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Advancing Near-Infrared Light Sources: Enhancing Chromium Emission through Cation Substitution in Ultra-Broadband Near-Infrared Phosphors
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Cite this: Chem. Mater. 2023, 35, 23, 10228–10237
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https://doi.org/10.1021/acs.chemmater.3c02466
Published November 17, 2023

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Abstract

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The growing interest in the use of near-infrared (NIR) radiation for spectroscopy, optical communication, and medical applications spanning both NIR-I (700–900 nm) and NIR-II (900–1700 nm) has driven the need for new NIR light sources. NIR phosphor-converted light-emitting diodes (pc-LEDs) are expected to replace traditional lamps mainly due to their high efficiency and compact design. Broadband NIR phosphors activated by Cr3+ and Cr4+ have attracted significant research interest, offering emission across a wide range from 700 to 1700 nm. In this work, we synthesized a series of Sc2(1–x)Ga2xO3:Cr3+/4+ materials (x = 0–0.2) with broadband NIR-I (Cr3+) and NIR-II (Cr4+) emission. We observed a substantial increase in the intensity of Cr3+ (approximately 77 times) by incorporating Ga3+ ions. Additionally, our investigation revealed that energy transfer occurred between Cr3+ and Cr4+ ions. Configuration diagrams are presented to elucidate the behavior of Cr3+ and Cr4+ ions within the Sc2O3 matrix. We also observed a phase transition at a pressure of 20.2 GPa, resulting in a new unknown phase where Cr3+ luminescence exhibited a high-symmetry environment. Notably, this study presents the pressure-induced shift of NIR Cr4+ luminescence in Sc2(1–x)Ga2xO3:Cr3+/4+. The linear shifts were estimated at 83 ± 3 and 61 ± 6 cm–1/GPa before and after the phase transition. Overall, our findings shed light on the synthesis, luminescent properties, temperature, and high-pressure behavior within the Sc2(1–x)Ga2xO3:Cr3+/4+ materials. This research contributes to the understanding and potential applications of these materials in the development of efficient NIR light sources and other optical devices.

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Introduction

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The growing interest in the use of near-infrared (NIR) radiation, spanning both NIR-I (700–900 nm) and NIR-II (900–1700 nm), stems from its vast and remarkable applications in fields such as food science, security, and biomedicine. Consequently, there is an urgent need to address the development of a new generation of NIR light sources. (1−4) As a new generation of NIR light sources, NIR phosphor-converted light-emitting diodes (pc-LEDs) are anticipated to replace traditional tungsten halogen lamps due to their low cost, high efficiency, and compact design. (5) These pc-LEDs utilize commercial high-power blue LED chips in combination with inorganic phosphors to achieve the desired near-infrared wavelength. Phosphors activated by transition metals (such as Cr3+, Fe3+, Mn2+, and Ni2+) (6−13) and lanthanide ions (including Yb3+, Er3+, and Eu2+) (14−20) are considered up-and-coming candidates for the new generation of NIR pc-mini/micro-LEDs. Additionally, Bi3+-activated materials (21) and zero-dimensional hybrid antimony chlorides (22) stand out as intriguing alternatives for NIR applications. These NIR light sources can be easily integrated into smartphones or wearable devices, enabling a wide range of functional applications.
In recent years, Cr3+-activated broadband NIR phosphors have garnered significant research interest owing to their exceptional characteristics, including high efficiency, high thermal stability, and tunable spectra. (23−28) These phosphors offer a broad range of emission wavelengths in the near-infrared region. Additionally, Cr4+ has emerged as a promising luminescent center with broadband emission spanning from 1100 to 1700 nm. There are indeed numerous examples of broadband NIR Cr4+-activated phosphors, some of which include Li2ZnGeO4:Cr4+, (29) Li2CaGeO4;Cr4+, (30) Y3Al5O12:Cr4+, (31) and Y2SiO5:Cr4+, (32,33) as well as the coexistence of Cr3+ and Cr4+ in MgSiO4, (34,35) and CaGa4O. (36,37) Recently, Wang et al. (18) demonstrated the potential application of Sc2O3:Cr3+,Cr4+ phosphor as temperature sensors. By codoping Yb3+ ions, they achieved a continuous NIR ultra-broadband emission spectrum from 650 to 1600 nm. This development opens up possibilities for utilizing these phosphors in spectral analysis applications.
Scandium oxide (Sc2O3) holds promising potential as a host material for the incorporation of luminescent dopants. Sc2O3 is regarded as a rare-earth sesquioxide because of similar chemical behavior. (38) Until now, researchers have identified five distinct structural modifications in rare-earth sesquioxides. (39−41) Three phases are designated as follows: hexagonal phase with space group P3m1 and seven-coordinated cations (A phase); monoclinic phase with space group C2/m, where each cation is surrounded by six or seven anions (B phase); and cubic phase with space group Ia3̅ with six-coordinated cations (C phase). These phases are observed at room temperatures and atmospheric pressures. The two other phases are formed at very high temperatures: H phase (hexagonal, P63/mmc) and X phase (cubic, Im3̅m). (42) Yusa et al. noted that under high pressures and temperatures the B phase of Sc2O3 undergoes the phase transition to the Gd2S3 structure. (43) The irreversible phase transition from cubic space group Ia3̅ (referred to as the C phase) to monoclinic space group C2/m (B phase) is calculated to occur at 15 GPa. The same research group has experimentally shown it to occur at 32 GPa. (44) Other experimental studies show that phase transitions occur at 25–28 GPa. (45) Yusa et al. (43) found that the corundum phase, not previously seen in Sc2O3, can only be synthesized as a recovered product from the Gd2S3 phase.
Introducing Cr3+ ions into the Sc2O3 matrix leads to the generation of a broad emission band within the NIR-I region. Furthermore, the presence of Cr4+ ions in the matrix also gives rise to emission within the NIR-II region. Several studies have investigated the luminescence properties of pure Sc2O3:Cr3+. (46−48) However, there is a lack of research specifically focusing on mixed-ion materials. The modification of the matrix through cation substitution enables the tuning of emission characteristics to meet the specific application requirements.
In the previous study, we investigated the new Ga2(1-y)Sc2yO3 materials synthesized in the monoclinic crystal structure with the space group C2/m. (49) We showed that y could reach around 0.44 and Sc could not be doped more in the structure. In this study, we demonstrate that even though Sc cannot be doped in the Ga2O3 structure for more than 44%, it is still possible to incorporate Ga into the Sc2O3 structure.
Understanding the luminescence mechanisms of Sc2O3 codoped with Cr3+ is essential for further optimizing its luminescence properties for commercial applications. This study focuses on investigating the luminescence properties of the Sc2(1–x)Ga2xO3:Cr3+/4+ solid solution. We will examine the thermal analysis of this material and explore the influence of the crystal field effects on the optical properties of Cr3+ and Cr4+ ions.

Results and Discussion

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Structural Analyses

The synchrotron X-ray powder diffraction of Sc2(1–x)Ga2xO3:Cr3+/4+ (SGOC) with x = 0–0.20 is characterized, as shown in Figure 1a. No impurity peaks are found for x = 0–0.05, and only slight impurity peaks are detected for x = 0.10. By contrast, more impurities could be seen for x = 0.15–0.20, as shown by the asterisk symbol in Figure 1a. The impurity can be attributed to Ga1.17Sc0.83O3 because the position of the impurity peak aligns with the standard peak position of the Ga1.17Sc0.83O3 phase attracted from ICSD-422271. For the x = 0–0.10 samples, the diffraction peaks shift toward higher angles because the ionic radius of Sc3+ (0.745 Å; CN = 6) (CN denotes coordinated number) is bigger than that of Ga3+ (0.62 Å; CN = 6). (50) Notably, the diffraction peak position only shifts to higher angles for x = 0–0.10 samples, and it does not shift for x = 0.10–0.20, indicating that Ga3+ ions cannot be incorporated into the Sc2O3 structure anymore. The structural information and standard XRD diffraction patterns of Sc2O3 and Ga1.17Sc0.83O3 are extracted from the crystallographic information framework (CIF) with COD-1008928 and ICSD-422271, respectively. Ga1.17Sc0.83O3 possesses the same structure as Ga2O3, as shown in Figure S1 in the Supporting Information (SI).

Figure 1

Figure 1. (a) XRD patterns of the SGOC with x = 0–0.2. (b) Crystal structure of Sc2O3. (c) Coordinated environment of Sc1 and Sc2 sites in Sc2O3. (d) Rietveld refinement of the SGOC with x = 0. (e) Lattice parameters of SGOC with x = 0–0.2. The asterisk symbol in (a) indicates the diffraction peaks from the Ga1.17Sc0.83O3 impurity phase.

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The crystal structure of Sc2O3 is shown in Figure 1b. Sc2O3 crystallizes in a cubic structure with the space group of Ia3̅ (no. 206). There are two Sc3+ sites in the structure, and both are coordinated by six O2– ions to form 6-fold polyhedrons, as shown in Figure 1c. The detailed coordination environment of Sc1 and Sc2 in [ScO6] polyhedrons is shown in Table S1 in SI. The bond lengths of the six Sc1–O1 bonds are identical and equal to 2.10314 Å. On the other hand, the six Sc2–O1 bonds are different, with an average bond length of 2.0978 Å. The distortion index can be utilized to realize the bond length deviation by the following formula (51):
D=1n∑i=1n|li−lav|lav
(1)
where D is the distortion index; lav is the average bond length; li is the bond length between the center ion and ith coordinated ions; n is the number of the coordinated ions. The calculated D values of Sc1 and Sc2 sites equal 0 and 0.00604, respectively. Despite the small D of the Sc2 site, the coordinated environment between the Sc1 and Sc2 sites is different. As shown in Table S1 in the SI, four O2– ions are in the same plane, while the other two O2– ions are offset from the vertical direction for the Sc1 site. On the contrary, despite the 6-fold coordination, the coordinated environment of the Sc2 site differs from the regular octahedron coordination, which is more distorted. One cannot observe four O2– ions in the same plane around the Sc2 site. To quantify the degree of distortion in bond angles, the bond angle variance can be calculated by the following equation (52):
σ2=1m−1∑i=1m(φi−φ0)2
(2)
where σ2 is the bond angle variance; m is the number of bond angles, which equals the number of faces ×3/2; φi is the ith bond angle; and φ0 is the ideal bond angle for the selected polyhedron, which equals 90° for an octahedron. σ2 calculated for Sc1 and Sc2 are 180.5362 and 246.4595 (degree), (2) respectively, indicating that the coordinated environment of Sc2 is much more distorted than that of Sc1. Although the bond angle variance in the Sc1 site is less than the Sc2 site, it is still much more than most of the octahedral coordination. Take the Ga2O3 and Ga1.17Sc0.83O3 structures as examples, the bond angle variances calculated from their CIF are 41.9175 and 48.2954, respectively. This result indicates that the [CrO6] polyhedrons in Sc2O3 will still be more distorted than [CrO6] polyhedrons in Ga2O3 and Ga1.17Sc0.83O3. The bond angle variance is beneficial in analyzing the distortion behavior and compensates for the disadvantage of the distortion index, which considers the bond length. One should note that the electronic transition for the Cr ion belongs to d–d transitions, for which the bond angles could potentially affect the luminescent properties.
To further understand the structural properties, the Rietveld refinements of SGOC with x = 0–0.20 are conducted, as shown in Figures 1d and S2 in the SI. The refined parameters, atomic positions, occupancies, and displacement parameters of SGOC with x = 0–0.20 are shown in Tables S2 and S3. The experimental data fit well with the Sc2(1–x)Ga2xO3 patterns. As expected, the lattice parameter, a, decreases in x = 0–0.10 and stays nearly constant in x = 0.10–0.20, as shown in Figure 1e. In our previous study, we have achieved the pure phase of Ga2(1–y)Sc2yO3:Cr3+ from the Ga2O3 structure as the Sc3+/Ga3+ ratio is lower than 44%, as shown in Figure S3 in SI. (49) By contrast, in this study, we can obtain the pure phase of Sc2(1–x)Ga2xO3 from the Sc2O3 structure as the Sc3+/Ga3+ ratio is higher than 94%. Furthermore, there is no significant preference for Ga3+ ions when incorporated into the Sc1 and Sc2 sites, as shown in Table S3.

