Preparation of Compensation Ions Codoped SrTiO 3 : Pr 3 + Red Phosphor with the Sol-Gel Method and Study of Its Luminescence Enhancement Mechanism

SrTiO 3 :Pr is themost representative titanate matrix red phosphor for field emission display (FED).The red luminous efficiency of SrTiO 3 :Pr will be greatly improved after the compensation ions codoping, so SrTiO 3 :Pr red phosphor has been a research focus at home and abroad. SrTiO 3 :Pr, SrTiO 3 :Pr, Mg, and SrTiO 3 :Pr, Al 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 phosphor is single SrTiO 3 cubic phase, the main emission front is located at 614 nm, corresponding to Pr ions 1D → 3H transition emission. SrTiO 3 :Pr, Mg and SrTiO 3 :Pr, Al phosphor luminescence intensity is enhanced, but the main luminescence mechanism is not changed. Acceptor impurity x =Mg, Al will replace Ti bit after being doped into the crystal lattice to form XTi − charge compensation corresponding defect centers PrSr + to reduce the demand of Sr or Ti 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 red phosphor.


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][2][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  = 0.680,  = 0.311, 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%Pr3+ : Al ( = 0∼ 0.35) to improve the performance of red phosphors [5] luminescence.When  = 0.26, 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 2 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.

Advances in OptoElectronics
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.

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 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%Pr3+ , 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(O h1 ) as space group to establish the crystal cell; crystal cell parameters are  =  =  = 3.9051 Å,  =  =  = 90 ∘ .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 kpoints grids are used.

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 Ti * -O * reduces to 0.1902 nm, the 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 The band gap increases from 3.2 eV to 4.25 eV after being 25%Al 3+ -doped.It introduced the impurity levels about −7 eV of the valence band and it is shown in Figure 1.As the doping concentration of Al increases, the energy band tends to degeneration and becomes more curved, the electronic effective mass becomes smaller, and it is more benefiting to transfer.
Figure 2 shows the DOS.It can be seen that the DOS peak is moved to right in the top of valence band.Fermi level enters into the valence band because the DOS peak of O p-stated electron is moved to left, and the contribution of O p electrons to the conduction band weakens or even disappears.The DOS peak is moved to left of Sr s-stated and d-stated electron, while p-stated is moved to right.The DOS peak of Ti d-stated electron is lessened and shifted to right.The contribution of Al, Mg of s-stated and p-stated electron is significant to the conduction band.Thus, the second impurity atoms will exclude the contribution of Ti d-stated electron to the conduction band, and the greater the doping concentration, the more apparent.Intensity (CPS)

