gate electrical measurement results of PMMA- and paraffin-transferred graphene devices. a Optical image of a typical graphene field-effect transistors (FETs) array with increasing channel length. b Transfer characteristics comparison of two field-effect transistors fabricated with PMMA- and paraffin-transferred graphene. The Dirac voltage of device fabricated on paraffin-transferred graphene is much smaller and closer to zero. c Two-terminal field-effect (electron) mobility distribution of graphene FETs fabricated with PMMA- and paraffin-enabled transfer process as a function of channel length. Each average value was extracted from 10 graphene FETs. d Dirac voltage distribution of 100 graphene FETs fabricated with PMMA- and paraffin-assisted transfer. In b–d, error bars indicate standard deviations, blue-color round-shaped symbols represent paraffin-transferred graphene, and red-color square-shaped symbols represent PMMA-transferred graphene

Fig. 4

Materials characterization of the PMMA- and paraffin-transferred graphene on Si/SiO₂ substrate. a, b Typical AFM height profile images of graphene film transferred with a PMMA and b paraffin support layers. c, d Correlation map of the Raman G and 2D peak positions of graphene transferred with c PMMA and d paraffin support layers. A total of 3600 Raman spectra were taken from each type of transferred graphene and the corresponding G peak position (${\mathrm{\omega }}_{\mathrm{G}}$), 2D peak position (${\mathrm{\omega }}_{2\mathrm{D}}$), and 2D peak’s full width at half maximum (Γ_2D) were extracted. The black circle represents the G and 2D peak positions of an intrinsic graphene, where graphene has neither doping nor strain

Fig. 5

Chemical structures, formulae, and isosurfaces of a PMMA dimer and a paraffin molecule. Chemical structures and formulae of a PMMA and b paraffin, and the isosurfaces of the Fukui function for c a PMMA dimer and d a paraffin molecule. Carbon atoms in green, hydrogen atoms in gray, oxygen atoms in red. Yellow isosurfaces represent those regions of the molecule that undergo electrophilic attack upon chemical reaction, and purple represents regions that undergo nucleophilic attack

Fig. 6

Adsorption ability of PMMA radical and paraffin molecule on graphene. a Adsorption of a paraffin molecule and PMMA radical on hexagonal graphene flakes with hydrogen-functionalized edges and a vacancy defect. Only the PMMA radical formed a covalent bond with the defective graphene flake, as verified by b the shared HOMO and LUMO of the bonded structure. Color code in the side view of a: carbon atoms in dark green, hydrogen atoms in gray, oxygen atoms in red. Color code in the top view of a: paraffin in red, PMMA radical in dark blue. Color code in b, the light green corresponds to regions of space where the phase of the wave function is positive, and the blue color corresponds to regions of space where the phase of the wave function is negative