Since its discovery in 2004, graphene has always been used and studied with a small amount of imperfection such as surface wrinkles, folds, or adlayers. Now, a new study has grown large-area, single-crystal graphene that has none of these flaws - and it may be the most perfect sample synthesized so far.
The new breakthrough was done by researchers from the Institute of Basic Science's (IBS) Center for Multidimensional Carbon Materials (CMCM), led by director Rod Ruoff together with graduate students from the Ulsan National Institute of Science and Technology (UNIST) - both institutes in South Korea.
Researchers present their findings in the article "Single-Crystal, Large-Area, Fold-Free Monolayer Graphene," appearing in the online portal for Nature, August 25.
Small Flaws, Huge Effects in Graphene
In a report from the IBS, Director Ruoff credits the pioneering breakthrough to a number of different factors. One of them was human ingenuity and the other was the capability of the CMCM team to create large-area, single-crystal Cu-Ni (111) foils, a binary alloy on which graphene was synthesized through a chemical vapor deposition (CVD) process using a mix of hydrogen and ethylene in an argon gas stream.
The effort was the result of student Meihui Wang and Dr. Ming Huang from IBS' CMCM, together with Ruoff, conducting experiments of growing graphene, single-crystal and single-layer samples, using "homemade" Cu-Ni (111) at different temperatures.
Researchers had previously reported single-crystal adlayer-free graphene films that were achieved using methane in CVD with Cu (111) foils at temperatures of about 1320 Kelvin. In graphene synthesis, adlayers are small bumps or "islands' in regions of the material that has another layer of graphene within them. However, these almost always included long "folds" that come as a result of tall wrinkles. These wrinkles, in turn, are created when the grown graphene is cooled down from the CVD temperature down to room temperature. This temperature shift induces wrinkles that create an undesirable drop in the performance of a graphene field-effect transistor (GFET), especially if the fold lies in the active region of the transistor material.
These folds also cause another undesirable effect on the graphene material: the presence of cracks adversely affects the mechanical strength of the material. The "super-strong" material can turn extremely brittle because of these cracks, according to a study from Rice University and the Georgia Institute of Technology.
Growing the Most Perfect Graphene
To arrive at what might be the most perfect graphene sample ever grown, CMCM scientists first tried a series of "cycling" experiments that involved altering the temperature immediately after graphene grows at 1320. They found out that the folds started appearing around 1020 K during the cooling process.
This discovery prompted researchers to try and grow graphene on Cu-Ni (111) foils at different temperatures near 1020 K. They found that large-area, single-crystal graphene with no folds, and adlayers were possible 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," explains Seong Won Kyung, CMCM senior research fellow who also installed the LEED equipment used in the study.
Researchers were able to create the "most perfect" sample up to five foils, or a dimension of 4 cm by 7 cm within a 6-inch diameter homemade quartz furnace.
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