Synthesis and near-infrared luminescence of La₃GaGe₅O₁₆:Cr³⁺ phosphors

Abstract

A novel Cr³⁺-doped La₃GaGe₅O₁₆ phosphor was synthesized by a solid-state reaction, and the phase formation and microstructure, near-infrared (NIR) photoluminescence (PL) properties and PL thermal stability were investigated in detail. The excitation spectrum of La₃GaGe₅O₁₆:Cr³⁺ at 270, 415 and 573 nm corresponded to three spin-allowed Cr³⁺ d–d intra-transitions of ⁴A₂–⁴T₁ (⁴P), ⁴A₂–⁴T₁ (⁴F), and ⁴A₂–⁴T₂ (⁴F), respectively, among which the main red emission peak observed at 700 nm is identified, which is due to the ²E–⁴A₂ transition from Cr³⁺ ions. It is further proved that the dipole–dipole interactions result in the concentration quenching of Cr³⁺ in La₃GaGe₅O₁₆:Cr³⁺ phosphors, and the energy-transfer distance at the quenching concentration was around 26.90 Å. Moreover, thermal quenching luminescence results reveal that La₃GaGe₅O₁₆:Cr³⁺ exhibits good thermal stability. The above results indicate that La₃GaGe₅O₁₆:Cr³⁺ has potential practical applications in luminescence solar concentrators with broad-band absorption.

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1. Introduction

Recently, solar cells have attracted much attention because they are promising devices for inexpensive and large-scale solar energy conversion. However, the efficiency in the commercial Si solar cells is just 15%, which means that most of sunlight energy is lost.¹ One of the main energy loss in the conversion of solar energy to electricity is attributed to the so-called spectral mismatch between incident solar photon energies and the energy gap (E_g) of the cells.² As we know, the main energy of the solar emission dispersed on the earth surface is in visible light region, but E_g of the Si cell is suitable to the absorption in far-red and near-infrared regions. Consequently, luminescent solar concentrator (LSC) was developed that can collect solar photons and transfer them to the photons matching E_g of the Si solar cells. Moreover, when luminescent materials with efficient UV-Vis absorption and deep-red and near-infrared emissions are applied in LSCs, the efficiency of Si cells would be significantly enhanced.³ Therefore, studies on new phosphors with deep-red and near-infrared emissions are highly desirable.

Interest in Cr³⁺ activated inorganic materials is widespread because of its deep red colors and bright photoluminescence. Cr³⁺ is the most stable state of chromium element because of its narrow-band emissions (usually near 700 nm) due to the spin-forbidden ²E–⁴A₂ transition, or a broadband emission (650–1600 nm) due to the spin-allowed ⁴T₂–⁴A₂ transition, which strongly depends on the crystal-field environment of the host lattices.^4–6 Therefore Cr³⁺ can be used as a luminescent dopant with red emission in LSCs. This dopant also has its specific use in modern technologies, for example, tunable solid-state lasers, high temperature sensors, and high-pressure calibrants.^7–9 Recently, various Cr³⁺ doped host materials has been widely investigated by several research groups and its emission in various compounds like Y₃Al₅O₁₂,¹⁰ MgAl₂O₄,¹¹ Cs₂NaScCl₆ (ref. 12) and many others have also been studied.

As for the development of Cr³⁺-activated optical materials with tunable red and NIR emission, gallogermanates were usually used as the host materials since Cr³⁺ ions can substitute for Ga³⁺ ions in distorted octahedral sites. Moreover, suitable host crystal-field strength around Cr³⁺ will help to achieve intense deep red emission.¹³ As we know, the reported Cr³⁺-doped gallogermanates include M₃Ga₂Ge₄O₁₄:Cr³⁺ (M = Sr or Ca),¹⁴ La₃Ga₅GeO₁₄:Cr³⁺,¹³ and Zn₃Ga₂Ge₂O₁₀:Cr³⁺.⁵ In 1998, G. Adiwidjaja firstly reported the new gallogermanate of lanthanum compound, La₃GaGe₅O₁₆, as a single crystal form and revealed its chemical composition and detailed crystal structure.¹⁵ However, to the best of our knowledge, there are no further reports on this compound and their potential application as optical materials hosts except for our recent work.¹⁶ In this paper, we have found that Cr³⁺-doped lanthanide gallogermanate garnet (La₃GaGe₅O₁₆:Cr³⁺) phosphors fabricated by a solid-state reaction method can exhibit remarkable red emission peak at 700 nm. And the energy transfer mechanism, photoluminescence decay curves and the temperature dependent PL behaviour have also been investigated.

