Energy Conversion and Transfer in the Luminescence of CeSc₃(BO₃)₄:Cr³⁺ Phosphor

Abstract

Novel near-infrared (NIR) phosphors are in demand for light-emitting diode (LED) devices to extend their suitability for new applications and, in turn, support the sustainable and healthy development of the LED industry. The Cr³⁺ has been used as an activator in the development of new NIR phosphors. However, one main obstacle for the Cr³⁺-activated phosphors is the low luminescence efficiency due to the spin-forbidden d-d transition of Cr³⁺. The rare-earth (RE) huntite minerals that crystallize in the form of REM₃(BO₃)₄ (M = Al, Sc, Cr, Fe, Ga) have a large family of members, including the rare-earth scandium borates of RESc₃(BO₃)₄. Interestingly, in our research, we found that the luminescence efficiency of Cr³⁺ in the CeSc₃(BO₃)₄ host, whose quantum yield was measured at 56%, is several times higher than that in GdSc₃(BO₃)₄, TbSc₃(BO₃)₄, and LuSc₃(BO₃)₄ hosts. Hereby, the energy conversion and transfer in the luminescence of CeSc₃(BO₃)₄:Cr³⁺ phosphor were examined. The Stokes shift of electron energy conversion within the Cr^{3+ 4}T_2g level for the emission at 818 nm and excitation at 625 nm in CeSc₃(BO₃)₄ host was evaluated to be 3775.1 cm⁻¹, and the super-large splitting energy of the ²F_5/2 and ²F₇₂ sub-states of the Ce³⁺ 4f¹ state, about 3000 cm⁻¹, was found in CeSc₃(BO₃)₄ host. The typical electronic thermal vibration peaks were observed in the excitation spectra of CeSc₃(BO₃)₄:Cr³⁺. On this basis, the smallest phonon energy, around 347.7 cm⁻¹, of the CeSc₃(BO₃)₄ host was estimated. Finally, the energy transfer that is responsible for the far higher photoluminescence of Cr³⁺ in CeSc₃(BO₃)₄ than in other hosts was proven through the way of Ce³⁺ emission and Cr³⁺ reabsorption.

1. Introduction

Light-emitting diodes (LEDs) have achieved big success in lighting and display applications [1,2]. To support the sustainable and healthy development of the huge LED industry, which ranges from substrate epitaxy, LED chip fabrication, and LED package to end-use applications, it is urgent to open up new fields of application [1,2]. For this aspect, the red and near-infrared LEDs show a bright future for applications in infrared photography, imaging, photomedicine, biomedicine, sensors, detectors, photosynthetic agriculture, and as a backlight for testing instruments [3,4,5,6,7].

Compared with multiple-chip LEDs, the phosphor-converted LEDs have many distinct advantages in terms of an easily tunable emission wavelength, low cost of manufacturing, and a short period research and development cycle. Moreover, the technique of fabricating blue LED chips is relatively more mature than that of other kinds of ultraviolet, near ultraviolet, and deep ultraviolet chips. In addition, the instruments used to grow blue LED chips, mainly metal–organic chemical vapor deposition (MOCVD), are abundant, and the key raw materials for fabricating blue LED chips are available. Nevertheless, due to the large Stokes shift, it is a challenge to convert the blue emission of LED chips into red and near-infrared photons by using phosphors. Both Eu²⁺ and Ce³⁺ have been widely used as activators to obtain yellow, green, and red emissions, but it is very hard to obtain near-infrared emissions from Eu²⁺ and Ce³⁺ under the excitation of blue light [8,9,10]. Comparatively, the Cr³⁺, whose d orbital has a relatively small crystal field splitting energy, is promising for obtaining red and near-infrared emissions under blue excitation [5,6,7]. However, the d-d transition of Cr³⁺ is spin-forbidden, and accordingly, the absorbance of Cr³⁺ is low, which results in the luminescence of the Cr³⁺-activated phosphor with poor efficiency. Recent progress achieved in the NIR phosphors activated by Cr³⁺ is described in the latest review papers on references [11,12,13]. Generally, the absorbance and luminescence efficiency of the Cr³⁺-activated phosphors are far below those of the Eu²⁺- and Ce³⁺- activated phosphors. To enhance the luminous efficiency of the Cr³⁺-activated phosphors, the absorbance of the phosphor should be improved above all.