Results and Discussion
4.1.Luminescent Properties. Figure 3(a) shows the PL excitation spectra of the first group of samples Pr 3+ single doped SrTiO 3 :Pr 3+ red phosphor.As we can see from Figure 3(a), the luminescence peak appears at the wavelength of 610 nm; the luminescence peak can be attributed to 1D 2 → 3H 4 transition of Pr 3+ .Experimental results show that the sample of SrTiO 3 :0.002molPr3+ luminous intensity is the highest, which is consistent with other related experiments.Generally, samples' luminescence intensity is not high enough; charge compensation principle described in the relevant literature can explain this phenomenon.Therefore, we introduce compensation ions codoping.the third groups of samples Al 3+ codoped SrTiO 3 :Pr 3+ red phosphor.Experimental results show that samples have a luminescence peak which appears at the wavelengths of 611 nm and 614 nm, and the optimum ratio of the samples is Pr 3+ 0.2%mol; Mg 2+ 10%mol, Pr 3+ 0.2%mol; Al 3+ 25%mol, Pr 3+ 0.2%mol.Next, we choose separately the maximum luminous intensity of samples in the first three sets of samples to compare with each other, and the result is shown in Figure 4. Experimental results show that different compensation ions codoping does not cause the change of the position of the luminescence peak.And the position of the luminescence peak ranges between 610 nm and 615 nm; however, different compensation ions codoping will improve luminous intensity of the luminescence peak of SrTiO 3 :Pr 3+ red phosphor in varying degrees.Luminous intensity of Al 3+ codoped SrTiO 3 :Pr 3+ red phosphor at the luminescence peak is max and increases by appropriately 2 times compared with the sample of Pr 3+ single doping.Thus, trivalent Al has a better luminous enhancement function.Finally, we have researched the annealing temperature of Al 3+ codoped SrTiO 3 :Pr 3+ red phosphor, and Figure 5 shows the experimental result.As can be seen from Figure 5, the best annealing temperature is 950 ∘ C.
When there is only one impurity Pr 3+ in SrTiO 3 crystal, Pr 3+ replaces Sr 2+ to generate positively charged impurity defect centers Pr Sr + , which can deter electron transitions of the intra-4f from the 1D 2 → 3H 4 of Pr 3+ ions [1,13].In the sample preparation process, it will produce some oxygen vacancies O by Pr 3+ mostly is compensated by the oxygen vacancies O 2− ; the effect is not very obvious.Due to these reasons, luminous intensity of SrTiO 3 :Pr 3+ materials without codoping is not high.Thus, we choose to introduce the second compensation ions codoping to mainly compensate the positive charge posted by Pr Sr + .When the acceptor impurity X is mixed into the crystal lattice, it will replace Ti to form X Ti − charge compensation pair corresponding to Pr Sr + , which reduces the demand of Sr 2+ or Ti 3+ vacancy.Meanwhile, when Sr is doped to Pr, it will cause lattice distortion.The energy generated by the 4f-5d transition is very sensitive to the crystal electric field changes around the Pr ion.After the incorporation of different acceptor impurities, it will change the crystal electric field distribution, thereby weakening or promoting 4f-5d transition energy transfer.
Obviously, as we see in Figure 6, after Mg codoping, there is a certain influence on the electric field around Pr 3+ , but it is not obvious.When Al is codoped, there is a very distinct effect on the electric field around Pr 3+ .Figure 6 is the Electron Density Difference of samples.diffraction peaks is different: the samples' diffraction peak intensity of Al codoping is max, and the samples' diffraction peak intensity of Mg codoped is min.Based on the JADE software, we get two parts of the sample data shown in Table 2.

XRD. Figures 7(a
As we can see from Table 2, the diffraction angle has a slight increase from Pr 3+ single-doped to Al 3+ codoped.This phenomenon indicates that a part of Al and Mg at least is homogeneously incorporated into the SrTiO 3 lattice.

Conclusions
In a word, Mg 2+ , Al 3+ -doped SrTiO 3 :Pr 3+ red phosphors and different annealing temperatures SrTiO 3 :Pr 3+ :Al 3+ are synthesized by the sol-gel method and the luminescence enhancement mechanism of compensation ions codoping is investigated.It can be seen from the experimental results that the compensation ions codoping can significantly improve SrTiO 3 :Pr 3+ red phosphors' luminous intensity.When the excitation light is 350 nm and the gap width is 5 nm, the position of the luminescence peak ranges between 610 nm and 615 nm.Trivalent Al luminescence enhancement is the
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+ .In prepared SrTiO 3 :Pr 3+ , SrTiO 3 :Pr 3+ , Mg 2+ , and SrTiO 3 :Pr 3+ , Al 3+ , SrTiO 3 lattice absorbed ultraviolet leading to electron transition from O 2p occupied valence bands to Ti 3d unoccupied conduction bands.Based on the above discussion, the Ti-O and Pr-O band is enhanced and the corresponding bond lengths decrease by Al(Mg) codoping.So, the charge transfer efficiency is improved, which indirectly indicates that the energy transfer efficiency is improved.Therefore, the luminous efficiency of SrTiO 3 :Pr 3+ , Al 3+ is improved.
) and 7(b), respectively, show the two sets of samples SrTiO 3 :Pr 3+ : SrTiO 3 :Pr 3+ , Mg 2+ and SrTiO 3 :Pr 3+ , Al 3+ XRD patterns.It can be seen from the figure that the overall XRD diffraction peak indicates that SrTiO 3 crystal grows along the (110) crystal orientation.Experimental diffraction peak coincides with the standard SrTiO 3 sample card (pdf number: 35-0734).The diffraction angles of the samples increase after Al(Mg) codoping as shown in

Figure 6 :Figure 7 :
Figure 6: The Electron Density Difference of samples.
*and O * are the atoms that are near the impurity atoms (Al and Mg).

Table 2 .
However, there exists a impurity peaks.Based on JADE software, the impure peak is caused by TiO 2 , which shows that a bit of TiO 2 exists in the crystal lattice and it causes the lattice changes.And the intensity of the

Table 2 :
The first set of samples (110) diffraction peak characteristic values and parameters.