碱金属辅助下硅基和锗基类沸石同素异构体的理论设计与性能预测

Due to the non-renewability of the fossil energy and growing environmental risks, clean and sustainable energy resources, such as solar and wind are becoming increasingly popular. The material dominates the solar cells market currently is the crystalline or poly-crystalline silicon, which is the alpha phase formed under the ambient condition. To be used as a solar photovoltaic material, alpha silicon has its intrinsic disadvantages, for instance, the light absorption efficiency for alpha-silicon is relatively low due to its indirect band gap; the optical band gap for alpha-silicon is too large and only captures a small fraction of the solar spectrum. Therefore, to find a new silicon based material which has a small direct band gap is one of the goals being pursued. Recently, researchers prepared a Si-Na compound with high pressure method, this precursor transform into a zeolite-like structure after removing the Na atoms. This zeolite-like silicon has a quasi-direct band gap about 1.3 eV, which is close to the optimal value for photovoltaic materials. In this project, using crystal structure searching algorithms combining with ab initio calculations and classical force fields, we propose to search new alkali metal compounds of silicon and germanium, as well as new zeolite-like silicon and germanium structures by removing the alkali metals. And study the influence to the structures and properties of zeolite-like phases from the size of the alkali metal atoms.

由于化石能源的不可再生和日益严重的环境问题，以太阳能、风能等为代表的清洁可持续能源越来越受人关注。目前在太阳能电池市场上占主导地位的材料是单晶硅或多晶硅，即常压下所形成的alpha相。然而alpha硅作为光伏材料有着一些本征的不足：其一，alpha硅是一种间接带隙的材料，其吸收光子的效率较低；其二，alpha硅的光学带隙太大，与太阳光谱只有小部分的重合。寻找一种带隙较小的直接带隙硅基材料一直是人们努力的方向之一。最近，人们发现用高压方法制备出的一种硅-钠化合物，其去除钠原子后所形成的类沸石多孔硅结构是一种准直接带隙半导体，其带隙值约为1.3eV，与作为光伏材料的最佳带隙值相接近。本项目拟用第一性原理计算和经典力场方法结合晶体结构搜索方法，寻找硅和锗的碱金属化合物及其去除碱金属原子后所形成的类沸石多孔低密度结构，并研究不同尺寸的碱金属原子对类沸石结构及其性质的影响。

高压是探索和合成新材料的重要手段，而基于第一性原理计算的晶体结构搜索也在加速新材料的发现上起了关键作用。在前期研究的基础上，我们提出用第一性原理计算和晶体结构搜索方法，探索用高压方法合成碳硅锗等四族元素与碱金属等金属元素构成的化合物，并以此化合物为前驱物，进一步处理得到新型元素同素异构体。在本项目中，我们用晶体结构搜索方法发现可以用碱金属钾和硅在高压下合成KSi化合物，然后再在常压下把钾蒸发掉，就可以得到多孔的单质硅，这种多孔硅具有很好的光吸收性能。文章发表在【Appl. Phys. Lett. 111, 173904 (2017)】。其次，我们发现钾和碳可在高压下合成C4K,并可以用它为前驱物，剥离出单层的T-graphene,这是一种由四八环构成的单层二维碳基超导体。理论预言其超导转变温度达到约20K。结果发表在【Chin. Phys. Lett. (Express letter) 36, 097401 (2019)】。另外，我们还预言了水氦化合物及其在高温高压下的多重超离子态【Nature Physics 15, 1065–1070 (2019)】以及TaAs的高压相是一个新的Weyl半金属态【Phys. Rev. Lett. 117, 146402 (2016)】。本项目取得了丰硕的成果，项目期间在国际高水平学术刊物上总共发表项目主持人孙建教授为通讯作者并且标注了本项目的文章共33篇，包括Nature Physics 1篇、PRL 1篇、PNAS 1篇、CPL express letter 1篇、Science Bulletin封面文章 1篇和PRB/PRM 15篇。