Strategies to approach high performance in Cr 3 + -doped phosphors for high-power NIR-LED light sources

Broadband near-infrared (NIR)-emitting phosphors are key for next-generation smart NIR light sources based on blue LEDs. To achieve excellent NIR phosphors, we propose a strategy of enhancing the crystallinity, modifying the micromorphology, and maintaining the valence state of Cr 3 + in Ca 3 Sc 2 Si 3 O 12 garnet (CSSG). By adding ﬂ uxes and sintering in a reducing atmosphere, the internal quantum ef ﬁ ciency (IQE) is greatly enhanced to 92.3%. The optimized CSSG:6%Cr 3 + exhibits excellent thermal stability. At 150 °C, 97.4% of the NIR emission at room temperature can be maintained. The fabricated NIR-LED device emits a high optical power of 109.9 mW at 520 mA. The performances of both the achieved phosphor and the NIR-LED are almost the best results until now. The mechanism for the optimization is investigated. An application of the NIR-LED light source is demonstrated.


Introduction
NIR spectroscopy has good penetration for organic matter, and it has drawn attention for application in monitoring foods and medicines, bioimaging, and night vision [1][2][3][4][5][6] . Smart NIR light sources, an emerging field, are proposed to be combined with smart phones to achieve convenient and fast applications [7][8][9] . In contrast to traditional tungsten filament lamps and halogen lamps, only light-emitting diodes (LEDs) that have a solid state and a small size are suitable for smart NIR devices. However, NIR-LED chips can only give narrow NIR emissions, which limits their applications [10][11][12] . Broad NIR-emitting phosphor-converted (pc) LEDs, adopting the technology of pc-white LEDs [13][14][15][16][17][18][19] , are believed to be the best solution. White LEDs are commonly based on blue-LED chips. Thus, how to achieve broad NIR phosphors that can be efficiently excited by blue light is one of the most important challenges 20 .
Recently, a number of NIR phosphors were realized  . Among them, Cr 3+ usually manifests a high efficiency, and the IQE can reach 58-75% [31][32][33][34][35][36][37] . The radiant power is 14.7-5 4 . 2 9m Ww h e nd r i v e na t1 0 0 -130 mA [34][35][36][37][38][39][40] .Ah i g h radiant power is beneficial for monitoring and detection. Liu et al made great progress in improving the radiant power, which was enhanced from 7-18.2 mW 42,43 to 65.2 mW 44 when driven at 350 mA. To achieve high radiance, a highpower LED chip operating at a high current is usually needed. In this situation, the large amount of heat will result in a high temperature on the surface of the chip. Thus, the greatest challenge is how to make NIR phosphors have excellent thermal stability to overcome the thermal quenching effect, in addition to a high QE for NIR phosphors.
We note that Cr 3+ has a high QE in garnets 34,35 .T h e silicate garnet Ca 3 Sc 2 Si 3 O 12 (CSSG) is a promising host for Ce 3+ due to its excellent thermal stability and high QE [45][46][47] . Fortunately, Cr 3+ can be excited by blue light and show broadband NIR emission in CSSG 39 . Unfortunately, the reported luminescence (IQE: 12.8%) and thermal stability were low because the Cr 3+ luminescence suffered from impurities and oxidation of Cr 4+ when the material was sintered in air 39 . Based on our previous experience, we prop o s eas t r a t e g yt oo p t i m i z eC S S G : C r 3+ by enhancing the crystallinity, modifying the micromorphology, and maintaining the valence state of Cr 3+ . By adding fluxes and sintering in a CO reducing atmosphere, the IQE is greatly enhanced to 92.3%. At 150°C, 97.4% of the NIR emission at room temperature can be maintained, indicating excellent thermal stability. When combined with a high-power 460 nm blue chip, the estimated radiant power of the fabricated pc-LED even reaches 109.9 mW when driven at 520 mA. The properties of both the optimized CSSG:Cr 3+ and the achieved NIR-LED are almost the best results to date as far as we know. Benefiting from the high radiant power, the pc-NIR-LED device has good application potential in nightvision technology.
In this work, mechanisms for optimization were investigated. The electron-phonon coupling (EPC) mechanism in CSSG that usually determines the Cr 3+ luminescence is revealed for the first time. Many types of Cr 3+ -doped NIR phosphors have been discovered. We believe that this work provides an effective strategy to optimize and discover more Cr 3+ -doped NIR phosphors using only a simple method. Moreover, this work will advance the development and application of next-generation smart NIR light sources.
CSSG:Cr 3+ synthesized in an air atmosphere exhibits a weak NIR emission (Figs. S2-5), which could be attributed to a lower crystallinity and oxidation of Cr 3+ 39 . For the optimized Cr 3+ concentration, that is, CSSG:3%Cr 3+ , its IQE and external quantum efficiency (EQE) are quite ab e d c Fig. 1 Optimization of the NIR emission. a Coordination of (Sc/Cr)O 6 in CSSG. b Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of CSSG:3%Cr 3+ . c Relative PL intensities of the samples with and without fluxes sintered in air and a CO atmosphere. d PL intensities of CSSG:3%Cr 3+ , xwt% Li 2 CO 3 (x = 0.5, 0.8, 1, 1.5, 2, 3, 4, 5, 6). e PL intensities of CSSG:y%Cr 3+ , 1 wt% Li 2 CO 3 (y = 1, 2, 3, 4, 5, 6). low, only~12.8% and 4.8%, respectively (Fig. S6). To enhance the luminescence by improving the crystallinity, fluxes of NH 4 F, CaF 2 ,H 3 BO 3 , LiF, and Li 2 CO 3 were added during the synthesis under the air condition. The NIR emission of the flux-free sample was set as the normalized standard. As Fig. 1c shows, H 3 BO 3 , LiF, and Li 2 CO 3 enhance the luminescence, while NH 4 F and CaF 2 decrease the luminescence. Thus, H 3 BO 3 , LiF, and Li 2 CO 3 were selected, and CSSG:3%Cr 3+ was sintered in a CO reducing atmosphere to further maintain the state of Cr 3+ .Itis noted that the luminescence is greatly enhanced by 2~3 times. Li 2 CO 3 has the best effect, and the optimal amount is 1 wt% (Fig. 1d). Correspondingly, the IQE and EQE of CSSG:3%Cr 3+ reach~77.8% and 15.5%, respectively. Moreover, by adding 1 wt% Li 2 CO 3 and sintering in CO, the Cr 3+ concentration is optimized to 6% (Fig. 1e). Then, the achieved EQE of CSSG:6%Cr 3+ even reaches 21.5%. Here, the QE is the measured result. Due to the limitation of the measurement, up to 850 nm, the results are smaller than the actual values, which will be illustrated later. Therefore, the actual QE is almost the best value among NIR phosphors as far as we know.

