Graphene is a 1-atom-thick layer of tightly bonded carbon atoms arranged in a hexagonal lattice. Graphene the world’s first 2D nanomaterial, is widely regarded as the “wonder material” of the 21st century due to the combination of its extraordinary properties. As a single layer of graphite, it is the thinnest material (monoatom thick), transparent, 200 times stronger than steel, yet as flexible as rubber, more conductive than copper, excellent thermal conductor and impermeable to moisture and gases. Graphene is also extraordinarily light at 0.77 mg/m2, which is roughly 1,000 times lighter than 1 m2 of paper. It is fire resistant yet retains heat.
One of the most recent advancements is the development of graphene nanoribbons (GNRs) layers of graphene with ultra-thin width of less than 50 nm. GNRs were originally discovered by Mitsutaka Fujita whilst examining the nanoscale size and edge effects of graphene.
Graphene nanoribbons are honeycomb-like structures and, compared to graphene and carbon nanotubes, are the lesser known carbon-based semiconductor family member. Graphene nanoribbons exhibit unique electronic and magnetic properties that do not appear in two-dimensional graphene.
GNRs have inherited almost all of the attractive properties of graphene and carbon nanotubes, with an additional advantage of having a tunable band gap. These structures exhibit semi-conducting and metallic electronic structures with band gaps that can be tuned across wide ranges.
The controlled edge orientation of GNRs has been studied using a scanning tunneling microscope (STM). Studies revealed that the electronic states of GNRs mainly depend on the edge structures – armchair or zigzag structures. Certain nanoribbons have gapped magnetic phases through which a semi-metallic state can be induced.
Production of Graphene Nanoribbons
Large amounts of width-controlled GNRs are produced through a process called graphite nanotomy. In this process, graphite is cut with a diamond knife to produce graphite nanoblocks which are then exfoliated to produce GNRs.
Another production method involves unzipping or cutting multi-walled carbon nanotubes in a solution with the help of sulfuric acid and potassium permanganate. Plasma etching of nanotubes partly embedded in a polymer film also produces GNRs.
Researchers have also proposed a method of growing GNRs on silicon carbide substrates using ion implantation followed by laser or vacuum annealing.
Most recently, Tohoku University developed a new method to produce defect-free GNRs. Their bottom-up fabrication method produces GNRs with periodic zigzag-edge regions.
GNRs are considered as one of the most promising models for future nanoelectronics. Due to their unique structure and properties, GNRs are widely used in a large number of applications, from spin and valley filters to chemical sensors.
Large-scale integrated circuits (LSIs) that use silicon semiconductors are used in a wide range of electronic devices, anywhere from computers to smartphones. They are actually supporting our lives and almost everything else these days. However, although LSIs have improved device performance by reducing the size of the devices, LSI miniaturization is approaching its limit. At the same time, commercial demand continues to put pressure on companies to make higher performing smartphones at smaller sizes, while industry pressure is demanding large-scale manufacturing with smaller equipment.
GNRs are commonly used in the following applications: Field effect transistors, Schotkky diodes, P-N junctions, Light emitting diodes, Solar cell systems, Liquid crystals, and Transparent conductive electrodes
However, researchers feel that the fine-tuning of the electronic band gap of GNRs will increase GNR’s future potential applications. They also hope that fabricating high-quality GNR samples with accurate control of the edge structures will stimulate the progress of research on GNRs.
Next-generation of miniaturized electronics, reported in June 2020
“Silicon semiconductors are giving us better performance at smaller sizes. However, we are reaching the limit in how small we can make devices. Thus, we have high expectations for the performance of graphene nanoribbons, which have semi-conducting properties that are only one atom thick—a 2-D material,” he notes.
Armchair-type graphene nanoribbons, which are promising type of nanoribbon for device application, display width-dependent band gap. They can be classified into three subfamilies (3p, 3p + 1, 3p + 2), their band gaps being inversely proportional to the width of those families. Basically, wider armchair-edge graphene nanoribbons belonging to the 3p + 2 subfamily have the smallest bandgaps among different graphene nanoribbons, having considerable potential to be exploited in GNR-based devices.
So far, 13-armchair graphene nanoribbons belonging to the 3p + 1 subfamily with a band gap of more than 1 eV have been reported, but Sato, Yamada and colleagues show the synthesis of a 17-graphene nanoribbon belonging to the 3p + 2 subfamily, which have even smaller bandgaps.
The graphene nanoribbon synthesis was based on the bottom-up approach, called “on-surface synthesis,” and a dibromobenzene-based molecule was used as a precursor for on-surface graphene nanoribbon synthesis.
