Time Resolved Photoluminescence Properties Of Low-Dimensional Semiconductors And Mixed-Dimensional Heterostructures

Low-dimensional(including two-dimensional(2D),one-dimensional(1D)and zero-dimensional(0D))semiconductor materials have great potential applications in the field of nano-optoelectronics due to the quantum confinement effect and the surface properties.So far,low-dimensional semiconductor materials have been applied in field effect transistors(FETs),light-emitting diodes(LED),photodetectors,photovoltaic devices and flexible electronic devices.To improve the performance of these optoelectronic devices and expand the applications of low-dimensional semiconductor materials in new devices,we need to deeply understand the carrier behavior and dynamics in these materials,so as to understand the inherent physical properties,such as structural characteristics and defect density,and provide the theoretical guidance for the design of optoelectronic devices.In recent years,by constructing low-dimensional heterostructures,some functions beyond a single low-dimensional semiconductor can be realized,and the overall performance of the optoelectronic devices can also be greatly improved.Among these low-dimensional semiconductor materials,2D materials have become an important part of the low-dimensional heterostructures due to their excellent photoelectric properties and easy integration.2D-2D,1D-2D,and 0D-2D heterostructures can be formed by integrating 2D materials with lowdimensional semiconductor materials.As an extension of 2D materials,2D-2D heterostructures have many novel physical properties and have shown their advantages in LED,photodetectors and optical modulators.Compared with 2D2D heterostructures,1D-2D and 0D-2D mixed-dimensional heterostructures have unique advantages,such as stronger optical absorption,higher coupling freedom and more localized interface carrier interactions.Therefore,the study of these mixed-dimensional heterostructures is very beneficial for the improvement and regulation of local photoelectric properties in 2D materials.Based on the above background,the study of carrier behavior in lowdimensional semiconductor materials and their mixed-dimensional heterostructures will not only help understand the photoluminescence(PL)properties in these structures but also help design the device structure.In this dissertation,we used the time-resolved spectroscopy to study the photogenerated carrier behavior of 1D CsPbBr3 nanowires in the stimulated emission process,the interaction of interfacial carriers in 1D-2D heterostructure composed of CsPbBr3 nanowire and MoSe2 monolayer,and the interaction of interfacial carriers in 0D2D heterostructures composed of CdSe/ZnS core-shell quantum dots(QDs)and MoS2.The specific research content and achievements are as follows.1.To study the photogenerated carrier behavior in low-dimensional semiconductors and their mixed-dimensional heterostructures,a streak camera system and a TCSPC system were designed and constructed.By optimizing the spatial optical path,using femtosecond pulsed laser as excitation source and selecting appropriate grating,we have obtained the best spectral resolution and time resolution of the streak camera.Additionally,by designing and optimizing the excitation and collection optical path of the TCSPC system,the functions of vector-polarized laser excitation,second-order photon correlation measurements,steady-state PL spectrum and time resolved PL measurements,and spatial scanning imaging of corresponding optical signals have been realized in this system.2.The interplay and reversible transitions between the excitonic and electron-hole plasma(EHP)states of CsPbBr3 nanowires in the stimulated emission process have been revealed at room temperature.We investigated the carrier behavior of nanowires based on one-photon energy and two-photon energy excitation using a streak camera system.A red-shift of the lasing gain profile with increasing excitation fluence is observed,suggesting the transition from the excitonic state to EHP state that responsible for the lasing.Whereas,a blue-shift of the gain profile with time decay represents the opposite direction of the former process.Moreover,the individual lasing modes show a blue-shift as excitation fluence increasing and a red-shift as time elapsing,due to the carrier density dependence of the refractive index.3.The fluorescence blinking behavior of MoSe2 in CsPbBr3/MoSe2 1D-2D heterostructure was observed due to the mechanism of carrier trapping by supertrap in CsPbBr3 nanowires.We first demonstrated the fluorescence blinking phenomenon of the nanowire by the PL intensity time trajectories,and thus confirmed the existence of the super-trap in the nanowire.Subsequently,mixeddimensional heterostructures of CsPbBr3/MoSe2 and CsPbBr3/hBN/MoSe2 were constructed simultaneously.The charge transfer process in CsPbBr3/MoSe2 heterostructure and the energy transfer process in CsPbBr3/hBN/MoSe2 heterostructure were proved by steady-state PL,transient PL and PL scanning imaging.Moreover,the PL intensity time trajectories and transient PL at different excitation powers proved that the fluorescence blinking behavior of MoSe2 in CsPbBr3/MoSe2 heterostructure is not caused by the intermittent charge transfer and energy transfer process,but should be attributed to the super-trap in nanowire randomly capturing the photogenerated carriers in MoSe2.4.The single photon energy transfer is realized between a single CdSe/ZnS core-shell quantum dot(QD)and MoS2,and the fluorescence blinking behavior of MoS2 in heterostructure is observed at room temperature.We first constructed a 0D-2D mixed dimensional heterostructure consisting of the QDs and MoS2 atomic layers.The second-order photon correlation measurements indicate that the single-photon nature of the QDs in heterostructures still maintains after energy transfer with MoS2.Based on the transient PL and PL time trajectories of QD/MoS2 heterostructure,we attribute the fluorescence blinking of MoS2 in heterostructure to the single photon energy transfer process.In addition,we also constructed a heterostructure formed by MoS2 bilayer(21-MoS2)and QDs.Due to the small energy transfer efficiency between QD and 21-MoS2,the fluorescence blinking behavior of 21-MoS2 in heterostructure was less prominent.