Tuning The Photoluminescence Properties Of Carbon Dots Toward Advanced Applications

Luminescent carbon dots?CDs?have attracted wide attention as the superior and universal next generation of optical functional material because of their unique combination of a number of key merits,including facile preparation,outstanding physicochemical stability,easy functionalization,excellent biocompatibility,with tunable fluorescence?FL?,up-conversion PL?UCPL?,phosphorescence?Phos?,delayed fluorescence?DF?,electroluminescence,chemiluminescence and so on.Up to now,despite that much breakthrough has been achieved in CD-relevant research field,scientists are still facing some challenges:i)most of reported CDs only show intense emission at blue-to green-light regions,and it is hard to prepare long-wavelength?i.e.yellow and red regions?emitting CDs.Meanwhile,the existing multi-color emitting CDs are usually achieved under different excitation wavelengths while it is difficult to realize that under a single excitation wavelength;ii)Recent years,the research on CDs mainly focus on the regulation of their fluorescent properties,preparation of long-wavelength-emitting CDs and exploration on their potential applications.However,studies on the realization and regulation of other types of photophysical properties?phosphorescence,delayed fluorescence,up-conversion luminescence?of CDs are rare;iii)So far,long-lived emission of CDs were obtained by fixing them within certain matrices,which highly impedes their applications,thus the synthesis of bare CDs with long lifetime room temperature phosphorescence?RTP?is highly desired;iv)At present,the application of CDs is mainly considered as a substitute for traditional luminescent materials,the lack of novel application with competitive advantage is not conductive for obtaining full potential performance of CDs.Given this,based on the existing research conclusions of CDs,this article starts from the origin and mechanism of the CDs'luminescence,and combines the generation process and microstructure control of CDs,by regulating the energy level structure of CDs and their composite for adjusting their photoluminescence properties.In this process,we successfully realized the controllable preparation for multi-color emissive CDs,the regulation of RTP of CDs and delay fluorescence of CDs,and the achievement of ultralong lifetime RTP from bare CDs,exploring a series of advanced applications of CDs based on their properties.The main content of this thesis are concluded as follows:1.Study on the regulation of CDs energy level structure and fluorescenceAccording to the origin and mechanism analysis of the luminescence from CDs:the carbon nuclear state,surface state and molecule state are the main sources of luminescence of CDs.Therefore,using hydrothermal method,by selecting raw materials and controlling reaction process,regulating the nitrogen?N?doping content of CDs and surface oxidation degree,also the introduction of oxygen-containing functional groups?-COOH?,resulting in the formation of carbon core defect,specific bonding and surface state that introduced by doping elements and surface functional groups.These affects lead to the reduction of band gap width of CDs and red shift of emission wavelength,thereby successfully preparing multi-color emissive CDs,realizing the controllable preparation of super bright yellow emissive CDs,and trichromatically fluorescent CDs that excited under a single wavelength,also the highly emissive green and red CDs.2.Regulation of RTP and DF emission from CDs by tuning the energy gap between the lowest triplet and singlet stateInspired by the strategy of incorporating the CDs into matrices,two composites with long lifetime room temperature afterglow were facilely prepared via fixing the m-CDs onto polyvinyl alcohol?PVA?and colloidal nanosilica?nSiO₂?through hydrogen and covalent bonds,which named m-CDs-PVA and m-CDs@nSiO₂,respectively.Therefore,both of hydrogen and covalent bond could be employed as an option to fix and rigidify triplet emission species and prevent the excited triplet deactivation while stabilizing the excitation triplet.The investigation into the effects of different types of bonds imposed on the energy structure and optical properties demonstrate that the covalent bonding is stronger than the hydrogen bonding,which can be able to reduce the energy gap between the lowest triplet and singlet state(?E_ST)from⁰.5 eV for m-CDs-PVA film to⁰.3 eV for m-CDs@nSiO₂,endowing them with RTP and delayed fluorescence,respectively.Therefore,we successfully achieved the controllable types of long-lived emission from RTP to delayed fluorescence.3.Study on the regulation of carbonization degree control on CDs energy level structure and phosphorescence emission performanceInspired by the general requirements for obtaining effective RTP from traditional luminophors and CDs,we proposed the following three key conditions for preparing RTP CDs directly:i)the amorphous polymer-like structures could act as matrices which are beneficial for embedding and immobilizing luminophors;ii)the special functional groups on the surface of CDs could form hydrogen or halogen bonds for further stabilizing the excited triplet species;iii)in order to facilitate the ISC process for effectively populating triplet excitons,some special elements?e.g.,N,P,and halogens?that favoring n??*transitions should be doped in CDs.Based on this,a facile,quick?within 5 min?and gram-scale?2.8 g,yield 70%?method was developed to prepare CDs via microwave-assisted heating of ethanolamine and phosphoric acid.These CDs exhibit ultralong RTP?URTP?emission with a lifetime of 1.46 s?more than 10 s to naked eye?,which present the longest RTP lifetime for CDs-based materials to date.Further investigations confirmed that the doping of N and P elements and formation of hydrogen bonds in the interior of the CDs were critical for the URTP emission.Moreover,the formation process of URTP-CDs could be disassembled into two steps.At first,the crosslinked enhanced emission?CEE?type of CDs?exhibit FL emissive only,F-CDs?were prepared from ethylenediamine and phosphoric acid by a simple heating treatment?180oC?.Importantly,in the next step,F-CDs are found to produce URTP?P-CDs?after further heating treatment?280oC?.This is the first example to achieve the conversion of a FL material to URTP by means of an external heating stimulus.Further studies reveal that the further carbonization of F-CDs could form more compact cores for effective intraparticle hydrogen bonds,which resulted in the self-immobilization of the excited triplet species and responsible for the observed URTP.4.Exploring the applications based on the CDs photoluminescence performanceTo ensure the great potential of these CDs for practical applications,firstly,the cytotoxicity of three primary color fluorescent CDs was evaluated.The low cytotoxicity of these CDs made them excellent biomarkers for multi-color cellular imaging.Secondly,after dispersing the multi-color FL CDs in solid polymers to prevent the aggregation-coursed quenching?ACQ?,numerous applications such as full-color emission turning,multi-color light-emitting diodes?LEDs?for display and white LED for lighting were demonstrated.Moreover,the up-conversion PL?UCPL?of m-CDs,o-CDs and p-CDs were also carried out and had been further confirmed to be two-photon luminescence.Meanwhile,for the first time,triple-mode emissive?that is,FL,UCPL,and RTP?material?m-CDs-PVA?was prepared based on the intrinsic FL and UCPL of m-CDs and RTP of m-CDs-PVA.On the basis of the unique triple-mode emission of m-CDs-PVA,a triple-mode emission ink in the field of advanced anti-counterfeiting is proposed and demonstrated.Besides,the URTP-CD also has been applied in the fields of anti-counterfeiting for improved identifiability of encrypted tags and labels.In addition,given that the long afterglow features of m-CDs@nSiO₂ in water dispersion and oxygen insensitivity,while the RTP of m-CDs-PVA could be easily quenched by vapour,a moisture-related strategy for high-level information protection is proposed and demonstrated.Likewise,the thermally induced conversion from FL to RTP feature of F-CDs was also potential for an advanced security ink for stimuli-responsive information encryption and anticounterfeiting.