Photoluminescence Analysis

To examine the basic optical properties, room temperature (RT) photoluminescence excitation (PLE) and photoluminescence (PL) spectra of SGOC for x = 0–0.20 are shown in Figure 2a,b, respectively. Upon NIR emission observation at 800 nm, the PLE spectra consist of two excitation bands typical for Cr3+ ions in 6-fold octahedral coordination. The high energy band that peaked at 470 nm corresponds to 4A2 → 4T1 transition, while the lower energy band that peaked at 660 nm is related to the 4A2 → 4T2 transitions of Cr3+ ions. Upon excitation at 473 nm, a broadband emission extending from 650 to 1100 nm (NIR-I) with a maximum at 830 nm is observed (Figure 2b). This emission corresponds to the 4T2 → 4A2 spin-allowed transition of Cr3+ ions in a weak crystal field. Excitation at 450 nm effectively stimulates NIR-I Cr3+ emission, suggesting its potential application in NIR-LEDs. Additionally, the less intense band appears at a longer wavelength (NIR-II), 1100–1600 nm, as shown in Figure S4a in SI. This emission can also be excited by 980 nm, and the corresponding emission spectra are shown in Figure 2b with a dashed line. Kück et al. (47) observed identical emission spectra for Sc2O3:Cr3+. The RT full width at half-maximum (fwhm) values for Cr3+ and Cr4+ ions are 2402 and 2075 cm–1, respectively. Figure 2a shows the excitation spectra of the NIR-II emission observation at 1300 nm (represented by the dashed line). The two excitation bands at 470 and 660 nm correspond to the excitation of Cr3+ ions, indicating energy transfer between NIR-I (Cr3+) and NIR-II (Cr4+). Two additional overlapped bands, extending from 650 to 1050 nm, are observed in the NIR-II excitation spectra. Although most papers show that Cr4+ ions prefer to occupy tetrahedral sites, (30,32,34,37,53) some studies (54) show that Cr4+ can also occupy the octahedrally coordinated sites. Both 6-fold coordinated Sc1 and Sc2 sites are distorted, but the distortion of Sc2 sites is much more significant. We can consider the 6-fold coordinated Sc1 site as a distorted octahedron. In contrast, the Sc2 site cannot be regarded as a typical octahedron, and the Cr4+ ions may occupy this strongly distorted 6-fold coordinated Sc2 site. The most likely NIR-II emission is related to the Cr4+ ions in disordered 6-fold coordination. According to the Tanabe-Sugano diagram for d2 electron configuration, (55,56) 650–1050 nm excitation bands can be attributed to the 3T1 → 3T2 and 3T1 → 3A2 (or 3T1 → 3T1) transitions of Cr4+ ions. Hence, the emission extending from 1100–1600 nm with a maximum of 1300 nm corresponds to the 3T2 → 3T1 emission of Cr4+.

Figure 2

Figure 2. Room temperature (a) PLE spectra upon Cr3+ and Cr4+ observation at 800 and 1300 nm, respectively, and (b) normalized PL spectra upon excitation at 473 (Cr3+) and 980 nm (direct Cr4+ excitation). (c) Unnormalized room temperature PL spectra upon excitation at 473 nm. (d) Arbitral emission intensity of Cr3+ (dots) and Cr4+ (diamonds) emission. (e,f) Evidence of the energy transfer mechanism.

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Figure S4b shows the x-dependent position of the 4A2 → 4T1,4T2, and 4T2 → 4A2 transitions of Cr3+ and 3T2 → 3T1 of Cr4+ in SGOC (Sc2(1–x)Ga2xO3:Cr3+/4+). A minimal blue shift of the broadband emission of Cr3+ and Cr4+ ions and excitation bands of Cr3+ ions is observed with an increase in the x value of up to 0.10. The emission band shift should be interpreted based on the crystal field theory, assuming that incorporating smaller Ga3+ in place of Sc3+ ions results in an increase in the crystal field strength Dq in the vicinity of Cr3+/Cr4+ ions. This leads to an increase in the energy of the 4T1 and 4T2 states with respect to the 4A2 ground state for Cr3+ ions. Similarly, for Cr4+ ions, the energy of 3T2 and 3A2 (or 3T1) increases with respect to the 3T1 ground state. In other words, this shift is due to the increase in the crystal field with increasing Ga concentration, which is also confirmed by XRD. The XRD measurements clearly demonstrate that Ga3+ ions are incorporated into the crystal structure only up to x = 0.10. Consequently, the increase in crystal field strength is limited to x = 0.10, and there is no further change for higher x values. As a result, the shift in emission and excitation bands is solely observable up to x = 0.10. The crystal field parameter Dq and Racah parameters B and C for Cr3+ are discussed in SI.
Figure 2c shows arbitral emission intensity upon excitation at 473 nm, while Figure 2d shows the integrated emission intensities of Cr3+ and Cr4+ for different x concentrations upon excitation at 473 and 980 nm, respectively. The intensity of Cr3+ emission increases significantly (around 77 times) from x = 0 to x = 0.20. In the case of Cr4+ emission, the intensity hardly changes and only slightly decreases for samples with a higher x concentration. Additionally, the photoluminescence spectra at 100 K in Figure S4c in SI show that the relative intensity of Cr3+ to Cr4+ increases with x. The ionic radius of Sc3+ (0.745 Å) is indeed bigger than that of Cr3+ (0.615 Å). Additionally, the ionic radius of Cr3+ is similar to that of Ga3+ (0.62 Å). These variations in ionic radii lead to a decrease in the average size of the octahedra, potentially facilitating the doping of Cr3+ ions. As a result, in a Ga3+-doped sample, Cr3+ may exhibit a higher likelihood of occupying these sites, which can contribute to an increase in the intensity of luminescence.
As shown in Figure S4d, the decay profiles of Cr3+ luminescence are multiexponential for materials without Ga (x = 0) and for low x concentration, and they start to become near single-exponential for materials with higher x (Ga) concentration. The opposite behavior could be expected: increasing multiexponential behavior with increasing x due to the different surroundings around the Cr3+ ion. The observed effect on the luminescence kinetics suggests a decrease in energy transfer with increasing x. The increase in the intensity of Cr3+ luminescence in a Ga3+-doped sample can indeed lead to a situation where the observability of energy transfer decreases in the decay profile. This may suggest that the energy transfer remains consistent across all samples, as evidenced by only slight shifts in the excitation and emission spectra, which should not significantly change the energy transfer efficiency. The heightened prominence of Cr3+ emissions can give the impression of a more single-exponential decay even when multiple energy transfer pathways might be involved. This effect arises because the contribution of the energy transfer process to the decay profile becomes relatively weaker in the presence of stronger Cr3+ luminescence.
Because of the multiexponential behavior, the average decay time was calculated for all studied materials using the equation:
τav=∫tI(t)dt∫I(t)dt
(3)
where I(t) is the luminescence intensity at time t. The calculated average decay times of Cr3+ ions are shown in Figure S4e. The average decay time increases with increasing x, ranging from approximately 17 to 37 μs. The decay profiles of Cr4+ shown in Figure S4f in the SI are relatively similar for all samples. The calculated average decay time using eq 3 is approximately 0.42 μs, 2 orders of magnitude shorter than the decay time of Cr3+ ions.
For a closer look at the energy transfer mechanism, the Cr4+ PLE spectra and the Cr3+ PL spectra are presented in Figure 2e. It can be observed that the Cr3+4T2 → 4A2 emission overlaps with the Cr4+ excitation bands. This overlapping implies the potential for energy transfer from the lowest excited 4T2 state of the Cr3+ ion to the 3T2 or 3T2/3A2 excited states of the Cr4+ ions. Consequently, the NIR-II luminescence from 3T2 to 3T1 of Cr4+ is observed as shown in Figure 2f.

Oxidation State of the Chromium Ions

The oxidation state of the activators will significantly affect the luminescent properties. To determine the oxidation state of the activators in the bulk powder materials, the Cr K-edge X-ray absorption near edge structure (XANES) spectra of the SGOC with x = 0–0.20 are measured, as shown in Figure 3a. All the absorption edges of x = 0–0.20 samples are close to the standard patterns of Cr3+ and Cr4+, while they differ from the standard patterns of Cr6+. This effect reveals the possibility of Cr3+ and Cr4+ coexisting in the SGOC phosphors. Besides, there is no intense pre-edge peak at around 5,993 eV, proving the lack of Cr6+ in the materials. On the other hand, the peak shape of x = 0–0.10 samples of peak maximum at around 6,007 eV is similar, while the ones of x = 0.15–0.20 samples are different. This result indicates that Cr may exist in Ga1.17Sc0.83O3 and contribute to the XANES spectra when x is higher than 0.15. Furthermore, the Cr K-edge k2-weighted Fourier transform of extended X-ray absorption fine structure (EXAFS) spectra of the SGOC with x = 0–0.20 is analyzed, as shown in Figure 3b. The oscillation patterns between them are roughly similar; however, one can still observe the subtle change in oscillation patterns for x = 0.15–0.20 samples, supporting the hypothesis that the Cr in Ga1.17Sc0.83O3 may contribute to XANES and EXAFS.

Figure 3

Figure 3. Cr K-edge XANES spectra and (b) Cr K-edge k2-weighted Fourier transforms of EXAFS spectra.

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Temperature-Dependent Photoluminescence

Measurements were conducted while the temperature was varied to enhance comprehension of both radiative and nonradiative processes. Due to the presence of impurities in x = 0.15–0.20 samples, the subsequent analysis of luminescent property in this study will mainly focus on the x = 0 and 0.10 samples.
Figure 4a,b shows the temperature-dependent steady-state PL emission of Cr3+ ions upon excitation at 473 nm and Cr4+ ions upon excitation at 980 nm for x = 0.10, respectively. To mitigate the overlap of Cr3+ emission with Cr4+ emission, we decided to investigate the temperature dependence of the emission intensity of Cr4+ under near-infrared 980 nm excitation. The measurements were performed in the temperature range of 100–450 K. The temperature-dependent luminescence of Cr3+ and Cr4+ for x = 0 is shown in Figure S5a,b, respectively. For both samples, emission bands become broadened with increasing temperature. The temperature dependence fwhm of the Cr3+ (represented by red dots) and Cr4+ (represented by green dots) emissions for x = 0.10 is shown in Figure 4c. For temperatures exceeding 400 K, the emission intensity becomes too weak to accurately determine the fwhm of the Cr4+ emission. The fwhm value increases with an increasing temperature. We have utilized the hyperbolic cotangent law describing the temperature dependence of the emission band (fwhm) given by equation: (57)
fwhm(T)=22ln2Sℏωcothℏω2kT
(4)
where T is temperature, S is the Huang–Rhys parameter, ℏω is the energy of effective phonon, and k is Boltzmann’s constant.

Figure 4

Figure 4. Temperature dependence of the luminescence of Cr3+ and Cr4+ in SGOC. The temperature-dependent emission spectra are (a) upon excitation at 473 nm for Cr3+ and (b) at 980 nm for Cr4+ for x = 0.10. (c) Temperature dependence of fwhm for x = 0 and 0.10. (d) Total emission intensity of Cr3+ and Cr4+ for x = 0 and 0.10. The purple circles and green dots represent Cr4+ in x = 0 and x = 0.10, respectively. The cyan circles and red dots represent Cr3+ in x = 0 and x = 0.10, respectively. Solid lines represent the fitting to the experimental data using formulas (4 and 5). Configuration coordinate diagrams for the (e) Cr3+ and (f) Cr4+ ions for x = 0.10.

