Spectral Properties and Energy Transfer of a Potential Solar Energy

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Spectral Properties and Energy Transfer of a Potential Solar Energy Converter Lei Zhou, Weijie Zhou, Fengjuan Pan, Rui Shi, Lin Huang, Hongbin Liang, Peter A. Tanner, Xueyan Du, Yan Huang, Ye Tao, and Lirong Zheng Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00763 • Publication Date (Web): 29 Mar 2016 Downloaded from http://pubs.acs.org on March 31, 2016

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Chemistry of Materials

Spectral Properties and Energy Transfer of a Potential Solar Energy Converter Lei Zhoua, Weijie Zhoua, Fengjuan Pana, Rui Shia, Lin Huanga, Hongbin Liang,*a Peter A. Tanner*b, Xueyan Duc, Yan Huangc, Ye Taoc, Lirong Zhengc a

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. E-mail: [email protected]; b

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

Road, Tai Po, Hong Kong S.A.R., P.R. China E-mail: [email protected]; c

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

Sciences, Beijing 100039, P.R. China ABSTRACT: The energy transfer between Ce3+ and Eu2+ has been investigated in the host Ca3Sc2Si3O12 (CSS), prepared by a modified sol-gel method. Excitation and emission measurements from the near infrared to the vacuum ultraviolet spectral regions have been performed upon CSS, Ce3+-doped CSS, Eu2+-doped CSS and Ce3+, Eu2+-co-doped CSS, at various concentrations, including experiments at temperatures range of 15-460 K. The energy transfer efficiency from Ce3+ to Eu2+ can approach 90% and the Ce3+ donor decay curves for different Eu2+ acceptor concentrations in the co-doped system were fitted by the Inokuti-Hirayama method, indicating that it is energy transfer induced by electric dipole interaction. The use of the Ce3+, Eu2+ couple in the CSS host as a wideband harvester with an emission profile tailored to the response of the silicon solar cell in solar energy conversion suffers from two main drawbacks relating to valence instability and emission quenching of Eu2+. Possible solutions are suggested. ___________________________________________________________________________________

1

ACS Paragon Plus Environment


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1. INTRODUCTION The World Energy Outlook 2015 released in November 20151 calls for the understanding of the implications of the shifting energy landscape for economic and environmental goals and for energy security. Oil price collapse, geopolitical instability, trends in CO2 emissions and energy inefficiency all provoke us to seek renewable and stable energy resources. Recent developments in flexible copper indium gallium selenide,2 transparent solar cells,3 upconversion multicolor tuning4 and lanthanide ion-doped quantum dots5 provide optimism in this respect. Our goal has been to harness as much of the solar flux as possible and herein we report a wideband harvester with an emission profile tailored to the response of the silicon solar cell. The cubic silicate garnet host Ca3Sc2Si3O12 (CSS) is endowed with unique and versatile spectroscopic properties. Its structure comprises Ca2+ in dodecahedral (8-) and Sc3+ in octahedral 6-coordination to oxygen.6 Upon doping with Eu3+, the bands of the transition 5D0 → 7F4 located between 800-900 nm are unusually strong,7 attributed to the selection rules for the distorted EuO8 coordination.8,9 The Tb3+-doped CSS exhibits strong green luminescence with a lifetime of 3 ms and a rather slow population of 5D4 from 5D3.7 The afterglow of the scintillator CSS:Pr3+ can be removed by co-doping with Mg2+ and hence the performance is improved.10 A phosphor with color rendering index Ra > 90 for white light-emitting diode (wLED) use was synthesized by co-doping CSS with Ce3+, Mn2+ and a charge-compensating lanthanide ion.11 Perhaps the most striking property of the host is that it can extend the 2


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luminescence of divalent europium into the near infrared spectral region12 and this is exploited in the present work. The synthesis of the CSS host material may be problematic due to the presence of various secondary phases when using solid state methods.6,8 Wu et al. have proposed that the substitution of Sc3+ by Al3+ (below 40%) in CSS:Ce3+ in solid state synthesis at 1450 oC can inhibit the formation of the impurity phases Sc2O3 and CeO2, improve crystallinity, and enhance the photoluminescence intensity.13 Recently, a freeze-drying method and increased heat treatment temperature have been utilized to reduce the presence of secondary phases.14 The X-ray diffraction pattern of a sample prepared by sol-gel combustion synthesis and subsequently fired at 1100 oC does not exhibit other phases.15,16 Several candidates are available for the doping of this host with divalent lanthanide ions, of which Eu2+ and Yb2+ are the most promising.17 The luminescence and excitation spectra of some Ce3+-activated silicate garnets have previously been reported at room temperature.18,19 The Ce3+ electronic energies were interpreted on the basis of two factors: the centroid shift from the free ion and the 5d crystal field splitting. There have been numerous studies regarding of the energy transfer between Ce3+ and Eu2+ in various solid-state hosts. To our knowledge, the aim of these studies has been the development of near-ultraviolet-pumped white light emitting diodes (wLEDs). For example, the Ca1.65Sr0.35SiO4:Ce3+, Li+, Eu2+ phosphor has been recommended as a candidate for color-tunable blue-green components of wLEDs, with emission between 465-550nm;20 Ba2ZnS3:Ce3+, Eu2+ has application as a blue-converting phosphor for wLEDs, with emission maximum 3



intensity at ~650 nm.21 The Ca4(PO4)2O:Ce3+, Eu2+ wLED phosphor has Eu2+ emission maximum at 650 nm.22 The format of this work is as follows. The structure and luminescence of the host material, luminescence of Ce3+ or Eu2+-doped systems, and the energy transfer from Ce3+ to Eu2+ in the CSS host material are presented and discussed in Section 2. A comment is included in Section 3 concerning the applicability of Ce3+-Eu2+ co-doped CSS in the field of solar energy conversion. Some conclusions from the experimental results and data analyses are made in the final Section 4.

