若干低对称性石墨烯量子点光电性质的DFT与模型哈密顿量研究
DFT and Model Hamiltonian Study of Optoelectronic Properties of Some Low-Symmetry Graphene Quantum Dots
AI总结：
本文结合TDDFT与PPP-CI模型研究三个边缘氢化低对称性GQDs的电子、光学吸收及自旋间隙，揭示对称性影响并验证屏蔽参数PPP计算与实验和TDDFT吻合。
摘要：
我们研究了三个具有$C_{2v}$和$C_{2h}$点群对称性的低对称性石墨烯量子点（GQDs）的电子和光学性质。为计算线性光学吸收谱，我们同时采用了第一性原理含时密度泛函理论（TDDFT），以及与组态相互作用（CI）方法耦合的电子相关Pariser-Parr-Pople（PPP）模型。在PPP-CI方法中，我们使用屏蔽参数和标准参数分别进行计算，并通过对基态和激发态采用多参考单-双组态相互作用，有效地纳入电子相关效应。我们假设GQDs的边缘由氢原子饱和，从而使其实际上成为多环芳烃（PAHs）：二苯并[bc,ef]蔻（亦称苯并(1,14)双蒽，C$_{30}$H$_{14}$），以及两个化学式均为C$_{36}$H$_{16}$的同分异构体化合物二萘并[8,1,2abc;2,1,8klm]蔻和二萘并[8,1,2abc;2,1,8jkl]蔻。这两个异构体具有不同的点群对称性，因此本研究也有助于理解对称性对光学性质的影响。三个GQDs吸收谱的共同特征是：代表光学带隙的第一个峰具有低至中等强度，而强峰出现在更高能量处。对于每个GQD，采用屏蔽参数进行的PPP模型计算均与相应PAH的实验结果以及TDDFT计算吻合良好。为进一步量化电子相关效应的影响，我们还计算了三个GQDs的单重态-三重态间隙（自旋间隙），发现其数值显著。
英文摘要：
We have studied the electronic and optical properties of three low-symmetry graphene quantum dots (GQDs), with the point-group symmetries $C_{2v}$, and $C_{2h}$. For the calculations of linear optical absorption spectra, we employed both the first-principles time-dependent density-functional theory (TDDFT), and the electron-correlated Pariser-Parr-Pople (PPP) model coupled with the configuration-interaction (CI) approach. In the PPP-CI approach, calculations were performed using both screened and standard parameters, along with efficiently incorporating electron correlation effects using multi-reference singles-doubles configuration-interaction for both ground and excited states. We assume that the GQDs are saturated by hydrogen atoms at the edges, making them effectively polycyclic aromatic hydrocarbons (PAHs) dibenzo[bc,ef]coronene (also known as benzo(1,14)bisanthene, C$_{30}$H$_{14}$), and two isomeric compounds, dinaphtho[8,1,2abc;2,1,8klm]coronene and dinaphtho[8,1,2abc;2,1,8jkl]coronene with the chemical formula C$_{36}$H$_{16}$. The two isomers have different point group symmetries, therefore, this study will also help us understand the influence of symmetry on optical properties. A common feature of the absorption spectra of the three GQDs is that the first peak representing the optical gap is of low to moderate intensity, while the intense peaks appear at higher energies. For each GQD, PPP model calculations performed with the screened parameters agree well with the experimental results of the corresponding PAH, and also with the TDDFT calculations. To further quantify the influence of electron-correlation effects, we also computed the singlet-triplet gap (spin gap) of the three GQDs, and we found them to be significant.
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