English

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  0   Introduction

  In recent years, energy transfer up-conversion (UC) and down-conversion(DC) researches have become hot topics in the material luminescence study ^[1-8], and attracted more and more attention due to their potential applications in the fields of optical comm-unication, electronic display systems, temperature sensors and UC laser ^{[3, 6-8]}. Owing to the abundant energy levels and narrow emission spectra lines, rare earth (RE) ions play a great role in the energy transfer UC/DC ^{[1, 6]} process. Among all the rare earth ions, Er³⁺ can emit characteristic green light originating from ²H_11/2-⁴I_15/2, ⁴S_3/2-⁴I_15/2 transitions, meanwhile, its energy levels are placed homogeneously in the energy space in different hosts, and the lifetimes of some energy levels are long enough, thus Er³⁺ is competent for UC/DC luminescence center. However, the absorption cross sections of Er³⁺ at output wavelength of comm-ercial LDs (typical one is around 980 nm) is small, so Yb³⁺ ion is often adopted as sensitizing center to improve the UC luminescence properties of Er³⁺ doped materials ^[9-16]. In recent years, most studies on Er³⁺/Yb³⁺co-doped systems have focused on exploring the UC mechanisms and developing novel hosts ^[17-19].

  Many RE-doped titanate hosts have been studied, such as MgTiO₃, La₂Ti₂O₇, SrTiO₃ and so on ^[20], except the La₂TiO₅ with pyrochlore structure. However, in our early investigations, La₂TiO₅ is proved to be a low-phonon host for many doping rare earth ions, which act as optical centers that convert the energy from absorbed UV and IR photons into visible light. In up/down-conversion process, the low phonon energy and high chemical stability are the critical factors for a preeminent host. Low phonon energy can prevent the non-radiative energy loss, which due to multi-phonon relaxation of exited rare earth dopants, and enhance the energy transfer efficiency ^[21-22]. As a new host material, La₂TiO₅ also has high chemical stability. In this research, the mechanism of UC/DC luminescence of the Er³⁺ doped and Er³⁺/Yb³⁺ co-doped samples are investigated through the excitation/emission spectra, dependence of emission intensity on Er³⁺ concentration, dependence of UC emission intensity on the pump power, the sensitization effect of Yb³⁺, the lumines-cence decay curve and quantum yield. To our best knowledge, it is the first report on the UC/DC lumine-scence of Er³⁺ ion in La₂TiO₅ host.

  1   Experimental

  1.1   Synthesis

  All the samples were prepared by sol-gel method. All reactants used in the synthesis were of analytical reagent grade. In a typical experiment, Er³⁺ doped and Er³⁺/Yb³⁺ co-doped La₂TiO₅ phosphors were prepared using lanthanum acetate [La(CH₃COO)₃], tetrabutyl titanate [Ti(OC₄H₉)₄], erbium nitrate hexahydrate [Er(NO₃)₃·6H₂O] and ytterbium nitrate hexahydrate [Yb(NO₃)₃·6H₂O] as starting reagents. The Er(NO₃)₃·6H₂O, Yb(NO₃)₃·6H₂O and La(CH₃COO)₃ in stoichio-metric propertions were dissolved into 30 mL distilled water, then 5 mL nitric acid was added to the above solution under vigorous stirring, which marked as solution-A. A certain amount of Ti(OC₄H₉)₄ and 30 mL ethanol were uniformly mixed, which marked as solution-B. Solution-A was added to solution-B under stirring and then the pH value of the mixed solution was adjusted to about 3 by the addition of an appropriate amount of nitric acid. After stirring for 2 h, transparent sol was obtained. Subsequently, the sol was dried at 80 ℃ for 2 h until it was transformed into xero-gel. Er³⁺ doped and Er³⁺/Yb³⁺ co-doped La₂TiO₅ samples were obtained by further sintering at 1 100 ℃ for 3 h in air.

  1.2   Instruments and measurement

  X-ray diffraction pattern was obtained for the powder specimen on DX2500 diffractometer using a graphite monochromator Cu Kα radiation (40 kV, λ=0.154 06 nm). The scanning rate was 0.04°·min^-1 with 2θ in the range of 10° to 70°. The powder morphology was characterized by a zeiss supra 55 scanning electron microscope(SEM). The photolumine-scence excitation and emission spectra were recorded on F-7000FL spectroflurophotometer with spectral slit width of 5 nm in range 220~800 nm. A 980 nm laser were used as a source of NIR excitation measurements. All measurements were made at room temperature. The lifetime and quantum efficiency were recorded using an Edinburgh instruments FLS920P. The quantum efficiency was measured by a 2 Port 150 mm BaSO4 coated integrating sphere which fits directly into the sample chamber.

