Graphene bandgap. Courtesy of Lawrence Berkeley National Laboratory
Graphene—a material comprised of a single layer of carbon atoms, arranged in a 2D honeycomb-shaped lattice—exhibits superb optical, mechanical and electrical properties. Discovered in 2003, it has been found to have an extremely high electron mobility (250,000 cm2/Vs), and an extremely high thermal conductivity (5,000 W/m K), among other desirable properties. Graphene is well-suited to the electronics industry because of its high electrical conductivity. But researchers have aspired to use it as an unconventional transistor material, an idea that has eluded the scientific world since its discovery. The main reason graphene cannot be used as a transistor material is because it has a zero bandgap, making it more a conductor than a semiconductor.
To produce switching devices, like transistors, a bandgap is needed. The bandgap is the energy difference between the valence and the conduction bands in semiconductors and conductors. The existence of the bandgap allows the movement of electrons from the valence to the conduction bands and vice-versa, so the material can switch electrical currents on and off in devices such as in transistors. In graphene the valence and conduction bands meet, similar to typical metals, as shown in the figure.
Scientists have recently tried to create a gap by teasing the two bands apart. Some attempts have been able to produce bandgaps in the order of 100 milli-electron-volts (meV), but this is not enough to create electronic devices. A typical semiconductor’s bandgap is larger than 0.5 eV.
A group of researchers at the Center for Multidimensional Carbon Materials (CMCM), within the Institute of Basic Science (IBS) in South Korea have developed a method to add hydrogen to graphene in order to improve its electronic and semiconductor behavior. Understanding how graphene chemically reacts with other materials will increase its utility as a future semiconductor material.
IBS researchers combine hydrogen – a reaction that is expected will increase the bandgap of graphene – by using a known method called “Birch-type reaction.” This is based on lithium dissolved in ammonia to introduce hydrogen into graphene through the formation of C-H bonds.
The IBS researchers found that hydrogenation takes place at a high rate in single-layer graphene, while it proceeds slowly in multi-layer graphene. But the most important find regarding changes in electronic properties is a significant change in optical as well as electric properties. This is a good sign for the future, given that hydrogenation is a simple and easy process.
"A primary goal of our Center is to undertake fundamental studies about reactions involving carbon materials. By building a deep understanding of the chemistry of single-layer graphene and a few layer graphene, I am confident that many new applications of chemically functionalized graphene could be possible, in electronics, photonics, optoelectronics, sensors, composites, and other areas," said Rodney Ruoff, the main researcher and CMCM director.
Their findings were published in the Journal of the American Chemistry Society in October 2016. An abstract of the article can be found here: http://pubs.acs.org/doi/abs/10.1021/jacs.6b08625.