2. Experimental

2.1 Materials and synthesis

A series of rare earth-doped La₃Ga_1−xGe₅O₁₆:xCr³⁺ phosphors were synthesized by high temperature solid-state reaction. The starting materials were analytical reagent grade La₂O₃ (99.995%), Ga₂O₃ (99.9%), GeO₂ (99.9%) and Cr₂O₃ (99.9%). Firstly, the starting materials were thoroughly mixed in an agate mortar according to the given stoichiometric amounts. Then, the mixture was put into an alumina crucible and calcined in a muffle furnace at 1250 °C for 5 h in air. After the samples were cooled to room temperature, the phosphors were finally obtained.

2.2 Characterization methods

The phase structures of the as-prepared samples were carried out on a SHIMADZU model XRD-6000 diffractometer using Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 30 mA. Microstructure observation was observed using a scanning electron microscopy (SEM) (HITACHI, S-3400). Room temperature excitation and emission spectra were measured on a fluorescence spectrophotometer (F-4600, HITACHI, Japan) with a photomultiplier tube operating at 400 V, and a 150 W Xe lamp used as the excitation lamp. The temperature-dependence luminescence properties were measured on the same spectrophotometer, which was combined with a self-made heating attachment and a computer-controlled electric furnace. The decay curves were recorded on a spectrofluorometer (HORIBA, JOBIN YVON FL3-21), and the 460 nm pulse laser radiation (nano-LED) was used as the excitation source.

3. Results and discussion

3.1 Phase formation and microstructure

The diffraction pattern is usually used to identify the crystal structure and the phase purity of the sample. Fig. 1(a) shows the XRD patterns of as-prepared La₃GaGe₅O₁₆, La₃Ga_0.97Ge₅O₁₆:0.03Cr³⁺ and La₃Ga_0.85Ge₅O₁₆:0.15Cr³⁺ phosphors. From Fig. 1(a), we can know that all of the diffraction peaks are in good agreement with the reported triclinic phase of La₃GaGe₅O₁₆ (JCPDS 89-0211) and no detectable impurity phase is observed, which indicates that single-phased La₃GaGe₅O₁₆ powder can be obtained by the solid-state method. Furthermore, based on the consideration of the similar effective ionic radius and valence of cations, Cr³⁺ ions prefer to occupy the Ga³⁺ sites in the host. SEM image of the selected La₃Ga_0.97Ge₅O₁₆:0.03Cr³⁺ phosphor is presented in Fig. 1(b), from which the phosphor particles were well dispersed, and their mean size was about 3–8 μm, and the uniform distribution and small particles verified that the as-prepared phosphors can be potential in the future application.

Fig. 1 (a) XRD patterns of as-prepared La₃GaGe₅O₁₆, La₃Ga_0.97Ge₅O₁₆:0.03Cr³⁺, and La₃Ga_0.85Ge₅O₁₆:0.15Cr³⁺ phosphors, the standard data for La₃GaGe₅O₁₆ (JCPDS card no. 89-0211) is shown as a reference. (b) SEM image of the selected La₃Ga_0.97Ge₅O₁₆:0.03Cr³⁺ phosphor.

