Research On The Preparation And Luminescence Properties Of Light-emitting Diodes Based On Liquid-type Carbon Dots

Colloidal quantum dots have attracted much attention of researchers because of their adjustable color and high fluorescence efficiency for the wide applications in photovoltaic cells,light-emitting diodes(LEDs),and bioluminescent labeling.As a novel quantum dot material,carbon dots have the advantages of low cost,abundant raw materials,low toxicity,good biocompatibility,and stable light emission.However,the carbon dots in the solid state had weak or no luminescence.And the emission of the carbon dots mainly located in the blue-green region.Unlike quantum dots,the multicolor emission cannot be obtained by just adjusting the particle size.In view of the above problems,firstly self-made glass boxes which were equipped with carbon dots in liquid type were used as phosphor.Then,multicolor LEDs from blue to red were obtained by adjusting concentration and changing solvent.White light emitting diodes were obtained by carbon dots and other high-efficiency quantum dots to make up for the long-wavelength emission.The detailed contents are shown as follows:(1)The excitation-dependent photoluminescent carbon dots were synthesized from N-(?-aminoethyl)-?-aminopropyl methyldimethoxy silane(AEAPMS)and anhydrous citric acid by pyrolysis.Liquid-type LEDs with blue,green,yellow,and red colors were obtained by increasing the concentration of carbon dots in the toluene solvent,accompanied by reabsorption and reemission of light.The highest luminous efficiency is 44.36 lm/W for the green LED(converted into a current efficiency of 2.75 cd/A),which is higher than the corresponding solid-state carbon-based LED.Moreover,the four different color LEDs exhibited good stability under continuous operating conditions.(2)The carbon dots exhibit stable fluorescence and no optical blinking,but the emission is mainly in blue region,the quantum yield for long-wavelength emission is low.In order to prepare warm white LEDs with high color rendering index,the green and red-emitting Cd Se-based quantum dots solution were mixed with blue-emitting carbon dots solution to prepare white-light phosphor,which was excited by an ultraviolet LED chip.By adjusting the ratio of three kinds of quantum dots,the relative intensities of the three main peaks of red,green,and blue were changed,and the correlated color temperature changed from 5001 K to 3447 K,so that a warm white LED was obtained with a color rendering index up to 91.In addition,the prepared white LED showed good stability with the increase of operating current.(3)Using diphenyl ether and p-phenylenediamine as raw materials,the carbon dots with quantum yield(QY)ranging from 9.2% to 34.5% were synthesized by pyrolysis,and the emission exhibited red shift with the increase of solvent polarity,but excitationindependent photoluminescence.The carbon dots were characterized by UV-vis absorption spectroscopy,photoluminescence spectra,transmission electron microscopy,infrared Fourier spectroscopy,and X-ray photoelectron spectroscopy.It was speculated that the solvent-dependent photoluminescence was due to the effect of the dipole on the surface electronic state.Liquid-type LEDs based on this kind of carbon dots provide a possibility for multicolor display.