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A High Efficiency Green Phosphor BaLuSiO :Tb : Visible Quantum Cutting via Cross Relaxation Energy Transfers Yongfu Liu, Jianxin Zhang, Changhua Zhang, Jun Jiang, and Haochuan Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11790 • Publication Date (Web): 12 Jan 2016 Downloaded from http://pubs.acs.org on January 17, 2016

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The Journal of Physical Chemistry

A High Efficiency Green Phosphor Ba9Lu2Si6O24:Tb3+: Visible Quantum Cutting via Cross Relaxation Energy Transfers Yongfu Liu,*, † Jianxin Zhang,†, ‡ Changhua Zhang,†, § Jun Jiang,*, † and Haochuan Jiang*, † †

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of

Sciences, Ningbo, 315201, China ‡

College of Electronic Information and Engineering, Hangzhou Dianzi University,

Hangzhou 310018, P. R. China §

Department of Chemistry, College of Science, Shanghai University,

Shanghai 200444, P. R. China

*Corresponding author *E-mail: [email protected] [email protected] [email protected] No. 1219 Western Zhongguan Road, Ningbo 315201, China Tel. 86-574-87619207, Fax. 86-574-86382329

ACS Paragon Plus Environment


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ABSTRACT Tb3+-doped phosphors are widely used in fluorescent lamps and plasma display panels (PDP) due to the strong VUV/UV light absorption of Tb3+. Most of these phosphors are fluorides. The quantum efficiency (QE) of these phosphors is more than 90%, however, it is still too low. Theoretically, one VUV/UV photon can convert two visible photons or more by visible quantum cutting (QC), and the QE can reach 200% or more. Usually, the oxides have a higher QE than fluorides. In this work, we obtained a novel oxide phosphor Ba9Lu2Si6O24:Tb3+ (BLS:Tb3+). Under the 251 nm UV-light excitation, QC processes occur in BLS:Tb3+ via cross-relaxation energy transfers (CRET) between Tb3+ ions, leading to intense green emissions around 552 nm. Based on an indirect method, the ideal QE is calculated nearly to be 171%. When the UV light absorption and energy losses were considered, the practical QE is near 144% estimated by a direct method. This value is much higher than that of the commercial phosphors of 90%, indicating the promising application of BLS:Tb3+ for fluorescent lamps and PDP. The CRET processes were investigated according to the luminescence spectra and decay curves.

Keywords: Ba9Lu2Si6O24:Tb3+, cross-relaxation energy transfer, visible quantum cutting, downconversiton


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1. INTRODUCTION With the development of the rising mercury-free fluorescent tubes and plasma display panels (PDP), the demand of vacuum ultraviolet (VUV)/UV-excited phosphors increases greatly.1-3 The quantum conversion efficiency from VUV/UV photons to visible photons of the commercial phosphors has more than 90%, however, the efficiency is still very low.2 Theoretically, one VUV/UV photon can convert more than one visible photon via downconversion. This process is commonly called as visible quantum cutting (QC).4 So the quantum efficiency (QE) of phosphors can be enhanced by QC and could be 200% or more. Visible QC has been detected in many phosphors.5-13 The greatest calculated QE in the visible spectral region reached 190% for LiGdF4:Eu3+ and 194% for BaF2:Gd3+,Eu3+.4, 10 Tb3+ is a promising activator for phosphors applied in the fluorescent tubes, PDP devices, and so on.2,

14

Because Tb3+ shows a strong VUV/UV excitation

characteristic due to its spin-allowed 4f-5d transition, and an intense visible emission can be achieved in an appropriate host lattice upon the VUV/UV excitation.2, 15 QC also exists in Tb3+ and can be realized by cross-relaxation energy transfer (CRET) processes between Tb3+ ions. There are two kinds of CRET processes for Tb3+.2 One is the well-known process by transitions 5D3 → 5D4 and 7F6 → 7F0, and the other is the process by transitions 7DJ → 5D1, 2, 3 and 7F6 → 5D4. Both of the two processes are strongly dependent on the Tb3+ concentration and lead to tunable emissions from blue to green region. These phenomena have been witnessed in many phosphors,16-25 such as

K2GdF5:Tb3+,16

GdPO4:Tb3+,18

CaSc2O4:Tb3+,19


BaGdB9O16:Tb3+,21


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Ca2Gd8Si6O26:Tb3+,22 Gd2O2S:Tb3+,25 and so on. For Eu3+ or Tb3+ doped phosphors mentioned above, most of them are the Gd3+-based or -doped phosphors and QC is always realized by constructing the Gd3+ → Eu3+ or Gd3+ → Tb3+ energy transfers. Wegh et al first proposed a formula to calculate the CRET efficiency (ηCRET) according to the energy transfer relationship in Gd3+-Eu3+ system.4 Lee et al modified the formula proposed by Wegh et al to calculate the ηCRET in Gd3+-Tb3+ system.16 Both of these formulas are based on the luminescence spectra of Eu3+ or Tb3+. On the other hand, ηCRET that used to reflect the extra QE in the QC processes can also be quantified in term of fluorescence lifetimes.26 So far, the calculated total QE reached a maximum of 189% for K2GdF5:Tb3+.16 However, the calculated QE was rough because they ignored the energy loss by non-relaxation transitions when they dealt with the QC processes. Xie et al.24 discussed and confirmed that Gd3+ played an important role in the QC processes for Gd3+-Tb3+ system. Really, the Gd3+-containing phosphors always have a high QC efficiency. However, Gd3+ is not always necessary to achieve a high QC efficiency. Previously, we discover a novel orthosilicate Ba9Lu2Si6O24 (BLS), which is an excellent photoluminescent material host.27, 28 In present work, we realize a high QC efficiency when Tb3+ ions are doped into the Gd3+-free host BLS. The calculated QE is 171% based on the luminescence spectra. As mentioned, the evaluation of the total QE that containing the QC processes is ideal and theoretical, because it ignoring the absorption of incident light and energy losses at defects and impurities. So this is an indirect evaluation for phosphors’ QE. A direct method by comparing the emitted


