Beyond T-graphene: Two-dimensional tetragonal allotropes and their potential applications

Breakthrough of graphene dictates that decreasing dimensionality of the semiconducting materials can generate unusual electronic structures, excellent mechanical, and thermal characteristics with remarkable stability. Silicene, germanene, and stanene are the next 2D stable counterparts of other elements belonging to the same group. Since these monolayers possess hexagonal symmetry, scientists had already explored the possibility in the post graphene era of whether hexagonal symmetry was the main and utmost criterion for achieving Dirac cone. This motivation gave birth to T-graphene, a tetragonal network comprised of carbon atoms. However, T-graphene is not the only candidate for exhibiting Dirac fermion. In recent days, tetragonal monolayers of Si and Ge, i.e., T-Si and T-Ge, have been predicted to be stable. These 2D tetragonal allotropes remarkably possess double Dirac cones in their electronic band structure. As these monolayers possess buckling similar to silicene and germanene, the electronic bandgap can be easily introduced in the presence of an external electric field. Another technique to open bandgap is to apply strain in hydrogenated tetragonal networks. Tunable electronic properties in these tetragonal systems make them efficient for optoelectronics as well as thermoelectric applications. Moreover, due to delocalized π electrons, quantum dot systems comprised of tetragonal Si and Ge network show remarkable characteristics in the field of nonlinear optics. Recently, based on theoretical calculations, a bilayer T-graphene system is predicted with excellent mechanical strength relative to its monolayer variant. Not only group-IVA, group-VA elements also exhibit stable monolayer structures. Rather than T-graphene, T-Si, and T-Ge, these monolayers, however, possess intrinsic semiconducting properties, which enable them as a potential candidate for optoelectronic applications. Furthermore, several possible routes have been introduced to realize these systems experimentally. In this topical Review, we would critically explore the recent advancements of 2D tetragonal networks containing group-IVA and VA elements and their possible application perspectives in the field of thermoelectrics and nano-photonics.