Spectral Properties and Energy Transfer between Ce3+ and Yb3+ in

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Spectral Properties and Energy Transfer Between Ce # Yb in the CaScSiO Host: Is It an Electron Transfer Mechanism? 3

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Lei Zhou, Peter A. Tanner, Lixin Ning, Weijie Zhou, Hongbin Liang, and Lirong Zheng J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b04641 • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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The Journal of Physical Chemistry

Spectral Properties and Energy Transfer Between Ce3+ → Yb3+ in the Ca3Sc2Si3O12 Host: Is It an Electron Transfer Mechanism? Lei Zhou†, Peter A. Tanner*‡, Lixin Ning§, Weijie Zhou†, Hongbin Liang*†, and Lirong Zheng¶ †

MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic

Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, P.R. China. ‡

The Hong Kong Institute of Education (The Education University of Hong Kong, designate), 10 Lo

Ping Road, Tai Po, Hong Kong S.A.R., P.R. China §

Center for Nano Science and Technology, Department of Physics, Anhui Normal University, Wuhu,

Anhui 241000, P.R. China ¶

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

Sciences, Beijing 100039, P.R. China

ABSTRACT: The downshifting from Ce3+ blue emission to Yb3+ near infrared emission has been studied in the garnet host Ca2.8-2xCe0.1YbxNa0.1+xSc2Si3O12 (x = 0-0.36). The downshifting does not involve quantum cutting but one incident blue photon is transferred from Ce3+ to Yb3+ with an energy transfer efficiency up to 90% when x = 0.36 for the Yb3+ dopant ion. For x ≤ 0.15, a multiphononassisted electric dipole – electric quadrupole mechanism of energy transfer dominates, whilst for the highest concentration of Yb3+ employed, the electron transfer mechanism is confirmed. A temperaturedependent increase of the Ce3+ → Yb3+ energy transfer rate does not exclusively indicate the electron transfer mechanism. The application of the material to solar energy conversion is indicated.

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ACS Paragon Plus Environment


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1. INTRODUCTION The energy transfer from Ce3+ to Yb3+ has been studied several times with the noble aim of quantum cutting from blue to infrared. A study in 2008 reported a downconversion quantum efficiency of 174% for this energy transfer process in borate glass, since the Ce3+ 5d1 → 4f1 emission is located at approximately twice the energy of the Yb3+ 4f 13 2F5/2→ 4f13 2F7/2 transition, with no intervening energy levels of single Yb3+ or Ce3+ ions.1 In fact this quantum cutting viewpoint is still partly held in recent studies, for example in order to explain the energy transfer efficiency in Ba2Y(BO3)2Cl:Ce3+,Yb3+ 2 and in Gd3(Al,Ga)5O12:Ce3+,Yb3+.3 However, it had been earlier suggested, such as from the luminescence study of LiYbF4:Ce3+,4 that the quenching of Ce3+ emission was due to an electron transfer process so that one ultraviolet photon could only be converted into one infrared photon. These authors commented that electron transfer between ions “a” and “b” will readily occur when there is an opposing valency change. Setlur and Shiang used Marcus Theory of electron transfer to rationalize the energy transfer between Ce3+ 5d1 donors and Yb3+ acceptors.5 The parameters β (a decay constant, related to the tunneling barrier) and λ (the reorganization energy in the Franck-Condon charge exchange) were elucidated. In the case of this electron transfer reaction involving Ce3+ 5d1(1) → 5d0 Ce4+ and Yb3+ 4f13 → 4f14 Yb2+, the free energy change was given as: ∆G0 = EPI + ECTB – Eg where EPI is the energy required to raise the electron from 5d(1) to the conduction band (CB); ECTB is the charge transfer energy of Yb3+; and Eg is the band gap energy of the host lattice. Thus, as long as the 5d(1) level lies above the Yb2+ ground state energy in the vacuum referred binding energy (VRBE) diagram, the transfer process 2


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is energetically possible.6,7 Notice that in this case the electron transfer involves two spin doublet initial states and two spin singlet final states. Sontakke et al.6,7 also pointed out that even if the electron transfer process dominates the energy transfer process it may not lead to excitation of the acceptor ion. An increase in energy transfer efficiency from Ce3+ to Yb3+ with increasing temperature up to 500 K was observed and was attributed to the thermally-activated crossover from the CT state to the Yb3+ ground state which takes place besides the quantum tunneling process. From the quantitative analysis of configuration coordinate diagrams, Barandiarán et al.8 have provided an explanation for the temperature quenching of Yb3+ emission in YAG: Ce3+, Yb3+. The production of Yb2+ ions in the CT state leads to Yb2+-Yb3+ pairs. A nonradiative crossover with a small energy barrier of 200 cm-1 was proposed for the mixed valence pair strong quenching of Yb3+ emission. These authors also considered the problem of the representation of the mixed metal CT state Ce4+-Yb2+ and the Ce3+-Yb3+ states in a single diagram. The quenching of Ce3+ 5d1 → 4f1 emission in YAG above 110 K 9 was explained using this diagram. First, the Ce3+ 5d1 - Yb3+ 4f13 2F7/2 state can decay nonradiatively to the metal-metal charge transfer (MMCT) state Ce4+ 5d0 -Yb2+ 4f14 with a very small energy barrier (73 cm-1). Finally, the Ce4+-Yb2+ MMCT state can decay directly to the Ce3+ 4f1 - Yb3+ 4f13 2F5/2 dimer manifold, which can yield Yb3+ 2F5/2 → 2F7/2 emission with no rise time. A high temperature crossover is possible from the MMCT state to Ce3+ 4f1 - Yb3+ 4f13 2F7/2 but the low temperature quenching of the Yb3+ 2F5/2 emission was attributed to the formation of Yb3+ - Yb2+ pairs as mentioned before. The electron transfer process and the Dexter exchange mechanism 10 both involve an exponential dependence of the transfer probability upon donor-acceptor distance. 3



