Progress on defect and related carrier dynamics in two-dimensional transition metal chalcogenides
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摘要: 原子级厚度的单层或者少层二维过渡金属硫族化合物因其独特的物理特性而被寄希望成为下一代光电子器件的重要组成部分。然而，二维材料的缺陷在很大程度上影响着材料的性质。一方面，缺陷的存在降低了材料的荧光量子效率、载流子迁移率等重要参数，影响了器件的性能。另一方面，合理地调控和利用缺陷催生了单光子源等新的应用，因此，表征、理解、处理和调控二维材料中的缺陷至关重要。本文综述了二维过渡金属硫族化合物中的缺陷以及缺陷相关的载流子动力学研究进展，旨在梳理二维材料中的缺陷及其超快动力学与材料性能之间的关系，为二维过渡金属硫族化合物材料特性和高性能光电子器件的相关研究提供支持。Abstract: Because of their unique physical properties, the monolayer and few-layer two-dimensional transition metal chalcogenides with atomic-level thickness are expected to play an important role in the next generation of optoelectronic devices. However, defects in two-dimensional materials affect their properties to a great extent. On one hand, defects reduce the fluorescence quantum efficiency, carrier mobility and other important device parameters. On the other hand, the control and utilization of defects have given birth to new techniques such as using single-photon sources. Therefore, it is very important to characterize, understand, handle and control the defects in two-dimensional materials. In this review, the research progress on defects and its related carrier dynamics in two-dimensional transition metal chalcogenides is summarized. This paper aims to sort out the great influence of defects and their related ultrafast dynamics on material performance in two-dimensional transition metal chalcogenides, and to support studies on fundamental physical properties and high-performance optoelectronic devices.
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Key words:
- two-dimensional materials /
- transition metal chalcogenides /
- defects /
- carrier dynamics
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图 1 二维TMDCs中点缺陷的高分辨表征。 （a−f） STEM-ADF对MoS2中6种点缺陷的表征[85]；（g−j）4种常见的TMDCs材料中缺陷的STM表征，深蓝色的点代表原子缺失或存在受体杂质，亮黄色的点代表存在给体杂质[86]；（k）新制备的MoS2与（l）在大气中放置8个月后的光学显微镜照片的对比[90]；（m−t）超高分辨STM和非接触式AFM对WS2中点缺陷的表征[88]
Figure 1. High resolution characterization of point defects in 2D TMDCs. (a−f) STEM-ADF characterization of six kinds of point defect in MoS2[85]; (g−j) STM characterization of defects in four common TMDCs materials, where dark blue points represent atom defects or receptor impurities, bright yellow points represent donor impurities[86]; comparison of optical microscope pictures between (k) fresh MoS2 and (l) fresh MoS2 after 8 months of atmospheric exposure[90]; (m−t) ultra-high resolution STM and non-contact AFM characterization of point defects in WS2[88]
图 2 缺陷的稳态光谱学研究。 （a）电子束、等离子体、紫外光可以在TMDCs中产生缺陷；（b）单层WSe2随氩等离子体处理时间变化的荧光光谱[103]；（c）使用氩等离子和电子束处理单层WSe2后的荧光光谱[103]；（d）单层MoS2随电压（上图）、泵浦功率（中图）、温度（下图）改变的荧光光谱[110]；（e）缺陷捕获激子的模型（上图），束缚态激子/激子发光占比（左侧坐标轴）与带电激子/激子发光占比（右侧坐标轴）随栅压的变化（下图）[110]；（f）DFT计算得到的存在缺陷的WS2的能带结构（左图）和跃迁偶极矩（右图）[115]；（g）（h）随着离子束处理强度增加，单层MoS2拉曼光谱的变化[120]；（i）单层WSe2中缺陷的EL光谱[130]
Figure 2. Steady state spectroscopic studies of defects. (a) The defects can be produced in TMDCs by electron beam, plasma and ultraviolet irradiation; (b) fluorescence spectrum of monolayer WSe2 as it changes with varying argon plasma treatment[103]; (c) fluorescence spectrum of monolayer WSe2 after argon plasma and electron beam treatment[103]; (d) fluorescence spectrum of monolayer MoS2 as it changes with voltage (above), pump power (center) and temperature (below)[110]; (e) defect capture exciton model (above), the bound exciton/exciton PL intensity ratio (left axis) and the trion/exciton PL intensity ratio (right axis) vary with the gate voltage (below)[110]; (f) band structure (left) and the transition dipole moment (right) of defected monolayer WS2 calculated by DFT[115]; (g)(h) Raman spectrum of monolayer MoS2 with respect to ion beam treatment intensity[120]; (i) electroluminescence spectra of defects in monolayer WSe2[130]
图 3 缺陷态发光的抑制。 （a）使用“超级酸”处理单层MoS2并使用含氟聚合物进行封装[134]；（b） 使用“超级酸”处理单层MoS2前后的荧光光谱[134]；（c）使用静电掺杂调节单层MoS2的PLQY的器件结构和原理图[142]；（d）改变栅压和泵浦功率时MoS2的PLQY的变化[142]；（e）使用激光照射处理MoS2的示意图（上图）和发生的氧气物理吸附与化学吸附（下图）[146]；（f）对久置的MoS2进行激光照射时荧光光谱随照射时间的变化[147]
Figure 3. Suppression of defect state luminescence. (a) Treating the monolayer MoS2 with "super acid" and encapsulating it with CYTOP[134]; (b) fluorescence spectrum before and after treatment[134]; (c) device structure and schematic diagram of PLQY tuning of the monolayer MoS2 through electrostatic doping[142]; (d) PLQY of monolayer MoS2 changes with the gate voltage and pump power[142]; (e) schematic diagram of the MoS2 treated by laser irradiation (above) and occurred oxygen physisorption and chemisorption (below)[146]; (f) fluorescence spectrum of aged MoS2 changing with time irradiated by laser[147]