  2   Results and discussion

  2.1   Structure and morphology characterization

  The XRD patterns for all the samples were measured to identify the crystal structure of the samples obtained from the sol-gel method. It was found that the profiles of the XRD patterns are the same. The top part in Fig. 1 shows the XRD pattern for 1% Er³⁺/10% Yb³⁺ (molar fraction) co-doped La₂TiO₅ phosphor as representative. All diffraction peaks are found in good agreements with PDF#75-2394 card shown in the bottom part of Fig. 1. No any extra diffraction peaks belonging to other compounds in addition to La₂TiO₅ are found. It indicates that the Er³⁺ doped and Er³⁺/Yb³⁺ co-doped La₂TiO₅ phosphors can be achieved via the simple sol-gel method and the Er³⁺ and Yb³⁺ doping at present level did not cause observable change in the host structure. Fig. 2 illustrates the SEM image of the La₂TiO₅ phosphor co-doped with 1% Er³⁺and 10% Yb³⁺ (molar fraction). The SEM image reveals that the average grain size of the phosphor particles is in the range of 100~300 nm. The shape of the particle is cubic, and some particles are agglomerated together due to partial melting of samples during annealing process.

  Figure 1.  XRD patterns of 1% Er³⁺/10% Yb³⁺ co-doped La₂TiO₅ phosphors (top) and the pattern plotted by using the data reported in PDF#75-2394 card (bottom)

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  Figure 2.  SEM image of 1% Er³⁺/10% Yb³⁺ co-doped La₂TiO₅ phosphor

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  2.2   DC/UC luminescence and mechanisms

  Fig. 3 shows the excitation spectra of 1% Er³⁺ (molar fraction) doped La₂TiO₅ by monitoring 546 nm. Six excitation bands are observed at 217, 272, 378, 408, 453 and 490 nm. In which, the first two broad excitation bands originate from the charge transitions of O-Ti and O-Er and the later four narrow bands are observed due to the f-f transitions from ground state ⁴I_15/2 to excited states ⁴G_11/2 (380 nm), ²H_9/2 (410 nm), ⁴F_3/2 (456 nm) and ⁴F_7/2 (490 nm), respectively. The intense CT bands indicate there are strong interaction existing between the doping Er³⁺ ions and the host. Among all the excitation bands, the band at 378 nm shows the highest intensity. Thereby, the DC emission spectra were recorded at 378 nm.

  Figure 3.  Excitation spectra of La₂TiO₅:1% Er³⁺ at λ_em=546 nm

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  It is well known that the doping concentration of luminescent center is an important factor affecting the luminescent properties of the phosphors. In order to analyse the concentration quenching behavior of La₂TiO₅ :Er³⁺ phosphors, the emission spectra of La₂TiO₅:Er³⁺ with (0.5%~7% molar fraction) Er³⁺ ion is measured and shown in Fig. 4. Two main emissions at 524 (²H_11/2-⁴I_15/2) and 546 nm (⁴S_3/2-⁴I_15/2) and one extremely weak emission band at 660~680 nm (⁴F_9/2-⁴I_15/2) are observed. The dependence of emission intensity (546 nm) as a function of the Er³⁺ doping concentration (0.5%~7% molar fraction) is shown in inset of Fig. 4. The emission peak position is almost the same for the different concentration of Er³⁺ in the samples indicating that the coordination surroundings of the doping Er³⁺ ions is independent for this concentration range. It can also be seen that, with the increase of Er³⁺ concentration up to 1% molar fraction, the relative emission intensity goes on increasing and above the concentration the intensity begins to decrease. This decrease in emission intensity may be caused by the concentration quenching or multi-phonon relaxation after cross relaxation (CR) of Er³⁺ ion.

  Figure 4.  Emission spectra of La₂TiO₅:xEr³⁺ (x=0.5%~7%) at λ_ex=378 nm

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  While taking into the concentration quenching of Er³⁺ ion, it means there exists the energy transfer between Er³⁺ ions when the R(Er-Er) in the La₂TiO₅ host is shorter than the critical transfer distance (R_c). Blasse has suggested that the R_c can be calculated using the formula 1 ^[30]:

  Where, V is the volume of the unit cell, X_c is the critical concentration of Er³⁺ ion, and N is the number of formula units per unit cell. In La₂TiO₅ host, V is 0.249 3 nm3 and N is 2. So R_c is obtained to be about 0.31 nm. As the calculated R_c is shorter than 0.5 nm, it implies energy exchange interaction is the major mechanism responsible for concentration quenching of La₂TiO₅:Er³⁺.