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Although the exchange mechanism also involves orbital overlap, it is based upon Coulomb repulsion. The spectral overlap between donor emission and acceptor absorption also figures in the Dexter exchange transfer probability, just as in resonant transfer by multipole-multipole interactions.11 Sontakke et al. have distinguished the occurrence of electron transfer from that of multipolar interactions in that the former depends on the absolute energy of the electron in donor and acceptor states, whereas the later mechanism needs the resonant condition of donor-acceptor electronic transitions.7 In addition, it is important to take into account physical distance of donoracceptor pairs when investigating the appropriate dominant mechanism of energy transfer since electron transfer involves wavefunction overlap. The energy gap between the Ce3+ donor and Yb3+ acceptor states is large so that if multipolar interactions are operative the transfer is nonresonant. Multiphonon-assisted nonresonant energy transfer was considered by Miyakawa and Dexter12 and Yamada et al.13 In this case, since there is no observable overlap between the emission of the donor and absorption of the acceptor, the lineshape function was expanded into multiphonon components and the resonant transfer rate was modified by an exponential factor. From this theory, in the present case of Ce3+ - Yb3+ transfer, assuming a difference of ~5500 cm-1 between the donor and acceptor nonradiative transitions and an illustrative effective phonon energy of ~900 cm-1 for the host, the energy transfer rate is expected to increase by a factor of 2.5 from 77 K to 500 K. As we have stressed in a previous study concerning Ce3+-Eu2+ energy transfer,14 the Ca3Sc2Si3O12 (CSS) host conveys unique properties to doped lanthanide ions. For example, in the 5D0 emission spectrum of Eu3+ the strongest transition is to the terminal 5D4 J-multiplet, and the emission of Eu2+ in this host is located at near infrared (NIR) wavelengths. In the present work the transfer mechanism from Ce3+ to 4


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Yb3+ in this host has been investigated. As mentioned above, current opinion holds the view that this energy transfer involves the charge transfer state Ce4+- Yb2+. Although the fitted curves for this mechanism in Y3Al5O12:Ce0.001Ybx (x = 0.01, 0.04) in Ref. 5 (Fig. 2) are in good agreement with the experimental decay curves, those in Y3Al5O12:Ce0.01Ybx in Ref. 9 (Fig. 3: being based upon x = 0.1) are not. This prompted our investigation using an alternative host lattice. The experimental details are presented in the Supporting Information (SI). We commence with a review of the structure and properties of the CSS host and the spectra of this host doped with Yb3+. Then, the energy transfer process between Ce3+ and Yb3+ is presented and discussed. Some conclusions are formulated, together with the potential applications of this study. The extensive figures and additional information in the SI should be at hand when reading the manuscript.

2. RESULTS AND DISCUSSION 2.1. Structure. The compound Ca3Sc2Si3O12 (CSS) crystallizes in the cubic system with the Ia-3d space group (No. 230).15 In the structure, each Ca2+ is surrounded by 8 O2- ions to form a distorted dodecahedron (D2 point group symmetry) with four long Ca-O distances of 2.5660(14) Å and four short Ca-O distances of 2.4324(11) Å. Each Sc3+ is coordinated with six equidistant oxygens at the distance of 2.1062(15) Å to form an octahedron.15 The ionic radii of eight-fold coordinated Ce3+ and Yb3+ are 114.3 pm and 98.5 pm,16 respectively, so that it is suggested that Ce3+/ Yb3+ occupy the Ca2+(VIII) (112.0 pm) site because of similar ionic sizes (Refer to SI). The Na+ ion serves to preserve electroneutrality and is envisaged to occupy the adjacent Ca2+ site to Ln•Ca .

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Ca2.88Yb0.03Ce0.03Na0.06Sc2Si3O12

Relative intensity (arb. units)

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Ca2.94Yb0.03Na0.03Sc2Si3O12

ICDD PDF-2 card #72-1969 Ca3Sc2Si3O12

20

30

40

50

60

70

80

2θ (degree)

Figure 1. Representative XRD patterns of samples. The X-ray diffraction (XRD) patterns of samples Ca3-2xYbxNaxSc2Si3O12 (x = 0.001-0.45) [hereafter, CSS:YbxNax] and Ca2.8-2xCe0.1YbxNa0.1+xSc2Si3O12 (x = 0-0.36) [hereafter, CSS:Ce0.1YbxNa0.1+x] were measured at room temperature (RT). Figure 1 shows the X-ray diffraction patterns of some representative samples. All of the samples comprise a single phase and they are consistent with the standard file of Ca3Sc2Si3O12 (CSS) (ICDD PDF-2 card #72-1969). Notice that the peak splitting for these samples indicates the presence of both Cu Kα1 and Kα2, where the intensity of the side peak is half of that of the main peak. The CSS host excitation and emission spectra have previously been presented14 and comprise features at 206 nm and 385 nm, respectively. 2.2. Luminescence of Yb3+ in Ca3Sc2Si3O12. The 4f13 2F7/2 ground state and 2

F5/2 excited state energy levels of Yb3+ all transform as the Kramers doublet Γ5 in the

double group D2 and all transitions between these levels are allowed by the forced electric dipole mechanism. 6


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λem = 1037 nm 3 K λem = 1037 nm 3 K

DRS RT

λex = 289 nm 3 K

1→7 1→6 1↔5 7 6 5

2

F5/2

4 3 2 1

250

300

2

F7/2

350 800

850

900

950

1000

1050

1100

Wavelength (nm)

Figure 2. Excitation and emission spectra at 3 K and DRS at room temperature of CSS:Yb0.27Na0.27.

The excitation and emission spectra of CSS:Yb0.27Na0.27 at 3 K are illustrated in Figure 2 and compared with the room temperature diffuse reflectance spectrum (DRS). A broad excitation band in the UV range is ascribed to the ligand to metal charge transfer (CT) absorption. The zero phonon line, i.e., the 2F7/2 ↔ 2F5/2 lowest energy resonant transition line (1↔5), is at 969 nm in the low temperature emission spectrum, with phonon structure at 107, 253 cm-1 to lower energy. A sharp band at 1037 nm corresponds to another zero phonon line. The observed peaks at 883 and 897 nm in the 3 K excitation spectrum are assigned to the transitions from the lowest ground level 2F7/2(1) to the excited levels 2F5/2(6,7).