The rare-earth (RE) scandium borates RESc₃(BO₃)₄ have a calcite-derived huntite structure, crystallized in the trigonal R32 space group [14,15,16]. In the crystal lattice of RESc₃(BO₃)₄, the Sc atom provides an ideal site for the activator Cr³⁺ because of their similar ion radii (Sc³⁺, 0.81; Cr³⁺, 0.69) and charge balance. Surprisingly, we found out that the luminescence efficiency of CeSc₃(BO₃)₄:Cr³⁺ is several orders of magnitude higher than that of GdSc₃(BO₃)₄:Cr³⁺, TbSc₃(BO₃)₄:Cr³⁺, and LuSc₃(BO₃)₄:Cr³⁺. This exceptional phenomenon mechanism was revealed in this work with energy conversion and transfer in the luminescence of CeSc₃(BO₃)₄:Cr³⁺ phosphor.

2. Experimental Procedure

The phosphors RE(Sc_0.975Cr_0.025)₃(BO₃)₄ (RE = Ce, Gd, Tb, and Lu), abbreviated as RESc₃(BO₃)₄:Cr³⁺ below, were synthesized with a two-step solid-state reaction. The high-purity CeO₂ (99.99%, Kepu, Ganzhou), Gd₂O₃ (99.99%, Kepu, Ganzhou), Tb₄O₇ (99.99%, Kepu, Ganzhou), Lu₂O₃ (99.99%, Kepu, Ganzhou), Sc₂O₃ (99.99%, Kepu, Ganzhou), H₃BO₃ (99.8%, Sinopharm), and Cr(NO₃)₃·9H₂O (99.95%, Luoen, Shanghai) were used as raw materials. Firstly, the stoichiometric raw materials were thoroughly ground with a high-speed vibration ball mill. Next, the ball-milled mixtures were pre-fired at 500 ℃ for 2 h, and then the pre-fired products were ground again using the agate mortar and pestle. Finally, the phosphors were synthesized at 1250 ^oC for 8 h in ambient air. Crystal structures of the phosphors were examined with the X-ray diffractometer (PANalytical, X-Pert PRO MPD) using a Cu target. The morphology was tested by scanning electron microscopy (JEOL, JSM-6490LV). The excitation and emission spectra of the samples were tested by a fluorescence spectrometer (Hitachi, F4600). The test sample was loaded into a self-made sample tank, pressed, and scraped flat to guarantee that the excitation and emission spectra were measured under the same conditions. The absorption spectra were collected with a UV–visible near infrared spectrophotometer (Shimadzu Corporation, Kyoto City, Japan, Shimadzu, UV-3600). All samples’ excitation and emission spectra were tested after the samples were synthesized and preserved for 8 months. The fluorescence quantum yield was measured with an absolute photoluminescence quantum yield measurement system (Hamamatsu Photonics K. K., Hamamatsu City, Janpan, Hamamatsu, Quantaurus-QY plus C13534-31), and the accumulation wavelength range is 300–1600 nm. The band gap, reflective spectra, absorption spectra, and the optical parameter of refractive index were calculated using the CASTEP module of Materials Studio 2019 software by employing the crystal structure of Ce₃Sc(BO₃)₄ with PDF # 38-0720 as the initial structure model. In calculating the properties of RE’Sc₃(BO₃)₄ (RE’ = Gd, Tb, Lu), the Ce atom was replaced with Gd, Tb, and Lu, respectively. Before performing computations, the crystal structure was optimized with the generalized gradient approximation (GGA).