  Because the absorption cross sections of Er³⁺ at output wavelength of commercial LDs (laser diode, typical one is about 980 nm) is small, thus Yb³⁺ is often co-doped in phosphors as sensitizing center to improve the UC luminescence properties of Er³⁺. The emission intensity at 546 nm reaches its maximum with 1% molar fraction concentration of Er³⁺, so the samples with various Yb³⁺ and fixed Er³⁺ concentration were prepared. UC fluorescence emission spectra of La₂TiO₅:Er³⁺/Yb³⁺ under the excitation of 980 nm at same condition are shown in Fig. 5. The strong green emissions (524, 546 nm) and red emission (673 nm) are observed in the Er³⁺/Yb³⁺ co-doped samples and the red emission is significantly stronger than the counterpart of Er³⁺ doped samples. It can also be observed that the green and red emissions of Er³⁺/Yb³⁺ co-doped samples can be adjusted by changing Yb³⁺ concentration. The variation of UC intensity as a function of Yb³⁺ concentration is shown in top right inset of Fig. 5. The UC emission intensity decreases with the increase in the content of Yb³⁺, which indicates that the energy transfer from Yb³⁺ to Er³⁺ keeps going down due to the Yb³⁺ concentration quenching. The UC luminescence intensities (I) of the green and red emissions of La₂TiO₅:Er³⁺/Yb³⁺ were measured as a function of the pump power(P), respectively. In Fig. 6, the slopes of the curves ln (I) versus the ln(P) were fitted to be 2.12 and 2.15, respectively, both close to 2. The results mean that there is a two-photon process for the UC excitation.

  Figure 5.  UC emission spectra under 980 nm excitation of La₂TiO₅ phosphors doped with fixed concentration of Er³⁺ and various Yb³⁺ concentration

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  Figure 6.  ln-ln plots of the UC emission intensity at 546 nm versus the pump-power of La₂TiO₅:1% Er³⁺/10% Yb³⁺

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  The schematic energy level diagrams of Er³⁺, Yb³⁺ and the different excitation routes for UC and DC are shown in Fig. 7 ^{[7, 23-25]}. When the samples of La₂TiO₅:Er³⁺ are excited at 378 nm, the luminescent level ²H_11/2 and ⁴S_3/2 are populated via multi-phonon nonradiative transitions(NRT) from upper levels and then the radiant transitions from ²H_11/2 and ⁴S_3/2 to the ground state produce the green emissions at 524 and 546 nm, respectively. While during the UC process, the Er³⁺ ion in the ground state absorbs one 980 nm photon (GSA) and is initially excited to ⁴I_11/2 level. And then the excited Er³⁺ absorbs another photon to be excited to ⁴F_7/2 level, which is an excited-state absorption(ESA) process. Subsequently, the emission from ⁴F_7/2 level to the ground state is observed at 490 nm and luminescent levels ²H_11/2 and ⁴S_3/2 are populated by nonradiative transition from ⁴F_7/2, resulting the strong green emissions observed in UC spectra. The possible cross relaxation process CR1, CR2 and CR3 are shown in Fig. 7 as reported in references ^{[7, 26-29]}. Thereby, the luminescent level ⁴F_9/2 can be populated by nonradia-tive transition from ²H_11/2 and ⁴S_3/2 or by CR2 route. And the red emission from ⁴F_9/2 to the ground state is enhanced via CR2 in UC process. Yb³⁺ ion is chosen as the sensitizer to enhance the photon absorption property at 980 nm of La₂TiO₅:Er³⁺/Yb³⁺ phosphor. In this case, the Er³⁺ is mainly excited from the ground state to the ⁴I_11/2 level via energy transfer (ET1). Then the upper level ⁴F_7/2 is populated via another energy transfer (ET2) process resulting the emission at 490 nm obviously observed and the populations of ²H_11/2 and ⁴S_3/2 by multi-phonon nonradiative from ⁴F_7/2 level, which increases the green emission intensity at 524 and 546 nm. For the red emission, besides the nonradiative relaxation from ⁴F_7/2, ²H_11/2 and ⁴S_3/2, the ⁴F_9/2 can be populated by the cross relaxation route from ⁴I_9/2 level and energy transfer (ET3) process of ⁴I_13/2-⁴F_9/2, rather than the CR2 process, which enhances the red emission at 673 nm comparing to the counterpart of La₂TiO₅:Er³⁺ as shown in Fig. 7.