2. RESULTS AND DISCUSSION

Relative intensity (arb. units)

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Ca2.54Ce0.2Eu0.06Na0.2Sc2Si3O12

Ca2.97Eu0.03Sc2Si3O12

Ca2.94Ce0.03Na0.03Sc2Si3O12

Ca3Sc2Si3O12

ICDD PDF 2 card # 72-1969 Ca3Sc2Si3O12

20

30

40

50 60 2θ θ (degree)

70

80

Figure 1. Representative XRD patterns of samples at room temperature.

2.1. Structure refinement. The X-ray diffraction (XRD) patterns of samples Ca3-2xCexNaxSc2Si3O12

(x

=

0.001-0.2),

Ca3-xEuxSc2Si3O12

4


(x

=

0-0.09),

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Ca2.8-2xCe0.1EuxNa0.1Sc2Si3O12 (x = 0-0.09), Ca2.94-2xCexEu0.06NaxSc2Si3O12 (x = 0-0.2) were measured at room temperature. The Na+ ions from Na2CO3 serve as charge compensators for Ce3+. Figure 1 shows the X-ray diffraction patterns of some representative samples. All of the samples were verified to comprise a single phase and they are consistent with the standard file of Ca3Sc2Si3O12 (ICDD no.72-1969), without other impurities such as Sc2Si2O7, Sc2O3, SiO2 and Ca2SiO4 etc. The compound Ca3Sc2Si3O12 (CSS) crystallizes in the cubic system with the Ia-3d space group (No. 230).13 In the structure, each Ca2+ is surrounded by 8 O2- ions to form a distorted dodecahedron (D2 point 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.13 The coordination environments are depicted in Figure S1. The crystal data of CSS was used as the initial model for structural analysis and the rare earth ions were assumed to be dispersed randomly in this host lattice. The refinement patterns of the three representative samples (CSS host; CSS:0.03Ce; CSS:0.03Eu) were processed using the software TOPAS23 and the results for the CSS host at room temperature are shown in Figure S2, with the values of Rwp, Rp, RB being in the range of 2.1% - 4.4%, indicating a good fitting quality. Table S2 also presents the crystallographic data and refined structure parameters for the three systems. The ionic radii of eight-fold coordinated Ce3+ and Eu2+ are 114.3 and 125.0 pm,24 respectively, so that it is suggested that Ce3+/Eu2+ occupy the Ca2+(VIII) (112.0 pm24) site because of similar ionic sizes. In fact, Shimomura et al.6 have shown from X-ray absorption 5



fine structure analysis that the Ce3+ ion replaces the Ca2+ position of the CSS host crystal, and mentioned that a Ca2+ vacancy could compensate for the charge of two Ce3+ ions. X-ray absorption near edge spectra6 showed that all Ce ions are trivalent, in agreement with the calculated value of 2.9 from the bond valence model.25 The SEM images of the CSS precursor and final product at the magnification of 100000× were measured and these are shown in Figure S3. The precursor calcined at 700 oC exhibits irregular fragmented morphology within the size of several hundred nm. The final product sintered at 1400 oC for 6 h comprises oblatoid grains united together with the entire size being ~ 20 µm. The larger particle size obtained in the present study than when firing at 1100 oC for 2 h15,16 results from the growth at higher temperature and holding time in the reducing atmosphere. We consider that the higher temperature and holding time is required in order to remove impurity phases and obtain purer samples (Figure S4).

0.7

Ca3Sc2Si3O12 15 K

λem = 385 nm λex = 206 nm

0.6

6.02 eV (206 nm)

0.5

3.23 eV (385 nm)

Intensity (arb. units)

0.8

λ ex = 206 nm; λ em = 385 nm τ = 5.5 µs

(b) 25

0.4 Normalized intensity (arb. units)


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0.3 0.2 0.1

6.74 eV (184 nm)

(a)

0.0 100

200

300

400

500

600

50 Time (µs)

75

λ em = 385 nm 15 K 70 K 120 K 296 K

(c) 125 150 175 200 225 250

Wavelength (nm)

Wavelength (nm)

Figure 2. (a) Excitation and emission spectrum of Ca3Sc2Si3O12 at 15 K using synchrotron radiation: λex = 206 nm; λem = 385 nm. (b) The decay curve Ca3Sc2Si3O12 6


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(λex = 206 nm; λem = 385 nm) at room temperature. (c) The excitation spectra (λem = 385 nm) at different temperatures. 2.2 The luminescence of Ca3Sc2Si3O12 host. The vacuum ultraviolet (VUV) spectroscopy of pure Ca3Sc2Si3O12 was investigated as a prerequisite for the study of the luminescence properties of doped samples. The emission spectrum upon synchrotron radiation excitation at 206 nm and the VUV excitation spectrum by monitoring at the wavelength of 385 nm emission at 15 K are shown in Figure 2(a). A broad excitation band was observed with a maximum at 206 nm (6.02 eV) and is associated with near-defect excitons. At higher energy, a broad shoulder band (184 nm, 6.74 eV) is present and is assigned to self-trapped exciton (STE) absorption. Upon 206 nm excitation, the emission exhibits a broad band with maximum at 385 nm. The spectrum is similar to that reported by Ivanovskikh et al.26 who pointed out that the broad emission band in the time-integrated excitation spectrum is attributed to excitons localized near defects or to direct electron–hole recombination. These authors also reported several other weak bands which are not present in Figure 2(a). Figure 2(b) shows the monoexponential decay curve of the emission intensity I, fitted to Eq (1). ln(I) = lnA - kt

(1)

where the slope k is the reciprocal of the intrinsic lifetime (τ0) and A is a constant. The lifetime is fitted to be 5.5 µs. The decay is little slower than that previously reported for CSS at 10 K (2 µs27) and also in comparison with the decay time of self-trapped exciton luminescence in pure LaCl3 (3.5 µs at room temperature28). 7



Figure 2(c) shows the normalized excitation spectra of Ca3Sc2Si3O12 at different temperatures between 15 K and 296 K. The shape and position of the band remain unchanged relative to the situation at 15 K and are insensitive to temperature variation.