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300 K - 460 K



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λex = 289 nm

a

900

950

1000

1050



1100

Wavelength (nm)

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1.5

1.0

0.5

0.0

86.7%

300 350 400 450 Temperature (K)

Ca2.46Yb0.27Na0.27Sc2Si3O12 λex = 289 nm, λem = 969 nm

300 K - 460 K

b 0

2

4

6

Time (ms)

Figure 3. (a) Emission spectra of Yb3+ in Ca2.46Yb0.27Na0.27Sc2Si3O12 at different temperatures between 300 and 460 K. The inner figure at the top right shows the integrated intensities as a function of temperature. (b) The decay curves of Yb3+ emission in this temperature range.

2.3. Temperature Variation of Emission Intensity and Lifetime of CSS:Yb0.27Na0.27. The emission spectra of CSS:Yb0.27Na0.27 at different temperatures in the range from 300 - 460 K are presented in Figure 3(a), using the excitation wavelength of 289 nm. The Yb3+ emission intensities are reasonably constant up to 400 K, without wavelength shifts, and decrease to 87% of the initial intensity at 300 K when the temperature increases to 460 K. These results are shown graphically in the right hand inset of the figure and reveal a good thermal stability of Yb3+ in the CSS host. Experimentally, the NIR emission intensity of YAG:Yb3+ has been demonstrated to increase in the temperature range from 130 K to 300 K when the excitation is into the CT band.17

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Under 289 nm excitation, the 2F5/2 Yb3+ lifetime of CSS:Yb0.27Na0.27 changes very little from 0.97 ms at 3 K to 0.96 ms at 300 K. Figure 3(b) shows the decay curves of 2F5/2 emission in the temperature range from 300 K to 460 K, using 289 nm excitation. All of the curves can be fitted by monoexponential decay and the lifetimes up to 440 K are plotted against (1/T) in Figure S6. Arrhenius and Mott-Seitz activation energy plots both give an energy of 0.20 eV (1642 cm-1) which is rather higher than the barrier value calculated for YAG:Yb3+.8 Two mechanisms that account for the temperature dependence of CT emission have been put forward: thermally activated cross-over to the 4f13 ground state of Yb3+ and photoionization of the CT state with the escape of the ligand hole to the valence band.18 In the present case, no CT emission is observed from CSS:Yb3+ and yet excitation into the CT band produces 2F5/2 emission. The quenching of CT emission for Yb3+ situated on the larger Ca2+ site therefore is attributed to crossing of the CT state potential energy curve with that of Yb3+ 2F5/2 so that the intensity and lifetime of 2F5/2 emission are sensibly constant up to 400 K. However, the 2F5/2 lifetime quenching is associated with a thermally-assisted quenching mechanism, either for 2F5/2 or its populating CT state. The Yb2+ ground state is far below the conduction band (CB), as shown in Figure 4, for thermal ionization to occur in this temperature range. Barandiarán et al.8 associate the thermal quenching of Yb3+ emission with the presence of Yb2+-Yb3+ dimers. If these are present at the concentrations employed in our experiments, then we would also expect to see the presence of Yb3+-Yb3+ dimers and their associated emission at ~500 nm.

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0 CB

-2

VRBE (eV)

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-4 -6

Eex

EPL(Ce)

1 2

3

Eg

4 ECTB

-8

-10 -12 -14

VB

5

Ce 1

Yb 13

Eu 6

number of electrons n in Ln3+ 4fn

Figure 4. Vacuum-referred binding energy (VRBE) diagram for Ce3+, Eu2+, Eu3+ and Yb2+, Yb3+ in CSS. CB conduction band; VB valence band; red line, Ce3+ 5d1(1) state; blue line 4fN Ln3+ ground state and excited state; green line 4fN+1 Ln2+ ground state; black line Eu2+ lowest 4f65d1 state; Eex excitonic energy. Arrow 1: the energy of the 4f1 - 5d1(1) transition of Ce3+ (2.58 eV, ZPL); Arrow 2: the host band gap energy, 7.35 eV; Arrow 3: the energy of the CT band of Eu3+, 4.86 eV; Arrow 4: the energy of the CT band of Yb3+, 4.29 eV; Arrow 5: the energy difference between 2F5/2 and 2F7/2, 1.28 eV. The Coulomb correlation energy, i.e., energy difference between the ground state of Eu2+ and that of Eu3+ (6.93 eV) and the centroid shift (1.4 eV) are not shown. The diagram has been modified from that given in Ref. 14 where the energy of the lowest 4f65d1 state was calculated empirically from that of 5d1(1) Ce3+. Herein the spectral energy has been taken.

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intensity (arb. units)

Ca3-2xYbxNaxSc2Si3O12 Relative intensity

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RT λex = 289 nm

x=0 x = 0.001 x = 0.03 x = 0.15 x = 0.27 x = 0.36 x = 0.45

925

950

0.0 0.1 0.2 0.3 0.4 0.5 content (x)

975

1000

1025

1050

1075

1100

Wavelength (nm) Figure 5. The emission spectra (λex = 289 nm) for CSS:YbxNax (x = 0 - 0.45) at RT. The inset shows the intensity-concentration dependence of emission.

