Excitation Wavelength Dependent Luminescence of LuNbO4:Pr3+

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Excitation Wavelength Dependent Luminescence of LuNbO4: Pr3+ - Influences of Intervalence Charge Transfer and Host Sensitization Chunmeng Liu, Fengjuan Pan, Qi Peng, Weijie Zhou, Rui Shi, Lei Zhou, Jianhui Zhang, Jun Chen, and Hongbin Liang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09806 • Publication Date (Web): 01 Nov 2016 Downloaded from http://pubs.acs.org on November 2, 2016

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The Journal of Physical Chemistry

Excitation Wavelength Dependent Luminescence of LuNbO4: Pr3+ − Influences of Intervalence Charge Transfer and Host Sensitization Chunmeng Liu, Fengjuan Pan, Qi Peng, Weijie Zhou, Rui Shi, Lei Zhou, Jianhui Zhang, Jun Chen, Hongbin Liang* MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China. * To whom correspondence should be addressed. E-mail: [email protected], Tel.: +86-20-84113695, Fax: +86-20-84111038.

ACS Paragon Plus Environment

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ABSTRACT A series of LuNbO4:Pr3+ phosphors was prepared by a solid-state reaction method at high-temperature. Rietveld refinements were performed based on powder X-ray diffraction (XRD) data. Diffuse reflectance spectra (DRS), UV-vis photoluminescence (PL), time-resolved emission spectra (TRES) and fluorescence decays were utilized to study the luminescence and host sensitization processes of Pr3+ in LuNbO4. Excitation wavelength dependent luminescence of LuNbO4:Pr3+ was investigated and explained in consideration of the processes of nonradiation relaxation via cross-relaxation, multi-phonon relaxation and crossover to the intervalence charge transfer (IVCT) state. Furthermore, the host sensitization of Pr3+ emission in LuNbO4 was confirmed and the energy transfer efficiency from host to Pr3+ increased with increasing Pr3+ doping concentration/temperature. Because the change of emission intensities for both blue from the host and red from 1D2 is sensitive to temperature, a large variation of emission color is observed between RT and 500 K.

1. Introduction Due to its interesting luminescent properties, the f-d transitions, the photon cascade emission (PCE) and the persistent luminescence of Pr3+ have been extensively investigated for various potential applications. Under X/γ-ray excitation, scintillators activated by Pr3+ have been developed for detecting applications based on the fast radiative decay rate of f-d transition.1-3 Upon VUV light excitation, Pr3+ doped phosphors show photon cascade emission (PCE) when the energy of the lowest 5d state is higher than that of 1S0 level. This feature is important for displays and lighting, because in principle a phosphor with quantum efﬁciency over 100 % might be attained.4-6 Under UV light excitation, Pr3+ doped phosphorescent materials usually show red emission originates from the intra-4f 1D2-3H4 transition.7-9 All these properties and applications


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depend on the 4f/5d energy levels of Pr3+ in a specific host lattice, the compositions and structure of the host compounds, and the excitation conditions. Recently, red emission of Pr3+ has attracted considerable interests because of the proper intervalence charge transfer (IVCT) energy of Pr3+-TMn+ (TMn+ = d0 transition metal ion, such as Ti4+, V5+, Nb5+, Ta5+, etc.).10-12 In these cases, the 1D2 state can be populated through IVCT channel and by-passing the population of 3P0 level. Although the correlations between red emission of Pr3+ and Pr3+-TMn+ IVCT energies in various host compounds have been reported,10, 13

the host sensitization and the radiative transition channels under different excitation conditions

are needed to be clearly studied. LuNbO4 belongs to the family of the fergusonite structured niobates and tantalates with general formula ABO4 (A = La, Gd, Y, Lu; B = Nb, Ta). It has the potential to be a good host lattice material for luminescence of rare earth ions (RE3+) due to the good chemical stabilization, thermal stability, and self-activated luminescent center NbO43- group.14-17 In the present paper, we investigate the luminescence properties of LuNbO4: Pr3+ upon different excitation wavelengths. It is of interest and significance to find that a low-lying Pr3+-Nb5+ IVCT state provides an extra channel for 3P0 quenching except for the multi-phonon relaxation (MPR) and cross-relaxation (CR) processes. Besides, the host sensitization for Pr3+ emission in LuNbO4 is verified, and the energy transfer from host to Pr3+ becomes more efficient with the temperature increase, resulting in the optimal Lu0.995Pr0.005NbO4 phosphor having a large variation in hue from RT to 500 K.

2. Experimental section A series of Lu1-xPrxNbO4 (x = 0-0.1) polycrystalline samples was prepared by a hightemperature solid-state reaction technique using raw materials Lu2O3 (99.99 %), Pr6O11 (99.99 %)


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and Nb2O5 (99.99 %). Appropriate amounts of these starting materials were weighed and thoroughly ground in an agate mortar using alcohol as the mixing medium. Then the mixtures were pressed into thin round tablets with dimension about 5 mm (height) × 12.5 mm (diameter) under a pressure of 150 MPa. These tablets were calcined at 1250 °C for 5 h in thermal-carbon reducing atmosphere. Finally, the as-obtained samples were cooled down to RT by switching off the muffle furnace and ground into powders for subsequent analysis. The phase purity of the samples was examined by X-ray diffraction (XRD) using a D8 ADVANCE powder diffractometer with Cu Kα radiation (λ = 1.5418 Å) at room temperature (RT). High quality XRD data for Rietveld refinement were collected over a 2θ range from 10°to 100° at an interval of 0.02°. Structural refinement of XRD data was performed using Topas Academic software.18 Diffuse reflection spectra were recorded using a Cary 5000 UV-vis-NIR spectrophotometer equipped with a double out of plane Littrow monochromator using BaSO4 powder as a standard reference. The luminescence spectra in UV-visible range and fluorescence decay curves at different temperatures were measured with an Edinburgh FLS 920 spectrometer combined fluorescence lifetime and steady-state spectrometer, which was equipped with a time-correlated single-photon counting (TCSPC) card. A 450 W xenon lamp was used as the excitation source for the UV-visible spectra recording, the excitation photons for luminescence decay curves and time-resolved emission spectra (TRES) were provided by a 60 W μF flash lamp with a pulse width of 1.5-3.0 μs. The Fourier transformation infrared (FT-IR) spectrum was collected on a Nicolet 6700-FTIR spectrometer with OMNIC software.

