Scientists create graphene in an oxygen-free environment, successfully bridging

"Under anaerobic conditions, the graphene we prepared has set a new standard for quality, which can be compared with the quality of graphene obtained by mechanical exfoliation, marking a milestone in the field of graphene preparation," said Yan Xingzhou, an undergraduate alumnus of the University of Electronic Science and Technology and a doctoral student at Columbia University in the United States.

In the study, he and his team developed a feasible method to bridge the gap between the quality and repeatability of graphene.

Currently, the research group has demonstrated that the synthesis process of graphene is extremely sensitive to trace amounts of oxygen.

In an anaerobic environment, the growth dynamics of graphene can be predicted by a compact model.

By eliminating the oxygen content in the synthesis environment, such a graphene can be synthesized on a single-crystal copper surface: not only is the measurable surface free of contamination, but the electrical properties are as good as those of the intrinsic graphene exfoliated from bulk graphite.For the relevant paper, the reviewer believes that this work is a breakthrough in the field of graphene, and it is expected that this method will be adopted by various laboratories and industrial institutions in a short period of time, leading to a new revolution.

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In terms of application prospects:

Graphene can serve as a basic component, integrated into many applications. For example, graphene has great potential to act as an alternative to metals in future electronic devices.

As the size of transistors becomes smaller, the thinner interconnects connecting the transistors are also shrinking, but there will still be a certain limit. Unlike traditional metals such as copper and gold, graphene can conduct electricity better at the nanoscale.

In addition, graphene has been proven to be an ideal material for condensed matter physics research. Since electrons can move from their atomic lattice in graphene with minimal scattering, graphene is an excellent platform for studying electron-electron interactions.And these interactions can give rise to new phenomena in condensed matter physics, such as superconductivity, the fractional quantum Hall effect, and so on.

Therefore, the large-scale production of graphene can achieve larger device sizes and is expected to enable mass production for applications in electrochemical sensing and optical sensing.

In addition, graphene is also a flexible, tough, and transparent two-dimensional material. At present, it has been used in wearable electronic devices for healthcare and high-performance sensing applications.

On the other hand, graphene also has the potential to act as a support film in cryo-transmission electron microscopy for determining protein structures.

What are the challenges in achieving commercial-scale production of graphene?It is understood that graphene is the most conductive material at the atomic level, and can play a role in various fields such as electronics, optoelectronics, sensors, semiconductor manufacturing, and basic condensed matter physics research.

Graphene is only one atomic layer thick in two dimensions. Due to the unique structure of graphene, electrons can move within graphene as if they were weightless.

In addition, the electron mobility of graphene is 200 times that of typical metals (copper). At the same time, it also has the characteristics of transparency and flexibility, and is the strongest known mechanical two-dimensional material.

However, producing high-quality graphene on a commercial scale still poses certain challenges.

So far, the production of high-quality graphene with a size of tens of micrometers still relies on the "mechanical exfoliation" method, which is to separate graphene from graphite using tape.The success rate of the aforementioned process is somewhat random, hence it is used for large-scale production.

Chemical Vapor Deposition (CVD) is one of the most promising methods for the industrialized mass production of graphene, but its quality has always been less than that of graphene mechanically exfoliated from graphite.

At the same time, another disadvantage of this method is the lack of reproducibility, which implies that there may be hidden variables in this method that have not yet been discovered.

In this study, Yan Xingzhou and others found that to prepare ultra-high-quality graphene, it is necessary to add a trace amount of oxygen during the strict removal of the growth process.

This requirement is extremely demanding, requiring the oxygen content in the preparation environment to be controlled below one part per million.So, how did Yan Xingzhou participate in and complete this research?

4 years, 1600 experiments, and a Nature paper

Yan Xingzhou said that when he was a sophomore at the University of Electronic Science and Technology, he first came into contact with graphene and two-dimensional materials in Professor Li Xuesong's research group at the university.

"At that time, Professor Li had just returned to the country and joined the university. He is a pioneer in the chemical vapor deposition of graphene and has published related papers in Science," said Yan Xingzhou.At that time, Li Xuesong's research group had just started, and Yan Xingzhou was able to come into contact with knowledge about the preparation of graphene in that laboratory, and gradually came into contact with two-dimensional materials.

Later, Yan Xingzhou joined the research group of Professor James Hone at Columbia University in the United States.

