Study on the mechanism of high energy transfer efficiency of blue light excited Cr<sup>3&#x002B;</sup>, Nd<sup>3&#x002B;</sup> co-doped near infrared phosphors

Ganggang Guo; Tao Yin; Tao Yin; Mengrui Dong; Jianju Nie; Yayuan Zhang; Zhenyang Liu; Fenghe Wang; Fenghe Wang; Li Guan; Li Guan; Li Guan; Xu Li

doi:10.1364/OE.494516

1. Introduction

NIR spectroscopy is a non-destructive, fast and convenient technique for detection and analysis [1–3]. In recent years, the development and application of new NIR light sources are becoming more and more abundant. With the increasing maturity of NIR phosphor-converted light emitting diodes (pc-LEDs) technology, the advantages of energy saving, environmental protection and miniaturization are gradually emerging [4–6]. Trivalent chromium ion (Cr³⁺) as the activation center is the mainstream choice for NIR phosphors recently, but in most hosts, the main peak value of its emission spectrum is less than 1000 nm [7–12]. It is well worthwhile to develop NIR phosphors with longer emission wavelengths. Since Nd³⁺ has a wide distribution of absorption peaks in the UV-vis-NIR band, it can achieve energy transfer with different ions [13]. Nd³⁺ co-doped phosphors with Bi³⁺, Ce³⁺, Eu³⁺, Ho³⁺, Mn⁴⁺, and Yb³⁺, respectively, have been investigated [14–19].

Due to the overlap between the emission spectrum of Cr³⁺ and part of the absorption band of Nd³⁺, the doping of Cr³⁺ and Nd³⁺ to achieve NIR luminescence with new characteristics is a novel direction. Viktor Anselm et al. prepared SrGa₁₂O₁₉:Cr³⁺, Nd³⁺ phosphors and investigated the energy transfer and thermal quenching behavior [20]. Yiling Wu et al. developed La₃Ga₅GeO₁₄: Cr³⁺, Nd³⁺ phosphors with NIR long persistent phosphorescence and NIR-to-NIR Stokes luminescence [21]. Abbi L. Mullins et al. synthesized LaGaO₃:Cr³⁺, Nd³⁺ phosphors with high relative sensitivity by detecting the characteristic emission of Cr³⁺, Nd³⁺ at different temperatures, which can be used in the field of temperature measurement [22]. Nevertheless, the energy transfer mechanism between Cr³⁺ and Nd³⁺, Nd³⁺ and Nd³⁺ and the mutual influence in the system with Cr³⁺, Nd³⁺ co-doping as activator is still obscure.

In this work, we have synthesized La₂CaZrO₆:Cr³⁺, Nd³⁺ phosphors by high-temperature solid-phase method. The occupancy of Nd³⁺ in LCZO crystals is discussed in detail by first-principles calculations and Rietveld refinement. The X-ray photoelectron spectroscopy (XPS) shows that trivalent neodymium (Nd³⁺) ions are present in LCZO crystals as the main valence state, and the morphological changes of LCZO: Nd³⁺/Cr³⁺ are also analyzed. The emission spectra of LCZO:Cr³⁺, Nd³⁺ under 471 nm excitation cover an ultra-wide range of 700-1400 nm, with three distinct emission peaks at 907 nm, 1084 nm, and 1373 nm corresponding to the ⁴F_3/2→⁴I_9/2, ⁴F_3/2→⁴I_11/2, and ⁴F_3/2→⁴I_13/2 transitions of Nd³⁺, respectively. The concentration-dependent emission spectra of ⁴F_3/2→⁴I_9/2, ⁴F_3/2→⁴I_11/2 and ⁴F_3/2→⁴I_13/2 electronic transitions of Nd³⁺ ions are observed, and the concentration dependence of LCZO:Nd³⁺ spectra are discussed to reveal the energy transfer mechanism between Nd³⁺ ions. In addition, the UV–vis–NIR diffuse reflectance spectra, Kubelka-Munk (K-M) and Urbach tail theories and fluorescence lifetime analysis elucidate the interaction between Cr³⁺, Nd³⁺ and their effect on LCZO crystals. Further, the energy transfer mechanism between Cr³⁺ and Nd³⁺, Nd³⁺ and Nd³⁺ is revealed in the Cr³⁺, Nd³⁺ co-doped system. This study provides insights into the development of novel Nd³⁺ emission-dominated NIR phosphors and demonstrates their promising applications in several fields.

2. Experimental section

2.1. Materials and synthesis

The samples La₂CaZrO₆:Cr³⁺, Nd³⁺ are synthesized using the conventional high-temperature solid-phase method. Lanthanum oxide (La₂O₃, 99.99%, Aladdin), calcium carbonate (CaCO₃, A.R.), zirconium oxide (ZrO₂, 99%, Aladdin), chromium oxide (Cr₂O₃, 99. 95%, Aladdin), neodymium oxide (Nd₂O₃, 99%, Aladdin) were weighed in proportion to their chemical formula as raw materials. The raw materials were sintered at 1500°C for 8 h under a reducing atmosphere of H₂ (10%) and N₂ (90%) to prevent the oxidation of Cr³⁺. After cooling to room temperature, the samples were reground into powder for study.

