Preparation,structure Regulation And Performance Optimization Of Two Red Luminescent Materials

In recent years,fluorescent nanomaterials have attracted great attentions due to their wide applications in bioimaging,display,lighting and catalysis.As one of the primary colors?red,green and blue?,the red emitters are required for full-color applications and white light-emitting diodes?WLED?.Therefore,it is essential to develop new red emitters and improve their luminance and efficiency.In this dissertation,a series of upconversion luminescence of rare earth doped materials and nitrogen doped carbon quantum dots were designed and fabrication.The related optical properties of these nanomaterials were systematically investigated.Samples of Yb/Er?x/1 mol%?: Bi OCl?x=0,1,5,10,20 and 30?were prepared via a template-free route simply by introducing water into ethylene glycol containing reactants at room temperature.The as-prepared samples were calcinated in air at 300,400,500 and 600? for 2h.XRD patterns verify that all the synthesized products with different post-sintering temperatures are assigned to pure tetragonal Bi OCl phase.Post-sintering on the product was found to have no obvious influence on the size and morphology but greatly enhance Er3+ upconversion luminescence probably owing to the improvement in crystallinity of nanoplatelets.The optimal Yb3+ concentration was 20 mol%,and further increasing Yb3+ content induced a great decrease in UC intensity due to the concentration quenching of luminescent activators.The optimal post-sintering temperature was determined to be 500?.Using the similar procedure,pure tetragonal Yb/Er: Bi OBr sample was prepared.Different from those of the routine Ln3+-doped materials with multiple-band UC emissions,the investigated Yb/Er: Bi OX?X=Cl,Br?products exhibited intense intrinsic near-single-band red UC emission under near-infrared?980 nm or 1530 nm?laser excitation.The formation of [Cl-Bi-O-Bi-Cl] layered structure in the Bi OX crystal lattice and the resulted short ionic distances among Ln3+ dopants in the [Cl-Bi-O-Bi-Cl] layer which will induce great population of Er3+ 4I11/2 and 4F7/2 states,were proposed responsible for the impressive upconversion emission behaviors.Carbon quantum dots?CDs?were prepared by a facile solvothermal or hydrothermal route.Citric acid?CA?was chosen as the C source,urea was selected as the N source,and DMF was used as the solvent.The sealed autoclave vessel was heated at 160 OC and held for different times?1 h,3 h,6 h,12 h,24 h and 48 h?,and the samples were denoted as CD-1,CD-3,CD-6,CD-12,CD-24 and CD-48,respectively.These CDs exhibit multi-band emissions with an increase in reaction time.The shifts in emission wavelength of the CDs,which are induced by varying the excitation light,constitute true color tuning since all the emission bands have comparable intensities.Eye-visible blue,yellow and red luminescence can be realized under the excitation of 355 nm,430 nm and 515/550 nm light.The quantum yields?QYs?for these blue,yellow and red emissions were determined to be 49.2%,30.6% and 30.3%,respectively.UV-Visible absorption spectra,photoluminescence?PL?and PL excitation?PLE?spectra indicate that these CD samples possess both multi-state absorption and emission features.The impact of C and N sources as well as solvents on optical properties of the prepared CDs was further investigated.It is concluded that the observed emission behavior of the CDs is due to the synergistic role of CA/urea/DMF since it is impossible to achieve red emission when either CA,urea or DMF is replaced.All the as-prepared CDs are monodispersed with similar sizes in the range of 2–3 nm.The selected area electron diffraction?SAED?pattern of the CD-48 sample shows a clear diffraction ring which corresponds to the?100?plane lattice of graphite,and the high-resolution TEM?HRTEM?micrograph of an individual CD exhibits well-resolved lattice fringes with a typical d-spacing of 0.21 nm,thus confirming the crystalline feature of the investigated CDs.Structural characterizations and spectroscopic analyses verify that three diverse emitting states,i.e.,sp2 carbon core,C=O and C=N related surface defects,are responsible for the multi-state absorptions and tunable emissions of the carbon dots.With a decrease in p H from 7 to 1 the red PL disappears,while with an increase in p H from 7 to 13 the red PL becomes dominant.The red luminescence is retained for CD dispersed in the strong polarity solutions of water,methanol or ethanol,while it gradually weakens for the weak polarity solutions.This solvent effect,similar to the p H effect,can be attributed to the basic nature of the emitting states on the surface of the CDs.Benefited from their intense visible absorption and highly efficient red emission,these CDs are demonstrated to be beneficial to improve correlated color temperature?CCT?and color redering index?CRI?of traditional WLED and ultrasensitive sensing for Cu²⁺ ions.