Preparation And Application Of Solid-State Luminescent Carbon Dots

The evolution of human civilization has always been accompanied by the development and application of carbon materials.The new carbon materials,such as fullerenes,carbon nanotubes and grapheme,mainly used in the fields of communication,space exploration,biology,medicine,etc.,The diversified performance of carbon materials has always been a research hotspot of researchers.As a new star in the carbon material family,carbon nanodots(CDs),a type of carbon nanomaterial with the size of less than 10 nm,were discovered in 2004 and have attracted great attentions from scientific researchers around the world.The CDs have the characteristics of rich carbon source,low cost,simple preparation process and various synthesis methods,and have good water solubility,low toxicity,good biocompatibility,wide spectrum of luminescence and tunability.The applications of CDs involve many fields such as ion detection,cell labeling,drug delivery,light emitting diodes,solar cells and so on.From the beginning of the discovery of CDs to the present,some progress has been made in the study of CDs.However,there are still some problems that need to be solved,such as the uncertainty of the carbon emission mechanism and the research and application of CDs in solid state luminescence.Therefore,in this paper,according to the existing problems in the research of CDs at the current stage,the following work is carried out based on the luminescent mechanism of CDs,solid-state luminescence and laser phenomenon:1.Blue luminescent CDs were synthesized by the hydrothermal method.Blue-shifts of the maximum emission wavelength from 480 nm to 443 nm were observed when the concentration of CDs solution was decreased.The photoluminescence(PL)spectra of CDs at low concentration showed the excitation-independent behavior,which is very different from the previous reports.Two different emitting mechanisms might work:the intrinsic luminescence from sp~2-carbon networks can be responsible for the shorter-wavelength part of emission(excitation-independent)at low concentration,and the high polarity of nano-sized clusters leaded to the excitation-dependent behaviour of the longer-wavelength part at high concentration of CDs solution.2.The CDs were prepared by microwave irradiation method and the as-prepared CDs emitted green fluorescence under UV light irradiation.CDs-NaCl hybrid crystals are obtained by incorporating the CDs into NaCl matrix through a simple process.The embedded CDs have added the luminescence centers into NaCl.When irradiated with ultraviolet light,the hybrid crystal still emits green fluorescence,realizing solid state fluorescence based on CDs.3.The phosphorescence with an average lifetime of 314 milliseconds(ms)is achieved after the 365nm light was turned off.The C=O groups on the surface of the CDs are considered as the origin of phosphorescence,and the NaCl crystal plays the role to solidify and block the surface oxygen,then promoting phosphorescence.4.Optical gain and lasing phenomenon has been observed from hybrid crystals.When the pump power is low,a weak spontaneous emission can be observed from the hybrid crystal,while the lasing action was observed under high pump power.The lasing threshold is found to be 0.08 mW and the corresponding Q factor is calculated to be 447.The tiny cubic crystal in hybrid crystals offers the whispering gallery mode(WGM)resonant cavity for lasing emission.This result has provided a new approach for realizing lasing materials.5.The CDs with blue light emission were prepared by high temperature pyrolysis method,and CDs were compounded with filter papers to prepare test papers based on CDs.The test papers had phosphorescent light emission.Because the test paper's phosphor intensity is sensitive to temperature and halogen ions,the test paper can be used to test different temperatures and identify three different halogen ions(Cl~-,Br~-,I~-).