Spectroscopy and Luminescence Dynamics of Ce3+ and Sm3+ in

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Spectroscopy and Luminescence Dynamics of Ce and Sm in LiYSiO Rui Shi, Jinzhong Xu, Guokui Liu, Xuejie Zhang, Weijie Zhou, Fengjuan Pan, Yan Huang, Ye Tao, and Hongbin Liang

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12501 • Publication Date (Web): 17 Feb 2016 Downloaded from http://pubs.acs.org on February 20, 2016

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The Journal of Physical Chemistry

Spectroscopy and Luminescence Dynamics of Ce3+ and Sm3+ in LiYSiO4

Rui Shi†, Jinzhong Xu†, Guokui Liu‡,*, Xuejie Zhang†, Weijie Zhou†, Fengjuan Pan†, Yan Huang¶, Ye Tao¶, Hongbin Liang†,*

†

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of

Environment and Energy Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China ‡

Chemical Science and Engineering Division, Argonne National Laboratory,

Argonne, Illinois 60439, USA ¶

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese

Academy of Sciences, Beijing 100039, P.R. China

* E-mail: [email protected] (Guokui Liu); [email protected] (Hongbin Liang)

*

To whom correspondence should be addressed.

ACS Paragon Plus Environment


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Abstract The lithium yttrium silicate series of LiY1-xLnxSiO4 exhibits superb chemical and optical properties, and with Ln = Ce3+, Sm3+, its spectroscopic characteristics and luminescence dynamics are investigated in the present work. Energy transfer and non-radiative relaxation dramatically influence the Ln3+ luminescence spectra and decay dynamics, especially in the Ce3+-Sm3+ co-doped phosphors. It is shown that thermal-quenching of the blue Ce3+ luminescence is primarily due to thermal ionization in the 5d excited states rather than multi-phonon relaxation, whereas cross-relaxation arising from electric dipole-dipole interaction between adjacent Sm3+ ions is the leading mechanism that quenches the red Sm3+ luminescence. In the co-doped systems, Ce3+-Sm3+ energy transfer in competing with the thermal quenching enhance the emission from Sm3+. The combined influences of concentration quenching, thermal ionization, and energy transfer including cross-relaxation on the luminescence intensity of single-center and co-doped phosphors are analyzed based on the theories of ion-ion and ion-lattice interactions.

Keywords: LiYSiO4; Ce3+; Sm3+; Luminescence; Energy transfer; Kinetics


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1

Introduction Within the 4f5 electronic configuration, Sm3+ usually exhibits orange-red

emission arising from 4G5/2-6HJ (J = 5/2, 7/2, 9/2, 11/2) transitions.1,2 For color rendering purposes, Sm3+ is used to offset the deficiency of red component in white light-emitting diodes (LEDs) and field emission displays (FEDs).3 From the application point of view, Sm3+ is also a promising activator for orange-red lasers for important applications in iatrology, astronomy, telecommunication and remote sensing.4,5 However, the visible emission of Sm3+ is often self-quenched by cross relaxation (CR) processes between adjacent Sm3+ ions. How to enhance its luminescent intensity is a challenge for the utilization of this rare-earth resource in luminescent materials. On the other hand, Ce3+ in 4f1 configuration has attractive optical properties due to the parity-allowed 4f-5d transitions tunable in the visible region. The luminescence of Ce3+ is often used as a reference to predict the vacuum referred binding energy (VRBE) of other rare earth ions in same host lattices, which can provide significant guidance to understanding the energy transfer (ET) processes of lanthanides.6,7 Furthermore, the large cross section of the 4f-5d transition and wide bandwidth induced by vibronic coupling make Ce 3+ an efficient sensitizer for the luminescence of other lanthanide or transition metal ions.8-10 For example, the sensitization of Tb3+ with Ce3+ has been applied in commercial green-emitting phosphors (Ce, Tb)MgAl10O19 and (La, Ce, Tb)PO4. Due to the combined influence of the crystal field strength, the nephelauxetic



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effect (covalency and spectroscopic polarization) and the electron-vibrational (vibronic) interaction, the wavelength of Ce3+ luminescence varies in a wide spectral range in different host compounds.11 Accordingly, it is expected that in a proper host compound the emission of Ce 3+ would occur near 400 nm, which matches well the dominant 6H5/2-6P3/2 excitation of Sm3+. Consequently, in a host co-doped with Ce3+ and Sm3+, Ce3+ performing as a blue-green emitter and at the same time as an efficient sensitizer to enhance the Sm 3+ red-emission would facilitate luminescence rendering and high quality white light emission. LiYSiO4 is an ideal host compound for this purpose, because the intense and broad emission of LiYSiO4:Ce3+ with a maximum at ~400 nm meets well the above criterion.12,13 In addition, the appropriate distance between two nearest adjacent Y3+ ions (~4.0516 Å) is suitable for weakening the cross relaxation processes of Sm3+.14,15 Although this host compound shows other excellent properties such as good chemical stability, easy preparation and single site occupancy for Y3+, information on the luminescence properties of doping lanthanide ions into LiYSiO4 is limited.12,13 To fully understand the combined impacts of the doping concentration, the Ce 3+-Sm3+ energy transfer (ET) and the Sm3+-Sm3+ cross relaxation (CR) on luminescence of Sm3+ in LiYSiO4, the luminescence and energy transfer kinetics of Ce 3+ and Sm3+ in lithium yttrium silicate LiYSiO4 were systematically investigated in the present work.

