Enhanced Near-Infrared Luminescence in Ba₃Y(BO₃)₃: Nd³⁺ by Codoping with Ce³⁺

Ba₃Y_1–0.01−x (BO₃)₃: 0.01Ce³⁺, xNd³⁺ (x = 0, 0.015, 0.025, 0.035, 0.045, 0.055) samples were prepared via solid-state reaction. All the starting materials including BaCO₃ (A.R.), CeO₂ (A.R.), H₃BO₃ (A.R., excess 5 mol% to compensate the loss), Y₂O₃ (99.99%) and Nd₂O₃ (99.9%), were weighted by stoichiometric ratio and milled in an agate mortar. It was then fully ground and transferred to corundum crucible. After pre-heating at 500 °C for 5 h, the mixtures were grinded and sintered at 1000 °C for 5 h in CO reducing atmosphere. The final powder products were obtained through orderly cooling down and thoroughly grind.

The crystal structures and phase were carried out on a Rigaku-Dmax 3 C powder diffractometer using Cu-Ka (λ = 1.54056 Å) radiation in the range 10^o ≤ 2θ ≤ 60^o. The luminescence spectra were measured by an Edinburgh FLS 920 fluorescence spectrometer equipped with a 450 W xenon lamp and near infrared photomultiplier tube (PMT) was cooled by liquid nitrogen system. The decay curves of Ce³⁺ ion were examined by a 375 nm nanosecond nF 900 flash-lamp. The tests were carried out more than three times to decrease measure error.

Figure 1a depicts the XRD patterns of the different doped samples calcined at 1000 °C for 5 h and the standard profile for Ba₃Y(BO₃)₃ (JCPDS 51-1849) used as reference. It has been reported that Ba₃Y(BO₃)₃ has two structures. The low-temperature phase is hexagonal crystal system (P6₃ cm), which will be converted into the high-temperature phase (trigonal, R 3) under the condition of 1148 °C high-temperature synthesis. ^16–18 The comparison shows that the obtained compound has the structure of low-temperature Ba₃Y(BO₃)₃ phase belonging to an hexagonal lattice (space group: P6₃ cm). The diffraction peaks of all phosphors are consistent with those in the standard card JCPDS 51-1849, which indicates that the prepared phosphors are pure phases and the doped rare earth ions of Ce³⁺ and Nd³⁺ completely enter the lattice of matrix without any extra peaks. Figure 1b shows the schematic illustration of Ba₃Y(BO₃)₃ structure plotted with Diamond software. Structurally, the Ba₃Y(BO₃)₃ is composed of the BO₃ triangle and the YO₆ octahedron (the blue groups), as well as the BaO₉ polyhedron. ¹⁹ The plane formed by BO₃ triangle divides the Y and Ba sites. Furthermore, Ba with nine oxygens occupies four lattice sites as shown the ball-and-stick structure model in Fig. 1c. There are two types of Y atom coordinates to 6 O, forming two different YO₆ octahedrons as shown in Fig. 1d. In addition, the Ce³⁺ and Nd³⁺ (R_Ce³⁺ = 1.01 Å, R_{Nd ³⁺} = 0.98 Å, CN = 6) ions can be expected to take up the sites of Y³⁺ (R_{Y ³⁺} = 0.90 Å, CN = 6) in this system due to their closer ionic radius and identical valency.

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Figure 2 exhibits the photoluminescence spectra of Ba₃Y(BO₃)₃: 0.01Ce³⁺ and Ba₃Y(BO₃)₃: 0.01Ce³⁺, 0.015Nd³⁺. For Ce³⁺ singly doped phosphor, the excitation band ranges from 280 to 400 nm including two significant broad bands assigned to the 4 f−5d transition. ²⁰ Under the excitation of 356 nm, the sample produces the typical double-peak emission of Ce³⁺ ions in 370−600 nm originating from two luminescence centers. ²¹

