纳米孔缺陷导致单层黑磷电荷局域极大抑制非辐射电子-空穴复合的时域模拟

通常认为缺陷加速黑磷的非辐射电子-空穴复合，阻碍器件性能的持续提高。实验打破了这一认识。采用含时密度泛函理论结合非绝热分子动力学，我们发现P-P伸缩振动驱动非辐射电子-空穴复合，使纳米孔修饰的单层黑磷的激发态寿命比完美体系延长了约5.5倍。这主要归因于三个因素。一，纳米孔结构不但没有在禁带中引入深能级缺陷，而且由于价带顶下移使带隙增加了0.22 eV。二，除了带隙增加，纳米孔减小了电子和空穴波函数重叠，并抑制了原子核热运动，从而使非绝热耦合降低至完美体系的约1/2。三，退相干时间比完美体系延长了1.5倍。前两个因素战胜了第三个因素，使纳米孔结构激发态寿命延长至2.74 ns，而其在完美体系中约为480 ps。我们的研究表明可以制造合理数量和形貌的缺陷，如纳米孔，降低黑磷非辐射电子-空穴复合，提高光电器件效率。这一研究对于理解和调控黑磷和其它二维材料的激发态性质有重要意义。

Charge Localization Induced by Nanopore Defects in Monolayer Black Phosphorus for Suppressing Nonradiative Electron-Hole Recombination through Time-Domain Simulation

Black phosphorus (BP) is a promising candidate for photovoltaic and optoelectronic applications owing to its excellent electronic and optical properties. It is believed that defects generally accelerate non-radiative electron-hole recombination in BP and hinder improvement of device performance. Experiments defy this expectation. Using state-of-the-art ab initio time-dependent density functional theory combined with non-adiabatic molecular dynamics, we investigate the non-radiative electron-hole recombination in monolayer (MBP) and MBP containing nanopore defects (MBP-ND). We demonstrate that non-radiative electron-hole recombination is promoted by the P-P stretching vibrations, and the recombination time of MBP-ND is approximately 5.5 times longer than that of the MBP system. This is mainly attributed to the following three factors: First, the nanopore creates no mid-gap state when increasing the bandgap by 0.22 eV owing to the downshift of the valence band maximum, caused by the decrease in the inter-layer P-P bond length, thereby weakening the antibonding interaction. Second, the nanopore reduces the overlap of electron and hole wave functions by diminishing the charge densities near the defect. Simultaneously, the nanopore significantly inhibits the thermal-driven atomic fluctuations. The increased bandgap correlated with the decreased wave function overlap and slowed thermal motions of the nuclei in the MBP-ND system reduces the non-adiabatic coupling by a factor of approximately 2 with respect to the pristine system. Third, the slow atomic motions weaken the electron-vibrational interaction and decrease the intensity of the major vibration mode at 440 cm⁻¹, which is the main source for creating non-adiabatic coupling, leading to loss of coherence formed between a pair of electronic states via non-adiabatic coupling and causing electron-hole recombination that results in a 1.5-fold increase in the coherence time in the MBP-ND system with respect to the MBP system. Consequently, the increased bandgap and decreased non-adiabatic coupling compete successfully with the prolonged coherence time, extending the excited-state lifetime to 2.74 ns in the system containing nanopore defects, which is only 480 ps in the pristine system. These phenomena arise owing to a complex interplay of the unusual chemical, structural, electrostatic, and quantum properties of BP with and without nanopore defects. This study is of great significance for understanding the excited-state properties of BP. The detailed mechanistic understanding of the prolonged charge carriers lifetime of MBP decorated with nanopore defects provides key insights for defect engineering in BP and other 2-dimensional materials for a broad range of solar and electro-optic applications by reducing the non-radiative charge and energy losses.