Crystal structures and micromorphology
To clarify the enhancement of the optimized CSSG:Cr 3+ compared with the initial phosphor, the crystal structures and micromorphology were studied. For CSSG:3%Cr 3+ sintered in air (Fig. 2a), the X-ray diffraction (XRD) pattern mainly shows a garnet phase of CSSG (PDF # 72-1969), but a few impurity phases of SiO 2 and Sc 2 O 3 are also observed. These impurity phases can also be observed in the SEM-EDS mapping images. Ca, Sc, Si, O, and Cr are inhomogeneously distributed in the square region, in which Sc is rich and Si and Ca are absent.
For CSSG:3%Cr 3+ sintered in CO with 1 wt% Li 2 CO 3 (Fig. 2b), the SiO 2 and Sc 2 O 3 impurities are quite low in the XRD pattern. The distributions of elements are more homogeneous. In addition, the diffraction peak intensity of the CSSG phase increases (Fig. 2c). These results demonstrate that the enhanced crystallinity of the CSSG phase is one of the reasons for the improvement of the Cr 3+ NIR emissions.
The CSSG:3%Cr 3+ phosphor has a particle size of~10 μm (Fig. 2d). The particle displays a small variation in the cathodoluminescence (CL), as the CL image shows. The bright region (point I) and the dark region (point II) are only different in CL intensity. Their normalized CL spectra are almost the same, having an emission peak at~760 nm and an FWHM of 1583-1590 cm −1 (93-94 nm), similar to the photoluminescence (PL) spectrum in Figs. S2-S5.