“There are many methods to synthesize graphene nanoribbons, but to produce atomically precise graphene nanoribbons, we decided to use the bottom-up approach. The important point is that the structure of the precursor can define the ultimate structure of graphene nanoribbons if we use the bottom-up approach,” explains NAIST’s Dr. Hironobu Hayashi, who also contributed to the study.
Scanning tunnel microscopy and spectroscopy by Dr. Junichi Yamaguchi at Fujitsu. Ltd. and non-contact atomic force microscopy by Dr. Akitoshi Shiotari and Prof. Yoshiaki Sugimoto at The University of Tokyo confirmed the atomic and electronic structure of the acquired 17-armchair graphene nanoribbons. Additionally, the experimentally obtained bandgap of 17-armchair graphene nanoribbons was found to be 0.6 eV, and this is the first demonstration of the synthesis of graphene nanoribbons having a band gap smaller than 1 eV in a controlled manner.
High-Speed, Low-Power Nanoscale Data Storage reported in Dec 2021
Researchers learned that slicing graphene along the edge of its honeycomb lattice creates one-dimensional zigzag graphene strips or nanoribbons with exotic magnetic properties. Many researchers have sought to harness nanoribbons’ unusual magnetic behavior into carbon-based, spintronics devices that enable high-speed, low-power data storage and information processing technologies by encoding data through electron spin instead of charge. But because zigzag nanoribbons are highly reactive, researchers have grappled with how to observe and channel their exotic properties into a real-world device.
Now, as reported in the journal Nature, researchers at Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley have developed a method to stabilize the edges of graphene nanoribbons and directly measure their unique magnetic properties.
The team co-led by Felix Fischer and Steven Louie, both faculty scientists in Berkeley Lab’s Materials Sciences Division, found that by substituting some of the carbon atoms along the ribbon’s zigzag edges with nitrogen atoms, they could discretely tune the local electronic structure without disrupting the magnetic properties. This subtle structural change further enabled the development of a scanning probe microscopy technique for measuring the material’s local magnetism at the atomic scale.
“Prior attempts to stabilize the zigzag edge inevitably altered the electronic structure of the edge itself,” said Louie, who is also a professor of physics at UC Berkeley. “This dilemma has doomed efforts to access their magnetic structure with experimental techniques, and until now relegated their exploration to computational models,” he added.
Guided by theoretical models, Fischer and Louie designed a custom-made molecular building block featuring an arrangement of carbon and nitrogen atoms that can be mapped onto the precise structure of the desired zigzag graphene nanoribbons.
To build the nanoribbons, the small molecular building blocks are first deposited onto a flat metal surface, or substrate. Next, the surface is gently heated, activating two chemical handles at either end of each molecule. This activation step breaks a chemical bond and leaves behind a highly reactive “sticky end.”
Each time two “sticky ends” meet while the activated molecules spread out on the surface, the molecules combine to form new carbon-carbon bonds. Eventually, the process builds 1D daisy chains of molecular building blocks. Finally, a second heating step rearranges the chain’s internal bonds to form a graphene nanoribbon featuring two parallel zigzag edges.
“The unique advantage of this molecular bottom-up technology is that any structural feature of the graphene ribbon, such as the exact position of the nitrogen atoms, can be encoded in the molecular building block,” said Raymond Blackwell, a graduate student in the Fischer group and co-lead author on the paper together with Fangzhou Zhao, a graduate student in the Louie group.
The next challenge was to measure the nanoribbons’ properties.
“We quickly realized that, to not only measure but actually quantify the magnetic field induced by the spin-polarized nanoribbon edge states, we would have to address two additional problems,” said Fischer, who is also a professor of chemistry at UC Berkeley.
First, the team needed to figure out how to separate the electronic structure of the ribbon from its substrate. Fischer solved the issue by using a scanning tunneling microscope tip to irreversibly break the link between the graphene nanoribbon and the underlying metal.
The second challenge was to develop a new technique to directly measure a magnetic field at the nanometer scale. Luckily, the researchers found that the nitrogen atoms substituted in the nanoribbons’ structure actually acted as atomic-scale sensors.
Measurements at the positions of the nitrogen atoms revealed the characteristic features of a local magnetic field along the zigzag edge.
Calculations performed by Louie using computing resources at the National Energy Research Scientific Computing Center (NERSC) yielded quantitative predictions of the interactions that arise from the spin-polarized edge states of the ribbons. Microscopy measurements of the precise signatures of magnetic interactions matched those predictions and confirmed their quantum properties.
“Exploring and ultimately developing the experimental tools that allow rational engineering of these exotic magnetic edges opens the door to unprecedented opportunities of carbon-based spintronics,” said Fischer, referring to next-generation nano-electronic devices that rely on intrinsic properties of electrons. Future work will involve exploring phenomena associated with these properties in custom-designed zigzag graphene architectures.
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