【作者】 张杰；

【导师】 徐文；

【作者基本信息】 云南大学 ， 凝聚态物理， 2021， 博士

【摘要】 由于具有奇特的物理性质（如自旋和谷自由度、高的电流开/关比等）,二维过渡金属硫化物（two dimensional transition metal dichalcogenides,2D TMDs）已成为凝聚态物理、材料科学、光电子学领域的研究热点,并在器件应用中被寄予厚望。单层二硫化钼（MoS₂）作为2D TMDs的代表,其偏振相关光荧光、谷霍尔效应及激子吸收特性等都已得到较多研究。需要指出的是,单层MoS₂的上述多种有趣性质以及大量潜在应用,都与其基本光电特性密不可分。基于单层MoS₂光电特性（尤其是宽谱光电特性）的重要性,以及当前相关工作存在的不足,本论文较为系统地研究了有/无光泵浦作用下的单层MoS₂在可见光、近红外和太赫兹（THz）频段的基本光电特性,重点探讨了光泵浦作用下单层MoS₂的光吸收行为以及自由载流子与激子形成之间的相互作用过程。本文主要研究内容和结果包括:（1）针对包括单层MoS₂在内的薄膜材料/衬底系统,提出了更为普适的改进多重反射模型,建立了薄膜材料光学参数（如光电导率）与其透射/反射之间的转换关系。在模型中,考虑了入射光偏振、入射角度以及衬底多重反射带来的影响,能够更灵活地从薄膜/衬底系统的光响应中提取出薄膜光学参数。本论文分别用单层二维材料（石墨烯和单层MoS₂）在不同频段的透射实验中证实了模型的有效性,也表明了该模型可扩展到其他二维电子材料或薄膜的研究中。（2）对单层MoS₂的宽谱光电响应特性进行了实验研究。测量了样品在可见光（350～800 nm）、近红外（1.2～1.9μm）以及THz（0.3-2 THz）波段的光透射特性,得到对应的光电导率,并分析了单层MoS₂光吸收的基本特性,以及电子的带内和带间跃迁过程。此外,通过对比,初步论证了衬底对这些性质的影响。上述结果完善了单层MoS₂光电参数的宽谱信息,为光电器件设计提供了重要参考。（3）通过连续光泵浦-光探测实验,研究了光激发下单层MoS₂光吸收特性。实验观测到随泵浦光功率增大导致吸收峰蓝移和吸收高度变化,通过建立唯象模型,讨论了其机理,发现这些现象与光泵浦产生的激子吸收以及激子-激子交互作用密切相关。此外,我们还发现这些效应会受到衬底的影响,例如:与蓝宝石衬底相比,石英衬底的介电屏蔽效应和带电激子（trion）效应会极大压制上述现象。这些结果不仅加深了我们对单层MoS₂中激子对可见光响应特性的认知,还为将单层MoS₂用于可见光频段的全光调制提供了科学研究依据。（4）通过连续光泵浦-近红外/THz探测实验,研究了单层MoS₂在光泵浦作用下的光电特性。对于近红外频段,观测到单层MoS₂的光电导率实部可随泵浦光强度的增大而减小,此反常现象是由于激子的形成与光生载流子的激发竞争导致的。而对于THz频段,观察到随着泵浦光功率增大至80 m W,透射率逐渐降低,然而,当泵浦功率增加到120 m W,透射率增大。基于Drude-Smith模型拟合,可获得载流子浓度随泵浦功率的增大而先增大后减小,弛豫时间先减小后增大。这些都与激子对自由载流子光响应特性的调制有关。上述结果显示了光泵浦作用下单层MoS₂中的光响应特性蕴含着丰富的物理过程,为调控单层MoS₂长波光电特性提供了更多的自由度。综上所述,本论文从可见光、近红外到太赫兹的宽谱范围内,重点研究了单层MoS₂的光学响应以及光泵浦调控规律,获得了新的重要实验结果。对全面深入理解单层MoS₂的基本光电特性及相关物理过程具有重要科学研究价值,也为其在光电子学器件中的应用提供了科学研究基础。更多还原

