Research On Structure Manipulation And Optical Properties Of Low-dimensional Cadmium-based Semiconductor Nanomaterials

Low-dimensional semiconductor nanomaterials are a new type of semiconductor material with excellent optical properties.Due to the influence of the quantum confinement effect,these artificially synthesized special materials with a size smaller than one hundred nanometers have some special physical properties that bulk materials do not have.At present,the most widely studied low-dimensional semiconductor nanomaterials are colloidal semiconductor quantum dots(QDs).Its advantages such as adjustable wavelength,high efficiency,solution processability,and low-cost preparation have made it important in the display field.In order to further realize the manipulation of the optical properties of low-dimensional semiconductor nanomaterials,designing and optimizing the structure and shape are two important technological means.However,the current research on the influence of these factors on the optical properties of low-dimensional nanomaterials is still limited.This thesis focuses on the group cadmium-based semiconductor nanomaterials with different core-shell structures and shapes as the research object.A variety of laser spectroscopy research methods were used to study the changes in optical properties,such as luminous efficiency,luminescence stability,Auger recombination,optical gain,and surface defects,etc.By further mastering the method of adjusting the optical properties of low-dimensional semiconductor nanomaterials,the improvement of their optical properties and the expansion of their application prospects can be achieved,and the theoretical and technical support for the structural design of materials can be provided.According to the structure and shape of low dimensional semiconductor nanomaterials,the specific research contents of this thesis are as follows:(1)CdSe/CdS single-layer core-shell QDs with good luminescence efficiency and stability are fabricated and their optical properties are studied.Mixed with polymers,the active micro optical waveguide structure doped with QDs is realized by the pulling method.The photoluminescence signal is received in the end of microfiber after single-point excitation to explore the effects of polarization excitation,fiber diameter,and substrate refractive index on the optical properties of the waveguide of this structure.The blue light emitting diode directly excite the QDs-doped polymer microfiber,realizing a small-sized and compact active optical waveguide structure,and expanding the application of QDs in miniaturized optoelectronic integrated devices.(2)Through coating the CdSe/CdS core-shell QDs with wide band gap ZnS material to form a multi-shell core-shell QDs structure.The effect of ZnS coating on the optical properties of QDs is studied by steady-state spectroscopy and temperature dependent photoluminescence spectroscopy.Benefit from the ZnS shell,the luminescence efficiency and stability of QDs are improved,and the internal defects are significantly reduced.Subsequently,a low-threshold laser action achieve through these multi-shell core-shell QDs combined with a whispering gallery mode resonant microcavity,the application of QDs in the field of micro-nano lasers is explored.(3)The core-shell structure of QDs is further improved with gradient alloyed shell.The femtosecond transient absorption spectroscopy is used to study the biexciton dynamic process,and the biexciton Auger recombination lifetimes are calculated through variable excitation power density measurement.Due to the structure of the gradient alloyed shell,the interface barrier changes from sharp to smooth,and a long biexciton Auger recombination lifetime of 1268.2±67.5 ps is obtained.Due to the significantly suppressed Auger recombination,the ultra-low threshold(6.9?J/cm~2)amplified spontaneous emission in blue QDs is achieved.The gradient alloyed shell structure has an obvious regulatory effect on the Auger recombination process,and further improves the optical gain of the QDs,making it more suitable for applications in the laser field.(4)Taking two-dimensional CdSe nanoplatelets with a thickness of 4monolayers as the research object,the influence of the shape on the optical properties is studied.The special optical properties of nanoplatelets are studied through steady-state spectroscopy and transient absorption spectroscopy,and their two-dimensional sheet-like structure is confirmed.Using temperature dependent photoluminescence spectroscopy,the origin of its dual emission peaks at low temperatures is explored,and the mechanism is analyzed.It is believed that the low-energy peak originates from surface state-related emission.The coating of the nanoplatelets by the polydimethylsiloxane(PDMS)achieves the suppression of the surface state-related emission.The experimental results show that the nanoplatelets structure makes the influence of the surface on the optical properties more obvious.In this thesis,a variety of spectroscopic methods are used to study the influence of core-shell structure and shape on the optical properties of low-dimensional semiconductor nanomaterials.The research results are of great significance for the subsequent improvement and optimization of the optical properties of materials,and expand the application of low-dimensional semiconductor nanomaterials on the optoelectronic integrated devices and optical gain media.