Trans. Nonferrous Met. Soc. China 22(2012) 1156-1160

Effect of Er substituting sites on upconversion luminescence of Er³⁺-doped BaTiO₃ films

CHEN Lei, WEI Xian-hua, FU Xu

State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials,

Southwest University of Science and Technology, Mianyang 621010, China

Received 22 June 2011; accepted 15 February 2012

Abstract: Erbium-doped BaTiO₃ films on LaNiO₃/Si substrates were fabricated by sol-gel method. The crystalline structure, morphologies and upconversion (UC) luminescence properties of films were respectively investigated by X-ray diffraction (XRD), atomic force microcopy (AFM) and photoluminescence (PL). The results indicate that both of the microstructure and luminescence are found to be dependent on Er³⁺ substituting sites. The samples with A-site substitution have smaller lattice constants, larger grains and smoother surface than those with B-site substitution. The photoluminescence spectra show that both of the samples have two stronger green emission bands centered at 528 and 548 nm and a weak red emission band centered at 673 nm, which correspond to the relaxation of Er³⁺ from ²H_11/2, ⁴S_3/2, and ⁴F_9/2 levels to the ground level ⁴I_15/2, respectively. Compared with B-site doped films, A-site doped films have a stronger integrated intensity of green emissions and a weaker relative intensity of red emissions. The differences could be explained by the crystalline quality and cross relaxation (CR) process.

Key words: Er³⁺ doping; BaTiO₃ thin films; upconversion photoluminescence; sol-gel method

1 Introduction

Ferroelectric thin films for integrated optics have received much attention due to their potential applications in second-harmonic generation, high speed modulators and gain devices. Barium titanate (BaTiO₃) is a good thin film integrated optic host because of its superior electronic and electro-optic properties, compared with other ferroelectric materials. For example, it has high electro-optic coefficient (820 pm/V at 632.8 nm, higher than 30.8 pm/V of LiNbO₃) [1,2], low half-wave voltage (310 V at 632.8 nm, lower than 2940 V of LiNbO₃) [3], low-loss waveguides (≤(4 ±2) dB/cm) and high solid solubility of rare earth ions [3,4]. Er³⁺ as an active ion can meet the request used for upconversion (UC) phosphors, planar waveguide and structural probe [5-8]. A lot of work about UC properties has been reported on Er³⁺-doped BaTiO₃ films [3, 4, 6, 9]. It is noted that both of the valence state and the radius of Er³⁺ ion are intermediate between those of Ba²⁺ ion and Ti⁴⁺ ion. As a result, Er³⁺ can occupy either A- or B-site depending on Ba/Ti mole ratio [8,10-12]. It is well-known that crystal field caused by structure symmetry of the host materials would result in different perturbation in terms of Er³⁺ inner shell transitions. Therefore, UC photoluminescence should be dependent on the excited-state dynamics of the Er³⁺ ions and their interactions with the host matrix [8,13]. Unfortunately, little attention has been paid to the effect of Er substituting sites on UC photoluminescence properties. Recently, the UC properties of Er³⁺-doped BaTiO₃ ceramics were correlated with doping concentrations, phase structure and doping sites [8,14]. Furthermore, the emission from substituted Er³⁺ ions might be considered structural probe for the ferroelectrics. In this work, the influence of substitution site of Er³⁺-doped BaTiO₃ films on their photoluminescence properties is investigated. Er³⁺-doped BaTiO₃ thin films with different doping sites are deposited on LaNiO₃/Si substrate by sol-gel method.

2 Experimental

Er³⁺-doped BaTiO₃ films were prepared by a modified sol-gel method. According to the charge compensation mechanism, the charge neutrality should be maintained through the vacancy formation of barium and oxygen for those samples with A-site and B-site substitution of Er³⁺, respectively. So, the formulas should be respectively Ba_1-3x/2Er_xTiO₃ and BaTi_1-xEr_xO_3-δ. Here the value of x is 0.03, which has been proved to show a strong UC photoluminescence [15]. The whole process of preparing the solution is described in Fig. 1. Barium acetate [Ba(Ac)₂], titanium butoxide  [Ti(C₄H₉O)₄, Ti(O-Bu)₄], erbium acetate [Er(Ac)₃] were used as starting materials; acetly acetone [AcAc, C₅H₈O₂], acetic acid (HAc) were used as solvents of Ti(O-Bu)₄ and Ba(Ac)₂ respectively. 2-methoxyethanol was used as solvent for the whole solution. The concentration of the solutions was adjusted to 0.4 mol/L through the addition of ethylene glycol. A light brown-yellow and transparent precursor appeared in the final sol. The LaNiO₃/Si substrates were first cleaned with acetone and ethanol, later rinsed with deionized water in a sonic bath. Spin-coating of the solutions was performed at 500 r/min for 18 s and 4000 r/min for 30 s on LaNiO₃/Si substrates. After coating, each layer was dried at 200℃ for 10 min and then baked at 400℃ for 10 min in air. This process was repeated until the desired thickness of about 500 nm was attained. Finally, the samples were annealed in oxygen at 750℃ for 2 h.