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Ca2.8Na0.1Ce0.1Sc2Si3O12 RT F

λex = 440 nm

G

λem = 550 nm (VUV)

H

λem = 550 nm (UV) A C

D

B E

100

200

300 400 500 Wavelength (nm)

600

700

Figure 3．VUV-UV excitation spectra (λem = 550 nm) of Ca2.8Ce0.1Na0.1Sc2Si3O12 and the UV emission spectrum of Ca2.8Ce0.1Na0.1Sc2Si3O12. The features in the excitation spectrum at wavelengths longer than 350 nm are beyond the collection range of the experimental setup at BSRF so that the excitation spectrum in the 280-530 nm range was recorded with a spectrometer in our laboratory and normalized on the band D.

2.3. Spectra of Ca2.8Ce0.1Na0.1Sc2Si3O12 at room temperature (RT). The VUV-UV excitation spectra of Ca2.8Ce0.1Na0.1Sc2Si3O12 at room temperature are shown in Figure 3. The far-left curve is the excitation spectrum using synchrotron radiation in the 125-350 nm range when monitoring 550 nm emission of Ce3+. Six excitation 8


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bands (A-F) are distinguished in the 125-530 nm range as labeled in Figure 3. The bands A (182 nm, 6.81 eV) and B (215 nm, 5.77 eV) are analogous to the exciton absorptions observed in the VUV-UV excitation spectrum of Ca3Sc2Si3O12 in Figure 2, but with different relative intensities and a red-shift of the latter by several nm. With an excess of 8% for exciton binding energy,29 the band gap of CSS:Ce3+ is estimated to be 7.35 eV at room temperature. The three lower energy bands C (239 nm, 5.18 eV), D (308 nm, 4.02 eV) and F (440 nm, 2.82 eV) are attributed to 4f-5d transitions of Ce3+, in line with previous studies.12,30 In CSS-Ce the Ce3+ ion occupies the D2 symmetry Ca2+ site, so that the 5d level is initially split into a lower doublet (Eg) and a higher triplet (T2g) by the cubic component of the crystal field and then further split by the distortion from cubic symmetry. A first-principles study27 has calculated the energies of the 4f1 and 5d1 levels of Ce3+ in Ca3Sc2Si3O12 with different types of charge compensation mechanisms. Assignments for the 5d(1)-5d(5) energy levels of Ce3+ in garnet host lattices (mostly expressed as vibronic band maxima) are collected in Table 1 and in the present case the assignments for 5d(1)-5d(3) appear secure. There is some disagreement in the literature concerning the assignment of the higher 5d levels in YAG:Ce3+. From the first principles calculation, the crystal field splitting (CFS) of Ca3Sc2Si3O12:Ce3+, Na+ is calculated to be about 27773 cm-1 (3.44 eV).

Table 1. Calculated (calc.) and observed (obs.) energies (in eV) for selected garnets doped with Ce3+. Values refer to band maxima except for the calculation of Ref. 32 for 9



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YAG:Ce3+ which refers to zero phonon line energies. In YAG:Ce3+, the 5d(1) and 5d(2) zero phonon lines are at 0.15 eV and 0.11 eV to low energy of the excitation band maxima, respectively.32 Measurement temperatures are included, where appropriate.

Ref.

System

5d(1) 5d(2) 5d(3) 5d(4) 5d(5)

This, 15 K, obs. Ca2.8Ce0.1Na0.1Sc2Si3O12 2.82

4.03

5.19

12, 77 K, obs.

Ca2.97Ce0.03Sc2Si3O12

2.79

4.03

5.23

27, calc.

Ca3Sc2Si3O12:Ce3+, Na+

2.87

3.94

5.59

6.10

31, obs./calc.

Y3Al5O12:Ce3+

2.71

3.65

5.50

~5.50 6.06

32, calc.

Y3Al5O12:Ce3+

2.53

3.57

5.39

6.09

6.31

7.51

A discussion of the calculation of the centroid shift is included in the Supporting Information and the calculated value is 1.42 eV (11455 cm-1). Hence, as also remarked by Berezovskaya et al.,12 the observed relatively long-wavelength position of the lowest 5d state for Ce3+ in CSS is attributed to the low 5d centroid shift energy from the Ce3+ free ion level and the large crystal field splitting of the 5d1 configuration in this host (Figure S5). In the emission spectrum of Ca2.8Ce0.1Na0.1ScSi3O12 excited by 440 nm radiation (Figure 3), a broad band with peak maximum at 505 nm and a shoulder at 550 nm are observed, corresponding to the electronic transition from the relaxed lowest 5d state to the 4f1 J-multiplets 2F5/2 and 2F7/2 of Ce3+, respectively. Although there is a change in total intensity of the emission bands, the shape and position are unchanged for 10


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excitation wavelengths in the range of 410-480 nm (Figure 4) so that is considered that Ce3+ occupies only one crystal site: that of Ca2+, in CSS. 40

Ca2.8Ce0.1Na0.1Sc2Si3O12 RT

500

λem = 390 nm

30 20 10

5d(1) 1 1.0

400

0.7

300

400

λem = 550 nm 100

0.5

300

5d(2)

0.2

0 500 150

5d(2)

5d(1)


Excitation wavelength (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


50

0

400 500 600 700 Emission wavelength (nm)

300

400

0 500

Wavelength (nm)

Figure 4. The excitation spectra map of Ca2.8Ce0.1Na0.1ScSi3O12 by monitoring different emission wavelengths. The right-hand figures show the representative excitation spectra by monitoring 390 and 550 nm emissions, respectively.

