The cold isostatic press pretreatment process was adopted to prepare fine rare earth oxysulfide up-conversion phosphors with spherical shape, narrow size distribution and high luminescence efficiency. The upconversion optical characteristics and brightness of the blue (Y2O2SYb,Tm), green (Y2O2S: Yb,Er), red (Y2O3Yb,Er) emitter were also investigated, and a novel method was successfully developed for the brightness measurement of upconversion luminescence (UPL). It is shown that a white color can be obtained by the appropriate mixture of these primary blue, green and red emissions components. The Er3+ ions exhibit different upconversion mechanism in Y2O2S and Y2O3 host materials. The rare earth oxysulfide is an efficient upconversion matrix. The UPL brightness of Y2O2S: Yb,Er is 6.5 times higher than that of Y2O3: Yb,Er, and Y2O2S: Yb,Er shows UPL brightness of 1100 cd/m2 under 5.56 W/cm2 power density using a 980 nm laser diode.
The synthesis and up-conversion luminescent properties of YTaO4:Er3+ and YTaO4:Er3+/Yb3+ are re- ported for the first time. According to the measurement results of up-conversion spectra, Yb3+ co-doping can remarkably enhance the green (2H11/2/4S3/2→4I15/2) and red (4F9/2→4I15/2) emissions, but de- press the infrared emission (4I9/2→4I15/2). With the increase of the Yb3+ concentration, the intensity of green emission increases, after that, when the Yb3+ concentration increases continuously, the intensity of green emission decreases, while those of the red and infrared emissions increase and decrease alternately. In addition, the up-conversion mechanisms of Er3+ doped and Er3+/Yb3+ co-doped YTaO4 are also discussed. It is found that the transform of up-conversion mechanism from two-step energy transfer to cooperating sensitization takes place when Yb3+ concentration is increased up to 12 mol%. With the further increase of Yb3+ concentration, the energy-back-transfer gradually becomes the dominant up-conversion mechanism, which results in the quenching of the green emission and slight increasing of the red and infrared emissions.
ZnS：Cu, Eu nanocrystals with an average diameter of 80 nm are synthesized using a hydrothermal approach at 200 ℃. The photoluminescence （PL） properties of the ZnS：Cu, Eu nanocrystals before and after annealing, as well as the doping form of Eu, are studied. The as-synthesized samples are characterized by X-ray diffraction, scanning electron microscopy, inductively coupled plasma-atomic emission spectrometry, and the excitation and emission spectra （PL）. The results show that both Cu and Eu are indeed incorporated into the ZnS matrix. Compared with the PL spectrum of the Cu mono-doped sample, the PL emission intensity of the Cu and Eu-codoped sample increases and a peak appears at 516 nm, indicating that Eu3＋ ions, which act as an impurity compensator and activator, are incorporated into the ZnS matrix, forming a donor level. Compared with the unannealed sample, the annealed one has an increased PL emission intensity and the peak position has a blue shift of 56 nm from 516 nm to 460 nm, which means that Eu3＋ ions reduce to Eu2＋ ions, thereby leading to the appearance of Eu2＋ characteristic emission and generating effective host-to-Eu2＋ energy transfer. The results indicate the potential applications of ZnS：Cu, Eu nanoparticles in optoelectronic devices.
