二维层状黑磷激发态动力学的超快光谱研究

Due to the limitation of photodetector, it is always difficult to study the optical properties of near-infrared two-dimensional (2D) semiconductor materials. Recently, a new 2D semiconducting material, few-layered black phosphorus (BP), has received considerable research attention due to its remarkable properties. Unlike the common 2D materials, such as transition metal sulfides with band gap above 1 eV (visible region), BP exhibits a tunable direct band gap, which can be modified from 1.8 eV for monolayer to 0.3 eV for bulk. Besides, BP also processes high carrier mobility (1000 cm2 V-1 s-1), strong anisotropy and polarization characteristics. These interesting and functional features make BP a promising candidate for photoelectric detector. However, due to the limitation of detector and the poor sample air stability, the excited state dynamic properties of BP have not been reported so far, which determine the response efficiency of photoelectric devices in practical applications. With the increase of the layer number, the band gap, exciton binding energy and electron/hole concentration of the upper and lower surfaces in black phosphorus will change, so the corresponding carrier dynamics will be different. By using the ultra-high time-resolved transient spectroscopy, this project aims to acquire the information of carrier dynamics of mechanical exfoliated BP with different layers. Through changing the energy and power of pump laser, we can achieve the following two aspects respectively, (1) the hot electron relaxation process of BP, the threshold value and the efficiency of multi-exciton generation; (2) The recombination process of edge excitons, especially the rate of auger recombination. By comparing the excited state characters of BP with different layers, we can summarize the influence of the layer number and obtain the basic dynamics information of BP. This could enrich the understanding of photoelectric conversion and extend the materials for practical applications.

继石墨烯与过渡金属硫化物之后，半导体黑磷因带隙从可见至近红外(1.8eV-0.3eV)大范围可调、高载流子迁移率以及强各项异性和偏振特性而成为激动人心的光电转换材料。但作为半导体材料最核心的性质，黑磷的激发态动力学性质包括热电子的驰豫过程和带边激子的复合过程等依然未知，而正是这些动力学性质决定了光电器件的性能和效率。此外，层数是二维材料中独特而又普适的调控自由度，黑磷的直接带隙属性也为研究层数对其动力学过程的影响提供了理想的研究平台。该项目以机械剥离的层状黑磷为研究对象，利用微区超高时间分辨瞬态光谱技术，通过改变激光能量及激发功率等，系统研究不同层数黑磷的激发态载流子动力学过程，包括:(1)黑磷的热电子驰豫过程以及多激子产生阈值和效率；(2)带边激子的复合过程速率和机理，尤其是俄歇复合过程的速率常数。通过分析不同层数黑磷的激发态动力学性质，揭示层数对二维层材料动力学过程的影响规律。

相较于石墨烯和过渡金属硫化物等二维层状材料，黑磷(BP)因其独特的光学性质而被广泛的应用在光电器件研究和应用中。具体表现为高效的载流子迁移率、可见至近红外能量范围的可调节带隙以及独特的各向异性及偏振光学特性。本项目主要利用显微超快瞬态光谱吸收/反射技术，结合稳态吸收和荧光发射光谱手段，详细深入的研究了不同层数黑磷的激发态载流子动力学过程，主要包括热电子弛豫过程以及带边激子复合过程。实验中，通过改变激发光能量和功率，我们首次证实了层状黑磷可以十分有效地实现多激子产生，这主要是由于较强的库伦相互作用、较高的态密度以及黑磷较小且相近的电子/空穴有效质量。此外，多激子产生在不同层数黑磷中也表现出了明显的层数依赖性，即随着层数的增加，屏蔽作用增强，库伦相互作用减弱，因而多激子产生的阈值逐渐增加，效率也逐渐降低。最后，我们也成功在BP/MoS2异质结中实现了多激子接近100%的有效转移。这一系列结果也更加促进了二维层状材料在实际光电转换中的应用和发展。