2.4. Spectral impurity bands in Ca2.8Ce0.1Na0.1ScSi3O12. The weak Band E (Figure 3: 330-360 nm) has not yet been assigned. This band is evident in the excitation spectra of Liu et al.16 for CSS:Ce3+ samples prepared by the gel-combustion method in air and by solid-state reaction method, but not in the spectrum of CSS:Ce3+ prepared by the gel-combustion method in carbon. The band was attributed by Ding et al.27 to the second 4f → 5d transition of Ce3+, with Sc3+ substituting Si4+ in its local environment. Our XANES shows that cerium is present in the 3+ oxidation state in our samples (Figure S6). In order to further investigate whether the broad absorption band E corresponds to defect or impurity absorption present beyond the limit of XRD detection, the continuous excitation spectrum was recorded by monitoring the 11



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emission from 340 nm to 700 nm, as shown in Figure 4. The remarkable intense absorption between 400 - 500 nm corresponds to the vibronic band of the 4f - 5d(1) transition of Ce3+. The transition to 5d(2) shows weak absorption intensity (as marked in Figure 4), whilst three additional weaker bands at 296, 330 and 360 nm appear when monitoring emission at ~390 nm (Figure 4: top right-hand panel), which is at a similar wavelength to that of the host emission (Figure 2). These excitation bands are more prominent when monitoring the emission at 390 nm than at 550 nm, as shown Figure 4, right-hand panels, Figure S7 and Figure S8. Furthermore, excitation at 330 nm or 360 nm gives an emission band ~390 nm in addition to the Ce3+ emission at longer wavelength (Figure S9). The former emission band is shifted a few nm to higher energy, at 385 nm, when exciting at 237 nm and 206 nm at 15 K (Figure S10). The decay curves of the Ce3+ and defect site emissions are compared in Figure S11. The life time of this defect site emission is 20±2 ns which is shorter than the 67 ns decay of Ce3+. The defect sites may arise from aliovalent substitutions near Ce3+ ions since their population leads not only to self-trapped excitonic emission but also to Ce3+ emission. Since the relevant spectral features of these defect sites are at high energy, their presence does not interfere with our study on Ce3+- Eu2+ energy transfer and application.

12


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Normalized intensity (arb. units)

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λex = 440 nm

Ca3-2xCexNaxSc2Si3O12 RT

x = 0.001 x = 0.03

λem = 550 nm

x = 0.10 x = 0.15

a

x = 0.20

300

400

500

600

700

Wavelength (nm) λex = 440 nm, λem = 550 nm

b 100

200 300 Time (ns)

400

500

Figure 5. (a) The normalized excitation spectra (λem = 550 nm) and emission spectra (λex = 440 nm) for Ca3-2xCexNaxSc2Si3O12 (x = 0.001-0.2) at room temperature. (b) The corresponding decay curves of these samples.

2.5. Concentration and temperature dependence of Ce3+ emission in Ca3-2xCexNaxSc2Si3O12. The effect of Ce3+ concentration upon the room temperature emission spectrum (a) and lifetime (b) of CSS:Ce3+, Na+ is illustrated in Figure 5. The increase in Ce3+ concentration markedly broadens features in the excitation spectrum but exhibits a smaller effect upon the emission bands. Notably, the defect site bands located at 330-360 nm are absent in the samples x = 0.001 and 0.03. It is remarkable that the Ce3+ lifetime remains monoexponential and within the range of 67.0±0.7 for x = 0.001-0.1 (as also reported for CSS:Ce3+ by Shimomura et al.6), and only decreases to 62.6±0.1 for x = 0.2. This feature is taken to indicate the absence of migration between Ce3+ ions and consequent 13



journey to killer sites.

Ca2.8Ce0.1Na0.1Sc2Si3O12 1.0

λex = 440 nm

0.8

0.2

320 K 340 K 360 K 380 K 400 K 420 K 440 K 460 K 480 K

0.0 400

450

0.6 0.4


1.2 Normalized intensity (arb. units)

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1.0 0.8 0.6 0.4

81.1%

0.2 0.0 300 350 400 450 500 Temperature (K)

500 550 600 Wavelength (nm)

650

700

Figure 6. Normalized emission spectra of Ce3+ in Ca2.8Ce0.1Na0.1Sc2Si3O12 at different temperatures between 320 and 460 K. The inner figure shows the integrated intensities as a function of temperature. The normalized emission spectra of Ca2.8Ce0.1Na0.1Sc2Si3O12 at different temperatures in the range from 320-460 K are presented in Figure 6, using the excitation wavelength at 440 nm. Two phenomena can be found from the comparison of these emission spectra. First, self-absorption decreases the intensity of the higher energy emission as the temperature increases (refer to the broadening of the excitation band in Figure S12). Second, there is a broadening of the emission band and a slight shift to lower energy at higher temperatures. However, as depicted in the inset of the figure, the integrated emission intensity of Ce3+ decreases only down to 81% of the initial intensity when the temperature rises from 320 K to 480 K. This shows the lack of thermal quenching and good thermal stability of Ca2.8Ce0.1Na0.1Sc2Si3O12. Shimomura

et

al.6

have

pointed

out

that

the

thermal

14


performance

of

Page 15 of 37

photoluminescence of CSS:Ce3+ is far superior to that of the commercial phosphor (Y,Gd)3Al5O12:Ce3+. Figure S12 depicts the corresponding changes in the excitation spectrum of Ca2.8Ce0.1Na0.1Sc2Si3O12 when the temperature increases from 320 K to 480 K. The 5d(1) band broadens at low energies due to transitions from thermally-occupied crystal field levels of 2F5/2. The defect site energy levels at 320-380 nm act as traps and they are emptied with increasing temperature.

0 CB

-1 -2

3+

Ln E

LS

-3

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


1

-4

HS

-5 -6

4 5

3

2

2+

Ln G

-7 -8 -9 -10

3+

Ln G La

Ce

Pr

Nd

0

1

2

3

Pm Sm

4

5

Eu

Gd

Tb

6

7

8

Dy

Ho

Er

Tm

Yb

Lu

VB

9 10 11 12 13 14 3+

n

number of electrons n in Ln 4f

Figure 7. VRBE scheme for lanthanide Ln3+ 4fn and 4fn-15d states (blue color), and Ln2+ 4fn states (red color) in Ca3Sc2Si3O12. The excited energy levels of lanthanide ions are taken from Refs. 33,34. Refer to the text for explanation. 2.6. Vacuum referred binding energy scheme (VRBE) scheme. The vacuum referred binding energy (VRBE) scheme for Ln3+ 4fn and 4fn-15d states and Ln2+ 4fn states in Ca3Sc2Si3O12 is displayed in Figure 7 by using published methods.33,34 Various approximations are involved, including the use of the same band gap for all 15



