Eu3+-doped Gd2O2CN2 was firstly synthesized by a classical solid-state reaction of Li2CO3, Eu2O3 and GdF3 under NH3 gas flow in the presence of graphite at low firing temperature. Powder X-ray diffraction (XRD) analysis indicated that Gd2O2CN2: Eu3+ crystallizes in a trigonal-type structure with space group P-3m1. Gd2O2CN2: Eu3+ shows a sharp red emission band peaking at 626 nm under excitation at 300 nm at room temperature. PL spectra indicates that Eu3+ doped Gd2O2CN2 samples emit the typical emission peaks at 614 nm and 626 nm originated from the hypersensitive electric dipole transition (5D0→7F2) of Eu3+ ions. The optimized doping concentration of Eu3+ ions was found to be 7.5 at. %, and the critical transfer distance was calculated to be 10.907 Å.
© 2015 Optical Society of America
Recently, much attention has been paid on investigating red emitting phosphors owing to their potential applications in X-ray mammography , in display devices [2,3 ] and especially in white light emitting diodes (WLEDs) [4–6 ]. Eu3+- doped luminescent materials as the main and outstanding red-emitting phosphors, such as Eu3+ doped Y2O3 , Y2O2S , R2(MoO4)3 (R = La, Y, Gd)  and NaEu(WO4)2 , have been studied for decades due to the transition of 5D0-7FJ (J = 1-6) of Eu3+. However, these excellent red emitting phosphors can hardly fulfill the demands for novel high-performance materials. Therefore, the exploration of novel luminescent host materials remains a meaningful work.
The crystal structures of rare earth oxysulfides RE2O2S and oxycyanamides RE2O2CN2 are closely related  and both consist of RE2O2 2+ layers and their interleaving anions. Eu3+ doped RE2O2S has been widely used as phosphor for CRT . The luminescence properties of Y2O2CN2: Eu3+ are quite similar to those of the commercially used red emitter Y2O2S: Eu3+ .Therefore, oxycyanamide compounds are considered to be efficient host candidates for good luminescence performance. Rare earth dioxymonocyanamides (RE2O2CN2, RE = La, Ce, Pr, Nd, Sm, Eu, Gd)  were prepared by nitriding a mixture of rare earth oxide in flowing ammonia at 950 °C. The luminescent properties of RE2O2CN2:M3+ (RE = Y, Gd and La, M3+ = Tb3+, Eu3+, Pr3+, Er3+ and Er3+/Tb3+) have been previously studied [11–14 ]. Eu3+ doped Gd2O2CN2 was firstly prepared by sol-gel method by Takeda et al., but the luminescence intensity was weak because of its low crystallinity and the suppression of concentration quenching was not recognized because of the presence of impurities for high Eu-doping concentration . Thus, we propose here to further investigate preparation and photoluminescence properties of pure micrometric Gd2O2CN2:Eu3+ phosphors.
In this paper, a series of Eu3+ doped Gd2O2CN2 samples with 1-3μm particle size were successfully prepared for the first time by a classical solid state route using GdF3, Li2CO3 and Eu2O3 as raw materials. The phase structures of the samples were determined by powder X-ray diffraction (XRD). Luminescence properties and the concentration quenching characteristics were also investigated in detail.
Powder samples with the general formula (Gd1-xEux)2O2CN2 [x = 0.005(GOCN-1), 0.02(GOCN-2), 0.035(GOCN-3), 0.05(GOCN-4), 0.075(GOCN-5) and 0.10 (GOCN-6)] were prepared starting from high purity GdF3 (99.99%), Eu2O3 (99.99%), Li2CO3 (99.99%), and active carbon (CARBIO 12 SA—ref: C1220 G 90) as raw materials [shown in Eq. (1)]. All starting materials were weighted in the proper stoichiometries, and finely mixed in an agate mortar. The mixture was placed at the end of a graphite boat, while active carbon was put in the upcoming flowing gas at the other end. After that, the mixture was fired at 600°C for 9 h, then 750°C for 12 h and finally cooled down to room temperature under NH3 atmosphere in a tubular furnace. The sintered samples were further washed with distilled water to remove LiF by-products (determined by XRD) from the reaction product and dried at 120 °C in air. Finally, the as-prepared fine powders were collected for characterization.