3. Results and discussion 3.1 Structural characterization.


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Figure 1. Experimental (crosses) and calculated (red solid line) XRD patterns and their difference (blue solid line) for Lu0.995Pr0.005NbO4. The magenta ticks mark the Bragg reflection positions. The inset shows the projection of LuNbO4 along the a-axis. Dark green, gray and red spherical balls represent Lu, Nb and O atoms, respectively. Figure 1 displays the results of Rietveld refinement for Lu0.995Pr0.005NbO4 implemented with the crystallographic information file (ICSD no. 109182) reported by Keller et al.19 The experimental, calculated and difference results of the XRD refinement of Lu0.995Pr0.005NbO4 are depicted by the black crosses, red and blue lines, respectively; the as-obtained goodness of fit parameters Rwp = 3.39 %, Rp = 2.21 %, and RB = 0.97 % can ensure the sample phase purity. The final refined structural parameters for Lu0.995Pr0.005NbO4 are summarized in Table S1 (Supporting Information). This compound exhibits a monoclinic crystal system with space group I2/a, and its lattice constants were determined to be a = 5.2297(1) Å, b = 10.8270(1) Å, c = 5.0416(1) Å, β = 94.4029(6), and the cell volume 284.621(5) Å3, being slightly larger than those of LuNbO4 synthesized in our study [a = 5.2280(1) Å, b = 10.8231(1) Å, c = 5.0407(1) Å, β = 94.4139(5) and V = 284.372(4) Å3], which is consistent with the substitution of larger Pr3+ (1.126 Å) for Lu3+ (0.977 Å) at 8-fold coordination. The inset of Figure 1 presents the crystal structure


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of LuNbO4 as viewed along a-axis. The structure consists of a three-dimensional network make up of NbO4 tetrahedra as well as irregular oxygen polyhedra of lutecium atoms. Lu is coordianted by 8 oxygen atoms with C2 point symmetry, while Nb is coordinated by four oxygen atoms. The Lu-O and Nb-O interatomic distances in Lu0.995Pr0.005NbO4 are listed in Table S2 (Supporting Information). The average distance between Lu and O atom is 2.4314 Å. Pr3+ is confirmed to be incoporated into Lu3+ site due to the similar ionic radii.

Figure 2. (a) XRD patterns of Lu0.995Pr0.005NbO4 as a function of temperature; (b) magnified XRD patterns in the region from 29.5 to 31.5 deg. for Lu0.995Pr0.005NbO4; (c) unit cell parameters of Lu0.995Pr0.005NbO4 show a contraction in the lattice constants a and β and an expansion in b, c and V with temperature increases. Figure 2a shows the XRD patterns of Lu0.995Pr0.005NbO4 as a function of temperature from 300 to 600 K. With increases of temperature, the XRD patterns of Lu0.995Pr0.005NbO4 maintain the monoclinic structure (space group I2/a). However, the diffraction peaks shift slightly to the lower angle side with increases in temperature, as shown in Figure 2b. This observation indicates that the increase in temperature causes a change of lattice constants, which can be calculated by


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structure refinement based on the powder XRD data. As shown in Figure 2c, the unit cell volume V clearly expands with the increases in temperature, which is due to the obvious increase of the lattice constants b and c. Herein, when we linearly fit the expansion rates of lattice constants b and c and the shrinkage rate of a, it can be found that b-axis and c-axis have evident expansion with slopes about 1.11E-4 and 0.99E-4, a-axis shows the slight contraction only with slope 2.36E-5. Meanwhile, the angle β seems to slowly decrease. These observations suggest that the increase in temperature expands the Lu0.995Pr0.005NbO4 lattice anisotropically. 3.2 Diffuse reflection spectra

Figure 3. The UV-vis diffuse reflectance spectra of Lu(1-x)PrxNbO4 (x = 0, 0.005, 0.01, 0.02, 0.05) at RT. Insets a and b are the digital photos of LuNbO4 and Lu0.995Pr0.005NbO4, respectively. The insert c shows a plot of [ln{(Rmax - Rmin)/(R - Rmin)}]2 against energy (eV) for LuNbO4, where R is reflectance, to estimate the band gap. Figure 3 shows the UV-vis diffuse reflectance spectra of LuNbO4 and Lu1-xPrxNbO4 (x = 0.005, 0.01, 0.02, 0.05) at RT. Since the samples were prepared in thermal-carbon reducing ambient, they are with slight gray body-color as displayed in the figure insets a and b, accordingly broad absorptions present in the visible region. For LuNbO4, the absorption band near 275 nm is the


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niobate host absorption;14, 17 the excitation band in Figure 4 can further confirm the assignment of this band. By employing the method of Kumar et al.,20 as shown in the figure inset c, the optical band gap of LuNbO4 is estimated about 4.4 eV, which is similar to that of other niobates such as CaNb2O6 (4.8 eV), LaNbO4 (4.8 eV), GdNbO4 (4.6 eV), YNbO4 (4.4 eV).21,

22

For

LuNbO4:Pr3+, the absorption peaks observed in the vicinity of 440-500 nm and 610 nm correspond to the f-f transitions of Pr3+ from 3H4 ground state to 3P0,1,2, 1I6 and 1D2 multiplets, respectively. In addition, the shoulder band located at lower energy side with respect to the host absorption band was observed, which is attributed to Pr3+ + Nb5+↔ Pr4+ + Nb4+ intervalence charge transfer (IVCT) state.11, 13 3.3 Photoluminescence properties. 1.2

1.2

Intensity (a.u.)

