Synthesis Of Highly Efficiency Blue Emissive Quantum Dots And Their Applications In White LED

Light-emitting diodes（LEDs）,as the emissaries of the 21st century,have the advantages of low energy consumption,small size,long service life and environmental friendliness.They are widely used in the fields of display and lighting fields.Quantum dots（QDs）is the general name of a class of low-dimensional nanoscale semiconductor materials,which has the advantages of wide excitation wavelength range,continuously adjustable emission wavelength,narrow and symmetrical fluorescence emission peak。Traditional QDs contains heavy metal elements,which limits its development and application.Metal free QDs based on sulfur and carbon have the advantages of non-toxicity and good biocompatibility,so they are used in cell imaging,analytical detection,display lighting and other fields.However,they still have many shortcomings,such as stability,luminous color control performance and large-scale preparation need to be improved.Sulfur quantum dots（SQDs）have a wide range of raw materials,abundant reserves,antibacterial,etc.,but lack of simple synthesis method,and its quantum yield is only 3.8%lower than the traditional quantum dots,which limits the application in LED.Carbon dots（CDs）has the advantages of stability,non-toxicity and simple preparation method.A variety of excellent luminescent properties of CDs can be synthesized by liquid phase method.However,its optoelectronic device applications are limited by severe aggregation caused quenching（ACQ）.In this paper,aiming at the above problems,from the perspective of material synthesis,to solve the key scientific problems in the application of blue emissive SQDs and CDs in white LED.High-efficiency fluorescent materials are obtained and used in the construction of high-quality white LED.The main research contents and results are as follows:（1）Fluorescent SQDs with high quantum yield（23%）and adjustable luminous color was synthesized by H₂O₂ assisted synthesis top-down approaches.Its synthesis process is that with the participation of polyethylene glycol（PEG）,sulfur powder is dissolved into sulfur dots（S dots）under alkaline environment,and H₂O₂ is used to assist in etching the polysulfide on the surface of S dots to obtain passivated SQDs.Through the above strategy,SQDs size and luminous color can be skillfully controlled by controlling the concentration of injected H₂O₂.Furthermore,the luminescence mechanism of SQDs was studied by means of transient fluorescence spectrum,X-ray photoelectron spectroscopy（XPS）,fourier transform infrared spectrometer（FTIR）and other characterization methods,and it was clarified that SQDs fluorescence was the result of the interaction between intrinsic luminescence and surface state.Through H₂O₂ treatment,surface defects can be eliminated,nonradiative transitions can be reduced,and luminous efficiency can be improved.The SQDs synthesized by this method has good water solubility,high luminous efficiency and simple synthesis method,which has a great application prospect in biological imaging,photoelectric devices,analysis and detection and many other fields.（2）A one-pot hydrothermal method for in-situ synthesis of high-efficiency blue emissive boron-nitrogen-doped CDs（BN-CDs）powder was proposed to solve the problem of low quantum yield caused by ACQ effect of CDs.In this method,citric acid,boric acid and urea were used as raw materials.The polymerization and carbonization of citric acid and urea occurred under hydrothermal conditions,and CDs was formed.Further,BN-CDs powder with fluorescence quantum yield up to 80.1%was obtained by freeze-drying,and its quantum yield was 3.2 times of that in solution state（25%）.Through a series of structure and morphology characterization,it was confirmed that the structure of the BN-CDs powder was composite structure of CDs and BNO matrix.The fluorescence mechanism was further studied.It was found that the ACQ effect of CDs was overcome by the introduction of electron-deficient boron atoms and the combined action of the BNO matrix with high refractive index formed around CDs.At the same time,boron atom attacks C=O to form C-B,which introduces boron-related surface state emission,inhibits surface non-radiative transition,improves luminous efficiency,and forms a new transition path,resulting in blue shift of luminous color.The BN-CDs synthesized by this method have highly solid-state luminous efficiency,and the preparation method is simple and easy to prepare on a large scale.It has great application prospects in the field of optoelectronic devices.（3）The key to the construction of white LED is to obtain high-efficient phosphors that are highly matched with the luminescent chip.This article use the GaN UV chip as excitation light source,SQDs,CDs as blue phosphor,combined with the orange light copper nano clusters（Cu NCs）to construct the white LED,which effectively avoids the potential harm to human eyes caused by excessive blue light of blue chip-based white LED.Meanwhile,orange emissive Cu NCs wide luminous spectrum is used to improve the color rendering index of white LED.A series of blue and orange LEDs were obtained through the optimization of packaging process.By changing the proportion of blue emissive SQDs and orange emissive Cu NCs,the white LED was finally obtained with CIE chroma coordinate of（0.33,0.32）,CCT of 5624 K and high color rendering index of（91）.White LED was prepared by using the prepared high quantum yield（80.1%）blue emissive BN-CDs and orange Cu NCs。The CIE chromaticity coordinate can be adjusted from（0.16,0.12）to（0.47,0.44）,optimized white light LED device is（0.33,0.32）,and the CRI is 89,and the CCT is 5624 K.The above work provides material and technical reference for the construction of high-quality white light LED.Meanwhile,SQDs,CDs and Cu NCs are all non-toxic materials with rich crustal content,which effectively avoids the trouble caused by rare earth and heavy metal elements.