2.4. Concentration Dependence of Emission Intensity and Lifetime of CSS:YbxNax. The emission spectra of CSS:YbxNax at RT with ascending Yb3+ concentration are illustrated in Figure 5. The 2F5/2 → 2F7/2 emission line wavelengths exhibit negligible change whereas the intensity reaches a maximum value at x = 0.27, as depicted in the inset. Under CT excitation, the emission decay curves of the 2F5/2 state as a function of Yb concentration are shown in Figure S7. The lifetimes are monoexponential and increase up to the concentration x = 0.15 and then decrease (Figure S8). The concentration quenching of YAG:Yb3+ at >10% Yb has been attributed to migration of the excitation energy to traps and killer sites (Yb2+, Yb-OH) which increase in concentration together with Yb3+.18

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(a) Ca2.8Ce0.1Na0.1Sc2Si3O12

RT

λem =550 nm

Relative intensity

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λex =440 nm

(b) Ca2.46Yb0.27Na0.27Sc2Si3O12 λem = 969 nm λex = 289 nm

(c) Ca2.26Ce0.1Yb0.27Na0.37Sc2Si3O12 λem = 969 nm λex = 440 nm λex = 440 nm 200

300

400

500

600

700

800

900

1000

1100

Wavelength (nm)

Figure 6. Excitation and emission spectra of CSS:Ce0.1Na0.1 (a), CSS:Yb0.27Na0.27 (b) and CSS:Ce0.1Yb0.27Na0.37 (c) at room temperature.

2.5. Energy Transfer Between Ce3+ and Yb3+ in Ca3Sc2Si3O12. The photoluminescence (PL) and excitation (PLE) spectra of CSS:Ce0.1Na0.1 (a), CSS:Yb0.27Na0.27 (b) and CSS:Ce0.1Yb0.27Na0.37 (c) are plotted in Figure 6 for the investigation of the energy transfer properties of Ce3+ and Yb3+ in the CSS phosphor. We have previously described the spectra of CSS:CexNax in detail and do not repeat here.14 There are two intense excitation bands at 308 and 440 nm ascribed to the 4f 2

F5/2 → 5d(1),(2) transitions of Ce3+ and the 5d(1) → 2F5/2,7/2 4f1 emission band peaks

at 505 nm (Figure 6(a)). The spectra of CSS:Yb0.27Na0.27 from Figure 2 are shown for comparison in Figure 6(b). Figure 6(c) demonstrates that the excitation spectrum of Yb3+ emission in CSS:Ce0.1Yb0.27Na0.37 comprises bands due to both Ce3+ and Yb3+; and that the excitation of Ce3+ at the Yb3+ non-absorbing wavelength of 440 nm leads to Yb3+ emission. These two features confirm that energy transfer from Ce3+ to Yb3+ occurs in the CSS host. 12


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3+

Ca2.8-2xCe0.1YbxNa0.1+xSc2Si3O12

Ce emission 3+ Yb emission

Integrated intensity (arb. units)

λex = 440 nm

Relative intensity

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RT

0.0 0.1 0.2 0.3 0.4 x = 0 Yb content (x) x = 0.001 x = 0.03 x = 0.15 x = 0.27 x = 0.36

450

500

550

600

650

Relative intensity

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900 950 1000 1050 1100

Wavelength (nm)

Figure 7. Visible and NIR emission spectra of CSS:Ce0.1YbxNa0.1+x samples (x = 0 - 0.36) under 440 nm excitation at RT. The inset depicts the variation of integrated Ce3+ and Yb3+ emission intensities as a function of Yb3+ dopant concentration.

The RT emission spectra covering the visible and NIR spectral range for Ce3+ and Yb3+ co-doped samples, with fixed Ce3+ concentration, of CSS:Ce0.1YbxNa0.1+x under 440 nm excitation are presented in Figure 7. In the visible region (450-700 nm), the Ce3+ broad band emission intensity decreases with x whereas the emission intensity of Yb3+ increases up to x = 0.03 and then decreases, as presented graphically in the inset to the figure. This quenching concentration for Yb3+ is considerably lower than that for CSS:YbxNax. As mentioned above, the concentration quenching of Yb3+ emission in YAG has been attributed partly to the presence of Yb2+,18 and this could

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also be the case in CSS co-doped with Ce3+ and Yb3+ due to the presence of more Yb3+-Yb2+ pairs, consistent with theoretical considerations.8



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λex = 440 nm, λem = 550 nm

RT

x=0 x = 0.001 x = 0.03 x = 0.15 x = 0.27 x = 0.36

100

200

300

400

500

Time (ns)

Figure 8. The decay curves of Ce3+ emission under 440 nm excitation for samples CSS:Ce0.1YbxNa0.1+x (x = 0 - 0.36) at RT.

2.6. Luminescence Decay in Ce, Yb Doubly-doped CSS Samples. To gain insight into the energy transfer process, the decay curves of the Ce3+ 5d1→ 4f1 emission at 550 nm upon Yb3+ ion concentration change in CSS:Ce0.1YbxNa0.1+x (x = 0 - 0.36) at RT were measured and these are shown in Figure 8. With ascending Yb3+ concentration, the lifetime of the 5d(1) state decreases and gradually deviates from monoexponential behavior (R2adj changes from 0.9984 for x = 0 to 0.9772 for x = 0.36). Therefore, the curves were fitted by the double-exponential equation: I = A e

+ A e

(1)

Where τ1 and τ2 are the slow and fast components of the luminescent lifetime, respectively; A1 and A2 are the corresponding fitting parameters. The average lifetimes can be further evaluated by the following equation: τ = (A τ + A τ

)/(A + A )

(2)

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The fitting results for the samples are presented in Table 1, together with goodness of fit. The Ce3+ average lifetime is reduced from 66.8 ns to 7.5 ns with increasing Yb dopant ion concentration and it follows a monoexponential relationship with dopant ion content, as shown in the top panel of Figure 9. The energy transfer rate kET increases linearly with Yb3+ dopant ion concentration x, indicating a direct transfer from one Ce3+ to one Yb3+, and not a transfer to two Yb3+, with the transfer efficiency rising as high as 88.8% (Table 1, final column).

