中文摘要：

利用多体格林函数理论，本文研究了二维CN体系（包括triazine 和tri-s-triazine）的激发态特性。通过GW方法，我们计算了准粒子的能量。考虑电子－空穴相互作用，通过求解Bethe-Salpeter 方程，我们获得了激发态能量和光谱。我们发现，在这两种CN体系的价带中，σ 轨道和π 轨道之间的交换作用非常强烈。由于占据的σ 轨道和π 轨道之间的准粒子修正量非常不同，因此，为了得到准确的带隙值和光谱，我们需要对这两种轨道开展精确的GW计算。与单层的CN体系相比，双层结构中层与层之间的范德华相互作用使带隙值降低了0.6 eV，而光吸收谱红移了0.2 eV，这是由于双层结构具有更小的激子束缚能。我们计算的吸收峰的位置与实验结果符合很好。实验中的吸收峰主要是由深能级的π 轨道到π＊轨道的跃迁形成的。π → π＊跃迁和σ → π＊跃迁之间的耦合能够在长波长范围产生弱的吸收尾巴，如果调整入射光的极化方向，由σ →π＊跃迁产生的高强度的吸收峰将会在更低能量处出现。

英文摘要：

The excited-state properties of two-dimensional carbon nitrides, including triazine and tri-s-triazine, were investigated using many-body Green＇s function theory. Their quasiparticle energies were calculated with the GW method. By solving the Bethe-Salpeter equation （BSE）, which takes into account electronhole interactions, excitation energies and optical spectra were obtained. Strong interactions, mainly originating from exchange interactions, were found between σ and TT orbitals in the valence bands of the two carbon nitrides. The quasiparticle corrections for occupied （7 and TT orbitals are quite different, so both of them need to be calculated at the level of the GW method to obtain accurate results for both band gaps and optical spectra from the BSE. Compared with monolayer carbon nitrides, interlayer van der Waals interactions in bilayer systems lower the band gap by 0.6 eV, while the optical absorption spectrum red shifts by 0.2 eV. This is because of the smaller exciton binding energy in bilayer systems. Our calculated positions of the absorption peaks are in good agreement with experiments. The absorption peaks in experiments are dominated by transitions from deep rr orbitals to π＊ orbitals. Coupling between π→ π＊ and σ→π＊ transitions can lead to a weak absorption tail in the long wavelength region. If we tune the polarization direction of the incident light, new strong absorption peaks originating from σ→ π＊ transitions emerge at lower energies.