Effect of Eu doping on the structure, morphology and luminescence properties of ZnO submicron rod for white LED applications

Eu3+-doped ZnO submicron rod has been prepared by a hydrothermal method. Synthesized rods are characterized by X-ray diffraction, scanning electron microscopy and photoluminescence spectra (PL). The results indicated structures and morphology of rods affected by the Eu3+ doping concentration. 5D0–7FJ (J = 1, 2 and 3) of Eu3+ emission appeared, and energy transfer from the ZnO host and deep defects of ZnO to Eu3+ in the ZnO:Eu is occurred. Strong and white emission was observed in the sample with 15% Eu.


Introduction
Today, the clean and renewable energy sources are very important because of limitations of fossil fuel and environmental problems. Semiconductor materials with different structures and morphology are used in various research and development fields of clean and renewable energy due to their unique characteristics [1][2][3]. The structure, morphology and properties of semiconductor materials are bound not only to precursor materials and preparation methods, but also to dopants and annealing temperatures [4][5][6]. Among the semiconductor materials, ZnO with a wide band gap (3.37 eV) is an II-VI semiconductor and it has a large exciton binding energy of 60 meV [7][8][9][10]. ZnO exhibits excellent physical and chemical properties which make it a unique host material for numerous devices by doping with different rare earth ions. Rare earth (RE) doping of ZnO can expand its optical and electronic properties because of their narrow and intense emission lines due to intra-4f transitions [11,12]. Among them, the Eu 3+ -doped ZnO (ZnO:Eu) raised a lot of interest because of the red emission from the Eu 3+ [13]. As a new energy source in the field of lighting, ZnO doped with rare earth ions is utilized for advanced full-color display and white LEDs lighting applications. For fabrication of white-light emitters, n-UV (370-410 nm) LEDs are usually combined with red/green/blue tricolor phosphors [14,15]. However, using two or three different phases of phosphors leads to decrease luminous efficiency. A single full-color emitting phosphor for n-UV LED is under an obligation to enhance luminous efficiency and reproducibility of white LED [16,17]. Pure ZnO generally emits blue-green light, because of its inherent defects. Furthermore, bright red emission from Eu 3+ is expected and this needs the efficient energy transfer between the ZnO matrixes to Eu 3+ emission centers. However, due to the considerable difference of chemical properties between Eu 3+ (0.98 Å) and Zn 2+ (0.77 Å), it is hard to substitute Eu 3+ ions for Zn 2+ ions in ZnO [18]. The emissions of Eu 3+ ions are sensitive to the local environment or symmetry of the host materials [19]. In recent years, many researchers indicate that Eu 3+ ions can incorporate into the ZnO matrix under suitable conditions because they observe the efficient red emissions from Eu 3+ -doped ZnO [11]. Therefore, several preparation techniques have been used to synthesize the Eu-doped ZnO such as sol-gel [20], microwave-assisted combustion [21], solidstate reaction [22], magnetron sputtering [23], hydrothermal [19,24,25], chemical vapor deposition [26,27] and electrodeposition [28]. Particularly, the hydrothermal method is considered as an attractive preparation process of Eu-doped ZnO due to the simplicity, low cost, the possibility of mass 1 3 production and the environmental friendliness. However, there are few reports of ZnO:Eu as a white powder. In this paper, Eu-doped ZnO is synthesized using hydrothermal method. Detailed investigations of Eu doping on morphological and crystalline structures of synthesized ZnO and PL properties and the energy transfer mechanism are discussed.

