New 2D Semiconductor Found to Have 1.7 Electron Volt Bandgap

June 07, 2019 by Scott McMahan

A research team headed by chemist Dr. Michael J. Bojdys at Humboldt University Berlin collaborating with a team from Helmholtz-Zentrum Berlin have examined the electrical properties a new 2D semiconductor they recently synthesized in the carbon-nitride family.

Triazine-based graphitic carbon nitride (TGCN) is a semiconductor that is expected to be highly suitable for optoelectronics applications, according to the researchers.

Its structure, which is reminiscent of graphene, is two-dimensional. Unlike graphene, however, the conductivity running perpendicular to its 2D planes is 65 times higher than along the planes themselves.

TGCN is made up of only carbon and nitrogen atoms, and it can be grown as a brown film on a quartz substrate. Similar to graphene the atoms form hexagonal honeycombs in a two-dimensional structure

However, with graphene, the planar conductivity is excellent, while its perpendicular conductivity is very poor. According to the findings, TGCN it is exactly the opposite.

With a band gap of 1.7 electron volts, TGCN is a candidate for optoelectronics applications.

Whether in solar cells, LEDs, or in transistors, what is important is the band gap, the difference in energy level between electrons in bound state of the valence band and the mobile state of the conduction band.

Applying electrical Voltage or light can raise charge carriers from the valence band into the conduction band. Band gaps of between one and two electron volts are best for optoelectronics.

Analysis of transport properties

HZB physicist Dr. Christoph Merschjann subsequently studied the charge transport properties of TGCN samples utilizing time-resolved absorption measurements in the femtosecond to nanosecond range at the JULiq laser laboratory, a joint lab between HZB and Freie Universität Berlin.

These kinds of laser experiments can connect observed macroscopic electrical conductivity measurements with theoretical models and simulations of microscopic charge transport.

From this approach, Dr. Merschjann was able to infer how the charge carriers travel in the material.

"They do not exit the hexagonal honeycombs of triazine horizontally but instead move diagonally to the next hexagon of triazine in the neighboring plane. They move along tubular channels through the crystal structure."

This electrical transport mechanism might explain why the electrical conductivity perpendicular to the planes is significantly higher than that along the planes.

However, this explanation is likely insufficient to explain the measured factor of 65.

"We do not yet fully understand the charge transport properties in this material and want to investigate them further," Merschjann added.

"TGCN is, therefore, the best candidate so far for replacing common inorganic semiconductors like silicon and their crucial dopants, some of which are rare elements", says Bojdys. "The fabrication process we developed in my group at Humboldt-Universität, produces flat layers of semiconducting TGCN on an insulating quartz substrate. This facilitates upscaling and simple fabrication of electronic devices."

Reference Publication

Noda, Y., Merschjann, C., Tarábek, J., Amsalem, P., Koch, N., Bojdys, M. J. Directional Charge Transport in Layered Two‐Dimensional Triazine‐Based Graphitic Carbon Nitride. Angewandte Chemie (May 9, 2019, International Edition) DOI: 10.1002/anie.201902314