980nm激发下Y2O2S∶Ho3+,Yb3+纳米晶可见和红外发光特性研究

摘要

Near-spherical Y2 O2 S∶Ho3+,Yb3+ nanocrystals (NCs)were prepared by coprecipitation method fol-lowed by a solid-gas sulfuration technic.The nanocrystals are in hexagonal structure,and the particle size was 40 nm on average.Under the excitation of 980 nm laser diode (LD),more systematic researches on lumines-cence properties of Y2 O2 S∶Ho3+,Yb3+ NCs in a broad range from 450 nm to 1 500 nm have been performed. According to the luminescence measurement results,three different emissions located at 545,655,1 180nm can be observed,which are derived from 5 F4/5 S2→5 I8 ,5 F5→5 I8 ,5 I6→5 I8 transitions,respectively.The upconver-sion luminescence (UCL)efficiency of Ho3+ in the nanocrystals can be improved obviously by co-doping Yb3+. However,because of the high-energy vibration quantum produced by surface adsorption,the 5 F4/5 S2→5 F5 and 5 I6→5 I7 multi-phonon relaxations of Ho3+ are very efficient,and the red emission could be held up hardly.In addition,the increasing content of Yb3+(no more than 6 mol%)can not only improve the population of 5 F4/5 S2 and 5 I6 energy levels,but also restrain the 5 I6→5 I7 multi-phonon relaxation in a certain degree,which results in an enhancement on green and infrared (IR)emissions,but a decrease on red emission.Nevertheless,the inten-sity ratio of green to red emission (IG/IR)obtained in Y2O2S∶Ho3+,Yb3+ NCs was only 3.75,which was much smaller than that of micron Y2O2S (46.6).This was because of another possible populating process (5 F4/5 S2→5 F5 )of 5 F5 energy level.When Yb3+ content was higher than 6 mol%,(5 F4/5 S2 ,2 F7/2 )→(5 I6 ,2 F5/2 ) inverse energy transmission make the green and red emission intensities decrease sharply,but this process was beneficial to the population of 5 I6 energy level and leads to an enhancement on IR emission intensity.These changes weakens the I G/I R ratio,and give rise to a remarkablely increase on I IR/I R .%采用共沉淀法,结合固-气硫化工艺制备出六角晶系结构的类球形 Y2 O2 S∶Ho3+,Yb3+纳米晶,平均粒径约40 nm。在980 nm LD 激发下,系统研究了纳米晶在450~1500 nm 宽波段范围内的发光特性。根据上转换(UCL)和下转换(DCL)测试结果, Y2 O2 S∶Ho3+,Yb3+纳米晶发射峰位于545,655和1180 nm,分别源于5F4/5S2→5I8、5F5→5I8和5I6→5I8辐射跃迁。Yb3+离子的掺杂可显著提高 Ho3+的上转换发光效率。由于纳米晶表面吸附产生的高能振动量子显著提高了5 F4/5 S2→5 F5和5 I6→5 I7多声子弛豫的发生几率,使得655 nm 红光发射很难得到抑制。随Yb3+浓度不断增大,Yb3+→Ho3+能量传递效率提高。这不仅可以增大5 F4/5 S2和5 I6能级的粒子布居数,使绿光和红外光发射增强,而且能在一定程度上抑制5 I6→5 I7多声子弛豫过程,间接削弱红光发射强度。但5 F5能级的另一布居途径(5 F4/5 S2→5 F5)使得Y2 O2 S∶Ho3+,Yb3+纳米晶的绿红光荧光分支比(IG/IR)值仅能达到3.75。当浓度高于6%(摩尔分数)时,(5F4/5S2,2F7/2)→(5I6,2F5/2)能量反传递过程导致绿光和红光发射大幅降低,而5 I6能级布居数的增大却增强了红外发射强度。上述变化导致IG/IR 增势减弱,红外/红光荧光分支比(IIR/IR)不断增大。