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photons to the incident or absorbed photons should be adopted to evaluate the practical QE of phosphors. In this work, we estimate the QE for BLS:Tb3+ by a direct method, as well as an indirect method based on the luminescence spectra. The difference of QE based on these two methods is discussed. When the absorption and energy losses are considered, the practical QE is still near 144%. The QE is much higher than the value for most commercial phosphors (90%), indicating a promising application of BLS:Tb3+for fluorescent lamps and PDP. The luminescence properties, the origin of the high QE and the QC processes and the CRET processes between Tb3+ ions are investigated according to the luminescence, energy level scheme, and fluorescence decays of Tb3+ in the BLS matrix. 2. EXPERIMENTAL SECTION Synthesis. Samples of Ba9(Lu2-xTbx)Si6O24 (x = 0.02 - 1.20) were prepared by the conventional high temperature solid state reaction. The starting materials (BaCO3, 99.8%; Lu2O3, 99.99%; SiO2, 99.9%; Tb3O4, 99.99%) were all weighted out in the desired stoichiometry and thoroughly mixed using an agate mortar and pestle for 40 min. The mixed powders were placed in an alumina crucible and sintered in a tube furnace under reducing atmosphere (95%N2+5%H2) at 1500°C for 4h with heating and cooling rates of 2°C/min. Each mixture was subsequently ground to a fine powder with an agate mortar and pestle for further analysis. Characterization. X-ray powder diffraction (XRD) data were collected for all samples on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.54056 Å). To perform the crystalline structure and the phase purity of samples, XRD profiles



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were recorded in the range of 10-80° with a step size of 0.02° (2θ). All the operations were carried out at 40 KV and 40 mA. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were obtained on a Hitachi F-4600 spectrometer equipped with a 150W Xenon lamp under a working voltage of 350 V. Both the excitation and emission slits were set at 5.0 nm. The CIE chromaticity coordinates at various Tb3+ concentrations were calculated based on the measured PL spectra. The internal and external QEs based on the direct method and absorbance were measured by using the QE-2100 quantum efficiency measurement system from Otsuka Electronics. The fluorescence decay curves of Tb3+ were measured by the Horiba Fluorolog FL3-111 spectrometer with a scintillate Xenon lamp. Photographs of the as-prepared samples under the 254 nm UV-light were taken by the Nikon D7100 digital camera. All the measurements were carried out at room temperature. 3. RESULTS AND DISCUSSION Crystal Structure. Figure 1 shows the XRD patterns for BLS:xTb3+ (x =0.02 ~ 1.20) with various Tb3+ concentrations. All the samples exhibit a single crystal phase similar with the Ba9Sc2Si6O24 (BSS, PDF # 82-1119) structure. As demonstrated in our previous work, BLS crystallizes into a rhombohedral structure that same with BSS.27 Therefore, the BLS crystal structure does not change with the substitutions of Tb3+ for Lu3+ in the BLS matrix. Tunable Luminescence. Figure 2 displays the PLE spectra of BLS:2%Tb3+ monitored at 380 and 552 nm. Both of them exhibit a strong excitation band peaking at 251 nm and a weak excitation band peaking at 282 and 302 nm. In addition, the


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excitation band for the 552 nm shows some peaks from 325 to 525 nm. These excitation bands and peaks include the f-d transitions and f-f transitions of Tb3+. The f-d transitions of Tb3+ include spin-allowed (SA) and spin-forbidden (SF) transitions. Usually, the f-d transition of Tb3+ is high than that of Ce3+ in the same host lattice with the energy difference of ~ (13.2 ± 0.92) × 103 cm-1.29 In our previous work, it was found that the f-d excitation band of Ce3+ in BLS is around 400 nm (25000 cm-1).27 The energy difference between Ce3+ and the strong excitation band of Tb3+ (251 nm, ~ 39841 cm-1) is about 14.8 × 103 cm-1 in the present case, which is near the average 5d energy difference between Ce3+ and Tb3+. Therefore, the strong excitation band peaking at 251 nm is assignable to the SA 4f-5d transitions (7F6-7DJ, 7DJ is corresponding to the low-spin state) of Tb3+. It is also known that the first SF 4f-5d transition (7F6-9DJ) of Tb3+ often occurs as a weak satellite peak at the long-wavelength side (~ 6300 ± 900 cm-1) of the first SA 4f-5d excitation band.29 Clearly, the weak band with peaks at about 282 and 302 nm (~ 5500 cm-1 lower than the SA f-d transition) belong to the lowest SF f-d transition (7FJ-9DJ, 9DJ is corresponding to the high-spin state) of Tb3+ in the host lattice. The excitation peaks in the range of 325-525 nm correspond to the f-f transitions of Tb3+. Figure 3 shows the PL spectra of BLS:xTb3+ (x = 0.02-1.20) with different Tb3+ concentrations under the 251 nm excitation. The emission spectra ranging from 350 to 475 nm are attributed to emissions from 5D3 to 7FJ (J = 6, 5, 4 and 3) of Tb3+. The emission spectra ranging from 475 to 725 nm are attributed to emissions from 5D4 to 7

FJ (J = 6, 5, 4, 3, 2, 1 and 0) of Tb3+. The blue emissions from 5D3 to 7FJ dominate the



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spectrum, and the peak locates at 380 nm (5D3 → 7F6) at a low Tb3+ concentration (x = 0.02). The blue emissions decrease with the increase of Tb3+ concentrations, while the green emissions from 5D4 to 7FJ dominate the spectra and the emission peak changes into 552 nm (5D4 → 7F5) after x = 0.10. The blue emissions almost disappear and only the green emissions maintain in the spectra at a high Tb3+ concentrations (x > 0.40). These phenomena result from the enhancement of the Tb3+ concentration-dependent CRET processes between Tb3+ ions and will be detailed according to the energy level scheme in following section. BLS:xTb3+ shows a tunable emission from blue to green, depending on the Tb3+ concentrations, as the CIE chromaticity diagram and digital images shown in Figure 4, in which the CIE coordinates and their positions are indicated. The dependence of emission peak intensities at 380 nm (5D3 → 7F6, I380 nm) and at 552 nm (5D4 → 7F5, I552 nm) on the Tb3+ concentrations upon 251 nm excitation is shown in Figure 5(a). I380 nm decreases rapidly with increasing Tb3+ concentrations, and the intensities are almost near zero after x = 0.60 due to cross-relaxation between Tb3+ ions. I552

nm

enhance greatly with increasing Tb3+ concentrations and nearly

maintain a maximum from x = 0.60 to 1.00. The concentration quenching only occurs at a high value of x = 1.20. Based on a direct method, we estimated the total internal and external QEs for BLS:xTb3+ including the 5D3 and 5D4 emissions from 350 to 725 nm. These results are depicted in Figure 5(b). It can be found that the QEs and the 5D4 emission intensities exhibit a similar dependence on the Tb3+ concentrations, and the QE drops down apparently when x = 1.20 due to the Tb3+ concentration quenching