ions, the use of band maxima instead of zero phonon line energies, and the separations of high and low-spin states. The required data for the construction in the CSS host are (i) the energy of the lowest 4f - 5d transition of Ce3+ (arrow 1: 2.84 eV); (ii) the host band gap energy (arrow 2: 7.35 eV); (iii) the energy of the charge transfer band of Eu3+ (arrow 3, see Figure S13: 4.86 eV); (iv) the Coulomb correlation energy, i.e., energy difference between the ground states of Eu2+ and that of Eu3+ (not shown, 6.93 eV); and (v) the centroid shift (Figure S5: 1.4 eV). The exchange splitting energy between the low spin and high spin 4f - 5d transitions of Tb3+ in CSS is taken from Velázquez et al.14 (arrows 4 and 5: 0.79 eV). The ground state of Eu2+ is found to be above the Fermi energy level in Figure 7, indicating that the Eu2+ is unstable in this host with respect to oxidation. This fact is supported by the calculated sum of the bond valences (Eq. (1) in Ref. 35) coordinating Eu2+ in the CSS host (equal to the oxidation number of Eu) and found to be 3.1 in the present case from the Ca-O bond distances in CSS.


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

Ca2.94Eu0.06Sc2Si3O12 RT

λex = 360 nm λex = 520 nm

λem = 873 nm a 300

400

500

600

700

800

900

1000

1100

1200

Wavelength (nm) λex = 405 nm, λem = 873 nm τ ~ 95.1 ns

b 150

300

450

600

750

900

1050

Time (ns)

Figure 8. (a) The excitation and emission spectra of Ca2.94Eu0.06Sc2Si3O12 at room temperature. (b) The decay curve of Eu2+ emission in Ca2.94Eu0.06Sc2Si3O12 excited by 16


Page 16 of 37

Page 17 of 37

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a laser light source at room temperature.

2.7. Luminescence of Ca3-xEuxSc2Si3O12. The excitation and emission spectra of Ca2.94Eu0.06Sc2Si3O12 at room temperature are shown in Figure 8(a). In this figure and subsequently, the Eu2+ concentrations, x, are taken as the nominal amounts added in the syntheses. A broad absorption band with maximum at 510 nm occupies the range of 450-720 nm, with a stronger higher-energy band at 353 nm having a shoulder at 412 nm. These features correspond to the 4f7→ 4f65d transitions of Eu2+ ions. The lower-energy band is derived from excitation to the 5d Eg orbital and 4f6 core states which are at lower energies than in Eu3+ since the divalent ion is larger. The complexity of the energy levels in addition to the progressions in totally-symmetric vibrational modes are responsible for the broad band. The higher-energy band corresponds in addition to excitation to the 5d T2g orbital and 4f6 core states. Upon excitation at 360 nm or 520 nm (Figure 8a), a relatively intense band in the range from 700 nm to 1100 nm is observed, with peak maximum at 873 nm. Low temperature spectra taken at 13 K by varying the emission and excitation wavelengths show similar spectral shape and position and are more clearly resolved than the room temperature spectra (Figure S14). The emission band corresponds to the transition from the lowest 4f65d state to the 4f7 ground state (8S7/2) of Eu2+.36 Note the absence of Eu3+ emission. Dorenbos37 has given an empirical equation relating the energies of the lowest 5d bands of Ce3+ and Eu2+. Using the energy of the Ce3+ emission band (2.46 eV), the peak maximum for Eu2+ emission is estimated to be at 590 nm, 17



assuming the same Stokes shift, and this is far from the observed peak wavelength. The decay curve of Eu2+ emission in Ca2.94Eu0.06Sc2Si3O12 was excited by 405 nm radiation (Figure 8(b)) and can be well fitted by the monoexponential Eq. (1) with the

Ca3-xEuxSc2Si3O12 RT

Intensity

lifetime of 95.1 ns. Relative 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 18 of 37

0.00

0.04

0.08

Concentration x

λex = 520 nm

873 nm

700

800

x=0 x = 0.001 x = 0.01 x = 0.03 x = 0.06 x = 0.09

900 1000 Wavelength (nm)

1100

1200

Figure 9. Emission spectra of the samples Ca3-xEuxSc2Si3O12 (x = 0 – 0.09) at room temperature. The inset shows the intensity-concentration dependence of emission.

Figure 9 displays the emission spectra for Ca3-xEuxSc2Si3O12, where x varies from 0 to 0.09. The change of Eu2+ dopant ion concentration has negligible effect on the location of the emission peak. The inset shows the luminescence intensity of Ca3-xEuxSc2Si3O12 as a function of Eu2+ concentration x between 0 and 0.09 under 520 nm excitation at room temperature. With increasing Eu2+ concentration, the emission intensity increases gradually and reaches a maximum when the Eu2+ concentration is x = 0.06, after which quenching occurs. Figure S15 shows the corresponding excitation 18


Page 19 of 37

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spectra which also exhibit the maximum intensity at x = 0.06. Therefore, in Ca3-xEuxSc2Si3O12 the optimal doping concentration of Eu2+ is x = 0.06. In addition, the decay curves for different doping contents x in Ca3-xEuxSc2Si3O12 (Figure S16) do not show a significant lifetime change. The temperature-dependence of the Eu2+ emission in Ca2.94Eu0.06Sc2Si3O12 is rather unusual (Figure S17(a)). The emission intensity increases from 13 K up to 50 K and then decreases. The initial increase is attributed to the thermal population of Eu2+ excited states.38 A further increase in temperature produces photoionization. The Eu2+ ground state has been located in Figure 7 at 4.02 eV (from the CT transition of Eu3+). The 4f – 5d zero phonon line energy of roughly 1.8 eV (Figure 8a) therefore places the lowest Eu2+ 5d level at ~2.22 eV, i.e. ~0.66 eV (~5300 cm-1) below the conduction band. The activation energy fit to T > 150 K in Figure S17(b) gives a linear plot with the value of 0.05 eV (400 cm-1). Part of the discrepancy in value with 0.66 eV arises from the uncertainty of location of the Eu2+ ground state (from the Eu3+ charge transfer band maximum) and the band gap (from the excitation spectra, Figures 2(a), 3, rather than from absorption measurements) and another part from the omission of other parameters in the activation energy fit (see, for example, Ref. 38). However, it can be concluded that the temperature stability of Eu2+ emission is poor.