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From the fitting, we have obtained the value of the Huang–Rhys factor for Cr3+ and Cr4+, S = 6.8 ± 0.2 and 3.6 ± 0.1, respectively, and effective phonon energy, ℏω = 310 ± 6 cm–1 for Cr3+, and 402 ± 7 cm–1 for Cr4+. Quantity Sℏω is the energy of electron lattice relaxation whose calculated value equals 2106 ± 40 and 1447 ± 25 cm–1 for Cr3+ and Cr4+, respectively. The magnitude of the electron–lattice coupling can be described by the Stokes shift, which is equal to 2Sℏω. This parameter describes the energy difference between PLE and PL maximum bands (indicated by arrows in Figure 2a,b, cyan for Cr3+ and magenta for Cr4+). The Sℏω calculated from PLE and PL spectra for Cr3+ is 1715 ± 300 cm–1, which is smaller than those calculated from temperature-dependent fwhm. In the case of Cr4+, Sℏω is equal to 1415 ± 240 cm–1, which agrees with those obtained from temperature-dependent fwhm. The obtained parameters allow us to construct the one-dimensional configurational coordinate diagrams of the ground state (represented by black parabola) and excited states (represented by color parabolas) of Cr3+ and Cr4+ presented in Figure 4e,f. The Sℏω values were taken from the temperature-dependent fwhm. It should be noted that the results discussed above refer to the x = 0.10 sample. However, the relatively small changes observed in the PLE and PL spectra suggest that similar diagrams would be accepted for the other considered samples.
Figure 4d shows the temperature-dependent Cr3+ and Cr4+ emission intensity of x = 0 (empty dots) and 0.10 (filed dots) samples. In both samples, the emission intensity of Cr3+ increases to 275 K and then decreases at higher temperatures. In contrast, for Cr4+, the emission begins to diminish significantly above 100 K. The slight increase in Cr3+ emission intensity can be interpreted as an increase in the absorption coefficient, which is related to the increasing phonon population. (58) The experimental data of temperature dependence of Cr3+ emission intensity were fitted in the temperature range ≥275 K, while for Cr4+ in all temperature ranges. The following formula, inclusive of the single deexcitation process, was used to describe the temperature-dependent intensity behavior:
I(T)=I(0)1+Aexp(−EAkT)
(5)
where I0 is the PL intensity at 0 K, EA is the activation energy, and A is the relative probability of the nonradiative deexcitation processes. The fitting curves are represented as solid lines in Figure 4d. The obtained parameters for x = 0 and 0.10 are as follows: A = 4070 ± 900; 3270 ± 1760, and EA = 2540 ± 70 cm–1; 2760 ± 170 cm–1, respectively. The activation energy corresponds to nonradiative quenching of Cr3+ luminescence. The activation energy EA and parameter A for Cr4+ ions (roughly the same for both samples) are 530 ± 45 cm–1 and 150 ± 50, respectively. This activation energy represents the nonradiative quenching of Cr4+ luminescence, which is approximately five times smaller than that of Cr3+. Considering the configuration diagrams, we can certainly say that the nonradiative processes are not solely attributed to the thermally induced nonradiative relaxation process directly from the excited state to the ground state: 4T2 → 4A2 transition in the case of Cr3+ and 3T2 → 3T1 transition for Cr4+. The intersections of the ground and excited states, represented by parabolas, occur at much higher energies for Cr3+ (∼18,000 cm–1) and Cr4+(∼9000 cm–1), which are much greater than that obtained from fitting the temperature dependence of intensity (2540–2760 cm–1).
Figure S6 shows the temperature dependence of luminescence kinetics in the μs range. Figure S6a,b shows the luminescence decay of Cr3+ emission upon excitation at 470 nm for x = 0 and 0.10 samples, respectively. Figure S6c shows the luminescence decay time of Cr4+ emission upon excitation at 980 nm. Due to the nonexponential decay, the average decay time was calculated using eq 3. Figure S6d,e shows the temperature dependence of the average decay time of Cr3+ and Cr4+ luminescence. The decay time values for Cr3+ at 10 K for x = 0 and 0.10 are 27 and 37 μs, respectively. For Cr4+, the decay time at 100 K is 1.34 ms. The decay time of Cr3+ is typical of Cr3+ in weak crystal fields. (49,59,60) Similar decay time of Cr4+ was found in the references. (32) For x = 0.10, the decay times of Cr3+ emissions remain stable up to 300 K and then decrease with increasing temperature. The temperature behavior of decay times aligns with the emission intensity behavior (Figure 4d). In the case of the x = 0 sample, where the energy transfer is more prominent, the decay times decrease slightly up to 300 K, followed by a more rapid decrease. The significant reduction in the decay time is related to nonradiative quenching of Cr3+. The shorter decay time observed at low temperatures for the x = 0 sample is attributed to an energy transfer process. The decrease in the decay time observed for Cr4+ emission is associated with nonradiative quenching. In both cases, the process of luminescence quenching remains unclear. However, we can certainly say that it is not solely attributed to the thermally induced nonradiative relaxation process directly from the excited state to the ground state (parabolas intercrossing). Instead, it could be attributed to either an ionization transition to the conduction band (specifically in the case of Cr3+) or defects or closely lying charge transfer (CT) states. In contrast, for Cr4+, these states are situated deeper within the bandgap, eliminating the possibility of an autoionization process.

Pressure-Dependent Photoluminescence

To further understand the crystal field influence on Cr3+ and Cr4+ ion luminescence in SGOC, high-pressure experiments were conducted. The x = 0.10 sample was selected for further study due to its consistent demonstration of the highest luminescence intensity of Cr3+ ions among all of the samples containing a pure phase of Sc2O3. RT pressure-dependent PL spectra of Cr3+ and Cr4+ for x = 0.10 are presented in Figure 5a,c, respectively. The excitation wavelength for Cr3+ luminescence was 473 nm, while for Cr4+, it was 980 nm. At atmospheric pressure, only broadband emissions are simultaneously observed for Cr3+ and Cr4+ luminescence. The strong dependence of the 4T2 state of Cr3+ ions on crystal field strength leads to a substantial increase in the energy of the 4T2 → 4A2 transition with pressure and a slight decrease in the energy of 2E → 4A2 transition, (24) as shown in the schematic energy structure diagram present in Figure 5e (left panel). For Cr3+ luminescence, the maximum broadband (4T2 → 4A2) emission spectra shift toward a shorter wavelength (higher energy). Above 12.3 GPa, the broadband emission disappears, and only narrow line emission is observed. This is explained by the crossing of two energy levels: 4T2 (black line) and 2E (blue line), as shown in Figure 5e, left panel (the energy levels represented by the black line is related to the energy of the 4T2 level reduced by Sℏω, while the dashed line by 2Sℏω). At room temperature, the thermal energy is too low to induce emission from the 4T2 state, and the lowest excited 2E state becomes the only emitting state. A similar effect is described in the references. (61−63) At a pressure of 20.2 GPa, a change in emission spectra is observed. A phase transition likely causes such phenomena. (43−45) It is worth noting that the new phase is metastable and the phase no longer persists when the pressure is released.

Figure 5

Figure 5. Pressure dependence of the luminescence of Cr3+ and Cr4+ in SGOC, x = 0.10. (a) Normalized RT emission spectra at different pressure for Cr3+ emission excited with 473 nm. (b) Cr3+ luminescence after a phase transition. (c) Normalized RT emission spectra at different pressures for Cr4+ emission excited with 980 nm. (d) Pressure-dependent emission shift in the energy scale. (e) Schematic pressure dependence of the selected energy levels of the Cr3+ and Cr4+ ions before phase transition. Black solid and dashed gray lines are related to the energy of the 4T2 level reduced by Sℏω and 2Sℏω, respectively.

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The Cr3+ emission in the new phase consists of seven well-resolved emission lines at 681.7, 691.9, 703.0, 713.3, 725.1, 737.5, and 746.6 nm, which are clearly shown in Figure 5b. The line at 713.3 is likely the zero phonon line (ZPL), while the lines at shorter wavelengths are anti-Stokes phonon sidebands marked as υ1’- υ3’, and the line at longer wavelengths (lower energies) are Stokes phonon sidebands marked as υ1- υ3. Surprisingly, we obtained well-resolved emission spectra at RT and high pressures after the phase transition. That emission spectrum is unusual for Cr3+ ions and can be found mostly for Mn4+-doped fluorides (11,64) but not found for Cr3+ emission so far. The most similar Cr3+ spectrum can be found in the high symmetric structure of ZnGa2O4:Cr3+ (65) and K2NaGaF6:Cr3+, (66) but it is still far from what is observed in this paper. This suggests that Cr3+ in the new phase is in a high-symmetry environment.
As mentioned in the Introduction section, there are three main phases found in rare-earth sesquioxide: hexagonal (P3m1); monoclinic, (C2/m); and cubic (Ia3̅). (39−41) Another structure of sesquioxide found in the literature is a corundum-like structure (trigonal, R3̅c). (43) The two other phases hexagonal (P63/mmc) and cubic (Im3̅m) are formed at very high temperatures. (42)
The Sc2(1–x)Ga2xO3:Cr3+/4+ sample synthesized in this work adopts a cubic structure, with a space group Ia3̅. Among the listed structures, only a monoclinic arrangement with the space group C2/m, featuring either six or seven-coordinated cation sites, or a trigonal structure (R3̅c) with six-coordinated cation sites, is the viable candidate for the obtained phases. In the other structures, the requisite 6-fold coordination sites are where the Cr3+ ions can be located. Interestingly, the luminescence behavior of Cr3+ in similar materials (such as β-Ga2–xScxO3:Cr3+ with a monoclinic C2/m structure) and corundum-like structures is reported in the literature. (8,24,67) However, these materials exhibit emissions different from those observed in this study. This discrepancy underscores that the exact origin of unusual Cr3+ emission following phase transition remains not fully understood.
The energies of the 4T2 and 2E transition to the 4A2 ground state of Cr3+ versus pressure before and after the phase transition are presented in Figure 5d. As pressure increases, the ZPL lines shift toward a longer wavelength (lower energy). The values of pressure shifts before phase transition were estimated for the ZPL (2E → 4A2) and broadband emission (4T2 → 4A2) and are −5 ± 2 and 126 ± 1 cm–1/GPa, respectively, which are typical for the pressure shifts of these transitions. (24,59,61−63,68) The pressure shift of the Stokes and anti-Stokes phonons of the Cr3+ emission of the new high-pressure phase is shown in Figure S7. It is seen that the pressure shift is roughly the same for all emission lines. The ZPL shift rate after the phase transition is estimated to be −12.3 ± 0.1 cm–1/GPa.
The schematic energy structure diagram for Cr4+ ions is presented in Figure 5e (right panel). The pronounced sensitivity of the 3T2 state of Cr4+ ions to the crystal field strength results in a significant rise in the energy of the 3T2 → 4T1 transition under pressure, along with a minor reduction in the energy of the 1E → 4T1 transition, similar to Cr3+ ions. For Cr4+ luminescence, a linear shift of the broadband emission toward higher energies is observed with increasing pressure. The calculated pressure shift is presented in Figure 5d. It is worth noting that only a few studies in the literature have conducted high-pressure investigations on NIR Cr4+ emission. (31,33,35) The linear shift of Cr4+ is estimated to be 83 ± 3 and 61 ± 6 cm–1/GPa before and after the phase transition, respectively. Although we conducted a high-pressure experiment up to 36 GPa, we did not reach the pressure value and observed the crossover of the 3T2 and 1E states for Cr4+ in RT. Although the schematic energy diagram indicates a crossing between 3T1 and 1E states around 20 GPa, as shown in Figure 5e, this phenomenon is not observed in RT pressure dependence of emission spectra. Moreover, as pressure increases further, a phase transition takes place. It is important to note that the schematic diagram shown in Figure 5e will no longer accurately represent the energy diagram of the sample in the new phase.
Figure S8a in the SI shows the pressure-dependent decay profiles of Cr3+ luminescence up to 25 GPa and upon excitation at 473 nm. The pressure-dependent decay profiles were taken from the whole emission range (taken from the maximum of the luminescence band, they show a similar trend). The decay profiles are multiexponential, and the average decay times τav were calculated using eq 3. The calculated decay time values are presented in Figure S8b. Pressure-induced increase of crystal field strength causes an increase in the energetic separation of 4T2 and 2E excited states as shown in Figure 5e, lowering the thermal occupation of the higher 4T2. Since the thermal occupation of the 4T2 state with μs lifetime affects the adequate decay time of the 2E → 4A2 emission, the increase of pressure leads to rapid elongation of the decay time of luminescence.