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effects. These results indicate that the strong green light emissions of BLS:Tb3+ mainly come from the 5D4 emissions of Tb3+. The internal QE (ηint) is also called as fluorescence quantum yield (ΦF), which is the ratio of emitted photons by the number of only absorbed photons.30 The external QE (ηext) is the ratio of emitted photons by the number of incident photons, called as absolute quantum yield as well.31-34 Their relationship can be described as following:

=

=

=

(1)

where Ninc, Nabs and Nem are the number of incident, absorbed and emitted photons, respectively. A is the absorbance. Equation (1) is a direct method to evaluate the practical QE, which can be obtained by the QE-2100 quantum efficiency measurement system mentioned in the Experimental Section. The higher of the QE value, the better of luminescent materials. For down-converting photoluminescent materials, QE is commonly less than 100% due to the limit of absorbance and energy losses by non-radiative mechanism. However, the estimated values for BLS:xTb3+ listed in Table 1 display that the ηint even more than 100% when x exceeds 0.02 and reaches a maximum of 143.5% at x = 0.60. The ηext also reach a maximum of 118.3% at x = 1.00. The maximum values for ηint and ηext do not appear at the same Tb3+ concentration. This could be ascribed to the different absorbance for different Tb3+ concentration. The actuality that both the ηint and ηext exceed 100% indicates that QC, which is considered as an effect way to improve the QE, should occur in the BLS:xTb3+ system. Quantum Cutting. For Tb3+, QC has been confirmed in many phosphors.16-25 Most of



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the compositions contain Gd3+ ions in the material hosts. Their remarkable characteristic is that the emission ratio of 5D4 to 5D3 is higher when the 5d energy level of Tb3+ is excited than that when the 6IJ level of Gd3+ is excited. In the BLS:xTb3+ system, the host does not contain Gd3+ ions. Before demonstrating the QC process, the level of 5D3 is selected to be excited. Figure 6 shows the PL spectra of BLS:xTb3+ (x = 0.02 – 1.20) under the 352 nm excitation, and the 5D3 → 7F6 emission intensity (I380

nm)

is set as the normalized standard. It is clear that the 5D4 green

emission is dominant for all the Tb3+ concentrations. This is different from the PL spectra under the 251 nm excitation (Figure 3), in which the 5D3 blue emissions are dominant at low Tb3+ concentrations (x = 0.02 and 0.06). The green emission from the magnetic dipole 5D4 → 7F5 transition splits into two peaks around 544 and 552 nm due to the Stark splitting of Tb3+. The peak intensity at 552 nm is the maximum value for all the Tb3+ concentrations under the 251 nm excitation; while the peak intensity at 544 nm is the maximum for Tb3+ at low concentrations (x = 0.02 - 0.40) under the 352 nm excitation, and the maximum shifts to the 552 nm peak only at high Tb3+ concentrations (x = 0.60 – 1.20). Therefore, the value of r in Figure 6 means the ratio of the emission intensity at 544 or 552 nm to that at 380 nm. One can find that r increases from x = 0.02 to 1.00 and declines at x = 1.20, exhibiting a similar changing rule with the emission intensities and QEs in Figure 5. The value of r increases greatly with the increasing Tb3+ concentrations under the 352 nm excitation. The reason should be assigned to the CRET processes between Tb3+ ions through the transitions 5D3 → 5D4 and 7F6 → 7F0. The schematic energy


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levels of Tb3+ in the BLS matrix are depicted in Figure 7 to elucidate this CRET process. When an electron is excited into the 5D3 level (process 1), it yield the 5D3 blue emissions (process 2) after the multiphoton relaxation. The 5D4 state also can be populated by the cross relaxation (process 3) and then yield the 5D4 green emissions (process 4). Because the energy difference between 5D3 and 5D4 (~ 5800 cm-1) of Tb3+ is close to that between 7F0 and 7F6 (~ 6000 cm-1) in the BLS:Tb3+ samples. If two Tb3+ ions are located close enough, a resonance condition can exist where the conversion of an electron from the 5D3 to the 5D4 state was matched by the promotion of an electron from the 7F6 to the 7F0 state (process 3). The electron in the 7F0 level could than fall down to the ground state by a nonradiative process. It is noted that the energy difference between 5D3 and 7F0 (~ 20300 cm-1) is close to that between 5D4 and 7

F6 (~ 20500 cm-1). Therefore, another cross relaxation (process 5) also can exist in

the BLS:Tb3+ samples if the Tb3+ ions are close enough, which further gives rise to the 5D4 green emissions (process 6). With increasing Tb3+ concentrations, the distance between Tb3+ ions becomes close, and then the resonance energy transfers easily occur through the cross relaxation processes. The elevating of the emission ratio in Figure 6 evidences the stepped-up cross relaxation effects on the luminescence properties of BLS:Tb3+ and indicates an increase in the resonance energy transfer efficiency upon the 5D3 excitation. The high quenching concentration for BLS:Tb3+ should also be attributed to the CRET interactions. For the emission spectra upon the 5d excitation (Figure 3), the ratio values of I552 nm to I380 nm for BLS:xTb3+ are listed in Table 2. The 5D4 green emissions further display



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a higher ratio upon the 5d excitation (λex=251 nm) than upon the 5D3 excitation (λex=352 nm) at high Tb3+ concentrations (x = 0.60 - 1.20), compared with the r values in Figure 6 and Table 2. These results further indicate that QC processes, leading to the QEs more than 100%, must exist in BLS:xTb3+. The QC processes under the 5d excitation can be descripted based on the energy levels of Tb3+ in Figure 7 as well. Electrons can be excited into the 7DJ state (5d levels) under the 251 nm excitation (process 7). Some of the excited electrons will relax to the 5D3 state by multiphoton relaxation (process 8), and then experience from process 2 to 6, giving rise to the blue and green emissions. On the other hand, we note that the energy difference between the lowest 7DJ level and 5D4 (~ 19300 cm-1) is close to that between 5D4 and 7F6 (~ 20500 cm-1). When the excited electrons populate from the 7

