High-speed roll-to-roll manufacturing of graphene using a concentric tube CVD reactor

The integration of two-dimensional (2-D) materials with applications that demand cost-effective large-area production requires understanding of how lab-scale synthesis methods can be translated to continuous manufacturing processes. For thin films of graphene, such promising applications include transparent electrodes for displays and photovoltaics, high-performance filtration membranes, and thermal imagers1,2,3,4,5,6,7. Direct synthesis of graphene on substrates by chemical vapor deposition (CVD) has emerged as a highly attractive technique for these applications because of its compatibility with thin film processing tools, and its potential scalability to large areas1. As a result of continued research efforts, the electrical transport of graphene synthesized by CVD on substrates is approaching that of exfoliated graphene, and a growing portfolio of CVD recipes can be applied to substrates of increasing size (centimeter to wafer scale) and diversity (e.g., metallic thin films deposited on Si and quartz, in addition to metal foils)2,8,9,10,11,12. Several systems and methods for roll-to-roll (R2R) graphene production have been presented in the academic literature13,14,15,16,17. Early on, Hesjedal and colleagues used a modified tube furnace for R2R production of multi-layer graphene on Cu foil (25 μm thick, 1 m length) at 1–40 cm/min14. Yamada and colleagues presented a custom microwave plasma CVD system and reported complete coverage of multilayer graphene at a feed rate of 30 cm/min using Cu foil with 294 mm width15. While the plasma-enhanced process enabled low temperature growth (>400 °C), this also limited the graphene quality and domain size. More recently, Kobayashi and colleagues produced high quality, predominantly single-layer graphene on Cu foil (230 mm wide, 36 μm thick) at 10 cm/min using a R2R CVD system that resistively heated the Cu foil fed between two electrode rollers17. Following subsequent transfer, graphene coverage of 89–98% was reported on the final substrate which was a polyethylene terephthalate (PET) film. In parallel with these efforts, notable progress has been made on batch-style CVD growth. In 2010, Bae et al. produced uniform graphene films on 30" diagonal Cu foils that were wrapped around a 7.5” diameter quartz tube placed for static processing inside an 8” diameter quartz tube within a tube furnace. The graphene films were subsequently transferred to PET following a wet chemical etch of the Cu13. A similar technique was used by Vlassiouk et al. to produce 40" diagonal films of graphene that were subsequently transferred to PET16. Despite these achievements, it is still necessary to advance continuous production of 2D materials to reflect a rigorous understanding of the underlying process physics, and to enable high-quality layer-controlled production at a high rate. For graphene in particular, the design of the R2R CVD system is critical to establish such understanding, and design principles that should be captured in an effective system include: thermal and fluidic uniformity over the substrate; efficient mixing and use of the feedstock gases; sealed and controlled gas atmospheres and thermal zones (e.g., as seen in carbon fiber production)18,19,20; and throughput that is compatible with upstream and downstream processes (e.g., integration with patterning operations)15,21. Also, graphene growth using CVD requires sequential heating in an inert or reducing atmosphere followed by hydrocarbon exposure, and substrate handling and the transitions between zones must consider this requirement22. Practically, there also exists a need to understand the dependence of key graphene characteristics on the multi-dimensional parameter space of a continuous process (e.g., temperature, pressure, atmosphere composition, feed rate, quality) similar to parametric studies performed for batch-scale graphene growth using static reactor conditions22,23,24,25. This would, in turn, enable engineering of graphene characteristics (e.g., number of layers, domain size, quality) to meet both application-oriented needs and production specifications (e.g., cost, rate). We present a new reactor design for R2R CVD of 2-D materials on flexible substrates, and using a benchtop prototype of this reactor, we study R2R production of graphene on metal foils. The reactor has a concentric tube geometry, which achieves several desirable features for R2R CVD, including thermal and fluidic uniformity over the substrate due to the small gap, a rapid isothermal transition between the two internal atmospheres via downstream injection of the hydrocarbon precursor, and modularity due to its circular geometry. Using the concentric tube system, we find an inverse relationship between graphene film quality and production speed, which is governed by the nucleation and coalescence kinetics of graphene domains in combination with the residence time of the substrate. The downstream injection of the hydrocarbon into the CTCVD system yields roughly 2.7x and 1.8x increases in I2D/IG and IG/ID respectively, yet overall graphene quality is limited by the grain size and surface quality of the copper foil. Last, we study the influence of annealing time, reactor temperature, and cooling atmosphere, and find that additional annealing time (3 hours), increased reactor temperature (from 1000 °C to 1045 °C), and a He/H2 cooling atmosphere give a 1.9x, 1.4x and 1.9x improvement of I2D/IG, respectively.