New discovery in the world of semiconductors: scientists from FEE CTU and German institutions confirm the existence of another carbon allotrope

For many decades, there has been speculation about the existence of a fourth basic carbon allotropic form, which would differ from the first three allotropes (diamond, graphite and carbyne) in its structural form. Although scientists have observed this material, so far, they have not been able to elucidate its unusual crystal lattice or stabilize it as a separate form of carbon. This has now been done by a group of scientists, including Prof. Tomáš Polcar, Assoc. Prof. Antonio Cammarata and Andrey Bondarev from the Department of Control Engineering at the Faculty of Electrical Engineering of the CTU. Their research results are summarized in an article which was published in the Communications Materials journal in the prestigious scientific publishing house Nature in July 2024.

The extraordinary combination of high electrical conductivity and insulating capabilities

Fcc-carbon, as the material is named on the basis of its face-centered cubic crystal lattice, is characterized by an unusual hybridization of its valence orbitals which results in an extraordinary combination of high electrical conductivity and an ultra-wide band gap, which is, however, typical of insulators in particular. The band gap is an empty layer in the electron shell of an atom, located between the valence and conduction bands. To conduct an electric current, electrons must cross this band gap, so the wider it is, the better the element should insulate.

However, semiconductors also have a band gap, but it should be somewhat narrower than that of insulators. This new material, also partly thanks to its ultra-wide band gap, should be able to operate under high temperatures, frequencies or voltages, which is difficult for the standard silicon-based semiconductors. However, at the same time, fcc-carbon maintains levels of conductivity comparable to these commonly used semiconductors. What is also remarkable about this material is that it does not need to be doped to achieve its properties. In fact, additional components are commonly added to some materials to improve their semiconducting properties. Thus, fcc-carbon is the first intrinsic ultra-wide band gap semiconductor .

The industry is interested. The first devices could reach the market in 10 years

In the future, the material could thus find applications in high-power, high-frequency or high-temperature electronics. Due to its properties, it could also potentially be used in radio-frequency electronics, deep-UV optoelectronics, transparent electronics or quantum devices. However, this discovery is still a long way from practical applications and carbon electronics, which would run on carbon semiconductors instead of conventional semiconductors. Given the boom in artificial intelligence in materials science, it can be expected that such devices could potentially reach the market in about 10 years.

However, the time commitment does not deter scientists from further research. They plan to continue and to set up a consortium to work on this material at the European level. Both universities and industrial partners should be involved. "I believe that this is only the aperitif of what I am confident will be a great banquet," says Assoc. Prof. Antonio Cammarata from the Department of Control Engineering, who has been heavily involved in the research. There have already been offers of collaboration from industry, which only goes to show the great potential of this allotropic form of carbon.

The research started in October 2020, but several experiments had to be postponed due to the pandemic. The experiments were carried out at four different institutions: at the Faculty of Electrical Engineering of the CTU (X-ray photoelectron spectroscopy, transmission electron microscopy, etc.), at the Technical University of Berlin (Raman spectroscopy, Fourier-transform infra-red spectroscopy, etc.), at the Helmholtz-Zentrum Berlin research center (photo-electron yield spectroscopy) and at the Element Six GmbH company (high-resolution scanning electron microscopy).

The scientists were motivated by the mystery raised by experimental data from the Element Six GmbH company, namely how can carbon form a material with a high coordination number, a very wide band gap, and still be conductive? Therefore, solving this question was a challenge for the scientists, since, according to Assoc. Prof. Cammarata, they had to learn to work with new simulation techniques and theories and to account for unusual physics which is not expected for carbon. "But I am always excited about these kinds of challenges. And the unknown corners of physics have always fascinated me, just as simple things fascinate a child taking its first steps," says the researcher of his passion for research.

An opportunity for students who want to get involved in materials science research

Assoc. Prof. Cammarata is happy to share his enthusiasm for materials science with the students at our faculty, where he teaches several courses on the subject. For undergraduate and graduate students, he offers a course called Elements of Atomistic Simulations, where he teaches the basics of classical and quantum mechanics for designing in-silico experiments in materials science. Doctoral students can then enroll in a more advanced course called Computational Methods for Materials Science to expand their knowledge in this area.

These courses are a good start for students interested in getting involved in materials research. A PhD student at the Department of Control Engineering, Ing. Matúš Kaintz knows all about it, as he took the PhD course mentioned above as part of his Master's degree in Cybernetics and Robotics, since the course for Bachelor's and Master's students did not exist at that time. Even though Ing. Matúš Kaintz had first encountered materials science and quantum mechanics while writing his bachelor thesis under the supervision of Assoc. Prof. Cammarata, this course significantly deepened his theoretical knowledge of computational methods and showed him new research techniques. "I believe that courses led by Assoc. Prof. Cammarata can be eye-opening and useful for many students, even those who might not have initially considered materials science as a career path, just as it was for me," recalls Ing. Kaintz, who is now continuing his doctoral studies at the Faculty of Electrical Engineering of the CTU. His research focuses on another carbon allotrope, namely diamond, which also has potential applications in the field of electronics and optoelectronics.

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