Annealing free, clean graphene transfer using alternative polymer scaffolds

Abstract

We examine the transfer of graphene grown by chemical vapor deposition (CVD) with polymer scaffolds of poly(methyl methacrylate) (PMMA), poly(lactic acid) (PLA), poly(phthalaldehyde) (PPA), and poly(bisphenol A carbonate) (PC). We find that optimally reactive PC scaffolds provide the cleanest graphene transfers without any annealing, after extensive comparison with optical microscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Comparatively, films transferred with PLA, PPA, and PMMA have a two-fold higher roughness and a five-fold higher chemical doping. Using PC scaffolds, we demonstrate the clean transfer of CVD multilayer graphene, fluorinated graphene, and hexagonal boron nitride. Our annealing free, PC transfers enable the use of atomically-clean nanomaterials in biomolecule encapsulation and flexible electronic applications. * Correspondence should be addressed to lyding@illinois.edu and epop@stanford.edu. Cu has proven the most fruitful platform for large-area graphene growth, as the low carbon solubility promotes monolayer growth. 8 Nevertheless, most applications using CVD-grown graphene require that the films be transferred to insulating substrates. The predominant graphene transfer approach is by using a poly(methyl methacrylate) (PMMA) scaffold. [12][13][14][15][16][17] In this method, the PMMA polymer coats the graphene, supporting it during Cu removal, underside contaminant cleaning, and placement on its destination substrate. 18, 19 However, PMMA removal from graphene after film transfer has proven challenging. 15 Approaches to remove it by high-temperature Ar/H2 forming gas annealing, 14, 20, 21 O2 based annealing, 15, 22, 23 and in situ annealing 16, 24, 25 have been marginally successful in removing PMMA without affecting the graphene. Furthermore, these processes are all at high-temperature, excluding graphene applications with low thermal budgets, including uses in flexible electronics and biomolecule encapsulation. Another process separated the graphene from the PMMA support by an Au interfacial layer, 26 but that process is subject to effective interfacial Au-graphene wetting. Recent transfer results using thermal release tape (TRT), [27][28][29] poly(bisphenol A carbonate) (PC), 30, 31 and sacrificial polymer release layers 26 required elevated temperature (over 100°C) during transfer and differed considerably in terms of surface contamination and graphene area coverage. To exploit the intrinsic properties of large-area graphene, a room temperature transfer process that comes off more cleanly than the established methods is needed. In print atIn this study, we compare the transfer of graphene with the conventional PMMA polymer scaffold with alternative poly(lactic acid) (PLA), poly(phthalaldehyde) (PPA), PC, and bilayer PMMA/PC scaffolds. We choose both PLA and PPA as scaffolds as they can supposedly be removed by modest heating or acid exposure. Further, we choose PC from its heightened reactivity as a condensation polymer and it...

Cleaning Transferred Graphene for Optimization of Device Performance

“…Nonetheless it has been reported that some PMMA residues still remain on the surface after thermal annealing, resulting in scattering and defects . In the absence of improved (i.e., low residue) graphene transfer techniques using PMMA or other polymers, the other option for wafer‐scale residue removal is wet‐chemical cleaning typically using organic solvents such as acetone, chloroform, etc . A major issue is that the presence and distribution of these polymer residues heavily depends on experimental conditions, the quality of graphene, etc., and therefore varies significantly from one process to another.…”

Recommended Methods to Study Resistive Switching Devices

“…The two most widespread methods to synthesize 2D materials applied to RS devices are chemical vapor deposition (CVD) and liquid-phase exfoliation. [89][90][91] A solution commonly employed is to synthesize the 2D material on the most suitable substrates (metallic foils for graphene [80] and h-BN [85][86][87] and SiO 2 or sapphire for 2D transition metal dichalcogenides (TMDs) [81][82][83] ) and transfer it on the desired sample using different methods, [92][93][94] being the wet transfer with the assistance of a polymer scaffold the most used by the RS community. The problem is that the temperature used for the growth is typically >700 °C, which prevents growing the 2D material on wafers with existing integrated circuits due to diffusion problems; the maximum temperature allowed for complementary metal-oxidesemiconductor (CMOS) back-end of line integration is typically 450 °C.…”

“…[7] CVD can be used to grow high quality graphene, [80] molybdenum disulfide (MoS 2 ), [81] molybdenum diselenide (MoSe 2 ), [82] tungsten disulfide (WS 2 ), [83] tungsten selenide (WSe 2 ), [84] and hexagonal boron nitride (h-BN), [85][86][87] among many others. [94] However, three main issues need to be taken into account: i) if the 2D layered material is too thin (e.g., monolayer) and the top electrodes are very large (>10 4 µm 2 ), [95] the 2D material below the TE is likely to contain cracks (see Figure 3a). [88] Recently, thermally assisted conversion of metallic films at CMOS back-end compatible temperatures has been demonstrated to yield promising layered films, such as platinum diselenide (PtSe 2 ).…”

Toward clean suspended CVD graphene

“…14 Recently, an alternative polymer scaffolds have been used instead of PMMA, which do not require an additional annealing after the polymers are dissolved. 15 However, some of the copper etchants, such as ammonium persulfate, can promote a crosslinking in polymers, resulting in a high concentration of residues on the graphene layer. 16 Alternatively, the direct transfer method is based on an adhesion between a Gr/substrate stack and a flexible perforated carbon membrane induced by capillary forces of a drying solvent such as IPA.…”