Graphene Transfer

Harrick Plasma → Applications → Materials → Graphene → Graphene Transfer

Graphene is commonly deposited by chemical vapor deposition (CVD) on a growth substrate followed by transfer onto a target substrate appropriate for its specific application. Plasma treatment can be applied to support and facilitate various graphene transfer methods.

Introduction

Graphene, a single atomic layer of carbon with a hexagonal crystal structure, has been heavily investigated in the past few decades for its many unique material properties. With its high electrical and thermal conductivity, near optical transparency, and high mechanical strength, graphene is considered a very versatile 2D material for a broad range of applications, including energy storage, optical displays, and sensors [1-3].

A common way to produce graphene is by CVD, whereby a hydrocarbon gas or carbon-containing precursor is used in combination with a catalytic substrate (e.g. copper Cu foil) to decompose the precursor and deposit graphene onto the growth substrate.

The graphene must then be transferred onto a desired, target substrate suitable for a specific application. For example, a flexible, transparent polymer substrate such as polydimethylsiloxane (PDMS) may be favored to fabricate graphene-based wearable electronics. As such, high fidelity transfer of graphene from the growth substrate to the target substrate (and its final point of use) is a very critical processing step in the device fabrication protocol.

Various methods have been used to transfer graphene, including wet transfer (release of graphene from the growth substrate through immersion in wet chemistry solution), dry transfer (delamination without wet chemistry), or a combination of different techniques. In a few cases, direct growth of graphene on the target substrate has also been investigated [3].

Plasma treatment can facilitate graphene transfer by cleaning surfaces on a nanoscale level and altering surface chemistry to enhance film adhesion. With relatively low applied powers of 10’s watts, Harrick Plasma cleaners are well-suited to gently clean surfaces and treat graphene. In this note, we explore examples of the use of plasma treatment to increase surface hydrophilicity and prepare substrate surfaces for reliable graphene transfer by various transfer techniques.

Wet transfer with su-8

Hiranyawasit et al. developed a variation of the wet transfer method (with polymethyl methacrylate (PMMA) as the supporting layer for graphene grown on Cu foil) to transfer graphene onto PDMS substrates using SU-8 photoresist as an intermediate adhesion layer. Surfaces were O₂ plasma treated during various stages of the transfer protocol to promote uniform coating and strong adhesion between film layers, and successful graphene transfer onto PDMS.

In their work, SU-8 was applied as an intermediate layer to enhance adhesion between graphene and PDMS. Graphene adheres strongly to SU-8 but, in contrast, SU-8 does not adhere well to PDMS. To overcome this deficiency, PDMS was O₂ plasma-treated to render the surface hydrophilic prior to spin-coating SU-8 (without plasma treatment, SU-8 did not uniformly coat the PDMS surface). Following UV-curing and hard-baking of the SU-8, the SU-8/PDMS substrate was further plasma treated to increase SU-8 surface hydrophilicity prior to collecting the graphene/PMMA film stack from an ammonium persulfate solution (used to dissolve the Cu foil on which graphene was grown), thereby transferring graphene/PMMA onto SU-8 coated PDMS. The assembled structure was then soaked in acetone to remove the top layer of PMMA, resulting in the final film structure of graphene/SU-8/PDMS substrate.

As a comparison, the researchers prepared graphene on bare (untreated) PDMS and O₂ plasma-treated PDMS. In both controls, no SU-8 intermediate layer was used. For the untreated PDMS, the graphene wrinkled and delaminated from the substrate, while for the plasma-treated PDMS, the graphene coated the substrate uniformly but ultimately peeled off during subsequent electrical resistance measurements.

The resulting graphene/SU-8/PDMS structure exhibited sheet resistance (R_S) of 1147 ± 184 Ω/sq, comparable to that of graphene transferred onto rigid substrates (e.g. Si, glass). Their work demonstrated the feasibility of developing graphene-based devices on PDMS, with possible applications for flexible electronics.

DRY RELEASE transfer 

Kinoshita et al. devised a dry release method to transfer graphene and hexagonal boron nitride (hBN) layers to construct h-BN/graphene multilayer structures, using polypropylene carbonate (PPC) film as the intermediate supporting layer.

The 2D materials exhibit strong adhesion to PPC at room temperature, however, the adhesion significantly weakens at > 40°C (the glass transition temperature of PPC). As a result, graphene and hBN layers adhere strongly to room temperature PPC surfaces but readily release from PPC with slight heating at 70°C. In this way, transfer and release from PPC can be performed with minimal polymer contamination and without use of solvents.

Air plasma treatment was utilized at different stages of the heterostructure fabrication protocol to render surfaces hydrophilic and enhance adhesion to subsequent layers. In their protocol, a thin PDMS/glass slide stack was prepared as a supporting, intermediate structure to facilitate the dry transfer process. The PDMS sheet (0.4 mm thick) was air plasma treated prior to attaching the glass slide to ensure a strong bond. Similarly, the top side of the PDMS/glass structure was plasma-treated to increase hydrophilicity and enhance adhesion of PPC to PDMS prior to receiving the PPC structure with attached 2D layer. Plasma treatment enabled strong attachment between the multiple layers and ensured mechanically stable structures for handling during the multistep dry transfer process.

direct graphene synthesis

Although both wet and dry methods are effective in transferring graphene onto target substrates, researchers are also investigating direct growth of graphene on the desired substrate to further simplify the fabrication process. In addition, as-grown graphene quality can be preserved by eliminating sacrificial polymer layers and metallic growth substrates, which can be sources of contamination and potentially degrade graphene electrical properties.

do Nascimento Barbosa et al. demonstrated growth of graphene by semi-atmospheric CVD directly on the target substrate (glass). CVD process conditions were tuned, namely vacuum pressure and methane flux, to synthesize homogeneous, large area bilayer graphene on glass.

Following deposition, plasma treatment was applied for up to 20 min to remove graphene from one side of the substrate prior to further characterization by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and optical transmittance measurements. The resulting optical transmittance spectra were comparable to that of conventional graphene bilayers and suggested that the plasma cleaning step did not significantly alter the as-grown graphene quality. Plasma cleaning can be utilized to remove residual graphene as part of the fabrication protocol while maintaining graphene’s as-grown properties.

Thus, these publications illustrate how plasma treatment can be a beneficial processing step to facilitate wet and dry graphene transfer or direct growth to ultimately produce high-quality graphene layers on the desired substrate.

Supplemental References (Do not report using Harrick Plasma instruments)

[1] Ullah S, Yang X, Ta HQ, Hasan M, Bachmatiuk A, Tokarska K, Trzebicka B, Fu L and Rummeli MH. “Graphene transfer methods: A review.” Nano Res. (2021) 14(11): 3756-3772.

[2] Lee HC, Liu W-W, Chai S-P, Mohamed AR, Aziz A, Khe C-S, Hidayah NMS and Hashim U. “Review of the synthesis, transfer, characterization and growth mechanisms of single and multilayer graphene.” RSC Adv. (2017) 7: 15644-15693.

[3] Chen Y, Gong X-L and Gai J-G. “Progress and Challenges in Transfer of Large-Area Graphene Films.” Adv. Sci. (2016) 3: 1500343.