DJ state into the 5D4 state, the electrons at the 7F6 ground state also can be promoted

into the 5D4 state by resonance cross relaxation (from process 9 to 5) if two Tb3+ ions are close enough. That is to say, if the cross relaxation from process 9 to 5 occurs, one electron at the 7DJ state can produce two excited electrons at the 5D4 state and then result in two 5D4 green-emitting photons (process 6 and 10). This further means the QE of Tb3+ can exceed 100% and even reach 200% upon excitation to its 5d states in theory through the CRET process. However, the CRET process that resulting QC in BLS:Tb3+ is not based on the transitions 7DJ → 5D1, 2, 3 and 7F6 → 5D4, while it is based on the transitions 7DJ → 5D4 and 7F6 → 5D4. This 7DJ → 5D4 and 7F6 → 5D4 transition has been observed in Tb/Gd-doped CsCdBr3 crystals, but this transition is quite rare.35


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QC process can exist in the BLS:xTb3+ samples under the 5d excitation. However, it is interesting that the emission ratio of 5D4 to 5D3 under the 5D3 excitation is much higher than that under the 5d excitation at low Tb3+ concentrations (x ≤ 0.40) by comparing the r values in Figure 6 and Table 2. The QC performance is only marked at high Tb3+ concentrations (x ≥ 0.60). These phenomena could mean that at low Tb3+ concentrations the cross relaxation from process 5 to 5 under excitation into the 5D3 level is more outstanding than the QC from process 9 to 5 and 10 under excitation into the 5d levels. That is to say, at low Tb3+ concentrations, the cross relaxation interaction from process 5 to 5 is weaker when excited into the 5d levels than that excited into the 5D3 levels. Thus, in Figure 3 the 5D3 blue emissions are dominant at low Tb3+ concentrations, which is different from the 5D3 emissions in Figure 6. By the way, the band shape of 5D3 emissions in Figure 6 is little different from that in Figure 3 as well. This maybe results from the unknown mechanisms during the CRET processes within Tb3+ ions or results from the experimental error. For the Gd3+-based materials, the QC efficiency (ηQC) of Tb3+ can be estimated with the following equation proposed by Wegh et al.4 and modified by Lee et al.16 =

( /) ( /)!" ( /)

%$(2)

Where R(5D4/rest) is the ratio of the PL intensity of 5D4 to that attributed to 5D3 of Tb3+ and 6P7/2 of Gd3+; the subscript indicates excitation from Tb3+ or Gd3+. For the Gd3+-free sample, such as BLS:Tb3+ in present work, the QC efficiency is calculated based on the processes in Figure 7. Under the 5D3 excitation that without QC, the 5D3 emission is named as (5D3)0 including process 2 (P2), and the 5D4



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emission is named as (5D4)0 including process 4 (P4) and CRET from process 5 to 6 (P5-6). Thus the (5D3)0 and (5D4)0 emissions without QC can be expressed as following (& '( )) = *+

(3)

(& ', )) = *, + *&

(4)

.

/) = (& ', )) ⁄(& '( ))

(5)

where R0 is the “normal branching ratio” when excited at the 5D3 level of Tb3+. Under the 5d excitation with QC, the 5D3 and 5D4 emissions are named as (5D3)5d and (5D4)5d, respectively. They can be expressed by (& '( )&1 = *+

(6)

(& ', )&1 = *, + *&

.

+ *2

& .

+ *%)

/&1 = (& ', )&1 ⁄(& '( )&1

(7) (8)

where P9-5-6 is the QC from process 9 to 5 and 6, R5d is the branching ratio when excited at the 5d levels of Tb3+. Based on the discussion about QC, we know that P9-5-9 is equal to P10. So the Equation (7) can be changed as (& ', )&1 = *, + *&

.

+ 2 × *%) = (& '( )&1 × /) + 2 × *%)

(9)

Based on the Equation (9), the extra 5D4 emission (Nem(extra)) inducing from QC is obtained by 56(7) = *2

& .

%

= *%) = + 8(& ', )&1 − (& '( )&1 × /) :

(10)

Before the calculation, some essential premises are often proposed: the UV absorption of phosphors should not be taken into account, and possible nonradiative losses due to energy migration at defects and impurities in samples must be ignored.4, 16 These premises mean an ideal internal QE of 100% in a sample without QC. In this


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condition, we can obtain that 57; = 56 = *+ + *, + *&

.

+ *%) %

= (& '( )&1 + (& '( )&1 × /) + + 8(& ', )&1 − (& '( )&1 × /) :

(11)

thus the extra efficiency from QC, ηQC, will be expressed by =

()

=

? A( )" @

( )" ×B C ? @

( )"$( )" ×B $ 8( )" ( )"×B :

=

? A( )" ( )"×B C @ ? ( )"$ 8( )"$( )" ×B : @

=

? A( )" ⁄( )" B C @ ? %$ 8( )"⁄( )"$B: @

=

? ( B ) @ " ? %$ (" B ) @

(12)

The R5d and R0 can be obtained from the r values in Table 2 and Figure 6. Based on the r values and Equation (12), ηQC and the total internal QE (ηint-cal) were calculated and listed in Table 2. ηQC improves from 8.04% for x = 0.60 to 70.7% for x = 1.20 due to the strengthened CRET. For overall calculation of internal efficiency involved the QC processes, it should be the sum of an ideal internal QE (100%) and the extra ηQC. The total ηint-cal reaches a maximum of 171% at x = 1.20 via QC. Tb3+ → Tb3+ CRET. Figure 8(a) shows the fluorescence decay curve for the BLS:2%Tb3+ sample monitoring the 5D3 → 7F6 emission (λem = 380 nm) under the 5d excitation (λex = 251 nm). It can be well fitted by a double-exponential function with chi-sq = 1.26. The difference between experiment and theory is presented in Figure 8(b). The decay curve is decomposed into a fast decay with a lifetime of τ1 = 0.375 ms and a slow decay with a lifetime of τ2 = 2.754 ms. The component of the slow decay is nearly close to a single-exponential function, indicating that it should be the



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intrinsic 5D3 decay of Tb3+ in the BLS host. The fast decay at the beginning of the curve is resulted from energy transfers from Tb3+ to a neighboring Tb3+ through the CRET processes. However, the CRET is not obvious due to a low Tb3+ concentration and a larger distance between Tb3+ ions. The cross relaxation effects can be strengthened by decreasing the distance between Tb3+ ions. Figure 9 represents the 5D3 decay curves (λem = 380 nm, λex = 251 nm) of BLS:xTb3+ (x = 0.02 – 1.20) for various Tb3+ concentrations. The fast decay components within the initial 2 ms range strongly evidence for the existence of the Tb3+ → Tb3+ CRET. Furthermore, it is obvious that the decays speed up and depart from single exponential function greatly with increasing Tb3+ concentrations due to the strengthened cross relaxation effects. In this situation, we define an average fluorescence lifetime for Tb3+ as I