Powder X-ray diffraction (XRD) data were recorded using a Bruker AXS D8 Advance diffractometer (Voltage 50 kV, current 40 mA, Cu-Ka) with a step width of 0.02°. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured by a Fluorolog-3-P UV-vis-NIR fluorescence spectrophotometer (Jobin Yvon, longjumeau, France) with a 450 W Xenon lamp as the excitation source. The surface morphology and particles size of the phosphor samples were examined by a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi High-Technologies) with high voltage of 15 kV. The BET-specific surface area was measured by ASAP 2460 surface area and porosity analyzer made by Micromeritics Instrument Corporation. The FTIR spectrum was measured in transmission mode using a KBr standard (Bruker, Model vector 22). The color chromaticity coordinates were obtained according to Commission Internationale de I’Eclairage (CIE) using Radiant Imaging color calculator software. All spectroscopic measurements were carried out at room temperature.
3.Results and discussion
Eu2O2CN2 and Gd2O2CN2 have the same crystal structure based on a trigonal unit cell with the space group P-3m1 and the linear CN2 2- ions lay perpendicular to RE2O2 2+ (RE = Eu and Gd) layers . The Eu3+and Gd3+ ions are both coordinated with four oxygen and three nitrogen atoms in a seven coordinated geometry with the oxygen and the metal in the same plane. Thus, Eu3+ ions can partially substitute for by Gd3+ ions to form a (Gd1-xEux)2O2CN2 (x = 0.005-0.10) solid solution as illustrated by XRD patterns presented in Fig. 1 . The characteristic diffraction peaks of all samples can be ascribed to the trigonal structure of Gd2O2CN2 (PDF#49-1169) with the space group P-3m1. No other impurity phase can be detected at the current doping concentrations. With the increase of Eu3+-doping concentration, the diffraction peaks of the samples slightly shift to lower diffraction angles compared with those of Gd2O2CN2 (PDF#49-1169), as shown in the second part of Fig. 1.The shift of diffraction angles can be attributed to the replacement of the smaller Gd3+ (r = 0.100 nm, VII) by relatively larger Eu3+ (r = 0.101 nm, VII), indicating a compacter lattice configuration. Meanwhile, the Eu3+ doping limit has been increased to 10 at. % compared to 4 at. % previously reported in sol-gel synthesis .
Figure 2 shows the IR spectra of Gd2O2CN2:Eu3+ samples with different concentration of Eu3+. All IR spectra samples show two typical absorption peaks in the vicinity of 652 and 2100 cm−1. These absorption peaks at ca. 652 and 2100 cm−1 were respectively assigned to the ν2 (bending vibration) and ν3 (asymmetric stretching vibration) modes of the CN2 2- ion which were comparable to the IR spectrum of RE2O2CN2 [16, 17 ] (RE = Ce, Pr, Nd, Sm, Eu, Gd), indicating the presence of CN2 2- ions in the Gd2O2CN2:Eu3+ samples. The other peaks around 400-500cm−1 have not been assigned as yet.
Figure 3 displays SEM images of GOCN-6, GOCN-4 and GOCN-5 samples. It can be noticed that the prepared samples with various Eu3+ concentrations exhibit similar morphology and particles size ranging from 1 to 3 μm. Meanwhile, the specific surface area of GOCN-4 is determined to be 0.64 m2/g.
The elementary composition of GOCN-4 is further confirmed by energy dispersive X-ray spectrometry (EDS), as shows in Fig. 4 . The energy dispersive spectrum reveals the presence of Gd, O, N and C elements and allows estimating the composition for the host matrix elements as Gd at. % = 27.32%, O at. % = 25.3%, N at. % = 22.95% and C at. % = 24.43% which are in rough agreement with the formula of host matrix Gd2O2CN2 except for the C at. %. The overestimation of the carbon content comes from the conductive adhesive used for preparation sample for EDS analyses.
Figure 5 illustrates the excitation (monitored by 626 and 614 nm) and emission (excited by 300, 395 and 467 nm) spectra of the GOCN-5 sample. The excitation spectra (Fig. 5a) exhibit a broad and intense band in the range from 250 to 350 nm with a maximum located at around 300 nm. This band is attributed to the ligand-to-metal charge transfer between O2- and Eu3+, the CTB (Charge-transfer band) of GOCN-5 corresponds to the electron transition from the 2p orbital of O2- to the 4f orbital of Eu3+ . The weak excitation bands at lower energy, i.e. at longer wavelengths, correspond to the expected 4f-4f transitions within the [Xe]4f6 configuration of Eu3+ and are located at 362 nm (7F1→5G3), 384 nm (7F0→5G2),395 nm (7F0→5L6), 417 nm (7F0→5D3) and 467 nm (7F0→5D2).
