Planar Hypercoordinate Motifs in Two-Dimensional Materials
Planar Hypercoordinate Motifs in Two-Dimensional Materials
复制标题
二维材料中的平面超坐标基序
DOI：
10.1021/acs.accounts.0c00025
复制
发表时间：
2020
影响因子：
18.3
通讯作者：
Chen Zhongfang
中科院分区：
文献类型：
--
作者：
Wang Yu;Li Yafei;Chen Zhongfang
碳元素是化学、物理和材料科学中最重要和最前沿的研究课题之一。许多基于碳的化学规则因此而建立。虽然自1874年以来，四配位碳的四面体偏好一直是有机化学的基石，但几乎世纪后，发现四配位碳能够采用称为平面四配位碳（ptC）的平面结构，它们被良好的π受体电子稳定（ptC的孤电子对的离域）或σ-供体（促进电子转移到缺电子键合）取代基或通过适当的空间强制机械地取代。这些突破常规的ptC物种的实验和理论研究成果刷新了我们对化学键的认识，并引发了对平面五配位碳（ppC）和平面六配位碳（phC）以及平面超配位硅等奇异物种的探索。由于最近对石墨烯及其类似物的广泛研究，人们对追求它们的扩展系统，特别是在二维（2D）空间中的扩展系统越来越感兴趣。虽然天然的二维层状晶体不包含任何平面超配位碳或硅，但理论上已经提出了几种具有平面或准平面超配位的二维纳米片。令人鼓舞的是，这些独特的平面构型具有相当的稳定性，其中一些甚至是全局最小结构，表现出巨大的实验实现潜力。由于材料的性质主要由其结构特性（例如，平面超坐标化学和二维纳米科学的结合不仅赋予这些打破规则的系统二维材料的优点，而且还可以提供各种有前途的性质和应用。例如，一个不寻常的负泊松比可以发现在ppC的含Be 5C 2和平面五配位硅（ppSi）的含CaSi单层，其中前者具有各向异性的狄拉克锥和后者是一个半导体与半导体工业所需的带隙。特别地，在理论预测之后不久，实验合成了一种含平面六配位硅（phSi）的Cu 2Si单层膜，并利用二维Dirac节线费米子对其进行了表征，这为实现高速、低损耗的纳米器件提供了一个平台。本文主要通过密度泛函理论（DFT）计算，综述了在设计具有平面超配位基序的二维材料方面的最新进展。我们描述了这一领域的主要成就，特别注意“自下而上”和“等电子取代”的设计策略。此外，还讨论了无限层中平面超坐标模体的基本稳定机制。我们希望这个帐户将激发更多的实验和理论努力，探索具有这种非常规化学键合的纳米材料。
ConspectusAs one of the most important and versatile elements, carbon renders itself as one of the most fundamental and cutting-edge topics in chemistry, physics, and materials science. Many carbon-based chemical rules were established accordingly. While the tetrahedral predilection of tetracoordinate carbon has been a cornerstone of organic chemistry since 1874, almost a century later tetracoordinate carbon was found to be able to adopt planar structures known as planar tetracoordinate carbon (ptC), which are stabilized electronically by good π-acceptor (delocalization of a lone electron pair of ptC) or σ-donor (promoting electron transfer to electron-deficient bonding) substituents or mechanically by appropriate steric enforcement. The experimental and theoretical achievements for the rule-breaking ptC species totally refreshed our understanding of chemical bonding and triggered exploration of peculiar molecules featuring planar pentacoordinate carbon (ppC) and planar hexacoordinate carbon (phC) as well as other outlandish species such as planar hypercoordinate silicon.While the planar hypercoordinate carbon chemistry has been gradually established for molecules in the past five decades, there is growing interest in pursuing their extension systems, especially in two-dimensional (2D) space as a result of the recent extensive studies of graphene and its analogues. Though the natural 2D layered crystals do not contain any planar hypercoordinate carbon or silicon, several 2D nanosheets featuring planar or quasi-planar hypercoordinate ones have been theoretically suggested. Encouragingly, these unique planar configurations possess decent stabilities, and some of them are even the global minimum structure, exhibiting great potential for experimental realization. As the nature of a material is mainly determined by its structural characteristics (e.g., dimensionality, crystallography, and bonding), the combination of planar hypercoordinate chemistry and 2D nanoscience not only endows these rule-breaking systems with the merits of 2D materials but also may offer various promising properties and applications. For example, an unusual negative Poisson’s ratio can be found in ppC-containing Be5C2and planar pentacoordinate silicon (ppSi)-containing CaSi monolayers, of which the former has an anisotropic Dirac cone and the latter is a semiconductor with a desirable band gap for the semiconductor industry. Specially, shortly after the theoretical prediction, a planar hexacoordinate silicon (phSi)-containing Cu2Si monolayer was experimentally synthesized and characterized with the 2D Dirac nodal line fermion, which offers a platform to achieve high-speed, low-dissipation nanodevices.In this Account, we review the recent progress, mostly by density functional theory (DFT) computations, in designing 2D materials with planar hypercoordinate motifs. We describe the key achievements in this field, paying special attention to the “bottom-up” and “isoelectronic substitution” design strategies. In addition, the fundamental stabilization mechanisms of planar hypercoordinate motifs in an infinite layer are discussed. We hope that this Account will inspire more experimental and theoretical efforts to explore nanomaterials with such unconventional chemical bonding.