Normalized intensity (a.u.)

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A

1.0

B

0.8

Eex

LuNbO4 em = 390 nm

1.0 0.8 0.6 0.4

EVC

0.2

Efa

0.0 3.0

3.5

4.0

4.5

5.0

5.5

Energy (eV)

0.6

LuNbO4

0.4

Lu0.995Pr0.005NbO4

em = 390 nm (a) em = 390 nm (b)

b 0.2

em = 613 nm (c)

c a

0.0 200

250

300

350

400

450

500

550

Wavelength (nm)

Figure 4. The highest height-normalized excitation spectra of LuNbO4 (a, λem = 390 nm) and Lu0.995Pr0.005NbO4 (b, λem = 390 nm; c, λem = 613 nm) at RT. The inset shows the highest heightnormalized excitation spectrum of LuNbO4 (λem = 390 nm) in energy scale (eV). Referring to the emission spectra shown below (Figure 5), the excitation spectra of LuNbO4 and Lu0.995Pr0.005NbO4 in Figure 4 were collected by monitoring the host emission at 390 nm (curves a and b) and emission of Pr3+ at 613 nm (curve c) at RT, respectively. In Figure 4 each


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spectrum has been normalized to its highest intensity for comparison. A broad band A with a maximum at ~262 nm appears in each spectrum, which is coincident with that in Figure 3 and can be unambiguously assigned to the niobate fundamental absorption. As displayed in the inset of Figure 4, three different energy values are labeled. The optical band gap Efa (energy for fundamental absorption) is the energy of the first sharp onset in the excitation or absorption spectrum of the pure compound, for LuNbO4 this is 4.38 eV. The peak energy of host exciton Eex is attributed to the creation of free excitons that can be considered as bound electron-hole pairs, which is observed at about 4.73 eV, so the band gap between the bottom of the conduction band and the top of the valence band EVC is evaluated to be around EVC = 1.08×Eex = 5.11 eV.23, 24 Two broad bands (A, B) and a series of sharp lines occur in the excitation spectrum of Pr 3+ 613 nm emission (curve c). These sharp lines correspond to the transitions from 3H4 ground state to 3

P0,1,2 and 1I6 excited states of Pr3+ as observed in Figure 3. The band B (~295 nm, ~33.9×103 cm-

1

) is ascribed to the IVCT transition between Pr3+ and Nb5+, which corresponds to the electronic

transition from the ground state of Pr3+ (3H4) to the bottom of conduction band.22 An empirical formula (1) to estimate the energy of Pr3+ + TMn+ ↔ Pr4+ + TM(n-1)+ IVCT transition has been proposed by Boutinaud,11, 13 which can be used to confirm the assignment of band B. IVCT (CM −1 ) = 58800 − 49800[χ(TM n+ )/𝑑𝑚𝑖𝑛 (Pr 3+ − TMn+ )]

(1)

where χ(TMn+) is the optical electronegativity of TMn+; 𝑑𝑚𝑖𝑛 (Pr3+−TMn+) is the shortest distance between TMn+ and Pr3+. Here, χ(Nb5+) is 1.85 and 𝑑𝑚𝑖𝑛 (Pr3+-Nb5+) obtained from Lu0.995Pr0.005NbO4 is 3.498 Å. Thus, the IVCT energy is estimated to be ~32.5×103 cm-1 (~308 nm) according to equation 1. The difference (~1.40×103 cm-1) between estimated value and experimental one is within the error margin (± 1500 cm-1) of this empirical formula. Here, occurrence of the low-lying 4f2 → 4f15d1 band of Pr3+ must be ruled out, because the crystal field


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splitting and the nephelauxetic effect required shifting the 4f15d1 band from a position at about 61,580 cm-1 in the free Pr3+ ion down to the position of the excitation band B would be too high in LuNbO4 host.25 1.2 LuNbO4

Normalized intensity (a.u.)

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ex = 261 nm (a)

1.0

Lu0.995Pr0.005NbO4 ex = 454 nm (b)

0.8

ex = 295 nm (c) ex = 261 nm (d)

0.6

RT

0.4

0.2

0.0

d a 300

b

c 400

500

600

700

800

Wavelength (nm)

Figure 5. The highest height-normalized emission spectra of LuNbO4 (a, λex = 261 nm) and Lu0.995Pr0.005NbO4 (b, λex = 454 nm; c, λex = 295 nm; d, λex = 261 nm) at RT. According to the observed host absorption, IVCT and f-f transitions of Pr3+ in Figures 3 and 4, we select excitation wavelengths 261, 295, and 454 nm to measure luminescence of LuNbO4 and Lu0.995Pr0.005NbO4. The highest height-normalized emission spectra of LuNbO4 (a, λex = 261 nm) and Lu0.995Pr0.005NbO4 (b, λex = 454 nm; c, λex = 295 nm; d, λex = 261 nm) at RT are presented in Figure 5. Firstly, we check luminescence of pure host compound LuNbO4 (curve a), a rather broad emission band with a maximum at about 390 nm and full width at half maximum (FWHM) about 5.73×103 cm-1 occurs in curve a of LuNbO4 upon 261 nm excitation, so the sample appears blue emission. This emission band can be ascribed to the recombination of self-trapped excitons (STEs) within the NbO43- group.16 Many other niobates show similar host emission, such as YNbO4, LaNbO4, and GdNbO4.15-17


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Then we talk about the luminescence of Pr3+ doped sample Lu0.995Pr0.005NbO4. Under 454 nm (3H4 → 3P2 transition) excitation at RT, the emission curve b consists of a series of transitions from 3P0 and 1D2 states of Pr3+ in visible range. The peak at 491 nm is ascribed to 3P0 →3H4 transition of Pr3+ and the strongest emission around 613 nm is assigned to 1D2→3H4 transition. This assignment will be further confirmed by time-resolved emission spectra as discussed later. Curve c shows the emission spectrum upon excitation at 295 nm (IVCT state), which is similar to curve b, also contains the transitions from 3P0 and 1D2 levels of Pr3+. However, it can be found that the intensity of 3P0→3H4 transition upon IVCT excitation is somewhat weaker than that upon 3

