Advances in OptoElectronics

Volume 2014 (2014), Article ID 674780, 9 pages

http://dx.doi.org/10.1155/2014/674780

## Preparation of Compensation Ions Codoped SrTiO_{3}:Pr^{3+} Red Phosphor with the Sol-Gel Method and Study of Its Luminescence Enhancement Mechanism

School of Information Science and Technology, Northwest University, Xi’an 710127, China

Received 30 June 2014; Revised 14 November 2014; Accepted 20 November 2014; Published 14 December 2014

Academic Editor: Jung Y. Huang

Copyright © 2014 Dan Guo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

SrTiO_{3}:Pr^{3+} is the most representative titanate matrix red phosphor for field emission display (FED). The red luminous efficiency of SrTiO_{3}:Pr^{3+} will be greatly improved after the compensation ions codoping, so SrTiO_{3}:Pr^{3+} red phosphor has been a research focus at home and abroad. SrTiO_{3}:Pr^{3+}, SrTiO_{3}:Pr^{3+}, Mg^{2+}, and SrTiO_{3}:Pr^{3+}, Al^{3+} phosphors are synthesized by a new sol-gel method. Crystal structure, spectral characteristics, and luminescence enhancement mechanism of the sample were studied by XRD and PL spectra. The results showed that after co-doped, SrTiO_{3}:Pr^{3+} phosphor is single SrTiO_{3} cubic phase, the main emission front is located at 614 nm, corresponding to Pr^{3+} ions 1D^{2}3H^{4} transition emission. SrTiO_{3}:Pr^{3+}, Mg^{2+} and SrTiO_{3}:Pr^{3+}, Al^{3+} phosphor luminescence intensity is enhanced, but the main luminescence mechanism is not changed. Acceptor impurity = Mg^{2+}, Al^{3+} will replace Ti bit after being doped into the crystal lattice to form charge compensation corresponding defect centers to reduce the demand of Sr^{2+} or Ti^{3+} vacancy. While Sr-doped Pr will make lattice distortion and transition energy of 4f-5d is very sensitive to crystal electric field changes around Pr atom. Doping different impurities will make electric field distribution around the icon have a different change. It increases energy transfer of 4f-5d transition and improves the luminous intensity of SrTiO_{3}:Pr^{3+} red phosphor.

#### 1. Introduction

Field emission display (FED) is a new display technology, with its high quality, low cost, large area of attractive advantages, achieving broad development prospects. Compared with cathode-ray tubes (CRT), FED has the high image quality. At the same time, FED has the thinness of liquid crystal display (LCD) and the large area characteristics of plasma display (PDP). FED has a considerable advantage in luminous efficiency, brightness, viewing angle, and power consumption way. In addition, FED also has a high resolution, fast response, high temperature resistant harsh, antivibration shock, weak electromagnetic radiation, and low production cost and is easy to implement digital display and so on.

Research found that SrTiO_{3}:Pr^{3+} [1–3] is the most representative for FED display titanate matrix red phosphor in the perovskite structure. SrTiO_{3}:Pr^{3+} phosphor generates red light when photoluminescence and cathode-ray excitation. And the red light’s coordinates is , , and it is very close to the American NTSC system providing an ideal red light. However, SrTiO_{3}:Pr^{3+} material has a problem that Pr^{3+} ion luminous efficiency is low, limiting its application. Study found that compensation ion doping can greatly improve SrTiO_{3}:Pr^{3+} material luminous intensity. For example, in SrTiO_{3}:Pr^{3+} material synthesis process, the luminous intensity will increase nearly 200 times more when adding Al(OH)_{3} or Ga_{2}O_{3}. Domestic Zhang et al. [4] studied Al^{3+}-doping of SrTiO_{3}:0.2%Pr^{3+}: Al ( = 0~0.35) to improve the performance of red phosphors [5] luminescence. When , its luminous intensity reaches the maximum; luminous intensity at this time is probably 20 times before. Yamamoto and Okamoto [6] used high temperature solid method to produce Al^{3+}-doped SrTiO_{3}:Pr^{3+} red phosphor and characterized and analyzed SrTiO_{3}:Pr^{3+} red phosphor. They have found generated strontium aluminate in the experiment, as well as proposing charge compensation theory. SrO layer disappears in SrTiO_{3} lattice, degree of crystallinity is improved, and luminous enhancement has also been improved to some extent.

In this experiment, we use the sol-gel method to produce compensating ions Mg^{2+}, Al^{3+} codoped SrTiO_{3}:Pr^{3+} red phosphor. Sol-Gel method has the product’s high uniformity, high purity, low firing temperature and the reaction easy control and so forth. Therefore, the project produces compensating ions doped SrTiO_{3}:Pr^{3+} red phosphor by the sol-gel method, improving the luminous intensity and persistence time by compensation ions doping method.