19



Ca2.94-2xCexEu0.06NaxSc2Si3O12 3+

Ce emission 2+ Eu emission

0.0

0.1

λex = 440 nm

x=0 x = 0.001 x = 0.03 x = 0.10 x = 0.20

0.2

Content (x)

400

500

600

800

RT

Relative 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 20 of 37

900

1000

1100

1200

Wavelength (nm)

Figure 10. Emission spectra of Ca2.94-2xCexNaxEu0.06Sc2Si3O12 under 440 nm excitation at room temperature. The inner figure shows the integrated emission area as a function of x.

2.8. Energy transfer between Ce3+ and Eu2+ in CSS. There is a large spectral overlap between the emission of Ce3+ (Figure 2) and the absorption of Eu2+ (Figure 8(a)), suggesting that efficient energy transfer may occur from the sensitizer Ce3+ to the activator Eu2+. In order to investigate this energy transfer, Ce3+ and Eu2+ co-doped samples Ca2.94-2xCexEu0.06NaxSc2Si3O12 and Ca2.8-2xCe0.1EuxNa0.1Sc2Si3O12 (where x is the nominal synthesis concentration) were prepared and the spectra and decay curves were

recorded

separately.

Figure

10

shows

the

emission

spectra

of

Ca2.94-2xCexEu0.06NaxSc2Si3O12 under 440 nm excitation at room temperature, as a function of the Ce3+ dopant ion concentration x. Note that this excitation wavelength does not exclusively populate Ce3+ because weak Eu2+ absorption also occurs at 440 nm. The emission intensity of Ce3+ gradually rises with x due to the increasing 20


Page 21 of 37

number of Ce3+ luminescence centers. By contrast, the emission intensity of Eu2+ increases up to x = 0.03 and then decreases. The variation of the integrated emission areas for Ce3+ and Eu2+ is shown in the inner panel of the figure. 1.2

Normalized intensity

Ca2.94-2xCexEu0.06NaxSc2Si3O12 1.0

(a)

λem = 870 nm

RT 0.8

0.6

x=0 x = 0.001 x = 0.03 x = 0.10 x = 0.20

0.4

0.2

350

400

450

500

550

600

650

700

750

Wavelength (nm) 0.9

(b)

0.8

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


0.7

Ca2.94-2xCexEu0.06NaxSc2Si3O12

0.6

0.5

0.4

0.3 0.00

0.05

0.10

0.15

0.20

3+

Ce content (x)

Figure 11. (a) The normalized excitation spectra (λem = 873 nm) of Ca2.94-2xCexEu0.06NaxSc2Si3O12 at room temperature. (b) Excitation intensity at 370 nm relative to that at 520 nm.

The excitation spectra, normalized at 520 nm, of Eu2+ emission (λem = 873 nm) from Ca2.94-2xCexEu0.06NaxSc2Si3O12 at room temperature (Figure 11(a)) show a 21



continuous increase of Ce3+ absorption at 450 nm as a function of Ce3+ dopant concentration. Notice that with increasing Ce3+ concentration, the higher energy Eu2+ absorption at 370 nm decreases in relative intensity relative to that at 520 nm, Figure 11(b). As shown in previously Figure 4, absorption at the former wavelength can give rise to the Ce3+ emission at 390 nm (and hence not Eu2+ emission). The excitation spectrum

of

the

Ce3+

emission

at

550

nm

in

the

co-doped

system

Ca2.8-xCe0.1Na0.1EuxSc2Si3O12 (Figure S18) does not show the presence of the Eu2+ absorption band at 368 nm so that nonradiative energy transfer from Eu2+ to Ce3+ does not occur. On the contrary, this excitation spectrum (Figure S18) and the emission spectrum, Figure S19, show that the emission of Ce3+ is quenched with the addition of Eu2+.

D

A

Ca2.8-xCe0.1EuxNa0.1Sc2Si3O12


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 37

RT

λex = 440 nm, λem = 550 nm x=0 x = 0.001 x = 0.01 x = 0.03 x = 0.06 x = 0.09 I-H fit

0

100

200 Time (ns)

300

400

Figure 12. The decay curves of Ca2.8-2xCe0.1EuxNa0.1Sc2Si3O12 and corresponding fitted curves using the I-H model at room temperature. Note the logarithmic scale of the ordinate. 22


Page 23 of 37

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The energy transfer rates were estimated from the decay curves of the Ce3+:5d1→ 4f1 emission at 550 nm in Ca2.8-xCe0.1Na0.1EuxSc2Si3O12 (x = 0-0.09) (Figure 12) at room temperature. With ascending Eu2+ concentration, the lifetimes decrease markedly and deviate from the monoexponential behavior of the sample x = 0. Therefore, the curves were fitted by the double-exponential equation: = A e

+ A e

(2)

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

)/(A + A )

(3)

The fitting results are presented in Table 2, with goodness of fit, for all the samples. The Ce3+ lifetime is drastically reduced from 66.8 ns and follows a monoexponential decay with Eu dopant ion concentration as shown in the top panel of Figure 13. The linear relationship of kET with Eu2+ dopant ion concentration x as shown in the bottom panel of this figure indicates a direct transfer from Ce3+ to Eu2+, with the transfer efficiency rising as high as 88.9% (Table 2, final column).

23



Table 2. Lifetimes, fitting parameters, energy transfer rates and efficiency for Ca2.8-2xCe0.1Na0.1EuxSc2Si3O12 from measurements of Ce3+ emission at 295 K. Eu2+

τ1

τ2

A1

A2

R2adj

τ

a

0

66.8 ± 0

66.8 ± 0

0.4874

0.4874

0.9984

66.8

0.0

0.0

0.001

18.0 ± 0.6

55.4 ± 0.5

0.2965

0.6886

0.9987

50.8

4.7

24.0

0.01

14.8 ± 0.3

48.1 ± 0.5

0.3855

0.5820

0.9984

42.4

8.6

36.5

0.03

6.9 ± 0.08

27.4 ± 0.2

0.5336

0.4332

0.9985

22.5

29.4

66.3

0.06

4.3 ± 0.04

17.1 ± 0.2

0.5995

0.2903

0.9985

12.8

63.4

80.9

0.09

2.6 ± 0.02

10.6±0.1

0.7053

0.2602

0.9990

7.4

120.4

88.9

a

kET (µs)-1

η (%)

kET = 1/τ - 1/τ0, where τ is the average lifetime at concentration x, and τ0 is at x =

Lifetime, τ (ns)

0.001. η = 100(1 - τ/τ0).

