New discovery allows for scalable production of fold-free and ad-layer free single-crystal graphene
A team of researchers led by Director Rod Ruoff at the Center for Multidimensional Carbon Materials (CMCM) within the Institute for Basic Science (IBS), including graduate students at the Ulsan National Institute of Science and Technology (UNIST), have achieved growth and characterization of large area, single-crystal graphene that has no wrinkles, folds, or adlayers. It can be said to be the most perfect graphene that has been grown and characterized, to date.
Director Ruoff notes, “This pioneering breakthrough was due to many contributing factors, including human ingenuity and the ability of the CMCM researchers to reproducibly make large-area single-crystal Cu-Ni(111) foils, on which the graphene was grown by chemical vapor deposition (CVD) using a mixture of ethylene with hydrogen in a stream of argon gas.” Student Meihui Wang, Dr. Ming Huang, and Dr. Da Luo along with Ruoff undertook a series of experiments of growing single-crystal and single-layer graphene on such ‘home-made’ Cu-Ni(111) foils under different temperatures.
The team had previously reported single-crystal and adlayer-free films of graphene which were grown using methane at temperatures of ~1320 Kelvin (K) degrees on Cu(111) foils. Adlayers refer to small “islands” of regions that have another layer of graphene present. However, these films always contained long “folds” that are the consequence of tall wrinkles that form as the graphene is cooled from the growth temperature down to room temperature. This results in an undesirable reduction in the performance of graphene field effect transistor (GFET) if the “fold” is in the active region of the GFET. The folds also contain “cracks” that lower the mechanical strength of the graphene.
CMCM researchers first implemented a series of ‘cycling’ experiments that involved “cycling” the temperature immediately after growing the graphene at 1320 K. These experiments showed that the folds are formed at or above 1020 K during the cooling process. After learning this, the team decided to grow graphene on Cu-Ni(111) foils at several different temperatures around 1020 K, which led to a discovery that large-area, high-quality, fold-free, and adlayer-free single-crystal graphene films can be grown in a temperature range between 1000 K and 1030 K. “This fold-free graphene film forms as a single crystal over the entire growth substrate because it shows a single orientation over a large-area low-energy electron diffraction (LEED) patterns,” noted SEONG Won Kyung, a senior research fellow in CMCM who installed the LEED equipment in the center. GFETs were then patterned on this single-crystal fold-free graphene in a variety of directions by UNIST graduate student Yunqing Li. These GFETs showed remarkably uniform performance with average room temperature electron and hole mobilities of 7.0 ± 1.0 × 103 cm2 V-1 s-1. Li notes, “Such remarkably uniform performance is possible because the fold-free graphene film is a single crystal with essentially no imperfections.”
Importantly, the research team was able to achieve “scaling up” of graphene production using this method. The graphene was successfully grown on 5 foils (dimension 4 cm x 7 cm) simultaneously in a 6-inch diameter home-built quartz furnace. “Our method of growing fold-free graphene films is very reproducible, with each foil yielding two identical pieces of high-quality graphene films on both sides of the foil,” and “By using the electrochemical bubbling transfer method, graphene can be delaminated in about 1 minute and the Cu-Ni(111) foil can be quickly readied for the next growth/transfer cycle,” notes Meihui Wang. Ming Huang adds, “When we tested the weight loss of Cu-Ni(111) foils after 5 runs of growth and transfers, the net loss was only 0.0001 grams. This means that our growth and transfer methods using the Cu-Ni(111) can be performed repeatedly, essentially indefinitely.”