Table 1. Lifetimes, τ (ns), Fitting Parameters, Energy Transfer rates, kET (µs)-1 and Transfer Efficiency, η (%) for CSS:Ce0.1YbxNa0.1+x from Measurements of Ce3+ Emission at 295 K.a τ1

τ2

A1

0

66.8 ± 0

66.8 ± 0

0.9169 0.9169

0.9984 66.8 0.0

0.0

0.001 69.2 ± 2.6 37.6 ± 3.6 1.2754 0.9291

0.9985 60.2 1.6

9.8

0.03

61.5 ± 0.9 24.7 ± 1.0 1.3456 1.6689

0.9987 49.3 5.3

26.3

0.15

41.4 ± 0.2 9.0 ± 0.1

0.9985 14.5 54.1

78.3

0.27

36.2 ± 0.3 7.6 ± 0.09 1.3436 86.8474

0.9981 9.6

89.6

85.7

0.36

37.1 ± 0.2 6.5 ± 0.08 1.3124 221.3842 0.9978 7.5

118.3

88.8

a

A2

R2adj

xYb

1.4758 33.3492

Τ

kET

η

kET = 1/τ - 1/τ0, where τ is the lifetime at concentration x, and τ0 is at x = 0; η = 100(1

- τ/τ0).

15


y = (58.1±3.2)exp[-x/(0.086±0.01)+(6.3±2.8)

60

Ca2.8-2xCe0.1YbxNa0.1+xSc2Si3O12RT

40 2

R

20

adj

= 0.9878

0

0.0 Energy transfer rate, -1 kET( µs )

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

Lifetime, τ (ns)


100

0.1

0.2

0.3

0.4

y` = (332.9±9.6)x`+ (-0.2±1.9)

50

R

2 adj

= 0.9958 Ca2.8-2xCe0.1YbxNa0.1+xSc2Si3O12 RT

0

0.0

0.1 0.2 Doping level, x

0.3

0.4

Figure 9. The Ce3+ emission lifetimes and the energy transfer rate as a function of Yb3+ dopant ion concentration in CSS:Ce0.1YbxNa0.1+x. 2.7. Electron Transfer. The analyses of the Ce3+ emission decay curves in the Ce3+,Yb3+ co-doped samples can provide information concerning the energy transfer mechanism. There is an assumption in such treatments that there is a random distribution of defects. The single-step energy transfer from Ce3+ to Yb3+ may be treated as a nonresonant multiphonon-assisted multipolar process or as an energy transfer process through a Ce4+-Yb2+ CT state. The electron transfer in the second case is permitted by point group selection rules,19 since the product of initial and final irreducible representations of the transfer process both correspond to Γ5(D2). From the VRBE diagram in Figure 4, the level of the Ce3+ 5d1(1) excited state is at -2.82 eV, which is higher in energy than the ground state of Yb2+ (-4.61 eV) so the charge transfer from the 5d1 state of Ce3+ to Yb3+ is energetically favorable (Fig. 4: ∆G = EPL(Ce) + ECTB – Eg = -1.89 eV). Under the electron transfer model, the luminescence decay of Ce3+ has been analyzed by assuming a random distribution of donors and acceptors within the host and taking each Ce-Yb distance as a spherical coordination shell.5 More recently, the variation of Ce3+ luminescence intensity with time I(t), has been given by:9 16


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I (t ) = e − t / τ 0

shells

∏ (1 − x + xe−Ct exp(− R / d ) )n i

(3)

i

i

where τ0 and x are as above; the parameters C and d represent the strength and range of energy transfer; ni is the number of cation sites at distance Ri; and the summation runs over the shells of cation sites surrounding the donor. The essential difference of the two approaches5,9 lies in the different ways of dealing with the acceptor concentration. In the equation of Setlur et al.,5 the concentration x is multiplied by the factor Z (the total number of sites within the shells), which means that only Z*x sites within k shells are occupied by the acceptors. In the second equation (3), the concentration x is taken into account for each site (within the brackets in the equation), which means that for each site the probability of being occupied by the acceptor is x. From an operational point of view, the second equation (3) is easier to be implemented and it was employed herein with the two variable parameters, including 1 shell or 10 shells, and the fitted results were found to be almost the same, as listed in Table 2. The fitting is good for 0.36 and fair for x = 0.27, but not for the other doping concentrations from x = 0.001 up to 0.15, for example, as shown for x = 0.15 in Figure 10(a). There are 4 near-neighboring Ca2+ sites in the 1st shell with the distance 3.7508 Å from each one to its neighbor. Neglecting the nearest-neighbor occupation by Na+, the probability that a Ce3+ ion has one Yb3+ neighbor within the first shell is given by 0.4x/9, which is 0.13% for x = 0.03 and 1.6% for x = 0.36. Since the CT tunnelling mechanism is largely based on wavefunction overlap (the fitted interaction range is d ~1.0 Å), only a Yb3+ in the first shell can be effective for electron transfer. The distances of Ca2+ sites range from 5.7294 Å to 10.6088 Å in the second to tenth shells. A Reviewer has pointed out that in the 1980’s, Hush discussed electron transfer 17



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Page 18 of 35

rates over long distances and showed that the expected exponential fall-off with donor-acceptor distance associated with “through-space” (overlap dependent) interaction could be far smoother (inverse power law) for “through-σ-bond” interaction.20,21 In the present case, the departure from exponential fall-off at long distances could arise from orbital mixing with neighboring atoms and mixing with delocalized states, including conduction band-like states. The decay curves for CSS:Ce0.1YbxNa0.1+x with x ≤ 0.36 were therefore reanalyzed by the use of Eq. (4): shells

q

I (t ) = e − γ 0t ∏ (1 − x + xe − Pt / Ri ) ni i

(4)

where the symbols are as before; P is a parameter and q represents the power. The fittings for x = 0.03, 0.15 using 10 shells are shown, for example, in Figures 10(b), (c). From Table 3, the fitted results with the use of only 1 shell are similar to those using the previous exponential equation: i,e., the results are good for x = 0.36, but not for the lower doping concentrations. However, with the inverse-power Equation (4), the 10-shell fitting is much better than the 1-shell fitting, especially for the low doping concentrations. This also contrasts with the 10-shell exponential fitting where the results are identical to the 1-shell exponential fitting. These results could show that for low doping concentrations, the inverse-power equation is more appropriate, pointing to a “through-bond” electron transfer mechanism. However, the fact that the index q is in the range of between 6-9 (except for x = 0.36) alternatively leads to the more likely conclusion that the transfer mechanism is multipolar in nature.