60

y = (52.9±6.7)exp[-x/(0.024±0.009)]+(7.2±6.2)

40

RT

Ca2.8-xCe0.1EuxNa0.1Sc2Si3O12 2

20

R

adj

= 0.9395

0

Energy transfer rate -1 kET( µ s)

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 37

0.00

0.02

0.04

0.06

0.08

0.10 RT

120

y` = ( 1269±100)x`- (2.6±4.6) 80 2

R 40

adj

= 0.9700 Ca2.8-xCe0.1EuxNa0.1Sc2Si3O12

0

0.00

0.02

0.04 0.06 Doping level, x

0.08

0.10

Figure 13. The Ce3+ emission lifetimes and the energy transfer rate as a function of Eu2+ dopant ion concentration in Ca2.8-xCe0.1EuxNa0.1Sc2Si3O12. 24


Page 25 of 37

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


Assuming a random distribution of Ce3+ around Eu2+ and without Ce3+-Ce3+ migration and Eu2+ - Ce3+ back-transfer as demonstrated above, the energy transfer dynamics can be further investigated by the Inokuti-Hirayama (I-H) model.39 The non-exponential

decay

of

donor

Ce3+

luminescence

is

attributed

to

a

multipole-multipole interaction between the donor Ce3+ ions and acceptor Eu2+ ions at different distances and can be described by the following equation:39

()

!

# $

= exp[−( ) − (1 − " ) ] ()

(4)

where '( is the acceptor concentration; ' is the critical concentration; and (1 − 3/*) is the gamma function. A value of 66.8 ns was taken for the lifetime (τ0) of Ce3+ as fitted above. The s value depends on the mechanism of transfer and for the following interactions it is: 6 (electric dipole-electric dipole or magnetic dipole-magnetic dipole); 8 (electric dipole-electric quadrupole); and 10 (electric quadrupole-electric quadrupole). Since the expression for energy transfer between a donor and acceptor can be written to involve individual matrix elements for the respective radiative transitions, 40 and both of the Ce3+ and Eu2+ transitions are from 4f to 5d states, the appropriate value of s is 6 (refer to the Supporting Information). Indeed, the decay curves plotted in Figure 12 closely resemble the plot for the regime of dipole-dipole energy transfer without donor migration.40 The fitted curves and the obtained fitting parameters are presented in Figure 12 and Table 3, respectively. The fits using other values of s are inferior (for s = 10, see Figure S20(a) for example).

25



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Page 26 of 37

Table 3. The fitting parameters and the CA/C0 ratios obtained from the I-H model.

Ca2.8-xCe0.1Na0.1EuxSc2Si3O12 CA(Eu2+)

Radj2

CA/C0

0

0.9984

-

0.001

0.9839

0.34

0.01

0.9973

0.55

0.03

0.9944

1.31

0.06

0.8466

2.10

0.09

0.9806

2.94

Neglecting the value for x = 0.001 in Table 3, the average value of C0, the critical transfer concentration, is x = 0.025±0.006. Taking the intrinsic lifetime τ0 as 66.8 ns, with the corresponding deactivation rate is 15 (µs)-1, then from the equation in the lower Figure 13, the value of C0 is calculated to be slightly lower at x = 0.014. There are 8x Eu2+ acceptors in the unit cell volume of 1837. 8 Å3, so that with the use of Eq. 25 in Ref. 39:

C0 =

3 4πR03

(5)

the value of R0 is determined to be 13 Å. This critical transfer distance, R0, represents the separation of an isolated Ce3+-Eu2+ pair when the energy transfer rate, kET = 1/τ0, i.e., the same rate as the spontaneous deactivation of Ce3+. The spectral overlap 26


Page 27 of 37

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calculation in the Supporting Information gives the value of R0 as 25 Å.

Figure 14. Spectral conversion design for solar cell applications, involving the AM 1.5 solar spectrum, spectral response of c-Si, and the PLE and PL spectra of Ca2.88Ce0.03Eu0.06Na0.03Sc2Si3O12.

3. APPLICATION TO SOLAR CELLS Figure 14 illustrates a schematic diagram for spectral conversion design in the solar cell application of the phosphor CSS:Ce,Eu. The AM 1.5 solar spectrum commences from the wavelength of 300 nm and shows the maximum absorption around 500 nm. When sunlight falls, the photons are firstly absorbed by the phosphor layer and converted to longer wavelengths more suitable for the response of the solar cell. The presence of Ce3+ ions makes up for the smaller absorption of Eu2+ in the range of 400-450 nm, so that the excitation spectrum of the Ce3+, Eu2+ co-doped sample Ca2.88Ce0.03Eu0.06Na0.03Sc2Si3O12 shows a continuous strong absorption from the UV through the visible spectral range and matches very well with the incident solar flux, implying a strong absorption of sunlight. Because the corresponding broad NIR 27



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

emission is located in the region of highest spectral response of the c-Si solar cell, the CSS: Ce, Eu phosphor could be a solar energy converter material for application together with the c-Si photovoltaic solar cell. The energy transfer efficiency from Ce3+ to Eu2+ is nearly 90%. The two main drawbacks have been identified herein: the poor valence stability of Eu2+ in the CSS host and the luminescence quenching with temperature increase.