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Comparing the single-doped and co-doped Ba₃Y(BO₃)₃ samples, it was obvious that under the monitoring of Ce³⁺ ion at 420 nm and Nd³⁺ ion at 1059 nm, the profile of excitation band in 280−400 nm was similar except for the difference in relative intensity. ²² In other words, the excitation spectrum of Ce³⁺ ion could be obtained by monitoring the 1059 nm emission of Nd³⁺, which verifies the occurrence of ET in the Ce³⁺- Nd³⁺ samples. Moreover, the visible excitation peaks of Nd³⁺ ions overlapping well with the emission of Ce³⁺ included 441, 473, 527, 582 and 698 nm are also observed by monitoring at 1059 nm, which should be assigned to the ⁴I_9/2 → ⁴D_5/2, ⁴I_9/2 → ²G_9/2, ⁴I_9/2 → ⁴G_7/2, ⁴I_9/2 → ⁴G_5/2 and ⁴I_9/2 → ⁴F_9/2 transition. ²³ Therefore, Nd³⁺ can broaden the range of absorption from ultraviolet region to visible and near-infrared region enhancing the utilization of solar spectrum. Under 356 nm excitation, the samples emit visible light from Ce³⁺ and Nd³⁺, as well as NIR emission from Nd³⁺ in 850−1500 nm peaked at 901 nm (⁴F_3/2 → ⁴I_9/2), 1059 nm (⁴F_3/2 → ⁴I_11/2) and 1340 nm (⁴F_3/2 → ⁴I_13/2). ²⁴ Here Ce³⁺ ions act as sensitizer for Nd³⁺ ions, Ce³⁺ ions absorb the energy of near-ultraviolet excitation light, which could transfer the 4 f electrons of ground state to the excited state, then emit blue violet light. Another part of the energy transfer to the nearby Nd³⁺, which emit visible light and NIR light in 850−1500 nm region.

Figure 3 displays the visible and NIR emission spectra of Ba₃Y(BO₃)₃: 0.01Ce³⁺, xNd³⁺ (x = 0, 0.015, 0.025, 0.035, 0.045, 0.055) samples. With the increase of Nd³⁺ doping, the visible emission of Ce³⁺ and Nd³⁺ ion reduce gradually. However, the NIR emission of Nd³⁺ ions rises first and then drops continuously after x = 0.035 because of concentration quenching. ^25,26 The gradual decrease of visible light emission intensity indicates the existence of ET from Ce³⁺ to Nd³⁺. The illustration clearly illustrates the relationship between normalized integrated emission and doping concentration, and the optimal doping concentration of Nd³⁺ ion is 0.035.

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In order to illustrate that Ce³⁺ ion could sensitize Nd³⁺ ion and the existence of ET process, Fig. 4 displays the NIR emission spectra comparison diagram of Ba₃Y(BO₃)₃: 0.035Nd³⁺ and Ba₃Y(BO₃)₃: 0.01Ce³⁺, 0.035Nd³⁺ samples. Under the 356 nm excitation, the emission intensity of Nd³⁺ singly doped phosphor is much weaker than that from corresponding Ce³⁺-Nd³⁺ sample. Furthermore, integrated intensity of the NIR emission in Ce³⁺-Nd³⁺ sample is 1.6 times of the single-doped sample when the Nd³⁺ doping content x = 0.035, which proves that effective energy transfer occurs. The phenomenon reveals that Ce³⁺ ion act as an efficient sensitizer for Nd³⁺ with strong broadband absorption, which could absorb the underutilized high-energy UV–Vis photons and convert to available low energy NIR photons by the ET process. The Ce³⁺-Nd³⁺ co-doped Ba₃Y(BO₃)₃ could serve as a potential DC solar spectral convertor based on spectra modulate to improve optimal NIR emission in 900−1100 nm region.

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Figure 5 shows the decay curves of Ce³⁺ emission depend on Nd³⁺ (x) dosage with the 375 nm excitation. The average lifetime (τ) could be calculated by: ²⁷

$$\begin{eqnarray}&&\tau =\displaystyle {\int }_{0}^{\infty }t{I}_{\left(t\right)}dt/\displaystyle {\int }_{0}^{\infty }{I}_{\left(t\right)}dt\end{eqnarray} \tag{ 1 }$$

