Optical Properties And Heterostructure Dynamics Of Two-Dimensional Transition Metal Chalcogenides And Perovskites

In the past half century,the rise of semiconductor mater ials has greatly promoted the great development of integrated circuitries science and technology.At present,the development of integrated circuitries has entered the post-Moore era,and the size of traditional silicon-based microelectronic devices has been continuously reduced to a few nanometers in order to improve the chip integration density and computing power.However,due to the decreasing size and increasing quantum effects,silicon-based chips gradually reach the physical limit.Therefore,it is u rgent to develop new integrated optoelectronic devices with small size,low power consumption,high speed and strong anti-interference ability to break through the bottleneck and challenge of silicon-based chips.Excellent semiconductor materials are the foundation for the construction of multifunctional and efficient new optoelectronic integrated devices,among which transition metal dichalcogenides（TMDCs）and perovskites（PVKs）are the star semiconductor materials.They have excellent physical properties in the fields of optics and electricity,which present good application prospects in high-density integrated circuits and optoelectronic micro/nano devices.It is expected to develop high-performance photodetectors,light-emitting diodes,high-efficiency light source devices and waveguide devices to make up for the shortcomings of traditional silicon semiconductor devices,continue Moore’s law,and expand the functions and applications of semiconductor devices.In recent years,TMDCs have attracted extensi ve attention of researchers due to their excellent photoelectric characteristics,such as,the band gap covering the visible-near infrared（1.0-2.1 e V）range,the direct band gap monolayers,novel valley degrees of freedom,large specific surface area and strong light interaction with matter.These properties indicate that TMDCs materials have great research potential in exploring new nano-optoelectronic devices.However,since the thickness of the ultrathin atomic layer limits its ability to absorb enough light for effective light-matter interactions,the photoelectric performance based on TMDCs still has a lot of room for improvement.Therefore,reasonable selection of another semiconductor material and its coupling to construct van der Waals heterojunctio n is a very feasible and important scientific mean for the design of high performance optoelectronic devices.The advantages of PVKs,such as adjustable band gap,ultra-high energy conversion efficiency,high light absorption to visible light,and long car rier diffusion length,make PVKs the best choice for constructing high-performance optoelectronic devices combined with TMDCs.This paper focuses on the two new semiconductor materials,two-dimensional transition metal chalcogenides and perovskites.First of all,I studied their unique optical properties respectively,and then constructed the van der Waals heterojunction to study the carrier and the energy transfer behavior of the device,which aims to reveal the basic physical properties of light and matte r mutual coupling effects,further improve the photoelectric device performance.The main innovative research results of this paper are as follows:1.I have revealed the dynamic photoluminescence（PL）enhancement and expansion behaviors regulated by oxygen in monolayer tungsten disulfide（WS₂）,and the PL brightening rate was successfully manipulated.I grown a monolayer WS₂triangle by a simple physical vapor deposition（PVD）one-step method.At atmospheric environment,I found the PL enhancement phenomenon at the edge through PL mapping,and the bright PL would gradually expand from the edge to the center over time.Optical studies and structural characterization techniques show that the PL enhancement is caused by p-doping introduced by oxygen chemisorption,which attenuates the many-body effect and non-radiative radiation.First principles calculations unveil the dynamic evolution process of the PL enhancement by oxygen chemisorption.In addition,I successfully manipulated the PL brightening rate by controlling the oxygen concentration and the temperature.Our results shed light on the special surface chemistry and related mechanisms in WS ₂ monolayers and deepen the understanding of their unique PL evolutionary behavior.2.I have studied the interlayer carrier dynamics of a single layer WS₂/PVKs van der Waals heterojunctions.Firstly,I synthesized Cs Pb Br₃ and Cs Sn Br₃ perovskites using a tube-in-tube setup chemical vapor deposition（CVD）system.The optical characterization techniques were used to study the PL and lasing waveguide behavior of Cs Pb Br₃ perovskite nanoplate-nanowire couplers and the typical thermal effect of Cs Sn Br₃ nanowires.Furthermore,the Van der Waals heterojunction s were constructed by combining the perovskites with the monolayer WS₂,which revealed the carrier transfer mechanism induced by type II band alignment,and the emission of interlayer excitons was also observed.This study combines the excellent physical characteristics of TMDCs and perovskites,which provides a good platform for exploring the interlayer coupling and related physical processes,and lays a foundation for the fabrication of high-performance integrated optoelectronic devices with vertical semiconductor heterostructures.3.I have explored the long-range dark state energy transfer mechanism under two-photon excitation in the WS₂/Cs Pb Br₃ van der Waals heterojunction.On the basis of carrier transfer phenomenon under one-photon excitation in WS₂/Cs Pb Br₃heterojunction in the previous chapter,I observed the PL enhancement of monolayer WS₂ using the two-photon pumping technique,which is quite different from that of one-photon excitation.Two-photon PLE and transient spectra revealed the dark-state resonant F?rster energy transfer mechanism from Cs Pb Br₃ to monolayer WS₂,and the thickness-dependent experiments of Cs Pb Br₃ further demonstrated this bottom-up hopping F?rster energy transfer process.In addition,by constructing van der Waals heterojunctions of WS₂ and RP-phase perovskite,I found that the two-photon induced energy transfer mechanism has certain universality.Finally,I successfully achieved an order of magnitude improvement in the near-infrared detection performance of monolayer WS₂ using this energy transfer mechanism.This work enriches the physical theory of energy transfer and provides a new scientific method for improving the near-infrared photoelectric characteristics of monolayer TMDCs.