2

Experimental


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Four series of powder samples with Ce 3+, Sm3+, singly doped and Ce3+-Sm3+ doubly doped were prepared by a high temperature solid-state reaction route using raw materials Li2CO3 (A.R.), Y2O3 (99.99%), SiO2 (99.99%), Sm2O3 (99.99%) and CeO2 (99.99%). The excess of 5% Li2CO3 were added to compensate the volatilization losing of Li+ at high sintering temperature. According to the nominal chemical formulas LiY1-xCexSiO4 (x = 0, 0.001, 0.01, 0.03, 0.05, 0.07, 0.1), LiY1-xSmxSiO4 (x = 0.001, 0.01, 0.03, 0.05, 0.09) and LiY0.99-xCe0.01SmxSiO4 (x = 0.001, 0.01, 0.05, 0.09), the stoichiometric amount of raw materials were ground thoroughly in an agate mortar and shifted to an alumina crucible. Then under CO reducing atmosphere arising from the incomplete combustion of thermal carbon, they were heated to 1373 K in 3 h and kept at this temperature for reaction about 10 h. Finally, the samples were gradually cooled down to room temperature (RT) and ground into powder. The phase purity of all final samples was examined by X-ray powder diffraction using Cu K radiation (λ = 0.15405 nm) on a BRUKER D8 ADVANCE powder diffractometer at RT. The data were collected with the scanning speed 10°·min-1 and the scanning angle range 10°-70°. The XRD patterns of representative samples LiY1-xLnxSiO4 (Ln = Ce, Sm, Ce-Sm) at RT are shown in Figure S1, which are in good agreement with the relevant diffraction pattern and no second phase can be found, indicating that each



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sample is purity phase and the substitution of Y3+ by Ln3+ do not significantly influence the crystal structure. The UV excitation and emission spectra as well as the luminescence decay curves were measured on an Edinburgh FLS 920 combined fluorescence lifetime and steady state spectrometer. The temperature control system was equipped with a CTI-Cryogenics instrument. The excitation source for steady-state spectra was a 450W Xe900 lamp, and that for luminescence decay curves was a 150W F900 lamp with a pulse width of 1 ns and pulse rate of 40-100 kHz or a 60W μF flash lamp with a pulse width of 1.5-3 μs and pulse rate of 50 Hz, respectively. The vacuum ultraviolet (VUV) excitation and corresponding emission spectra were measured at the VUV spectroscopy experimental station on beam line 4B8 of Beijing Synchrotron Radiation Facility (BSRF). A 1 m Seya monochromator (1200 g·mm-1, 120-350 nm, 1 nm bandwidth) and an Acton SP-308 monochromator (600 g·mm-1, 330-900 nm) were used for the excitation and emission spectra measurement. The vacuum of measurement was about 2.4×10-5 mbar. The signal was detected by a Hamamatsu H8259-01 photon counting unit and corrected by the excitation intensity of sodium salicylate (o-C6H4OHCOONa) measured simultaneously in the same condition.

3

Results and discussion


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3.1 Luminescence Properties of LiY1-xCexSiO4

1.0

ex= 348 nm (3.56 eV)

(c)

1.0

2

Fit Curve ( F5/2) 2

Fit Curve ( F7/2)

0.8

Total Fit

0.6

Normalized Intensity (a.u.)

2

F7/2

0.4

0.8 2

F5/2

0.2 0.0

0.6

0.4

3.4

3.2

3.0 2.8 Energy (eV)

2.6


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LiY0.999Ce0.001SiO4, 15 K

5d1

em= 400 nm

4f-5d1 (~3.54 eV) 4f-5d2 (~3.93 eV) 4f-5d3 (~4.07 eV) 4f-5d4 (~6.03 eV) 4f-5d5 (~6.71 eV)

2.4

Fit Curve

5d2

5d3

(b) 5d5

Host Absorption (~8.05 eV)

5d4

(a)

0.2 5d5 9

8

7

6

5d4

0.0 10

9

8

7

6

5

4

Energy (eV)

Figure 1. (a) VUV excitation spectrum of Ce 3+ in LiY0.999Ce0.001SiO4 of 400 nm emission at 15 K; (b) the enlargement in 5.0-9.5 eV region of curve a; (c) emission spectrum of Ce 3+ under 348 nm excitation.