【Abstract】 In recent years,the investigation of two-dimensional（2D）transition metal dichalcogenides（TMDs）has become one of the focused fields of research in condensed matter physics,material science,and optoelectronics due to their peculiar physical properties（such as spin and valleytronics properties,direct band gap structure,high current switching ratio,etc.）and to their potential application in advanced electronic,optical and optoelectronic devices.Particularly,monolayer（ML）molybdenum disulfide（MoS₂）is the representative of 2D TMDs.Its novel physical effects（such as the polarization dependent photoluminescence,spin polarization,valley hall effect,exciton absorption characteristics,to mention but a few）have been studied extensively.It should be pointed out that the above-mentioned interesting properties and the related potential applications of ML MoS₂ are closely related to its basic optoelectronic properties.Therefore,the study of the basic optoelectronic properties of ML MoS₂ in wide spectrum is of great importance and significance.In this thesis,the optoelectronic response of ML MoS₂ to visible,near-infrared and terahertz（THz）irradiations is systematically studied.We focus on the light absorption behavior of ML MoS₂ and the interaction between free carrier and exciton formation.The main results and outcomes from this research work are summarized as follows:（1）For a thin film/substrate system,an improved multiple reflection model is proposed and developed,in which the conversion relationship between optical coefficients（i.e.,the optical conductivity）and transmission/reflection of thin film materials is established.In this model,the effects of the polarization of incident light,the incident angle and the multiple reflections from the substrate are taken into account.Thus,the optical coefficients of the thin film can be extracted more flexibly from the measurement of optical response of the thin film/substrate system.In this study,the transmission experiments for graphene and ML MoS₂ on the substrate in different frequency regimes are undertaken to verify the validity of this model.The developed theoretical model can be extended to the study of other 2D electronic materials and devices.（2）The optoelectronic properties of ML MoS₂ in the absence of external optical pumping are studied experimentally in different spectrum regimes.In this study,the optical transmission characteristics of the samples in the visible（350～800 nm）,near infrared（1.2～1.9μm）and THz（0.3～2 THz）bandwidths are measured and the corresponding optical conductivities are determined.The basic characteristics of the light absorption in ML MoS₂ and the related intra-and inter-band electronic transitions are observed and analyzed.Additionally,the effect of substrate on these properties is demonstrated by comparison with the results obtained for ML MoS₂on different substrates.These results can help us to gain an in-depth understanding of the basic optoelectronic properties of ML MoS₂ in wide spectrum regimes and can provide a scientific backup for the design of ML MoS₂based optoelectronic devices.（3）The light absorption behavior of the ML MoS₂ is studied in the presence of continuous optical excitation measured by pump-probe experiments.The phenomena of blue shift of the absorption peak and the change of absorption height caused by the increase in pump power are observed.The mechanism is discussed on the basis of a phenomenological model established by this study.It is found that these phenomena are closely related to exciton absorption and exciton-exciton interaction induced by optical pumping.In addition,it is shown that these effects can be modulated by the presence of the substrate.For example,the dielectric screening effect and trion effect of quartz substrate,which are different from sapphire substrate,can great suppress the above-mentioned phenomena.These important findings can not only deepen our understanding of the response characteristics of excitons in monolayer MoS₂ for visible light,but also provide a scientific research basis for the application of ML MoS₂ to all-optical modulation in visible band.（4）Based on the measurement of continuous optical pump and near infrared/THz probe,the influence of optical excitation on optoelectronic response of ML MoS₂ is studied.For the case of near infrared irradiation,it is observed that the real part of the optical conductivity of ML MoS₂ decreases with increasing pump power,which is due to the competition between the formations of excitons and photogenerated carrier excitation.In the presence of THz irradiation,it is observed that the transmittance decreases with the increase of pump power to 80 m W while increases with the increase of pump power to120 m W.Based on the Drude Smith model,the carrier concentration first increases and then decreases with the increase of pump power,on the contrary,the relaxation time first decreases and then increases.All these are related to the modulation of free carrier light response by excitons.Above these interesting and important findings indicate that,in the presence of optical pumping,the physical processes for optical response characteristics in ML MoS₂ are abundant and diverse and,on the other hand,provide more freedom for us to control the long-wavelength optoelectronic characteristics of ML MoS₂.In short,in this thesis the basic optoelectronic properties of ML MoS₂ have been studied in the visible,near-infrared and THz bandwidths in the absence and presence of external optical pumping.Some interesting and important new experimental results have been observed,analyzed and discussed.The research outcomes from this study can make a significant contribution to comprehensive and in-depth understanding of the basic optoelectronic characteristics and related physical processes of ML MoS₂ and can provide a scientific research basis for the application of ML MoS₂ as novel and advanced optoelectronic devices.更多还原