Graphene produced by chemical vapor deposition finds applications in a wide range of technologies. However, the transfer of monolayer graphene from the native substrate (commonly Cu foils) to a desired substrate remains challenging. In this study, we report a solvent-free transfer technique for monolayer graphene using a sublimable supporting coating, cyclododecane (CDD). After spin-coating CDD on graphene grown on Cu foil, we rapidly heat the stack at 80 °C to melt and recrystallize the CDD coating. The recrystallized coating top surface becomes composed of larger and interconnected CDD crystals, which form a mechanically strong coating that takes almost 2 h to sublimate completely. Having a bottom surface that closely conforms to graphene's topography, the CDD coating can effectively support graphene during the whole transfer process without compromising its structural integrity, maintaining its superior electrical and optical properties. In this way, CDD becomes capable of transferring monolayer graphene over square centimeter areas. Unlike traditional polymer-based techniques, after transfer, the CDD coating sublimates completely, without the need of any post-transfer cleaning treatment. Our work shows that the CDD coating can be an alternative transfer medium for the efficient and clean transfer of monolayer graphene. This technique paves the way for the widespread adoption of graphene in various applications, including those not compatible with solvents.

Graphene has been the focus of intensive research in a wide range of applications in electrical, catalytic, biomedical, and energy storage fields,1–4 owing to its remarkable properties, such as electrical and thermal conductivity, mechanical strength, chemical stability, and biocompatibility.1,2,5–7 Chemical vapor deposition (CVD) is one of the promising methods to grow graphene with high-quality and uniformity in a wafer-scale.8–10 CVD graphene is typically grown on catalytic metal foils (e.g., Cu, Ni) and, as such, it needs to be transferred to the desired substrates (e.g., SiO2/Si, polymer) for device manufacturing.11,12 This crucial transfer process needs to be carefully optimized to avoid mechanical damage and impurities,13–16 which can easily degrade the quality and alter the properties of the graphene samples.17,18 To retain the integrity of the graphene film during the etching of the growth substrate and provide mechanical stability during transfer, supporting materials, such as PMMA and PDMS, have been used. Although the PMMA-assisted wet transfer has been widely used owing to minimized defects and impurities over large areas,19–21 the PMMA transfer process leaves behind considerable residue and induces cracks and wrinkles that degrade the electrical and surface properties.18,19,22–24 To avoid formation of defects during the PMMA transfer process, the flexibility and mechanical robustness of the PMMA layer was increased by additionally coating the second PMMA layer.22 In addition, to eliminate the PMMA residue, thermal annealing was used.19 However, it has been known that complete removal of the PMMA residue is still challenging.25 

Atomic-thick films, such as graphene, can be easily damaged by tears and cracks if any strain is induced during the transfer or cleaning processes. Such damage will limit the electrical and mechanical properties of the films, while impurities may introduce unwanted doping through the formation of charge-scattering centers.15,18,22 The transfer issue is currently the most outstanding obstacle to a widespread commercialization of graphene-based films and devices.13,14 Various research groups have been working to optimize the transfer methods, proposing several approaches with their respective advantages and disadvantages.13,14,26–33 In general, all polymer-assisted wet transfer approaches entail a final cleaning step with organic solvents (e.g., acetone) and/or by high-temperature annealing to fully remove the polymeric layer, inevitably introducing impurities and defects into graphene. This cleaning step has always been a liability in the fabrication of graphene-based technology, thus motivating the search for alternative transfer techniques. Wax-assisted transfer have shown significant improvements over traditional methods in the transfer of clean monolayer graphene with low defects and doping.34,35 Wax-based materials, such as paraffin and cyclododecane (CDD), have low polarity and high rigidity, and they are easy to mix in common solvents. Paraffin, a by-product of the oil refining process, has good thermal properties and interacts weakly with graphene, enabling the transfer of wrinkle- and doping-free monolayer graphene.36,37 Paraffin can be finally removed in organic solvents (e.g., chlorobenzene) or by a mild thermal treatment (140–200 °C). CDD is a nontoxic, environmentally friendly organic compound found in plant oils, which offers valuable properties (low density of 0.82 g/cm3 at 80 °C, high vapor pressure of 1.33 kPa at 100 °C, low melting point of 60.7 °C, and boiling point of 247 °C), make it a promising supporting coating for clean graphene transfer.38 Previously, the use of CDD in the fabrication of multi-layer graphene-based PV39–41 and optical applications have been demonstrated.42 The radical advantage of CDD over the PMMA and paraffin is that CDD can be completely removed by sublimation in ambient conditions. Therefore, there is no need for additional cleaning processes, such as solvent rinsing and thermal annealing. Nevertheless, low mechanical robustness of the loosely bound CDD microcrystals has so far limited its use to few- and multi-layer graphene, more mechanically robust than monolayer graphene.38 In this paper, we explore the CDD-assisted transfer of clean and crack-free monolayer graphene on the square centimeter scale, by engineering the CDD crystallization via mild heating. We validate this method by electronic transport and electron microscopy measurements. Recrystallized CDD coated on graphene guarantees a long-lasting support, enabling the transfer to any substrate without compromising the structural integrity of graphene. Importantly, no solvents are required at last for the removal of the supporting CDD layer. This green transfer method is suitable to fabricate graphene-based devices on solvent-sensitive substrates, as in the case of biomedical or biosensing applications.