Fig. 1 Process of preparing Er³⁺-doped BaTiO₃ solutions

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of dried gel were carried out on METTLER TOLEDO (TGA/SDTA 851^e) from the room temperature to 800℃ in an oxygen flux with a heating rate of 10℃/min. The crystal structure of the thin films was analyzed by an X-ray diffractometer (XRD, Philips X’pert MPD Pro) utilizing Cu K_α (wavelength: λ=  1.540 6 ?) radiation. Atomic force microscope (AFM, Seiko SPI 3800) was used to investigate the grain size and surface roughness. The photoluminescence spectra were characterized by the use of Edinburgh FLSP920 spectrofluorophotometer at room temperature. The excitation light for UC luminescence was a 980 nm laser diode with a power of 500 mW. The excitation beams were applied directly onto the film surface and the signals were collected in the front of the films.

3 Results and discussion

3.1 DTA and TG

Figure 2 shows the TGA, DrTGA (derivative of TGA) and DTA results of BaTiO₃ dry gel. The first peak of DrTGA (the inset of Fig. 2) appears at about 100℃, indicating the loss of water in dry gel. Significant mass losses of about 1.6 mg and 2 mg are observed in temperature range of 100-300℃ and 300-450℃, respectively. These may be due to the decomposition and combustion process of the organic compounds, which are in agreement with the exothermic peaks in DTA curve. The two peaks at 600℃ and 700℃ corresponding to the mass loss of about 0.5 mg and 0.3 mg represent the complete decomposition of the organic compounds and the formation of crystallized BaTiO₃, respectively. Therefore, the treatment temperatures of films are selected as 400℃ for the baking process and 750℃ for the final annealing process.

Fig. 2 Thermal analysis results of BaTiO₃

3.2 X-ray diffraction patterns and atomic force microscopy image

Figure 3(a) presents the XRD patterns of Er³⁺-doped BaTiO₃ thin films annealed at 750℃. Only the diffraction peaks from phase-pure crystalline BaTiO₃ is observed besides the existence of the information of the substrates in both cases. The result implies that Er³⁺ ions were doped efficiently into different sites of BaTiO₃ host. There are minor shifts of the diffraction peak (200) between the Er³⁺-doped samples, as shown in Fig. 3(b), meaning that the lattices of the films could be slightly deformed because of the doping of impurity ions. In  two cases, no obvious splitting of cubic (200) into tetragonal (200) and (002) reflections at about 45° can be observed. It remains difficult to assign the crystal structure of BaTiO₃ films to either cubic or tetragonal symmetry using conventional X-ray diffractometer due to line broadening [16]. Otherwise, it should be induced by the size effect. Here the BaTiO₃ lattice was regarded as cubic phase with the lattice parameter of 3.9820 ? for the undoped sample (JCPS Card No.74—1956). The computed lattices are respectively 3.9745 ? and 3.9872 ? from the XRD results for the A-site and B-site doped films, indicating the contraction and expansion of the lattice [8,17].

Fig. 3 XRD patterns of films doped with 3% Er³⁺ for A- and B-site BaTiO₃ substitution annealed at 750℃ (a) and enlarged (200) peak (b)

Moreover, the FWHM of B-site substitution sample is larger than that of A-site substitution one, indicative of the smaller grains. It can be further proved by AFM analysis shown in Fig. 4. The films of A-site substitution have a well-crystallized and dense surface with the average grain size of about 60 nm and the RMS roughness of 4.0 nm. On the other hand, the samples of B-site substitution show the smaller grain size of about 40 nm and the larger RMS value of 8.4 nm (not shown here). The difference of the crystalline quality should be associated with the oxygen-deficient nonstoichiometry in B-site doped films.

3.3 UC photoluminescence

The PL spectra of BaTiO₃ films with different Er³⁺ ion site substitutions are measured under excitation of 980 nm, as shown in Fig. 5. The typical UC emission consists of two strong green bands located at 528 and 548 nm corresponding to ²H_11/2/⁴S_3/2→⁴I_15/2 transitions, and a weak red emission band at 673 nm associated with ⁴F_9/2→⁴I_15/2  transition of the Er³⁺ ions, respectively. The intensity ratio of 528 and 548 nm (I₅₄₈/I₅₂₈) is about 1.35 for both of the cases, while the relative ratios of green and red (I₅₄₈/I₆₇₃) are respectively 9.25 and 4.83 for A-site substitution and B-site substitution. There is no obvious shift of the emission peaks. These results indicate that the crystal field surrounding the erbium ions can change the relative probability of the transitions from the excited state to the ⁴I_15/2 ground state. The UC luminescence mechanisms are mainly determined by the process of excited state absorption (ESA) and energy transfer (ET). As shown in Fig. 6, under the excitation of 980 nm, through ESA or ET process, Er³⁺ ion can populate the ⁴F_7/2 level. Subsequently, the Er³⁺ ion then relaxes nonradiatively to the ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels by multiphonon relaxation, from which the strong green ²H_11/2/⁴S_3/2→⁴I_15/2 emissions and  the weak red ⁴F_9/2→ ⁴I_15/2 occur. The ESA approach only occurs during excitation, whereas the ET can happen both during and after the excitation. For the 673 nm red emission, due to the large energy gap between ⁴S_3/2 and ⁴F_9/2 levels, the nonradiative relaxation probability from the ⁴S_3/2 level to the ^4F_9/2 level is quite low. Therefore, the intensity of the red emissions is weaker than that of green emissions [18].

