Preparation And Properties Of Mn⁴⁺-doped Red-emitting Phosphors For Plant Lighting LEDs

It is well known that light is the bas ic factor of plant growth and development.Light can not only provide the energy needed for plant photosynthes is but also regulate plant life activities,such as seed germination,seedling formation,flowering,bearing,and other processes.Therefore,plant illumination technology is an indispensable part of modern agric ultural development;espec ially for areas lac king land resources and light conditions,plant illumination is particularly important.At present,the light-emitting diodes(LEDs)possess many advantages,such as energy s avings,low cost,high effic iency,environmental friendliness,and long lifetimes.Espec ially,LEDs can adjust the spectral composition by us ing various phosphors to match with the spectrum of plant photosynthes is and photomorphogenes is.So,nowadays LEDs have been widely applied in indoor plant growth.It is known that p hytochrome regulate the growth and development of plants,whic h have two types–P_R and P_FR,P_R absorbs red light peaked at 660 nm and P_FR absorbs far-red light centered at about 730 nm.Mn⁴⁺–doped oxide–based red phosphors possess broad absorption bands in the range of 200–600 nm and produce intens e red emiss ions in the 650–800 nm wavelength range.Consequently,Mn⁴⁺–doped oxide–based red phosphors have bright prospects for indoor plant growth LEDs.This paper mainly studies the Mn⁴⁺–doped oxide–based red phosphors applied for plant lighting by choos ing the suitable matrix,co-doped ions,etc.,and adopting a high-temperature solid-state reaction method to obtain Mn⁴⁺–doped oxide–based red phosphors with good chemical stability,excellent luminescence performance and good spectral matching.The main research contents are as follows:(1)CaGdAlO₄:Mn⁴⁺(CGA:Mn⁴⁺)phosphors were prepared by a high-temperature solid-state reaction route.CGA:Mn⁴⁺phosphors had two broad exc itation bands in the 250–550 nm region.CGA:Mn⁴⁺phosphors give intens itive red emiss ion in 715 nm due to the²E_g?⁴A_2g trans ition of Mn⁴⁺ions.The optimal doping concentration of Mn⁴⁺was about 0.2 mol%.The CIE chromatic ity coordinates of CGA:0.2%Mn⁴⁺s ample were(0.7132,0.2866),and the lifetimes of CGA:x Mn⁴⁺phosphors monitored at 715 nm s howed a decrease with inc reas ing Mn⁴⁺doping concentration.The CGA:0.2%Mn⁴⁺sample had internal quantum effic iency as high as 45%.Finally,a deep-red LED device was prepared by integrating a 365 nm near-UV chip and CGA:0.2%Mn⁴⁺phosphors.These results indic ated that the CGA:Mn⁴⁺phosphors could be promis ing deep red-emitting phosphor candidates for application in plant growth LEDs.(2)SrGdAlO₄:Mn⁴⁺(SGA:Mn⁴⁺)phosphors were synthes ized by a high-temperature solid-state reaction method for the first time.The crystal structure and luminescence properties were investigated on the bas is of X-ray diffraction,exc itation and emiss ion spectra,luminescence decay curves and temperature-dependent photoluminescence spectra.When exc ited by 353 nm,the SGA:Mn⁴⁺phosphors showed a narrow red emiss ion band in the 650–800nm wavelength range with peaks at 709 and 725 nm.The optimal Mn⁴⁺doping concentration was about 0.1 mol%.The CIE chromatic ity coordinates of the SGA:0.1%Mn⁴⁺samp le were(0.7064,0.2934),whic h were located in far-red region.Furthermore,the emiss ion band of SGA:0.1%Mn⁴⁺sample was well-matched with the absorption spectrum of phytochrome P_FR.The results indic ated the SGA:Mn⁴⁺red phosphors had potential application value in plant lighting.(3)A series of CaYAlO₄:Mn⁴⁺,Mg²⁺(CYA:Mn⁴⁺,Mg²⁺)phosphors were prepared by a high-temperature solid-state reaction method.The crystal structure,morphology and luminescence properties of the phosphors were studied systematic ally.The CYA:Mn⁴⁺,Mg²⁺phosphors had two broad exc itation band in the 250–600 nm wavelength range with peaks at 348 and 469 nm.When exc ited by 348 nm,all the CYA:Mn⁴⁺,Mg²⁺samples emitted far-red light around 710 nm.The addition of Mg²⁺ions enhanced luminescence intens ity due to the charge compensation mechanis m.Importantly,the emiss ion band of the CYA:0.4%Mn⁴⁺,1%Mg²⁺phosphors had an ideal spectral overlap with the absorption band of phytochrome P_FR.The CYA:0.4%Mn⁴⁺,1%Mg²⁺phosphors exhibited a high internal quantum effic iency value of 48%.These results proved that CYA:Mn⁴⁺,Mg²⁺far-red-emitting phosphors are suitable for near ultraviolet excitation for plant growth LEDs.(4)A series of novel Mn⁴⁺-doped Ba₂Gd TaO₆(BGT)red-emitting phosphors were successfully synthes ized.The exc itation spectra of BGT:Mn⁴⁺samples exhibited two broad exc itation bands from 250 to 600 nm with peaks at358 and 469 nm.Exc ited at 358 nm,these samples showed an intense deep-red emiss ion band peaking at 688 nm.The major mec hanis m of energy transfer among Mn⁴⁺activators in BGT:Mn⁴⁺was dipole-dipole interaction.The values of the internal quantum effic ienc y for all BGT:Mn⁴⁺samples were measured,and the highest value could reach up to 62%.(5)Ba LaMgSbO₆:Mn⁴⁺(BLMS:Mn⁴⁺)double-perovskite phosphors were successfully synthes ized.The concentration quenc hing mec hanis m of Mn⁴⁺was systematic ally studied.Moreover,the luminescence properties of different Mn⁴⁺doped phosphors reported rec ently were compared.The thermal stability and thermal quenching mechanis m of the samples were analyzed by temperature-dependent emiss ion spectra.Impress ively,the BLMS:0.6%Mn⁴⁺sample showed an outstanding internal quantum efficienc y of 83%.