Studies On Ultrafast Spectroscopy Of 2D Transition Metal Dichalcogenides

Human development is inseparable from the progress of material science.Nowadays,we are in the information age,the material science has achieved unprecedented development.The preparation and processing technology of monocrystalline silicon materials has promoted the development of large-scale integrated circuits.The rise of semiconductor compounds has promoted the progress in the fields of microelectronics and optoelectronics.The breakthrough of high-temperature and high-strength structural materials has promoted the development of the aerospace industry.This shows that materials science is the foundation of high-tech development and represents a country's industrial and technological strength.Among these materials,nanomaterials acting as a material with unique physical and chemical properties,have been widely used in the fields of information technology,electrochemical physics and life sciences.Among nanomaterials,since the monolayer graphene material was first successfully obtained by mechanical exfoliation in 2004,the research on two-dimensional materials represented by graphene has developed rapidly.In the past ten years,it has become an important research object in the fields of chemistry,physics,materials science,and optoelectronics,etc.2D transition metal chalcogenides(2D TMDCs),as a new generation of 2D materials,known as“inorganic graphene”,have superior physical characteristics,such as large direct band gap,easy adjustment of band gap,high carrier mobility,high exciton binding energy,good mechanical properties,and good industrial application with a low cost,etc.In recent years,it has been widely used in the research fields of catalysis,quantum logic gates,entangled photon sources,spintronic devices,gas sensors and photovoltaic devices,ect.For the semiconductor-based devices,excited state species such as exciton,trion,biexciton,and carrier have a decisive impact on the device performance.However,due to the short history of in-depth research on two-dimensional materials,there are still many physical phenomena that cannot be accurately explained.There are still many physical properties need to be studied.The dynamic properties of the corresponding excited state species also need to be further characterized.Therefore,only if we have a deeper understanding of the excited species in 2D TMDCs,can we fundamentally promote the wider application of 2D TMDCs in the future.In this dissertation,we employ femtosecond transient absorption spectroscopy to study the ultrafast photophysical properties of two typical 2D TMDCs materials prepared by CVD technique:monolayer MoS₂ and monolayer WS₂.The research results are as follows:1.In the field of quantum information and quantum computing,the construction of quantum coherent state is a crucial step.Among them,the quantum coherent state system constructed from biexciton in semiconductor materials has great application potential due to its advantages of high stability and easy integration into devices.The photophysical properties of exciton and biexciton in monolayer MoS₂ is investigated in detail with the help of transient absorption spectroscopy.The biexciton of B exciton state would appear under high excitation photon energy(3.10 e V)according to the transient absorption spectrum.As the intensity of probe laser increases,it is observed that the biexciton could form on A and B exciton state simultaneously.The corrsponding binding energy of the biexciton gradually dissipates with time.As the intensity of probe laser increases,the percentage of biexciton in the excited state species gradually increases.In addition,by fitting the transient absorption spectra,we confirm that short-range interactions play a dominant role in the biexciton-biexciton annihilation process.These results help to broaden the applications of quantum logic gates and entangled photon sources based on 2D TMDCs.2.The existence of hot carriers can significantly improve the efficiency of photodetectors,photoelectric devices of solar cells and photocatalytic processes.However,the materials that have been studied so far,the hot carrier lifetime is less than one picosecond,which is difficult to extract in practical applications.Therefore,extending the cooling lifetime of hot carriers is of great significance for the effective utilization of hot carriers.Thus,the hot carrier dynamic in monolayer MoS₂ is investigated in detail with the help of transient absorption specstroscopy.Through analysing the transient absorption spectrum,hot carrier can simultaneously exist in different excitonic states,and the lifetime of hot carrier can reach several picoseconds.For the C-excitonic state,the initial energy of hot carrier can increase by increasing the excitation photon energy.As a result,the hot carrier cooling lifetime is gradually prolonged to several picoseconds due to the existence of hot phonon bottleneck effect.When the initial carrier energy is low,hot carrier can be generated through Auger heating effect,demonstrated by that the increasement of absorbed photon flux can enhance the carrier-carrier interaction to reach higher energy states.When combine these two effects,the hot phonon bottleneck effect induced by high absorbed photon flux is gradually weakened owing to the Auger recombination,which becomes apparent with the excitation photon energy.The similar hot carrier cooling phenomenon can also be observed in A/B-X,owing to the rich energy band structure of monolayer MoS₂.These results clarify the characteristics of hot carriers in the complex energy band structure system,which will help promote the further development of 2D TMDCs in the fields of photodetection and catalysis,etc.3.Hot carriers in semiconductor materials can improve the performance of their optoelectronic devices.In fact,the core is the cooling and diffusion process of hot carriers,which determines whether hot carriers can be effectively extracted at the material interface.In this part,the characteristics of hot carrier in C-excitonic state of monolayer WS₂ is investigated in detail by transient absorption specstroscopy.The transient absorption spectrum shows that due to the existence of hot phonon bottleneck effect,the hot carrier cooling lifetime can be prolonged to several picoseconds with the increasement of hot carrier density.In addition,the characteristics of the transient absorption spectrum of hot carrier is significantly different from those of normal carrier,indicating that the absorption coefficients between hot carrier and normal carrier are different.Finally,a modified Lennard-Jones model is established to study the diffusion process of hot carrier and estimate the average distance of hot carriers by fitting peak shift of transient absorption spectra.These results broaden the understanding of the cooling and diffusion process of 2D TMDCs hot carriers,and promote the application of 2D TMDCs in the field of optoelectronics.