图 4 (a) TMDCs中的载流子动力学和(b)缺陷捕获过程
Figure 4. (a) Carrier dynamics and (b) defect capture process in TMDCs
图 5 基于瞬态吸收光谱对TMDCs缺陷相关的载流子动力学的研究 。（a）使用泵浦-探测方法对缺陷态进行探测的示意图；（b）泵浦光和探测光打在样品的相同位置，两束光通过位移台改变到达的时间差[62]；（c）缺陷捕获载流子的快通道（左图）和慢通道（右图）的示意图[62]；（d）不同温度下单层MoS2对探测光的透射率随时间的变化[62]；（e）不同功率下少层MoTe2载流子寿命的变化[63]；（f）不同功率下单层MoSe2对探测光的反射率随时间的变化[159]
Figure 5. Studies on defect related carrier dynamics of TMDCs based on transient absorption spectroscopy. (a) Schematic diagram of detecting the defect state by using the pump-probe method; (b) pump light and the probe light illuminate the same position of the sample. The time difference of two light beams changing through optical path difference[62]; (c) schematic diagram of the fast channel (left) and the slow channel (right) of the defect trapping process[62]; (d) the transmittance of the monolayer MoS2 to the probe light varying with time under different temperatures[62]; (e) change of carrier lifetime of MoTe2 at different pump powers[63]; (f) change in reflectivity of the monolayer MoSe2 with respect to time at different pump powers[159]
图 6 基于时间分辨荧光光谱对TMDCs缺陷相关的载流子动力学的研究。 (a)时间分辨荧光对缺陷态进行探测的原理图；(b)本征WS2自由激子的荧光衰减曲线[157]；(c)使用氩离子束轰击产生缺陷后WS2的自由激子（蓝线）和束缚态激子（红线）的荧光衰减曲线[157]；(d)使用激光对单层WS2进行预处理后（圆圈区域），高功率（上图）和低功率（下图）的荧光强度对比，两张图中的强度都用各图的最高强度进行了归一化[148]；(e)单层WS2的荧光在不同激发功率下的荧光衰减曲线，蓝线为预先使用激光照射，红色为未提前处理[148]
Figure 6. Studies of defect-related carrier dynamics in TMDCs using TRPL spectroscopy. (a) Schematic diagram of TRPL to detect the defect state; (b) PL decay curve of free exciton energy in pristine monolayer WS2[157]; (c) PL decay curves of the free exciton (blue line) and the bound exciton (red line) energy in the defective monolayer WS2 after being irradiated by an argon ion beam[157]; (d) comparison of the PL intensities of the monolayer WS2 pretreated by laser (circle area) at a high pump power (above) and a low pump power (below)[148]; (e) PL decay curve of the monolayer WS2 under different excitation powers. The blue line represents the sample before being irradiated by laser, and the red line presents that not having been pretreated[148]
图 7 基于时间分辨ARPES和PEEM对TMDCs缺陷相关的载流子动力学的研究。 (a)时间分辨ARPES和PEEM对缺陷态进行探测的原理图；(b)时间分辨ARPES对动量空间进行探测的原理图[175]；(c)时间分辨PEEM对样品表面光电子成像的原理图[64]；(d)理论计算得到的MoS2的动量空间与探测的动量区域（灰色阴影）（上图），时间分辨APRES实验测得的动量空间不同位置光电子发射信号的上升和下降时间[161]；(e)脉冲光激发后MoS2动量空间谷间散射和缺陷捕获的示意图[161]；(f)泵浦探测零时刻PEEM对单层WS2的实空间成像（上图），三种覆盖单层WS2的不同区域的光电子发射信号强度（下图）[64]；(g)三种区域光电子发射信号强度随时间的变化[64]；(h)光电子发射信号强度的能量分布随泵浦探测时间差的变化[64]
Figure 7. Study of defect related carrier dynamics in TMDCs based on time-resolved ARPES and PEEM. (a) Schematic diagram of time-resolved ARPES and PEEM for detecting the defect state; (b) schematic diagram of time-resolved ARPES for detecting momentum space[175]; (c) schematic diagram of time-resolved PEEM for photoelectron imaging of the sample surface[64]; (d) momentum space of the MoS2 calculated by theoretcally. The detectable momentum area (gray shadow) (above). Characteristic photo emission intensity rise times and fall times as a function of momentum (below)[161]; (e) schematic diagram of intervalley scattering and defect trapping process of hot electrons in the momentum space of MoS2 after being excitated by optical pulse[161]; (f) PEEM image of the WS2 at a 0 ps time delay (above), photoelectron intensity along the yellow cross-cut line (below)[64]; (g) photoelectron emission intensities as a function of the time delay in the three regions[64]; (h) variation of energy distribution of photoelectron emission intensity at different pump-probe time delay[64]
图 9 TMDCs缺陷对谷间散射的影响 。（a）理论计算得到的MoSe2的偏振荧光光谱，使用右旋光（蓝色）激发，得到左旋光和右旋光（红色）的荧光[79]；（b） WSe2束缚态激子的积分荧光光谱和不同光子能量的谷偏振度[78]；（c）WSe2束缚态激子瞬态荧光和谷偏振度随时间的变化[78]
Figure 9. The effect of defects on intervalley scattering in TMDCs. (a) Calculated PL spectrum of MoSe2 excited by right-handed light (blue line), observing PL from the left and the right (red line)[79]; (b) integrated PL spectrum of bound exciton in defective WSe2 and their degree of valley polarization[78]; (c) decay of PL and valley polarization degree of bound exciton in defective WSe2[78]
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