Graphene and laser - a new revolution in materials science

Introduction: Graphene is the lightest material ever produced. Its strength is 100 times that of steel. It is better than copper's conductivity and flexibility, and it is largely transparent.

Often, tape is not considered a scientifically groundbreaking advance. But when the University of Manchester physicist Andre Geim and Konstantin Novoselov (the two won the Nobel Prize in Physics in 2010) and colleagues in the "Science The magazine published their research results - the use of scotch tape to peel off a single atomic sheet of carbon atoms from a piece of graphene, a study that slowly opened the prelude to the material science revolution.

In the 11 years since the above-mentioned Manchester research team published its research results, the research results in related fields have grown exponentially. Last year, global researchers published more than 15,000 research results on graphene. This phenomenon is quite reasonable: graphene is the lightest material ever produced, its strength is 100 times that of steel, its conductivity and flexibility are better than copper, and it is largely transparent. The researchers envisioned every future graphene-based product, from next-generation computer chips and flexible displays to batteries and fuel cells.

However, graphene may not achieve its great impact in the future through its own as an ideal material, but rather a product derived from it. Although graphene has many dazzling advantages, it also has disadvantages, especially the inability to act as a semiconductor—the cornerstone of microelectronics. Now, chemists and materials scientists are working hard to cross graphene and look for other materials. They are synthesizing two other two-dimensional sheet materials that combine flexibility and transparency with electronic properties that graphene cannot match, and they have turned some of them into fast electronic and optical devices that are lightweight and flexible. They hope that these materials can serve as the backbone of the future industry.

Graphene, opening a new horizon for 2D materials

In a sense, 2D materials are not entirely new technologies. Researchers have developed atomic-form thin-film materials using molecular number epitaxy (MBE) machines since the 1960s. But MBE machines are often used to store materials such as silicon and gallium arsenide—the atomic structures of these crystalline materials tend to form three-dimensional structures. From this perspective, the atomic layer made by the MBE machine is like a piece of cheese, a two-dimensional version of a three-dimensional material.

Graphene is different, it is more like a paper in a book, said Yi-HsienLee, professor of materials science at National Tsinghua University in Hsinchu, Taiwan. To the surprise of scientists, when they studied graphene at close range, they found that there were no conductive and optical features in bulk graphene. “The biggest lesson is that graphene is not that different,” said Zhang Yuanbo, a condensed matter physicist at Fudan University in Shanghai, China. However, the researchers said, “Graphene brings two-dimensional materials to the spotlight.”

When talking about high-tech equipment, the aura of graphene fainted. Most of the materials believed to be valuable in the electronic age are semiconductors, and graphene is more like a metal conductor. “Graphene is indeed an invaluable material,” says David Tomanek, a condensed matter physicist at Michigan State University. “But it doesn’t go along with the electronics industry.”

However, graphene opens the eyes of scientists, allowing them to focus on the new world of planar electronics. They saw materials similar to graphene but with new optoelectronic characteristics. They designed a single layer of silicon (silicone), a single layer of germanium (decene), and a single layer of tin (tin); they created nitriding. An insulator made of boron, which has a cage-like lattice structure like graphene; they make high-efficiency catalyst monolayer metal oxides that can be used to control specific chemical reactions; they even enclose water molecules in two-dimensional sheets although the use of this is still unclear.

But for now, most research work around planar materials focuses on two materials: one is called a compound called molybdenum disulfide (MoS2); the other is a two-dimensional black phosphorus single crystal (or black) phosphorus) a single layer of phosphorus atoms. Both materials have attractive electronic properties, and the competition among their researchers is extremely intense.

Molybdenum disulfide, optical equipment preferred material

Among the two materials, molybdenum disulfide research is the first to start. Molybdenum disulfide was synthesized in 2008 and is a member of the large family of transition metal disulfide materials (TMDs). This seemingly "fancy" name represents their structure: a transition metal atom (ie, a molybdenum atom) and a pair of atoms from the 16th column of the periodic table, including sulfur and selenium. The family element is famous).

To the surprise of electronics manufacturers, all TMDs are semiconductors. They are nearly identical to the thinness of graphene (in molybdenum disulfide, two layers of sulfur atoms sandwich a layer of molybdenum atoms like a "sandwich"), but they have other advantages. One of the advantages with regard to molybdenum disulfide is the speed at which electrons travel in planar flakes, ie, electron mobility. The electron mobility of molybdenum disulfide is about 100 cm 2 /vs (that is, 100 electrons per square centimeter per volt second), which is much lower than the electron mobility of crystalline silicon of 1400 cm 2 /vs, but smaller than amorphous silicon and other ultra-thin semiconductors. The migration rate is better, and scientists are investigating these materials for future electronic products such as flexible displays and other flexible electronic products.

