Controllable Preparation And Luminescence Performance Of Highly-efficient Carbon Dot Materials

Carbon dots(CDs)are emerging optical nanomaterials with excellent optical properties,tunable photoluminescence(PL),high quantum yield(QY),and environmentally friendly,low toxicity,and good biocompatibility.Since their discovery in 2004,the photoluminescence quantum yield of CDs in solution can reach up to 90%with continuous research.However,reports on long-wavelength solid-state fluorescent CDs are relatively rare and have lower QYs.This is due to excessive resonance energy transfer or interlayer van der Waals interactions,leading to-interactions that result in severe self-quenching of CDs in solid-state environments.In addition,the study of the phosphorescence properties of CDs started relatively late and is still in the preliminary stage.In most reports,the phosphorescence properties of solid-state CDs are limited to short-wavelength(green,yellow light)emission,showing limited color and tunability with relatively short phosphorescence lifetimes.The main focus of this thesis is to address the aforementioned issues by conducting the following research:synthesis of a high quantum yield solid-state fluorescent carbon dot through polymer chain modification strategy and its application in constructing color temperature adjustable white-light emitting diodes(LEDs);successful preparation of room-temperature phosphorescent carbon dots with tunable full-color and wide-range lifetimes through the host-guest synergy of CDs and MOFs,and their application in information anti-counterfeiting and data encryption fields;preparation of high quantum yield fluorescent/phosphorescent single-component white light by regulating the intrinsic and defect states of CDs and its application in constructing color temperature adjustable white-light emitting diodes with high color rendering index(CRI).(1)Currently,the transformation of CDs from solution to solid state results in a significant quenching of their PL,and the QY of long-wavelength solid-state CDs is generally low.Therefore,obtaining high-QY solid-state fluorescent CDs is still highly desirable.In this study,we report a new structural strategy for synthesizing high-QY CDs with anti-self-quenching solid-state fluorescence emission that is significantly tunable from the visible to near-infrared regions.The relative QY of these CDs reaches up to 67.7%.Using the luminescent properties of the CDs,we also successfully fabricated high-performance cool,pure,and warm white LEDs,as well as a plant growth LED lamp that combines blue chips with deep red and near-infrared-emitting CDs.Under the same experimental conditions,plants irradiated by the plant growth LED lamp grew better in leaf and root length than those grown under daylight and white light LEDs.(2)Room temperature phosphorescence(RTP)has great potential in the fields of anti-counterfeiting,data encryption,sensing,etc.However,most of the reported RTP materials emit in short-wavelength regions(yellow,green)and have tunable lifetimes ranging from microseconds to milliseconds,which greatly limits their practical applications in the field of anti-counterfeiting.Therefore,it is urgent to develop RTP materials with full-color emission and a wide range of tunable lifetimes.In this study,we achieved full-color RTP tuning of carbon dots(CDs)by embedding them into a rigid structure of a metal-organic framework(MOF),which also allowed for tunable lifetimes in the range of milliseconds to seconds.In this material,CDs as the main material were embedded in the rigid MOF environment,which effectively suppressed their non-radiative transitions.The MOF as a guest material,with the addition of Zn2+,could effectively enhance the spin-orbit coupling between the triplet states of CDs,promote intersystem crossing,and achieve full-color RTP emission with a wide range of tunable lifetimes.We utilized its optical properties for a series of advanced anti-counterfeiting and information encryption 4D coding applications.(3)Currently,white light-emitting diodes(WLEDs)are mainly prepared by encapsulating different fluorescent CDs on the surface of LED chips.However,mixing different colors of multiple CDs in specific ratios increases the cost and difficulty of device fabrication.Therefore,it is crucial to develop white light materials and conduct research on their structure and performance to achieve high-performance,single-component WLEDs that can be excited in the near-ultraviolet region.In this study,we proposed an effective synthetic strategy that achieved single-component white CDs with a QY of up to 67.35%and adjustable color temperature(CCT)by modulating the size of the sp~2 structure.Under UV lamp irradiation,these CDs exhibited white light emission through the synergistic effect of FL and RTP.By combining these CDs with UV LED chips,we achieved WLEDs with a CCT that can be adjusted from 3376 to10803 K and a high color rendering index(CRI)of 81-85.Finally,we further prepared planar WLEDs based on transparent and uniform CDs/PVA composite films.This technique helps to dissipate heat and improve the conversion efficiency of LED chips.These WLEDs emit bright white light with a high luminous efficacy of 35.2 lm/W,which is comparable to some commercial WLED products.