| As a new kind of multifunctional nanomaterials,carbon dots(CD)have received widespread attention because of their excellent luminescent properties,particle surface effect,excellent stability,low toxicity,non-photobleaching,as well as superior biocompatibility.CD show a good application prospect in photocatalysis,photoelectric conversion,biological imaging and optical anti-counterfeiting.Although carbon dots possess the aforementioned advantages,they still have some disadvantages such as low quantum yield,poor color rendering index,weak long wavelength fluorescence and almost no luminescence in solid state.Based on this background,herein we used different precursors to prepare CD by hydrothermal/solvothermal method.The luminous performance of as-prepared CD is successfully tailored via doping different atoms,regulating the internal structure and surface functional groups,etc.The luminous mechanism of the CD is also investigated in detail.Furthermore,some applications of carbon dots in light emitting diodes(LEDs)are carried out.We propose a simple and feasible one-step hydrothermal method for preparing multicolor fluorescent carbon dots.Three colors of red,green and blue CD were prepared through a one-step hydrothermal method,using o-phenylenediamine and triaminomethane as precursors,adding concentrated sulfuric acid as a catalyst/passivating agent,and using the principle of different polarities of solvents.Using high-resolution transmission electron microscopy,X-ray diffraction and X-ray photoelectron spectroscopy,the molecular structure of carbon dots is investigated.We find that the emission of carbon dots is affected by the size of the carbon dots.The larger size of the carbon dots,the greater the degree of carbonization,and the size of the sp~2 conjugate domain also increases,which lead to the red-shift luminescence of CD.It provides a method and experience for the design and preparation of carbon dots with adjustable luminous color,which is of great significance to the research and development of the carbon dots with controllable luminescence.We add the light-emitting carbon dots to polyvinyl alcohol(PVA)solution to form a composite film,and encapsulate the film with an ultraviolet LED chip.Four kinds of LED devices with the relative color temperature(CCT:Correlated Color Temperature)from cold white light(8203K)to warm white light(3750K)and CIE(Commission Internationale de l’Eclairage)color coordinates from(0.296,0.307)to(0.391,0.388)were obtained by adjusting the amount of carbon dots in different films.In particular,the high color rendering index(CRI)of 96.5 is achieved for the white light-emitting diodes with color coordinates of(0.362,0.370)and CCT of 4650 K.In order to solve the problem that the solid carbon dots have weak fluorescence or almost no light emission due to the quenching of aggregation,a special hydrothermal method is employed,that is,the Para-polyphenylene lining is not placed in the stainless steel reactor during the reaction,only the Para-polyphenylene lining without lid is heated.A method to prepare gram scale CD was designed.Citric acid,ethylenediamine and phosphoric acid were used as precursors for the synthesis of multicolor solid fluorescent carbon dots.Keeping the amount of precursor unchanged,only changing the reaction temperature and reaction time,we successfully obtained blue,green and red solid-state CD.The aqueous solutions of the three kinds carbon dots show blue fluorescence.The solid-state quantum yields of the three carbon dots are 14%,11%and9%,respectively.We think that the introduction of N and P elements is mainly responsible for the generation of multicolor luminescent solid carbon dots.Solid fluorescence is derived from certain specific fluorescent groups containing N and P elements and n→π*electronic transitions between the conduction band and the valence band.In addition,TEM,XRD,infrared and Raman spectroscopy results of solid fluorescent carbon dots indicate that the emission peaks of carbon dots mainly depend on the particle size,graphitization and sp~2conjugation.The fluorescence emission of carbon dots based on the size of the conjugated structure is investigated.The multi-color light-emitting carbon dots that covers the entire visible light region from blue to red light is obtained,by using citric acid and urea as precursors,keeping the content of citric acid and urea unchanged,and adjusting the reaction temperature with the same heating time.The three-color fluorescent carbon dots of blue,green and red are selected as an illustration for further analysis.It is found that the emission peaks of the three carbon dots are located at 462,543 and 631 nm,respectively,and the quantum yields are 41%,31%and 19%,respectively.The structure of three typical fluorescent carbon dots,blue,green and red,were characterized by transmission electron microscopy and Raman spectra.The results show that the carbon dots have a highly conjugated core of sp~2 hybridization,and the emission peak position of the carbon dots depends on the degree of graphitization and sp~2 conjugation,showing the high crystallinity of the as-prepared carbon dots.Because the carbon dots have excellent water solubility,polyvinyl alcohol is used as a dispersant,and the carbon dots and polyvinyl alcohol are combined to form a composite film.The color coordinates of the carbon dot/polyvinyl alcohol composite film are located in blue,green,and red light regions,respectively,which is consistent with the fluorescence emission of the carbon dots in the aqueous solution.This confirms that the PVA disperses the carbon dots sufficiently,and the basic fluorescent properties of the carbon dots in the PVA are well preserved.The synthesis of multi-color fluorescent carbon dots and carbon dot/polyvinyl alcohol composite films covering the visible light range will promote the wide application of carbon dots in the fields of multi-color/white light-emitting diodes,biological imaging,and display lighting. |
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