4. CONCLUSIONS This study has comprised two facets: the spectral properties and energy transfer of Ce3+ and Eu2+ in the garnet host and the suitability for solar energy harvesting. Indeed, the CSS garnet host exhibits many unique characteristics. The energy levels of Ce3+ are subject to a strong crystal field so that the 4f1 -5d1 lowest energy zero phonon line is located in the blue spectral region. The Ce3+ ion occupies the Ca2+ site and charge compensation by Na+ has been included. The monoexponential emission decay does not exhibit noticeable quenching for Ce3+ concentration below x = 0.2 in Ca3-2xCexNaxSc2Si3O12, and the intensity only decreases by 20% at 460 K. By contrast, the emission of Eu2+-doped CSS is more sensitive to concentration and temperature quenching. However, the emission band is located in the near infrared spectral region and broad, intense 4f7 → 4f65d absorption bands span the visible and ultraviolet regions. The spectral overlap between Ce3+ emission and Eu2+ absorption promotes resonant energy transfer and the Ce3+ decay kinetics have been fitted by the Inokuti-Hirayama formalism for electric dipole – electric dipole energy transfer, in 28


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agreement with the mechanism deduced from selection rules. The broad, electric dipole allowed absorption spectrum of CSS:Ce3+, Eu2+ enables ~60% of the sunlight spectrum (from spectral overlap) to be utilized for solar energy conversion. The emission is more intense than that of Nd3+ or Yb3+ systems and it effectively targets the response of the c-Si solar cell. Although CSS:Ce3+ has excellent emission intensity and thermal performance, the drawbacks of valence instability and temperature quenching of emission when co-doping with Eu2+ have been found. Some suggestions are included for research to solve these problems which are related to host structure. First, the valence stability in a cubic garnet can be tuned by variation of the central cation size and the introduction of larger cation sites (e.g., Sr2+, Ba2+) may be beneficial.41 The thermal instability due to photoionization may be improved through the introduction of cations to improve structural rigidity (e.g., Mg2+, Al3+, Ge3+); to increase the host bandgap;42 and to utilize core-shell protected nanocrystals.43 A good host candidate could be Lu2CaMg2Si3O12, which exhibits longer wavelength Ce3+ emission.44,45 It is also important to modify the form of the solar cell device into a transparent glass or ceramic, or a phosphor in glass.46 We will pursue the above modifications.

ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21171176, U1232108, and U1432249).

29



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Page 30 of 37

SUPPORTING INFORMATION Experimental details; Centroid shift of Ce3+ in CSS; Energy transfer formulae; Figures and Tables.

REFERENCES (1)

International

Energy

Agency,

World

Energy

Outlook

2015,

http://www.worldenergyoutlook.org/weo2015/, sourced 19:30, 30 October 2015. (2) Xu, M.;Wachters, A. J. H.; van Deelen, J.; Mourad, M. C. D.; Buskens, P. J. P. A Study on the Optics of Copper Indium Gallium (di)Selenide (CIGS) Solar Cells with Ultra-Thin Absorber Layers. Opt. Express, 2014, 22, A425-A437. (3) Freitas, V. T.; Fu, L.; Cojocariu, A. M.; Cattoën, X.; Bartlett, J. R.; Le Parc, R.; Bantignies, J.-L.; Wong, M. C. M.; André, P. S.; Rute A S Ferreira, R. A. S.; Carlos, L. D. Eu3+-Based Bridged Silsesquioxanes for Transparent Luminescent Solar Concentrators. ACS Appl. Mater. Interfac., 2015, 7, 8770-8778. (4) Wang, F.; Liu, X. Upconversion Multicolor Fine-Tuning: Visible to Near-Infrared Emission from Lanthanide-Doped NaYF4 Nanoparticles. J. Am. Chem. Soc., 2008, 130, 5642-5643. (5) Martín-Rodríguez, R.; Geitenbeek, R.; Meijerink, A., Incorporation and Luminescence of Yb3+ in CdSe Nanocrystals. J. Am. Chem. Soc., 2013, 135, 13668–13671. (6) Shimomura, Y.; Honma, T.; Shigeiwa, M.; Akai, T.; Okamoto, K.; Kijima, N. 30


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Photoluminescence and Crystal Structure of Green-Emitting Ca3Sc2Si3O12:Ce3+ Phosphor for White Light Emitting Diodes. J. Electrochem. Soc., 2007, 154, J35-J38. (7) Piccinelli, F.; Speghini, A.; Mariotto, G.; Bovo, L.; Bettinelli, M. Visible Luminescence of Lanthanide Ions in Ca3Sc2Si3O12 and Ca3Y2Si3O12. J. Rare Earths, 2009, 27, 555-559. (8) Bettinelli, M.; Speghini, A.; Piccinelli, F.; Neto, A. N. C.; Malta, O. L. Luminescence Spectroscopy of Eu3+ in Ca3Sc2Si3O12. J. Lumin., 2011, 131, 1026-1028. (9) Tanner, P. A. Some Misconceptions Concerning the Electronic Spectra of Tri-Positive Europium and Cerium. Chem. Soc. Rev., 2013, 42, 5090-5101. (10) Ivanovskikh, K. V.; Meijerink, A.; Piccinelli, F.; Speghini, A.; Ronda, C.; Bettinelli, M. VUV Spectroscopy of Ca3Sc2Si3O12:Pr3+: Scintillator Optimization by Co-Doping with Mg2+, ECS J. Solid State Sci. Technol., 2012, 1, R127-R130. (11) Liu, Y.; Zhang, X.; Hao, Z.; Wang, X.; Zhang, Tunable Full-Color-Emitting Ca3Sc2Si3O12:Ce3+, Mn2+ Phosphor Via Charge Compensation and Energy Transfer. Chem. Commun., 2011, 47, 10677-10679. (12) Berezovskaya, I. V.; Dotsenko, V. P.; Voloshinovskii, A. S.; Smola, S. S. Near Infrared Emission of Eu2+ Ions in Ca3Sc2Si3O12. Chem. Phys. Lett., 2013, 585, 11-14. (13) Wu, Y. F.; Chan, Y.H.; Nien, Y.T.; Chen, I.G. Crystal Structure and Optical Performance of Al3+ and Ce3+ Codoped Ca3Sc2Si3O12 Green Phosphors for White LEDs. J. Am. Ceram. Soc., 2013, 96, 234-240. (14) Velázquez, J. J.; Fernández-González, R.; Marrero-Jerez, J.; Rodríguez, V. D.; 31



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Unit

Cosubstitution

and

Tuning

of

Photoluminescence

in

the

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