少层PtSe_(2)中光生载流子超快动力学研究

Investigation of ultrafast photocarrier dynamics in few-layer PtSe_(2) thin films

二维PtSe_(2)具备宽可调带隙、高稳定性等优点,在新型光电器件方面具有极大应用价值。利用时间分辨太赫兹光谱研究了不同厚度PtSe_(2)中的光生载流子超快动力学,发现该材料瞬态太赫兹光电导的幅度及其激发光强度依赖性随材料厚度的增加呈现出显著的非线性增加趋势。通过太赫兹光电导频谱分析,获得了光生载流子浓度、散射时间、背散射因子等动力学参数,并结合激发波长依赖的太赫兹弛豫动力学,推测束缚激子和自由载流子的竞争是引起这种厚度非线性关系的主要原因。此外,基于光泵浦-光探测光谱证明了PtSe_(2)中的激子效应及半导体-半金属转变。该工作演示了层数对PtSe_(2)中非平衡态动力学的有效调控,对贵金属基二维材料在光电器件方面的应用具有指导意义。

Two-dimensional(2D)PtSe_(2) has the unique properties such as large-range tunability in band gap and high air stability,holding a great promise in the development of novel optoelectronic devices.In this work,the ultrafast photocarrier dynamics in 2D PtSe_(2) with different thicknesses have been studied by using time-resolved terahertz(THz)spectroscopy.It is found that both the amplitude of transient THz photoconductivity and its dependence on the excitation fluence of the material show a significant nonlinear increase with the increase of sample thickness.The dynamical parameters including photocarrier density,scattering time and backscattering factor are obtained through analyzing the THz frequency-dependent conductivities.In combination with the excitation-wavelength dependent THz relaxation dynamics,it can be inferred that the competition between bound excitons and free carriers is mainly responsible for the nonlinear thickness dependence.In addition,the exciton effect and thicknessinduced semiconductor-semimetal transition in PtSe_(2) are also revealed using the optical pump-optical probe spectroscopy.This work demonstrates the effective regulation on the nonequilibrium ultrafast dynamics of PtSe_(2) through varying the thickness of materials,and provides an important guideline for the optoelectronic applications of noble-metal based 2D materials.