3. Results and Discussion

The compounds RESc₃(BO₃)₄ (RE = Ce, Gd, Tb, Lu) are members of a large group of huntite-family rare-earth borates REM₃(BO₃)₄ (M = Al, Sc, Cr, Fe, Ga) that crystallize in the structure of huntite minerals, CaMg₃(CO₃)₄ [14,15,16]. From the X-ray diffraction (XRD) patterns of the RESc₃(BO₃)₄:Cr³⁺ (RE = Ce, Gd, Tb, Lu) phosphors shown in Figure 1a, we can see that the main diffraction peaks of CeSc₃(BO₃)₄:Cr³⁺ phosphor are consistent with standard PDF# 38-0720 of CeSc₃(BO₃)₄. When Ce is replaced with Gd, Tb, and Lu, the phosphors of GdSc₃(BO₃)₄:Cr³⁺, TbSc₃(BO₃)₄:Cr³⁺, and LuSc₃(BO₃)₄:Cr³⁺ exhibit a similar diffraction pattern to that of CeSc₃(BO₃)₄:Cr³⁺, as Ce, Gd, Tb, and Lu belong to the same rare-earth family elements. Furthermore, the minor peaks indicate the presence of minor intermediate compounds of TbBO₃ and ScBO₃, as well as the remaining unreacted CeO₂ raw material. Indeed, the melting point of B₂O₃, about 450 ^oC, is very low, while the melting point of rare-earth oxides is far higher. During the synthesis process of RESc₃(BO₃)₄, the rare-earth orthogonal borates REBO₃ usually appear first, followed by the formation of RESc₃(BO₃)₄. For this reason, we always find that borates such as TbBO₃ and ScBO₃ always accompany the RESc₃(BO₃)₄ phases. The three-dimensional structure of CeSc₃(BO₃)₄, shown in Figure 1b, exhibits non-centrosymmetry with the space group R32 in the trigonal system. In the crystal lattice of CeSc₃(BO₃)₄, both Sc and Ce are sixfold coordinated, as shown in Figure 1b. Considering ionic radium and charge balance, Cr³⁺ should take the place of Sc in the crystal lattice of RESc₃(BO₃)₄. Except for pure hosts, the concentration of Cr³⁺ doped in each RESc₃(BO₃)₄ sample was nominally fixed at 2.5 atm% of Sc in the crystal lattice. Thus, the luminescence properties of Cr³⁺ in the RESc₃(BO₃)₄ host could be explained with the Tanabe–Sugano diagram as shown in Figure 1c.

The scanning electron microscope (SEM) image presented in Figure 2 shows that the particles of CeSc₃(BO₃)₄:Cr³⁺ phosphor have a typical size of 2–10 μm, consisting of several small grains that have agglomerated together. Nonetheless, the characteristic trigonal profile of the grains could still be identified from the irregular particles, as indicated by the arrow in Figure 2.

The emission and excitation spectra of RESc₃(BO₃)₄:Cr³⁺ (RE = Ce, Gd, Tb, Lu) displayed in Figure 3 show that the luminescence intensity of CeSc₃(BO₃)₄:Cr³⁺ is nearly four times higher than that of LuSc₃(BO₃)₄:Cr³⁺ and more than 10 times higher than that of GdSc₃(BO₃)₄:Cr³⁺ and TbSc₃(BO₃)₄:Cr³⁺. This phenomenon is very interesting. The measured quantum yield and absorbance of CeSc₃(BO₃)₄:Cr³⁺ are 65.8% and 24.4%, respectively.