"Professor Hone is also a pioneer in the field of two-dimensional materials, and his research has greatly promoted the progress of the entire field of two-dimensional materials," said Yan Xingzhou.

At that time, Professor Hone highly valued Yan Xingzhou's background in the preparation of graphene and asked him to take over a topic that the team was working on, aiming to explore the path to improving the quality of graphene.

When Yan Xingzhou first entered the laboratory, the work of preparing graphene had not yet entered the right track. At that time, the biggest trouble in the group was that the reaction of preparing graphene was very difficult to control, and the reproducibility of the experiment was very poor.Initially, they realized that the quality of graphene was related to some impurities or contaminants, but they were not sure exactly what it was.

After collaborating with a research group from the University of Montreal in Canada, they decided to upgrade the vacuum device used to prepare graphene in order to improve the vacuum system and reduce impurities in the gas.

On the other hand, the research group also made progress in obtaining an atomically flat and clean copper surface. After combining the two, they found that the growth rate of graphene was extremely fast.

During the system verification phase, they found that the growth rate of graphene had increased tenfold, and it could even be prepared without a hydrogen gas environment.

This phenomenon is very unusual because most previous studies would use a large amount of hydrogen gas as a protective gas.At this point, they realized that oxygen was the key to the rapid growth of graphene, and the reason why traditional methods relied on a large amount of hydrogen was due to the difficulty in controlling the trace amounts of oxygen.

Later, they began to explore the relationship between trace oxygen and the growth rate of graphene, and introduced trace oxygen into the experimental environment in a controllable manner. Subsequently, they found that the growth rate and quality of graphene showed a significant decline.

The graphene devices prepared in an oxygen-free environment, however, could achieve unprecedented conductivity. Under room temperature conditions, the obtained graphene devices reached the theoretical limit.

However, at this time, they had not yet obtained low-temperature data. Because, in a low-temperature environment, due to the weakening of atomic vibrations, the movement of electrons would be significantly scattered by material defects.

Therefore, they began to make devices for low-temperature testing. At the same time, they found that the growth principle of graphene in an oxygen-free environment could be easily understood.More carbon sources will accelerate growth, while an excess of hydrogen will reduce the growth rate of graphene.

This marks that the preparation of graphene is no longer a "black box," but can be designed based on growth conditions.

Later, they found through atomic force microscopy: the cleanliness of the graphene surface is related to trace oxygen.

Previous studies have found: the surface of synthesized graphene often has trace amorphous carbon. Because this is an impurity, it is considered to be a byproduct of the reaction.

This study clearly points out: oxygen content below one millionth is the source of this impurity.In fact, before the amorphous carbon on the surface of graphene was observed under an atomic force microscope, the team's speculation was still in a hypothetical state.

Yan Xingzhou once speculated: Under certain conditions, oxygen may lead to the production of amorphous carbon.

"I didn't expect to be right. The moment I saw the solid evidence on the computer screen is very memorable. You should know that everything is unpredictable under an atomic force microscope, and the scale of the evidence I was looking for is less than one nanometer," he said.

And when he and another co-first author saw the quantum Hall effect in graphene for the first time in the measurement, that is, the first time the quantum phenomenon appeared in their data, it was also very exciting. This means the research was successful.

"During the research period, I myself and co-first author Jacob Amontree conducted more than 1600 experiments in four years, and countless failures and attempts were made to achieve the current understanding," Yan Xingzhou said.Ultimately, the relevant paper was published in Nature[1] with the title "Reproducible graphene synthesis by oxygen-free chemical vapour deposition."

Jacob Amontree and Xingzhou Yan from Columbia University in the United States are the co-first authors.

Professor Katayun Barmak and Professor James Hone from Columbia University in the United States, along with Professor Richard Martel from the University of Montreal in Canada, are the co-corresponding authors.

Of course, new challenges are waiting for Xingzhou Yan and others. Next, the research team will focus on the high-quality transfer of graphene.

It is important to note that graphene grows on the surface of copper, and to realize the widespread application of graphene, it must be transferred to other substrates, such as silicon wafers.Due to the two-dimensional nature of graphene, achieving an atomically clean transfer is extremely difficult, so they will explore how to achieve high-quality transfer of graphene. Currently, the relevant work is underway.