Diagnosis of the valence state of Cr 3+
To diagnose the change in the ionic valence of Cr, X-ray photoelectron spectroscopy (XPS), diffuse reflection (DR), and electron paramagnetic resonance (EPR) results for CSSG:3%Cr 3+ sintered in air and CO are given in Fig. 3. The binding energies at 99, 344, 399, and 528 eV are from Si-2p, Ca-2p, Sc-2p, and O-1s, respectively. The binding energy at 40 eV for Cr-3p is detected in the two samples 48 .
The changes in Cr are not apparent in the XPS spectra. However, in the DR spectra (Fig. 3c), the absorption band of Cr 4+ (at~1140 nm) is clearly identified for the sample sintered in air in addition to the absorption band of Cr 3+ (at~460 and 640 nm) 34,35 . This means that some Cr 3+ ions are oxidized into Cr 4+ even though Cr 2 O 3 is used as the raw material. For the sample sintered in CO, the Cr 4+ absorption band almost disappears, and only the Cr 3+ absorption band is observed. This means that Cr 3+ is well maintained in the CO reducing atmosphere.
In the 3d 3 electronic configuration of Cr 3+ , three electrons occupy the d orbitals and give rise to a total spin of S = 3/2. 4 A 2g (F) is the ground state for Cr 3+ . In an octahedral crystal field, the 4 F state splits into a singlet orbital 4 A 2g and two orbitals 4 T 1g and 4 T 2g , thus causing EPR signals 49 . As Fig. 3d shows, the sharp peaks at g = 3.8-3.9 belong to the isolated Cr 3+ ion, and the peaks at approximately g = 2 represent the first neighbor Cr 3+ -Cr 3+ pair [42][43][44]49 . The EPR intensity for the sample sintered in CO is stronger than that for the sample sintered in air, demonstrating an increased ratio of Cr 3+ . Combining the DR and EPR results, it is claimed that Cr 3+ can be maintained and increased in the reducing atmosphere, which is another reason for the improvement of the Cr 3+ NIR luminescence.

