| Two-dimensional transition metal dichalcogenides?TMDs?as a new generation of two-dimensional nanomaterials,have been widely studied in the field of optoelectronic devices over the past several years,due to their relatively large direct bandgap,high mobility and large exciton binding energy and so on.However,at this stage the quantum efficiency of TMDs-based optoelectronic devices preserves a poor level of 0.01%-1%,which cannot satisfy the demands of practical applications.Hence,it is urgent to understand the basic reasons for this poor quantum efficiency of TMDs-based optoelectronic devices,and meanwhile to effectively improve their quantum efficiency.In this thesis,from the perspective of improving the quantum efficiency of common two-dimensional TMDs materials such as MoS2,WSe2etc.,we have studied the ultrafast carrier dynamics of two-dimensional TMDs crystals in detail,especially in non-radiative recombination processes,and then fundamentally revealed the reasons for the low quantum efficiency in TMDs materials.On this basis,the fluorescence intensities of two-dimensional TMDs materials were significantly enhanced by effectively regulating the carrier behaviors to reduce the the recombination channels of photogenerated carriers with low fluorescence efficiency and to increase the recombination channels with high fluorescence efficiency.The main results are as follows:?1?A detailed study on high-energy C exciton dynamics in monolayer MoS2 was presented by femtosecond transient absorption?TA?spectroscopy.Different from previous reports,the C exciton exhibits a slow relaxation process within tens of picoseconds.From TA spectroscopy,we have proposed a brand-new model of C exciton relaxation and confirmed that the relatively slow cooling of C exciton is mainly limited by the rates of intervalley transfer.In addition,an anomalous photoluminescence quenching with decreasing temperature was observed in multilayer WSe2.Based on TA measurements,we have confirmed the reason of the photoluminescence quenching is that defect-assisted Auger processes dominate the photogenerated carrier recombination,and observed two distinct Auger processes caused by two different deep midgap defect-levels.Based on the Auger recombination model,the two Auger rates including a fast one and a slow one,were quantitatively estimated at 6.69±0.05×10-2 cm2 s-1 and 1.22±0.04×10-3 cm2 s-1,respectively.?2?Based on the above understandings of nonradiative processes of photogenerated carriers,we have first put forward the model of thermo-driven transition from trions to excitons,to reduce the number of trions?charged excitons?with low fluorescence efficiency.Subsequently,this model has been proved well in monolayer MoS2 grown by chemical vapor deposition?CVD?,resulting in a nearly 3-fold enhancement of photoluminescence.In addition,a controlled gas-molecules doping of monolayer MoS2 was achieved via atomic-layer-deposited Al2O3 films.The deposited Al2O3 films,in the shape of nanospheres,can effectively control the contact areas between ambient atmosphere and MoS2 that allows precise modulation of gas molecules doping.Using this doping method,the range of carrier concentration in monolayer MoS2 was estimated from 9.2×1012 cm-2 to 3.6×1013 cm-2 based on the mass action model.?3?Besides the control of carrier concentration,we have also modulated the energy states of photogenerated carriers and proposed a new mechanism of light emission.That is high-temperature induces photogenerated carriers transferring from indirect bandgap to direct bandgap,leading to an around 6.5-fold enhancement of direct transiton emission at 640 K comparing with room temperature of 300 K.Both theoretical and experimental results have confirmed that the high-temperature fluorescence enhancement can be attributed to thermally-activated intervalley transfer of photogenerated carriers,rather than a common mechanism of thermal-expansion-induced interlayer decoupling. |
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