Protomene: Better than GaN/Graphene?

Article By : Nitin Dahad, EE Times

Carbon allotrope protomene could have a serious future in optoelectronics and semiconductors, say researchers.

LONDON — An Abu Dhabi based company claims to have identified an allotrope of carbon that may be more suitable than gallium nitride (GaN) for opto-electronic components and applicable to more semiconductor device applications than carbon nanotubes (CNT) and graphene.

A paper on the topic, published in the scientific journal Carbon, shows the structure of a new carbon allotrope, protomene, which researchers say offers promise of tremendous advancements in the electronics industry as a single material. 

A team of international experts working on this advancement is led by Emirati brothers, Mohamed and Rashed Al Fahim at Alfields LLC. It is part of the United Arab Emirates government strategy to address innovation and future technologies needs for the “Fourth Industrial Revolution,” launched in September 2017.

“Protomene and its tremendous properties have been on the wish lists of forward thinking innovators and manufacturers for decades, and we will deliver it,” said Larry Burchfield, an American nuclear chemist who serves as chief science officer at Alfields.   

“We’ve graduated from the ‘pipe dream’ stage to eventually making a certain and beneficial impact upon the world of semiconductors, opto-electronics, coatings, and energy conservation,” Burchfield said.

Alfields says this probably marks the first new carbon allotrope classification since that of fullerenes by Nobel Laureates Robert F. Curl Jr., Sir Harold W. Krotoand Richard E. Smalley in 1996, and the most significant advancement since graphene by Nobel Laureates Andre Geim and Konstantin Novoselov in 2010.

Work toward the actual manufacturing of protomene will move forward in collaboration with the Khalifa University of Science and Technology in Abu Dhabi.

Protomene qualifies as a new potentially useful direct gap semiconductor. The energy band gap is very close to that of GaN, which is approximately 3.4 eV at room temperature. As a result, protomene possesses similar semiconducting properties to GaN, which may enable it to have applications for high-power and/or high-frequency electronic devices with large breakdown voltages.

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The electronic states of protomene in an energy region around the top of the valence band (dashed line). Left: the DFT-LDA Kohn-Sham band structure along the path in the first Brillouin zone highlighted in the inset at the right side. Right: the density of electronic states of these bands.
Source: Carbon

Considering that it is a binary system, the control of GaN composition is challenging during the crystal growth, whereas protomene is a single-element carbon allotrope and defect control might be easier than for GaN. Due to the gap amplitude near the blue end of the visible spectrum, protomene may find applications in opto-electronic components, such as blue or UV-light generating LEDs, or as a UV filter in optics.

Furthermore, the well-defined direct gap suggests that protomene may have more application in semiconductor devices than CNTs and graphene in terms of energy band gap. Indeed, one obstacle to overcome in the fabrication of CNTs is controlling whether the CNT is metallic and semiconducting. Protomene instead is predicted to be semiconducting up to a transition temperature.

Thermal expansion in protomene is likely to play against the interplane bonding. As the temperature is raised, a structural phase transition is likely to occur from a low-temperature semiconducting 48-atoms cell structure to the high-temperature metallic phase characterized by a 24-atoms cell structure. As this transition is approached, the band gap would close rapidly, much faster than the slow decay, also associated to thermal expansion, in diamond and silicon.

Accordingly, this phase transition would provide a sensitive temperature-controlled optical filter. The eventual transition to the high-temperature no-dimers metallic phase of protomene also has potential temperature-controlled optical and electric switching applications.

The quest for new allotropes of carbon has been an increasingly active field of research for several decades. Broad interest is fueled by the wide range of structural and electronic properties of carbon allotropes.

Carbon possesses three energetically competitive different types of orbital hybridization (sp, sp2, and sp3). This allows carbon atoms to combine with each other in an exceptional number of ways.

The sp3 configuration gives rise to three-dimensional networks with insulating properties along with high stiffness, as in cubic and hexagonal diamond. In contrast the sp (linear) and sp2 (planar) hybridizations can be responsible for flexible structures such as carbyne and graphene, which often come with small electronic interband gaps or even metallic properties. Intermediate hybridizations are quite frequent as well, as in the fullerenes and the nanotubes.

Protomene is based on a new stable carbon structure combining sp2 and sp3 hybridizations, with six atoms out of 24 being able to adopt a perfectly planar sp2 geometry, from which they can then move out of the plane to build comparably weak bonds with partner atoms in the next vertically stacked lattice cell. It’s thought that this extra bond formation will lower the total energy by approximately 1 eV per bond, and therefore induce a substantial change in electronic properties.

— Nitin Dahad is a European correspondent for EE Times.