18


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Relative intensity

1.0


RT

λex = 440 nm, λem = 550 nm

0.8

x=0 x = 0.15 x = 0.36

0.6

fitted x = 0 fitted x = 0.15 fitted x = 0.36

0.4

0.2

(a) 100

1.0

200

Time (ns)

300

400

Ca2.74Ce0.1Yb0.03Na0.13Sc2Si3O12

0.8

RT

λex = 440 nm, λem = 550 nm

0.6 0.4 0.2 0.0 1.0(b) Ca Ce Yb Na Sc Si O 2.5 0.1 0.15 0.25 2 3 12

Relative Intensity

Relative intensity

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

0

λex = 440 nm, λ200 = 550 nm 300 100 em

0.6

Time (ns)

RT

400

0.4 0.2 0.0

(c) 0

100

200

300

Time (ns) 19


400


1


Relative intensity

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 35

λex = 440 nm, λem = 550 nm

0.1

RT

0.01

x = 0.36 0.001

1E-4

s=

(d) 0

100

200

6 8 10 300

Time (ns)

20


400

500

Page 21 of 35

1


Relative intensity

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


x = 0.03 0.1

R

2 adj

= 0.99804

0.01

(e) 0

100

200

300

400

500

Time (ns)

Figure 10. Fitting results for CSS:Ce0.1YbxNa0.1+x: (a) (x = 0, 0.15, 0.36) using Equation (3); (b),(c) (x = 0.03, 0.15) using Equation (4); (d) Inokuti-Hirayama fits for x = 0.36; (e) Dornauf-Heber fit for x = 0.03 (s = 8). Notice the linear ordinate in (a-c) and logarithmic scales in (d), (e). The fitted lines are shown in each case.

Table 2. Fitting Results for CSS:Ce0.1YbxNa0.1+x Using Eq. (3) [C (ns)-1, d (Å)]. 1 shell x

C

10 shells D

R

2

C

adj

R2adj

d

0.27

5.9±9.7

0.85±0.32

0.9826

6.2±9.4

0.84±0.29

0.9845

0.36

1.4±1.7

1.1±0.4

0.9930

1.3±1.7

1.1±0.4

0.9930

Table 3. Fitting Results for CSS:Ce0.1YbxNa0.1+x Using Eq. (4) [P (ns)-1 (Å)q].

1 shell x 0.001 0.03

P 316±1331 14±459

R2adj

q -20±50 2±25

0.9772 0.9539

21



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Page 22 of 35

0.15

2.9±11.8

2.5±3.0

0.9426

0.27

4.1±4.4

3.1±0.8

0.9836

0.36

54±20

5.4±0.3

0.9930

10 shells x

P

q

Radj2

0.001 0.03

231087±1546030 823±89

5.8±2.9 6.34±0.06

0.9918 0.9982

0.15

419±13

6.64± 0.02

0.9985

0.27

5263±296

8.61±0.04

0.9961

0.36

1.08E13±9.6E11

25.05±0.07

0.9930

2.8. Multipolar Energy Transfer. In the first scenario mentioned above, the energy transfer from Ce3+ to Yb3+ may be treated as a nonresonant multiphononassisted process. The electronic part of the transfer expression is scaled by various methods.12,13,22 It has been previously demonstrated14 that the Ce3+ lifetime of CSS:Cex remains monoexponential and within the range of 67.0 ± 0.7 ns for x = 0.001 −0.1, and only decreases to 62.6 ± 0.1 ns for x = 0.2. A further indication from the present study that donor-donor migration can be neglected is the nonmonoexponential donor decay in the presence of the acceptor. Hence the electronic factor for energy transfer was firstly treated by Inokuti-Hirayama theory,11 just as in the study of Yamada et al.,13 to determine the energy transfer mechanism. Our fittings of the decay curves according to electric dipole-electric dipole (dd, s = 6), electric dipole-electric quadrupole (dq, s = 8) and electric quadrupole-electric quadrupole (qq, s = 10) energy transfer11 are displayed in Figure S9 for CSS:Ce0.1YbxNa0.1+x and the parameters for the best fits, s = 8, are listed in Table S1. The three fits for x = 0.36 using different values of s are shown in detail in Figure 10(d) and there is not a great 22


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deal of difference between them. For s = 8, from Table S1, the critical transfer distance (i.e., for an isolated donor-acceptor pair separated by R0, the energy transfer occurs with a rate equal to the reciprocal donor lifetime11) is 7.0 Å, 6.3 Å, and 5.8 Å for x = 0.15, 0.27 and 0.36, respectively. The corresponding radiative Ce3+ (5d1 → 4f1) and Yb3+ (2F7/2 → 2F5/2) transitions are electric dipole and electric quadrupole allowed, respectively, and the nonradiative SLJ selection rules are similar (Refer to the SI for a theoretical derivation of the importance of the electric dipole-electric dipole and electric dipole-electric quadrupole mechanisms). To check if the continuum approach of Inokuti and Hirayama is valid, and to include the exchange interaction, an additional analysis was made using the Dornauf-Heber approach,23 which takes into account the discrete donor-acceptor distances in the CSS host and utilizes only one variable parameter, as described in the SI. The electric dipole-electric quadrupole interaction fits (s = 8) were superior, with a critical interaction radius23 of 6.5±1.0 Å. The Dornauf-Heber fit for x = 0.03 (s = 8) is displayed in Figure 10(e).