Figure 1 (a) represents a VUV excitation spectrum of Ce3+ in LiY0.999Ce0.001SiO4 of 400 nm emission at 15 K. Three bands were observed below 5.0 eV. Their peaks were estimated to be ~3.54 eV (~350 nm, ~28.6×103 cm-1), ~3.93 eV (~315 nm, ~31.7×103 cm-1), and ~4.07 eV (~305 nm, ~32.8×103 cm-1) by fitting the spectrum with a sum of three Gaussian functions. The enlargement in 5.0-9.5 eV region is plotted in curve b. The spectrum in this range also contains three bands with maxima at ~6.03 eV (~206 nm, 48.6×103 cm-1), ~6.71 eV (~185 nm, 54.1×103 cm-1), and ~8.05 eV (~154 nm, 64.9×103 cm-1), respectively. It is clear that these bands correspond to the silicate lattice absorption (~8.05 eV, ~154 nm) and five Ce3+ 4f-5d



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excitation bands.16-18 Accordingly, the Ce3+ 5d centroid energy in LiYSiO4 shifts down approximately 1.49 eV (~12.0×103 cm-1) with respect to that in gaseous state (~6.35 eV, ~51.2×103 cm-1).19 This value is in the typical range of ~10500-12650 cm-1 for Ce3+ in most silicates.20 The crystal field splitting (CFS) of the Ce3+ 5d levels is closely correlated to the shape of coordinated polyhedral, the average Ce-O bond length and site symmetry of Ce3+ in the host lattice.19 According to the crystal structure data, Ce3+ substituting Y3+ should locate at the centre of a distorted octahedron (CN = 6). This characteristic structure results in a shorter average Ce-O bond length in comparison with most of other silicates. Therefore, Ce3+ would experience a stronger crystal field and the crystal field splitting (CFS) for the Ce3+ 5d states in LiYSiO4 (~3.17 eV, ~26.0×103 cm-1) is larger than that in most of other silicates.21 With 348 nm (~3.56 eV, ~28.7×103 cm-1) excitation, a broad emission band centred at 400 nm (~3.10 eV, ~25.0×103 cm-1) was observed as shown in Fig. 1 (c), and attributed to the transitions from the lowest 5d state (denoted as 5d1) of Ce3+ to its 2

F5/2 and 2F7/2 spin-orbit states of the 4f1 configuration. The 2F5/2 and 2F7/2 bands aren’t

separated in the emission spectrum, which is primarily because of multi-phonon vibronic transitions similar to that of Ce3+ in Y3Al5O12.22 Given that the energy levels of these two multiplets are expected to separate only by approximately 2000 cm-1 (~0.16 eV) and the typical value of the Huang-Rhys vibronic parameter is about 1 for the 5d-4f transitions, progressions of multiple vibronic bands would effectively obscure not only the crystal-field splitting of the 2F5/2 and 2F7/2 multiplets but also the


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gap between them. Moreover, the Stokes shift ΔEs of Ce3+ in the Y3+ site is calculated to be ~3.74×103 cm-1 (~0.46 eV), which is close to the value of Ce3+ in most compounds.23

0.9

(a) LiY Ce SiO 1-x x 4

em = 420 nm

RT Normalized Intensity (a.u.)

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0.6

0.1% 1% 3% 5% 7% 10%

0.3 0.0 260 1.2

280

300

320

340

360

ex = 348 nm

(b)

0.1% 1% 3% 5% 7% 10%

0.9 0.6 0.3 0.0 360

380

380

400

420

440

460

480

500

Wavelength (nm)

Figure 2. The highest height normalized excitation (a), and emission (b) spectra of LiY1-xCexSiO4 with different Ce3+ contents at RT.

The normalized excitation and emission spectra of LiY1-xCexSiO4 as a function of doping concentration x = 0.001, 0.01, 0.03, 0.05, 0.07 and 0.1 are shown in Figure 2 (a, b) for comparison. With the increase of Ce3+ content, both the lowest f-d excitation band and the emission peak shift to longer wavelength, indicating lowering of the energy level of the emitting Ce3+ 5d state. Assuming that substitution of the larger Ce3+ for Y3+ with a smaller ionic size is accompanied by lattice expansion, the crystal-field splitting (CFS) should increase in some extent, thus, lowering the emitting state and inducing red shift in both excitation and emission spectra. It is also