P2 excitation when each curve is normalized to the height of 1D2→3H4 transition, which

suggests that the direct pumping in the IVCT state contributes to by-passing the population of 3P0 level.11 In addition, the host emission is undetectable in curve c, implying that the Pr3+ cannot transfer excitation energy to host emission centers via IVCT state. Curve d displays the emission spectrum of Lu0.995Pr0.005NbO4 after 261 nm excitation into the host-related absorption. The sharp peaks in 490-700 nm range are the transitions from 3P0 and 1D2 states of Pr3+ as observed in curves b and c, indicating the energy transfer occurs from host to Pr3+. The broad luminescence band with a maximum at about 390 nm in this curve is ascribed to the host emission, which is coincident with the spectrum of LuNbO4 as shown in curve a. Its presence indicates that the host sensitization process of Pr3+ in Lu0.995Pr0.005NbO4 is not complete. 3.4 Luminescence under 3H4 → 3P2 transition excitation at 454 nm.


11

1

D2

90

3

H4

1

td = 50 s

60

0.8

ex = 454 nm

30

0.6 0.4

550

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0

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1200

td = 6 s

800

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0 450 200

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Wavelength (nm)

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P0

-150 0

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Wavelength (nm)

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H4

3

P1 3P2

3

H5 3H6

80

P0

td = 2 s

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40 500

550

F2

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700

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3

0 450

.)

500

Intensity (a.u

Intensity (a.u.)

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120

0 450

Intensity (a.u.)

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Intensity (a.u.)


700

750

Wa 6 5 0 vele ngt 6 0 0 h (n m)

40

20

550

10

500

0

30 s) e ( Tim

800

Wavelength (nm)

Figure 6. Time-resolved emission spectra of the Lu0.995Pr0.005NbO4 sample (λex = 454 nm) at RT. In order to clarify the origin of f-f emission peaks of Pr3+, time-resolved emission spectra (TRES) of Lu0.995Pr0.005NbO4 were measured by excitation into the 3P2 level of Pr3+ with a 454 nm UV light. The emission spectra collected at different delay times (td) are shown in Figure 6. When td = 2 μs, the emission peaks that appear at around 490, 535, 563 and 661 nm, can be assigned, on the basis of the energy level scheme (Figure 7), to 3P0→3H4, 3P1→3H5, 3P2→3H6 and 3

P0→3F2 transitions, respectively. Because only a small part of the electrons have been relaxed

from 3P2 to 1D2 level within this short time, the transitions from the 1D2 state almost cannot be observed. As the extension of delay time, the peaks appearing at around 610 nm can be reasonably assigned to the transitions from the 1D2 level to the different Stark components of the 3

H4 ground state. When td = 6 μs, the emission intensity of 3P0 and 1D2 is strongest. Then, the

emission intensities of 3P0 and 1D2 decrease gradually with further increase of the delay time. The emission due to transitions from 3P0 decays much faster than that from 1D2, since the 1D2→3H4 transition is spin forbidden, but 3P0→3H4 transition is spin allowed. When td = 50 μs, the characteristic emission of 1D2→3H4 is dominant, a little 3P0 emission can be observed.


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1

D2

3

454 nm

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H4

3

P2

3

P0

MPR

ex = 454 nm

3

3

1

P1 + I6

1

D2

3

P0

CR non resonant

P0

1

G4

3

3

3

H4

F4 3 CR F2

F2

3

F3

3

H6

resonant

3

H5 H4

3

x = 0.001 x = 0.003 x = 0.005 x = 0.01 x = 0.02 x = 0.05 x = 0.10 500

550

600

650

700

750

Wavelength (nm)

Figure 7. Emission spectra of Lu(1-x)PrxNbO4 (x = 0.001, 0.003, 0.005, 0.01, 0.02, 0.05, 0.10) upon 454 nm excitation at RT. The inset shows the schematic energy levels of Pr3+ in LuNbO4, the wine solid line represents the excitation process, the dash lines describe the non-radiative processes. Emission spectra of samples Lu(1-x)PrxNbO4 (x = 0.001-0.10) upon excitation at 454 nm in the 3

P2 level of Pr3+ are displayed in Figure 7. It is clear that the relative intensities of 1D2→3H4 and

3

P0→3H4, 3F2 transitions increase and reach the maxima at Pr3+ concentrations x = 0.005 and x =

0.05, respectively, and then decrease due to the concentration quenching. The quenching concentration of 3P0 state is apparently higher than that of 1D2 multiplet. The energy transfer through multi-phonon relaxation (MPR) and cross-relaxation (CR) processes plays a key role in the observations. The MPR of 3P0→1D2 contributes to 3P0 quenching but its doping concentration dependency is weakly, which will be discussed in detail in next section. Several CR channels have been reported for 3P0 of the types 26, 27 [3P0, 3H4] → [1G4, 1G4]

(i)

[3P0, 3H4] → [1D2, 3H6]

(ii)


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as shown in the figure inset. Process (i) is not resonant, since the two involved transitions occur at energies differing by about 950 cm-1, and process (ii) is nearly but not fully resonant, both are favored by phonon assistance. The cross relaxation involving the 1D2 state (the inset of Figure 7) [1D2, 3H4] → [1G4, 3F4]

(iii)

is in practice a resonant process, so that strong concentration and weak temperature dependences are expected for this process (iii), and the quenching concentration of 1D2 emission is lower than

Lu(1-x)PrxNbO4 x = 0.001 x = 0.005 x = 0.01

x = 0.02 x = 0.05 x = 0.10 the fitted line

Ln{-ln[I(t)/I(0)]-(t/t0)}

that of 3P0 emission.

Logarithmic intensity (a.u.)

