Investigation On The Controlable Synthesis And Fluorescence Properties Of Graphene Quantum Dots

Graphene is a kind of material with zero band gap and its band gap can be opened up when it is cut into graphene quantum dots(GQDs),which not only inherit excellent properties such as good conductivity and large surface area of graphene but also have good optical properties.There are a variety of functional groups on the surface of GQDs,through which the band gap of GQDs can be adjusted within a large range.Along with their good biocompatibility,chemical and thermal stability,GQDs will be a compitative substitute for rare earth phosphor as light converter in LED.This paper mainly focuses on the preparation,characterization,performance improvement and application of graphene quantum dots.By an acid-free one-step redox reaction,GQDs with uniform size were prepared.Their average size is 3.21 nm.They have similar structure with graphite,and their height is between 0.5~2 nm,which is equivalent to the thickness of 1~4 layers of graphene sheet.In order to regulate the emission wavelength and to improve the quantum yield of GQDs,we modified the as-prepared GQDs with 2,3-diaminonaphthalene(DAN)and a kind of silane coupling agent(AEAPTMS)to get amine modified GQDs labeled as D-GQDs and A-GQDs,respectively.The average sizes of D-GQDs and A-GQDs are 3.5 nm and 3.75 nm,which are both larger than that of GQDs,and the height of D-GQDs also increased to 1~3 nm.After the optimation of preparation and modification parameters,and the emission wavelength of GQDs was changed from 500 nm to 617 nm and 550 nm,respectively,and the quantum yield was increased from 23.5 % to 52.4 % and 35.3 %,respectively.In order to achieve solid-state-lighting,the as-prepared GQDs were dispersed in PVA gel to prepare GQDs/ D-GQDs/ PVA films and A-GQDs/PVA films.The transmittance of GQDs/ D-GQDs/ PVA film is high in the visible light and low in ultraviolet light.It has good thermal stability emits white light at the excitation of 380 nm with the absolute QY of 5.9 %.As to A-GQDs/PVA film,it emits yellow light at the excitation 400 nm light with the absolute QY of 14.6 %.Finnally,White LED devices were fabricated by assembling GQDs/D-GQDs/PVA films and A-GQDs/PVA films with 380 nm UV chip and 400 nm chip respectively.The stability of these two LED devices under different current is good.The CIE coordinates of the device based on GQDs/D-GQDs/PVA film is(0.3399,0.4043),the color temperature is 5290 K and the color rendering index is up to 77.3.The CIE coordinate of LED device based on A-GQDs/PVA film is(0.3406,0.3705),which is warm white light,the color temperature is as low as 5216 K,and the color rendering index is 74.2.The luminescent properties of these two kinds of WLED both have good prospects in the field of LED display.