Fundamental principles for calculating charged defect ionization energies in ultrathin two-dimensional materials
Fundamental principles for calculating charged defect ionization energies in ultrathin two-dimensional materials
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DOI：
10.1103/physrevmaterials.2.124002
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发表时间：
2018-08
影响因子：
3.4
通讯作者：
T. Smart;Feng Wu;M. Govoni;Y. Ping
中科院分区：
文献类型：
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作者：
T. Smart;Feng Wu;M. Govoni;Y. Ping
2D材料中的缺陷已成为量子发射器和可扩展光电应用的杰出候选者。但是，很难用常规的第一原理方法来模拟其表征其行为的几种物理特性，例如带电的缺陷电离能，主要是因为降低尺寸降低引起的弱和各向异性介电筛选。我们建立了基本原理，以精确有效地计算超薄2D材料中带电的电离能和电子结构。我们建议将真空水平用作缺陷电荷过渡水平（CTL）的参考，因为它使结果对理论水平不敏感，这与常用的带边缘位置不同。此外，我们通过强制执行广义的Koopmans的局部缺陷状态来确定混合功能中Fock交换的比例，以确定2D材料的精确带隙和带边缘位置。我们发现，获得的Fock Exchange的分数从$ h $ h $ -bn的0.2到单层$ h $ h $ -bn的0.4不等，其频段差距也与实验结果非常吻合并计算出GW的结果。这些方法的组合允许对缺陷电离能（CTL和带边缘位置之间的差异）进行可靠，有效的预测。我们通过几个示例来激励和概括这些发现，包括单层不同的缺陷到氮化氢硼（$ h $ -bn），单层MOS $ _2 $和Graphane。最后，我们表明，增加了$ h $ bn的层数系统地降低了缺陷电离能，主要通过CTL向真空转移的CTL，传导带minima几乎保持不变。
Defects in 2D materials are becoming prominent candidates for quantum emitters and scalable optoelectronic applications. However, several physical properties that characterize their behavior, such as charged defect ionization energies, are difficult to simulate with conventional first-principles methods, mainly because of the weak and anisotropic dielectric screening caused by the reduced dimensionality. We establish fundamental principles for accurate and efficient calculations of charged defect ionization energies and electronic structure in ultrathin 2D materials. We propose to use the vacuum level as the reference for defect charge transition levels (CTLs) because it gives robust results insensitive to the level of theory, unlike commonly used band edge positions. Furthermore, we determine the fraction of Fock exchange in hybrid functionals for accurate band gaps and band edge positions of 2D materials by enforcing the generalized Koopmans' condition of localized defect states. We found the obtained fractions of Fock exchange vary significantly from 0.2 for bulk $h$-BN to 0.4 for monolayer $h$-BN, whose band gaps are also in good agreement with experimental results and calculated GW results. The combination of these methods allows for reliable and efficient prediction of defect ionization energies (difference between CTLs and band edge positions). We motivate and generalize these findings with several examples including different defects in monolayer to few-layer hexagonal boron nitride ($h$-BN), monolayer MoS$_2$ and graphane. Finally, we show that increasing the number of layers of $h$-BN systematically lowers defect ionization energies, mainly through CTLs shifting towards vacuum, with conduction band minima kept almost unchanged.