Material preparations
ZnO:Eu 3+ was synthesized using hydrothermal method. All reactants were of reagent grade. ZnO doped with different Eu 3+ concentrations was synthesized. In a typical experiment, 35 mL of 0.05 M zinc acetate (Zn(CH 3 COO) 2 ·2H 2 O) was dissolved in distilled water, and 2.1 mL of 13.3 M ammonia (NH 3 ), 5.25 mL of 0.05 M Eu(NO 3 ) 3 6H 2 O were mixed under magnetic stirring. The molar ratio of Eu/Zn is 0.05(5%), 0.10(10%), 0.15 (15%) and 0.20(20%) in the mixed solution, respectively. The resulting precipitate was enclosed in a Teflon-lined autoclave with 50 mL capacity. The closed autoclave was placed into the furnace at a preset temperature of 200 °C for 12 h and then cooled to room temperature naturally. The resulting precipitate was filtered off and washed 1 time in distilled water. The final product was dried in air at 60 °C for 12 h.

Materials characterization
Crystal structures of the ZnO:Eu were analyzed by using an X-ray diffraction (XRD) (XRD-6000. Shimadzu Corporation). The morphology of the samples was characterized using scanning electron microscopy (SEM) (Hitachi S4800 SEM). Photoluminescence (PL) measurements were taken by HITACHI F-4600 spectrophotometer.

