Creation of the most perfect graphene
New discovery allows for scalable production of fold-free and ad-layer
free single-crystal graphene

Date:
August 25, 2021
Source:
Institute for Basic Science
Summary:
Researchers 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.



FULL STORY ==========================================================================
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.

The next exciting challenge was thus eliminating these folds.

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 x 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."
In the process of achieving fold-free single-crystal graphene, the
researchers also discovered the reasons behind the formation of these
folds. High- resolution TEM imaging was performed by student CHOE Myeonggi
and Prof. LEE Zonghoon (a group leader in CMCM and professor at UNIST)
to observe the cross- sections of the samples grown above 1040 K. They discovered that the deadhesion, which is the cause of the folds, is
initiated at the "bunched step edge" regions between the single crystal Cu-Ni(111) plateaus. "This deadhesion at the bunched step edge regions
triggers the formation of graphene folds perpendicular to the step edge direction," noted co-corresponding author Luo.

Ruoff further notes that "We discovered that step-bunching of a Cu-Ni(111)
foil surface suddenly occurs at about 1030 K, and this 'surface
reconstruction' is the reason why the critical growth temperature of
fold-free graphene is at ~1030 K or below." Such large-area fold-free single-crystal graphene film allows for the straightforward fabrication
of integrated high-performance devices oriented in any direction over
the entire graphene film. These single-crystal graphene films will be
important for further advances in basic science, which will lead to
new applications in electronic, photonic, mechanical, thermal, and
other areas. The near-perfect graphene is also useful for stacking,
either with itself and/or with other 2D materials, to further expand
the range of likely applications. Given that the Cu-Ni(111) foils can
be used repeatedly and that the graphene can be transferred to other
substrates in less than one minute, the scalable manufacturing using
this process is also highly promising.

========================================================================== Story Source: Materials provided by Institute_for_Basic_Science. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Wang, M., Huang, M., Luo, D. et al. Single-crystal, large-area,
fold-free
monolayer graphene. Nature, 2021 DOI: 10.1038/s41586-021-03753-3 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210825113619.htm

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