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May 20, 2023

Study identifies a new synthesis technique to attain monolayer honeycomb SiC

March 13, 2023 feature

March 13, 2023 feature

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by Ingrid Fadelli , Phys.org

Silicon carbide (SiC) is a hard crystalline compound of silicon and carbon that rarely occurs in nature and is generally synthetically produced. In addition to being used to create ceramic plates, bulletproof vests and other commercial products, SiC is a semiconductor, a material that has a moderate electrical conductivity, ranging between that of conductors and insulators.

Physicists and material scientists have been investigating the properties of this semiconductor for decades. Like other materials, SiC can exist in different physical forms (i.e., allotropes), and its 2D allotrope has so far remained elusive and primarily hypothesized.

According to theoretical predictions, the 2D allotrope of this semiconductor would have a large direct band gap of 2.5 eV and a high chemical versability, and would be stable in ambient conditions. So far, however, this was not empirically verified, as existing studies only reported disordered nanoflakes of 2D SiC.

Researchers at Lund University, Chalmers University of Technology and Linköping University were recently able to synthesize monocrystalline epitaxial monolayer honeycomb SiC on top of ultrathin transition metal carbide films placed on SiC substrates. Their paper, published in Physical Review Letters, introduces a promising technique for the large-area and bottom-up synthesis of SiC's elusive allotrope.

"Our collaborators are interested in studying thin transition metal carbide films on SiC substrates," Craig Polley, one of the researchers who carried out the study, told Phys.org. "It was already known that graphene can be grown 'through' overlayers on SiC, and the hope was to do this and create a graphene encapsulation layer over the metal carbide films. Hence, the original point where we got involved was to study the properties of this grown graphene layer."

Initially, Polley and his colleagues were thus trying to investigate the properties of a graphene encapsulation layer formed over metal carbide films. However, while trying to characterize the properties of this layer using a technique known as ARPES (angle resolved photoemission spectroscopy), they observed very striking and fascinating spectra that did not resemble those observed in graphene.

"It eventually turned out that there was no graphene on the samples," Polley said.

"It took a lot of measurements and calculations before we could identify what this mysterious surface was, and we were pleasantly surprised when it turned out to be honeycomb SiC, as that was never our plan!"

Polley and his colleagues are yet to understand all the details of the process that underpins the successful growth of monolayer honeycomb SiC. Nonetheless, they were able to identify a technique that enables its synthesis.

Essentially, this technique entails placing a thin film of transition metal carbide on top of a SiC substrate. When this material stack is annealed to high enough temperatures, the SiC decomposes, while the metal carbide remains intact, and the Si and C atoms migrate to the surface.

"If you anneal hot enough the Si leaves and the C recrystallizes into graphene—and this is a well-known technique to grow high quality graphene layers on plain SiC," Polley explained. "But for the right annealing conditions, it turns out that Si and C not only remain on the surface but recrystallize into honeycomb SiC. Until now there was no known method to create large area, single crystal honeycomb SiC, so we were surprised that it works at all!"

The researchers also conducted further analyses to verify that the unique surface they observed was in fact the 2D phase of SiC. Once they confirmed this, they studied its characteristics, to validate previous theoretical predictions. Interestingly, they found that in this 2D phase, the SiC was almost planar and stable at high temperatures (up to 1,200 °C in vacuum).

"The main contributions here are the discovery of a new synthesis technique, and the in-depth detective work that went into conclusively identifying this mystery surface as honeycomb SiC," Polley said.

This recent study by Polley and his colleagues is merely a first step in the experimental investigation of SiC's 2D allotrope, as additional work will be required to effectively isolate the layer they observed from its underlying substrate. Nonetheless, the synthesis technique they uncovered is a notable milestone that paves the way towards this goal.

"One of the things we're interested in learning more about is whether there's anything one could do to decouple it from the substrate, for example by trying to intercalate some other species like hydrogen between the SiC and the TaC," Polley added. "That trick works with graphene on SiC, but this is now a new material and unexplored territory."

More information: C. M. Polley et al, Bottom-Up Growth of Monolayer Honeycomb SiC, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.076203

Journal information: Physical Review Letters

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