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1.0 0.0 -1.0 -2.0 -3.0 -4.0

3/s ~ 0.53 3/s ~ 0.53 3/s ~ 0.52

x = 0.01 x = 0.02 x = 0.05

s ~ 5.69 (D-D interaction)

2.0

3.0

4.0

5.0

lnt

0

20

40

60

80

100

Time (s)

Figure 8. Decay curves of the 1D2 emission measured as a function of the doping concentrations upon 454 nm excitation at RT, the red solid squares denote the fitting results by the InokutiHirayama model. The inset shows the fitting results based on equation 5. Figure 8 represents the decay curves of 1D2 emission upon 454 nm excitation in Lu(1-x)PrxNbO4 (x = 0.001-0.10) samples. For lower concentrations of Pr3+ (x = 0.001, 0.005) the decay curves are found to be nearly exponential, while for the higher Pr3+ concentrations (x > 0.005) the decay curves gradually deviate from exponential and the decay time is shortened mainly due to the resonant cross-relaxation (CR) process (iii) as mentioned above. This non-exponential


14

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characteristic of the fluorescence decay curve of the 1D2 emission can be described by the model developed by Inokuti and Hirayama:28 I(t) = I(0)exp[−

t 𝑡0

− Q · 𝑡 3/s ]

(2)

where t is the time after excitation, t0 is the intrinsic decay time of isolated donor in the absence of acceptors; s denotes the multipolar effect parameter that the value s = 6, 8, 10 represents dominant mechanism being dipole-dipole (D-D), dipole-quadrupole (D-Q), and quadrupolequadrupole (Q-Q) interaction, respectively. Q is the macroscopic parameter that determines the effect of multipolar interaction in decay property: Q=

4𝜋 3

3

(𝑠)

Γ(1 − ) ∙ 𝐶𝐴 ∙ (𝐶𝐷𝐴 )3/𝑠 𝑠

(3)

where Γ is the gamma function; CA is the acceptor content; CDA denotes the CR kinetic parameter defined by the following: (𝑠)

𝑃𝑆𝐴 =

𝐶𝐷𝐴 𝑠 𝑅𝑆𝐴

(4)

where RSA denotes the distance between donors and acceptor, and PSA represents the energy transfer probability between donor and acceptor. When PSA is equal to 1/t0, RSA(k) and CA(k) are called “critical distance” and “critical doping concentration”, respectively. For the purpose of determining the type of multipolar effect, the experimental decay curves were fitted using equation 5 and the results are plotted in the figure inset.29, 30 𝐼(𝑡)

ln (−𝑙𝑛 (

𝐼(0)

𝑡

3

𝑡0

𝑠

) − ) = 𝐵 + ∙ lnt

(5)

where B is a control factor. t0 value is roughly estimated to be ~ 31.5 μs from the exponential decay curve of Lu0.999Pr0.001NbO4. The plot of equation 5 yields straight line with the slope equal to 3/s, s value is therefore estimated to be about 5.69, implying that the main multipolar effect is dipole-dipole interaction for Pr3+ in LuNbO4. Then, we fitted the decay curves by means of


15


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Page 16 of 33

equation 2 as shown in Figure 8. According to the fitting results (all Adj. R2 data are beyond 0.99), Q values increase from 2.9×10-3 for x = 0.01 to 1.7×10-2 for x = 0.10, demonstrating that the energy transfer through multipolar interaction becomes more remarkable with the Pr3+ doping concentration increase. The kinetics parameter CDA and critical doping concentration CA(k) are estimated to be 2.01 × 10-47 m6·s-1 and ~ 0.54 mol%, respectively. With increasing Pr3+ doping concentrations from 0.001 to 0.10, the values of PSA increase from 8.72× 103 s-1 to 1.13× 106 s-1, revealing that the CR process becomes more efficient gradually. 3.5 Luminescence under IVCT excitation at 295 nm. 1

D2

3

H4

ex = 295 nm

3

3

P0

3

P0

H4

3

F2 x = 0.001 x = 0.003 x = 0.005 x = 0.01 x = 0.02 x = 0.05 x = 0.10

400

450

500

550

600

650

700

750

Wavelength (nm)

Figure 9. Emission spectra of Lu(1-x)PrxNbO4 (x = 0.001, 0.003, 0.005, 0.01, 0.02, 0.05, 0.10) upon excitation at 295 nm. Figure 9 displays the emission spectra of Lu(1-x)PrxNbO4 (x = 0.001-0.1) upon excitation at 295 nm corresponding to the Pr3+-Nb5+ IVCT state. Two obvious characteristics have been seen: Firstly, the emission intensity ratio of 1D2 to 3P0 [R(1D2/3P0)] is larger upon excitation at 295 nm than that upon excitation at 454 nm at each doping concentration. Secondly, with increase of Pr3+ doping concentration, it can be found that a similar tendency of concentration quenching behavior upon 295 nm excitation (Figure 9) and 454 nm (Figure 7) excitation. The first


16

Page 17 of 33

characteristic can be accounted for by the fact that the direct pumping in the IVCT state is followed by radiationless relaxation to the 1D2 level, by-passing partly the population of 3P0 level.11,

13, 27

The second feature implies that after return to 3P0 and 1D2 states, relaxation

processes are the same in principle for Figures 7 and 9. I/Imax 1

ex = 295 nm

3

P0

1

D2

the fitting curve 0.1 100

200

300

400

500

Temperature (K)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


78K 100K 150K 200K 250K 300K 350K 400K 450K 500K

450

500

550

600

650

700

750

Wavelength (nm)

Figure 10. Emission spectra of Lu0.995Pr0.005NbO4 at different temperatures from 78 to 500 K upon excitation at 295 nm. The inset depicts the temperature behaviors of the integrated intensities of the 3P0 and 1D2 emissions, the solid line denotes the fitting result via equation 8. The temperature dependence of the emission spectra of Lu0.995Pr0.005NbO4 in the range of 78500 K upon excitation at 295 nm is plotted in Figure 10. The integrated intensities of the 1

D2→3H4 and 3P0→3F2 transitions as a function of the temperature are given in the figure inset.

