The fascinating electronic and optoelectronic properties of freestanding graphene and the possible inclusion of novel two-dimensional (2D) systems in silicon-based electronics have driven the search for atomic layers consisting of other group-IV elements Si, Ge, Sn, and Pb, which form similar hexagonal lattices and are isoelectronic to graphene. The resulting 2D crystals silicene, germanene, stanene and plumbene, referred as Xenes, but also their functionalized counterparts, e.g. the hydrogenated sheet crystals, named as Xanes, silicane, germanane, and stanane, are in the focus of this review article. In addition, halogenated Xenes are investigated. The consequences of the larger atomic radii on the atomic geometry, the energetic stability, and possible epitaxial preparations are discussed.

In the case of honeycomb atomic arrangements, the low-energy electronic excitations are ruled by almost linear bands. Spin–orbit coupling opens small gaps leading to Dirac fermions with finite effective masses. The linear bands give rise to an absorbance of the Xenes determined by the finestructure constant in the long-wavelength regime. While for vanishing photon energies the excitonic influence is still an open question, saddle-point excitons and excitons at $M_0$ van Hove singularities appear at higher frequencies. After opening substantial fundamental gaps by hydrogenation, the absorption edges of the Xanes, silicane, germanane, and stanane, are dominated by bound excitons with extremely large binding energies. Other chemical functionalizations, but also vertical electric fields, yield electronic structures ranging from topological to trivial insulators. Even a quantum spin Hall phase is predicted at room temperature. The topological character and the possible quantization of the spin Hall conductivity are studied versus gap inversion, chemical functionalization, and Rashba spin–orbit interaction. The drastic changes of the electronic properties of Xenes with chemical functionalization, interaction with the substrate, and external perturbations, open future opportunities for tailoring fundamental properties and, therefore, interesting applications in novel electronic and optoelectronic nanodevices.

独立石墨烯迷人的电子和光电特性以及可能在硅基电子产品中包含新型二维 (2D) 系统，推动了对由其他 IV 族元素 Si、Ge、Sn 和 Pb 组成的原子层的研究，它们形成类似的六边形晶格并且与石墨烯等电子。由此产生的二维晶体硅烯、锗烯、stanene 和 plumbene，称为 Xenes，以及它们的功能化对应物，例如氢化片状晶体，命名为 Xanes、silane、germanane 和 stanane，是这篇评论文章的重点。此外，研究了卤化的氙。讨论了较大原子半径对原子几何形状、能量稳定性和可能的​​外延制备的影响。

在蜂窝原子排列的情况下，低能电子激发几乎由线性带支配。自旋轨道耦合打开了导致具有有限有效质量的狄拉克费米子的小间隙。线性带产生由长波长区域中的精细结构常数确定的 Xene 的吸光度。而对于消失的光子能量，激子的影响仍然是一个悬而未决的问题，鞍点激子和激子在${米}_0$范霍夫奇点出现在更高的频率。在通过氢化打开大量基本间隙后，Xane、硅烷、锗烷和锡烷的吸收边缘由具有极大结合能的束缚激子支配。其他化学功能化，以及垂直电场，产生从拓扑绝缘体到普通绝缘体的电子结构。甚至在室温下预测量子自旋霍尔相位。研究了自旋霍尔电导率的拓扑特征和可能的量化与间隙反转、化学功能化和 Rashba 自旋轨道相互作用。Xenes 的电子性质随着化学功能化、与底物的相互作用和外部扰动的剧烈变化，