The Growth And Morphology Control Of Red Microcrystal For A New Type LED Artificial Photosynthetic Illumination

With the rapid development of LED for artificial illumination application, a660nm-featured red emission materials and devices are significant to achieve the desired lightquality of artificial illumination. The ever-grown attention has been paid to the use ofluminescent phosphors, especially CaS:Eu as a new solution for artificial plant cultivationlight, owing to either the broad spectral versatility or the effective cost. Other than the mostcomplex and costly epitaxial growth by MOCVD to produce AlGaInP based red chip, a routeof microwave (MW) electromagnetic field assisted solvothermal growth was explored hereinin terms of inhomogeneous dielectric property to allow the formation of crystal nucleus andgrown. A660nm-featured red-shift emission of Eu2+doped CaS microcrystal was observed.The main achievements in this work are summarized as follows.A. Compared to solid state reaction (SS), a distinct red-shift to660nm-featured longerwavelength of blue-green excitable (Ca,Mg)S:Mn2+, Eu2+microcrystals were observed in MWsolvothermal synthesis procedure. Investigation of the influence of Mn2+and Mg2+on theemission of Eu2+and the relationship between luminescence and morphology with differentcontent of Mg2+. This red-shift is probably due to a combined effect from Mg2+codoping andstrain accumulation at grain boundaries. In particular, MW electromagnetic field suppressesgrain growth and enables a reconstruction of atoms at the inner grain boundary at which thefield strength was intensified around Eu2+ion in host lattice.B. The effects of varied initial reaction duration, solvent and surfactant to themicrocrystalline morphology and luminescence were examined experimentally. Up addedethanol to aqueous solution, the solubility of Ca2+and S2-is lowered so as to lead to theformation of nanometer sized nucleation to form micrometer sized crystals under MWenvironment. The ethanol prevents the surface of the particle non-briding hydroxyl grouphydrogen bond formation and agglomeration and has the steric effect to decrease theprobability of collisions among the particles. Both MW electromagnetic field and ethanolenable the formation of small sized crystals in nano scale and well-dispersed particles inmicrometer scale.C. It is proposed that the intensified strain by MW at grain boundary around Eu2+enablesa red-shift of the position of PL peak. The codoped Mg2+intensifies the small sized effect. Inthe growth of microcrystal, the local stress distribution is likely to be uneven, thus the strainproduced by the same sample of different grains is not completely the same and lead to theposition of PL peak changed.