The integrated emission intensity of the 1D2→3H4 transition is nearly constant before about 450 K and then shows a bit of decrease, whereas that of the 3P0→3F2 transition is characterized by a plateau in the 200 - 350 K range and followed by a marked decrease to 500 K. It is obvious that the thermal-quenching is relatively weaker for 1D2, but that is more obvious for 3P0. Three possible non-radiative de-excitation pathways cooperatively lead to these observations: (1) cross-


17


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Page 18 of 33

relaxation (CR), (2) multi-phonon relaxation (MPR) from 3P0 to 1D2 level and (3) crossover to the IVCT state. We firstly discuss the possibility of de-excitation pathway through the CR channel. The absence of [1D2, 3H4] → [1G4, 3F4] CR process for the 1D2 level has been confirmed by the exponential decay of this low doping sample Lu0.995Pr0.005NbO4 at RT as demonstrated in Figure 8. It is widely accepted that the [1D2, 3H4] → [1G4, 3F4] channel is more efficient than the [3P0, 3

H4] → [1D2, 3H6] channel, due to a smaller energy mismatch in the former case. Therefore, CR

process is also inefficient for 3P0 level in Lu0.995Pr0.005NbO4 at RT. Herein, it is worth to mention that the phonon-assisted cross relaxation processes (i) and (ii) of 3P0 multiplet are expected to be enhanced with increase of temperature. Then we consider the multi-phonon relaxation (MPR) from 3P0 to 1D2 levels. Usually 4~5 phonons are required to bridge the energy gap between 3P0 to 1D2 (ΔE ≈ 3500 cm-1). The maximum phonon energy ℏ𝜔𝑚𝑎𝑥 obtained from the FT-IR spectrum of LuNbO4 is about 807 cm1

as shown in Figure S1 (Supporting Information). So it is expected that the phonon energy meets

this criterion for MPR from 3P0 to 1D2. Besides, the MPR rate (WNR) can be roughly estimated using the modified exponential energy-gap equation 6 of Van Dijk and Schuurmans:31 (6)

𝑊𝑁𝑅 = 𝛽𝑒𝑙 exp[−𝛼(Δ𝐸 − 2ℏ𝜔𝑚𝑎𝑥 )]

where βel = 107 s-1 and α = 4.5 (±1) ×10-3 cm.10, 32 Under these conditions, the multi-phonon relaxation rate is calculated to be 2.1×103 s-1. The value of WNR will increase with increasing temperature according to7 ℏ𝜔𝑚𝑎𝑥

𝑊𝑁𝑅 (𝑇) = 𝑊𝑁𝑅 (0) [1 + (𝑒𝑥𝑝 (

𝑘𝑇

−1 𝑝

)) ]

(7)

where p is the number of phonons involved, and k is the Boltzmann constant (0.696 cm-1). Using equation 7, we calculated that the MPR rate is increased to about 3.3×103 s-1 at 500 K. This


18

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probability remains much smaller than the typical 3P0 non-radiative rate ~7.0×106 s-1.32 Thus, MPR certainly contributes to 3P0 quenching but its influence is limited. Thirdly, we infer that the cross-over to the IVCT state would have indispensable contribution to the thermal-quenching of the 3P0 emission in Lu0.995Pr0.005NbO4. 7, 10, 32 The trends in the inset of Figure 10 are typically observed when the thermal depopulation of the 3P0 state takes place through a cross-over to Franck–Condon shift states.7,

11, 27

Following the Struck and Fonger

model,33 the temperature behavior of the 3P0 emission intensity is described by equation 8: 𝐼(𝑇) 𝐼0

= [1 + 𝐴𝑒𝑥𝑝(−

∆𝐸 𝑘𝐵 𝑇

−1

)]

(8)

where A is close to 107 and ∆E is the activation energy from the emitting state to its crossover with the quenching state, kB is Boltzmann’s constant. The fitted result is plotted in the inset of Figure 10 as solid line. We obtained the value of ∆E is approximately 2490 cm-1 (0.31 eV). This energy barrier of 2490 cm-1 corresponds to the gap separating from the 3P0 level to its crossover with the IVCT state. Taking into account this energy (∆E) and the results discussed above, it is possible to construct the single configurational coordinate diagram, as shown in Figure 11. After excitation to IVCT state, relaxation from this state results in the population of the excited 1D2 level (red arrow) and the partial quenching of the 3P0 emission (green arrow). It is worth to mention that the activation energy ∆E value is only an indication of the magnitude of the energy barrier due to the presence of other non-radiative processes contributing to the depopulation of the emitting level. With the increase of temperature, more electrons might overcome this energy barrier, the probability of depopulating of the 3P0 level by cross-over to the IVCT state is enhanced (blue arrow), leading to a weaker emission from 3P0.7, 27 Meantime, the influences of multi-phonon relaxation and phonon-assisted cross relaxation processes of 3P0 state cannot be excluded, since the rates of these two processes also increase with increase of temperature. In


19


addition, it seems that another possibility, the thermal population of 1I6 multiplet and then quick relaxation to 1D2 state at higher temperatures, cannot be excluded. Hence the 3P0 emission shows a sharp decrease at higher temperatures due to the cooperative effects of these factors.

E

1

IVCT

P0

D2

3

E

613 nm

3

491/661 nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 33

H4

Q

Figure 11. Schematic configuration coordinate diagram of LuNbO4:Pr3+. The green and red arrows signify the non-radiative relaxation processes from IVCT to 3P0 and 1D2 levels, respectively. The blue arrow signifies the thermal quenching of 3P0 emissions. 3.6 Luminescence under host excitation at 261 nm. The energy transfer from host to Pr3+ has been observed in the UV-vis spectra. Herein, concentration/temperature dependence of LuNbO4:Pr3+ emission spectra and host emission decay curves excited at 261 nm were measured to investigate the host sensitization effect for Pr3+ in LuNbO4.