2.9. Temperature-dependence of Emission Lifetime and Emission Intensity. The temperature-dependence of emission intensity of the Ce3+ 5d(1) → 4f1 emission under the excitation of 440 nm as a function of temperature between 77-500 K and the corresponding variation of decay lifetime are displayed in Figure 11(a) and (b), respectively for CSS:Ce0.1Yb0.03Na0.13. There is a decrease of Ce3+ emission intensity, and a gradual increase of Yb3+ emission intensity with increasing temperature. The integrated intensities of Ce3+ and Yb3+ emissions with varying

23



3+

Ce emission 3+ Yb emission

0

100

200 300 400 Temperature (K)

500

Temperature 77 K 150 K 200 K 250 K

500

600

700

300 K 350 K 400 K 450 K 500 K

900

1000

1100


a

Intergrated intensity (arb. units)

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 24 of 35


λex = 440 nm, λex = 550 nm 77 K 150 K 200 K 250 K 300 K 350 K 400 K 450 K 500 K

b

0

Wavelength (nm)

100 200 300 400 500

Time (ns)

Figure 11. (a) Temperature-dependent visible and NIR emission spectra of CSS:Ce0.1Yb0.03Na0.13. The inset shows the integrated intensities of Ce3+ and Yb3+ emissions at different temperatures. (b) The decay curves of Ce3+ emission at 550 nm in CSS:Ce0.1Yb0.03Na0.13 at different temperatures.

temperature are displayed in the inset of the figure. Notice that the variation of integrated emission intensity differs from that of YAG:Ce3+,Yb3+ where although the Ce3+ intensity also decreases with temperature, the Yb3+ emission intensity increases up to ~150 K and then decreases.9 The decrease was attributed to excitation migration to killer sites9 attributed to Yb2+-Yb3+ pairs.8 In Figure 11(b), the decay lifetime decreases from 53 ns at 150 K to 39 ns at 500 K. A simple analysis of the intensity and lifetime data of Ce3+ in CSS:Ce0.1Yb0.03Na0.13 follows the use of the modified Arrhenius equation: y(T) = y0/(1+ Dexp(-EA/kT))

(5)

where y represents intensity or lifetime at temperatures T and 150 K; D is a constant; k 24


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is the Boltzmann constant; and EA is the potential barrier (or activation energy) for thermal quenching. The fits are shown in Figures S11(a),(b) and indicate the value of EA as 0.039±0.003 eV (315 cm-1) for the thermally-activated Ce3+ to Yb3+ energy transfer. One can enquire concerning the physical meaning of the energy barrier of the thermally-activated process which quenches the Ce3+ emission. It could arise from the Ce3+ → Yb3+ energy transfer, in addition to the quenching of Ce3+ emission in the CSS host: by migration to trap killer sites and/or thermal ionization to the CB. Hence it is necessary to distinguish these processes. From Figure 4, the energy barrier from the Ce3+ ground state to that of Yb2+ is ~0.86 eV (6950 cm-1) which is rather high. The temperature quenching of the emission intensity of CSS:Ce0.1Na0.1 in the range from 320-480 K occurs by a factor of 1.23.14 In the present case, for CSS:Ce0.1Yb0.03Na0.13 the corresponding figure is 1.49 so that an additional mechanism contributes about 20% to the quenching of Ce3+ emission. Sontakke et al.7 have proposed that the energy transfer efficiency η(T) can provide a distinction between electron transfer and multipolar interactions, where in the present case: η(T) = 1 – [τ(Ce-Yb)T/τ(Ce)T]

(6)

where the subscripts T refer to measurements of lifetimes at different temperatures.

25



0.20

Energy transfer efficiency η(T)

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

0.15

0.10

0.05

0.00 0

100

200

300

400

500

Temperature (K)

Figure 12. Plot of Eq. (6) for CSS:Ce0.1 and CSS:Ce0.1YbxNa0.1+x with a linear fit as in Fig. 3 of Ref. 7 as a guide to the eye.

The appropriate plot is displayed in Figure 12 and the increasing trend of transfer efficiency with temperature could indicate that the energy transfer occurs by an electron transfer mechanism. However, there is a fundamental difference between the energy transfer Ce3+ → Tb3+ studied in Ref. 7 (where the transfer is resonant and the transfer rate is temperature-independent) and Ce3+ → Yb3+ herein, where the transfer is nonresonant. In the present case the transfer can also occur by a multipolar mechanism but can be multiphonon-assisted: this would then introduce the temperature-dependence of energy transfer rate.

2.10. Multiphonon-assisted Energy Transfer. The IR spectrum of the CSS host is shown in Figure S11 and the phonon absorption is strong up to ~1100 cm-1. In addition to references given in the Introduction, the temperature-dependence of multiphonon-assisted energy transfer has been discussed by Wassam and Fong.24 In 26


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the weak coupling limit, the energy transfer rate is given by:

k ET

( 2 π )1 / 2 2  Eω  = J exp[− K (2nm + 1)] ×  m  2 h  h 

−1 / 2

 E exp−  hω m

  E  ln − 1  Khω m (nm + 1)  (7)

where E is the energy difference between the donor and acceptor transitions, representing the energy that needs to be partitioned into vibrational degrees of freedom; J2 is the electronic coupling parameter, assumed to be constant; K = Lm g m2 ; ωm represents the frequency of normal modes of degeneracy Lm which accept the energy mismatch between donor and acceptor; gm is a dimensionless parameter representing equilibrium displacement between excited and ground adiabatic states. The temperature-dependence is contained in the phonon occupation number, nm: nm =

1 (exp(hω / k B T ) − 1)

(8)

where kB is the Boltzmann constant. The fits to the temperature variation of the energy transfer rate from Ce3+ to Yb3+ (kET) employed two variable parameters:24 one of these represented the electronic factor (assumed to be constant) and the other parameter was K = Lmgm2. The choice of the ‘effective phonon energy’ hω is somewhat arbitrary and it should represent a Ce-O mode. For CSS:Ce0.1, the energy transfer rate, kET(Ce)T is deduced from the equation: kET(Ce)T = 1/τT – 1/τ0

(9)

For CSS:Ce0.1Yb0.03Na0.13, the energy transfer rate kET(CSS:Ce,Yb)T includes kET(CSS:Ce)T in addition to kET(Ce → Yb)T. The Ce → Yb energy transfer rate at various temperatures is therefore given by: ∆kT = kET(Ce → Yb)T = kET(CSS:Ce,Yb)T - kET(CSS:Ce)T

(10)

The values of ∆kT are plotted by subtraction in Figure 13 as black squares. The 27



energy difference between the Ce3+ 5d1(1) → 4f1(7) (20833 - 3800 cm-1) and Yb3+ 4f13(1) → 4f13(5)/ 4f13(1) → 4f13(7) (0 – 10326/11315 cm-1) nonradiative transitions (i.e., ~6700/5720 cm-1) is at least six phonons. An illustrative value of 6000 cm-1 was chosen. Two values for the effective phonon energy of 600 cm-1 (black curve) and 300 cm-1 (red curve) are plotted in the figure, with the respective values of K being 0.1 and 4.9, respectively. Taking into account the errors in the plot of ∆k, it is reasonable that the temperature dependence of the energy transfer rate can be explained by multiphonon emission.