Figure 3a presents the emission spectra of RESc₃(BO₃)₄:Cr³⁺ (RE = Ce, Gd, Tb, Lu) under the excitation of 471 nm, in which the broadband emission is attributed to the ⁴A₂-⁴T₂(⁴F) transition of Cr³⁺. By monitoring the emission of Cr³⁺ at 822, 830, and 866 nm, the excitation spectra of RESc₃(BO₃)₄:Cr³⁺ (RE = Ce, Gd, Tb, Lu) are given in Figure 3b–d, respectively, in which two strong excitation bands in regions of 400–575 nm and 575–800 nm are attributed to the electron transitions of ⁴A₂-⁴T₁(⁴F) and ⁴A₂-⁴T₂(⁴F), respectively [11,12,13]. Moreover, minor excitation bands in the region of 200–400 nm are observed, which should be caused by host excitation (as discussed below) in addition to the high-level excitation of Cr³⁺ with the ⁴A₂-⁴T₁(⁴G) transition. Many electronic vibration peaks were observed in Figure 3b–d, particularly in Figure 3d as indicated by arrows, which for one side suggests the presence of strong thermal vibration of the crystal lattice on Cr³⁺ luminescence and for the other side suggests that the crystal lattice of RESc₃(BO₃)₄:Cr³⁺ (RE = Ce, Gd, Tb, Lu) has an un-rigid structure. Through the electronic vibration peaks displayed in Figure 3d, the phonon energy of CeSc₃(BO₃)₄:Cr³⁺ phosphor could be evaluated, in which the reciprocal of peak wavelengths corresponds to energy. According to quantum mechanics, the difference between two neighboring peaks corresponds to the energy of coupled phonons, nћω, i.e., n times the smallest one ћω, where n is the natural number. Hereby, the smallest phonon energy ћω, which is about 347.7 cm⁻¹, was evaluated from the neighboring peaks at 718.3 nm and 736.7 nm as marked with pink arrows in Figure 3d. Using the relation of phonon energy of E = ħω, where ħ = h /2 π (h is the Planck constant, h = 6.626 × 10⁻³⁴ J·s = 4.1369 × 10⁻¹⁵ eV·s; ħ is the reduced Planck constant, ħ = 1.055 × 10⁻³⁴ J·s = 6.582 × 10⁻¹⁶ eV·s) and ω = 2 πν (radians per unit time), the angular frequency ω was evaluated at 4.26 × 10²¹ (rad/ S).

To reveal the mechanisms of the far higher photoluminescence of CeSc₃(BO₃)₄:Cr³⁺ phosphor than others, the emission and excitation spectra of RESc₃(BO₃)₄:Cr³⁺ (RE = Ce, Gd, Tb, Lu) were first normalized. After normalizing the emission intensity, the spectra in Figure 4a show that the emission peaks red shift upon the change of rare-earth elements in the order of Ce-Lu-Gd-Tb. However, the variation in emission peak is not evident. With the strongest excitation at 471 nm normalized to 1.0, the excitation spectra upon the emission at 822 nm are shown in Figure 3b. As indicated by the line in Figure 4b, the relative excitation intensity in the 550–700 nm region in the order of Tb-Gd-Lu-Ce is opposite to that presented in Figure 3a for luminescence red shift. To some extent, this phenomenon suggests that the relaxation of electrons from the ⁴T₁(⁴F) to ⁴T₂(⁴F) level that produces NIR emission results in the low-efficiency luminescence of the phosphors.

The element Lu, due to the inert electronic structure of [Xe]4f¹⁴, is widely used as a component for phosphor hosts, such as the green phosphor of Lu₃Al₅O₁₂:Ce³⁺ for white LEDs [17]. The element Gd³⁺, which has a 4f semi-complete orbital electronic structure, i.e., [Xe]4f⁷, is not only used as a component of phosphor hosts but is also frequently used as a sensitizer, such as a phosphor (Y,Gd)BO₃:Eu³⁺ for plasma display panels. Gd³⁺ is a good innate sensitizer due to the large energy difference between the ground state, ⁸S, and the lowest excited level, ⁶P. Thus, Gd³⁺ could play the role of an intermediate to transfer energy through the sublattice [18]. Ce³⁺ and Tb³⁺ are commonly used as phosphor activators to obtain various color emissions, but they can also be used as components of hosts, such as the traditional green phosphor LaPO₄:Ce³⁺,Tb³⁺ for fluorescent lamps and the yellow phosphor Tb₃Al₅O₁₂:Ce³⁺ for white LEDs [19,20]. Herein, the rare-earth elements Ce, Gd, Tb, and Lu were all used as the components of hosts for the activator Cr³⁺. Surprisingly, the photoluminescence of Cr³⁺ in the CeSc₃(BO₃)₄ host is far higher than that in GdSc₃(BO₃)₄, TbSc₃(BO₃)₄, and LuSc₃(BO₃)₄ hosts. To further reveal the mechanisms, the emission, excitation, and absorption spectra of the CeSc₃(BO₃)₄ host without doping Cr³⁺ were studied.

By exciting the CeSc₃(BO₃)₄ host with two different wavelengths of 323 and 358 nm, nearly the same configuration emission spectra were obtained, as shown in Figure 5a, which confirms that the emissions originate from the same mechanism. By monitoring the emission at 450 and 472 nm, the excitation spectra are shown in Figure 5b, in which two excitation peaks were observed. In Figure 5b, the two excitation peaks should originate from the 4f–5d transition of Ce³⁺. The spin–orbital coupling effect on the outer one electron in the 4f orbital splits the ground state of the Ce³⁺ electron into two states, ⁴F_5/2 and ⁴F_7/2. Thus, the two excitation peaks at 323 and 358 nm in Figure 5b were attributed to the ⁴F_5/2–5d and ⁴F7_/2–5d transitions of Ce³⁺. Accordingly, the emission spectra of the CeSc₃(BO₃)₄ host presented in Figure 5a were attributed to the 5d–4f transition of Ce³⁺. By plotting the emission and excitation spectra of the CeSc₃(BO₃)₄ host together, Figure 5c shows that there is almost no overlap between them, suggesting the CeSc₃(BO₃)₄ host has a weak crystal field. The excitation peaks of the ⁴F_5/2–5d and ⁴F_7/2–5d transitions at 323 and 358 nm, respectively, in Figure 5b, show that the energy difference between the ⁴F_5/2 and ⁴F7_/2 states is about 3000 cm⁻¹, which is equivalent to 9–10 phonons (as above-obtained, 347.7 cm⁻¹) in CeSc₃(BO₃)₄:Cr³⁺ phosphor. The energy difference between the ⁴F_5/2 and ⁴F7_/2 states in CeSc₃(BO₃)₄:Cr³⁺ phosphor is far higher than that in the rigid-structure phosphors such as Y₃Al₅O₁₂:Ce³⁺ [17], also indicating the soft structure of CeSc₃(BO₃)₄.

The absorption spectrum of the CeSc₃(BO₃)₄ host is shown in Figure 5d, in which a broadband consisting of several peaks was observed. By plotting the absorption and the excitation spectra of CeSc₃(BO₃)₄ together, the absorption peaks of the characteristic ⁴F_5/2–5d and ⁴F7_/2–5d transitions of Ce³⁺ were easily identified. The extra absorption within 200–300 nm may be caused by the electron transitions within the Sc-O or Sc-BO₃ bonds. Therefore, Figure 5d suggests that the minor excitation band within 200–400 nm should be caused by the CeSc₃(BO₃)₄ host, but the ⁴A₂-⁴T₁(⁴P) transition of Cr³⁺ cannot be excluded.

Figure 5d shows that the low energy cutoff of the absorption spectrum is approximately 400 nm, suggesting the band gap of the CeSc₃(BO₃)₄ host is about 3.1 eV. The calculated band gap of CeSc₃(BO₃)₄ shown in Figure 6a is about 0.494 eV. The result is comparable with the calculated value of 0.317 eV of CeSc₃(BO₃)₄ band gap shared in the Materials Project with ID mp-16097 [21]. Due to the inherent nature of the first principle calculation, the calculated band is usually smaller than the experimental one [22]. Nevertheless, the calculated band still could provide some important information for understanding the mechanism. Figure 6d shows that the Fermi level consists of a line, which should originate from the 4f orbital of CeSc₃(BO₃)₄ and is consistent with the experimental results in Figure 5d. The refractive index is an intrinsic parameter that determines the optical behavior of reflection and absorption. As the result shown in Figure 6b, the refractive index changes with photon energy. However, the blue excitation and NIR emission have photon energies less than 5 eV. As shown in Figure 6c,d, there is a significant difference in the reflection and absorption spectra among the REc₃(BO₃)₄ hosts within the energy range of 5–50 eV. Yet, there is almost no difference in the energy below 5 eV, which further indicates that Ce plays a special role in improving the luminescence efficiency of CeSc₃(BO₃)₄:Cr³⁺.

By plotting the emission of the CeSc₃(BO₃)₄ host and the excitation of the CeSc₃(BO₃)₄:Cr³⁺ phosphor together, the spectra are shown in Figure 7. In Figure 7, a good match between the emission of the CeSc₃(BO₃)₄ host and the ⁴A₂-⁴T₁(⁴F) excitation of Cr³⁺ in the CeSc₃(BO₃)₄:Cr³⁺ phosphor suggests that there is an energy transfer from the CeSc₃(BO₃)₄ host Ce³⁺ to the activator Cr³⁺. With the intensity normalized, the overlap of Cr³⁺ absorption and Ce³⁺ emission is 99% of Ce³⁺ emission, suggesting an efficient pathway from Ce³⁺ emission to Cr³⁺ re-absorption.

However, the excitation spectra in Figure 3b–d show that the energy transfer from the host to Cr³⁺ is not efficient since the relative intensity of excitation in the 200–400 nm range is very low. Moreover, no overlap between the emission and excitation spectra of Ce³⁺ suggests that the Stokes shift is very high. As a result, the energy transfer between variant Ce³⁺ ions is almost impossible. This phenomenon could be explained by the low phonon energy in CeSc₃(BO₃)₄. Thus, the energy transfer pathway from Ce³⁺ to Cr³⁺ through electron resonance effect was excluded. By combining with the absorption spectrum in Figure 5d, we get to know that the energy transfer from Ce³⁺ to Cr³⁺ in the CeSc₃(BO₃)₄:Cr³⁺ phosphor under the excitation of blue light is thus through coaction of Ce³⁺ emission and Cr³⁺ reabsorption. Accordingly, the mechanism of the energy transfer from the CeSc₃(BO₃)₄ host to the Cr³⁺ activator and energy conversion processes in the CeSc₃(BO₃)₄:Cr³⁺ phosphor could be depicted with Scheme 1.

4. Conclusions

In summary, the luminescence efficiency of the CeSc₃(BO₃)₄:Cr³⁺ phosphor is 3–20 orders of magnitude higher than that of GdSc₃(BO₃)₄:Cr³⁺, TbSc₃(BO₃)₄:Cr³⁺, and CeSc₃(BO₃)₄:Cr³⁺ phosphors. The measured quantum yield and absorbance of CeSc₃(BO₃)₄:Cr³⁺ are 65.8% and 24.4%, respectively. The characteristic peaks of electronic vibrations were observed in the excitation spectra of CeSc₃(BO₃)₄:Cr³⁺, on which the smallest energy of the CeSc₃(BO₃)₄ phonons was evaluated at 347.7 cm⁻¹. The splitting energy of the ²F_3/2 and ²F_3/2 sub-states of the Cr³⁺ 4f¹ state in CeSc₃(BO₃)₄ was determined at 3000 cm⁻¹, and the Stokes shift of electrons within the emission (818 nm) and excitation (625 nm) states of the Cr³⁺ 4T_2g level was 3775.1 cm⁻¹. All these parameters suggest that CeSc₃(BO₃)₄ has a super-soft structure. The self-emission of the CeSc₃(BO₃)₄ host, which originates from the Ce³⁺ 5d–4f transition, was confirmed. However, the Stokes shift between the ground state and the excited states of Ce³⁺ in the CeSc₃(BO₃)₄ host is too big, resulting in the excitation energy hardly being able to transfer through the electron resonance effect among variant Ce³⁺ ions. Therefore, the energy transfer that is responsible for the intensified luminescence of Cr³⁺ in the CeSc₃(BO₃)₄ host was mainly through the Ce³⁺ emission and Cr³⁺ reabsorption. This work sheds light on developing new highly efficient Cr³⁺-activated phosphors.