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conceivable that the nephelauxetic effect may also contribute to the energy level shift.18 Moreover, variation in vibronic interaction also contributes to the shifts of the excitation and emission maxima. Usually, a stronger vibronic interaction should broaden both excitation and emission bands, therefore induces effectively a blue shift in excitation and a red shift in emission.22, 24 After measured each curve in Figure 2, we obtained a slightly increase in Stokes shift of approximately 100 cm-1 with an increase of Ce3+ concentration from 0.1 to 10%. The red shift in both emission and excitation spectra and insignificant increase in Stokes shift suggest that vibronic interaction is less impacted by doing Ce3+ in the lattice. It is also noticed that, due to different radii of Ce3+ and Y3+, a higher Ce3+ concentration could induce lattice defects, therefore, leads to inhomogeneous broadening and shift the emission band to the long-wavelength side. The excitation spectra of a concentrated sample with 10% Ce3+ (LiY0.9Ce0.1SiO4) were collected at different emission wavelengths (400, 420, 445 and 460 nm) at RT as shown in Figure S2. The results indicated there was no obvious inhomogeneous broadening, so its influence on the shift of the emission band can be neglected. The luminescence decay curve of LiY0.999Ce0.001SiO4 strictly obeys the exponential characteristic, which can be fitted by equation (1), (1) where I(t) denotes the luminescence intensities at time t, and τ is the decay time. The fitted decay time of the Ce3+ f-d transition is ~38.1 ns. The decay curves of samples with different doping contents (0.001, 0.01, 0.03, 0.05, 0.07, 0.1) shown in Figure S3


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are all exponential and have the same decay times of ~38±0.8 ns, indicating that concentration quenching does not occur for Ce3+ content below x = 0.1 in LiY1-xCexSiO4. The excitation and emission spectra of LiY0.999Ce0.001SiO4 at different temperatures are shown in Figure S4 (a, b), respectively. The lowest Ce3+ 5d excitation band is nearly invariant at different temperatures, which suggests that temperature has little influence on the coordination surroundings of Ce3+ in the investigated temperature range. For the emission spectra, the relative intensity ratio of the transitions to the Ce3+ 2F5/2 and 2F7/2 states changes slightly with increasing temperature, which is related to somewhat stronger self-absorption at higher temperature due to thermal broadening of the excitation and emission bands. As a result, the integrated intensity of Ce3+ emission decreases gradually with increasing temperature as shown in the inset of Figure S4.

38 Decay Time (ns)

1000

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


100

300 K 330 K 360 K 390 K 420 K 450 K 480 K 500 K

10

36 E=~0.65 eV

34

32

300

350 400 450 Temperature (K)

500

LiY0.999Ce0.001SiO4 ex=348 nm, em=400 nm 1 0

100

200

300

400

Time (ns)

Figure 3. The decay curves of LiY0.999Ce0.001SiO4 at different temperatures; the inset shows the temperature-dependent decay time and the fitting result using equation (2).



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Figure 3 displays the decay curves of LiY0.999Ce0.001SiO4 at different temperatures. A small increase of the measured decay time between 300-390 K is likely related to self-absorption.25 When temperature is higher than 390 K, the decay time of the Ce3+ luminescence undergoes a rapid decrease. It is worth mentioning that, as shown in the inset of Figure S4, the decrease of the luminescence intensity as a function of temperature starts at 300 K where the decay time is almost invariant below 400 K. Generally, luminescence intensity can be affected by temperature-dependent oscillator strength and distributed scattering centers, which leads to possible thermally activated non-radiative energy relaxation at higher temperature.26-28 Accordingly, we believe that the variation of decay time with temperature in the inset of Figure 3 would give a more reliable characterization of thermal quenching. When we consider that the decrease of the decay time is mainly caused by thermal-ionization of the 5d electron,18, 29

the dependence of decay time τ on temperature T (K) would be given by equation

(2), (2) where τ(0) is intrinsic decay time of the Ce3+ excited state, A is a pre-exponential factor denoting the rate constant for the thermally activated escape25, ΔE is the energy gap between the 5d1 energy level and exciton level,30 k is Boltzmann constant [8.6173324(78)×10-5 eV/K−1]. With A = 5.3×105 and ΔE = 0.65 eV, the fitting result is plotted in the inset of Figure 3.


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It is obvious that thermal-ionization from the lowest 5d1 state into the conduction band at 0.65 eV (~5.24×103 cm-1) above is a strongly collective operation of a number of phonons. In competition, multi-phonon relaxation down to the 4f states would also contribute. To evaluate the possibility of multi-phonon relaxation, a comparison of experimental decay data and simulation is shown in Figure S5. The simulation was made using the following energy gap law (3) 31, 32 (3) where τ(0) is the decay time of Ce3+ at 0 K, hν is the effective phonon energy, p is the number of phonons, and k is Boltzmann constant as given above. In silicate lattice, we assume that the high-energy vibrational modes have an average energy at hν = ~800 cm-1.33 In our case, the emitting state (the lowest 5d state) is approximately at ~28000 cm-1 and the 2F7/2 is at ~3000 cm-1, so the energy gap is ~25000 cm-1. Therefore, the p=ΔE/hν is estimated to be ~31, which is the number of phonons involved to bridge the energy gap between the lowest 5d state and 2F7/2 ground state. Shown in Figure S5, the simulation predicts that the decay time is expected to significantly drop with temperature near 300 K, which is in contradiction with the experimental results and suggests that in this system the ion-lattice interaction induced multi-phonon relaxation is weak. Nevertheless, in competing with the mechanism of thermal ionization into CB, multi-phonon relaxation down to the

2

F5/2,

7/2

states should have some

contribution at elevated temperature region.



