GeH/π层间弱相互作用调控锗烯电子结构的机制

锗烯是继石墨烯、硅烯发现以来最重要的二维纳米材料之一, 以其优异的物理化学性质迅速得到人们的广泛关注. 然而, 锗烯具有的零带隙能带特点(狄拉克点)极大程度地限制了其在微电子纳米材料方面的应用. 本文采用范德华力修正的密度泛函计算方法, 研究了锗烯、锗烷、锗烯/锗烷的几何和电学性质. 研究发现, 锗烯和锗烷可以通过弱相互作用形成稳定的双层结构, 并在锗烯中打开一个85 meV的带隙. 电子结构分析表明, Ge-H/π 的存在破坏了锗烯子晶格的对称性, 从而在狄拉克点上打开一个带隙. 差分电荷密度图分析表明有部分电荷从H原子的s轨道转移至Ge的p_z轨道. 该电荷转移机制增强了锗烯与锗烷之间的相互作用力, 是形成锗烯/锗烷双层二维纳米结构的主要原因. 进一步研究还发现, 锗烷/锗烯/锗烷的三明治结构无法在锗烯中打开带隙. 这是由于两侧的锗烷对夹层的锗烯作用力等价, 无法破坏锗烯的子晶格对称性, 所以无法打开锗烯带隙. 最后, 所有计算结果都在高精度杂化密度泛函HSE06计算精度下得到进一步验证. 因此, 本文从理论上提出了一种切实可行的打开锗烯狄拉克点的方法, 为锗烯在场效应管和其他纳米材料中的应用提供了理论指导.

Mechanisms on the GeH/π interactions in germanene/germanane bilayer for tuning band structures

Germanene, one of the most important two-dimensional materials after graphene and silicone have been discovered, is attracting wide attentions due to its many excellent physical properties. Since a suitable band gap is needed for the electronics and optoelectronics, the lack of a band gap has essentially restricted the practical applications of germanene in macroelectronics. In this article, density functional theory calculations with van de Waals corrections is utilized to study the geometric and electronic properties of germanene (Ge), germanane (GeH) and germanene/germanane (Ge/GeH) bilayer. The band gaps for Ge and GeH are zero and 1.16 eV, respectively. For the Ge/GeH bilayer, a considerable binding energy of 273 meV/unit cell is obtained between Ge and GeH layers. This value is smaller than that of Ge bilayer (402 meV/unit cell), but larger than that of GeH bilayer (211 meV/unit cell), indicating a considerable GeH/π bonding. This means that Ge and GeH layers could be combined steadily by the interlayer weak interactions. Meanwhile, a band gap of 85 meV is opened, which is contributed to the breaking of the equivalence of the two sublattices in the Ge sheet, yielding a nonzero band gap at the K point. Charge density difference indicates that the electrons on the s orbital of H transfer to the Ge_p orbital, enhancing the interlayer interactions. It should be noted here that the van de Waals corrections are pretty important for the geometric and electronic properties of the Ge/GeH bilayer. Without the van de Waals corrections, the binding energy of the Ge/GeH bilayer is reduced from 273 meV/unit cell to only 187 meV/unit cell, severely underestimated the strength of the weak forces between Ge and GeH layers, resulting in a much smaller band gap of 50 meV. Interestingly, no band gap is obtained for the sandwich structure GeH/Ge/GeH, in which the equivalence of two sublattices in germanene is kept. Finally, all the results are confirmed by the high accurate hybrid functional calculations. At the Heyd-Scuseria-Ernzerhof level, the band gap of Ge/GeH bilayer is 117 meV, slightly larger than 85 meV at the Perder-Burke-Ernzerhof level. Our work would promote utilizing germanene in microelectronics and call for more efforts in using weak interactions for band structure engineering.