特殊构型Si2N2分子团簇电致激发特性的密度泛函理论研究∗

摘要

In order to understand in depth the electroluminescence mechanism, the influences of the external electric field on the geometric and electronic structure in ground state, the molecular vibrational spectra of Si2N2 molecule with Cs special symmetry are studied by density functional theory with B3LYP exchange-correlation prescription at the aug-cc-pVTZ basis set level. Following each optimization, the vibrational frequencies are calculated and all optimized structures are stable. The results show that the molecular vibrational Stark effect, i.e., red-shift for the low-frequency modes and blue-shift for the high-frequency modes are observed with the increase of the applied field strength. The energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), the energy gap between HOMO and LUMO of Si2N2 molecule diminish with the increase of external field. A time-dependent density functional theory is used to investigate the excited properties of Si2N2 (Cs) molecule. The calculated absorption spectra of Si2N2 molecule with Cs symmetry are in agreement with the experiment values. The analysis reveals that the absorption spectrum wavelength increases in the visible region with a concomitant increase in the electronic transition oscillator strengths in the course of the increase of the external electric field strength. The results reveal that the excited properties of Si2N2 molecule can be easily tuned by the external electric field, which indicates that the silicon nitride is an interesting optoelectronic functional material. These investigations on the various properties of Si2N2 molecule with Cs symmetry under an external electric field are useful to understand the electroluminescence mechanism for silicon nitride used in molecular electronics.%以在可见光区有吸收峰的Cs构型的Si2N2分子团簇为研究对象，利用密度泛函B3LYP方法，在aug-cc-pVTZ基组水平下优化得到了处于不同外电场中的Si2 N2分子团簇的稳定结构。分析发现：在不同的外电场中， Si2N2分子构型对称性没有发生改变，均为Cs对称性，且都有6种振动模式；随着外电场强度的逐渐增大， Si2 N2分子振动频率较低的前三种振动模式的频率略有减小，而后三种振动模式的频率逐渐增加；随着外电场强度的逐渐增大，在一定电场范围内最高占据分子轨道与最低空分子轨道的能隙值出现振荡，之后能隙值随着外电场强度的增大而减小。在此基础上，采用含时密度泛函TD-B3LYP方法研究了外电场对Si2 N2分子吸收谱的影响规律。计算得到的吸收谱范围在紫外-可见光区，这与实验值相符合。随着外电场强度的逐渐增大，在可见光区吸收谱发生红移，最大跃迁振子强度逐渐增大。结果表明，施加外电场有利于Si2 N2分子在可见光区的吸收，也有利于操控分子特定激发态的电子状态，进而调节相应的跃迁光谱特性，可达到获得所需特定波长的要求。