Preparation Of Organosilicon-Functionalized Carbon Dots And Their Application In White LED

Solid-state lighting in the form of white LED (WLED), owning to their energy conservation, environmental protection, long lifespan and high economic efficiency have attracted widespread attention in recent years. At present, phosphors, quantum dots (QDs) are mostly used as conversion materials in commercial WLED. Phosphors are often prepared in harsh conditions such as high temperature and pressure. Phosphors usually contain expensive rare earth elements, which will do harm to the environment. In addition, size of phosphors are inhomogenous which will cause light blocking, leaking, and even relatively low optical properties. Although QDs exhibit several advantages such as high color index and tunable emission, they will show photobleaching or optical scintillation which is negative to WLED in the long term. And for all we know, QDs are often non-environmentally friendly due to existence of heavy metal that will cause huge harm even at relatively low level. Further, poor compatibility between phosphors/QDs and encapsulation materials (silicone or polymer matrix) seriously influenced optical properties and stability of the device. Photoluminescent carbon dots (CDs) have received wide attention on account of plenty of advantages such as good biocompatibility, high photostability, eco-friendliness and easy functionalization. Organosilicon materials which possess excellent high and low temperature resistance, weather resistance, high transmittance and refractive index, have widely applied as LED packaging materials. This thesis mainly focuses on preparation of three kinds of organosilicon-functionalized fluorescent CDs including green emissive silicone oil-functionalized CDs, green and red emissive silane-functionalized CDs (SiCDs) based on easy functionalization of CDs and its application in WLED. In addition, silicone-functionalized CDs can be self-curing or co-curing which are expected to have dual functions of luminescence and encapsulation in WLED. This strategy will effectively avoid the traditional compatibility problems between luminescence and packaging materials. Organosilicon-functionalized CDs are potential to provide opportunity to simplify the preparation process and improve optical performances of WLED. The main research contents in this thesis are summarized as follows:(1) Aminopropylmethyl silicone oil-functionalized CDs (AMS-CDs) and their application in dichromatic WLED:AMS-CDs were prepared via one-step solvothermal method, and their emission wavelength can reach 550 nm at 460 nm excitation which can meet the requirement of commercial WLED. AMS-CDs can realize self-or co-crosslinking with AMS to form CDs crosslinked silicone rubbers (CDs-SRs), due to rich carboxyl and amino groups on AMS-CDs surface. Thus, AMS-CDs are potential to be conversion and packaging materials at the same time in WLED device. The as-prepared AMS-CDs-based WLED have CIE coordinates of (0.33,0.28) which are close to the pure white light (0.33,0.33). Doping concentration of AMS-CDs in CDs-SRs can be controlled from 10-100 wt%. The as-obtained CDs-SRs with high transparency can be bent for several cycles. Furthermore, AMS-CDs-based encapsulation layer shows good thermal stability. Hence, AMS-CDs have potential application in the field of WLED.25 wt% CDs-SR is investigated to have the maximum bending deflection of 9.58 mm, the curvature radius of 26 mm, low bending modulus of 5.2 MPa, shore hardness of 20 A which is similar to the reported silicone elastomer. Give this, CDs-SRs are potential to be applied in the flexible devices.(2) Red emissive CDs and red LED:Red emissive CDs were prepared by one-step solvothermal method in which p-phenylenediamine (PPD) was carbon source. By adjusting reaction conditions and ratios of reactants, the resulting CDs with high QY of 25% were obtained. QY of CDs was similar with that of the reported CDs with red emission, but preparation method in this thesis is quite simple. We conclude NaOH plays important roles in high QY and the red-shift of PL spectra. CDs with higher QY of 32.8% and SiCDs with red emission was obtained by further purifying CDs with QY of 25%. Further, we got red LED by combining blue LED chips and red emissive CDs which are potential to be used in lighting and display.(3) SiCDs and their application in trichromatic WLED:Color rendering index (CRI) can be tuned from 69 to 88 by controlling ratio of the SiCDs with green and red emission, which is higher than that of green emissive SiCDs-based dichromatic WLED (CRI is 56-58). CRI of the prepared trichromatic WLED can meet the requirement of indoor ligting. Color temperature (Tc) can be tuned in the range of 5592-8732 K, luminous efficiency can be tuned to reach 54.4 1m/W. CDs with red emission are significant to improve CRI value and red-shift of EL spectra. CIE coordinates of the prepared WLED were the same as pure white light. This result indicates SiCDs show good prospect in the application of trichromatic WLED. SiCDs with green and red emission have similar functional alkoxysilane group which can co-condensate with each other to form 3D crosslinking structure. What’s more, environmental SiCDs are potential to be luminescence and encapsulation layer simultaneously in trichromatic WLED. This strategy will help to avoid traditional compatibility problem between luminescence and encapsulation materials in WLED. The as-prepared polymerizable SiCDs are hopeful to provide new opportunity to simplify the preparation process and improve optical performances of WLED.