Fig. 4 AFM micrograph of 3% Er³⁺-doped BaTiO₃ film with A-site

Fig. 5 Integrated intensity of BaTiO₃ doped with 3% Er³⁺ for A- and B-site substitutions

Fig. 6 Possible UC photoluminescence mechanisms of 3% Er³⁺-doped BaTiO₃ films

The integrated intensity of A-site doped sample is much stronger than that of B-site doped BaTiO₃ film. It is mainly affected by the crystalline condition which A-site substitution has a better crystalline. But there is no difference between them in the value of I₅₄₈/I₅₂₈, which indicates that the ratio of I₅₄₈ to I₅₂₈ has no great importance in different substituting positions and crystalline conditions. As for the large difference of I₅₄₈/I₆₇₃, it could be understood by cross-relaxation (CR) shown in Fig. 6. As we known, subband gap defect levels could participate in the relaxation process and change the probability of radiative recombination [8, 19, 20]. Compared with A-site sample, B-site substitution has higher defect density which could enhance ET probability of CR process, i.e. ²H_11/2+⁴I_15/2→⁴I_9/2+⁴I_13/2. Then the Er³⁺ ions at ⁴I_13/2 state populate the ⁴F_9/2 state through ESA, which enhances the relative intensity of the red emission [8]. Furthermore, the population of ⁴S_3/2 state will decrease because a part of Er³⁺ ions populate the ²H_11/2 state [21], which also weakens the integrate intensity of the green emission. Therefore, the green-to-red ratio of B-site doped sample decreases relatively to that of A-site substitution.

4 Conclusions

1) Er³⁺-doped BaTiO₃ nano-luminescent films were fabricated on LaNiO₃ substrate by sol-gel method. The films only have the diffraction peaks of BaTiO₃ and the peaks (200) shift for Er³⁺ doped A-site or B-site film, which indicates that Er³⁺ ions are doped efficiently into different sites of BaTiO₃ host.

2) B-site doped film has a worse crystalline than A-site substitution, which should be attributed to the oxygen-deficient nonstoichiometry in B-site doped films.

3) Strong green UC emissions at 528 and 548 nm were obtained under the excitation of a 980 nm laser diode. The different UC photoluminescence properties were observed for Er³⁺-doped samples with different lattice sites. A-site doped film has a stronger integrated intensity of green emissions and a weaker relative intensity of red emissions, which could be related to the better crystalline and the weaker CR process respectively.

Acknowledgements

The authors are grateful to Dr. J. H. HAO and Y. ZHANG (The Hong Kong Polytechnic University) for technical support and useful discussion during the photoluminescence measurements.

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Er³⁺掺杂位置对Er³⁺:BaTiO₃薄膜上转换发光的影响

陈 磊，魏贤华，傅 旭

西南科技大学 四川省非金属复合与功能材料重点实验室－省部共建国家重点实验室培育基地，绵阳621010

摘  要：采用溶胶-凝胶法在LaNiO₃/Si衬底上制备Er³⁺掺杂BaTiO₃薄膜。通过XRD、AFM和PL图谱分别研究薄膜的晶体结构、形貌以及上转换发光性能。结果表明，薄膜的微观结构和发光性能与Er³⁺掺杂晶格的位置有关。A位掺杂薄膜较B位掺杂薄膜具有较小的晶格常数和较好的结晶。PL光谱表明：A位掺杂的薄膜和B位掺杂的薄膜都于528 nm和548 nm处获得较强的绿色上转换发光以及在673 nm处获得较弱的红光，分别对应Er³⁺离子的²H_11/2→⁴I_15/2，⁴S_3/2→⁴I_15/2和⁴F_9/2→⁴I_15/2能级跃迁。相对于B位掺杂的薄膜，A位掺杂样品有较强的绿光发射积分强度以及较弱的红光发射相对强度。这种差异可以通过薄膜的结晶状况和交叉弛豫机制来进行解释。

关键词：Er³⁺掺杂；BaTiO₃薄膜；上转换发光；溶胶-凝胶法

(Edited by YANG Hua)

Foundation item: Project (2009AA035002) supported by the High-tech Research and Development Program of China

Corresponding author: WEI Xian-hua; Tel: +86-816-2419201; E-mail: weisansao@yahoo.com.cn

DOI: 10.1016/S1003-6326(11)61299-5