3.2 Luminescence properties and the corresponding mechanism

Cr³⁺ belong to 3d³ electronic configurations and the splitting of the energy levels in octahedral symmetry can be expressed by well-known Tanabe–Sugano energy level diagram as shown in Fig. 2(a). The d³ configuration gives rise to two quartet terms and ⁴F and ⁴P with ⁴F term lying lower to ⁴P as per Hund's rule. In addition to these quartet terms, there are several doublet terms. The ²G term splits into ²E level, and the ⁴A₂, ⁴T₂, and ⁴T₁ levels come from the 4F term.^17,18 Moreover, the luminescence and excitation spectra depend on the relative positions between ⁴T₂ and ²E levels, which is shown in Fig. 2(b) as a configurational coordinate diagram. When Cr³⁺ ions locate in the intermediate field sites, ⁴T₂ is above the ²E level, resulting in the R lines emission (assigned to ²E–⁴A₂ transition). And when Cr³⁺ ions locate in the weak field sites, ⁴T₂ is under the ²E level, which causes the band emission (assigned to ⁴T₂–⁴A₂ transition). Therefore, Fig. 3 shows the normalized excitation and emission spectra of La₃Ga_0.97Ge₅O₁₆:0.03Cr³⁺ at room temperature, which can be used to understand the corresponding transitions of Cr³⁺. Accordingly, Fig. 4 shows schematic energy levels of Cr³⁺, the corresponding excitation spectrum and electron transitions for the Cr³⁺-doped La₃GaGe₅O₁₆ phosphor is also given based on the measurement results in Fig. 3. It is found that, under excitation at 415 nm, Cr³⁺-doped La₃GaGe₅O₁₆ phosphor exhibits a broadening ²E–⁴A₂ emission peaking at 700 nm that superimposes on a broad emission band extending from 600 to 900 nm. The broadening of the ²E–⁴A₂ emission is probably caused by the electron–phonon coupling because the phonon energy of the Cr³⁺ dopant matches well with that of the gallate-containing host,¹⁹ whereas the broadband emission may be ascribed to the ⁴T₂–⁴A₂ transitions from some disordered Cr³⁺ ions in the gallogermanate system.²⁰ The excitation spectrum monitored at 700 nm covers a very broad spectral region (from 200 to 650 nm) consists of three main excitation bands originating from the inner transitions of Cr³⁺, including the 270 nm band originating from the ⁴A₂–⁴T₁ (⁴P) transition, the 415 nm band originating from the ⁴A₂–⁴T₁ (⁴F) transition and 573 nm band originating from the ⁴A₂–⁴T₂ (⁴F) transition.²¹ Based on the above PLE spectrum for La₃Ga_0.97Ge₅O₁₆:0.03Cr³⁺, it proves that the obtained sample has excellent n-UV and visible excitation properties, which in turn means that this kind of phosphor has the potential as a promising NIR-emitting phosphor system for LSCs with broad-band absorption.

Fig. 2 (a) Tanabe–Sugano diagram for Cr³⁺ in La₃Ga_0.97Ge₅O₁₆. (b) Mechanistic configurational coordinate diagram illustrating different emission channels. The solid and dashed arrows represent the emissions from the Cr³⁺ ions located in the intermediate and weak field sites, respectively.

Fig. 3 Normalized excitation and emission spectra of La₃Ga_0.97Ge₅O₁₆:0.03Cr³⁺ phosphor at room temperature.

Fig. 4 Schematic energy levels of Cr³⁺, the corresponding excitation spectrum and electron transitions for the Cr³⁺-doped La₃GaGe₅O₁₆ phosphors.

Fig. 5 presents the emission (λ_ex = 415 nm) spectra of the La₃Ga_1−xGe₅O₁₆:xCr³⁺ phosphors with different concentrations of Cr³⁺ (ranging from 0.01 to 0.15) at room temperature. As is shown in Fig. 5, this series of samples all produce a strong and broad-band NIR emission with a peak at around 700 nm, and the emission spectra have no obvious changes in the spectral configuration except for the emission intensity. Furthermore, the inset shows the Cr³⁺ content dependent PL intensities, and Cr³⁺ concentration (x) in La₃Ga_1−xGe₅O₁₆:xCr³⁺ reaches a saturation point at x = 0.03. After that, the PL intensity begins to decrease with increasing Cr³⁺ concentration due to the concentration quenching effect.

Fig. 5 PL spectra for the La₃Ga_1−xGe₅O₁₆:xCr³⁺ (x = 0.01, 0.03, 0.05, 0.08, 0.10 and 0.15) phosphors, and the inset shows Cr³⁺ concentration dependence of PL intensity.