The CDD can be easily dissolved in hexane and other aprotic/apolar solvents. When cast and dried at ambient conditions, it begins to sublimate due to the cyclic alkane atomic structure (Fig. S1). CDD (dissolved in hexane—40 wt. %) spin-coated on a graphene-grown copper foil. We analyzed the structural evolution of the CDD coating after spin-coating on the CVD graphene on Cu. As-coated CDD consists of aggregated crystals of a few micrometer size [Fig. 1(a)]. The CDD crystals begin to sublimate and become progressively smaller with time, leaving a few micrometer-sized areas of Gr/Cu exposed to air after 20 min and then completely disappearing in less than 1 h [Fig. 1(a)]. Indeed, this short sublimation time is not enough for the whole transfer process that includes Cu etching and several rinsing steps. To investigate this point, we observed the evolution of CDD after transferring graphene from Cu to SiO2/Si (Fig. S2). Right after the transfer, the CDD coating leaves larger exposed and unprotected graphene regions, which appear torn after the full CDD sublimation (Fig. S2). This proves that the CDD coating cannot guarantee a continuous and uniform physical support to graphene during the whole transfer process.

FIG. 1.

Microscopic analysis of the CDD-assisted transfers. CDD (dissolved in hexane at 40 wt. %) was spin-coated on Cu/Gr and photographed over 3 h. (a) As-coated CDD fully sublimated in 60 min. (b) The recrystallized CDD (R-CDD) displays a more uniform and constant structure (consisting of bigger and denser CDD crystals), which translated in a lower sublimation rate.

FIG. 1.

Microscopic analysis of the CDD-assisted transfers. CDD (dissolved in hexane at 40 wt. %) was spin-coated on Cu/Gr and photographed over 3 h. (a) As-coated CDD fully sublimated in 60 min. (b) The recrystallized CDD (R-CDD) displays a more uniform and constant structure (consisting of bigger and denser CDD crystals), which translated in a lower sublimation rate.

Close modal

In order to create a homogeneous and continuous CDD coating and at the same time increase in the sublimation time, we recrystallized the spin-coated CDD placing the CDD/Gr/Cu stack on a hot plate at 80 °C for 2 s (immediately removing it for cooling it under ambient condition). The selected heating temperature is slightly higher than the melting point of CDD and, thus, appropriate for recrystallization. The melted CDD readily recrystallized after removal from the hot plate, exhibiting larger (tens of μm) and denser CDD crystals, which form a well-knit coating [Fig. 1(b)]. The increased crystal size and their close interconnection translates in a reduced surface area for the recrystallized CDD (R-CDD), which, in turn, leads to a reduced CDD mass flow due to sublimation. Because the sublimation speed of CDD is fast at the edge, graphene is exposed consistently from the edge. This melting-recrystallization process also favors the physical adhesion between the bottom of the CDD coating and graphene, improving the contact between the two materials. The R-CDD coating demonstrates a uniform sublimation rate over the entire area, supporting and protecting graphene for 1.5 h (and possibly reducing the p-type doping usually induced by the Cu etchant).43 The full process flow of the R-CDD-assisted transfer is depicted in Fig. 2. Differently from the CDD coating [whose crystals progressively shrink in time, Fig. 1(a)], the R-CDD coating slowly recedes from the edges, leaning no cracks in the transferred graphene [Figs. 1(b) and S3].

FIG. 2.