20

Page 21 of 33

1.0

ex = 261 nm

0.9



12.0

Relative intensity (106, a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

T = 1 - IS/IS0

0.7

10.0

0.6 0.5 0.00 0.02 0.04 0.06 0.08 0.10

8.0

concentration (x)

Lu(1-x)PrxNbO4

6.0

x=0 x = 0.001 x = 0.003 x = 0.005 x = 0.01 x = 0.02 x = 0.05 x = 0.10

4.0

2.0

0.0

350

400

450

500

550

600

650

Wavelength (nm)

Figure 12. Emission spectra of Lu(1-x)PrxNbO4 (x = 0-0.10) upon 261 nm excitation at RT. The inset shows the dependence of the energy transfer efficiencies (ηT) on Pr3+ concentration (x). Figure 12 shows the emission spectra as a function of the Pr3+ doping concentration in Lu(1x)PrxNbO4

(x = 0.001-0.1) under 261 nm excitation at RT. With increase of Pr3+ doping

concentration, the intensity of host emission monotonically decreases, whereas the emission intensity of the 1D2→3H4 transition of Pr3+ first increases to a maximum corresponding to x = 0.005, and then decreases with a further increase of Pr3+-doping concentration. Two factors cooperatively lead to these observations: i) the energy transfer from the host to Pr 3+ will increase the emission intensity of Pr3+ and decrease the host emission intensity; ii) concentration quenching is the main reason for the decrease of 1D2 emission intensity with increase of Pr3+ concentration after x = 0.005, this value is compatible with that in section 3.4. The energy transfer efficiency (ηT) from host to Pr3+ in LuNbO4 matrix can be approximately calculated using the formula ηT = 1 – IS/IS0, where IS0 and IS correspond to the luminescence intensities of the host in the absence and presence of Pr3+, respectively. As illustrated in the inset of Figure 12, the values of ηT are plotted as a function of the Pr3+ doping concentration (x), which shows that the ηT values gradually increase to reach 93.9 % when x is 0.10, indicating that


21


the energy transfer from host to Pr3+ becomes more and more efficient with an increase in Pr3+ concentration. 1

1.0

3

D 2  H4

7.0 2.0

6.0

0.5 1.0 0.0 100

200

300

400

T(K)

500

5.0 4.0 3.0 2.0

6

niobate

Intensity (10 , a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

1.0 0.0 100 K 150 K 200 K 250 K 300 K 325 K 350 K T 375 K 400 K 450 K 500 K 350

400

450

500

550

600

650

700

Wavelength (nm)

Figure 13. Emission spectra of Lu0.995Pr0.005NbO4 at different temperatures from 78 to 500 K. The inset shows temperature dependence of 1D2→3H4 and of the niobate integrated emission intensity in Lu0.995Pr0.005NbO4 upon excitation at 261 nm. Figure 13 shows the temperature dependent behavior of Lu0.995Pr0.005NbO4 luminescence under host excitation (261 nm) in the temperature range of 100-500 K. The integrated emission intensities of the host (niobate) as a function of temperature are displayed in the figure inset. It is obvious that the host emission decreases with increasing temperature, and is not totally quenched until 500 K. To get an insight into the intensity of temperature-dependent host emission, the fluorescence decay curves of host emission in Lu0.995Pr0.005NbO4 were collected in the temperature range of 100-350 K, and a comparison was made with that of the LuNbO4 as shown in Figure S2 (Supporting Information). It can be observed that the decay curves of LuNbO4 at different temperatures are close to a single-exponential function (Figure S2a), and the decays become faster with increasing temperature due to the thermal quenching.34,

35

In

Lu0.995Pr0.005NbO4 (Figure S2b) the host decay curves deviate from exponential, indicating the


22

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


existence of energy transfer from the host to Pr3+. Furthermore, the decay of host emission in Lu0.995Pr0.005NbO4 is much faster than that in LuNbO4 especially at higher temperatures, providing the evidence that the efficiency of this energy transfer increases with increasing temperature. As shown in the inset of Figure 13, the intensity of 1D2 emission at 613 nm slightly decreases from 100 to 250 K, increases from 250 to 450 K and keeps nearly unchangeable between 450 and 500 K. The behavior in 100 to 250 K can be tentatively accounted for by the variation of the thermal population of the Stark levels of the emitting 1D2 state.36 The similar phenomena of the increase of 1D2 emission (herein it is in the temperature range of 250 to 450 K) have been reported in Pr3+-doped YNbO4, R1/2Na1/2TiO3 (R = La, Gd, Lu, and Y), CaTiO3 and NaGdTiO4,11-12, 37-38, and the reason was attributed to the thermal-activated host sensitization. It is possible that the observation in 450 to 500 K is due to the competitive balance of energy transfer as mentioned below. The excited state populations of host and activator can be described by the rate equations12, 39-41 𝑑𝑛𝐻 ⁄𝑑𝑡 = 𝑊𝐻 − 𝛽𝐻 𝑛𝐻 − 𝜔𝐻𝐴 𝑛𝐻 − 𝜔𝑛𝑟 𝑛𝐻 ′ 𝑑𝑛𝐴 ⁄𝑑𝑡 = 𝜔𝐻𝐴 𝑛𝐻 − 𝛽𝐴 𝑛𝐴 − 𝜔𝑛𝑟 𝑛𝐻

(9) (10)

here 𝑛𝐻 and 𝑛𝐴 are the concentrations of excited states of host and activator, βH and βA are the fluorescence decay rates of the host and activator, respectively, and WH is the pumping rate of ′ the host. The ωHA is the energy transfer rate between host and activator. 𝜔𝑛𝑟 and 𝜔𝑛𝑟 are the

non-radiation transfer rate from host and activator to quenching centers, respectively. Because of the large difference in energy between the activator emission and the host absorption, no back transfer from activator to host is present. Accordingly, the increased emission intensity of 1D2 with increasing temperature in Lu0.995Pr0.005NbO4 is thought to be originated from an enhanced


23


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Page 24 of 33

ωHA. The radiative transition of 1D2 is dominated by a competitive process of receiving energy from host and transferring energy to quenching centers. The higher the ωHA is, the stronger the 1

D2 emission intensity will be. Therefore, it is reasonable that in Lu0.995Pr0.005NbO4 the

competitive process reaches balance at 450-500 K resulting in the maximum emission intensity at this range of temperature. Because of the temperature-dependent relative intensity of host emission (blue) and 1D2 emission (red), Lu0.995Pr0.005NbO4 has a temperature sensitive emission color from RT to 500 K. The International Commission on Illumination (CIE) chromaticity coordinates and the photographs of Lu0.995Pr0.005NbO4 at different temperatures are shown in Figure 14. As can be seen, the coordinates of Lu0.995Pr0.005NbO4 systematically shifted from violet-blue to purple and then to pink with increasing temperature, the emission of Lu0.995Pr0.005NbO4 has a large variation in hue between RT to 500 K.


24

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Figure 14. CIE chromaticity diagram and photographs of Lu0.995Pr0.005NbO4 at different temperatures under 261 nm excitation. The line is a guide for the eye.

4. Conclusions The XRD Rietveld refinement analyses for the Pr3+ doped LuNbO4 prepared by a hightemperature solid-state reaction method confirm the high phase purity of the samples and reveal that Pr3+ was effectively incorporated into the Lu3+ site, and indicate that the lattice of Lu0.995Pr0.005NbO4 was expanded and distorted as temperature increases. On the basis of the diffuse reflection spectrum and excitation spectrum of LuNbO4, the optical band gap is determined to be about 4.4 eV, and the band gap between the bottom of the conduction band and the top of the valence band is evaluated to be 5.11 eV. By means of TRES, the origins of emission peaks from Pr3+ are clarified. In view of Pr3+ luminescence dependencies on excitation wavelengths, it is demonstrated that a low-lying Pr3+-Nb5+ IVCT state provides an extra quenching channel for 3P0 level besides the multi-phonon relaxation and cross-relaxation processes. UV-vis excitation and emission spectra and decay curves confirm the host sensitization of Pr3+ emission in LuNbO4, and the energy transfer efficiency from host to Pr3+ increases with increasing Pr3+ doping concentration/temperature. The tunable emission in a large color gamut (from violet-blue to pink) is obtained along with the temperature increases from RT to 500 K because of the change of relative intensity of the blue host emission and red 1D2 emission. Therefore, further optimized Lu0.995Pr0.005NbO4 may have a promising application as a fluorescence thermometer in temperature range between RT and 500 K.

ASSOCIATED CONTENT Supporting Information


25


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Page 26 of 33

The refined structural parameters for Lu0.995Pr0.005NbO4; FT-IR spectrum of LuNbO4; temperature-dependent decay curves of host emission in LuNbO4 and Lu0.995Pr0.005NbO4 measured with an excitation at 261 nm and monitored at 390 nm. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * To whom correspondence should be addressed. E-mail: [email protected], Tel.: +86-20-84113695, Fax: +86-20-84111038. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT The work is financially supported by the National Natural Science Foundation of China (U1432249, 21671201, and U1632101), the National Key Research and Development Project of China (2016YFA0202001), and the Postdoctoral Science Foundation of China (2015M582466). REFERENCES 1.

Zych, A.; de Lange, M.; de Mello Donegá, C.; Meijerink, A. Analysis of the Radiative

Lifetime of Pr3+ d-f Emission. J. Appl. Phys. 2012, 112, 013536.


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Leveraging Solar Energy through Persistent Dual Red Phosphorescence: Preparation, Characterization, and Density Functional Theory Study of Ca2Zn4Ti16O38:Pr3+. J. Phys. Chem. C 2010, 114, 7196-7204.


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Wang, B.; Lin, H.; Xu, J.; Chen, H.; Lin, Z. B.; Huang, F.; Wang, Y. S. Design,

Preparation, and Characterization of a Novel Red Long-Persistent Perovskite Phosphor: Ca3Ti2O7:Pr3+. Inorg. Chem. 2015, 54, 11299-11306. 10. Boutinaud, P.; Pinel, E.; Oubaha, M.; Mahiou, R.; Cavalli, E.; Bettinelli, M. Making Red Emitting Phosphors with Pr3+. Opt. Mater. 2006, 28, 9-13. 11. Boutinaud, P.; Cavalli, E.; Bettinelli, M. Emission Quenching Induced by Intervalence Charge Transfer in Pr3+- or Tb3+-doped YNbO4 and CaNb2O6. J. Phys.: Condens. Matter 2007, 19, 386230. 12. Zhang, S.; Liang, H. B.; Liu, C. M. Increased

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D2 Red Emission of Pr3+ in

NaGdTiO4:Pr3+ Due to Temperature-Assisted Host Sensitization and Its Color Variation. J. Phys. Chem. C 2013, 117, 2216-2221. 13. Boutinaud, P.; Mahiou, R.; Cavalli, E.; Bettinelli, M. Red Luminescence Induced by Intervalence Charge Transfer in Pr3+-Doped Compounds. J. Lumin. 2007, 122–123, 430-433. 14. Mahesh, S. K.; Rao, P. P.; Francis, T. L.; Reshmi, V. R.; Koshy, P. Intense Red Line Emitting Phosphor LuNbO4:Eu3+ for White Light Emitting Diode Applications. Mater. Lett. 2014, 120, 115-117. 15. Liu, X. M.; Lü, Y.; Chen, C.; Luo, S. L.; Zeng, Y. H.; Zhang, X. Q.; Shang, M. M.; Li, C. X.; Lin, J. Synthesis and Luminescence Properties of YNbO4:A (A = Eu3+ and/or Tb3+) Nanocrystalline Phosphors via a Sol–Gel Process. J. Phys. Chem. C 2014, 118, 27516-27524.


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TOC Graphic:


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Excitation Wavelength Dependent Luminescence of LuNbO4:Pr3+

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