  Figure 7.  Schematic energy level diagrams of the Er³⁺, Yb³⁺ and the proposed UC/DC mechanisms

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  ET1: ⁴I_15/2(Er³⁺)+²F_5/2(Yb³⁺)→⁴I_11/2(Er³⁺)+²F_7/2(Yb³⁺)

  ET2: ⁴I_11/2(Er³⁺)+²F_5/2(Yb³⁺)→⁴F_7/2(Er³⁺)+²F_7/2(Yb³⁺)

  ET3: ⁴I_13/2(Er³⁺)+²F_5/2(Yb³⁺)→⁴F_9/2(Er³⁺)+²F_7/2(Yb³⁺)

  The kinetic decay curves of the level ⁴S_3/2 was measured. Fig. 8 shows the decay time profile of 1% molar fraction Er³⁺ doped La₂TiO₅ phosphor. The moni-toring wavelength is at 546 nm with a 378 nm excitation. The decay curve can be well fitted by a bi-exponential equation as

  Figure 8.  Decay curve of 1% Er³⁺doped La₂TiO₅ phosphor observed at 546 nm, excited at 378 nm

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  where I(t) is the luminescence intensity, t is the time after excitation, and τ is the decay time constant. A and B are coefficient constant. The fitting results are showed in Table 1. The lifetime values are found to be 26.05 and 79.32 μs for level ⁴S_3/2. Based on this, the average lifetime can be calculated using equation

  Table 1.  Fitting results of fluorescent decay curve for 1mol% Er³⁺doped La₂TiO₅

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  Table 1.  Fitting results of fluorescent decay curve for 1mol% Er³⁺doped La₂TiO₅

  In present case, the bi-exponential fitting implies the existence of multi-phonon nonradiative transitions from ⁴S_3/2, which is in agreement with the energy transfer process in schematic energy level diagrams ^[31-32]. The increase of initial stage of decay curve indicates the population process of ⁴S_3/2 level from upper energy levels. The quantum yield was measured with a value of about 41%. These results may be important for the fabrication of high-resolution optical detectors, solar cells and high-definition luminescent displays.

  3   Conclusions

  Phosphors of La₂TiO₅:Er³⁺/Yb³⁺ were synthesized via sol-gel method followed by thermal annealing at 1 100 ℃ and the X-ray diffraction pattern of pure La₂TiO₅, Er³⁺ doped and Er³⁺/Yb³⁺ co-doped La₂TiO₅ samples showed good agreement with the PDF#75-2394 card. All samples exhibit interesting characteristic green and red emissions. The Yb³⁺ sensitizes effectively the UC luminescence properties of Er³⁺ in La₂TiO₅ host. The strongest emission intensity at 546 nm is observed in 1% Er³⁺doped and 1% Er³⁺/10% Yb³⁺ co-doped phosphors. The mechanisms of energy absorp-tion, energy transfer, electron transition and nonradia-tive relaxation are discussed in details to explain the UC and DC emission spectra of the phosphors. The studies of decay curve indicate the existence of multi-phonon nonradiative transitions from ⁴S_3/2 level. The quantum yield is about 41%.

  Acknowledgment: This work was supported by Fundamental Research Foundation of Central Universities (Grant N142304007) and Basic Key Program of Applied Basic Research of Science and Technology Commission Foundation of Hebei Province in China (Grant No.15961005D and Grant No.14961108D).

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• 

  Figure 1  XRD patterns of 1% Er³⁺/10% Yb³⁺ co-doped La₂TiO₅ phosphors (top) and the pattern plotted by using the data reported in PDF#75-2394 card (bottom)

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  Figure 2  SEM image of 1% Er³⁺/10% Yb³⁺ co-doped La₂TiO₅ phosphor

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  Figure 3  Excitation spectra of La₂TiO₅:1% Er³⁺ at λ_em=546 nm

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  Figure 4  Emission spectra of La₂TiO₅:xEr³⁺ (x=0.5%~7%) at λ_ex=378 nm

  Inset shows variation of PL intensity with Er³⁺ content

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  Figure 5  UC emission spectra under 980 nm excitation of La₂TiO₅ phosphors doped with fixed concentration of Er³⁺ and various Yb³⁺ concentration

  Inset shows variation of intensity at 546nm with Yb³⁺ content; For a comparison, an up-converted spectra of 1mol% Er doped sample is included

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  Figure 6  ln-ln plots of the UC emission intensity at 546 nm versus the pump-power of La₂TiO₅:1% Er³⁺/10% Yb³⁺

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  Figure 7  Schematic energy level diagrams of the Er³⁺, Yb³⁺ and the proposed UC/DC mechanisms

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  Figure 8  Decay curve of 1% Er³⁺doped La₂TiO₅ phosphor observed at 546 nm, excited at 378 nm

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  Table 1.  Fitting results of fluorescent decay curve for 1mol% Er³⁺doped La₂TiO₅

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