D = E) F (G)HG

(13)

where I(t) is the fluorescence intensity at time t with normalized initial intensity.36-38 The fluorescence lifetimes at 5D3 (τ380

nm)

for various Tb3+ concentrations were

calculated and listed in Table 2. It is found that the lifetime, τ380 nm, decreases from 2.181 to 0.067 ms with the Tb3+ concentration increases from x = 0.02 to 1.20 due to CRET between Tb3+ ions. The CRET efficiency, ηCRET, can be calculated by the equation38, 39 M

JK = 1 − M

B

(14)

where τ is the average fluorescence lifetime of Tb3+ at 380 nm, that is τ = τ380 nm; τ0 is the intrinsic fluorescence lifetime of Tb3+ at 380 nm, that is τ0 = τ2 = 2.754 ms. The


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efficiency ηCRET increases greatly with increasing x, as listed in Table 2, and even reaches as high as 97.6% for x = 1.20. The energy transfer rate (W) can be calculated by W = 1/τ - 1/τ0.38, 39 The dependences of τ380 nm, ηCRET, and W are depicted in Figure 10(a), (b), and (c), respectively. The lifetime of τ380 nm decreases extremely with x increases from 0.02 to 0.20, after that the trend of decrease of τ380 nm becomes slow in Figure 10(a). Correspondingly, the efficiency of ηCRET elevates greatly from x = 0.02 to 0.20 in Figure 10(b), and then ηCRET almost reach a saturation after x = 0.40. Different from the changes of lifetime and efficiency, the energy transfer rate, W, almost increases proportionally with the x value and reaches 14.56 ms-1 for x = 1.20. These results state that the efficiency ηCRET will reach saturation at high Tb3+ concentrations, however, the transfer rate W still increases due to the strengthened cross relaxation interaction resulted from the more and more close distance between Tb3+ ions. The Tb3+ → Tb3+ CRET processes also can be evidenced by the fluorescence decay curves of the 5D4 → 7F5 transition (λem = 552 nm) of Tb3+ under the excitation at 5d (λex = 251 nm). As Figure 11(a) shown, the 5D4 decay curves for various x values in BLS:xTb3+ almost exhibit a single-exponential decay model, especial for high Tb3+ concentrations. When the decay curves are displayed in the range of initial 6 ms in Figure 11(b), one can clear see build-up processes in the curves, especial for the low Tb3+ concentrations. These strongly evidence the CRET between Tb3+ ions in BLS:xTb3+. The energy excited into the 5d levels of Tb3+ ion firstly transfers to the 5

D4 level of a neighbor Tb3+ ion via cross relaxation in the build-up process, and then



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 5D4 → 7F5 transition decay slowly. However, the initial rise processes were only detected at low Tb3+ concentrations (x = 0.02 – 0.80), and only simple single-exponential decay processes can be observed at high Tb3+ concentrations (x = 1.0 and 1.2). These phenomena indicate the CRET rate become more and more fast with the increasing Tb3+ concentrations, consisting with the calculated results in Figure 10(c). The single-exponential decay curve for high Tb3+ concentration should only be attributed to the intrinsic decay behavior of the 5D4 → 7F5 transition. To confirm this point, the decay curves of the 5D4 → 7F5 transition under the excitation at 5d (λex = 251 nm) and 5D4 levels (λex = 501 nm) are compared and depicted in Figure 12(a) for the BLS:120%Tb3+ sample. The reason that we chose 501 nm as an excitation resource is that the cross relaxation processes can be ignored and only the intrinsic decay can be obtained under this wavelength excitation. It is clear that the 5D4 decay curves under the 251 and 501 nm excitation almost parallel with each other. Both of the two curves can be well fitted by a single-exponential function based on the residuals in Figure 12(b) and (c). They even have a same fluorescence lifetime of τ = 5.907 ms. The same decay model and lifetime directly evidence that the CRET process can be ignored due to the faster cross relaxation rate and only the intrinsic 5D4 decay can be detected under the 5d excitation. The average lifetime for 5D4 transition under the 5d excitation were also calculated and listed in Table 2. The decrease of the average lifetime from 8.078 ms for x = 0.02 to 5.803 ms for x = 1.20 should also arise from the increase of the cross relaxation rate at high Tb3+ concentrations.


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The QE of BLS:Tb3+ calculated by the luminescence spectra can nearly reach 171% in theory. However, this method in term of spectra is indirect and ideal, because it ignores the absorption of the incident UV light and energy losses at defects and impurities. The practical QE for luminescent materials should be evaluated by a direct method. Clearly, the QE of BLS:Tb3+ drops down to 144% when the absorption and energy losses are considered. However, this value is still very higher than the commercial phosphors of 90%, indicating a promising application of BLS:Tb3+ for fluorescent lamps and PDP. 4. CONCLUSION We obtained a novel green phosphor BLS:Tb3+ for PDP and fluorescent lamps. With increasing Tb3+ concentrations, BLS:Tb3+ exhibits tunable emissions from blue to green region due to the CRET processes. Upon the 251 nm excitation, visible QC occurs via the CRET processes, leading to high efficient green emissions. The ideal QE that ignoring the absorption and energy losses was calculated by the indirect methods based on the spectra, which could reach 171%. When the absorption of incident UV light and energy losses were considered, the practical QE estimated by a direct method was near 144%. This value is much higher than most of commercial phosphors of 90%, indicating the promising applications BLS:Tb3+ for PDP and fluorescent lamps. ACKNOWLEDGEMENTS This work is financially supported by the National Natural Science Foundation of China

