Two-dimensional Janus layers offer unique, promising quantum characteristics

Named after the two-faced Roman god, Janus materials have the unique characteristic of having vastly different chemical compositions – and sometimes functionalities – on each side. Yagmurcukardes et al. review the predicted quantum properties of two-dimensional Janus layers and discuss their experimental challenges.

“Formations of Janus-type 2D layers may result in unique quantum phenomena,” said author Hasan Sahin. “They offer properties that lead to many important application areas in quantum sciences, energy conversion technologies and spintronics.”

The most obvious example of 2D Janus layers is graphene-based. One side of the material is made of graphene, giving it a wide bandgap suitable for optoelectronic applications. The other side consists of atoms from surface functionalization groups that can absorb onto the graphene, such as hydrogen, chlorine or fluorine.

This combination of individual layers leads to exciting quantum characteristics, including tunable electronic properties, colossal vertical electric fields and intrinsic polarization that can allow for quantum spin ratio manipulation. Because of these unique features, superlattices of 2D Janus layers are promising in energy conversion and piezoelectrics.

“We anticipate that their quantum properties will unlock novel applications that are not possible with other materials,” said Sahin.

Though two independent groups reported the experimental realization of 2D Janus materials in 2017, they remain difficult to synthesize. As a result, much of the work on these materials is driven by theory, since experimental studies are not readily reproducible.

“While a wealth of quantum and physical effects have been predicted, the experimental results are severely lacking,” Sahin said. “We anticipate essential progress in the fundamental understanding of these materials once the synthesis techniques become more accessible.”

Source: “Quantum properties and applications of 2D Janus crystals and their superlattices,” by M. Yagmurcukardes, Y. Qin, S. Ozen, M. Sayyad, F. M. Peeters, S. Tongay, and H. Sahin, Applied Physics Reviews (2020). The article can be accessed at https://doi.org/10.1063/1.5135306 .