Results and discussion
Morphology and structure analysis Figure 1 shows SEM images of the undoped, 5% and 15% Eu-doped ZnO materials synthesized by a hydrothermal method. It can become quite clear that undoped ZnO in Fig. 1a shows submicron rod and their diameter is about 200 nm (insert of Fig. 1a). The rod's end looks just like a taper. For ZnO doped with 5% Eu and 15%, shown in Fig. 1b, c, the diameter is about 300 nm (insert of Fig. 1b) and 600 nm (insert of Fig. 1c), respectively, and they have flat facets at both ends. From above results both crystallite size and their crystal growth are influenced significantly by Eu doping. The chemical potential of a crystal plane is determined by the average number of dangling bonds per atom over the entire crystal facet [29]. The different density of Eu 3+ on various crystal planes leads to the difference in chemical potential of crystal facets. The different chemical potential of crystal facets has different growth rates. The growth rate along the diameter is much quicker than that along the top side, which may lead the rod diameter to increase with Eu doping. Figure 2 shows XRD patterns of ZnO:Eu 3+ with different concentrations of Eu 3+ . It is clear that all of the XRD peaks of the samples match with the values of the standard card for zincite ZnO (JCPDS No. 36-1451). In total, 20% Eu-doped samples detected a weak impurity peak observed at 29.4 (marked with an asterisk) is assigned to Eu 2 O 3 (JCPDS No. 43-1009) [13]. ZnO peak intensity increases with increasing the doping concentrations from 0 to 5% and decreases with increasing the doping  Figure 2 shows the shift of (002) peak positions.] The common defects in ZnO such as oxygen, zinc vacancies and oxygen interstitials are produced in preparing process. The incorporation of Eu may lead to the formation of four kinds of configuration in ZnO's host lattice for ZnO:Eu Zn , ZnO:Eu I , ZnO:Eu Zn + V O and ZnO:Eu Zn + Zn I , respectively [30,31]. Considering structural stability with less lattice distortion, the substitutional Eu an oxygen interstitial to form (Eu Zn + O I ) complexes [30,31]. The XRD peak position of (002) moves to a slight higher angle as compared to that of ZnO, indicating that the lattice parameter is decreased after doping of Eu. A decrease in the lattice parameters is rather a sign of the formation of defect complexes [32]. Therefore, it shows that the Eu Zn + O I complexes are formed and dominant at the doping concentration which is less than 10%. The XRD (002) peak position of 15-20% samples moves to a slightly lower scattering angle. It indicated that a small amount of Eu 3+ ions has been successfully doped into the crystal lattice of ZnO host to form Eu Zn . The larger Eu 3+ substituted the Zn 2+ and results in the expansion of the unit-cell volume of the Eu-doped samples [16]. The intensity of the (002) diffraction peak increases with the Eu molar ratio until 5% and then decreases, at further increase in Eu doping. Figure 3 displays the PL excitation (PLE) spectra of the 15% Eu-doped ZnO rods. In the PLE, the monitoring emission was set at 618 nm, the observed peaks are centered at 380 nm belonging to ZnO, and 394 and 464 nm are ascribed to the direct excitation of the Eu 3+ ions from the ground state ( 7 F 0 ) to different excited levels ( 5 L 6 , 5 D 2 ) [20], and 532 nm is related to deep defects of ZnO, respectively. The strongest excitation peak is located at 394 nm. Figure 4 shows the PL spectra of Eu-doped ZnO submicron rods under the excitation of 394 nm. Series of characteristic Eu 3+ ion emissions centered at 593, 618 and 652 are, respectively, attributed to intra-4f transitions of 5 D 0 → 7 F 1 , 5 D 0 → 7 F 2 , 5 D 0 → 7 F 3 of Eu 3+ ions. The most prominent emissions are set at 593 nm and 618 nm. The peak set at 593-nm emission attributed to 5 D 0 → 7 F 1  transition is a magnetic dipole transition, which is not affected by the crystal environment of the host materials. The strongest emission peak is at 618 nm, attributed to 5 D 0 → 7 F 2 transition, which is a hypersensitive forced electric-dipole transition and the 5 D 0 → 7 F 2 transition with ΔJ = 2 is a forbidden transition in the free ion. It is noted only when the Eu 3+ ions are in a low symmetry position and no inversion center [10,18]. The red emission intensity was increasing in increasing the Eu 3+ concentration. The strongest 5 D 0 → 7 F 2 emission intensity of ZnO:Eu comes from the 15% Eu doping sample. To 20% Eu doping sample, the emission intensity decreased owing to concentration quenching. The transition of 5 D 0 → 7 F 1 is independent of the host lattice and therefore of the intensity ratio of red to orange emission. S increases with decreasing lattice symmetry, such as formula (1) [33] The calculated S Eu 3+ of different concentrations of Eu doping samples is as shown in Fig. 5. The value of S Eu 3+ is a maximum of 3.02 at 15% doping, and the S Eu 3+ value decreases as the doping concentration increases further. This is indicated that for 15% doping concentration more Eu 3+ ions are located in a low symmetry position. Figure 6 shows the PL spectra of undoped and 15% Eudoped ZnO samples under excitation of 394 nm. The broad blue-green emission around 455 nm is observed in the undoped sample. It is from the structural defects of ZnO host such as oxygen and zinc interstitials. After doping with Eu, the strongest Eu 3+ -related emission is observed and the blue-green emission intensity decreased. The Eu 3+ -related emission can also be observed under 380and 532-nm excitation (see Fig. 7). The result indicated (1) S Eu 3+ = ∫ (I) 618 nm d ∫ (I) 593 nm d that energy transfer from ZnO and deep defects of ZnO to Eu 3+ has appeared. Figure 8 shows the chromaticity coordinate of 15% Eu doping sample excited by 394 nm and shown on the CIE 1931 color diagram. The color coordinates of (0.25872, 0.26152) in 15% Eu doping ZnO are found to be set in the standard white-light oval, and the white light is achieved by the superposition of the blue-green emission from defects of ZnO and the red emission from Eu 3+ ions, which reveals that ZnO:Eu is a potential candidate for fabricating near (n)-ultraviolet (UV) or blue-converted LEDs white-light-emitting materials.

Conclusions
We have synthesized the ZnO:Eu submicron rods by a simple hydrothermal method. SEM and XRD studies reveal that the rod's diameter increases with the increase in the Eu concentration. The XRD peak positions gradually shifted toward higher scattering angle firstly and then shifted toward lower diffraction angle with increasing Eu doping. The Eu 3+ -related emissions appeared in Eu doping samples. The strongest red emission was obtained from 15% Eu 3+ doping; the sample due to the energy transfer from ZnO and deep defects of ZnO to Eu 3+ has appeared. The color coordinate (x, y) of 15% Eu doping phosphor is (0.2587, 0.2615) which is located on the standard white-light oval under the excitation at 394 nm. Doping forms of Eu are discussed. The results would be helpful for ZnO:Eu in optoelectronic device applications.