Since 2004, when British scientists "peeled" graphene from graphite layers using adhesive tape and later received the Nobel Prize in Physics, this two-dimensional material has become the highly acclaimed "king of new materials."
Graphene boasts ultra-high carrier mobility and excellent conductivity, making it an ideal candidate for future high-performance electronic devices and chips. However, its "zero bandgap" characteristic has been a "fatal flaw" limiting its applications. In contrast, quasi-one-dimensional materials like graphene nanoribbons with widths smaller than ten nanometers can open bandgaps through quantum confinement effects, addressing this limitation. Yet, producing high-quality graphene nanoribbons has always been challenging.
Recently, Professor Shi Zhiwen's team from Shanghai Jiao Tong University, in collaboration with Professor Ouyang Wengen's team from Wuhan University, Dr. Ding Feng's team from the Shenzhen Institutes of Advanced Technology of the Chinese Academy of Sciences, and Professor Michael Urbakh's team from Tel Aviv University in Israel, successfully "grew" the world's longest and highest-performing graphene nanoribbons in the laboratory and analyzed the mechanisms behind their exceptional performance. The relevant research has been published in Nature.
A Conceptual Diagram of Carbon-Based Chips Encapsulated with Graphene Nanoribbons
An Unexpected Discovery
Graphene nanoribbons possess the bandgap necessary for fabricating transistor devices, making them ideal materials for future integrated circuits.
How to transform two-dimensional graphene into one-dimensional graphene nanoribbons?
Physicists worldwide have attempted numerous methods, including cutting graphene into strips narrower than 5 nanometers to prepare nanoribbons. However, achieving such precision is challenging, and even if successful, the resulting graphene nanoribbons often suffer from lattice defects and disrupted edge structures, leading to inferior properties. Some scientists have tried growing graphene nanoribbons on single-crystal metal substrates, but the resulting lengths are only 20 to 30 nanometers, and after transfer, the substrates contain disordered effects such as charge impurities, rendering them impractical for real-world applications.
Over a decade ago, while pursuing his doctorate at the Institute of Physics, Chinese Academy of Sciences, Zhiwen Shi began researching graphene nanoribbons and published some methods for their fabrication, but did not completely solve the aforementioned problems.
In 2020, Zhiwen Shi, now leading doctoral student Bosai Lv at Shanghai Jiao Tong University, accidentally discovered another one-dimensional nanomaterial—carbon nanotubes—while growing on a hexagonal boron nitride substrate. These "fruits" grown on the substrate not only included carbon nanotubes but also some one-dimensional materials with heights of only a few nanometers.
They characterized these materials with atomic resolution and were surprised to find that they were graphene nanoribbons.
Interestingly, many graphene nanoribbons did not grow on the surface of the boron nitride substrate but were embedded within the boron nitride layers.
They collaborated with Feng Ding's team to analyze the formation mechanism of graphene nanoribbons and realized the importance of hexagonal boron nitride substrates.
"Previously, graphene nanoribbons grown on metal substrates required 'mechanical encapsulation' for device fabrication to transfer them elsewhere because the metal substrate would interfere with measurements. In contrast, hexagonal boron nitride is an excellent insulator. Graphene nanoribbons grown within hexagonal boron nitride layers mean they do not require any transfer, enabling 'in-situ encapsulation,' preserving their structure and properties from external environmental factors and micro/nanofabrication influences, thus exhibiting excellent performance," said Zhiwen Shi, the corresponding author of the paper.
Graphene nanoribbons grown epitaxially on hexagonal boron nitride atomic layers. Image courtesy of the interviewee.
Following this "serendipitous" discovery, Shi Zhiwen guided Lv Bosai to conduct experiments to "seed" graphene nanoribbons on hexagonal boron nitride substrates.
They conducted numerous experiments, exploring repeatedly, and finally identified the cultivation conditions required for the growth of graphene nanoribbons, including temperature, pressure, catalysts, and other factors.
"We placed some catalyst nanoparticles on hexagonal boron nitride crystal substrates, placed the substrates in a tube furnace, introduced methane gas into the furnace, and then raised the temperature to around 800 degrees Celsius to decompose methane into carbon atoms, allowing graphene nanoribbons to continuously grow on the catalyst," explained Lv Bosai, the first author of the paper, to Chinese Science Bulletin.
Using this method, researchers cultivated the world's longest graphene nanoribbons on hexagonal boron nitride substrates, with lengths reaching the sub-millimeter level, far exceeding previous reports. Meanwhile, their width is only 3-5 nanometers, and they are chiral, meaning they have a larger bandgap, making their properties more stable.
