Modulation On The Luminescent Properties Of Carbon Dots And Their Applications In Light-emitting Diodes

Carbon dots(CDs)as a luminescent material possess a number of distinct merits such as facile preparation,superior optical properties,biocompatibility,and low toxicity,which render them attractive for various applications in biomedical imaging,sensors,energy storage,and light-emitting devices.Especially,the observation of the room-temperature phosphorescence(RTP)emission from CDs which has been reported in recent years has largely broadened their application prospects.However,the photoluminescence mechanism for CDs remains unclear,which limits their further practical application.And how to effectively regulate and improve the optical properties of CDs,including the color-tunable photoluminescence,high photoluminescence quantum yield for the long wavelength range and the self-quenching-resistant ability,still remains an intense research topic.In addition,the reported room-temperature phosphorescence emission wavelengths of those CDs were constrained to the blue or the green region below 540 nm,these CDs based room-temperature phosphorescence materials have very low emission intensity,but the methods to regulate room-temperature phosphorescence emission and to enhance the emission intensity have been not realized so far.Based on above consideration,we have study the photoluminescence mechanism in detail in CDs,which prepared from urea and citric acid,and unraveled the contribution of intrinsic state and surface states to different fluorescent emissions.Then we proposed an effective strategy for modulating the emitting states of the CDs to obtain white light and red light emission.Finally,we developed a seeded growth method to achieve color-tunable photoluminescence and room-temperature phosphorescence from the CDs,and also enhanced the room-temperature phosphorescence emission intensity of those CDs through the step-by-step modification.In addition,the related application in white light-emitting diode(WLED)has also been explored.Specifically,there are five aspects introducing in detail in the following paragraphs.Firstly,CDs were synthesized via a solvothermal route from citric acid and urea.Through the oxidation and reduction treatment of pristine CDs,we found that the blue/green/red emissions originated from three diverse emitting states,i.e.the intrinsic state,and C=O-and C=N-related surface states,respectively.Secondly,based on the formation mechanism and optical properties of the pristine CDs,we proposed an effective strategy for modulating the emitting states of pristine CDs by introducing hexadecyltrimethyl ammonium bromide(CTAB).Consequently,white light emission with tunable correlated color temperature was realized.Furthermore,we dispersed CDs in a(PVP,polyvinylpyrrolidone)matrix for solid state films,where the solid-state quenching was effectively avoided.A white light-emitting QY of 38.7% was thus achieved through the inhibition of non-radiative electron-hole recombination as well as the cooperation between the intrinsic state of the carbogenic cores and the surface-related state of the organic ligands.Benefiting from the efficient white light-emitting solid films,WLEDs were fabricated.Thirdly,we developed highly red light emitting CDs by the step-by-step surface modifications of the pristine CDs.By introducing CTAB to modulate a red CD aqueous solution,the photoluminescence QY increases from 23.2% to 43.6%.Furthermore,we dispersed CDs in a PVP matrix for a solid-state film,where the solid-state quenching was effectively suppressed,and high QY(41.3%)of red light emission was achieved.The dramatic enhancement of the photoluminescence performance lies in the fact that the non-radiative paths of the electron-hole recombination and the aggregation in the CDs have been effectively avoided through surface modification.The red light emitting CDs were demonstrated to have promising application in white light-emitting diodes or as a red color converter in traditional YAG-based WLEDs to improve their correlated color temperature and rendering index.Fourthly,we developed a seeded growth method to produce colloidal CDs through controlling the number of seeds and reaction time,which is demonstrated to be an effective way to tune their optical properties.Color-tunable fluorescence of CDs with blue,green,yellow,and orange emissions under UV excitation is achieved by increasing the size of the seed CDs.Moreover,color-tunable room-temperature phosphorescence of CD powders is demonstrated in the broad spectral range of 500-600 nm.It is related to the presence of the nitrogen-containing groups at the surface of CDs,which form inter-particle hydrogen bonds to protect the CD triplet states from quenching,and to the existence of the PVP polymer chains at the surface of CDs.Such excellent properties make it possible for them have great potential in WLED and data security.Fifthly,based on the crosslink-enhanced emission(CEE)effect,we enhanced the emission intensity of RTP CDs through introducing polymer.To confirm the method can be generally applied to establishment of RTP in CDs,a series of enhanced RTP emission CDs have been obtained,indicating that the universality of our technology.Finally,we demonstrated an effective way to further enhance the RTP emission intensity of CDs by introducing melting urea,which would rigidify and decrease vibration and rotation of C=O/C=N bonds at the CDs surface to protect triplet states from being quenched.Our results are greatly important for the rational design of CD-based materials to realize efficient RTP emission.