It is accepted that energy quenching can be ascribed on the energy transfer between Cr³⁺ ions followed by energy transfer to traps or quenching sites. Therefore, in order to further confirm the process of energy transfer between Cr³⁺ ions in the La₃GaGe₅O₁₆:Cr³⁺ phosphor, the interaction type between sensitizers or between the sensitizer and activator can be calculated by the following equation:²²

in this equation x is the activator concentration which is not less than the critical concentration, I/x is the emission intensity (I) per activator concentration (x), K and β are constants for the same excitation condition for a given host crystal, and θ is an indication of electric multipolar character. It is previously reported that θ = 3 for the energy transfer among the nearest-neighbor ions, as θ = 6, 8 and 10 corresponds to dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions, respectively. To get a correct θ value, the dependence of lg(I/x) on lg(x) is plotted, and it yield a straight line with a slope equal to −θ/3. As shown in Fig. 6, the slope is −1.93. The value of θ can be calculated as 5.79, which is close to 6 that means the quenching is dipole–dipole interactions in La₃GaGe₅O₁₆:Cr³⁺ phosphors.

Fig. 6 The fitting curve of log(I/x) vs. log(x) in La₃Ga_1−xGe₅O₁₆:xCr³⁺ phosphors.

The critical distance of energy transfer R_c was calculated by using the concentration quenching method. R_c can be calculated using the relation given by Blasse:²³

here N is the number of molecules in the unit cell, V is the unit cell volume and x_c is the critical concentration. For La₃GaGe₅O₁₆ host, the crystallographic parameters are N = 2, V = 611.81 ų and x_c is 0.03. According to the above equation, the critical distance of energy transfer is estimated to be about 26.90 Å.

In order to further understand the concentration quenching in detail, the decay curves for the La₃Ga_1−xGe₅O₁₆:xCr³⁺ (x = 0.01, 0.03, 0.05, 0.08 and 0.15) phosphors excited at 460 nm and monitored at 700 nm are measured and depicted in Fig. 7. As shown in Fig. 7, it is found that the decay curves are well fitted with a second order exponential equation:²⁴

I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (3)

where, I is the luminescence intensity, A₁ and A₂ are constants, t is the time, τ₁ and τ₂ are rapid and slow lifetime values for exponential components, respectively. Based on the eqn (3), and we can obtain the A₁, A₂, τ₁ and τ₂ values based on the fitting of the decay curves. Therefore, the effective lifetime constant (τ*) can be calculated as:²⁵

τ* = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (4)

Fig. 7 Room temperature decay curves of La₃Ga_1−xGe₅O₁₆:xCr³⁺ phosphors with different Cr³⁺ contents x = 0.01, 0.03, 0.05, 0.08 and 0.15.

The effective decay time (τ*) were calculated to be 75.19, 69.66, 60.18, 47.65 and 40.29 μs for La₃Ga_1−xGe₅O₁₆:xCr³⁺ with x = 0.01, 0.03, 0.05, 0.08 and 0.15, respectively. The obtained result shows that the measured lifetime τ of Cr³⁺ emission decreases faster with higher doping Cr³⁺ concentration. The measured lifetime is also related to the total relaxation rate by:^26,27

where τ₀ is the radiative lifetime, A_nr is the nonradiative rate due to multiphonon relaxation, P_t is the energy transfer rate between Cr³⁺ ions, and τ is equivalent to τ*. With increasing Cr³⁺ concentration, the distance between Cr³⁺ ions decreases. Therefore, both the energy transfer rate between Cr³⁺–Cr³⁺ and the probability of energy transfer to luminescent killer sites increased. Consequently, the lifetimes are shortened as increasing Cr³⁺ concentration.²⁸