Laboratory measurements show that these graphene nanoribbons exhibit outstanding performance: the charge carrier mobility reaches 4,600 cm^2V^–1s^–1, and the on-off ratio can reach 10^6, setting the highest record achieved in ultra-narrow graphene nanoribbons to date.
"Charge carrier mobility and on-off ratio are both performance indicators that need to be emphasized when making electronic devices. A higher charge carrier mobility means faster device response, leading to higher computational speeds and lower energy consumption; a higher on-off ratio means more effective switching on and off," explained Shi Zhiwen to Chinese Science Bulletin, suggesting that these excellent performances are expected to play an important role in future nanoelectronic devices.
Shi Zhiwen and team members (Shi Zhiwen is in the front row on the right, and Lv Bosai is on the right).
The secret to growing long is "friction-free."
The world's longest high-performance graphene nanoribbon has been grown, but what is the mechanism behind it?
To answer this question, Shi Zhiwen contacted many experts and teams for collaboration. One of them was Professor Ouyang Wengen from Wuhan University. Shi Zhiwen saw a paper published by Ouyang Wengen and his collaborator Professor Michael Urbakh from Tel Aviv University in the journal "Nano Letters," discussing the remarkable ultra-low friction behavior of graphene nanoribbons on hexagonal boron nitride substrates. He believed this could be used to reveal the growth mechanism of nanoribbons observed in experiments and contacted Urbakh and Ouyang Wengen by email to collaborate.
To reveal the growth mechanism of graphene nanoribbons between hexagonal boron nitride layers, Ouyang Wengen led doctoral student Wang Sen to conduct large-scale molecular dynamics simulations of the experimental system.
They "moved" the laboratory research into the supercomputers at Wuhan University Supercomputing Center and the National Supercomputing Center in Tianhe, conducting continuous research for three years. They performed numerous tests and simulated various growth directions, widths, deformations, and paths of graphene nanoribbons. They found that the formation of ultra-long graphene nanoribbons is related to their "super-slip" properties when slipping between hexagonal boron nitride layers, exhibiting near-zero frictional losses.
"The metal particle catalysts at the edge of hexagonal boron nitride crystals act like 'roots,' absorbing carbon atoms produced by methane decomposition. The new carbon atoms push the already formed graphene nanoribbons forward, allowing them to continue growing. During this sliding process, the longer the graphene nanoribbons generated, the greater the resistance encountered by the new atoms as they move forward. When the thrust cannot overcome the resistance, the growth of the nanoribbons stops," explained co-corresponding author Ouyang Wengen to "Chinese Science Bulletin." "Only when the resistance is very low can they grow very long."
Ouyang Wengen stated that this explains why graphene nanoribbons in "zigzag" and "armchair" shapes grow longer than nanoribbons on surfaces in experiments. Especially for "zigzag" graphene nanoribbons, they grow along the "valleys" with the lowest friction between hexagonal boron nitride layers, creating a "green channel" that exhibits characteristics of chiral growth.
This computational simulation result was supported by theoretical analysis from the Urbakh team and first-principles calculations from the Ding Feng team.
Further experimental studies also confirmed that graphene nanoribbons inside hexagonal boron nitride crystals grow the longest because they encounter the least resistance when carbon atoms advance. Although graphene nanoribbons can also grow on the surface of crystals, they encounter greater resistance when growing on the surface, so the longest they can grow is only a few tens of micrometers.
Ouyang Wengen and team members (sixth from the left is Ouyang Wengen, seventh from the left is Wang Sen). Image provided by the interviewee.
Win-Win Cooperation, Showcasing Strengths
Lü Borsai explained that the growth rate of graphene nanoribbons, a new technology, is extremely fast, with hundreds of micrometers growing in just a few minutes. This means that this high-performance one-dimensional material can be easily prepared on a large scale and in batches.
"Very novel," "exciting," "this is an innovative approach" ... Five reviewers from the journal Nature highly praised this research, believing that it overcame the biggest obstacle to using graphene in electronic products (i.e., zero bandgap) and addressed various challenges in the preparation of graphene nanoribbons, resulting in graphene nanoribbons with clear semiconductor properties. This provides new insights into the synthesis of controlled materials in the future and has promising applications.
Regarding the reasons for the success of this research, Shi Zhiwen said that one person's knowledge and experience are limited, and good research cannot be achieved without the collaboration of interdisciplinary experts. The breakthrough in this research is thanks to the excellent collaborative model of showcasing each other's strengths. "During the three years of collaboration, the collaborative team from four units held online discussions almost every month, ensuring the smooth progress of the project."
Researchers hope that this achievement can play a role in high-performance transistor devices, future integrated circuits, and other areas.
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