Inspired by the success of graphene, various two dimensional (2D) non-hexagonal graphene allotropes having sp 2 bonded tetragonal rings in free standing (FS) (hypothetical) form and on different substrates have been proposed recently. These systems have also been fabricated after modifying the topology of graphene by chemical processes. In this review, we would like to in- dicate the role of tetra-rings and the local symmetry breaking on the structural, electronic and optical properties of the graphene system. The first principles computations have demonstrated that the tetragonal graphene (TG) allotope exhibits appreciable thermodynamic stability. The band structure of the TG nannoribbons (TGNRs) strongly depends on the size and edge geometry. This fact has been supported by the transport properties of TGNRs. The optical properties and Raman modes of this graphene allotrope have been well explored for the characterization purpose. Re- cently, a tight binding model has been used to unravel the metal to semiconductor transition under the influence of external magnetic fluxes. Even the introduction of transition metal atoms into this non-hexagonal network can control the magnetic response of the TG sheet. Furthermore, the collective effect of B-N doping and confinement effect on structural and electronic properties of TG systems have been investigated. We also suggest future directions to be explored to make the synthesis of T-graphene and its various derivatives/allotropes viable for verification of theoretical predictions. It is observed that these doped systems act as a potential candidate for carbon monox- ide (CO) gas sensing and current rectification device. Therefore, all these experimental, numerical and analytical studies related to non-hexagonal TG systems are extremely important from basic science point of view as well as for the device applications in sensing, optoelectronic and photonic devices.

中文翻译：

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四环在石墨烯系统中的作用及其可能的应用综述

受石墨烯成功的启发，最近提出了各种二维 (2D) 非六方石墨烯同素异形体，这些同素异形体具有独立 (FS)（假设）形式和不同基材上的 sp 2 键合四方环。这些系统也是在通过化学过程修改石墨烯的拓扑结构后制造的。在这篇综述中，我们想指出四环的作用和局部对称性破坏对石墨烯系统的结构、电子和光学特性的影响。第一性原理计算表明，四方石墨烯 (TG) 同素异位体表现出可观的热力学稳定性。TG 纳米带（TGNRs）的能带结构很大程度上取决于尺寸和边缘几何形状。这一事实得到了 TGNR 传输特性的支持。这种石墨烯同素异形体的光学特性和拉曼模式已被充分探索用于表征目的。最近，紧束缚模型已被用于解开外部磁通量影响下的金属到半导体的转变。即使将过渡金属原子引入到这种非六边形网络中，也可以控制 TG 片的磁响应。此外，还研究了 BN 掺杂和限制效应对 TG 系统结构和电子特性的集体影响。我们还建议探索未来的方向，以使 T-石墨烯及其各种衍生物/同素异形体的合成可用于验证理论预测。据观察，这些掺杂系统可作为一氧化碳 (CO) 气体传感和电流整流装置的潜在候选者。因此，所有这些与非六边形 TG 系统相关的实验、数值和分析研究从基础科学的角度以及在传感、光电和光子器件中的器件应用都极为重要。