(NSFC11404351),

Ningbo

Municipal

Natural


Science

Foundation


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(2014A610122), Ningbo Science and Technology Innovation Team (2014B82004), and the China Postdoctoral Science Foundation (2014M560497, 2015T80638). REFERENCES (1) Henderson, B.; Imbusch, G. F. Optical spectroscopy in inorganic solids; Clarendon: Oxford, 1989. (2) Blasse, G.; Grabmaier, B. C. Luminescent materials springer; Verlag: Berlin, 1994. (3) Ronda, C. R. Phosphors for lamps and displays: an applicational view. J. Alloys Compd. 1995, 225, 534-538. (4) Wegh, R. T.; Donker, H.; Oskam, K. D.; Meijerink, A. Visible quantum cutting in LiGdF4:Eu3+ through downconversion. Science. 1999, 283, 663-666. (5) Sommerdijk, J. L.; Bril, A.; de Jager, A. W. Two photo luminescence with ultraviolet excitation of trivalent praseodymium. J. Lumin. 1974, 8, 341-343. (6) Piper, W. W.; Deluca, J. A.; Ham, F. S. Cascade fluorescent decay in Pr3+-doped fluorides: achievement of a quantum yield greater than unity for emission of visible light. J. Lumin. 1974, 8, 344-348. (7) Srivastava, A. M.; Doughty, D. A.; Beers, W. W. Photon cascade luminescence of Pr3+ in LaMgB5O10. J. Electrochem. Soc. 1996, 143, 4113-4116. (8) Wegh, R. T.; van Loef, E. V. D.; Meijerink, A. Visible quantum cutting via downconversion in LiGdF4:Er3+, Tb3+ upon Er3+ 4f11 to 4f105d excitation. J. Lumin. 2000, 90, 111-122. (9) Khaidukov, N. M.; Lam, S. K.; Lo, D.; Makhov, V. N.; Suetin, N. V.


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Luminescence spectroscopy from the vacuum ultra-violet to the visible for Er3+ and Tm3+ in complex fluoride crystals. Opt. Mater. 2002, 19, 365-376. (10) Liu, B.; Chen, Y.; Shi, C.; Tang, H.; Tao, Y. Visible quantum cutting in BaF2:Gd, Eu via downconversion. J. Lumin. 2003, 101, 155-159. (11) Chen, Y.; Shi, C.; Yan, W.; Qi, Z.; Fu, Y. Energy transfer between Pr3+ and Mn2+ in SrB4O7:Pr, Mn. Appl. Phys. Lett. 2006, 88, 061906. (12) Nie, Z.; Zhang, J.; Zhang, X.; Ren, X.; Zhang, G.; Wang, X. Evidence for visible quantum cutting via energy transfer in SrAl12O19:Pr, Cr. Opt. Lett. 2007, 32, 991-993. (13) Wang, W.; Yang, P.; Cheng, Z.; Hou, Z.; Li, C.; Lin, J. Patterning of red, green, and blue luminescent films based on CaWO4:Eu3+, CaWO4:Tb3+, and CaWO4 phosphors via microcontact printing route. ACS Appl. Mater. Interfaces 2011, 3, 3921-3928. (14) Lü W.; Lv W.; Zhao Q.; Jiao M.; Shao B.; You H. Generation of orange and green emissions in Ca2GdZr2(AlO4)3:Ce3+, Mn2+, Tb3+ garnets via energy transfer with Mn2+ and Tb3+ as acceptors. J. Mater. Chem. C 2015, 3, 2334-2340. (15) Carnall, W. T.; Fields, P. R.; Rajnak, K. Electronic energy levels of the trivalent lanthanide aquo ions. III. Tb3+. J. Chem. Phys. 1968, 49, 4447-4449. (16) Lee, T. J.; Luo, L. Y.; Diau, E. W. G.; Chen, T. M.; Cheng, B. M.; Tung, C. Y. Visible quantum cutting through downconversion in green-emitting K2GdF5:Tb3+ phosphors. Appl. Phys. Lett. 2006, 89, 131121. (17) Tzeng, H. Y.; Cheng, B. M.; Chen, T. M. Visible quantum cutting in green-emitting BaGdF5:Tb3+ phosphors via downconversion. J. Lumin. 2007, 122-123,



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917-920. (18) Wang, D.; Kodama, N. Visible quantum cutting through downconversion in GdPO4:Tb3+ and Sr3Gd(PO4)3:Tb3+. J Solid State Chem. 2009, 182, 2219-2224. (19) Hao, Z.; Zhang, J.; Zhang, X.; Lu, S.; Wang, X. Blue-green-emitting phosphor CaSc2O4:Tb3+: tunable luminescence manipulated by cross-relaxation. J. Electrochem. Soc. 2009, 156, 193-196. (20) Chan, T. S.; Lin, C. C.; Liu, R. S.; Xie, R. J.; Hirosaki, N.; Cheng, B. M. Photoluminescent and thermal stable properties of Tb3+-doped Ca-α-SiAlON under VUV excitation. J. Electrochem. Soc. 2009, 156, J189-J191. (21) Zhang, H.; Wang, Y.; Tao, Y.; Li, W.; Hu, D.; Feng, E.; Nie, X. Visible quantum cutting in Tb3+-doped BaGdB9O16 via downconversion. J. Electrochem. Soc. 2010, 157, J293-J296. (22) Raju, G. S. R.; Park, J. Y.; Jung, H. C.; Moon, B. K.; Jeong, J. H.; Kim, J. H. Gd3+ sensitization effect on the luminescence properties of Tb3+ activated calcium gadolinium oxyapatite nanophosphors. J. Electrochem. Soc. 2011, 158, J20-J26. (23) Han, B.; Liang, H.; Huang, Y.; Tao, Y.; Su, Q. Vacuum ultraviolet-visible spectroscopic properties of Tb3+ in Li(Y, Gd)(PO3)4: tunable emission, quantum cutting, and energy transfer. J. Phys. Chem. C 2010, 114, 6770-6777. (24) Xie, M.; Tao, Y.; Huang, Y.; Liang, H.; Su, Q. The quantum cutting of Tb3+ in Ca6Ln2Na2(PO4)6F2 (Ln = Gd, La) under VUV-UV excitation: with and without Gd3+. Inorg. Chem. 2010, 49, 11317-11324. (25) Yan, X.; Fern, G. R.; Withnall R.; Silver, J. Effects of the host lattice and doping