2.2. Characterization and measurements

X-ray powder diffraction (XRD) were measured on a D8 X-ray diffractometer with nickel-filtered Cu Kα radiation (λ = 1.54056 Å) at 40 mA, 40 kV. The Rietveld refinement of the phase structure of the samples was done using FullProf software. The fluorescence lifetime, photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the phosphors were tested with an FLS980 transient fluorescence spectrometer (Edinburgh), which uses a 450 W Xe lamp as the excitation source and an external temperature controller to test the variable high temperature spectra. The diffuse reflectance spectra (DRS) of the samples were tested using Hitachi U-4100 UV–vis–NIR spectroscopy, utilizing BaSO₄ as a standard reference. The X-ray photoelectron spectra (XPS) of the samples were measured by ESCALAB 250Xi. The surface morphology of the phosphor was measured by field emission scanning electron microscopy (SEM, Nova NanoSEM450). The elemental distribution of the phosphor was tested using a high-resolution transmission electron microscope (HRTEM, JEPL-2100Plus), and the crystal plane spacing was calculated based on the obtained data using Gatan Digital Micrograph software. The external quantum yield (EQY) was measured by the Edinburgh FLS-1000. The pc-LED is prepared by using 470 nm blue light chip and phosphor LCZO:2%Cr³⁺, 1%Nd³⁺.

2.3 Computational details

All calculations were performed with the CASTEP code [23,24]. The relaxation and formation energy (E_form) of LCZO was calculated using generalized density functional theory and generalized gradient modified Perdew-Burke-Ernzerhof (PBE) [25]. All structures were converged to 0.01 eV/Å.

3. Results and discussion

3.1. Phase and morphology analysis of phosphors

The double perovskite oxide LCZO belongs to the monoclinic crystal system with the space group P21/n as shown in Fig. 1(a). Under the condition of charge conservation, Nd³⁺ (coordination number CN = 8, 1.109 Å) tends to replace La³⁺ (CN = 8, 1.16 Å) due to their similar ionic radii. The formation energy for the three possible occupancies of Nd³⁺ (replacing La or Ca or Zr) is calculated using the following equation [26,27]:

(1)$${E_f} = {E_{LCZO:Nd}} - {E_{LCZO}} - \; {\mu _{Nd}} + \; {\mu _s}$$

where E_{LCZO: Nd} denotes the total energy of LCZO: Nd³⁺, E_LCZO represents the total energy of LCZO, and µ_Nd and µ_S imply the chemical potentials of the Nd atom and the substituted atom, respectively. The results in Table 1 show that Nd³⁺ is most likely to replace La³⁺ due to its minimal formation energy (-8.072 eV).

Fig. 1. (a) Crystal structure of LCZO. (b) Cr³⁺, Nd³⁺ substitution schematic. (c) Rietveld refinement of XRD of LCZO: 2% Cr³⁺, 4% Nd³⁺. (d), (e) XRD patterns of LCZO: x Nd³⁺ (x = 0.1%∼8%). (f), (g) XRD patterns of LCZO: 2% Cr³⁺, y Nd³⁺ (y = 0%∼8%).

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Table 1. The calculated formation energies (E_f) of Nd³⁺ occupying different sites in LCZO

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Cr³⁺ ions tend to co-substitute Ca²⁺ and Zr⁴⁺ because Cr^3 +(0.615 Å) and Zr^4 +(0.72 Å) have similar radii and must satisfy the condition of charge conservation. The schematic diagram of Cr³⁺, Nd³⁺ substitution and the mechanism of their interactions are given in Fig. 1(b). Further, Fig. 1(c) depicts the Rietveld refinement results of the XRD of LCZO: 2% Cr³⁺, 4% Nd³⁺. The refinement parameters (R_p = 5.50%, R_wp = 9.54%, and R_exp = 3.70%) are less than 10%, which ensures the confidence of the refinement results. The atomic occupation information obtained from the XRD refinement results is shown in Table 2, which indicates that the Nd³⁺ and La³⁺ ions occupy the same lattice sites, and the lattice sites of the two Cr³⁺ ions match with Ca²⁺ and Zr⁴⁺, respectively.

Table 2. Atomic occupancy information of LCZO: 2% Cr³⁺, 4% Nd³⁺

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Fig. 1(d),1(f) show the XRD patterns of LCZO: x Nd³⁺ (x = 0.1%∼8%) and LCZO: 2% Cr³⁺, y Nd³⁺ (y = 0%∼8%), respectively, and the results show good synthesis compared with the calculated reference XRD and refinement results. As shown in Fig. 1(e), 1(g), Nd³⁺ doping in LCZO does not cause a significant shift in the XRD peak due to the close ionic radius of Nd³⁺ and La³⁺.

In addition, Fig. S1 shows the XRD patterns of LCZO: 4% Nd³⁺, zCr³⁺ (z = 0 ∼ 6%). With the increase of Cr³⁺ concentration, the position of the XRD peak shifts considerably toward the large angle direction (Fig. S1(b)), which implies that the crystal plane spacing will become smaller.