#### 2. Experiment Details

In this experiment, Sr(NO_{3})_{2} and Ti(OC_{4}H_{9})_{4} are used as the precursor; CH_{3}CH_{2}OH is the solvent and CH_{3}COOH is the stabilizer. In the process of sol configuration, we select Pr^{3+}, Al^{3+}, and Mg^{2+} to be doping elements to make SrTiO_{3}:Pr^{3+} red phosphor. Weigh moderate amount of Sr(NO_{3})_{2}, Pr(NO_{3})_{3}·6H_{2}O, and the impurities (Mg(NO_{3})_{2}·6H_{2}O or Al(NO_{3})_{3}·9H_{2}O) to a beaker marked A, and then add 15 mL of pure water to dissolve them and stir for about 20 min to manufacture solution A. Then, corresponding molar ratio of Ti(OC_{4}H_{9})_{4} and corresponding proportion of CH_{3}COOH and 10 mL of CH_{3}CH_{2}OH are mixed into another beaker marked solution B. The solution A is added into solution B dropwise keeping churning up the solution B about 30 min until the liquid becomes yellowish sol. After keeping churning up the solution B for 40 min, SrTiO_{3}:Pr^{3+} solution has been synthesized. (Sr) = (Ti) = 0.03 mol; the molar ratio of Ti(OC_{4}H_{9})_{4} and H_{2}O is 40 : 1; volume ratio of CH_{3}COOH and H_{2}O is 1.4 : 1; the amount of CH_{3}CH_{2}OH is 10 mL; aging time and temperature are 1.5 h/32°C, and the sol is turned into gel; dry the gel in 180°C for 8 h and the gel is turned into light yellow powder. The light yellow powder finally is annealed in air at 950°C for 1 hour and the SrTiO_{3}:Pr^{3+} red phosphor has been made.

In this experiment, the crystal structure and lattice constant of the SrTiO_{3}:Pr^{3+} red phosphor are investigated by powder X-ray diffractometer (XRD). Photoluminescence (PL) spectrum is taken by Hitachi F4500 luminescence spectrometer. And wavelength of the excitation light is 350 nm, the gap width is 5 nm, and scan range is from 525 nm to 650 nm.

The virtual crystal approximation (VCA) method is carried out to establish undoped SrTiO_{3} model and Al, Mg codoped SrTiO_{3}:Pr^{3+} nanophosphors valence-bond model using the CASTEP software package [7]. For example, to SrTiO_{3}:0.2%Pr^{3+}, 25%Al^{3+}, we set Sr and Ti atoms as mixture atoms in the crystal using the VCA method. When Pr^{3+} replaces Sr^{2+}, the relative concentration of Pr^{3+} is 0.2% and Sr^{2+} is 99.8%. And when Al^{3+} replaces Ti^{4+}, the relative concentration of Al^{3+} is 25% and Ti^{4+} is 75%. The interaction between nuclei and electrons is approximated with Vanderbilt ultrasoft pseudopotential [8] and the Perdew and Wang 91 parametrization [9] is taken as the exchange-correlation potential, which is the precise method used for the calculation of electronic structure at present [10]. From the electronic structure point of view (including the Band Structure, DOS, Mulliken population analysis) give a reasonable explanation for compensating ions Al, Mg, Li codoped SrTiO_{3}:Pr^{3+} system enhancement mechanism of luminescence. We use Pm3m() as space group to establish the crystal cell; crystal cell parameters are Å, . Plane wave basis with kinetic energy cutoff of 380 eV is used to represent wave functions. And the Brillouin Zone integration is approximated using the special k-points sampling scheme of Monkhorst-Pack [11] and 6 × 6 × 6 k-points grids are used.

#### 3. Theoretical Calculations

After compensating ion Al codoping, SrTiO_{3}:Pr^{3+} becomes a direct band gap semiconductor from an indirect band gap semiconductor, and the result is shown in Figure 1. Table 1 gives the calculated Mulliken population analysis for the prepared SrTiO_{3}:Pr^{3+}, SrTiO_{3}:Pr^{3+}, Mg^{2+}, and SrTiO_{3}:Pr^{3+}, Al^{3+} samples. When Pr^{3+} is single doped, the bond lengths of Sr–O and Ti–O are 0.2760 and 0.1952 nm, and the corresponding bond populations are −0.03 and 0.62, which indicates that the Sr–O band is an ionicity and the Ti–O band is a high degree of covalency. As we have seen, the bond length of Pr–O is 0.2700 nm, which is 0.006 nm less than Sr–O, and a positive population value is 0.12, which obviously indicates that there exists strong interaction between Pr–O and the Pr–O’s ionicity is weaker than Sr–O. In the case of SrTiO_{3}:Pr^{3+}, Mg^{2+}, considerable electron charge density near the Mg atom is redistributed. It is obvious that the calculated bond length of reduces to 0.1902 nm, the bond population of increases to 0.66, and the bond population of Ti–O also increases to 0.65. At the same time, the bond population of Pr–O in SrTiO_{3}:Pr^{3+}, Mg^{2+} is bigger than Pr–O band in SrTiO_{3}:Pr^{3+}. In the case of SrTiO_{3}:Pr^{3+}, Al^{3+}, the bond length of decreases to 0.1893 nm, smaller than in SrTiO_{3}:Pr^{3+}, Al^{3+}. Meanwhile, the bond population of increases to 0.70 and the bond population of Ti–O also increases to 0.68. As we all know, the calculated positive bond population in Mulliken population analysis indicates that there is strong attraction interaction between two atoms [12]. The larger the bond population is, the stronger the interaction is [12]. So, after codoping Mg^{2+} and Al^{3+} to SrTiO_{3}:Pr^{3+}, the interactions of Ti–O and Pr–O band are enhanced, and the interactions of Ti–O and Pr–O band in SrTiO_{3}:Pr^{3+}, Al^{3+} are stronger than those in SrTiO_{3}:Pr^{3+}, Mg^{2+}.