The emission spectra of GOCN-5 (Fig. 5b) at different excitation wavelengths are very similar both in shape and relative intensities. The strongest peak splits into two peaks at 614 and 626 nm which originates from the electric dipole transition 5D0→7F2 of Eu3+, indicating that Eu3+ occupies a site with no inversion center low symmetry in GOCN-5 . This transition is sensitive to crystal-structure and chemical surroundings. According to previous studies, the dominated emission of Y2O3:Eu3+ is located at 613 nm  and Y2O2CN2:Eu3+ shows red luminescence at 614 nm and 626.5 nm  which are both due to the 5D0→7F2 transition within europium. Meanwhile, the emitted radiation of Gd2O3:Eu3+ is dominated by the red emission peak at 612 nm . From the predominant peaks at 614 and 626 nm, it can be further proved the formation of the oxycyanamide host [13–15 ]. Some weak peaks can be observed at 580 nm, 590 nm, 594 nm and 653 nm, corresponding to the forbidden transition 5D0→7F0 (580 nm) and the magnetic dipole transitions 5D0→7F1 (590 nm and 594 nm) and 5D0→7F3 (653 nm).
Figure 6 shows the PL and PLE spectra of Gd2O2CN2: Eu3+ samples with various concentrations of Eu3+ ions. While the spectral shape and locations of excitation and emission peaks do not vary with the doping concentration of Eu3+ ions, the photoluminescence intensity strongly depends on it. With the increase of doped Eu3+ ions concentration, the excitation and the emission intensity increases gradually ranging from 0.5 to 7.5 at. % and decreases from 7.5 to 10 at. %. Thus the optimized Eu3+ ions doping concentration in Gd2O2CN2 host matrix is about 7.5 at. %. Considering the mechanism of energy transfer in phosphors, the concentration quenching can be explained in more detail by the critical distance (Rc) between Eu3+ ions which can be calculated by Eq. (2) :
For the Gd2O2CN2 host, V = 101.9 Å3, Xc = 0.075 and N = 2. Therefore, the average distances Rc between Eu3+ ions is calculated to be Rc = 10.907 Å when the optimized doping molar concentration is 7.5 at. %.
It is interesting to note that the optimized Eu3+ concentration in Gd2O2CN2 host matrix (7.5 at. %) is higher than that in Gd2O2S and Gd2O3 host matrix that is around 5 at. % . The suppression of concentration quenching is attributed to the two-dimensional character of the Gd2O2CN2 structure. The trigonal structure of Gd2O2CN2 consists of Gd2O2 2+ and CN2 2- layers. The Gd2O2 2+ layers are perpendicular to the c axis and the linear CN2 2- ions are parallel to the c axis . This kind of structure leads to a long interlayer distance between the Gd2O2 2+ slabs (≈0.57 nm) which contributes to the higher doping concentration of Eu3+ .
The color chromaticity coordinates have been calculated for the optimized sample Gd1.85Eu0.15O2CN2 under a 467 nm excitation (Fig. 7 ). The calculated values (0.6475, 0.3488) are very close to the CIE color coordinates of the red region, indicating Gd1.85Eu0.15O2CN2 phosphor is a promising red emitting phosphor for WLEDs application.
In this paper, pure phase (Gd1-xEux)2O2CN2(x = 0.005, 0.02, 0.035, 0.050, 0.075, 0.100) phosphors with space group P-3m1 have been prepared using GdF3, Li2CO3 and Eu2O3 as raw materials at low firing temperature (750 °C), for the first time. The Eu3+ doped Gd2O2CN2 phosphors exhibit a characteristic red emission. The strongest and second strongest peaks are located at 626 and 614 nm (5D0→7F2 transition) under excitation of 300, 395 and 467 nm. The strongest luminescent intensity of Gd2O2CN2:Eu3+ is obtained when the doping concentration of Eu3+ reaches 7.5 at. %. The optimized Eu3+ doping concentration in Gd2O2CN2 is higher than that in Gd2O3 and Gd2O2S host lattices, which is due to the 2D structure of the Gd2O2CN2 host matrix. The CIE chromaticity coordinates (0.6475, 0.3488) for Gd1.85Eu0.15O2CN2 phosphor are located in the red region. All the results indicate that Gd2O2CN2:Eu3+ is a promising red phosphor for white LEDs.
This project has been supported by the National Natural Science Foundation of China (NSFC) (No.51502091), and the Fundamental Research Funds for the Central Universities (WD1314055, WD1313009).
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