Here, I_(t) is fluorescence intensity. The decay rate of Ce³⁺ becomes faster and the decay curves shift to multi-exponential due to the doping of Nd³⁺ (x) would cause else decay pathway. ²⁸ Base on the fitted fluorescence attenuation curve, the decay lifetime of Ce³⁺ are calculated to be about 11.46, 7.27, 6.92, 6.55, 6.07 and 5.67 ns for x = 0, 0.015, 0.025, 0.035, 0.045 and 0.055, respectively. The ET efficiency (ETE, η_ETE) from Ce³⁺ to Nd³⁺ could be described as: ²⁹

$$\begin{eqnarray}&&{\eta }_{{\rm{ETE}}}=1-{\tau }_{x}/{\tau }_{0}\end{eqnarray} \tag{ 2 }$$

in which τ_x and τ₀ represent the lifetime of Ce³⁺ in the presence and absence of Nd³⁺ doping. By the calculation, the η_ETE are 36.6%, 39.6%, 42.8%, 47.0% and 50.5% for x = 0.015−0.055. With the addition of Nd³⁺ ions, the decay lifetime decreases monotonically and the corresponding ETE increases gradually. It is further proved that the occurrence of ET between Ce³⁺ and Nd³⁺. When the doping amount of Nd³⁺ ions is 0.055, the energy transfer efficiency reaches a maximum of 50.5%.

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Based on the above-mentioned results, the schematic diagram with probable energy transfer routes in Ba₃Y(BO₃)₃: Ce³⁺, Nd³⁺ sample are depicted in Fig. 6. The electrons on the 4 f levels of Ce³⁺ ions could be excited to the 5d excited state with the UV excitation, then some energy from the 5d₁ and 5d₂ excited states returns to the ground state through radiation transition along with emitting blue violet light. At the same time, the other part of the energy can be transferred to the acceptor energy levels of Nd³⁺ ions through ET. Some of the energy further relaxes to ⁴F_5/2 level mainly through non-radiatively decay, and then jumps to ⁴I_9/2, generating emission with wavelength of 769 nm. While the other part through the radiation-free relaxation relaxes to ⁴F_3/2 energy level, and then transits to ⁴I_J/2 (J = 9, 11, 13), respectively producing 900, 1059 and 1340 nm near-infrared emission. ^30–32 Therefore, Ce³⁺-Nd³⁺ ion pair can convert a high-energy near-ultraviolet photon to a near-infrared photon matching the silicon band gap through ET process, thus effectively enhance the utilization of high-energy photons.

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For better describing the spectra, Fig. 7 depicts the PLE (pink line) and NIR emission spectra (red area) of Ba₃Y(BO₃)₃: Ce³⁺, Nd³⁺ phosphor along with solar spectrum. According to spectral response curve of crystal silicon solar cells, most high-energy short-wave photons less than 500 nm are almost underutilized. ⁴ As can be seen, the co-doped phosphor absorbs inefficient sunlight less than 400 nm, and achieves intense dominant NIR long-wave photons around 1000 nm matches well with silicon solar cells. Moreover, the absorption range expands to the visible region by doping Nd³⁺ ions. Hence, Ba₃Y(BO₃)₃: Ce³⁺, Nd³⁺ phosphor is a potential solar spectral convertor, which could modify UV–Vis photons to beneficial NIR photons and reduce energy loss by spectra modulation.

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In summary, NIR emitting phosphor Ba₃Y(BO₃)₃: Ce³⁺, Nd³⁺ has been successfully prepared using solid state reaction. Under 356 nm excitation, Ce³⁺ ion act as an efficient sensitizer could absorb the underutilized high-energy photons in 280−400 nm and convert to optimal NIR photons (850−1100 nm) matching the silicon band gap. In particular, the NIR emission of Ce³⁺ −Nd³⁺ sample was improved in contrast to Nd³⁺ single-doped sample. Fluorescence lifetime properties and possible ET mechanism were also investigated, which further confirm the existence of ET process from Ce³⁺ to Nd³⁺. The highest η_ETE is 50.5% when the doping of Nd³⁺ is 0.055. The results indicate that Ba₃Y(BO₃)₃: Ce³⁺, Nd³⁺ is a promising NIR down conversion converter with broad absorption cross-section for silicon-based solar cells.

This work was supported by National Natural Science Foundation of China (No. 11804265), Natural Science Foundation of Shaanxi Province (No. 2020JQ-656, 2018JQ1048), Talents Foundation of Xi'an University of Architecture and Technology (No. RC1809), College Students' Innovation and Entrepreneurship Training Programs (No. S201910703018).