Silicon carbide (SiC) is a hard crystalline compound of silicon and carbon that is rarely found in nature and is usually produced synthetically. In addition to being used to make ceramic plates, bulletproof vests, and other commercial products, SiC is a semiconductor, a material with a moderate electrical conductivity that falls between that of conductors and insulators.
Physicists and materials scientists have been investigating the properties of this semiconductor for decades. Like other materials, SiC can exist in various physical forms (i.e., allotropes), and its 2D allotrope has so far remained elusive and treated mostly hypothetically.
According to theoretical predictions, the 2D allotrope of this semiconductor would have a large direct band gap of 2.5 eV and high chemical versatility, and would be stable under ambient conditions. So far, however, this has not been empirically verified, as existing studies have only reported disordered nanoflakes of 2D-SiC.
Researchers from Lund University, Chalmers University of Technology and Linköping University were recently able to synthesize monocrystalline epitaxial honeycomb SiC on ultra-thin transition metal carbide films placed on SiC substrates. Her work, published in Physical Verification Letterspresents a promising technique for the large-scale and bottom-up synthesis of the elusive allotrope of SiC.
“Our collaborators are interested in studying thin transition metal carbide films on SiC substrates,” Craig Polley, one of the researchers who conducted the study, told Phys.org. “It was already known that graphene could be grown ‘through’ cap layers on SiC and the hope was to do this and create a graphene encapsulation layer over the metal carbide films. Therefore, the initial point we got involved in was to study the properties of this grown graphene sheet.”
First, Polley and his colleagues therefore attempted to study the properties of a graphene encapsulation layer formed over metal carbide films. However, when they tried to characterize the properties of this layer using a technique known as ARPES (angle-resolved photoemission spectroscopy), they observed very striking and intriguing spectra that did not resemble those observed in graphene.
“It eventually turned out that the samples did not contain graphene,” Polley said.
“It took a lot of measurements and calculations before we were able to 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 have yet to understand all the details of the process underpinning the successful growth of single-layer honeycomb SiC. Nonetheless, they were able to identify a technique that would allow their synthesis.
Essentially, this technique involves placing a thin film of transition metal carbide on a SiC substrate. When this stack of material is annealed to sufficiently high 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 and C will recrystallize into graphene – and this is a well-known technique for growing high-quality graphene layers on plain SiC,” Polley explained. “But for the right annealing conditions, it turns out that not only do the Si and C remain on the surface, they recrystallize into honeycomb SiC. Until now, no method was known to produce large-area, single-crystal honeycomb SiC, so we were surprised that this is possible at all!”
Researchers also performed further analysis to verify that the observed unique surface was indeed the 2D phase of SiC. After confirming this, they studied their properties to validate previous theoretical predictions. Interestingly, they found that in this 2D phase, the SiC was nearly planar and stable at high temperatures (up to 1,200 °C in vacuum).
“The key contributions here are the discovery of a new synthetic technique and the thorough detective work that went into conclusively identifying this mysterious surface as honeycomb SiC,” said Polley.
This recent study by Polley and his colleagues is just a first step in the experimental investigation of the 2D allotrope of SiC, as more work is needed to effectively isolate the layer they observed from its underlying substrate. Nonetheless, the synthetic technique they discovered is a remarkable milestone that paves the way towards this goal.
“One of the things we’d like to learn more about is if there’s anything you could do to decouple it from the substrate, for example by trying to intercalate other species like hydrogen between SiC and TaC,” Polley added. “This trick works with graphene on SiC, but that’s new material and unexplored territory now.”
CM Polley et al., Bottom-Up Growth of Monolayer Honeycomb SiC, Physical Verification Letters (2023). DOI: 10.1103/PhysRevLett.130.076203
Physical Verification Letters
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