Zhang Yingjie, Sheng Weidong, Li Yang
State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai 200433, China.
Phys Chem Chem Phys. 2017 Aug 30;19(34):23131-23137. doi: 10.1039/c7cp04591c.
By using a configuration interaction approach with up to the fifth excitations taken into account, we study the excitonic effect in the optical absorption in graphene nanodots. While the many-electron states are either singlet or doublet in a triangular nanodot system, all the excited singlet states are found to be optically dark in the absorption. These dark excitons are shown to originate mainly from the geometric symmetry of the system and would remain inactive even when the electron-hole or sublattice symmetry is broken. The first excited state in most of the cases is found to be a dark singlet; however, the order of dark and bright excitonic states is shown to be quite sensitive to the strength of electron-electron interactions such as the dielectric screening from the substrate. All the double degeneracies in the excitonic spectrum are found to be lifted when the rotational symmetry is absent such as in the case of a trapezoidal nanodot; however, the first excited state is shown to still remain a dark exciton when there is a strong screening effect. In order that the optical gap of a graphene nanodot can be efficiently tuned by its dielectric environment, the geometric symmetry is revealed to be a crucial factor.
通过使用一种考虑了高达五次激发的组态相互作用方法，我们研究了石墨烯纳米点光吸收中的激子效应。在三角形纳米点系统中，多电子态要么是单重态要么是双重态，然而所有激发的单重态在吸收过程中都被发现是光学暗态。这些暗激子主要源于系统的几何对称性，即使电子 - 空穴或子晶格对称性被打破，它们仍然保持非活性。在大多数情况下，第一激发态被发现是一个暗单重态；然而，暗激子态和亮激子态的顺序被证明对电子 - 电子相互作用的强度非常敏感，例如来自衬底的介电屏蔽。当不存在旋转对称性时，如梯形纳米点的情况，激子谱中的所有双重简并都被消除；然而，当存在强屏蔽效应时，第一激发态仍然被证明是一个暗激子。为了使石墨烯纳米点的光学带隙能够被其介电环境有效地调节，几何对称性被揭示为一个关键因素。
