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.

中文翻译：

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超越 T-石墨烯：二维四方同素异形体及其潜在应用

石墨烯的突破表明，降低半导体材料的维度可以产生不寻常的电子结构、出色的机械和热特性以及显着的稳定性。硅烯、锗烯和锡烯是属于同一族的其他元素的下一个二维稳定对应物。由于这些单分子层具有六方对称性，科学家们已经在后石墨烯时代探索了六方对称性是否是实现狄拉克锥的主要和最高标准的可能性。这种动机催生了 T-石墨烯，一种由碳原子组成的四方网络。然而，T-石墨烯并不是展示狄拉克费米子的唯一候选者。最近几天，预测 Si 和 Ge 的四方单层，即 T-Si 和 T-Ge 是稳定的。这些二维四方同素异形体在其电子能带结构中具有双狄拉克锥。由于这些单层具有类似于硅烯和锗烯的屈曲性，因此在存在外部电场的情况下可以很容易地引入电子带隙。打开带隙的另一种技术是在氢化四方网络中施加应变。这些四方系统中的可调电子特性使它们在光电和热电应用中非常有效。此外，由于π电子离域，由四方Si和Ge网络组成的量子点系统在非线性光学领域表现出显着的特性。最近，基于理论计算，预测双层 T-石墨烯系统相对于其单层变体具有优异的机械强度。不仅是IVA组，VA 族元素也表现出稳定的单层结构。然而，与 T-石墨烯、T-Si 和 T-Ge 不同，这些单层膜具有固有的半导体特性，这使它们成为光电应用的潜在候选者。此外，已经引入了几种可能的途径来通过实验实现这些系统。在这篇专题综述中，我们将批判性地探讨包含 IVA 族和 VA 族元素的二维四方网络的最新进展及其在热电学和纳米光子学领域的可能应用前景。已经引入了几种可能的途径来通过实验实现这些系统。在这篇专题综述中，我们将批判性地探讨包含 IVA 族和 VA 族元素的二维四方网络的最新进展及其在热电学和纳米光子学领域的可能应用前景。已经引入了几种可能的途径来通过实验实现这些系统。在这篇专题综述中，我们将批判性地探讨包含 IVA 族和 VA 族元素的二维四方网络的最新进展及其在热电学和纳米光子学领域的可能应用前景。