10

kET(CSS:Ce,Yb)T

-1

Energy transfer rate (µs)

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 28 of 35

8

kET(CSS:Ce)T 6

4

2

∆kT

0

-2 0

100

200

300

400

500

Temperature (K)

Figure 13. Temperature variation of the energy transfer rate from Ce3+ to Yb3+ in CSS:Ce0.1Yb0.03Na0.13. The squares are experimental data and the full lines are from Eq. (7): E = 6000 cm-1; red curve hω = 300 cm-1, K = 4.9; black curve hω = 600 cm1

, K = 0.1.

28


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3. CONCLUSIONS The focus of the present study has been the energy transfer mechanism for Ce3+ → Yb3+ transfer in the remarkable Ca3Sc2Si3O12 host. It has been demonstrated that this transfer does not involve quantum cutting, although the efficiency of transfer approaches 90% for xYb3+ = 0.36. The mechanism for transfer involving a Ce4+ Yb2+ charge transfer state is feasible from energetic considerations (Figure 4). However, a certain degree of wavefunction overlap is required for electron exchange and this restricts the opportunity for this mechanism to be dominant to the appropriate donor-acceptor concentrations. In fact, it may not be appropriate to think of “the energy transfer mechanism” because there exists a competition between the major mechanisms of electric dipole-electric quadrupole (dq), electric dipole-electric dipole (dd) and electron transfer. It has been demonstrated that a nonresonant multipolar electric dipole-electric quadrupole mechanism is dominant at low acceptor concentrations. From the SI, for a Ce3+ ion with one Yb3+ ion in the first shell, the Ce3+ → Yb3+ dd/dq energy transfer ratio is calculated to be 0.22, assuming a random doping distribution. However, for the transfer between the same Ce3+ ion with a Yb3+ ion in the 35th shell, the dd/dq ratio is 6.07. The electric dipole-electric dipole transfer rate in the second case is 2.1×104 times weaker than in the first case so that the contribution to the overall energy transfer rate is negligible. When xYb3+= 0.36, the probability that a Ce3+ ion has a Yb3+ nearest neighbor is 1.6% and in this case the Ce3+ luminescence decay could be fitted by the electron transfer model. The competition between the electric dipole-electric quadrupole and electron transfer mechanisms therefore depends upon the acceptor concentration. We do not observe or

29



identify a metal-metal intervalence charge transfer band for the co-doped system.

Figure 14 illustrates the AM 1.5 solar spectrum from the wavelength of 280 through to 1200 nm and the response of solar cell compared with the excitation and emission spectra of CSS:Ce0.1Yb0.03Na0.13. The presence of Ce3+ ions increases the absorption in the visible and ultraviolet spectral ranges from 300-500 nm, near the maximum intensity of sunlight. Since the Yb3+ emission of this material is strong and matches very well with the highest spectral response of the c-Si solar cell, the CSS:Ce,Yb phosphor has a potential application for the c-Si solar cell.


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

The Am 1.5 solar spectrum PLE of CSS:0.1Ce,0.03Yb

Spectral response of c-Si PL of CSS:CSS:0.1Ce,0.03Yb

300 400 500 600 700 800 900 1000 1100 1200

Wavelength (nm) Figure 14. The AM 1.5 solar spectrum, spectral response of c-Si, and the PLE and PL spectra of CSS:Ce0.1Yb0.03Na0.13.

AUTHOR INFORMATION Corresponding Authors *(P.A.T.) E-mail: [email protected]. Telephone: +852 90290610 30


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*(L.H.) E-mail: [email protected]. Telephone: +86-20-84113695

ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (NSFC) (21171176, U1232108, and U1432249) at Sun Yat-sen University and NSFC Grant nos. 11174005, 11574003 at Anhui Normal University.

SUPPORTING INFORMATION Experimental measurements, syntheses. Evidence that Yb3+ substitutes at the Ca2+ site. Artefact in lifetime measurements. Decay curves and lifetimes. Inokuti-Hirayama fits. Analysis of Energy Transfer using the Dornauf-Heber model. Donor - acceptor shell distances and coordination numbers in CSS. FT-IR spectrum of CSS host. Contributions of dipole-dipole and dipole-quadrupole energy transfer mechanisms to the total energy transfer rate. This material is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES (1) Chen, D.; Wang, Y.; Yu, Y.; Huang, P.; Weng, F. Quantum Cutting Downconversion by Cooperative Energy Transfer from Ce3+ to Yb3+ in Borate Glasses. J. Appl. Phys., 2008, 104, 116105 (1-3). (2) Zhao, J.; Guo, C.; Li, T. Enhanced Near-infrared Emission by Co-doping Ce3+ in Ba2Y(BO3)2Cl: Tb3+, Yb3+ Phosphor. RSC Adv., 2015, 5, 28299-28309. (3) Luo, Z. H; Liu, Y. F.; Zhang, C. H; Zhang, J. X.; Qin, H. M.; Jiang, H. C.; Jiang, J. Effect of Yb3+ on the Crystal Structural Modification and Photoluminescence Properties of GGAG: Ce3+. Inorg. Chem., 2016, 55, 3040-3046.

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Energy transfer in a Yb3+-Ce3+ co-doped system 85x47mm (300 x 300 DPI)


Spectral Properties and Energy Transfer between Ce3+ and Yb3+ in

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