Temperature-dependent NIR emissions
Figure 4a-c shows the temperature-dependent luminescence of CSSG:6%Cr 3+ . The integrated PL intensity at 25°C is set as the normalized standard, and 97.4% can still be maintained at 150°C for the phosphor sintered in CO, whereas 85.6% can be maintained for the phosphor sintered in air. The temperature dependence of the emission intensity can be well fitted by the Arrhenius formula, and the activation energy is calculated to be ΔE = 0.336 eV for the optimized sample, compared with ΔE = 0.220 eV for the initial sample (Fig. 4b). When the temperature increases from 25 to 300°C, the peak position redshifts from~783 to~807 nm (Fig. 4d), attributed to the decreased CF caused by lattice expansion. The FWHM increases from 1483 to 1551 cm −1 (92.3 to 100.2 nm), attributed to the strengthened EPC effect that will be discussed in the following. Figure 4e shows the PLE and PL spectra of CSSG:6%Cr 3+ at 77 K with a step size of 0.05 nm. The transitions from 4 A 2g to 4 T 2g ( 4 F) and 4 T 1g ( 4 F) are centered at 15,670 cm −1 (~636 nm) and 22,050 cm −1 (~453 nm), respectively. The R-line is only detected in the PLE spectrum at 698.3 nm (~14,320 cm −1 ). The peak at ~713 nm (~14,030 cm −1 ) is observed in both the PL and PLE spectra, which is assigned to the zero-phonon line (ZPL) for the 4 T 2 ( 4 F) and 4 A 2g transition (Fig. S7). The energy gap between 2 E g ( 2 G) and the 4 T 2g ( 4 F) ZPL was evaluated to be~290 cm −1 , indicating a strong spin-orbit coupling (SOC) of the 2 E g ( 2 G) and 4 T 2g ( 4 F) states 50 . Thus, the fluorescent decay curve shows a biexponential model with a lifetime of~190 µs (Fig. S8-S9).
The PL spectrum has a maximum peak at~12,980 cm −1 (~770 nm) with a FWHM of 1560 cm −1 (~93 nm). The corresponding Stokes shift is~2700 cm −1 . The peak at 730 nm (~13,700 cm −1 ) is the phonon satellite of the ZPL. The energy difference between the ZPL and its phonon satellite is determined to be~330 cm −1 , which corresponds to one of the vibrational modes (ħw) that couple with the 4 T 2g ( 4 F) → 4 A 2g transition (Fig. S7).
A Stokes shift has a relationship of (2S + 1)ħw,where S is the Huang-Rhys parameter; thus, S is determined to be between 3 and 4, which is smaller than the value (~6) for LSGG:Cr 3+ 31 . Both the broadband emissive characteristic and the large Stokes shift indicate a stronger EPC for the 4 T 2g ( 4 F) → 4 A 2g transition in CSSG:Cr 3+ . A stronger EPC leads to a larger S value [28][29][30][31] . At a low temperature (77 K), the EPC effect is weak. Thus, the FWHM value at 77 K decreases bỹ 190 cm −1 compared with the FWHM of 1750 cm −1 (~110 nm) at RT.
It is worth noting that the ratio in the range of 650-725 nm increases and the emission shows a blueshift with increasing temperature. The small energy gap (2 90 cm −1 ) between 2 E g and the 4 T 2g ZPL leads to mixing of the 2 E g ( 2 G) and 4 T 2g ( 4 F) states. When the temperature increases, electronic transfer from the 4 T 2g state to the 2 E g state is strengthened with the assistance of EPC. Thus, the radiative transitions from the 2 E g ( 2 G) state increase, and blueshifts are observed. ab c d A Fig. 2 Crystal structures and micromorphology. a, b XRD patterns, SEM images, and EDS mappings for CSSG:3%Cr 3+ sintered in air and sintered in CO with 1 wt% Li 2 CO 3 . c XRD peak intensities. d CL image and normalized CL spectra at points I and II for CSSG:3%Cr 3+ sintered in CO. The insert shows the CL intensities at points I and II.

Performance of the fabricated NIR-LED device
The CSSG:6%Cr 3+ phosphor shows a green color (Fig. 5a). Based on the optimized phosphor and a high-power blue chip, an NIR-LED device was fabricated and is displayed in Fig. 5b-d. The electroluminescence (EL) spectra, optical powers, and conversion efficiencies of the device depending on the driving current (I)a r eg i v e ni nF i g .5e-h. The strong emission peak at~460 nm comes from the blue chip. The broad NIR emission band comes from CSSG:Cr 3+ .T h e optical powers of both the total radiance and NIR light increase with increasing current until reaching maxima of 97.8 and 64.7 mW, respectively, at 520 mA.
The conversion efficiency from the emitted blue light to NIR light (η NIR/blue light ) drops from 33.5 to 12.3% when the driving current increases from 100 to 600 mA. Correspondingly, the conversion efficiency from the input electronic power to NIR emissions (η NIR/input ) decreases from 7.2 to 2.3%. η NIR/input is much lower than η NIR/blue light .T h el o w e r photoelectric conversion efficiency (η blue light/input )f r o mt h e input electronic power to blue light should be responsible for this phenomenon because the η blue light/input of the used blue chip, ranging from 31.9 to 19.4% at 100-600 mA, is not very high (Fig. S9). If the used blue chip is efficient, then η NIR/input can be greatly enhanced further.
On the other hand, only 84.3% of the NIR emission can be detected due to the limitation of the measurement range, up to 850 nm (Fig. 5h). If the 15.7% unmeasured part is taken into account, then the actual NIR optical power should be 76.8 mW at 520 mA. Thus, the total optical power even reaches 109.9 mW, which is much higher than the performances reported until now 44 .
The QEs discussed above are the measured results. The QE and radiant power were measured by using the same spectrometer. If the unmeasured part is also taken into account, then the IQE and EQE can actually reach 92.3% and 25.5%, respectively, which are almost the best results as far as we know up to now. Figure 6 shows the application of the NIR-LED for night vision. Visible images taken by a visible camera are colorful when water, milk, and cups are illuminated by either fluorescent light or NIR light. The logo is clear under fluorescent light. However, only black-and-white images are captured by an NIR camera. When the NIR-LED is off, nothing can be captured. When the NIR-LED is on, the logo is much clearer when it is taken by the NIR camera than when it is taken by the visible camera, especially the ab cd logo on the surface of the glass filled with transparent water. These results indicate that the achieved CSSG:Cr 3+ phosphor enables the NIR-LED to have good application in night-vision technology. Potential applications in monitoring foods and medicines are also expected for such NIR phosphors and NIR-LED light sources.

