Crystal structure of Lu3(Sc, Mg)2(Ga, Si)3O12:Cr3+
The phase and purity of the as-prepared powder samples were examined by X-ray diffraction and the results are shown in Figure 1A. The x value reflects the degree of co-substitution of [Mg2+-Si4+] for [Sc3+-Ga3+]. All diffraction peaks were well matched with the standard data of the Lu3Sc2Ga3O12 cubic phase (PDF No. 54-1252, space group Ia-3d) and no obvious impurity phases were observed, indicating that the phase of the samples was not affected by a certain amount of chemical unit co-substitution due to the high structural tolerance of garnet-type compounds as shown in Figure 1C.
The shift of the strongest diffraction peak to a higher angle, as shown in Figure 1A, indicates that the cell shrinks with increasing x. With the substitution of [Mg2+-Si4+] for [Sc3+-Ga3+], the diffraction peak near 32.7° shifts slightly toward a higher angle [Figure 1A] due to the ionic radii of Mg2+ [0.72 Å, coordination number (CN) = 6] and Si4+ (0.26 Å, CN = 4) being smaller than that of Sc3+ (0.745 Å, CN = 6) and Ga3+ (0.47 Å, CN = 4), respectively[35]. The lattice parameter of the phosphors in the cubic system was calculated using the Bragg equation:
where d and a represent the interplanar spacing and lattice parameter, respectively. The wavelength of the incidence X-ray λ was 1.5406 Å. The diffraction peak position θ originating from the crystal face (hkl) was obtained from the strongest diffraction peak near 32.7° in Figure 1A, where h, k and l are the Miller indices, namely, 0, 2 and 4, respectively, in the calculation. The lattice parameter decreased from 12.32 to 12.23 Å with increasing x, as shown in Figure 1B. This indicates that the [Mg2+-Si4+] chemical unit had been successfully incorporated into the garnet-type structure. The shrinkage of the crystal cell usually enhances the crystal field strength of the octahedrally-coordinated Cr3+ center with a blueshift and narrowed emission band. In contrast, a completely opposite phenomenon in Lu3Sc2Ga3O12:Cr3+ can be observed after the [Mg2+-Si4+] chemical unit was introduced, as discussed below.
Photoluminescence properties of Lu3(Sc, Mg)2(Ga, Si)O12:Cr3+
The room-temperature emission (PL) and excitation (PLE) spectra of Lu3Sc1.98Ga3O12:0.02Cr3+ are shown in Figure 2A. Under the excitation of 440 nm blue light, Lu3Sc1.98Ga3O12:0.02Cr3+ exhibits a dark-red emission with a FWHM of 88 nm centered at 706 nm, which is attributed to the 2E→4A2g transition of Cr3+. When monitored at the 706 nm emission, Lu3Sc1.98Ga3O12:0.02Cr3+ gave three excitation bands in the UV, blue and red regions, which were assigned to the 4A2→4T1 (4P), 4A2→4T1 (4F) and 4A2→4T2 (4F) transitions of Cr3+, respectively. After the [Mg2+-Si4+] unit replaces the [Sc3+-Ga3+] couple in the structure, the emission band position gradually shifts from 706 to 765 nm. Furthermore, the FWHM is doubled to 176 nm [Figure 2B]. The Lu3Sc1.38Mg0.6Ga2.4Si0.6O12:0.02Cr3+ phosphor has a broadband NIR emission with stronger penetrability and crypticity, which is more conducive to its application in biological imaging and component analysis compared to Lu3Sc1.98Ga3O12:0.02Cr3+ with a narrowband visible dark-red emission.
The change in the spectrum can be divided into two stages, as shown in Figure 2C. In the first stage (x = 0.0-0.45), the emission band position rapidly redshifts and the FWHM increases significantly, mainly as a result of the enhancement of the broadband 4T2→4A2 emission, indicating that the excited electrons gradually tend to populate at the 4T2 state rather than the 2E state. The enhancement of the parity-forbidden 4T2→4A2 transition is a common phenomenon in garnet oxide solid solutions[36] and is usually caused by the disordered local crystal environment of Cr3+ due to the introduction of the chemical unit. In the second stage (x = 0.6-0.9), the values of the emission band position and FWHM increased slowly and finally converged to a constant. This may be caused by two competing factors. The first is the crystal field enhancement caused by the lattice contraction discussed previously, which usually engenders the blueshift and sharpens the Cr3+ emission. The second factor is the disordered local crystal environment, which usually engenders the redshift and broadens the Cr3+ emission.
Because of the lack of protection from an external shell, the orbital energy levels of the d orbital of Cr3+ are very sensitive to the influence of the host lattice. Therefore, to explore the further influence of the [Mg2+-Si4+] chemical unit substitution on the emission properties of Cr3+, it is necessary to quantitatively calculate the crystal field splitting parameters, including Dq, B and C. According to crystal field theory, the crystal field strength parameter Dq and Racah parameter B can be approximated by the following equations[37]:
Finally, the Racah parameter C can be calculated by:
where E(4T1g) and E(4T2g) are the energy levels of 4T1g(4F) and 4T2g(4F) for Cr3+, respectively, which can be obtained from the PLE spectra, and E(2E) is the equilibrium position of the sharp zero-photon line (ZPL) obtained from the PL spectra.
The Racah parameter B represents the repulsion between electrons in the 3d orbital of Cr3+. The value of the Racah parameter B of Cr3+ in the host is usually lower than in the free environment (B0 = 918 cm-1) and strongly depends on the covalency of the host. Accordingly, the specific crystal field parameters of Lu3Sc1.98-xMgxGa3-xSixO12:0.02Cr3+ (x = 0.0, 0.6 or 0.9) were calculated, as listed in Table 1. The value of Dq/B decreases from 2.85 to 2.65 with increasing x value, thus reducing the energy separation between 4T2g and 2E [Figure 2D]. The decreased B values indicate that the ionic character of the octahedra in which Cr3+ is located is enhanced due to the enlargement of the [Sc/MgO6] octahedra, even though the whole crystal cell shrinks under co-substitution[38]. Therefore, the excited electrons will tend to reside in the gradually redshifted 4T2g level due to the weakening crystal field and induction of a broad NIR emission (4T2→4A2). As shown in Figure 2E, with increasing x, the integral intensity of Lu3Sc1.98-xMgxGa3-xSixO12:0.02Cr3+ increased to 180% of the original value at first (x = 0.0-0.6) and then decreased (x = 0.6-0.9). Figure 2F shows the DR spectra of the Lu3Sc1.98-xMgxGa3-xSixO12:0.02Cr3+ series. The three absorption bands of Cr3+ in the UV-Vis region fit well with the three excitation bands of the PLE spectra. With the introduction of [Mg2+-Si4+] chemical units in the structure (x = 0.0-0.6), the reflectance at 440 nm reduces from 81.9% for x = 0.0 to 67.4% for x = 0.6, which indicates that more blue light was absorbed by the optimized phosphor.
The external quantum efficiency (EQE) of the designed phosphors is crucial for high-performance NIR LEDs and is the product of the internal quantum efficiency (IQE) and Abs[39,40]. The quantum yield of the samples was measured and the values of the EQE, IQE and Abs are shown in Figure 3A. The matrix garnet phosphor (x = 0.0) shows a high IQE of up to 81.4% and a low Abs (0.257) due to the parity-forbidden transition of Cr3+. The [Mg2+-Si4+] co-substituted phosphor (x = 0.6) demonstrated a higher EQE (28.1%) compared to the matrix phosphor (20.9%) on account of the improvement of the absorption ability from 25.7% (light green body color) to 38.4% (deep green body color). Excessive [Mg2+-Si4+] chemical units were harmful to the luminescence ability of the Lu3Sc1.98-xMgxGa3-xSixO12:0.02Cr3+ phosphors (x = 0.75 or 0.90), which is consistent with the trend of the integral intensity of the PL spectra [Figure 2E]. The decreased trend of the Abs of Lu3Sc1.98-xMgxGa3-xSixO12:0.02Cr3+ phosphors (x = 0.75 or 0.90) may be caused by more serious diffuse reflection due to the mismatched refractive index between the garnet phase and the impurities.
The experiment results show that the PL decay curves can only be fitted with a bi-exponential function [Figure 3B]:
where I(t) represents the emission intensity at a certain time t, A1 and A2 are constants and τ1 and τ2 are the luminescence decay times. According to the above equation, the decay times were determined to be 406.1, 218.4 and 195.3 μs for τ1 and 185.1, 58.6 and 56.0 μs for τ2, corresponding to the x = 0.0, 0.6 or 0.9 samples, respectively. It is known that Cr3+ generally exists stably at octahedral sites. Although Cr3+ will occupy both octahedral and dodecahedral sites in some garnet oxides, the PL band of Cr3+ at dodecahedral sites usually exhibits longwave PL (> 800 nm) due to the larger radius of the polyhedra[31]. Thus, the bi-exponential PL decay was more likely caused by one crystallographic site with multiple local crystal environments.
The two lifetimes with different orders can be attributed to the 2E→4A2g and 4T2g→4A2g transition, respectively. The 2E→4A2g transition is both parity-forbidden and spin-forbidden, while the 4T2g→4A2g transition is parity-forbidden but spin-allowed, which causes the former to have a longer lifetime. The transition from the 2E state becomes partially allowed due to the overlap between the wavefunctions of the 2E and 4T2 states, which explains the shorter decay time in the phosphors after co-substitution. This phenomenon suggests that the excited electrons are populated at both the 2E and 4T2g levels, which further indicates that Cr3+ in Lu3Sc1.98-xMgxGa3-xSixO12 may possess more than one set of Dq/B values, which also explains why the total crystal field strength calculated in Figure 2D was not at a medium level and the inhomogeneous broadening in the PL spectra. The bi-exponential decay of the matrix may be caused by anti-site defects, which were previously reported in Lu3Sc2Ga3O12:Cr3+[41]. Overall, co-substitution will induce a local structural distortion and then introduce sites with weaker crystal field strength, as well as the partial lifting of the forbidden 4T2g→4A2g transition due to the high symmetry. The phenomenon where two luminescence components in the inhomogeneous broadening PL spectra can be detected when monitored at a certain wavelength, indicating the nonnegligible thermal coupling of the 2E and 4T2g levels, as discussed below. The PL lifetimes decreased with increasing x [Supplementary Figure 1] due to the partial lifting of the restriction on the forbidden transition and the increase in the probability of the non-radiative transition.
To investigate the detailed luminescence mechanism herein, the low-temperature-dependent PL spectra and PL decay curves of LSMGS:Cr were recorded and compared in Figure 3C, Supplementary Figures 2 and 3. At 80 K, the PL spectra of LSMGS:Cr consist of a sharp ZPL, namely, the 2E→4A2g transition at 690 nm and the attached stokes sideband (705 and 720 nm) due to the participation of phonons. The 4T2g→4A2g transition presented as a broadband emission centered at 785 nm. With increasing temperature, the intensity of the 2E→4A2g/4T2g→4A2g transition enhanced at first and then decreased significantly when the temperature exceeded 125 K. This similar phenomenon related to the ZPL and Stokes sideband also exists in other transition metal-doped phosphors with the 3d3 electronic configuration, such as V2+ and Mn4+[42,43]. The dopants in the host introduce point defects, which can accept energy from not only photons but also phonons in the host lattice to produce luminescence. The latter, known as phonon-assisted emission, will be enhanced by more phonons according to the Bose-Einstein distribution law when heating from a low temperature. When the temperature is raised to a specific level, the large number of phonons induces serious electron-phonon coupling, which leads to thermal quenching.
The 4T2g→4A2g emission with the same behavior was abnormal compared to other Cr-doped phosphors. Generally, phosphors at low temperatures exhibit a stronger emission because the lattice vibration is inhibited, which weakens the electron-phonon coupling effect and causes a smaller probability of non-radiative transition. The abnormal 4T2g→4A2g PL behavior related to the temperature indicates that the thermal coupling effect between the 2E and 4T2g levels did exist in Lu3Sc1.98-xMgxGa3-xSixO12:0.02Cr3+ so that the phonon energy can transfer between these two levels by back-intersystem crossing/intersystem crossing (blsc/lsc)[44]. The phonon-assisted emission was also present in the solid solution with x = 0.45, as shown in Supplementary Figure 4. In addition, the PL decay also can be fitted with a bi-exponential function with a reduced lifetime compared with that at 80 K due to the increasing portion of 4T2g→4A2g emission during the blsc process and more serious electron-phonon coupling. In the Cr-doped phosphor, weakening of the crystal field strength caused by lattice thermal expansion with increasing temperature leads to a redshift in the PL spectra. In LSMGS:Cr, however, the emission center with a larger Stokes shift will suffer more serious thermal quenching, resulting in the 20 nm blueshift of the emission spectrum at high temperature, which is common in disordered local crystal environment, such as Cr-doped glasses[45], which further proves the disorder of the local structure in LSMGS:Cr. Even though the non-radiative transition to some extent exists in the LSMGS:Cr, it still retains 75% at 423 K of the initial intensity measured at room temperature and maintains fine color stability [Figure 3D and Supplementary Figure 5], which benefits from the high structural rigidity of the garnet.
In summary, the mechanism of [Mg2+-Si4+] co-substitution on the regulation of optical properties can be qualitatively explained, as shown in Figure 3E and F. The [Mg2+-Si4+] chemical units not only introduce more multiple local crystal environments but also reduce the energy level gap ΔEc between 4T2g and 2E, which is closely related to the crystal field strength. Thus, the energy level thermal coupling enhances, which allows part of the excited electrons at the 2E level can also populate into the 4T2g level through the back-intersystem crossing process with increasing x and eventually leads to the transition from a short wavelength sharp emission to a long wavelength broad emission. However, the weak crystal field strength simultaneously causes the reduction of the thermal activation barrier ΔEa, which causes more serious electron-phonon coupling and thermal quenching, thereby weakening the luminescence performance, which is consistent with the results of the luminescence analysis and thermal stability [Figures 2E and 3A; Supplementary Figure 6].
Application in a NIR pc-LED
To demonstrate the potential application of NIR pc-LEDs in night vision illumination and the penetrating imaging of biological tissue, a broadband NIR pc-LED device was designed and fabricated using the LSMGS:Cr (x = 0.6) phosphor on a blue light-emitting InGaN chip (440 nm), as shown in Figure 4A. The NIR output power continuously increases with the drive current and reaches 90.3 mW at 100 mA. Furthermore, the photoelectric efficiency of pc-LED drops from 8.73% to 5.74% due to the efficiency drop of LED chips, as shown in Figure 4B. Figure 4C shows that a bouquet can be vividly captured by a NIR camera using a 720 nm long-pass filter under the non-visible illumination of the NIR pc-LED, which shows the feasibility of its application in night vision. The veins of a human palm can be distinguished, as shown in Figure 4D, using NIR light to penetrate and a NIR camera to capture, because NIR light has a good penetrability through biological tissue and veins have a specific absorption of NIR light. This fundamental demonstration indicates that the LSMGS:Cr phosphors can be potentially applied in machine vision and non-destructive examination.
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