Schematic diagram of the R-CDD-assisted transfer process. CDD is spin-coated on the Gr/Cu and CDD/Gr/Cu is quickly heated at 80 °C to melt and recrystallize CDD. Cu is removed using an APS solution (20 wt. %). The R-CDD/Gr is scooped with a glass slide and rinsed in DI water for three times (15 min each time). The R-CDD/Gr is scooped onto the target substrate (Si/SiO2 in this case). R-CDD/Gr/SiO2/Si is placed in a vacuum chamber to fully remove the R-CDD and dry. Unlike other support materials (e.g., PMMA), no washing in solvents (as acetone) are required to remove the supporting coating nor any additional post-processes to remove chemical residues.

FIG. 2.

Schematic diagram of the R-CDD-assisted transfer process. CDD is spin-coated on the Gr/Cu and CDD/Gr/Cu is quickly heated at 80 °C to melt and recrystallize CDD. Cu is removed using an APS solution (20 wt. %). The R-CDD/Gr is scooped with a glass slide and rinsed in DI water for three times (15 min each time). The R-CDD/Gr is scooped onto the target substrate (Si/SiO2 in this case). R-CDD/Gr/SiO2/Si is placed in a vacuum chamber to fully remove the R-CDD and dry. Unlike other support materials (e.g., PMMA), no washing in solvents (as acetone) are required to remove the supporting coating nor any additional post-processes to remove chemical residues.

Close modal

The CVD monolayer graphene transferred by R-CDD was thoroughly evaluated (Fig. 3). Figure 3(a) shows the optical images of transferred CVD graphene with a size of 1 × 1 cm2 before and after sublimation. R-CDD graphene is clean and continuous, without cracks and voids [Fig. 3(b)]. At an atomic force microscopy (AFM) analysis, the surface of R-CDD graphene shows minimal residues of small size (in the nm range). The film also shows an unusually low density of wrinkles [Fig. 3(c)].18 Meanwhile, the graphene transferred by PMMA shows significant density of residues with size up to hundreds of nm [Fig. 3(d)]. The uniformity and cleanness of R-CDD graphene is further confirmed by scanning electron microscopy (SEM) [Fig. 3(e)], where graphene is confirmed intact, continuous and with few wrinkles. No residues ever appeared in the SEM analysis over several mm. The high-resolution transmission electron microscopy (HR-TEM) image of Fig. 3(f) shows the honeycomb structure of monolayer graphene, also confirmed by the fast Fourier transform (FFT) pattern highlighting the sixfold symmetry of graphene (inset). Figure 3(g) shows the Raman spectrum of R-CDD graphene, with the typical features of a monolayer film.44–47 The I2D/IG ratio of ∼2.16 (G peak at ∼1580 cm−1, 2D peak at ∼2700 cm−1) confirms the monolayer thickness of the film.22,45–49 The FWHM of 2D peak is ∼32 cm−1, higher than that usually reported for mechanically exfoliated monolayer graphene (∼24 cm−1), as expected for CVD graphene.46,47 The D peak can be observed at ∼1343 cm−1 (the same graph is plotted in logarithmic scale in Fig. S4(a) to aid the visualization). Similarly, the D peak appears at ∼1347 cm−1 in graphene from the same CVD batch but transferred with PMMA [Fig. S4(b)], proving that the defect band is related to the CVD graphene itself and not to the transfer method. Notably, the ID/IG ratio of R-CDD graphene is smaller than that of PMMA graphene [∼0.08 ± 0.02 vs ∼0.14 ± 0.02, Fig. S4(c)]. From these results, we can confirm that the R-CDD transfer induces no or fewer defect or residues than the PMMA transfer does. From the Raman spectra of graphene samples transferred by R-CDD and PMMA, we plotted the 2D vs G frequency graph to infer information on potential doping and strain [Fig. 3(h)].44,50 Both CVD-graphene films transferred by R-CDD and PMMA show slight doping and induced strain with respect to pristine graphene, probably due to the Cu etchant and a mild substrate effect.18,51–53 However, the R-CDD graphene exhibits smaller doping and strain when compared to the PMMA graphene. The UV–vis spectra in Fig. 3(i) demonstrate that the R-CDD graphene also exhibits higher transparency, remarkably close to the theoretical limit of graphene.54 Finally, we measured the sheet resistance of the R-CDD graphene by the van der Pauw method [Figs. 3(j) and 3(k)]. Metal electrodes were designed and patterned by e-beam lithography and deposited by e-beam evaporation (see details in Methods in the supplementary material). The R-CDD graphene showed a low sheet resistance of ∼400 Ω/□ under ambient conditions.40,53 These results indicate that the R-CDD transfer method is as effective as the PMMA method in the supporting monolayer graphene during transfer, but superior in terms of minimized residual effects. As a further significant difference, the absence of any solvent rinsing makes the R-CDD method eco-friendly and sustainable.