The XPS spectra of LCZO: 2% Cr³⁺, 4% Nd³⁺ samples are plotted in Fig. 2(a), where the peaks of elements La, Ca, Zr, O, Cr and Nd could be identified. The two peaks at 576.3 eV and 586.3 eV correspond to 2p_3/2 and 2p_1/2 of the Cr³⁺ ion, respectively, as shown in the inset in the upper left corner of Fig. 2(a) [28]. And, the peak is located at 982.9 eV corresponding to the major 3d_5/2 orbital of the Nd³⁺ ion as shown in the upper right inset of Fig. 2(a) [29].

Fig. 2. (a) XPS spectra of LCZO: 2% Cr³⁺, 4%Nd³⁺. (b), (c) The HRTEM image of phosphor LCZO:2% Cr³⁺, 4% Nd³⁺ particles and elemental distribution maps.

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The above results show that Cr³⁺, Nd³⁺ exist in phosphor LCZO as the main valence state. Furthermore, Fig. S2 shows the SEM images of phosphors LCZO: 2%Nd³⁺, 2%Cr³⁺, which shows that the phosphors have irregular particle shapes but the samples are relatively uniform in size. The size distribution of the phosphor LCZO: 2%Nd³⁺, 2%Cr³⁺ samples ranged from 0-30 µm, with an average particle size of 9.0 µm. Fig. 2(b) shows the high-resolution TEM (HR-TEM) images of LCZO: 2% Cr³⁺, 4% Nd³⁺ samples, the results of which show (221) crystal plane spacing of 0.139 nm. Fig. 2(c) shows the elemental distribution of LCZO: 2% Cr³⁺, 4% Nd³⁺ phosphor particles, in which La, Ca, Zr, O, Cr and Nd are uniformly distributed.

3.2. Photoluminescence properties and energy transfer mechanism of Cr³⁺, Nd³⁺ doped in LCZO

Fig. 3(a) depicts the UV-vis-NIR diffuse reflectance spectra of LCZO:Nd³⁺ in the range of 320∼1200 nm, where the absorption peak of Nd³⁺ at 360 nm corresponds to the ⁴I_9/2→⁴D_3/2 transition, 471 nm to the ⁴I_9/2→⁴G_9/2 transition, 530 nm to the ⁴I_9/2→⁴G_7/2 transition, 590 nm to the ⁴I_9/2→⁴G_5/2 transition, 687 nm to the⁴I_9/2→⁴F_9/2 transition, 750 nm to the ⁴I_9/2→⁴F_7/2 transition, 808 nm to the ⁴I_9/2→⁴F_5/2 transition, and 880 nm corresponds to ⁴I_9/2→⁴F_3/2 transition(Fig. 3(a), 3(b)). The absorption peaks gradually rise with the increase of Nd³⁺ concentration. Further, Fig. 3(b) also depicts the diffuse reflectance spectra of LCZO: 2% Cr³⁺, y Nd³⁺ (y = 0 ∼ 8%), where the absorption peaks of both Cr³⁺, Nd³⁺ are present. The diffuse reflectance spectra of LCZO:Cr³⁺ show absorption peaks at 471 nm and 687 nm corresponding to the ⁴A₂(F)→⁴T₁(F) and ⁴A₂(F)→⁴T₂(F) transitions of Cr³⁺, respectively. With the increase of Nd³⁺ concentration, the intensity of Cr³⁺ dominated absorption bands (⁴A₂(F)→⁴T₁(F) and⁴A₂(F)→⁴T₂(F) transitions) remain basically unchanged, while the Nd³⁺ dominated absorption bands are gradually enhanced on the basis of Cr³⁺.

Fig. 3. (a) Diffuse reflectance spectra of LCZO: x Nd³⁺ (x = 0 ∼ 8%). (b) Diffuse reflectance spectra of LCZO: 2%Cr³⁺, y Nd³⁺ (y = 0% ∼ 8%). (c) PL and PLE spectra of LCZO: Cr³⁺, LCZO: Nd³⁺. (d) PL and PLE spectra of LCZO: 2%Cr³⁺, 0.5%Nd³⁺.

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The optical band gap can be calculated using UV-vis-NIR absorption spectral data according to the Kubelka-Munk equations [30–32]:

(2)$$F(R )= \; \frac{{{{({1 - R} )}^2}}}{{2R}}$$

(3)$${({F(R )h\nu } )^n} = A({h\nu - {E_g}} )$$

where A, hv, and E_g denote the absorption constant, photon energy, and optical band gap, respectively, n = 2 for direct transition, 1/2 for indirect transition, R represents the reflection coefficient, and F(R) is the absorption coefficient. Fig. S3 shows that LCZO is a direct band gap material. Fig. S4 (a) shows that the optical band gap E_g of LCZO: x Nd³⁺ (x = 0%∼8%) decreases from 3.59 eV to 3.35 eV with increasing Nd³⁺ concentration, which is consistent with the variation of LCZO: 2% Cr³⁺, y Nd³⁺ (y = 0%∼8%) (Fig. S4(b)). This reduction in the band gap can be attributed to the appearance of new unoccupied electronic states in the gap below the conduction band edge due to the substitution of Nd³⁺ ions for La³⁺. The introduction of Cr³⁺ significantly enhances the absorption coefficients of Nd³⁺ at various energy levels as shown in the Kubelka-Munk (K-M) absorption spectra (Fig. S4(a), S4(b)).