Discussion
In conclusion, CSSG:Cr 3+ exhibits broad NIR emission from 700 to 900 nm under blue light excitation. The previously reported CSSG:Cr 3+ synthesized in air has low IQE (12.8%) and EQE (4.8%), limiting its performance in pc-NIR-LED devices. By adding fluxes and synthesizing in ac d b e Fig. 4   high-power blue chip, the fabricated NIR-LED showed a high optical power of nearly 110 mW at 520 mA, which is almost the best performance among NIR-LED light sources. Among the reported NIR phosphors, Cr 3+ is an important activator, and its luminescence is determined by both the selected host and the synthesis technology. We believe that this work provides an effective strategy to optimize NIR phosphors using only a simple but the best method. Thus, it will inspire more researchers to achieve much better performance of known NIR phosphors and advance development of next-generation smart NIR-LED light sources.

Synthesis
Samples with the nominal composition of Ca 3 Sc 2- x Si 3 O 12 :yCr 3+ , xwt% flux were synthesized by a hightemperature solid-state reaction. The starting materials of CaCO 3 (99.9%), Sc 2 O 3 (99.9%), SiO 2 (AR), and Cr 2 O 3 (99.95%) and fluxes of NH 4 F, CaF 2 ,H 3 BO 3 , LiF, and Li 2 CO 3 were weighed according to the nominal composition and then ground in an agate mortar for 30 min. After that, the powders were sintered at 1450°C for 3 h in air and a CO atmosphere.

Fabrication of pc-NIR-LEDs
NIR-LEDs were fabricated using the optimized NIR phosphor CSSG:6%Cr 3+ and high-power blue-LED chips (460 nm). The phosphors were thoroughly mixed with epoxy resin and then coated on the chips.
Characterization XRD patterns were measured by a Bruker D8 X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å) at 40 kV and 40 mA. DR spectra were measured by a LAMBDA 950. PL and PLE spectra at RT-300°C were characterized by a Hitachi F-4600. PL and PLE spectra at 77 K were measured by a Horiba FL-311 by dipping the sample in liquid nitrogen. EPR data were recorded on a Bruker E500 with the X-band frequencies (≈9.845 GHz) and a microwave power of 0.63 mW. XPS was performed on a Kratos Axis Ultra DLD. IQEs and EQEs were recorded by an Otsuka Photal Electronics QE-2100. A field-emission scanning electron microscope (FE-SEM, Hitachi S-4800) equipped with an energy dispersive X-ray spectroscopy (EDS) system and a CL system (MonoCL4, Gatan) was used to measure the morphology. EL spectra and performances of fabricated pc-NIR-LED devices were measured by an integrating sphere (Labsphere), and data were collected by a multichannel photodetector (MCPD-9800, Otsuka Photal Electronics). Visible images and NIR images were taken by a visible camera (SONY ILCE-7M2K) and an NIR camera (Work Power UC-500M), respectively.