Conclusions

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A series of new Sc2(1–x)Ga2xO3:Cr3+/4+ materials with x = 0–0.20 were synthesized, exhibiting broadband near-infrared (NIR-I and NIR-II) emission. X-ray powder diffraction reveals the absence of impurity peaks for x = 0–0.05. The diffraction peak position shift toward higher angles was observed for x = 0–0.10 indicating the incorporation of Ga3+ ions into the Sc2O3 structure. However, no further shifts were observed for x = 0.10–0.20, suggesting that Ga3+ could no longer be incorporated into the Sc2O3 structure. In a previous study, the pure phase of Ga2(1–y)Sc2yO3:Cr3+/4+ was achieved when the Sc3+/Ga3+ ratio was lower than 44%, originating from the Ga2O3 structure. Conversely, the pure phase of Sc2(1–x)Ga2xO3 was obtained from the Sc2O3 structure when the Sc3+/Ga3+ ratio exceeded 94%. The crystal structure of Sc2(1–x)Ga2xO3 consists of two 6-fold coordinated Sc3+ sites (Sc1 and Sc2), both exhibiting distortion, but the distortion of Sc2 sites is much more significant. Excitation at 450 nm effectively stimulates the studied phosphors, suggesting their potential application in NIR-LEDs. Under 490 nm excitation, Sc2(1–x)Ga2xO3:Cr3+/4+ demonstrates an ultra-broadband NIR emission band ranging from 650 to 1100 nm and 1100 to 1600 nm, originating from Cr3+ and Cr4+ ions, respectively, matching the NIR-I and NIR-II regions. The RT fwhm value for Cr3+ is 2402 cm–1, while for Cr4+ ions, it is 2075 cm–1. The strong distortion of the 6-fold coordinated Sc2 site suggests that Cr4+ ions may occupy this site. The intensity of Cr3+ emission increases significantly (around 77 times) from x = 0 to x = 0.20. This finding suggests the positive impact of Ga3+ on the enhancement of the luminescent properties of the material. Energy transfer between Cr3+ and Cr4+ ions occurs in the studied materials. Taking into account the configuration diagrams presented in this paper, we can certainly say that the nonradiative processes do not result from the thermally induced nonradiative relaxation process directly from the excited state 4T2 to the ground state 4A2. Instead, these processes could potentially be ascribed to either an ionization transition to the conduction band (specifically in the case of Cr3+) or the presence of defects or closely lying charge transfer (CT) states. Furthermore, a phase transition is observed at a pressure of 20.2 GPa. The emission spectra obtained after phase transition exhibit high resolution at high pressures, indicating that Cr3+ in the new phase exists in a high-symmetry environment. That emission spectrum is unusual for Cr3+ ions, and the origin of Cr3+ emission following the phase transition remains not fully understood. Additionally, we present the pressure-induced shift of the NIR Cr4+ luminescence in Sc2(1–x)Ga2xO3:Cr3+/4+. The linear shifts are estimated to be 83 ± 3 cm–1/GPa and 61 ± 6 cm–1/GPa before and after phase transition, respectively.

Experimental Section

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Gallium oxide (Ga2O3, 99.99%), scandium oxide (Sc2O3, 99.99%), and chromium oxide (Cr2O3, 99.99%) were purchased from Gredmann. To prepare the Sc2(1–x)Ga2xO3:Cr3+ samples, all the precursors were stoichiometrically weighed and mixed with an agate mortar for 30 min. The mixing precursors were then poured into alumina crucibles and placed in a muffle furnace. Then, all the samples were heated to 1200 °C for 5 h, with a heating and cooling rate of 5 °C/min. After the samples were cooled to room temperature, they were ground, and the final products were obtained.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.chemmater.3c02466.

  • Characterization; crystal structure; X-ray diffraction; Rietveld refinement; photoluminescence; and decay time (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Natalia Majewska - Institute of Experimental Physics, Faculty of Mathematics, Physics and Informatics, University of Gdansk, Wita Stwosza 57, 80-308 Gdansk, Poland;  Orcidhttps://orcid.org/0000-0002-1933-0355
    • Yi-Ting Tsai - Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan;  Orcidhttps://orcid.org/0000-0001-6280-3907
    • Xiang-Yun Zeng - Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was financially supported by the National Science Center Poland Grant Opus No. 2018/31/B/ST4/00924 and Preludium No. 2022/45/N/ST3/00576. S. Mahlik acknowledges support from the Foundation for Polish Science (IRAP project, ICTQT, Contract No. 2018/MAB/5, cofinanced by EU within Smart Growth Operational Programme). We thank the synchrotron X-ray characterization support from the National Synchrotron Radiation Research Center (NSRRC, Taiwan) with the beamlines of TPS BL19A1 and TPS BL44A1. M.H. Fang acknowledges support from the National Science and Technology Council of Taiwan (Contract No. 112-2113-M-001-039-MY3).

References

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This article references 68 other publications.