3.3 Thermal stability behavior

The thermal stability of phosphor is one of important parameters for the possible application. Fig. 8 shows the dependence of PL spectra of La₃Ga_0.99Ge₅O₁₆:0.01Cr³⁺, La₃Ga_0.97Ge₅O₁₆:0.03Cr³⁺ and La₃Ga_0.85Ge₅O₁₆:0.15Cr³⁺ on temperature excited by 415 nm light. The relative emission intensities of all phosphors decrease with increasing temperature in the range of 25 °C to 300 °C. We observed a decay of 12.9% for La₃Ga_0.99Ge₅O₁₆:0.01Cr³⁺, 20.5% for La₃Ga_0.97Ge₅O₁₆:0.03Cr³⁺, and 32.6% for La₃Ga_0.85Ge₅O₁₆:0.15Cr³⁺ at 100 °C. The thermal quenching of emission intensity can be explained by several mechanisms. A widely accepted mechanism is the electronic transition through the intersection between the ground and excited states. In other words this mechanism is described as a large displacement between the ground and excited state in the configuration coordinate diagram.²⁹ Moreover, the full-width at half-maximum (FWHM) of the PL spectra of all phosphors increases with increasing temperature, which can be described by using the Boltzmann distribution according to the following equation:³⁰where W₀ is the FWHM at 0 K, hw is the energy of the lattice vibration that interacts with the electronic transitions, S is the Huang–Rhys–Pekar parameter, and k is the Boltzmann constant. With the increasing of temperature, the bond length between luminescence center and its coordinated ions increase which cripple the intensity of crystal field and the interaction between electron and phonon play as the leading role, consequently the FWHM of the emission line is broadened.

Fig. 8 Temperature-dependent PL spectra of typical La₃Ga_0.97Ge₅O₁₆:0.03Cr³⁺. The inset shows the variation of the temperature dependent relative emission intensities for La₃Ga_1−xGe₅O₁₆:xCr³⁺ (x = 0.01, 0.03 and 0.15).

To better understand the temperature dependence of photoluminescence, the activation energy was calculated using the Arrhenius equation:^31,32

where I₀ is the initial PL intensity of the phosphor at room temperature, I_T is the PL intensity at different temperatures, c is a constant, ΔE is the activation energy for thermal quenching, and k is Boltzmann constant (8.62 × 10⁻⁵ eV). According to the equation, the plot of ln[(I₀/I_T) − 1] vs. 1/kT yields a straight line, and the activation energy ΔE is obtained from the slop of the plot. As shown in Fig. 9, ΔE is calculated to be 0.309, 0.236 and 0.234 eV for the La₃Ga_0.99Ge₅O₁₆:0.01Cr³⁺, La₃Ga_0.97Ge₅O₁₆:0.03Cr³⁺ and La₃Ga_0.85Ge₅O₁₆:0.15Cr³⁺ phosphors, respectively. From the results, we can find that the samples with low Cr³⁺ concentration has larger ΔE values corresponding to better thermal stability properties, which means that the sample with high Cr³⁺ concentration possible possess more de-activation channels originating from the enhanced interaction among the increasing Cr³⁺ ions.

Fig. 9 A ln[(I₀/I_T) − 1] vs. 1/K_BT fitting plot and the corresponding activation energy derived from the thermal quenching data of La₃Ga_1−xGe₅O₁₆:xCr³⁺ (x = 0.01, 0.03 and 0.15).

4. Conclusion

In summary, a novel near-infrared (NIR) phosphor, La₃GaGe₅O₁₆:Cr³⁺ was synthesized via the solid-state reaction. The La₃GaGe₅O₁₆:Cr³⁺ phosphor exhibited NIR emission peaked at 700 nm under the excitation of 415 nm UV radiation. The optimal doping concentration of Cr³⁺ is 3 mol%, and the concentration quenching mechanism is determined to be dipole–dipole interaction. Moreover, the temperature dependence of luminescence shows that the thermal quenching behavior is related to the Cr³⁺ doping concentration and the emission intensity of Cr³⁺ decrease with the increase of temperature and Cr³⁺ will decay faster with higher doping concentration. Preliminary studies showed that La₃GaGe₅O₁₆:Cr³⁺ has potential practical applications in luminescent solar concentrator with broad-band absorption and emission.

Acknowledgements

The present work was supported by the National Natural Science Foundations of China (Grant no. 51002146, 51272242), Natural Science Foundations of Beijing (2132050), the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0950), Beijing Nova Program (Z131103000413047), Beijing Youth Excellent Talent Program (YETP0635) and the Funds of the State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University (KF201306).

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