FIG. 3.

Analysis of the R-CDD-transferred graphene. (a) Photos of Gr/SiO2/Si before and after R-CDD sublimation. (b) Optical and (c) AFM image, with a comparison with (d) PMMA-transferred graphene. (e) SEM and (f) HRTEM images (FFT in the inset). (g) Raman spectrum and (h) correlations of frequencies of G and 2D Raman modes of R-CDD-transferred (red) and PMMA-transferred (blue) graphene. The two dashed lines families represent expected (ωG and ω2D) variations for graphene undergoing two different kinds of changes: “strain-free” condition with varying doping level (cyan lines) and “charge-free” condition under randomly oriented uniaxial stress (magenta lines).44,50 (i) Transmittance spectra. (j) Optical image of R-CDD-transferred graphene patterned for van der Pauw measurement and (k) I–V curves.

FIG. 3.

Analysis of the R-CDD-transferred graphene. (a) Photos of Gr/SiO2/Si before and after R-CDD sublimation. (b) Optical and (c) AFM image, with a comparison with (d) PMMA-transferred graphene. (e) SEM and (f) HRTEM images (FFT in the inset). (g) Raman spectrum and (h) correlations of frequencies of G and 2D Raman modes of R-CDD-transferred (red) and PMMA-transferred (blue) graphene. The two dashed lines families represent expected (ωG and ω2D) variations for graphene undergoing two different kinds of changes: “strain-free” condition with varying doping level (cyan lines) and “charge-free” condition under randomly oriented uniaxial stress (magenta lines).44,50 (i) Transmittance spectra. (j) Optical image of R-CDD-transferred graphene patterned for van der Pauw measurement and (k) I–V curves.

Close modal

In conclusion, the findings of our investigation further validate the potential of CDD in transferring CVD monolayer graphene to a desired substrate without compromising its structural integrity or intrinsic properties. Typically, CDD naturally sublimates at room temperature, without the need of any removal/cleaning process. However, this could translate to a neither uniform nor support constant support during the transfer process, with potential damage to monolayer graphene. To go beyond this limitation, we propose the recrystallization of the spin-coated CDD by a mild heating process, which increases the CDD crystal size and makes the coating more uniform, thus enabling monolayer transfer. The recrystallized CDD exhibited improved mechanical strength and a slower rate of sublimation. This optimization significantly improved the efficiency and reliability of the graphene transfer process. By eliminating the need for solvent processing steps, our technique promotes a green approach, circumventing the use of typical chemicals necessary in graphene transfer. This opens opportunities for the transfer to solvent-sensitive substrates, including organics, polymers, and biosystems. Moreover, the large-scale utilization of CDD for transfer allows the recollection of clean CDD after use. This can be achieved by solidifying the sublimated CDD on a cold surface during the mild heating-removal stage, enabling its full recycling. Our transfer method offers two significant advantages in terms of mechanical robustness and purity, so that it might be used for any 2D material made by CVD, thus expanding the potential applications in various fields, such as biomedicine and sensing.

See the supplementary material for the methods and additional Raman and microscopic analyses.

A.C. acknowledges the financial support of the project “2DM4EH” with reference DRI/India/0664/2020, funded by FCT—Science and Technology Foundation. G.H.L. acknowledges the support from the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT & Future Planning (Nos. 2021R1A2C3014316 and 2021H1D3A2A01045033), the Research Institute of Advanced Materials (RIAM), the Institute of Engineering Research (IER), the Institute of Applied Physics (IAP), and the Inter-university Semiconductor Research Center (ISRC) at the Seoul National University.

The authors have no conflicts to disclose.

Min Jung Kim: Data curation (equal); Formal analysis (equal); Investigation (equal); Writing – original draft (equal); Writing – review & editing (equal). Gabriel Moreira: Data curation (equal); Formal analysis (equal); Writing – review & editing (equal). Nicola Lisi: Conceptualization (equal); Formal analysis (equal); Writing – review & editing (equal). Namwon Kim: Formal analysis (equal); Validation (equal); Writing – review & editing (equal). Wooyoung Shim: Resources (equal); Supervision (equal); Writing – review & editing (supporting). Gwan-Hyoung Lee: Conceptualization (equal); Formal analysis (equal); Funding acquisition (equal); Resources (equal); Validation (equal); Writing – review & editing (equal). Andrea Capasso: Conceptualization (equal); Formal analysis (equal); Funding acquisition (equal); Resources (equal); Supervision (equal); Validation (lead); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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