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    Yao, Leqi; Shao, Qiyue; Shi, Meiling; Shang, Tianqi; Dong, Yan; Liang, Chao; He, Jinhua; Jiang, Jianqing
    Advanced Optical Materials (2022), 10 (4), 2102229CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Near-IR (NIR) phosphor-converted light-emitting diodes (pc-LEDs) hold great potential for applications ranging from night vision to non-destructive detection. However, it remains a long-standing challenge to develop NIR phosphors simultaneously with longer-wavelength broadband emissions and higher efficiency. Herein, ultra-broadband Ga4GeO8:Cr3+ (GGO:Cr3+) phosphors are developed, with the NIR emission covering 700-1300 nm. Furthermore, tunable emission bands peaking from 835 to 980 nm are achieved simply by varying the Cr3+ concn. Particularly, emission maxima (λmax) of GGO:xCr3+ shift from 850 to 900 nm without intensity loss when increasing x values between 0.02 and 0.10. An internal quantum yield of 60% is achieved for GGO:0.02Cr3+ (λmax ≈ 850 nm, full width at half max. (FWHM) ≈215 nm). The origin of tunable ultra-broadband emissions of GGO:Cr3+ is revealed on the basis of structural and time-resolved spectroscopic anal. The pc-LED fabricated by GGO:0.02Cr3+ exhibits a max. NIR output power of ≈56 mW at 400 mA drive current, and its application in high-penetration quality anal. of fruits is also demonstrated. The results indicate that GGO:Cr3+ phosphors have high promise for practical applications in NIR pc-LED devices.
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  8. 8
    Chen, K.-C.; Fang, M.-H.; Huang, W.-T.; Kamiński, M.; Majewska, N.; Leśniewski, T.; Mahlik, S.; Leniec, G.; Kaczmarek, S. M.; Yang, C.-W.; Lu, K.-M.; Sheu, H.-S.; Liu, R.-S. Chemical and Mechanical Pressure-Induced Photoluminescence Tuning via Structural Evolution and Hydrostatic Pressure. Chem. Mater. 2021, 33 (10), 3832– 3840,  DOI: 10.1021/acs.chemmater.1c01041
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    8
    Chemical and Mechanical Pressure-Induced Photoluminescence Tuning via Structural Evolution and Hydrostatic Pressure
    Chen, Kuan-Chun; Fang, Mu-Huai; Huang, Wen-Tse; Kaminski, Mikolaj; Majewska, Natalia; Lesniewski, Tadeusz; Mahlik, Sebastian; Leniec, Grzegorz; Kaczmarek, Slawomir M.; Yang, Chia-Wei; Lu, Kuang-Mao; Sheu, Hwo-Shuenn; Liu, Ru-Shi
    Chemistry of Materials (2021), 33 (10), 3832-3840CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    A chem. and mech. pressure-induced photoluminescence tuning method was developed through the structural evolution and hydrostatic pressure involving phase transition. A series of Ga1.98-xAlxO3:0.02Cr3+ phosphors were synthesized. Structural evolution reveals a crystal phase change with the incorporation of Al ions. The luminescent anal. shows the broad-to-sharp emission process with a high internal quantum efficiency value (>90%). The high-pressure study reveals the emission from the exchange-coupled Cr3+ pairs and the phase transition under high pressure. ESR indicates the distortion in the microstructures of the emission center. Finally, an ultra-broadband phosphor-converted light-emitting diode is achieved by utilizing the mixt. of Ga1.18Al0.8O3:0.02Cr3+ and Ga1.18Sc0.8O3:0.02Cr3+ phosphors with a bandwidth of 209 nm and an output power of 119 mW. This study provides insights into the effect of chem. and mech. pressure on the Cr3+-doped materials and the development of high-quality near-IR luminescent materials.
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  9. 9
    Liu, D.; Li, G.; Dang, P.; Zhang, Q.; Wei, Y.; Qiu, L.; Molokeev, M. S.; Lian, H.; Shang, M.; Lin, J. Highly Efficient Fe3+-Doped A2BB′O6 (A = Sr2+, Ca2+; B, B′ = In3+, Sb5+, Sn4+) Broadband near-Infrared-Emitting Phosphors for Spectroscopic Analysis. Light Sci. Appl. 2022, 11 (1), 112,  DOI: 10.1038/s41377-022-00803-x
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    9
    Highly efficient Fe3+-doped A2BB'O6 (A = Sr2+, Ca2+; B, B' = In3+, Sb5+, Sn4+) broadband near-infrared-emitting phosphors for spectroscopic analysis
    Liu, Dongjie; Li, Guogang; Dang, Peipei; Zhang, Qianqian; Wei, Yi; Qiu, Lei; Molokeev, Maxim S.; Lian, Hongzhou; Shang, Mengmeng; Lin, Jun
    Light: Science & Applications (2022), 11 (1), 112CODEN: LSAIAZ; ISSN:2047-7538. (Nature Portfolio)
    Abstr.: Near-IR (NIR)-emitting phosphor-converted light-emitting diodes have attracted widespread attention in various applications based on NIR spectroscopy. Except for typical Cr3+-activated NIR-emitting phosphors, next-generation Cr3+-free NIR-emitting phosphors with high efficiency and tunable optical properties are highly desired to enrich the types of NIR luminescent materials for different application fields. Here, we report the Fe3+-activated Sr2-yCay(InSb)1-zSn2zO6 phosphors that exhibit unprecedented long-wavelength NIR emission. The overall emission tuning from 885 to 1005 nm with broadened full-width at half max. from 108 to 146 nm was realized through a crystallog. site engineering strategy. The NIR emission was significantly enhanced after complete Ca2+ incorporation owing to the substitution-induced lower symmetry of the Fe3+ sites. The Ca2InSbO6:Fe3+ phosphor peaking at 935 nm showed an ultra-high internal quantum efficiency of 87. The as-synthesized emission-tunable phosphors demonstrated great potential for NIR spectroscopy detection. This work initiates the development of efficient Fe3+-activated broadband NIR-emitting phosphors and opens up a new avenue for designing NIR-emitting phosphor materials.
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  10. 10
    Song, P.; Qiao, B.; Song, D.; Cao, J.; Shen, Z.; Xu, Z.; Zhao, S.; Wageh, S.; Al-Ghamdi, A. Modifying the Crystal Field of CsPbCl3:Mn2+ Nanocrystals by Co-Doping to Enhance Its Red Emission by a Hundredfold. ACS Appl. Mater. Interfaces 2020, 12 (27), 30711– 30719,  DOI: 10.1021/acsami.0c07655
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    10
    Modifying the Crystal Field of CsPbCl3:Mn2+ Nanocrystals by Co-doping to Enhance Its Red Emission by a Hundredfold
    Song, Pengjie; Qiao, Bo; Song, Dandan; Cao, Jingyue; Shen, Zhaohui; Xu, Zheng; Zhao, Suling; Wageh, Swelm; Al-Ghamdi, Ahmed
    ACS Applied Materials & Interfaces (2020), 12 (27), 30711-30719CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    CsPbCl3:Mn2+ is a practical soln. for obtaining red-orange light inorg. perovskite nanocrystals since CsPbI3 is unstable. Increasing the concn. of Mn2+ is an effective way to enhance the orange-red emission of CsPbCl3:Mn2+. However, the relationship between emission intensity of the Mn2+ dopant and the concn. of Mn2+ is very chaotic in different studies. As a transition metal ion, the electronic states of Mn2+ are very sensitive to the crystal field environment. Here, the crystal field of the CsPbCl3:Mn2+ nanocrystals was adjusted by co-doping other cations, and the concn. of Mn2+ remained unchanged. Addnl., the crystal field strength of different samples was calcd. Compared with the CsPbCl3:Mn2+ nanocrystals, the red-orange peak in the fluorescence spectrum of CsPbCl3:Mn2+, Er3+ nanocrystals was red shifted from 580 to 600 nm and enhanced by 100 times successfully. The same expt. was carried out on CsPbCl3:Mn2+ nanoplatelets at the same time to confirm the changed crystal field around Mn2+. The effect of co-doping cations on the luminescence properties of Mn2+ is similar to that in nanocubes, and the mechanism was analyzed in detail.
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  11. 11
    Leśniewski, T. Evolution of the Full Energy Structure of Mn 4+ in Fluoride Phosphors under High Pressure Conditions. Phys. Chem. Chem. Phys. 2023, 25 (20), 14449– 14462,  DOI: 10.1039/D3CP01045G
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    11
    Evolution of the full energy structure of Mn4+ in fluoride phosphors under high pressure conditions
    Lesniewski, Tadeusz
    Physical Chemistry Chemical Physics (2023), 25 (20), 14449-14462CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    This paper analyzes the photoluminescence excitation and emission spectra of fluoride phosphors doped with Mn4+: KNaSiF6:Mn4+, Rb2GeF6:Mn4+, and Na3HTiF8:Mn4+ under high pressure conditions. From the optical spectra, the pressure-dependent energies of optically active 4T2, 4T1, and 2E crystal field subterms of Mn4+ have been detd. in the 0-30 GPa pressure range. A strong blueshift of the 4T2 and 4T1 subterms was found, as expected from the Tanabe-Sugano diagram for Mn4+ (d3). At the same time, the 2E emitting state exhibited a red shift under pressure - an effect opposite to the prediction of the Tanabe-Sugano diagram. This is a manifestation of the pressure-driven nephelauxetic effect, governed by pressure induced changes of Racah parameters, which demonstrates the necessity of taking into account the Racah parameters for a correct description of Mn4+ emission under pressure. The high pressure exptl. data allowed to det. the pressure dependence of crystal field strength parameter Dq and Racah parameters B and C. Finally, obtaining the pressure dependence of Dq and Racah parameters allowed to calc. the full energy structure of the d3 configuration of Mn4+ in KNaSiF6, Rb2GeF6, and Na3HTiF8 in the pressure range of 0-30 GPa. The calcns. reproduced the red shift of the 2E emitting state under pressure, as well as gave the pressure shift direction and magnitude for all crystal field subterms of Mn4+ up to 50 000 cm-1 (i.e. the equiv. of the Tanabe-Sugano diagram for high-pressure expts.). The approach presented in this paper can be easily extended for calcg. the energy structure of materials doped with isoelectronic Cr3+ as well as other transition metal ions.
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  12. 12
    Satpathy, A.; Huang, W.-T.; Chan, M.-H.; Su, T.-Y.; Kamiński, M.; Majewska, N.; Mahlik, S.; Leniec, G.; Kaczmarek, S. M.; Hsiao, M.; Liu, R.-S. Near-Infrared I/II Nanophosphors with Cr3+/Ni2+ Energy Transfer for Bioimaging. Adv. Opt. Mater. 2023, 11 (15), 2300321  DOI: 10.1002/adom.202300321
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    12
    Near-Infrared I/II Nanophosphors with Cr3+/Ni2+ Energy Transfer for Bioimaging
    Satpathy, Aishwarya; Huang, Wen-Tse; Chan, Ming-Hsien; Su, Ting-Yi; Kaminski, Mikolaj; Majewska, Natalia; Mahlik, Sebastian; Leniec, Grzegorz; Kaczmarek, Slawomir M.; Hsiao, Michael; Liu, Ru-Shi
    Advanced Optical Materials (2023), 11 (15), 2300321CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In the biomedical field, the use of fluorescence imaging in the second near-IR (NIR-II) region is growing rapidly because it imparts the advantages of reduced autofluorescence and low photon scattering. The advantage of reduced scattering is that it increases penetration depth in vivo and improves imaging clarity. Herein, this work uses mesoporous silica, a biocompatible template that can be easily modified, functionalized, and loaded with drugs for use in several bioapplications. The ZnGa2O4 spinel oxide system is integrated into mesoporous silica and different concns. of Cr3+ and Ni2+ are loaded in octahedral sites to obtain the highest emission intensity in the NIR-II region at 1285 nm via energy transfer from Cr3+ to Ni2+. Given that only a few nanophosphor systems with emission in the NIR-II region are available, this work attempts to establish emission in the NIR-II and NIR-I regions to obtain images in vivo with an increased penetration depth to 5 mm and improved clarity for bioimaging purposes. This system will open the door for biomedical research on other NIR-II nanophosphors.
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  13. 13
    Rajendran, V.; Chen, K.-C.; Huang, W.-T.; Kamiński, M.; Grzegorczyk, M.; Mahlik, S.; Leniec, G.; Lu, K.-M.; Wei, D.-H.; Chang, H. Unraveling Luminescent Energy Transfer Pathways: Futuristic Approach of Miniature Shortwave Infrared Light-Emitting Diode Design. ACS Energy Lett. 2023, 8 (5), 2395– 2400,  DOI: 10.1021/acsenergylett.3c00680
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    13
    Unraveling Luminescent Energy Transfer Pathways: Futuristic Approach of Miniature Shortwave Infrared Light-Emitting Diode Design
    Rajendran, Veeramani; Chen, Kuan-Chun; Huang, Wen-Tse; Kaminski, Mikolaj; Grzegorczyk, Maciej; Mahlik, Sebastian; Leniec, Grzegorz; Lu, Kuang-Mao; Wei, Da-Hua; Chang, Ho; Liu, Ru-Shi
    ACS Energy Letters (2023), 8 (5), 2395-2400CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    Phosphor-converted shortwave IR phosphor light-emitting diodes (pc-SWIR LEDs, 900-1700 nm) are promising next-generation portable light sources for spectroscopy, security, optical communication, and medical applications. A typical design strategy involves energy transfer from Cr3+ to Ni2+, and thus, energy transfer from Cr3+-Cr3+ pairs to Ni2+ ions is important but challenging. Here, we report a Sr1-xLaxAl5.92Cr0.08Ga6-xO19:xNi2+ (x = 0-0.09) series for the SWIR emissions range of 900-1600 nm due to an energy transfer from Cr3+ and Cr3+-Cr3+ pair to Ni2+. Short-range structural studies using ESR and magnetometry measurements reveal that Ni2+ ions likely exist as isolated Ni2+ ions and Cr3+-Ni2+ pairs rather than forming Ni2+-Ni2+ pairs. The fabricated prototype SWIR pc-LED delivers a radiant flux of 12.43 mW under a 350 mA driving current. This work provides insights into the codopant strategy for energy transfer and the design of promising next-generation SWIR phosphors.
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  14. 14
    Fang, M.-H.; Hsueh, H.-P.; Vasudevan, T.; Huang, W.-T.; Bao, Z.; Majewska, N.; Mahlik, S.; Sheu, H.-S.; Liu, R.-S. Dual-Emission Eu-Doped Ca2–xSrxPN3 Nitridophosphate Phosphors Prepared by Hot Isostatic Press. J. Mater. Chem. C 2021, 9 (26), 8158– 8162,  DOI: 10.1039/D1TC01207J