Fig. 3(c) shows the excitation and emission spectra of LCZO: Cr³⁺ and LCZO: Nd³⁺ simultaneously. Monitoring 861 nm, the excitation spectrum of phosphor LCZO:Cr³⁺ shows peaks of 471 nm, 651 nm corresponding to the ⁴A₂(F)→⁴T₁(F), ⁴A₂(F)→⁴T₂(F) transitions of Cr³⁺, respectively. Moreover, the LCZO: Cr³⁺ shows an ultra-broadband emission spectrum covering 700 to 1400 nm under 471 nm excitation. In addition, the emission peaks of LCZO: Nd³⁺ at 907 nm, 1084 nm and 1373 nm correspond to the electronic transitions of ⁴F_3/2→⁴I_9/2, ⁴F_3/2→⁴I_11/2 and ⁴F_3/2→⁴I_13/2 of Nd³⁺ at 822 nm excitation, respectively. The 4f electrons of Nd³⁺ are shielded by the outer 5s² and 5p⁶ electrons, and the overall energy levels of Nd³⁺ ions do not change greatly in different hosts, but the individual energy levels of Nd³⁺ can be divided into 2J + 1 sub-energy levels by the surrounding local crystal field. Compared with previous reports (Table S1), the wavelengths of the emission peaks at 1084 nm and 1373 nm corresponding to ⁴F_3/2→⁴I_11/2 and ⁴F_3/2→⁴I_13/2 of LCZO:Nd³⁺ are longer in the case of high concentration of Nd³⁺ doping (discussed in the later section).

Monitoring 1084 nm, a series of excitation peaks with 880 nm as the strongest peak can be observed, among which 880 nm, 894 nm, 902 nm and 915 nm correspond to the ⁴I_9/2→⁴F_3/2 transition of Nd³⁺. The excitation spectrum of Nd³⁺ and the emission spectrum of Cr³⁺ effectively overlap, which provides the possibility of energy transfer from Cr³⁺ to Nd³⁺.

Fig. 3(d) depicts the PL and PLE spectra of phosphor LCZO: 2% Cr³⁺, 0.5% Nd³⁺. The emission spectrum of LCZO: 2% Cr³⁺, 0.5% Nd³⁺ is dominated by the emission of Nd³⁺ at 471 nm excitation, which indicates that the energy transfer between Cr³⁺, Nd³⁺ occurs. The excitation spectrum of LCZO:2%Cr³⁺, 0.5%Nd³⁺ at 500∼1000 nm is obtained by monitoring at 1084 nm, which is basically consistent with the excitation spectrum of LCZO: Nd³⁺. The excitation spectrum of LCZO: 2% Cr³⁺, 0.5% Nd³⁺ at 890 nm is monitored and shows the characteristic excitation peaks of Nd³⁺ at 360 nm, 530 nm, 590 nm, 750 nm, 808 nm for Nd³⁺ and 471 nm for Cr³⁺, which can be perfectly matched with the absorption peaks in the DRS spectra.

Fig. 4 exhibits a schematic diagram of the energy levels of Nd³⁺ in LCZO, where detailed information can be seen in the fine emission spectrum of LCZO: 4% Nd³⁺ labeled in Fig. S5 and Table S2. The 695 nm, 890 nm, and 907 nm peaks in the emission spectra of LCZO: Nd³⁺ correspond to the ⁴F_9/2→⁴I_9/2 transition, ⁴F_3/2(R₁)→⁴I_9/2(Z₂) transition, and ⁴F_3/2(R₁)→⁴I_9/2(Z₄) transition, respectively. Fig. 5(a) shows the emission spectra of LCZO: x Nd³⁺ (x = 0.1 ∼ 8%) under 471 nm excitation. The integrated intensity of the emission spectra of LCZO: x Nd³⁺ increases with the concentration of Nd³⁺ and reaches a maximum at x = 6%. Further, the effect of Nd³⁺ concentration on the intensity of the electronic transitions from ⁴F_3/2→⁴I_J/J’ can be expressed in terms of the fluorescence intensity branching ratio β_J/J' [13]:

(4)$$\beta J/J' = \; \frac{{{A_{J - J'}}}}{{\mathop \sum \nolimits_{A_{J - J'}}}}$$

where A_J-J’ denotes the emission intensity of the ⁴F_3/2→⁴I_J/J’ transition, and the denominator is the sum of the emission intensities of the ⁴F_3/2 radiation reduction to all energy levels. Fig. 5(b) depicts the effect of concentration on the branching ratio of fluorescence intensity, where β_9/2 shows a decreasing trend, β_11/2 shows an increasing trend, while β_13/2 remains essentially unchanged. The rapid diffusion of the excitation energy corresponding to the resonant ⁴F_3/2 energy levels between the Nd³⁺ ions (Purple arrow in process (a) of Figure 4) leads to the variations in the emission intensity branching ratio. Fig. S6 (a), (c), (e) show the variation of the relative intensity of LCZO: xNd³⁺ (x = 0.1% ∼ 8%) at 907 nm (I₉₀₇) and 890 nm (I₈₉₀), at 1084 nm (I₁₀₈₄) and 1070 nm (I₁₀₇₀), and at 1373 nm and 1348 nm. The results in Fig. S6(b), (d), (f) reveal that the emission intensity ratios (I₉₀₇/I₈₉₀, I₁₀₈₄/I₁₀₇₀, I₁₃₇₃/I₁₃₄₈) increase with the rise of Nd³⁺ concentration. The relative intensities of the short wavelength portions of the ⁴F_3/2→⁴I_9/2, ⁴F_3/2→⁴I_11/2, ⁴F_3/2→⁴I_13/2 transition bands become weaker with increasing Nd³⁺ doping concentration, which can be attributed to the enhanced cross-relaxation {⁴F_3/2,⁴I_9/2}⇔{⁴I_15/2,⁴I_15/2} of Nd^3 +(blue arrow in process (b) of Figure 4) [33]. In addition, the re-absorption process between Nd³⁺ ions may also cause changes in the emission intensity branching ratio due to the relatively high absorption cross section of ⁴I_9/2→⁴F_3/2 (Fig. 3(a)) and the overlap between the excitation spectrum monitored at 1084 nm (⁴F_3/2→⁴I_11/2) and the emission spectrum produced by ⁴F_3/2→⁴I_9/2 (Fig. 3(c)).

Fig. 4. Schematic diagram of the energy levels of Nd³⁺ in LCZO.

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Fig. 5. (a) Emission spectra of LCZO: x Nd³⁺ (x = 0.1 ∼ 8%). (b) Branching ratio β_J/J’ as a function of concentration of Nd³⁺. (c) Emission spectra of LCZO: 2%Cr³⁺, y Nd³⁺ (y = 0%∼8%). (d) Fitted curves of I_s0/I_s versus C^n/3(n = 6, 8, 10) associated with LCZO: 2% Cr³⁺, y Nd³⁺ (y = 0% ∼ 8%).

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Fig. S7 shows the emission spectra of LCZO: 4% Nd³⁺, zCr³⁺ (z = 0∼6%) under 471 nm excitation. The results show that the introduction of Cr³⁺ does not change the shape and peak of the emission spectra, and the best emission intensity is obtained with 2% Cr³⁺ doping. Fig. 5(c) depicts the emission spectra of LCZO: 2% Cr³⁺, y Nd³⁺ (y = 0% ∼ 8%), the intensity of the emission spectrum reaches a maximum at 1% Nd³⁺ doping. In addition, the fluorescence intensity at 861 nm for LCZO: 2% Cr³⁺, y Nd³⁺ (y = 0 ∼ 8%) gradually decreases with increasing Nd³⁺ concentration as shown in Fig. S8, which implies that energy transfer between Cr³⁺ and Nd³⁺ occurs. The emission spectra of LCZO: 2% Cr³⁺, y Nd³⁺ (y = 0% ∼ 8%) at 471 nm excitation does not show an emission peak at 695 nm, which implies that the Cr³⁺ ions absorb the main excitation energy.

The central distance between ions will be less than the critical distance of ions at a high doping concentration of Nd³⁺ ions, and the critical distance of energy transfer can be calculated by the equation proposed by Blasse as follows [34]:

(5)$${R_c}\; \approx 2{\left[ {\frac{{3V}}{{4\pi {x_c}N}}} \right]^{1/3}}$$

where V (V = 288.125 Å³) refers to the cell volume of the sample material, R_c represents the critical distance between Nd³⁺, x_c(x_c= 0.06 for LCZO: Nd³⁺, x_c= 0.01 for LCZO: 2%Cr³⁺, yNd³⁺) represents the critical concentration, and N is the cation number in the unit cell (N = 3). The critical distance of LCZO: 2%Cr³⁺, yNd³⁺ (R_c = 26.37 Å) is larger than that of LCZO: Nd³⁺ (R_c’ = 7.26 Å), which can be seen from the fact that the introduction of Cr³⁺ leads to smaller grain plane spacing and as well as looser atomic spacing.

The interaction between Cr³⁺ and Nd³⁺ can be explained by Reisfeld's approximation and Dexter's multipole interaction theory. The type of interaction between the donor and acceptor ions can be determined according to the following equation [35]:

(6)$$\frac{{{I_{{s_0}}}}}{{{I_S}}}\; \alpha \; {C^{\frac{n}{3}}}$$

where I_s0 and I_s denote the integrated intensity of the emission spectra in the presence and absence of Nd³⁺, respectively; C is the critical doping concentration; and n = 6, 8, and 10 represent dipole-dipole (d-d), dipole-quadrupole (d-q), and quadrupole-quadrupole (q-q) interactions, respectively. As shown in Fig. 5(d), the best fit line (R² = 98.7%) is obtained for n = 6, which indicates that the d-d interaction of electrons plays a dominant role between Cr³⁺ and Nd³⁺.

To further investigate the effect of Cr³⁺ introduction on LCZO: Nd³⁺ phosphors, LCZO: 4% Nd³⁺, LCZO: 4% Nd³⁺, 2% Cr³⁺ are prepared under the same conditions, and their DRS spectra are shown in Fig. 6(a). The introduction of 2% Cr³⁺ ions significantly enhance the absorption of the phosphor in the UV-vis-NIR band, and the characteristic absorption of Cr³⁺ and Nd³⁺ coexists. The difference in the radii of Cr³⁺ ions with Ca²⁺ ions and Zr⁴⁺ ions and the charge compensation mechanism can cause defects centers in LCZO crystals.

Fig. 6. (a) Diffuse reflectance spectra of LCZO: 4% Nd³⁺, LCZO: 4% Nd³⁺, 2% Cr³⁺. (b) Diagram of Urbach tail in the energy band. (c) K-M function plot of reflection spectra of LCZO: 4% Nd³⁺, LCZO: 4% Nd³⁺, 2% Cr³⁺. (d) Plot of Ln(F(R)) versus hν for the width of Urbach tail determined by the slope.

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As shown in Fig. 6(b), these defects generate strong potential gradients in the LCZO host, leading to quantum mechanical penetration of the localized states into the forbidden band gap [36]. The relationship between [F(R)hν]² and energy can be obtained by substituting the data in Fig. 6(a) into Equation (3) as in Fig. 6(c). The doping of Cr³⁺ ions lead to a slight increase in the optical band gap of LCZO (3.56eV→3.58 eV), which is due to the Burstein-Moss effect [37].

The Urbach tail (E_u) width can be obtained from the absorption coefficient (α) versus the photon energy (hν) as follows [38]:

(7)$$\alpha = \; {\alpha _0}\; \textrm{exp}\left( {\frac{{h\nu }}{{{E_u}}}} \right)$$

(8)$$\ln (\alpha )= \ln ({{\alpha_0}} )+ \frac{{h\nu }}{{{E_u}}}\; $$

(9)$$\ln ({F(R )} )= \ln ({{R_0}} )+ \; \frac{{h\nu }}{{{E_u}}}$$

where α₀ is a constant, F(R) is proportional to α, and Equations (8) and (9) can be derived by taking the logarithm of equation (7). Fig. 6(d) depicts the plot between ln(F(R)) and energy for LCZO: 4% Nd³⁺, LCZO: 4% Nd³⁺, 2% Cr³⁺, where the value of Urbach tail (E_u) can be obtained from the inverse of the slope of the straight line after linear fitting of the data points. The results show that the Urbach energy value of LCZO: 4% Nd³⁺, 2% Cr³⁺ (E_u’ = 308 meV) is higher than that of LCZO: 4% Nd³⁺ (E_u = 302 meV). The introduction of Cr³⁺ ions lead to an increase in disordered atoms and an increase in structural bonding defects, which can produce localized states at or near the conduction band level, leading to an increase in Urbach tail (E_u) [39,40].

Fig. S9(a), (b) show the fluorescence lifetimes monitored at 861 nm for Cr³⁺ luminescence-dominated and 1084 nm for Nd³⁺ luminescence-dominated, respectively. With the increase of Nd³⁺ concentration, the fluorescence lifetime at 861 nm becomes smaller, while the fluorescence lifetime at 1084 nm shows a trend of first increasing and then decreasing, which implies the process of energy transfer from Cr³⁺ to Nd³⁺.

The energy transfer efficiency can be calculated by the reduction of the emission intensity of Cr³⁺ or the shortening of its lifetime as in the following equation [41,42]:

(10)$${\eta _{ET}} = 1 - \; \frac{{{I_{Cr}}}}{{{I_{C{r_0}}}}}$$

(11)$${\eta _{ET}}^{\prime} = 1 - \; \frac{{{\tau _{Cr}}}}{{{\tau _{C{r_0}}}}}$$

where I_Cr and I_Cr0 are the integrated intensities of the emission spectra of Cr³⁺ in the presence and absence of Nd³⁺, respectively, and τ_Cr and τ_Cr0 are the corresponding fluorescence lifetimes.

Fig. 7(a) shows that the energy transfer efficiencies calculated by the two methods are similar, while it can be seen that the energy transfer efficiency increases with increasing Nd³⁺ concentration, reaching 88.4% at 6% Nd³⁺ concentration according to the variation of Cr³⁺ emission spectral intensity. Fig. 7(b) exhibits the emission spectra of LCZO:1% Nd³⁺, LCZO: 6% Nd³⁺, LCZO: 2% Cr³⁺, LCZO: 2% Cr³⁺, 1%Nd³⁺ and their integrated intensity comparison at 471 nm excitation. The emission spectra of LCZO: 2% Cr³⁺, 1% Nd³⁺ have an integrated intensity of 847% relative to LCZO: 1% Nd³⁺, 457% relative to LCZO: 6% Nd³⁺, and 204% relative to LCZO: 2% Cr³⁺, and have a longer wavelength dual-peak emission. Moreover, Fig. S10 shows the external quantum yield of 27.8% for LCZO: %Cr³⁺,1%Nd³⁺.

Fig. 7. (a) Variation of emission spectral intensity with Nd³⁺ concentration, energy transfer efficiency calculated based on Cr³⁺ emission intensity, lifetime. (b) The emission spectra of LCZO:1% Nd³⁺, LCZO: 6% Nd³⁺, LCZO: 2% Cr³⁺, LCZO: 2% Cr³⁺, 1%Nd³⁺ at 471 nm excitation.

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Fig. 8 depicts the schematic diagram of the energy levels in LCZO: Cr³⁺, Nd³⁺ and the corresponding energy transfer processes. The phosphor LCZO: Cr³⁺, Nd³⁺ is excited at 471 nm (21231 cm^-1), and the Cr³⁺ ions absorb the main excitation energy and radiate a broadband emission spectrum of 700 (14285 cm^-1)-1400 nm (7142 cm^-1) as shown in process (1). The process (2) shows that electrons from the ⁴T₁ and ⁴T₂ excited states relaxing to the ²E metastable state can be transferred to the ⁴F_7/2 or ⁴S_3/2, ⁴F_5/2 or ²H_9/2, ⁴F_3/2 of Nd³⁺ and subsequently relax to the lower ⁴F_3/2 excited state of Nd³⁺ by a resonant energy transfer process. At 471 nm excitation, a small fraction of the ground state ⁴I_9/2 electrons of the Nd³⁺ ions are pumped to the ⁴G_9/2 excited state, and then they rapidly nonradiative relax to the ⁴F_3/2 energy level of Nd³⁺ as depicted by the red dashed arrow in process (3). As discussed earlier, there is rapid diffusion and reabsorption between the ⁴F_3/2→⁴I_9/2 energy levels of the Nd³⁺ ions (process (4) in the Fig. 8). The electrons on the ⁴F_3/2 excited state of the Nd³⁺ ions eventually return to the ⁴I_9/2, ⁴I_11/2, ⁴I_13/2 ground state, producing enhanced near-infrared emission.

Fig. 8. Schematic energy level diagrams of Cr³⁺, Nd³⁺ in LCZO, illustrating the involved energy transfer process.

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In addition, the temperature-dependent PL spectra of LCZO: 2% Cr³⁺, 1% Nd³⁺ are shown in Fig. S11(a). The intensity of the emission spectra decreases with increasing temperature, and the intensity of the emission spectrum at 373 K is 67% of that at room temperature. As the temperature increases, the emission peak at 822 nm is gradually shown, which arises from the ⁴F_5/2→⁴I_9/2 electron transition of Nd^3 + [43]. Fig. S11(b) depicts the emission peak intensity ratios at 907 nm and 890 nm for LCZO: 2% Cr³⁺, 1% Nd³⁺, where the value of I₉₀₇/I₈₉₀ decreases with increasing temperature. Fig. S11(c) shows β_5/2, β_9/2, β_11/2, and β_13/2 versus temperature for LCZO: 2% Cr³⁺, 1% Nd³⁺. The value of β_5/2 gradually rises with increasing temperature, while the values of β_9/2, β_11/2, and β_13/2 show a decreasing trend with increasing temperature. According to Boltzmann's distribution law, the population of (⁴F_5/2) states increase with temperature compared to the ⁴F_3/2 level [44]. The activation energy ΔE of the non-radiative relaxation of the phosphor can be obtained from the following equation [45]:

(12)$${I_T} = \; \frac{{{I_0}}}{{1 + c\; \textrm{exp}\left( {\frac{{\Delta E}}{{kT}}} \right)}}$$

where I_T is the intensity of the emission spectrum at a certain temperature, I₀ is the intensity of the emission spectrum at room temperature, c is a constant, and ΔE and k are the activation energy and Boltzmann constant, respectively. According to the plot of Ln(I₀/I_T-1) versus 1/kT in Fig. S11(d), the activation energy of LCZO: 2% Cr³⁺, 1%Nd³⁺ is 0.290 eV.

3.3. Preparation of NIR pc-LED and its multifunctional application

Face recognition is a common mode in biometrics and is used in public safety. In the field of face recognition, NIR light is more suitable for face recognition than UV and visible light due to its non-destructive characteristics to human body. Fig. S12 shows that the sample LCZO: 2% Cr³⁺, 1% Nd³⁺ can be maintained at 373 K at 67% of room temperature, which is higher than 64% of LCZO: 2% Cr³⁺, 2% Nd³⁺ and 46% of LCZO: 2% Cr³⁺, 4% Nd³⁺, so LCZO: 2% Cr³⁺, 1% Nd³⁺ is chosen as the NIR light source. Fig. S13 shows the thermograms of pc-LEDs prepared using LCZO:2%Cr³⁺,1%Nd³⁺ at currents ranging from 0 ∼ 300 mA, where the temperature of pc-LEDs rises with increasing current and reaches 99.3 °C at 300 mA. Fig. 9(a), (b) shows the photos of the NIR camera under NIR light and blue light irradiation, where the faces under NIR light can be clearly recognized, while the photos under blue light are very blurred. Fig. 9(c) shows that the visible light camera does not take good pictures in backlight conditions. Fig. 9(d), (e) shows the photographs of orange, pear, grape and dragon fruit taken by the visible camera under white fluorescent light irradiation and those taken by the NIR camera under NIR pc-LED irradiation, respectively. Fig. 9(f) exhibits a photograph of a NIR pc-LED applied to finger penetration, where blood vessels can be clearly identified, indicating its potential application in the field of biological tissue penetration imaging as well. The above applications show that NIR light sources with dual emission peaks have great potential for applications in several fields.

Fig. 9. (a), (b) NIR pc-LED for face recognition. (c) Photo taken by visible light camera in backlight environment. (d), (e)NIR pc-LED for night lighting of oranges, pears, grapes and dragon fruit. (f) NIR light source for angiography.

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4. Conclusion

In summary, we explore LCZO: Cr³⁺, Nd³⁺ phosphors with emission spectra covering the range of 700-1400 nm under 471 nm blue light excitation. It is important to note that the energy transfer efficiency of Cr³⁺ for Nd³⁺ is as high as 88.4%. The spectral integrated intensity of LCZO: Cr³⁺, Nd³⁺ at 471 nm excitation is 847% of LCZO: Nd³⁺ and 204% of LCZO: Cr³⁺. This work provides a new vision for the development of novel NIR phosphors with Nd³⁺ as the dominant activator. Finally, LCZO: Cr³⁺, Nd³⁺ phosphor and 470 nm InGaN chips were combined to make NIR pc-LED. The NIR light sources are used in face recognition, night illumination and angiography, showing great potential for applications.

Funding

National Natural Science Foundation of China (62175075, 62075055); Natural Science Foundation of Hebei Province (2020201030); Central Project Guide local science and technology for development of Hebei Province (216Z1104G, 226Z1103G).

Disclosures

The authors declare that they have no conflict of interest.

Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

Supplemental document

See Supplement 1 for supporting content.

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Occupied sites	E_f (eV)	E_LCZO:Nd (eV)	E_LCZO (eV)	µ_Nd (eV)	µ_S (eV)
Nd→Ca	0.933	-28324.527	-27589.02	-1740.76	-1004.32
Nd→Zr	3.080	-28033.570	-27589.02	-1740.76	-1293.13
Nd→La	-8.072	-28365.019	-27589.02	-1740.76	-972.833

Name	x	Y	z	U_iso	Occ
La	0.5217	0.55409	0.25426	0.961	0.960
Nd	0.5217	0.55409	0.25426	0.961	0.040
Ca1	0.50000	0.00000	0.00000	0.491	0.250
Zr1	0.50000	0.00000	0.00000	0.491	0.245
Cr1	0.50000	0.00000	0.00000	0.491	0.005
Ca2	0.00000	0.50000	0.00000	0.152	0.250
Zr2	0.00000	0.50000	0.00000	0.152	0.245
Cr2	0.00000	0.50000	0.00000	0.152	0.005
O1	0.20878	0.20176	-0.07227	0.951	1.000
O2	0.15844	0.82129	-0.00217	0.424	1.000
O3	0.43140	-0.01996	0.20039	0.987	1.000

Occupied sites	E_f (eV)	E_LCZO:Nd (eV)	E_LCZO (eV)	µ_Nd (eV)	µ_S (eV)
Nd→Ca	0.933	-28324.527	-27589.02	-1740.76	-1004.32
Nd→Zr	3.080	-28033.570	-27589.02	-1740.76	-1293.13
Nd→La	-8.072	-28365.019	-27589.02	-1740.76	-972.833

Name	x	Y	z	U_iso	Occ
La	0.5217	0.55409	0.25426	0.961	0.960
Nd	0.5217	0.55409	0.25426	0.961	0.040
Ca1	0.50000	0.00000	0.00000	0.491	0.250
Zr1	0.50000	0.00000	0.00000	0.491	0.245
Cr1	0.50000	0.00000	0.00000	0.491	0.005
Ca2	0.00000	0.50000	0.00000	0.152	0.250
Zr2	0.00000	0.50000	0.00000	0.152	0.245
Cr2	0.00000	0.50000	0.00000	0.152	0.005
O1	0.20878	0.20176	-0.07227	0.951	1.000
O2	0.15844	0.82129	-0.00217	0.424	1.000
O3	0.43140	-0.01996	0.20039	0.987	1.000

Study on the mechanism of high energy transfer efficiency of blue light excited Cr³⁺, Nd³⁺ co-doped near infrared phosphors

Abstract

1. Introduction