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concentration on the colour of Tb3+ cation emission in Y2O2S:Tb3+ and Gd2O2S:Tb3+ nanometer sized phosphor particles. Nanoscale 2013, 5, 8640-8646. (26) Vergeer, P.; Vlugt, T. J. H.; Kox, M. H. F.; den Hertog, M. I.; Eerden, J. P. J. M. van der.; Meijerink, A. Quantum cutting by cooperative energy transfer in YbxY1−xPO4:Tb3+. Phys. Rev. B 2005, 71, 014119. (27) Liu, Y.; Zhang, J.; Zhang, C.; Xu, J.; Liu, G.; Jiang, J.; Jiang, H. Ba9Lu2Si6O24:Ce3+: An efficient green phosphor with high thermal and radiation stability for solid-state lighting. Adv. Opt. Mater. 2015, 3, 1096-1011. (28) Song, K.; Zhang, J.; Liu, Y.; Zhang, C.; Jiang, J.; Jiang, H.; Qin, H. Red-emitting phosphor Ba9Lu2Si6O24:Ce3+, Mn2+ with enhanced energy transfer via self-charge compensation. J. Phys. Chem. C 2015, 119, 24558-24563. (29) Dorenbos, P. The 5d level positions of the trivalent lanthanides in inorganic compounds. J. Lumin. 2000, 91, 155-176. (30) Demas, J. N.; Crosby, G. A. The measurement of photoluminescence quantum yields. A review. J Phys. Chem. 1971, 75, 991-1024. (31) Brannon, J. H.; Magde, D. Absolute quantum yield determination by thermal blooming. Fluorescein. J. Phys. Chem. 1978, 82, 705-709. (32) Williams, A. T. R.; Winfield, S. A.; Miller, J. N. Relative fluorescence quantum yields using a computer controlled luminescence spectrometer. Analyst 1983, 108, 1067-1071. (33) Bindhu, C. V.; Harilal, S. S.; Varier, G. K.; Issac, R. C.; Nampoori, V. P. N.; Vallabhan, C. P. G. Measurement of the absolute fluorescence quantum yield of



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rhodamine B solution using a dual-beam thermal lens technique. J. Phys. D: Appl. Phys. 1996, 29, 1074-1079. (34) Joos, J. J.; Botterman, J.; Smet, P. F. Evaluating the use of blue phosphors in white LEDs: the case of Sr0.25Ba0.75Si2O2N2:Eu2+. J. Solid State Lighting 2014, 1:6. (35) May, P. S.; Sommer, K. D. Tb3+ luminescence in Tb-doped and Tb/Gd-doped CsCdBr3 crystals: 5D4 → 5D3 cross-relaxation rates in Tb3+ pairs. J. Phys. Chem. A 1997, 101, 9571-9577. (36) Liu, Y.; Zhang, X.; Hao, Z.; Luo, Y.; Wang, X.; Zhang, J. Generating yellow and red emissions by co-doping Mn2+ to substitute for Ca2+ and Sc3+ in Ca3Sc2Si3O12:Ce3+ green emitting phosphor for white LED applications. J. Mater. Chem. 2011, 21, 16379-16384. (37) Liu, Y.; Zhang, X.; Hao, Z.; Wang, X.; Zhang, J. Tunable full-color-emitting Ca3Sc3Si3O12:Ce3+, Mn2+ phosphor via charge compensation and energy transfer. Chem. Commun. 2011, 47, 10677-10679. (38) Liu, Y.; Zhang, X.; Hao, Z.; Luo, Y.; Wang, X.; Ma, L.; Zhang, J. Luminescence and energy transfer in Ca3Sc2Si3O12:Ce3+, Mn2+ white LED phosphor. J. Lumin. 2013, 133, 21-24. (39) Zhang, X.; Liu, Y.; Lin, J.; Hao, Z.; Luo, Y.; Liu, Q.; Zhang, J. Optical properties and energy transfers of Ce3+ and Mn2+ in Ba9Sc2(SiO4)6. J. Lumin. 2014, 146, 321-324.


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Figure Captions Figure 1. XRD patterns for BLS:xTb3+ (x =0.02 ~ 1.20). Figure 2. PLE spectra for BLS:0.02Tb3+ monitored at 380 and 552 nm. Figure 3. PL spectra of BLS: xTb3+ (x = 0.02-1.20) under the 251 nm excitation. Figure 4. CIE chromaticity diagram for the BLS:xTb3+ (x = 0.02 – 1.20) samples and digital images for x = 0.02 and 1.20 irradiated by the 254 nm UV-light. Figure 5. (a) Emission peak intensities for BLS:xTb3+ at 380 nm (5D3 → 7F6, I380 nm) and at 552 nm (5D4 → 7F5, I552 nm) and (b) internal and external QEs for BLS:xTb3+ as a function of Tb3+ concentrations upon the 251 nm excitation. Figure 6. PL spectra of BLS:xTb3+ (x = 0.02 – 1.20) under the 352 nm excitation. The 5

D3 blue emission intensity (I380 nm) is set as the normalized standard. The value of r is

the ratio of the 5D4 green emission intensity (I544 nm or I552 nm) to I380 nm. Figure 7. Schematic energy levels of Tb3+ and the excitation, emission, and cross-relaxation processes in the BLS:Tb3+ sample. Figure 8. (a) Fluorescence decay curve of BLS:2%Tb3+ obtained using direct excitation into 5d at 251 nm, monitoring the 5D3 → 7F6 emission at 380 nm. (b) Residuals of fit to a double-exponential function. Residual counts = experiment – theory. Figure 9. Fluorescence decay curves of the 380 nm emissions in BLS:xTb3+ with various Tb3+ concentrations (x = 0.02 – 1.20) under the 251 nm excitation. Figure 10. Average lifetimes for the 380 nm emission under the 251 nm excitation in BLS:xTb3+ (a), Dependences of efficiencies (b) and rates (c) of the cross-relaxation



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Page 26 of 38

energy transfers on the Tb3+ concentrations. Figure 11. Fluorescence decay curves of the 552 nm emissions in a full time scale (a) and a local time scale (b) in the BLS:xTb3+ samples with various Tb3+ concentrations (x = 0.02 – 1.20) under the 251 nm excitation. Figure 12. (a) Fluorescence decay curves of the 552 nm emission in BLS:120%Tb3+ under excitations into 5d (λex = 251 nm) and into 5D4 (λex = 501 nm). (b) and (c) are residuals of fit to an exponential function for the 251 nm and the 501 nm excitations, respectively.

Table 1 Internal QEs (ηint), external QEs (ηext), and absorbance (A) for BLS:xTb3+ under the 251 nm excitation. Table 2 The emission ratio (r) of 5D4 (I552

nm)

to 5D3 (I380

nm),

quantum cutting

efficiencies (ηQC), calculated internal QEs (ηint-cal), average lifetimes for the 380 nm emission (τ380

nm)

and the 552 nm emission (τ552

nm),

and efficiencies of

cross-relaxation energy transfer (ηCRET) of BLS:xTb3+ under the 251 nm excitation.


Page 27 of 38

Relative Intensity (arb. units)

x = 1.20 x = 1.00 x = 0.80 x = 0.40 x = 0.20 x = 0.10 x = 0.06 x = 0.02 Ba9Sc2Si6O24 PDF # 821119 10

20

30

40

50

60

70

2θ (degree) Figure 1

SA f-d

3+

BLS:0.02Tb

PLE

λex = 380 nm λex = 552 nm

Intensity (a. u.)