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    14
    Dual-emission Eu-doped Ca2-xSrxPN3 nitridophosphate phosphors prepared by hot isostatic press
    Fang, Mu-Huai; Hsueh, Hsi-Ping; Vasudevan, Thangaraji; Huang, Wen-Tse; Bao, Zhen; Majewska, Natalia; Mahlik, Sebastian; Sheu, Hwo-Shuenn; Liu, Ru-Shi
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2021), 9 (26), 8158-8162CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    Nitridophosphates are regarded as an emerging compd. class with applications in phosphor-converted light-emitting diodes. In this work, we aim to develop a series of Ca2-xSrxPN3:Eu2+ nitridophosphate phosphors through a solid-soln. strategy involving a hot isostatic press. Unexpected dual emissions in the red and IR regions are obsd., different from previous research results. The unique change in red and near-IR emission with the incorporation of Sr2+ is revealed and explained in detail through the structural anal. of the locally coordinated environment. Thermal properties including temp.-dependent photoluminescence and decay, as well as electron-lattice relaxation properties, are analyzed in detail. Furthermore, photobiomodulation based on the dual emission of Ca2-xSrxPN3:Eu2+ in the red and near-IR regions is proposed, demonstrating their potential practical applications. This study provides insight into the anal. of nitridophosphate phosphors and the development of novel luminescent materials.
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  15. 15
    Wang, H.; Xu, Y.; Pang, T.; Chen, B.; Xin, F.; Xing, M.; Tian, M.; Fu, Y.; Luo, X.; Tian, Y. Engineering Er3+-Sensitized Nanocrystals to Enhance NIR II-Responsive Upconversion Luminescence. Nanoscale 2022, 14 (3), 962– 968,  DOI: 10.1039/D1NR06945D
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    15
    Engineering Er3+-sensitized nanocrystals to enhance NIR II-responsive upconversion luminescence
    Wang, Hong; Xu, Yang; Pang, Tao; Chen, Baojiu; Xin, Fangyun; Xing, Mingming; Tian, Meng; Fu, Yao; Luo, Xixian; Tian, Ying
    Nanoscale (2022), 14 (3), 962-968CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    An Er3+-sensitized system with a high response to 1550 nm radiation in the second near-IR window (NIR II) has been considered for a new class of potential candidates for applications in bio-imaging and advanced anti-counterfeiting, yet the achievement of highly efficient upconversion emission still remains a challenge. Here, we constructed a novel Er3+-sensitized core-shell-shell upconversion nanostructure with a Yb3+-enriched core as the emitting layer. This designed nanostructure allows the Yb3+ emitting layer to more efficiently trap and lock excitation energy by combining the interfacial energy transfer (IET) from the shell (Er3+) to the core (Yb3+), high activator Yb3+ content, and minimized energy back-transfer. As a result, the NIR II emission at 1000 nm is remarkably enhanced with a high quantum yield (QY) of 11.5%. Based on this trap and lock-in effect of the excitation energy in the Yb3+-enriched core, highly efficient 1550 nm-responsive visible and NIR upconversion emissions are also achieved by co-doping with other activator ions (e.g., Ho3+ and Tm3+). Our research provides a new functional design for improving NIR II-responsive upconversion luminescence, which is significant for developing practical applications.
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  16. 16
    Shafei, R.; Maganas, D.; Strobel, P. J.; Schmidt, P. J.; Schnick, W.; Neese, F. Electronic and Optical Properties of Eu2+-Activated Narrow-Band Phosphors for Phosphor-Converted Light-Emitting Diode Applications: Insights from a Theoretical Spectroscopy Perspective. J. Am. Chem. Soc. 2022, 144 (18), 8038– 8053,  DOI: 10.1021/jacs.2c00218
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    16
    Electronic and Optical Properties of Eu2+-Activated Narrow-Band Phosphors for Phosphor-Converted Light-Emitting Diode Applications: Insights from a Theoretical Spectroscopy Perspective
    Shafei, Rami; Maganas, Dimitrios; Strobel, Philipp Jean; Schmidt, Peter J.; Schnick, Wolfgang; Neese, Frank
    Journal of the American Chemical Society (2022), 144 (18), 8038-8053CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    In this work, we present a computational protocol that is able to predict the exptl. absorption and emission spectral shapes of Eu2+-doped phosphors. The protocol is based on time-dependent d. functional theory and operates in conjunction with an excited-state dynamics approach. It is demonstrated that across the study set consisting of representative examples of nitride, oxo-nitride, and oxide Eu2+-doped phosphors, the energy distribution and the band shape of the emission spectrum are related to the nature of the 4f-5d transitions that are probed in the absorption process. Since the 4f orbitals are very nearly nonbonding, the decisive quantity is the covalency of the 5d acceptor orbitals that become populated in the electronically excited state that leads to emission. The stronger the (anti) bonding interaction between the lanthanide and the ligands is in the excited state, the larger will be the excited state distortion. Consequently, the corresponding emission will get broader due to the vibronic progression that is induced by the structural distortion. In addn., the energy sepn. of the absorption bands that are dominated by states with valence 4f-5d and a metal to ligand charge transfer character defines a measure for the thermal quenching of the studied Eu2+-doped phosphors. Based on this anal., simple descriptors are identified that show a strong correlation with the energy position and bandwidth of the exptl. emission bands without the need for elaborate calcns. Overall, we believe that this study serves as an important ref. for designing new Eu2+-doped phosphors with desired photoluminescence properties.
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  17. 17
    Qiao, J.; Zhou, G.; Zhou, Y.; Zhang, Q.; Xia, Z. Divalent Europium-Doped near-Infrared-Emitting Phosphor for Light-Emitting Diodes. Nat. Commun. 2019, 10 (1), 5267,  DOI: 10.1038/s41467-019-13293-0
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    17
    Divalent europium-doped near-infrared-emitting phosphor for light-emitting diodes
    Qiao Jianwei; Zhou Guojun; Zhou Yayun; Zhang Qinyuan; Xia Zhiguo; Qiao Jianwei; Zhou Guojun; Xia Zhiguo
    Nature communications (2019), 10 (1), 5267 ISSN:.
    Near-infrared luminescent materials exhibit unique photophysical properties that make them crucial components in photonic, optoelectronic and biological applications. As broadband near infrared phosphors activated by transition metal elements are already widely reported, there is a challenge for next-generation materials discovery by introducing rare earth activators with 4f-5d transition. Here, we report an unprecedented phosphor K3LuSi2O7:Eu(2+) that gives an emission band centered at 740 nm with a full-width at half maximum of 160 nm upon 460 nm blue light excitation. Combined structural and spectral characterizations reveal a selective site occupation of divalent europium in LuO6 and K2O6 polyhedrons with small coordination numbers, leading to the unexpected near infrared emission. The fabricated phosphor-converted light-emitting diodes have great potential as a non-visible light source. Our work provides the design principle of near infrared emission in divalent europium-doped inorganic solid-state materials and could inspire future studies to further explore near-infrared light-emitting diodes.
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  18. 18
    Wang, Y.; Shang, M.; Huang, S.; Sun, Y.; Zhu, Y.; Xing, X.; Dang, P.; Lin, J. Continuous Ultra-Broadband Near-Infrared Sc2O3-Based Nanophosphor Realized by Spectral Bridge of Cr3+-Yb3+-Cr4+ for Multiple Optical Applications. Adv. Opt. Mater. 2023, 11 (19), 2300517  DOI: 10.1002/adom.202300517
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    18
    Continuous Ultra-Broadband Near-Infrared Sc2O3-Based Nanophosphor Realized by Spectral Bridge of Cr3+-Yb3+-Cr4+ for Multiple Optical Applications
    Wang, Yining; Shang, Mengmeng; Huang, Shuai; Sun, Yixin; Zhu, Yiying; Xing, Xiaole; Dang, Peipei; Lin, Jun
    Advanced Optical Materials (2023), 11 (19), 2300517CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Cr3+-activated near-IR (NIR) phosphors are one of the most influential candidates for the new generation of intelligent NIR phosphor-converted light-emitting diode (pc-LED) light sources. For spectral anal., the broader the spectrum, the more useful information it contains. Herein, ultra-broadband Cr-activated Sc2O3 nanophosphors, which exhibit NIR ultra-broadband from 650 to 1600 nm, are developed by the sol-gel method. This ultra-broadband emission originates from the presence of Cr3+ and Cr4+, two luminescent centers confirmed by diffuse reflection spectra, ESR, time-resolved photoluminescence spectra, and temp.-dependent emission spectra. The nanophosphor exhibits excellent temp.-sensing performance with SA = 4.102% K-1 and SR = 1.855% K-1. Co-doping Yb3+ into Sc2O3 nanophosphors builds the Cr3+-Yb3+-Cr4+ model to bridge the spectral gap located at 1000 nm, forming a continuous NIR ultra-broadband spectrum ranging from 650 to 1600 nm for the first time. Moreover, the introduction of Yb3+ improves the thermal stability of nanophosphors from 29.49% to 50.65% at 150°C. The NIR transmission spectra of water, ethanol, and peanut oil demonstrate that Sc2O3:Cr3+-Yb3+-Cr4+ NIR nanophosphors favor potential applications in spectral anal. The construction of a Cr3+-Yb3+-Cr4+ spectral bridge for realizing NIR ultra-broadband phosphors based on cheap blue LEDs provides novel and effective insights.
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  19. 19
    Meng, M.; Zhang, T.; Wang, J.; Cheng, Z.; Liu, Y.; Qiao, X.; Wen, J.; Resch-Genger, U.; Long, W.; Ou, J. NaYF4:Yb3+/Tm3+@NaYF4:Yb3+ Upconversion Nanoparticles for Optical Temperature Monitoring and Self-Heating in Photothermal Therapy. ACS Appl. Nano Mater. 2023, 6 (1), 759– 771,  DOI: 10.1021/acsanm.2c05110
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    19
    NaYF4:Yb3+/Tm3+@NaYF4:Yb3+ Upconversion Nanoparticles for Optical Temperature Monitoring and Self-Heating in Photothermal Therapy
    Meng, Mingzhou; Zhang, Tianmei; Wang, Jiaoyu; Cheng, Zhenlong; Liu, Yuanli; Qiao, Xin; Wen, Jian; Resch-Genger, Ute; Long, Wen; Ou, Jun
    ACS Applied Nano Materials (2023), 6 (1), 759-771CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
    The core-shell NaYF4:Yb3+/Tm3+@NaYF4:Yb3+ upconversion nanoparticles were successfully prepd. by a solvothermal method, and a layer of mesoporous silica (mSiO2) was successfully coated on the periphery of the core-shell nanoparticles to transform their surface from lipophilic to hydrophilic, further expanding their applications in biol. tissues. The phys. phase, morphol., structure, and fluorescence properties were characterized by X-ray diffraction (XRD), field emission transmission electron microscopy (TEM), Fourier IR spectroscopy (FT-IR), ζ potential anal., and fluorescence spectroscopy. It was found that the material has a hexagonal structure with good hydrophilicity and emits intense fluorescence under 980 nm pump laser excitation. The non-contact temp. sensing performance of nanoparticles was evaluated by analyzing the upconversion fluorescence of Tm3+ (1G4 → 3F4 and 3F3 → 3H6) in the temp. range of 284-344 K. The abs. and relative sensitivities were found to be 0.0067 K-1 and 1.08% K-1, resp., with high-temp. measurement reliability and good temp. cycling performance. More importantly, its temp. measurement in phosphate-buffered saline (PBS) soln. is accurate. In addn., the temp. of the cells can be increased by adjusting the laser power d. and laser irradn. time. Therefore, an optical temp. sensing platform was built to realize the application of real-time monitoring of cancer cell temp. and the dual function of photothermal therapy.
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  20. 20
    Qiao, J.; Zhang, S.; Zhou, X.; Chen, W.; Gautier, R.; Xia, Z. Near-Infrared Light-Emitting Diodes Utilizing a Europium-Activated Calcium Oxide Phosphor with External Quantum Efficiency of up to 54.7%. Adv. Mater. 2022, 34 (26), 2201887  DOI: 10.1002/adma.202201887
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    20
    Near-Infrared Light-Emitting Diodes utilizing a Europium-Activated Calcium Oxide Phosphor with External Quantum Efficiency of up to 54.7%
    Qiao, Jianwei; Zhang, Shuai; Zhou, Xinquan; Chen, Weibin; Gautier, Romain; Xia, Zhiguo
    Advanced Materials (Weinheim, Germany) (2022), 34 (26), 2201887CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Near-IR (NIR) luminescence materials with broadband emissions are necessary for the development of light-emitting diodes (LEDs) based light sources. However, most known NIR-emitting materials are limited by their low external quantum efficiency. This work demonstrates how the photoluminescence quantum efficiency of europium-activated calcium oxide (CaO:Eu) NIR phosphor can be significantly improved and stabilized at operating temps. of LEDs. A carbon paper wrapping technol. is innovatively developed and used during the solid-state sintering to promote the redn. of Eu3+ into Eu2+. In parallel, the oxygen vacancies in the CaO lattice are repaired utilizing GeO2 decompn. Through this process, a record-high external quantum efficiency of 54.7% at 740 nm is obtained with a thermal stability greatly improved from 57% to 90% at 125°C. The as-fabricated NIR-LEDs reach record photoelec. efficiency (100 [email protected]%) and output power (100 mA @ 319.5 mW). This discovery of high-performance phosphors will open new research avenues for broadband NIR LED light sources in a variety of photonics applications.
