A new semiconductor of the family of carbon nitrides

Teams from Humboldt-Universität and Helmholtz-Zentrum Berlin have explored a new material in the family of carbon nitrides. Triazine-based graphitic carbon nitride (TGCN) is a semiconductor that should be ideally suited for optoelectronic applications. Its structure is two-dimensional and reminds graphene. Unlike graphene, however, the conductivity in the direction perpendicular to its two-dimensional planes is 65 times higher than in the planes themselves.

Some organic materials could be used in the same way as silicon semiconductors in optoelectronics. Whether in solar cells, light-emitting diodes or transistors, the important thing is the forbidden band, ie the difference in energy level between the electrons in the valence band (bound state) and the conduction band (mobile state). Charge carriers can be lifted from the valence band into the conduction band by means of a light or electrical voltage. This is the operating principle of all electronic components. The forbidden bands of one to two electron volts are ideal.

A team led by chemist Michael J. Bojdys of the Humboldt University of Berlin has recently synthesized a new organic semiconductor material from the family of carbon nitrides. Triazine-based graphitic carbon nitride (or TGCN) consists solely of carbon and nitrogen atoms and can be grown as a brown film on a quartz substrate. The combination of C and N atoms forms alveoli hexagonal similar to graphene, composed of pure carbon. As for graphene, the crystalline structure of TGCN is two-dimensional. With graphene, however, the plane conductivity is excellent, while its perpendicular conductivity is very low. In TGCN, it is exactly the opposite: the perpendicular conductivity is about 65 times greater than the plane conductivity. With a bandgap of 1.7 electron volts, TGCN is a good candidate for optoelectronic applications.

HZB physicist Christoph Merschjann then investigated the charge transport properties in TGCN samples using time-resolved absorption measurements in the nanometer range at fem, at the JULiq laboratory, a joint laboratory between HZB and Freie Universität Berlin. These types of laser experiments make it possible to relate macroscopic electrical conductivity to theoretical models and simulations of microscopic charge transport. With this approach, he was able to deduce how load carriers move in the material. "They do not go out horizontally from the hexagonal honeycomb nests of the triazine, but move diagonally to the next hexagon of the triazine in the neighboring plane.They move along tubular channels through the crystalline structure." This mechanism could explain why the electrical conductivity perpendicular to the planes is considerably higher than that along the planes. However, this is probably not enough to explain the actual measured factor of 65. "We do not yet fully understand the charge transport properties of this material and we want to study them further," adds Merschjann. ULLAS / HZB in Wannsee, the analytical laboratory used after JULiq, is currently preparing new experiments.

"TGCN is therefore the best candidate to date to replace common inorganic semiconductors such as silicon and their essential dopants, some of which are rare elements," said Bojdys. "The manufacturing process we developed in my group at Humboldt University is to produce flat layers of TGCN semiconductors on an insulating quartz substrate, which makes it easier to scale and the simple manufacture of electronic devices. "