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


f-f transition SF f-d

200

250

300

x100

350

400

Wavelength (nm) Figure 2


450

500

80


5

10000

5

7

D4→ FJ

x= 0.02 x= 0.06 x= 0.10 x= 0.15 x= 0.20 x= 0.40 x= 0.60 x= 0.80 x= 1.00 x= 1.20

7

D3→ FJ

Intensity (a. u.)

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

8000 6000 4000 2000 0

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700 650 ) 600 550 (nm 500 th 450 eng 400 el 350 Wav

Figure 3


Page 29 of 38

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x=1.20

x=0.02 (0.223, 0.237) x=0.06 (0.271, 0.383) x=0.10 (0.297, 0.453) x=0.15 (0.309, 0.498) x=0.20 (0.321, 0.537) x=0.40 (0.335, 0.580) x=0.60 (0.339, 0.592) x=0.80 (0.339, 0.596) x=1.00 (0.337, 0.597) x=1.20 (0.337, 0.598)

x = 0.02

Figure 4



10000 9000

(a)

Intensity (a. u.)

8000 7000

5

7

5

7

I552 nm ( D4- F5)

6000 5000 4000 3000

I380 nm ( D3- F6)

2000 1000 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

3+

Tb concentration (x) 150 140

(b)

130 120

QE (%)

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

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110 100 90

internal external

80 70 60 0.0

0.2

0.4

0.6

0.8

3+

Tb concentration (x) Figure 5


1.0

1.2

Page 31 of 38

60

PL λex= 352 nm

30 0 60 30

r = 61.5

x = 1.20

r = 68.4

x = 1.00

r = 59.2

x = 0.80

r = 49.3

x = 0.60

r = 34.8

x = 0.40

r = 15.0

x = 0.20

r = 13.4

x = 0.15

0 60 30 0 30 0

Relative Intensity (a. u.)

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


25 0 10 0 10 0 5

r = 8.1

x = 0.10

0 4 2

r = 4.8

x = 0.06

0 1

r = 1.6 x = 0.02

0 350

400

450

500

550

600

Wavelength (nm)

Figure 6


650

700


50000 45000

7

DJ

9

35000

8

30000 25000

9

DJ

9

D3

5

D4

5

DJ

5

3

D3

5

20000

5

2

15000 10000

7

DJ

40000

Energy (cm-1)

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

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1

D4

7 10

4

5

6

7

7

F0

5000

F0

3

0

7

3+

Tb

F6

Figure 7


7

3+

Tb

F6

Page 33 of 38

(a)

3+

λex = 251 nm, λem = 380 nm

BLS:2%Tb

Counts

10000

τ1 = 0.375 ms τ2 = 2.754 ms

1000

chi-sq = 1.26 100

Decay Fit

Residual counts

10

8

(b)

4 0 -4 -8 0

10

20

30

40

50

60

70

80

Time (ms) Figure 8 1

λex = 251 nm, λem = 380 nm Log Intensity (a. u.)

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


x = 0.02 x = 0.06 x = 0.10 x = 0.20 x = 0.40 x = 0.80 x = 1.00 x = 1.20

0.1

0.01

1E-3

0

4

8

12

Time (ms) Figure 9


16

20


2.5

(a)

average lifetime for 380 nm

2.0

τ (ms)

1.5 1.0 0.5 0.0 1.0 0.8

η

0.6

η = 1 - τ /τ0

0.4 0.2 16 12

-1

W (ms )

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 34 of 38

(b)

W = 1/τ - 1/τ0

8 4

(c)

0 0.0

0.2

0.4 3+

Tb

0.6

0.8

concentration (x)

Figure 10


1.0

1.2

Page 35 of 38

1

(a)

Log Intensity (a. u.)

λex = 251 nm, λem = 552 nm

x = 0.02 x = 0.06 x = 0.10 x = 0.20 x = 0.40 x = 0.80 x = 1.00 x = 1.20

0.1

0.01

1E-3

1E-4 0

10

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30

40

50

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80

Time (ms) 1

Log Intensity (a. u.)

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λex = 251 nm, λem = 552 nm (b)

0.8

x = 0.02 x = 0.06 x = 0.10 x = 0.20 x = 0.40 x = 0.80 x = 1.00 x = 1.20

0.6

0.4

0

1

2

3

4

Time (ms) Figure 11


5

6


3+

(a)

λem = 552 nm

BLS:120%Tb

Counts

10000

λex = 251 nm

1000

Fit 100

τ = 5.907 ms chi-sq = 1.19

λex = 501 nm Fit

10

Residual counts

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 36 of 38

5 0 -5

(b)

5 0 -5

(c)

0

10

20

τ = 5.907 ms chi-sq = 1.17

30

40

50

Time (ms) Figure 12


60

70

80

Page 37 of 38

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


Table 1 x

ηint (%)

ηext (%)

A (%)

0.02 0.06 0.10

98.8 115.9 117.7

65.2 86.4 91.6

66.0 74.6 77.8

0.15 0.20 0.40

126.1 135.6 139.6

99.7 104.7 112.1

79.1 77.2 80.3

0.60 0.80 1.00 1.20

143.5 141.5 141.9 131.4

117.8 117.9 118.3 107.7

82.1 83.3 83.4 82.0

Table 2 x

r

ηQC (%)

ηint-cal (%)

τ380 nm (ms)

τ552 nm (ms)

ηCRET (%)

0.02 0.06 0.10 0.15

0.34 0.96 1.76 2.76

/ / / /

/ / / /

2.181 1.759 1.362 1.021

8.078 7.831 7.737 7.529

20.8 36.1 50.5 62.9

0.20 0.40 0.60 0.80 1.00 1.20

4.87 18.3 58.1 120 164 363

/ / 8.04 33.6 40.8 70.7

/ / 108 134 141 171

0.666 0.266 0.136 0.094 0.073 0.067

7.081 6.525 6.286 6.224 6.092 5.803

75.8 90.3 95.1 96.6 97.3 97.6



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

Quantum Cutting

7

DJ

9

DJ

5

D3 5

5

D4

D4

7

7

7

7

F0

F0 F6 3+

F6

Tb

Cross Relaxation

TOC


Page 38 of 38

High Efficiency Green Phosphor Ba - American Chemical Society

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