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  21. 21
    Li, Y.; Xiong, P.; Lou, B.; Swart, H. C.; Ma, C.; Xia, Z. Near-Infrared Emission in Perovskite Oxides MZrO3:Bi (M = Ba, Sr) by Dopants’ Selective Site Occupancy. J. Lumin. 2023, 256, 119659  DOI: 10.1016/j.jlumin.2022.119659
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    21
    Near-infrared emission in perovskite oxides MZrO3:Bi (M = Ba, Sr) by dopants' selective site occupancy
    Li, Yuanyuan; Xiong, Puxian; Lou, Bibo; Swart, Hendrik C.; Ma, Chonggeng; Xia, Zhiguo
    Journal of Luminescence (2023), 256 (), 119659CODEN: JLUMA8; ISSN:0022-2313. (Elsevier B.V.)
    Near-IR (NIR) emitting materials are essential for spectroscopic anal., biol. imaging and special lighting sources. Herein, broadband NIR emission is achieved in the Bi3+ activated perovskite oxides MZrO3 (M = Ba,Sr), and the luminescence mechanism was revealed using the combination of the exptl. and first-principles studies. The valence state of the Bi3+ ions was confirmed to be trivalent in the XPS spectra and the formation energy calcns. The possibility of NIR emission from lower state bismuth is excluded as the redn. atm. treatment, and therefore the selective occupancy of Bi3+ in the Zr4+ and M2+ sites enables the luminescence discrepancy in different atmospheres, where the Bi3+ on M2+ sites accounts for visible emission while Bi3+ on Zr4+ sites shows NIR emission. For the visible emission of Bi3+, the existence of Ba2+ vacancy shows remarkable influence on the emission spectra. This work not only provides a novel method to develop Bi-activated NIR emitting materials, but also helps to understand the intrinsic NIR luminescence mechanism of Bi3+ ions in inorg. compds.
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  22. 22
    Su, B.; Geng, S.; Xiao, Z.; Xia, Z. Highly Distorted Antimony(III) Chloride [Sb2Cl8]2– Dimers for Near-Infrared Luminescence up to 1070 nm. Angew. Chem., Int. Ed. 2022, 61 (33), e202208881  DOI: 10.1002/anie.202208881
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    22
    Highly Distorted Antimony(III) Chloride [Sb2Cl8]2- Dimers for Near-Infrared Luminescence up to 1070 nm
    Su, Binbin; Geng, Shining; Xiao, Zewen; Xia, Zhiguo
    Angewandte Chemie, International Edition (2022), 61 (33), e202208881CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Zero-dimensional (0D) hybrid metal halides with unique compositional and structural tunability appear as an emerging class of luminescent materials, but near-IR (NIR) emitters therein are largely unexplored to date. This study presents three novel 0D hybrid antimony chlorines with edge-sharing [Sb2Cl8]2- dimers, showing unusual room-temp. broadband NIR emission with the max. emission wavelength up to 1070 nm. Photoluminescence studies and d. functional theory calcn. demonstrate that the emissions originate from the highly localized excitons, and that the confined [Sb2Cl8]2- dimers in these structures show low symmetry and a large degree of structural freedom. These hybrid antimony chlorines with [Sb2Cl8]2- dimers expand the range of new NIR materials in 0D metal halides.
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  23. 23
    Hsu, J.-Y.; Chung, R.-J.; Majewska, N.; Kreft, D.; Sheu, H.-S.; Lee, J.-F.; Mahlik, S.; Fang, M.-H. Probing Local Structural Changes by Sharp Luminescent Infrared Nanophosphor for Application in Light-Emitting Diodes. Chem. Mater. 2022, 34 (24), 11093– 11100,  DOI: 10.1021/acs.chemmater.2c03224
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    23
    Probing Local Structural Changes by Sharp Luminescent Infrared Nanophosphor for Application in Light-Emitting Diodes
    Hsu, Jia-Yu; Chung, Ren-Jei; Majewska, Natalia; Kreft, Dominik; Sheu, Hwo-Shuenn; Lee, Jyh-Fu; Mahlik, Sebastian; Fang, Mu-Huai
    Chemistry of Materials (2022), 34 (24), 11093-11100CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Cr3+-doped IR phosphors are promising candidates for next-generation phosphor-converted IR light-emitting diodes (LEDs) because they can, in principle, tune and convert the luminescence spectra from an LED chip. However, most studies focus on broad-band Cr3+-doped phosphors, and the control mechanism of Cr3+-doped phosphors with sharp line emissions remains ambiguous. Here, we report LiGa5(1-x)Al5xO8:Cr3+ phosphors with sharp line emissions. The luminescence anal. reveals the subtle change of the local structure around Cr3+, which cannot be well resolved by X-ray diffraction. The deviation between the temp.-dependent photoluminescence and decay profile is introduced as well. Furthermore, the morphologies of LiGa5(1-x)Al5xO8:Cr3+ phosphors with high aluminum concn. demonstrate their great potential for mini-LED applications. Finally, an LED package is constructed, and it reveals the potential for angiog. applications. This study opens up a new understanding and perspective for Cr3+-doped sharp emission phosphors and reveals their potential for LED applications.
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  24. 24
    Majewska, N.; Muñoz, A.; Liu, R.-S.; Mahlik, S. Influence of Chemical and Mechanical Pressure on the Luminescence Properties of Near-Infrared Phosphors. Chem. Mater. 2023, 35 (12), 4680– 4690,  DOI: 10.1021/acs.chemmater.3c00203
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    24
    Influence of Chemical and Mechanical Pressure on the Luminescence Properties of Near-Infrared Phosphors
    Majewska, Natalia; Munoz, Alfonso; Liu, Ru-Shi; Mahlik, Sebastian
    Chemistry of Materials (2023), 35 (12), 4680-4690CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The authors aim to compare the changes in the luminescence properties of Ga2O3:Cr3+ modified by Al or Sc ion substitution (chem. pressure) and hydrostatic pressure. The same behavior for Ga2-xAlxO3:Cr3+ and different behavior for Ga2-xScxO3:Cr3+ in terms of the optical properties under chem. and mech. pressure were found. Al substitution, which does not affect the chem. bond angles in the Cr3+ local environment and changes the crystal vol., like mech. pressure does are considered. As confirmed by Raman spectroscopy, the Sc ions cause lattice distortion and influence the chem. bond lengths and angles in the Cr3+ local environment. The energy structure diagrams of all levels of the d3 configuration of the Cr3+ ion as a function of pressure are calcd. by considering the pressure dependence of the Racah parameters. The energy structure diagrams presented in the paper show a decrease in the energy of the 2E, 2T1, and 2T2 excited levels with an increase in Dq. This does not align with the behavior predicted for these excited levels by the std. Tanabe-Sugano diagram. It seems correct that a high-pressure expt. involving Cr3+ and other transition metals should be interpreted using the method and diagrams presented herein.
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  25. 25
    Liu, B.-M.; Guo, X.-X.; Cao, L.-Y.; Huang, L.; Zou, R.; Zhou, Z.; Wang, J. A High-Efficiency Blue-LED-Excitable NIR-II-Emitting MgO:Cr3+,Ni2+ Phosphor for Future Broadband Light Source toward Multifunctional NIR Spectroscopy Applications. Chem. Eng. J. 2023, 452, 139313  DOI: 10.1016/j.cej.2022.139313
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    25
    A High-efficiency blue-LED-excitable NIR-II-emitting MgO:Cr3+,Ni2+ phosphor for future broadband light source toward multifunctional NIR spectroscopy applications
    Liu, Bo-Mei; Guo, Xiao-Xuan; Cao, Lu-Yu; Huang, Lin; Zou, Rui; Zhou, Zhi; Wang, Jing
    Chemical Engineering Journal (Amsterdam, Netherlands) (2023), 452 (Part_2), 139313CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
    The development of phosphor-converted light-emitting diodes (pc-LEDs) in the second near-IR window (NIR-II, 1000-1700 nm) represents an important, newly emerging, and dynamic field in NIR spectroscopy. Unfortunately, lacking efficient NIR-II phosphors that can be excited by com. blue LED chips impedes NIR spectroscopy applications. Herein, a donor-acceptor strategy is developed by introducing Cr3+ sensitizers to greatly enhance the blue-light excitation efficiency of NIR-II-emitting MgO:Cr3+,Ni2+ phosphor. Consequently, a high-efficiency blue-light-excitable MgO:Cr3+,Ni2+ phosphor that gives a NIR-II emission at 1335 nm with a full width at half-max. (FWHM) of 235 nm, is demonstrated. The product has a high internal quantum efficiency of 92.7 % and excellent thermal stability, maintaining 83.0 % of the room temp. emission intensity at 150 °C. Excitingly, the fabricated NIR-II pc-LED device shows a high NIR-II optical power (27.4 mW@350 mA). The performances of the achieved NIR-II pc-LED are almost the best results until now. Addnl., multifunctional applications including nondestructive detection and anti-counterfeiting of the NIR-II light source are demonstrated. These results are pretty crucial for the further development of NIR-II spectroscopy and imaging technol.
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  26. 26
    Yan, Y.; Shang, M.; Huang, S.; Wang, Y.; Sun, Y.; Dang, P.; Lin, J. Photoluminescence Properties of AScSi2O6:Cr3+ (A = Na and Li) Phosphors with High Efficiency and Thermal Stability for Near-Infrared Phosphor-Converted Light-Emitting Diode Light Sources. ACS Appl. Mater. Interfaces 2022, 14 (6), 8179– 8190,  DOI: 10.1021/acsami.1c23940
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    26
    Photoluminescence Properties of AScSi2O6:Cr3+ (A = Na and Li) Phosphors with High Efficiency and Thermal Stability for Near-Infrared Phosphor-Converted Light-Emitting Diode Light Sources
    Yan, Yu; Shang, Mengmeng; Huang, Shuai; Wang, Yining; Sun, Yixin; Dang, Peipei; Lin, Jun
    ACS Applied Materials & Interfaces (2022), 14 (6), 8179-8190CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Near-IR (NIR) phosphors are fascinating photoluminescence materials with applications in phosphor-converted light-emitting diodes (pc-LEDs) for night vision lighting, which are still restricted by low efficiency and thermal stability in the current research stage. In this work, AScSi2O6 (A = Na/Li) are chosen as hosts due to a larger band gap and a single octahedral site for Cr3+ doping. The NIR-emitting Cr3+-activated AScSi2O6:Cr3+ phosphors were successfully prepd. by a common high-temp. solid-state method. X-ray diffraction and Rietveld refinement confirm that the Cr3+ prefers to enter the Sc3+-octahedral lattice site in the AScSi2O6 structure. Under blue light excitation, AScSi2O6:Cr3+ phosphors exhibit broadband NIR emission from 700 to 1100 nm with a full width at half-max. of ~ 150 nm owing to the 4T2 → 4A2 electron transition of Cr3+. The photoluminescence properties were enhanced by adjusting the fluxes and sintering conditions, and highly efficient LiScSi2O6:Cr3+ NIR phosphors with external quantum efficiencies of 33.4% were obtained. Moreover, the optimized LiScSi2O6:Cr3+ exhibits excellent thermal stability (75% at 150°C) with an activation energy of 0.33 eV. Importantly, the fabricated NIR pc-LED with the highly efficient LiScSi2O6:Cr3+ phosphor demonstrates brighter NIR light and a higher luminous efficacy than the NaScSi2O6:Cr3+ phosphor in night vision.
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  27. 27
    Jiang, H.; Chen, L.; Zheng, G.; Luo, Z.; Wu, X.; Liu, Z.; Li, R.; Liu, Y.; Sun, P.; Jiang, J. Ultra-Efficient GAGG:Cr3+ Ceramic Phosphor-Converted Laser Diode: A Promising High-Power Compact Near-Infrared Light Source Enabling Clear Imaging. Adv. Opt. Mater. 2022, 10 (11), 2102741  DOI: 10.1002/adom.202102741
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    27
    Ultra-efficient Aluminum gadolinium gallium oxide:chromium ceramic phosphor-converted laser diode: a promising high-power compact near-infrared light source enabling clear imaging
    Jiang, Hangjie; Chen, Liyan; Zheng, Guojun; Luo, Zhaohua; Wu, Xianhui; Liu, Zehua; Li, Ruiyang; Liu, Yongfu; Sun, Peng; Jiang, Jun
    Advanced Optical Materials (2022), 10 (11), 2102741CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Near-IR (NIR) light is widely used in fields such as org. components detection and biol. imaging, due to its strong tissue penetration and non-destructive characteristics. To improve the detection sensitivity and spatial resoln. of bioimaging, it is important to obtain a new NIR light source with good thermal stability, high efficiency, and ultra-high optical output power, which currently remains a great challenge. In view of this, a Gd3Al2Ga3O12:Cr3+ (GAGG:Cr3+) NIR ceramic phosphor is proposed in this study. By optimizing the prepn. process of GAGG:Cr3+ ceramic phosphor and the concn. of Cr3+ ions, high external quantum efficiency (EQE = 70.1%) and excellent thermal stability (96.8%@150°C) are achieved. After coupling the GAGG:Cr3+ ceramic phosphor with a blue light-emitting diode (LED), the electro-optical conversion efficiency of the NIR ceramic phosphor converted LED (NIR-cpc-LED) reaches 31%@20 mA, which is higher than the previously reported values. Moreover, when coupled with a blue laser diode (LD), the optical output power of the NIR-cpc-LD reaches as high as 1652.6 [email protected] W. Therefore, GAGG:Cr3+ NIR-cpc-LD not only shows a great prospect in practical applications, but also provides a new soln. for high-power compact NIR light sources.
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    Back, M.; Ueda, J.; Brik, M. G.; Lesniewski, T.; Grinberg, M.; Tanabe, S. Revisiting Cr3+-Doped Bi2Ga4O9 Spectroscopy: Crystal Field Effect and Optical Thermometric Behavior of Near-Infrared-Emitting Singly-Activated Phosphors. ACS Appl. Mater. Interfaces 2018, 10 (48), 41512– 41524,  DOI: 10.1021/acsami.8b15607
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    28