Studies have shown that molybdenum disulfide is also very easy to make, even for making large pieces of two-dimensional materials. This allows engineers to test their performance in electronics at very fast speeds. For example, in 2011, a research team led by AndrasKis of the Swiss Federal Institute of Technology published an article in Nature-Nanotechnology, saying that they made the first transistors with a single layer of molybdenum disulfide that was only 0.65 nm thick. It turns out that those products and subsequent products have other unique properties than the more technologically advanced silicon-based counterparts.

In addition, molybdenum disulfide has other desirable properties, the direct band gap, which allows the material to convert electrons into photons and vice versa. This feature also makes molybdenum disulfide a good candidate for use in optical devices such as light emitters, lasers, photodetectors, and even solar cells. Some scientists say that this material is also rich in reserves, low in price, and non-toxic, so Yi-HsienLee believes: "The future is bright."

However, Tomanek believes that the electron transfer rate of molybdenum disulfide is still not high enough to make it competitive in the crowded electronics market. The reason for this is the structural characteristics of this material. When electrons move inside, they will explode in their structures when they encounter a large metal atom, thus reducing the migration speed.

But some scientists say that this stumbling block will be transient. Researchers are trying to circumvent these obstacles—by making a slightly thicker layer of molybdenum disulfide sheets that provide a path for compressed electrons to bypass roadblocks. "At that time, the migration problem of molybdenum disulfide will be solved," Yi-HsienLee said.

Black phosphorus, the new favorite of electronic equipment materials

The competitor of molybdenum disulfide, a two-dimensional black phosphorus single crystal (also known as black phosphorus), seems to make scientists more excited. A two-dimensional black phosphorus single crystal is one of three different crystal structures (or allotropes) that can be formed by pure phosphorus. The other two materials are white phosphorus used to make fireworks and red phosphorus used to make match heads.

The two-dimensional black phosphorus single crystal consists of wavy phosphorus atoms located on two planes, which were synthesized last year. But its properties have made it the darling of materials science, with an electron transfer rate of 600cm2/vs. Some researchers hope to further increase this rate; at the same time, its interband gap (the electrical volt required to pass current through the substance) It is tunable, that is, the electronics engineer can adjust the band gap by simply changing the stack of the two-dimensional black phosphorus single crystal, which is advantageous for designing the desired band gap according to specific requirements. "All of these properties make two-dimensional black phosphorus single crystals a super material," Tomanek said.

Researchers are advancing the commercialization of two-dimensional black-phosphorus single crystals at an extremely fast rate. On March 2 last year, Zhang Yuanbo and other colleagues at Fudan University published a report online in Nature-Nanotechnology, saying that they produced a crystal triode based on a two-dimensional black-phosphorus single crystal—a product in a computer logic circuit. Play a "heart" role. Two weeks later, Tomanek and colleagues also published a report on their use of two-dimensional black-phosphorus single-crystal transistors in the Journal of the American Chemical Society's Nano.

However, unfortunately, the two-dimensional black phosphorus single crystal is unstable in the air. “After 24 hours, we can see the bubbles on the surface of the material, and then the entire device will fail within a few days,” said Joon-SeokKim, a two-dimensional black phosphorus single crystal expert at the University of Texas at Austin. Experts say the culprit is water vapor, which reacts with phosphorus to convert phosphorus into phosphoric acid and cause corrosion. Still, Kim's research team and other researchers are still trying to solve this problem. For example, Kim said in a report from the American Physical Society in March that he and his colleagues could already keep transistors based on two-dimensional black-phosphorus single crystals for three months – by encapsulating them in alumina and Teflon in the compartment.

However, Yi-HsienLee believes that this method does not guarantee the long-term stability of the material. “You can add a protective layer to the product, but this only slows down the rate of aging.” He argues that the reason why two-dimensional black phosphorus single crystals are favored by some researchers is because this material is easy to use: Like graphene, it is easy to peel off the black phosphorus flakes with a transparent tape. “This is the same method,” says Yi-HsienLee. “But this does not mean that the prospect of two-dimensional black phosphorus single crystals is good.”

In the end, both materials may have a lot of room for development. "We are just getting started," said Florida State University physicist Luis Balicus. He said that over time, engineers will use the strong interaction of molybdenum disulfide with light to make solar cells, light emitters and other optical devices; at the same time enhance the high electron mobility of two-dimensional black phosphorus single crystals, and use it to make Electronic equipment.

Graphene and laser

1. New graphene photodetector

Spanish and American scientists have collaborated to develop a graphene-based photodetector converter that converts light into electrical signals in less than 50 femtoseconds (one-billionth of a second). Close to the limit of photoelectric conversion speed, will greatly promote the development of many fields.

Efficient photoelectric conversion technology, because it can convert the information carried by light into an electrical signal that can be processed in electronic circuits, plays an important role in many key technical fields from camera to solar cell, and is also an important data communication application support. Although graphene is a material with extremely high photoelectric conversion efficiency, scientists have not known how fast it reacts to ultrashort optical pulses.

Now, Professor Frank Koppons, a researcher at the Spanish Institute of Photonics, Nilke Van Hirst of the Institute of Advanced Studies in Catalonia, and Pablo Galliolo of the Massachusetts Institute of Technology -Hero, and a research team led by Liu Jinning, a professor of physics at the University of California, Riverside, developed the graphene-based photodetector converter that can light in less than 50 femtoseconds, converted to electricity, pushing the photoelectric conversion speed to the limit. The latest research has been published in the recently published journal Nature Nanotechnology.

To do this, the researchers used ultra-fast pulsed laser excitation and ultra-high sensitivity electronic readout methods. Researcher Kras-Jan Tell said: "The uniqueness of this experiment is that the ultra-fast pulse forming technology obtained from single-molecule ultrafast photonics is perfectly combined with graphene electronic technology, plus The nonlinear photo-thermoelectric reaction of graphene allows scientists to convert light into electrical signals in such a short period of time."

Researchers say that it is possible to quickly create photovoltages in graphene due to the ultra-fast and ultra-efficient correlation between all of the conduction band carriers in graphene. This correlation allows them to quickly create an electronic distribution using a rising electron temperature. As a result, the energy absorbed from the light can be efficiently and rapidly converted into heat of electrons. Subsequently, at the junction of the two graphene regions with two different dopings, the heat of the electrons is converted into a voltage. The experimental results show that this photothermographic effect occurs almost simultaneously, and the absorbed light can be quickly converted into an electrical signal.

Researchers say the latest research opens up a new pathway to ultrafast photoelectric conversion. Copps stressed: "The graphene photodetectors have amazing performance and can be used in many fields."

2. Graphene materials are expected to breed a new broadband laser

The international research team led by the Helsindorf Research Center (HZDR) in Helmholtz, Germany, is expected to promote the development of new broadband lasers in the dynamics of electrons in graphene under the action of strong magnetic fields.

The researchers placed graphene in a magnetic field of 4 Tesla, and under the action of the magnetic field, forced the electrons in the material to form a specific energy level, which is called the Landau level. The researchers then studied these energy levels with free electron lasers. Dr. Martin Mittendorff said: "The laser pulse excites electrons and makes it transition to a specific Landau level. Then, with time-delayed pulses to detect how the system works." The researchers found that the laser pumped some new electrons into the system. The Landau level, and then these Landau levels were gradually hollowed out in unexpected ways. They think this is caused by a collision between electrons.

Dr. Stephan Winnerl made an analogy to explain the process: “Imagine a librarian tidying up a book on a three-story shelf, she placed a book at a time, and placed the book on the lower shelf of the bookshelf in the middle shelf. Her son also helped her with her, each time taking two books from the middle shelf, one on the top shelf and one on the lower shelf. Her son very much hopes the middle shelf the number of books placed on it is reduced, even if the middle partition is the shelf where his mother wants to fill the book.” Winnerl said the researchers did not want the effect known as Auger scattering to be too strong or dissipated on the energy level electronics.

The placement of graphene in a magnetic field reveals many effects, and electrons have not been studied before in the kinetics of this system. Researchers believe that the phenomenon they found has the potential to achieve lasers, and that the resulting laser wavelengths can be arbitrarily tuned in the infrared and terahertz range.

Winnerl said: "For a long time, this Landau level laser was considered unachievable. But now, with graphene, the dream of semiconductor physicists is likely to become a reality."

3. Graphene or will replace SESAM as the core material of femtosecond fiber laser

Femtosecond fiber lasers are used in a wide range of applications, including laser imaging, holographic spectroscopy and ultrafast photonics, as well as laser processing, laser medical (such as ophthalmic surgery), and laserspecial. The traditional femtosecond fiber laser core device, semiconductor saturated absorption mirror (SESAM), is fabricated by semiconductor growth process, which is costly and has a monopoly of technology.

In the field of femtosecond fiber lasers, graphene is considered to be the best material to replace SESAM. The winners of the 2010 Nobel Prize in Physics predicted that graphene femtosecond fiber lasers are expected to be industrialized around 2018. In order to achieve real industrialization, it is necessary to solve a series of key technologies such as high-quality graphene preparation, large-scale low-cost graphene transfer, graphene and light field interaction, graphene saturated absorber packaging, and laser power stability control. After years of continuous research, Taizhou Junna New Energy Co., Ltd. successfully overcomes these key technologies and is the first to realize the commercialization of graphene femtosecond fiber lasers. The main performance indicators are higher than similar products, with high cost performance and strong performance market competitiveness.

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