In 2010, Geim and Novoselov won the Nobel Prize in physics for their work on graphene. This award has left a deep impression on many people. After all, not every Nobel Prize experimental tool is as common as adhesive tape, and not every research object is as magical and easy to understand as “two-dimensional crystal” graphene. The work in 2004 can be awarded in 2010, which is rare in the record of Nobel Prize in recent years.
Graphene is a kind of substance that consists of a single layer of carbon atoms closely arranged into a two-dimensional honeycomb hexagonal lattice. Like diamond, graphite, fullerene, carbon nanotubes and amorphous carbon, it is a substance (simple substance) composed of carbon elements. As shown in the figure below, fullerenes and carbon nanotubes can be seen as rolled up in some way from a single layer of graphene, which is stacked by many layers of graphene. The theoretical research on the use of graphene to describe the properties of various carbon simple substances (graphite, carbon nanotubes and graphene) has lasted for nearly 60 years, but it is generally believed that such two-dimensional materials are difficult to stably exist alone, only attached to the three-dimensional substrate surface or inside substances like graphite. It was not until 2004 that Andre Geim and his student Konstantin Novoselov stripped a single layer of graphene from graphite through experiments that the research on graphene achieved new development.
Both fullerene (left) and carbon nanotube (middle) can be regarded as being rolled up by a single layer of graphene in some way, while graphite (right) is stacked by multiple layers of graphene through the connection of van der Waals force.
Nowadays, graphene can be obtained in many ways, and different methods have their own advantages and disadvantages. Geim and Novoselov obtained graphene in a simple way. Using transparent tape available in supermarkets, they stripped graphene, a graphite sheet with only one layer of carbon atoms thick, from a piece of high-order pyrolytic graphite. This is convenient, but the controllability is not so good, and graphene with a size of less than 100 microns (one tenth of a millimeter) can only be obtained, which can be used for experiments, but it is difficult to be used for practical applications. Chemical vapor deposition can grow graphene samples with the size of tens of centimeters on the metal surface. Although the area with consistent orientation is only 100 microns [3,4], it has been suitable for the production needs of some applications. Another common method is to heat the silicon carbide (SIC) crystal to more than 1100 ℃ in vacuum, so that the silicon atoms near the surface evaporate, and the remaining carbon atoms are rearranged, which can also obtain graphene samples with good properties.
Graphene is a new material with unique properties: its electrical conductivity is as excellent as copper, and its thermal conductivity is better than any known material. It is very transparent. Only a small part (2.3%) of the vertical incident visible light will be absorbed by graphene, and most of the light will pass through. It is so dense that even helium atoms (the smallest gas molecules) cannot pass through. These magical properties are not directly inherited from graphite, but from quantum mechanics. Its unique electrical and optical properties determine that it has broad application prospects.
Although graphene has only appeared for less than ten years, it has shown many technical applications, which is very rare in the fields of physics and material science. It takes more than ten years or even decades for general materials to move from laboratory to real life. What’s the use of graphene? Let’s look at two examples.
Soft transparent electrode
In many electrical appliances, transparent conductive materials need to be used as electrodes. Electronic watches, calculators, televisions, liquid crystal displays, touch screens, solar panels and many other devices can not leave the existence of transparent electrodes. The traditional transparent electrode uses indium tin oxide (ITO). Due to the high price and limited supply of indium, the material is brittle and lack of flexibility, and the electrode needs to be deposited in the middle layer of vacuum, and the cost is relatively high. For a long time, scientists have been trying to find its substitute. In addition to the requirements of transparency, good conductivity and easy preparation, if the flexibility of the material itself is good, it will be suitable for making “electronic paper” or other foldable display devices. Therefore, flexibility is also a very important aspect. Graphene is such a material, which is very suitable for transparent electrodes.
Researchers from Samsung and chengjunguan University in South Korea obtained graphene with a diagonal length of 30 inches by chemical vapor deposition and transferred it to a 188 micron thick polyethylene terephthalate (PET) film to produce a graphene based touch screen . As shown in the figure below, the graphene grown on the copper foil is first bonded with the thermal stripping tape (blue transparent part), then the copper foil is dissolved by chemical method, and finally the graphene is transferred to the PET film by heating.
New photoelectric induction equipment
Graphene has very unique optical properties. Although there is only one layer of atoms, it can absorb 2.3% of the emitted light in the whole wavelength range from visible light to infrared. This number has nothing to do with other material parameters of graphene and is determined by quantum electrodynamics . The absorbed light will lead to the generation of carriers (electrons and holes). The generation and transport of carriers in graphene are very different from those in traditional semiconductors. This makes graphene very suitable for ultrafast photoelectric induction equipment. It is estimated that such photoelectric induction equipment may work at the frequency of 500ghz. If it is used for signal transmission, it can transmit 500 billion zeros or ones per second, and complete the transmission of the contents of two Blu ray discs in one second.
Experts from IBM Thomas J. Watson Research Centre in the United States have used graphene to manufacture photoelectric induction devices that can work at 10GHz frequency . Firstly, graphene flakes were prepared on a silicon substrate covered with 300 nm thick silica by “tape tearing method”, and then palladium gold or titanium gold electrodes with an interval of 1 micron and a width of 250 nm were made on it. In this way, a graphene based photoelectric induction device is obtained.
Schematic diagram of graphene photoelectric induction equipment and scanning electron microscope (SEM) photos of actual samples. The black short line in the figure corresponds to 5 microns, and the distance between metal lines is one micron.
Through experiments, the researchers found that this metal graphene metal structure photoelectric induction device can reach the working frequency of 16ghz at most, and can work at high speed in the wavelength range from 300 nm (near ultraviolet) to 6 microns (infrared), while the traditional photoelectric induction tube can not respond to infrared light with longer wavelength. The working frequency of graphene photoelectric induction equipment still has great room for improvement. Its superior performance makes it have a wide range of application prospects, including communication, remote control and environmental monitoring.
As a new material with unique properties, the research on the application of graphene is emerging one after another. It is difficult for us to enumerate them here. In the future, there may be field effect tubes made of graphene, molecular switches made of graphene and molecular detectors made of graphene in daily life… Graphene that gradually comes out of the laboratory will shine in daily life.
We can expect that a large number of electronic products using graphene will appear in the near future. Think about how interesting it would be if our smartphones and netbooks could be rolled up, clamped on our ears, stuffed in our pockets, or wrapped around our wrists when not in use!
Post time: Mar-09-2022