    Revisiting Cr3+-Doped Bi2Ga4O9 Spectroscopy: Crystal Field Effect and Optical Thermometric Behavior of Near-Infrared-Emitting Singly-Activated Phosphors
    Back, Michele; Ueda, Jumpei; Brik, Mikhail G.; Lesniewski, Tadeusz; Grinberg, Marek; Tanabe, Setsuhisa
    ACS Applied Materials & Interfaces (2018), 10 (48), 41512-41524CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    The increasing interest in the development of ratiometric optical thermal sensors led to a wide variety of new systems with promising properties. Among them, singly-doped ratiometric thermometers were recently demonstrated to be particularly reliable. With the aim to discuss the development of an ideal optical thermal sensor, a combined exptl. and theor. insight into the spectroscopy of the Bi2Ga4O9:Cr3+ system is reported showing the importance of an insightful anal. in a wide temp. range. Low-temp. luminescence anal. (from 10 K) and the temp. dependence of the lifetime study, together with the crystal field anal. and the modeling of the thermal quenching process, allow the estn. of key parameters such as the Debye temp. (cutoff frequency), the Huang-Rhys parameter, and the energy barrier between 2Eg and 4T2g. Addnl., by considering the reliable class of singly-doped ratiometric thermometers based on a couple of excited states obeying the Boltzmann law, the important role played by the abs. sensitivity was discussed and the great potential of Cr3+ singly-activated systems was demonstrated. The results may provide new guidelines for the design of reliable optical thermometers with outstanding and robust performances.
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    Zhou, H.; Cai, H.; Zhao, J.; Song, Z.; Liu, Q. Crystallographic Control for Cr4+ Activators toward Efficient NIR-II Luminescence. Inorg. Chem. Front. 2022, 9 (9), 1912– 1919,  DOI: 10.1039/D2QI00217E
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    Crystallographic control for Cr4+ activators toward efficient NIR-II luminescence
    Zhou, Hao; Cai, Hao; Zhao, Jing; Song, Zhen; Liu, Quanlin
    Inorganic Chemistry Frontiers (2022), 9 (9), 1912-1919CODEN: ICFNAW; ISSN:2052-1553. (Royal Society of Chemistry)
    Broadband near-IR (NIR) emitting phosphors have attracted great interest due to their potential applications in non-destructive examn. and bioimaging. However, most of the reported broadband NIR phosphors emit in the NIR-I region with a wavelength shorter than 950 nm, while rare-earth activated NIR-II phosphors can hardly meet the requirements because of their sharp emission. Herein, we successfully synthesized the broadband NIR-II phosphor Li2ZnGeO4:Cr4+. By employing the all-tetrahedron-built matrix, all the Cr ions are stabilized in the tetravalent state due to crystallog. control. This phosphor shows wide absorption from the red to near-IR region. Under 646 nm excitation, it exhibits broadband NIR-II emission peaking at 1218 nm with an FWHM of 220 nm at room temp. We also demonstrated the potential applications of Li2ZnGeO4:Cr4+ as an NIR-II light source in non-destructive examn. and bioimaging. This work provides a new strategy for exploring broad-band NIR-II luminescent materials.
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    Wang, D.; Zhang, X.; Wang, X.; Leng, Z.; Yang, Q.; Ji, W.; Lin, H.; Zeng, F.; Li, C.; Su, Z. Investigation of the Structural and Luminescent Properties and the Chromium Ion Valence of Li2CaGeO4 Crystals Doped with Cr4+ Ions. Crystals 2020, 10 (11), 1019,  DOI: 10.3390/cryst10111019
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    Investigation of the structural and luminescent properties and the chromium ion valence of Li2CaGeO4 crystals doped with Cr4+ ions
    Wang, Dongmei; Zhang, Xiaowei; Wang, Xinyu; Leng, Zhuang; Yang, Qianqian; Ji, Wen; Lin, Hai; Zeng, Fanming; Li, Chun; Su, Zhongmin
    Crystals (2020), 10 (11), 1019CODEN: CRYSBC; ISSN:2073-4352. (MDPI AG)
    Herein, we report on the growth of Cr4+-Li2CaGeO4 crystals by the flux growth method from the flux of LiCl, as well as on the effect of doping Li2CaGeO4 with Cr4+ ions on the NIR region spectral properties and crystal structure. The results quantified the occupancy of Cr4+ in Ge4+ sites. The emission spectrum presented broad bands in the NIR region, i.e., 1000-1500 nm excited by 980 nm, with max. peaks at 1200 nm at room temp. caused by the transition of 3T2→3A2 in Cr4+ ions. The lifetime decreased with the Cr4+ ion doping concn., specifically from 14.038 to 12.224 ms. The chem. compn. and the valence state of chromium in Li2CaGeO4 were analyzed using XPS, which showed that the chromium in Li2CaGeO4 was tetravalent and no trivalent chromium was found. Therefore, the Cr4+-Li2CaGeO4 crystal has a great potential and future in optical applications.
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    Hömmerich, U.; Shen, Y.; Bray, K. High-Pressure Luminescence Studies of Cr4+-Doped Laser Materials. J. Lumin. 1997, 72–74, 139– 140,  DOI: 10.1016/S0022-2313(97)00053-7
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    Kuleshov, N. V.; Mikhailov, V. P.; Scherbitsky, V. G.; Minkov, B. I.; Glynn, T. J.; Sherlock, R. Luminescence Study of Cr4+-Doped Silicates. Opt. Mater. 1995, 4 (4), 507– 513,  DOI: 10.1016/0925-3467(94)00120-0
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    Luminescence study of Cr4+ -doped silicates
    Kuleshov, N.V.; Mikhailov, V.P.; Scherbitsky, V.G.; Minkov, B.I.; Glynn, T.J.; Sherlock, R.
    Optical Materials (Amsterdam, Netherlands) (1995), 4 (4), 507-13CODEN: OMATET; ISSN:0925-3467.
    The results of optical absorption, luminescence, and lifetime measurements on Cr-doped Y2SiO5 and Gd2SiO5 crystals are reported. The dominant absorption and emission bands in both crystals are assigned to transitions on the Cr4+ ion in distorted tetrahedral sites. An addnl. near IR emitting center was obsd. in Y2SiO5.
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    Shen, Y.; Riedener, T.; Bray, K. L. Effect of Pressure on Site-Symmetry Distortions of Mn5+ and Cr4+ in Y2SiO5. Phys. Rev. B 2000, 61 (14), 9277– 9286,  DOI: 10.1103/PhysRevB.61.9277
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    Effect of pressure on site-symmetry distortions of Mn5+ and Cr4+ in Y2SiO5
    Shen, Yongrong; Riedener, Toni; Bray, Kevin L.
    Physical Review B: Condensed Matter and Materials Physics (2000), 61 (14), 9277-9286CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    High pressure is used to study the effects of nontetrahedral site distortions on luminescence properties of Cr4+ and Mn5+ in Y2SiO5. Luminescence spectra and lifetime measurements of Mn5+:Y2SiO5 as a function of pressure ≤120 kbar indicated that the local Mn5+ site distortion increased strongly at .ltorsim.50 kbar and increased only slightly at higher pressure. A crystal field energy level calcn. that included spin-orbit coupling effects was completed to quant. model the luminescence and lifetime data in the context of a D2d distortion model. The model provided excellent agreement with the spectral and lifetime data and showed that the 3T1 state has a significant influence on the lifetime of 1E luminescence. Luminescence studies of Cr4+:Y2SiO5 revealed a red shift at .ltorsim.40 kbar followed by a blue shift at higher pressure. These results are consistent with a pronounced increase in the Cr4+ site distortion at .ltorsim.40 kbar that levels off at higher pressures. The Cr4+:Y2SiO5 results also support assignment of the ambient-pressure luminescence spectrum to a transition from orbital components of the 3T2 state to the 3A2 ground state. No evidence of a 3T2-1E electronic crossover in Cr4+:Y2SiO5 was obsd. at ≤120 kbar. The results indicate more generally that the luminescence of 3d2 systems is controlled by competing cubic crystal field and distortion effects. The ability of modest pressures to alter the relative importance of these effects suggests that cubic crystal field and distortions vary widely from host lattice to host lattice at ambient pressure and are responsible for the complicated optical properties of 3d2 ions.
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  34. 34
    Nanai, Y.; Ishida, R.; Urabe, Y.; Nishimura, S.; Fuchi, S. Octave-Spanning Broad Luminescence of Cr3+, Cr4+-Codoped Mg2SiO4 Phosphor for Ultra-Wideband near-Infrared LEDs. Jpn. J. Appl. Phys. 2019, 58, SFFD02,  DOI: 10.7567/1347-4065/ab0b9f
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    Shen, Y.; Bray, K. L. Influence of Low-Symmetry Distortions on the Luminescence of Cr4+-Doped Forsterite. Phys. Rev. Lett. 2000, 84 (17), 3990– 3993,  DOI: 10.1103/PhysRevLett.84.3990
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    Influence of Low-Symmetry Distortions on the Luminescence of Cr4+-Doped Forsterite
    Shen, Yongrong; Bray, Kevin L.
    Physical Review Letters (2000), 84 (17), 3990-3993CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    By using pressure to vary the extent of nontetrahedral distortions of the Cr4+ site in Mg2SiO 4, the authors reveal the important influence of the T13 state on the emission properties of the T23 state. T13- T23 mixing has a pronounced effect on the line shape and radiative decay rate of emission from the T23 state and the extent of mixing depends critically on the magnitude of nontetrahedral distortions. The results provide an explanation for the wide variation of Cr4+ emission properties in different host lattices at ambient pressure and indicate that the tailoring of asym. distortions of luminescent centers represents an effective new strategy for tuning the linewidth of spectral transitions.
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    Hazenkamp, M. F.; Güdel, H. U.; Atanasov, M.; Kesper, U.; Reinen, D. Optical Spectroscopy of Cr4+-Doped Ca2GeO4 and Mg2SiO4. Phys. Rev. B 1996, 53 (5), 2367– 2377,  DOI: 10.1103/PhysRevB.53.2367
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    Optical spectroscopy of Cr4+-doped Ca2GeO4 and Mg2SiO4
    Hazenkamp, M. F.; Guedel, H. U.; Atanasov, M.; Kesper, U.; Reinen, D.
    Physical Review B: Condensed Matter (1996), 53 (5), 2367-77CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    Polarized single crystal absorption spectra and luminescence spectra of Cr4+-doped Ca2GeO4 and Mg2SiO4 are presented and discussed. The absorption spectra are analyzed using the angular overlap model (AOM). The agreement between the exptl. and the AOM-calcd. energies of the ligand field states is satisfactory. The ligand field parameters 10Dq and B are 8950 cm-1 and 540 cm-1 for Ca2GeO4 and 10 100 cm-1 and 560 cm-1 for Mg2SiO4, resp.,. The 3A2 → 1A1 transition appears as a Fano antiresonance in the spectrum of Cr4+-doped Mg2SiO4. The luminescence spectra are assigned to a transition from an excited triplet state, which is definitely not mixed with 1E contributions in Ca2GeO4. The luminescence spectra are surprisingly sharp. Possible explanations are discussed. The luminescence of Cr4+-doped Ca2GeO4 is much less quenched at room temp. than in the Mg2SiO4 host. Finally, the spectroscopic properties of Cr4+-doped materials are compared with those of other 3d2 ions in tetroxo coordination.
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  37. 37
    Zhang, J.; Luo, L.; Zhao, W.; Zhang, W.; Hu, Z. Luminescence Properties and Spectral Modulation of the Ultra-Broadband, Highly Efficient near-Infrared Luminescent Material CaGa4O7:Cr3+, Cr4+, Yb3+. Opt. Mater. 2023, 135, 113388  DOI: 10.1016/j.optmat.2022.113388
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    Luminescence properties and spectral modulation of the ultra-broadband, highly efficient near-infrared luminescent material CaGa4O7:Cr3+, Cr4+, Yb3+
    Zhang, Jian; Luo, Li; Zhao, Weiren; Zhang, Wei; Hu, ZhengFa
    Optical Materials (Amsterdam, Netherlands) (2023), 135 (), 113388CODEN: OMATET; ISSN:0925-3467. (Elsevier B.V.)
    Recently, near-IR phosphors have received a lot of attention due to their wide application in the field of detection and anal., and it is urgent to develop efficient broadband NIR phosphors. In this paper, a conventional high-temp. solid-phase method was used to prep. a novel near-IR phosphor CaGa4O7:Cr3+, Cr4+, which provides two emission bands ranging from 600 to 1000 nm and 1100-1600 nm. Although there are only four and five coordination sites in the CaGa4O7 (CGO) lattice, the doping of the Cr3+ ions in the CGO causes the Ga-O tetrahedral distortion into an octahedron, providing a favorable environment for the emission of the Cr3+ ions. Under blue light excitation, Cr3+ emits luminescence in the NIR I region from 600 to 1000 nm with a half-height width of up to 148 nm. With increasing Cr ion concn., more and more Cr4+ preferentially occupy the four coordinated Ga3+ sites, and the intensity of the ultra-broad emission band in the NIR II region (1100 nm-1600 nm) increases with a full width at half max. (FWHM) of 262 nm. To further optimize the emission spectrum, Yb3+ was codoped into CGO: Cr3+, Cr4+, the energy transfers from Cr3+ to Yb3+ in CGO was found, thus filling the gap between 1000 and 1100 nm in the CGO emission spectrum and obtaining a more complete ultra-wide spectrum. Moreover, the luminescence mechanism, energy transfer efficiency, and quantum yield are also studied in detail, and the ability of CaGa4O7:Cr3+, Cr4+, and Yb3+ luminescent materials to achieve ultra-broadband efficient near-IR emission was demonstrated.
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    Ubaldini, A.; Carnasciali, M. M. Raman Characterisation of Powder of Cubic RE2O3 (RE = Nd, Gd, Dy, Tm, and Lu), Sc2O3 and Y2O3. J. Alloys Compd. 2008, 454 (1), 374– 378,  DOI: 10.1016/j.jallcom.2006.12.067
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    Raman characterisation of powder of cubic RE2O3 (RE=Nd, Gd, Dy, Tm, and Lu), Sc2O3 and Y2O3
    Ubaldini, Alberto; