Xiangyu Menga,
Xiaoshuai Zhanga,
Xueli Shia,
Keliang Qiua,
Zhijun Wang*a,
Dawei Wangb,
Jinxin Zhaob,
Xue Lia,
Zhiping Yanga and
Panlai Li*a
aNational-Local Joint Engineering Laboratory of New Energy Photoelectric Devices, Hebei Key Laboratory of Optic-electronic Information and Materials, College of Physics Science & Technology, Hebei University, Baoding 071002, China. E-mail: wangzj1998@126.com; li_panlai@126.com
bHebei Key Laboratory of Semiconductor Lighting and Display Critical Materials, Baoding 071000, China
First published on 19th May 2020
A series of broad emission band near infrared materials Mg3Y2Ge3O12:Cr3+ (650–1200 nm) was prepared based on cation inversion. For trivalent chromium ions (Cr3+), garnet structural components can provide conditions for the occurrence of cation inversion. With an increase in Cr3+ concentration, the Mg2+ and Ge4+ cations are inverted to ensure valence equilibrium, which was explained by recording the low temperature spectrum of the structure and carrying out structural refinement. As a result, this structure provides a new luminescent center [GeO6] for Cr3+, leading to a secondary enhancement in emission intensity. The wavelength of the main peak was found to move from 771 to 811 nm, and the full width at half maximum (FWHM) was broadened from 180 to 226 nm. The lattice occupation, luminescence mechanism and the reasons behind the red-shift and broadening of the spectra were studied in detail. By analyzing the crystallinity and particle size distributions of the samples, as well as the Cr3+ ion energy level shift, it was determined that cation inversion is an effective method that can be used to tune the luminescence performance. Meanwhile, a super broad near infrared light emitting diode (LED) with a FWHM of 260 nm was obtained by combining a GaN chip with MYG:0.40Cr3+.
NIR phosphors have been studied extensively. For example, Kolesnikov et al. reported NIR phosphors with YVO4 as a matrix and Nd3+ as an activator. YVO4:Nd3+ exhibits two emission bands at around 808 and 880 nm when excited by light at 532 nm. Since these bands are thermally coupled over a wide temperature range, YVO4:Nd3+ can be used as a ratiometric luminescence thermal sensor. Singh et al. reported that ZnMgAl10O17:Er3+ emits NIR light in the ranging of 1450–1650 nm.9 At the same time, the introduction of Yb3+ ions was shown to improve the luminescence intensity of Er3+ by energy transfer, which does not affect the occupation of Er3+. Since the luminescence of rare earth ions originates from transitions within the 4f energy level, the emission of the ions is hardly affected by the change in the surrounding crystal field environment, due to the shielding effect of the 5s and 5p electron layers. Transition metal ions are a good choice as NIR activators because their emission spectra can be well adjusted by changing the crystal field environment. For example, the novel NIR luminescent material Li2ZnGe3O8:Mn2+ was prepared by our research group, with an emission spectrum ranging from 650 to 900 nm.10 However, the emission of Mn2+ ions is closer to the red light region, such as for Ca9−x−y−zMgxSryBazCe(PO4)7:Eu2+, Mn2+,11 and MgGeO3:Mn2+.12 The Cr3+ ion is also an excellent NIR activator, which shows different luminescence properties in different crystal field environments. For example, in a strong crystal field environment, the emission of Cr3+ originates from the spin forbidden transition 2E(2G) → 4A2(4F), it displays a sharp band spectrum (the peak is near 700 nm) and is slightly affected by the change in crystal field strength, according to its Tanabe–Sugano (3d3) diagram. Deng et al. prepared LiGa5O8:Cr3+ with enhanced persistent luminescence that has a narrow peak located at 700 nm.13 Cr3+ and Sn4+ ions prefer to occupy octahedral sites. At the same time, the introduction of Sn4+ can effectively improve afterglow performance. In a weak crystal field, the emission of Cr3+ originates from the spin-allowed transition 4T2 → 4A2, showing a broadband emission (probably in the range of 600–1000 nm) that is easily affected by the change in the crystal field environment.14,15 For example, the NIR phosphor Ca2MgWO6:Cr3+ was prepared by Meng et al., which exhibited a broadband peak at 803 nm when excited at 371 nm.16 In Ca2MgWO6, Cr3+ ions substitute Mg2+ sites and are located in a weak crystal field. Shao et al. reported the NIR material ScBO3:Cr3+ and obtained a NIR LED.8 In the host, the Cr3+ ions occupied the Sc3+ sites with relatively low crystal field strength and showed a broadband emission peak at 800 nm.
A garnet has a cubic structure with Iad symmetry, which can be described in terms of a 160 atom body-centered cubic unit cell.17 Its chemical formula is generally A3B2C3O12, in which the atoms A, B, and C occupy the lattice sites 24c, 16a, and 24d, respectively.18,19 The coordination numbers of atoms A, B and C are 8, 6, 4, respectively, and each tetrahedron is connected to four octahedra, providing multiple sites for active ions to occupy.20 Since the 1970s, Cr3+-doped garnet phosphors have been one of the most famous types of NIR phosphors, for example, YAG:Cr3+, which is a typical NIR material.21 In addition, Kiss and Duncan further studied the non-radiative energy transfer from Cr3+ to Nd3+ in YAG.22,23 Another characteristic of garnets is the cationic inversion that occurs between the tetrahedra and octahedra, which can be used to improve the luminescence performance of an activator, such as improving luminescence efficiency, broadening excitation or emission spectra.24–26 In order to improve the probability of wide-spectrum NIR emission, an inverse-garnet structure can be used as a host material. In a normal garnet, all three polyhedra are regular polyhedra. In order to increase the probability of wide-spectrum emission, the inverse-garnet structure is a good choice because its polyhedra are distorted. Therefore, in this work, Mg3Y2Ge3O12 with an inverse-garnet structure was chosen as the matrix and Cr3+ ions were selected as an activator to achieve broadband NIR emission. A series of NIR materials were prepared that can cover from 700 to 1200 nm. The full width at half maxima (FWHM) reached 226 nm and the emission intensity was enhanced due to cationic inversion. A NIR LED was prepared by combining MYG:0.40Cr3+ with a GaN chip.
3MgO + (1 − x/2) Y2O3 + 3GeO2 + x/2Cr2O3 → Mg3Y2−xGe3O12:xCr3+ |
Fig. 1 (a) Crystal structure of Mg3Y2Ge3O12; (b) XRD patterns of MYG:xCr3+ (x = 0, 0.03, 0.30) and the standard Mg3Y2Ge3O12 (ICSD #280049) crystal data; (c) X-ray Rietveld refinements for MYG. |
The EPR spectrum of MYG was measured to elucidate its structural characteristics at room temperature, as shown in Fig. 2(a). The EPR spectrum exhibits an intense resonance signal centered at g ∼ 2.00, which indicates that there are typical oxygen vacancies in the host.27,28 Generally, an oxygen vacancy can capture one electron, forming a sequential resonance source with S = 1/2, which results in the appearance of a stable EPR signal at around g ∼ 2.00. That is to say, the oxygen vacancies are formed in the process of MYG crystal growth. Furthermore, the thermoluminescence (TL) glow curve of MYG was measured in the range of 300–600 K after 10 minutes of ultraviolet lamp irradiation (254 nm), as shown in Fig. 2(b). There is a peak located at 395 K in the spectrum, which can be attributed to oxygen vacancies. For the host, the trap depth (E) can be obtained using the peak shape method, as shown in the following equation:29,30
ω = δ + τ | (1) |
Fig. 2 (a) EPR spectrum of MYG at room temperature; (b) TL spectra of MYG; (c) the process of the generation of oxygen defects. |
Generally, the broadband emission of Cr3+ is generated in intermediate or weak crystal field environments. The order of crystal field strength is tetrahedral > octahedral > dodecahedral, therefore, it is hard for Cr3+ to enter into a tetrahedral lattice with its strong crystal field, but it tends to enter octahedral [Mg2O6] and dodecahedral [Mg1/Y1O8] lattices.5,35 To gain further knowledge about the occupation of Cr3+ in MYG and structural information on MYG:xCr3+, the XRD Rietveld refinements of MYG:xCr3+ (x = 0.01, 0.03, 0.05, 0.07) were carried out using the GSAS program, the results of which are shown in Fig. 4(a). A model of MYG (ICSD #280049) was used as the initial structure to refine the samples in the 2θ range of 10–80°. The parameters of the refinement results (Rp, Rwp and χ2) were found to be in line with the standard, which indicates that the Rietveld refinement results are reliable. The atomic positions are shown in Table S2.†
The cell parameters a, b and c, as well as the cell volume, V, of the samples are shown in Fig. 4(b). The values of a, b, c and V gradually decrease, which can be attributed to the radius of Cr3+ (0.0615 nm) being smaller than those of Mg2+ (0.072 nm) and Y3+ (0.1019 nm). In Fig. 4(c), the occupying information of the Cr3+ ions at different Cr3+ concentrations is shown. It can be seen that the Cr3+ ions are only doped into the [Mg2O6] site and [Mg1/Y1O8] site, as we expected. Therefore, there are three luminance centers of Cr3+, which was also proven by the low temperature spectrum of MYG:0.03Cr3+ shown in Fig. S2.† Fig. 5(a) shows the PL spectrum of MYG:0.03Cr3+, which was divided into three sub-peaks with peaks at 748 nm (peak 1), 806 nm (peak 2) and 899 nm (peak 3) using a Gaussian multiple peak fitting technique. At the same time, the corresponding excitation spectra of MYG:0.03Cr3+, monitoring the three sub-peaks, were measured and are shown in Fig. S3.† Taking the energy of the 4A2 state to be equal to zero, the value of the crystal field strength, Dq, and the Racah parameter, B, can be calculated using the following eqn (2) and (3):36–38
E(4T2) = 10Dq |
(2) |
Fig. 5 (a) The PL and the three sub-peaks of MYG:0.03Cr3+; (b) fluorescence decay curves of Cr3+ emission in MYG:xCr3+ phosphors monitored at 748, 806 and 899 nm. |
The relative energy between the 4T1(4F) and 4T2(4F) levels is represented by the symbol ΔE. Therefore, the Racah parameter, B, can be calculated by simplifying eqn (2), in the following way:
(3) |
According to eqn (2) and (3), the values of Dq and Dq/B are 1594, 1574, 1557 and 2.38, 2.24, 2.13, indicating that the three sub-peaks arise from different luminescence centers. The value of Dq is inversely proportional to R5.39 Therefore, the bond lengths corresponding to peaks 1, 2 and 3 increase gradually. The average bond lengths of Mg2–O and Mg1/Y1–O are 2.065 and 2.390 Å, respectively. Because the radius of Mg1 (0.089 nm) is smaller than that of Y1 (0.1019 nm), the average bond length of Y1–O is longer than that of Mg1–O. The emission peaks of CrMg2, CrMg1 and CrY1 correspond to peaks 1, 2 and 3, respectively. Fig. 5(b) shows the fluorescence decay curves of Cr3+ emission in the MYG:xCr3+ phosphors monitored at 748, 806 and 899 nm, which were fitted using a double exponential equation:40
I(t) = I0 + A1e−t/τ1 + A2e−t/τ2 | (4) |
τ* = (A1τ12 + A2τ22) | (5) |
The lifetimes of the peaks located at 748, 806 and 899 nm are 25.3, 21.1 and 6.3 μs, respectively. Besides this, according to Fig. 4(c), with an increase in the Cr3+ concentration, the Cr3+ content in [Mg1/Y1O8] (CrMg1/Y1) increases, but first increases and then decreases in [Mg2O6] (CrMg2), which is the same change trend as for the emission intensity, as shown in Fig. 3(c). It can be inferred that in the range of 0 < x < 0.1, the luminous intensity of Cr3+ in [Mg2O6] determines the overall luminous intensity.
Because there are defects in the host, as seen from Fig. 2(b), the effect of defects on the luminescence intensity should be considered. Therefore, the TL spectra of the samples were measured and are shown in Fig. S4.† However, the TL peak was observed to gradually disappear, which can be attributed to the unequal substitution of Cr3+. The substitution process of Cr3+ in the host can be represented by the two equations:
(6) |
Cr3+ → Y3+ + CrY | (7) |
When Mg2+ is substituted by Cr3+, a Mg2+ cation vacancy occurs due to different valence states. However, when Y3+ is substituted by Cr3+, there are no cation vacancies. Because of the existence of oxygen vacancies, in MYG, the make up for the defects with an increase in the Cr3+ concentration, which results in the disappearance of the TL peak. In addition, the refined data shown Fig. 4(a) also indicate that the crystallinity improves.
The reflection spectra of MYG:xCr3+ (x = 0.01, 0.03, 0.05, 0.07) were measured and are shown in Fig. 6(b). The two absorption peaks arising from the 4T1(4F) and 4T2(4F) levels shift to a shorter wavelength with an increase in Cr3+ concentration. Using eqn (2) and (3), the values of Dq, B and Dq/B were obtained and are displayed in Table 1. It can be seen from Table 1 that the Dq/B values decrease with an increase in Cr3+ concentration. Together with the Tanabe–Sugano (3d3) diagram shown in Fig. 6(a), the energy difference between the energy levels 4T2 and 4A2 decreases as the value of Dq/B decreases, which leads to a red-shift in the emission spectra.
Fig. 6 (a) Tanabe–Sugano (3d3) diagram; (b) reflection spectra of MYG:xCr3+ (x = 0.01, 0.03, 0.05, 0.07). |
MYG:xCr3+ | Dq | B | Dq/B |
---|---|---|---|
x = 0.01 | 1587.30 | 648.25 | 2.45 |
x = 0.03 | 1589.83 | 651.24 | 2.44 |
x = 0.05 | 1594.90 | 671.25 | 2.38 |
x = 0.07 | 1600.00 | 677.59 | 2.36 |
The spectral broadening is related to the degree of energy splitting. An enhancement in the energy level splitting leads to spectral broadening, whereas in contrast, a reduction in the energy level splitting leads to spectral narrowing. The splitting degree of the energy level is positively correlated with the crystal field strength, Dq. According to Table 1, the value of Dq increases with an increase in the Cr3+ concentration, which leads to an enhanced splitting of the 4T2 energy level and spectral broadening.
(8) |
At the same time, a new luminescence center, is formed, which enhances the emission intensity and broadens the emission spectra. As the concentration of Cr3+ continues to increase, the value of Dq/B continues to decrease, resulting in the continued red shift of the spectrum. Therefore, it is necessary to further increase the concentration of Cr3+ ions to obtain NIR emission materials with a wider FWHM that are closer to long wave.
The PL spectra of MYG:xCr3+(x = 0.1, 0.2, 0.3, 0.35, 0.4) are shown in Fig. 8(a). The emission intensities of the samples are enhanced again, the spectra keep red shifting, and the FWHM continues to broaden. In order to show the spectral changes more clearly, the normalized spectra and the PL spectral change curves are shown in Fig. 8(b) and (c), respectively. It is obvious that the emission wavelength moves from 789 to 811 nm, and the FWHM extends 204 nm to 226 nm. The emission intensity first increases and then decreases, and the intensity is strongest at x = 0.30, which is about 1.5 times the emission intensity exhibited by MYG:0.03Cr3+. According to the experimental design, with an increase in the Cr3+ ion concentration, a new luminescence center CrGe should arise. The low temperature spectrum of MYG:0.30Cr3+ at 10 K was measured and is shown in Fig. S5.†
It can be seen that a new peak (peak 4) appears, which means that a new luminescence center is formed with an increase in Cr3+ concentration. The formation of the new luminescence center is due to Ge4+, with a smaller radius, entering the octahedron, i.e., cation inversion. In order to explore the change in sample properties in more detail, the XRD data of the samples were refined and the results are shown in Fig. 9(a). The refinement parameters (Rp, Rwp and χ2) all meet the requirements. The atomic positions are shown in Table S3.†
The important parameters of the refinement results are shown in Fig. 9(b)–(d), such as a/b/c, V, the occupancy of Cr3+ and the volume changes of the tetrahedron (V-[GeO4]), octahedron (V-[Mg2O6]), and dodecahedron (V-[Mg1/Y1O8]). From Fig. 9(b), it can be seen that the cell parameters (a/b/c and V) decrease, as observed in the samples with low Cr3+concentration. According to Fig. 9(c), the Cr3+ ions still substitute for the [Mg2O6] and [Mg1/Y1O8] sites. However, the concentration of CrMg2 increases despite a slight decrease at x > 0.3, which is important for enhancing the luminescence intensity. The increase in CrMg2 indicates that a new luminescence center arises in [Mg2O6], corresponding to peak 4 shown in Fig. S5.†
The source of the new luminescence center can be well observed by analyzing the polyhedral volume in the host shown in Fig. 9(d). Since the Cr3+ is in the positions of [Mg2O6] and [Mg1/Y1O8], and the radius of Cr3+ (0.0615 nm) is less than those of Mg2+ (0.072 nm, N = 6), (0.089 nm, N = 8) and Y3+ (0.1019 nm, N = 8), V-[Mg2O6] and V-[Mg1/Y1O8] will be decreased. As for [GeO4], there are two reasons for the change in volume. One reason is that the [GeO4] volume increases due to the tugging action of [Mg1/Y1O8] and [Mg2O6]. Another reason is the occurrence of cation inversion between Mg2 (0.057 nm, N = 4), (0.072 nm, N = 6) and Ge (0.039 nm, N = 4), (0.053 nm, N = 6). When x > 0.30, due to the decrease in the degree of cation inversion, the volume of [GeO4] decreases. Therefore, it can be inferred that with an increase in Cr3+ concentration (x < 0.30), the degree of cation inversion increases, which leads to an increase in the concentration of Cr3+ in the octahedron, which enhances the emission intensity. However, when x > 0.30, the degree of cation inversion begins to decrease and the emission intensity decreases.
In addition, the reflection spectra of MYG:xCr3+ (x = 0.10, 0.20, 0.30, 0.40) were measured and are shown in Fig. 10(a). Using eqn (2) and (3), the values of Dq, B and Dq/B were calculated and the results are shown in Fig. 10(b), where it can be seen that the value of Dq/B gradually decreases. Therefore, together with the information shown in Fig. 6(a), it was determined that the 4T2 level is close to the 4A2 level, which causes a red shift in the spectrum. The value of Dq gradually increases, which leads to spectral broadening. With an increase in Cr3+ concentration, the change in the crystal field environment is in line with the experimental design and requirements.
Fig. 10 (a) Reflection spectra of MYG:xCr3+ (x = 0.10, 0.20, 0.30, 0.40); (b) the curves of the changes in Dq, B and Dq/B. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra01742f |
This journal is © The Royal Society of Chemistry 2020 |