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Direct and Indirect Excitons in Two-Dimensional Covalent Organic Frameworks
马玉臣
School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China
Abstract:
Highly luminescent bulk two-dimensional covalent organic frameworks (COFs) attract much attention recently. Origin of their luminescence and their large Stokes shift is an open question. After first-principles calculations on two kinds of COFs using the GW method and Bethe-Salpeter equation, we find that monolayer COF has a direct band gap, while bulk COF is an indirect band-gap material. The calculated optical gap and optical absorption spectrum for the direct excitons of bulk COF agree with the experiment. However, calculated energy of the indirect exciton, in which the photoelectron and the hole locate at the conduction band minimum and the valence band maximum of bulk COF respectively, is too low compared to the fluorescence spectrum in experiment. This may exclude the possible assistance of phonons in the luminescence of bulk COF. Luminescence of bulk COF might result from exciton recombination at the defects sites. The indirect band-gap character of bulk COF originates from its AA-stacked conformation. If the conformation is changed to the AB-stacked one, the band gap of COF becomes direct which may enhance the luminescence.
Key words:  COF, luminescence, excitons, many-body Green’s function theory
FundProject:
二维共价有机骨架中的直接激子和间接激子
马玉臣
山东大学化学与化工学院
摘要:
具有较高发光效率的体相二维共价有机骨架材料(covalent organic framework,COF)最近在学术界引起广泛关注。COF的发光机制尚需要从理论上加以解释。利用基于GW方法和Bethe-Salpeter方程的第一性原理计算,我们研究了两种二维COF的激发态性质。单层COF是直接带隙材料,而体相COF呈现间接带隙。根据直接激子计算的体相COF的光学带隙和吸收光谱与实验一致。而由位于导带底的光生电子和位于价带顶的空穴形成的间接激子能量的理论计算值远低于实验荧光光谱的测量值。基于此,可以排除间接带隙COF材料的发光由声子主导的可能性。我们认为体相COF的发光可能源于缺陷处直接激子的复合。体相COF的AA堆叠结构导致其带隙是间接的。如果将堆叠方式由AA变成AB,体相COF将转变成直接带隙材料,它的发光效率可能会增强。
关键词:  共价有机骨架,发光,激子,多体格林函数理论
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