• Chinese Journal of Chemical Physics
  • Vol. 33, Issue 5, 05000569 (2020)
Sun Shan, Ma Hui-zhong, Zhang Xiao, and Ma Yu-chen*

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, the 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.

Ⅰ INTRODUCTION

After Yaghi and his colleagues made the breakthrough in 2005 on the synthesis of covalent organic framework (COF) materials [1], the study of COFs grows rapidly [2-4]. In two- and three-dimensional COFs, atoms are connected by various covalent bonds to construct the framework structure, giving this new kind of material broad prospective applications. Due to its high specific surface area, low density and diverse structure, COFs may be applied in the fields of gas storage [5, 6], catalysis [7], optoelectronic devices [8-12], etc. COFs can possess wide optical absorption in the visible range. Their highly ordered arrays may facilitate charge transport and exciton migration [13]. Together with the presence of large pores in their structures, COFs might find important applications in photocatalysis [14-16].

Copy Citation Text
Shan Sun, Hui-zhong Ma, Xiao Zhang, Yu-chen Ma. Direct and Indirect Excitons in Two-Dimensional Covalent Organic Frameworks[J]. Chinese Journal of Chemical Physics, 2020, 33(5): 569
Download Citation
Received: Jan. 7, 2020
Accepted: Feb. 10, 2020
Published Online: Apr. 21, 2021
The Author Email: Ma Yu-chen (myc@sdu.edu.cn)