0

CB

-1

E

-2 -3 -4

VRBE (eV)

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

2+

4f (Ln ) 2+ 5d (Ln ) 3+ 4f (Ln ) 3+ 5d (Ln LS) 3+ 5d (Ln HS) Exciton Level

-5 -6 -7 -8

LiYSiO4

-9 -10 -11

VB La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 0

2

4

6

8

10

number of electrons n in the 4f shell of Ln

12

3+

14

Figure 4. VRBE scheme of lanthanide 4f and 5d states in LiYSiO4.

To confirm the thermal ionization mechanism and the activation energy of 0.65 eV extracted from fitting the decay dynamics, the vacuum referring binding energy (VRBE) scheme for the lanthanide series in LiYSiO4 was constructed and shown in Figure 4. The method to construct VRBE scheme has been reported in previous literatures.19, 34, 35 To construct this scheme, several experimental data have been used, which include (1) the descending of 5d centroid energy of Ce3+ in Y3+ site with respect to that in free gaseous state (~1.49 eV); (2) the Eu3+-O2- charge transfer energy ~5.34 eV for Eu3+ in LiYSiO4 as shown in Figure S6; (3) the fundamental absorption energy ~8.05 eV of LiYSiO4 and (4) the lowest f-d transition energy of Ce3+ ~3.54 eV in Figure 1, respectively. Resulted from the construction of VRBE, the difference between the 5d energy level of Ce3+ and the bottom of conduction band is approximately 1.46 eV, which is larger than the activation energy of 0.65 eV evaluated from fitting the experimental temperature-dependent decay times. Such


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mismatch is possible because the construction of VRBE scheme is based on the state following Frank-Condon principle while the estimated activation energy corresponds to the energy difference between Ce3+ 5d1 state and the bottom of conduction band after lattice relaxation.20, 34-37 3.2 Properties of LiY1xSmxSiO4 100000 140000

LiY0.999Sm0.001SiO4, RT 4

6

em=564 nm ( G5/2- H5/2) 6

120000

6

6

ex=401 nm ( H5/2- P3/2)

6

80000

H5/2- P3/2 4

6

G5/2- H5/2

4

60000

6

G5/2- H7/2

80000 4

6

G5/2- H9/2

60000

40000

40000

4

Intensity (a.u.)

100000

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


6

G5/2- H11/2

20000 20000

(b)

(a) 0

0 300

400

500

600

700

Wavelength (nm)

Figure 5. (a) Excitation (b) emission spectrum of LiY0.999Sm0.001SiO4 at RT.

Figure 5 presents the excitation and emission spectra of Sm3+ in LiYSiO4 at RT, respectively. In the excitation spectrum of Sm3+ at 564 nm emission, a number of lines can be unambiguously assigned to transitions from the 6H5/2 ground state to the excited multiplets within the 4f5 electron configuration.38, 39 In the emission spectrum under 401 nm excitation, the lines are attributed to transitions from the 4G5/2 excited state to 6HJ (J = 5/2, 7/2, 9/2, 11/2) ground states of Sm3+, respectively. The excitation and emission spectra with different Sm3+ contents are measured under above conditions (and shown in Figure S7). All normalized spectra are invariant with increasing of the doping contents in spite of slight change of the



relative intensities, which may relate to the slight site symmetry distortion and lattice expansion of Sm3+ when the doping content increases. 1

(a)

LiY0.999Sm0.001SiO4

LiY0.99Sm0.01SiO4

LiY0.97Sm0.03SiO4

LiY0.95Sm0.05SiO4

LiY0.91Sm0.09SiO4

LiY0.85Sm0.15SiO4

3

In[-In(I(t)/I(0))-(t/)]

Fitting result by I-H method


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0.1

2

LiY0.99Sm0.01SiO4

3/s ~ 0.47

LiY0.97Sm0.03SiO4

3/s ~ 0.48

LiY0.85Sm0.15SiO4

3/s ~ 0.47

(b)

1 0 -1 SAveraged ~ 6.35 (D-D interaction)

-2 12

14

16

In t

ex=401 nm

0.01

em=564 nm

RT

0

5

10

15

20

Time (ms)

Figure 6. (a) Decay curves of Sm3+ with different contents at RT; the red squares denote the fitting results by Inokuti-Hirayama (I-H) model; (b) the fitting results based on equation (7).

Figure 6 shows the luminescence decay curves of Sm3+ in LiYSiO4 with different doping concentrations. It is clear that the decay curve of LiY0.999Sm0.001SiO4 is exponential and decay time is ~2.8 ms. With the increase of Sm3+ content, the curves deviate gradually from exponential. Generally, the decay dynamics of the luminescence from isolated emitting centre is expected to be exponential; and the non-exponential behaviour arises from energy transfer (ET) which is cross-relaxation (CR) in Sm3+ phosphors. Several CR channels have been previously reported: (4G5/2,6H5/2 - 6F11/2,6F5/2),40, 41 (4G5/2,6H5/2 - 6F9/2,6F9/2),42-44 (4G5/2,6H5/2 - 6F5/2,6F11/2),45, 46 4

( G5/2, 6H5/2 - 6F9/2,6F7/2),47, 48 (4G5/2,6H5/2 - 6F7/2,6F9/2).49, 50 With the increase of Sm3+

content, the average distance between Sm3+ becomes shorter, resulting in more


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efficient CR processes. Here, the Inokuti-Hirayama (I-H) model of donor to acceptor energy transfer 51 is applied in analysis of the decay curves of Sm3+ luminescence. (4) where I(0) denotes the intensity at t = 0, t0 denotes the donor decay time in the absence of acceptor; s denotes the multipolar effect parameter that the value s = 6, 8, 10 represents dominant mechanism being dipole-dipole, dipole-quadrupole or quadrupole-quadrupole interaction, respectively; Q is the macroscopic parameter that determines the effect of multipolar interaction in decay property: (5) is a gamma function; CA is the acceptor content; CDA denote the CR kinetic parameter defined by: (6) where RSA denotes the distance between donors and acceptor; 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 content”, respectively. For the purpose of defining the type of multipolar effect, the experimental decay curves were fitted using equation (7) and the results are plotted in the Figure 6(b). (7) where B is a control factor. The plot yield straight lines with the slope equal to 3/s. According to the fitting results (all Adj.R2 data are beyond 0.99, t0 values are approximately equal to 2.8 ms), s value is estimated to be about ~6.35, showing that



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the main multipolar effect is dipole-dipole interaction for Sm3+ in LiY1-xSmxSiO4.43 Q values increase from 4.51×10-4 for x = 1% to 2.61×10-3 for x = 9%, demonstrating that dipole-dipole interaction becomes more remarkable with the increase of doping content. Besides, the CA(k) value is calculated to be ~1.6%, which suggests a critical doping content. Accordingly, the CR kinetic microparameter CDA is evaluated to be ~3.55×10-50 m6·s-1, which is relatively larger than Sm3+ in other lattices, denoting the more efficient CR processes for Sm3+ occurring in LiYSiO4.39, 52 The decay curves of Sm3+ in LiY0.999Sm0.001SiO4 at different temperatures are shown in Figure S8. Sm3+ can be regarded isolated in this lowing doping sample and the CR effect can be negligible. All curves at different temperatures exhibit the exponential characteristics. The temperature dependence of decay time and integrated emission intensity of Sm3+ are shown in the inset of Figure S8. With the increase of temperature in the experimental region, the decay time of Sm3+ remains largely unchanged, which is accordant with the tendency of emission intensity. This behaviour is because of the large energy gap (~7407 cm-1) between the 4G5/2 emitting state and top state of the 6F11/2 multiplet, and also due to the inefficient multi-phonon relaxation in this compound. Also, because of the large energy difference between excited state 4G5/2 and exciton level, the thermal ionization effect of Sm3+ is negligible. More fundamentally, in comparison with the distinctive behaviours of Ce3+ in the same host, Sm3+ in the localized 4f states exhibits much weaker ion-lattice interactions. As a result, its spectroscopic properties and luminescence dynamics are less influenced by the host lattice, rather than its own electronic structure.


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3.3 Energy transfer between Ce3+ and Sm3+ in LiY0.99-xCe0.01SmxSiO4

em= 564 nm, LiY0.98Ce0.01Sm0.01SiO4 (a)

1.0

ex=348 nm, LiY0.98Ce0.01Sm0.01SiO4 (b1)

0.8

Enlargement of curve b1 Enlargement of curve b2

ex=348 nm, LiY0.99Sm0.01SiO4 (b2)

250000

RT

200000

(c)

0.6

150000

0.4

0.2

300000

100000

(a)

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


50000

4

10+5x10

(b1, b2) 0

0.0 250

300

350

400

450

500

550

600

650

700

Wavelength (nm)

Figure 7. (a) Excitation spectrum of LiY0.98Ce0.01Sm0.01SiO4 of Sm3+ 564 nm emission; (b1) emission spectrum of LiY0.98Ce0.01Sm0.01SiO4 upon 348 nm excitation, (b2) emission spectrum of LiY0.99Sm0.01SiO4 under 348 nm excitation; (c) is the enlargements in Sm3+ emission region of curves b1 and b2.

Figure 7 (a) shows the excitation spectrum of LiY0.98Ce0.01Sm0.01SiO4 of Sm3+ 564 nm emission. Clearly, this curve contains not only f-f transitions of Sm3+ but also 4f-5d excitation bands of Ce3+, implying the occurrence of energy transfer (ET) from Ce3+ to Sm3+ via 5d state of Ce3+. As shown in curves b1, b2 and plot c, the comparison of the emission spectra of singly doped sample LiY0.99Sm0.01SiO4 and the doubly doped sample LiY0.98Ce0.01Sm0.01SiO4 immediately confirms the existence of this energy transfer. The excitation wavelength 348 nm corresponds to the lowest 4f-5d excitation band of Ce3+, but Sm3+ has weak absorption at this wavelength as shown in Figure 5 by blue dotted arrow. The emission intensity of Sm3+ in doubly doped sample is evidently stronger than that in singly doped sample.



18

(a)

1

LiY0.989Ce0.01Sm0.001SiO4

16

(c)

LiY0.99Ce0.01SiO4




14



LiY0.94Ce0.01Sm0.05SiO4 LiY0.9Ce0.01Sm0.09SiO4

12 ex=348 nm, RT

4


10

Intensity (10 )

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

8 6 4 2 0 12

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ex=348 nm, em=401 nm, RT

0.1

0.01

LiY0.999Sm0.001SiO4

(b)

9

em=564 nm, RT

6 3 0 350

375

400

425

450

475

500

525

0

50

100

Wavelength (nm)

150

200

250

300

Time (ns)

Figure 8. (a) Emission of Ce3+ in LiY0.99-xCe0.01SmxSiO4 with different Sm3+ contents (x = 0.001, 0.01, 0.05, 0.09); (b) excitation spectrum of Sm3+ 564 nm emission in LiY0.999Sm0.001SiO4; (c) decay curves of Ce3+ in LiY0.99-xCe0.01SmxSiO4 with different Sm3+ contents (x = 0, 0.001, 0.01, 0.05, 0.09).

With the increase of Sm3+ contents in LiY0.99-xCe0.01SmxSiO4 (x = 0.001, 0.01, 0.05, 0.09), the emission of Ce3+ in Figure 8 (a) showed a gradually enhanced dip in the range of 400-425 nm. In consideration of the sufficient overlap between Ce3+ emission and Sm3+ f-f excitation bands as shown in Figure 8 (b), the enhanced dip in the spectra is clearly the result of the reabsorption of Sm3+. Besides, the Ce3+ emission intensities decrease rapidly with the increase of Sm3+ contents. All these phenomena demonstrate that the ET between Ce3+ and Sm3+ occurs effectively in the Ce3+-Sm3+ co-doped samples. This ET process can be further confirmed by Figure 8(c), the decay curves of Ce3+ in LiY0.99-xCe0.01SmxSiO4 samples at RT. For Ce3+ single doping sample, the decay


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curve is exponential. Co-doping of Sm3+ leads to the shortening of the decay time of Ce3+ in LiY0.99-xCe0.01SmxSiO4. When Ce3+ concentration is fixed but Sm3+ concentration increases, there would be more Sm3+ ions around Ce3+, which leads to more efficient energy transfer from Ce3+ to Sm3+. Thereby, the Ce3+ decay becomes faster with the increase of Sm3+ concentration.

3.0

(b)

LiY0.989Ce0.01Sm0.001SiO4 LiY0.98Ce0.01Sm0.01SiO4 LiY0.94Ce0.01Sm0.05SiO4

2.5

LiY0.9Ce0.01Sm0.09SiO4 4

Intensity (10 )

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


2.0

ex=348 nm, RT 1.5

1.0

0.5

0.0

550

575

600 625 Wavelength (nm)

650

Figure 9. Emission spectra of Sm3+ in LiY0.99-xCe0.01SmxSiO4 (x = 0.001, 0.01, 0.05, 0.09) at RT.

Figure 9 shows the emission spectra of Sm3+ in LiY0.99-xCe0.01SmxSiO4 with different Sm3+ contents (x = 0.001, 0.01, 0.05, 0.09) at RT. With increasing of the Sm3+ contents, the Sm3+ emission increases initially then decreases gradually. For the most diluted sample with Sm3+ doping concentration x = 0.001, the emission intensity is lowest because of the lowest Ce3+→Sm3+ ET efficiency in this sample. The critical doping content of Sm3+ in LiY1-xSmxSiO4 has been estimated to be ~1.6% as mentioned before. Accordingly, we consider that the increase of Sm3+ emission



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intensity with increasing Sm3+ concentration in low doping contents (0.1% to 1%) is attributed to the combined effects of two factors. The increase of ET efficiency would be a dominate factor, meanwhile the increase of Sm3+ absorption at 348 nm also contributes in some extents. When the Sm3+ content is beyond its critical level, the luminescence intensity of Sm3+ decreases gradually, because CR becomes dominant in the high doping range.

4

Conclusion Synthesized using a high temperature solid-state reaction method, LiYSiO4 doped

with Ce3+, Sm3+, and Ce3+-Sm3+ exhibits fascinating phosphor properties. In LiYSiO4:Ce3+, the blue photoluminescence exhibits no concentration quenching up to 10% Ce3+ doping and no thermal quenching below 450 K. These excellent luminescence properties are primarily due to the relatively long distance between neighbouring Y locations and weak ion-lattice interactions. As a result, multi-phonon relaxation takes action at a temperature much higher than that predicted by the energy-gap law, and thermal ionization in the excited 5d state plays a leading role. It is not surprising that because of multiple channels of near resonance or small energy mismatches, cross-relaxation is the most important mechanism to quench the red Sm3+ luminescence. What further accomplished in the present work is that the critical Sm3+ concentration in the LiYSiO4 host we calculated based on the I-H energy transfer theory (1.6%) is in good agreement with that derived from the experimental results.


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The present work shows that Ce3+-Sm3+ co-doped in LiYSiO4 is a good match for Ce3+ to perform as a blue emitting centre and a sensitizer of Sm3+ to enhance the red luminescence. Three factors influence the emission intensity of Sm3+ in the Ce3+-Sm3+ co-doped systems, which include the concentration of Sm3+, the CR processes between Sm3+, and the ET processes from Ce3+ to Sm3+. With fundamental understanding of the mechanisms that influence these factors, controlling and adjusting these factors in materials preparation can be achieved for developing advanced phosphors of particular properties. Because of the attractive luminescence properties of LiYSiO4 with lanthanide doping, LiYSiO4: Ce3+, Sm3+ phosphors are of potential for application in white light-emitting-diodes (LEDs) with balanced spectral profile in blue, green and red.

ASSOCIATED CONTENT Supporting Information XRD patterns of LiY0.99Ln0.01SiO4 (Ln= Ce, Sm, Ce-Sm) (Figure S1), the normalized excitation spectra of LiY0.9Ce0.1SiO4 at different monitoring emission wavelengths (Figure S2), the decay curves of LiY1-xCexSiO4 with different Ce3+ contents at RT (Figure S3), excitation and emission spectra of LiY0.999Ce0.001SiO4 at different temperatures and the variation of Ce3+ integrate emission intensity with temperatures (Figure S4), the comparison of experimental decay data and simulation curve (Figure S5), the VUV excitation spectrum of LiY1-xEuxSiO4 (x=0.01) (Figure S6), the excitation and emission spectra of LiY1-xSmxSiO4 under different doping



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contents at RT (Figure S7), the decay curves of LiY0.999Sm0.001SiO4 at different temperatures and the variation of decay time and integral emission intensity with temperature (Figure S8). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *G.K. Liu: e-mail, [email protected]; tel, +01-630-252-4630; fax, +01-630-252-4225. *H.B. Liang: e-mail, [email protected]; tel, +86-20-84113695; fax, +86-20-84111038.

Notes The authors declare no competing financial interest.

Acknowledgement The work is financially supported by the National Natural Science Foundation of China (21171176, U1232108, and U1432249), and the Natural Science Foundation of Guangdong Province (S2013030012842). G. L. acknowledges the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences through Grant DE-AC02-06CH11357 for funding the work on heavy elements chemistry, and travel support from the CAS/SAFEA International Partnership Program for Creative Research Teams.


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49 Lupei, A.; Tiseanu, C.; Gheorghe, C.; Voicu, F. Optical Spectroscopy of Sm3+ in C2 and C3i Sites of Y2O3 Ceramics. Appl. Phys. B 2012, 108(4), 909-918. 50 Sobczyk, M.; Szymański, D. Optical Properties of Sm3+-Doped Y2Te4O11. J. Lumin. 2015, 166, 40-47. 51 Inokuti, M.; Hirayama, F. Influence of Energy Transfer by the Exchange Mechanism on Donor Luminescence. J. Chem. Phys. 1965, 43(6), 1978-1989. 52 Solarz, P.; Ryba-Romanowski, W. Luminescence and Energy Transfer Processes of Sm3+ in K5Li2LaF10: Sm3+− K5Li2SmF10 Single Crystals. Phys. Rev. B: Condens. Matter 2005, 72(7), 075105(1-8).



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Spectroscopy and Luminescence Dynamics of Ce3+ and Sm3+ in

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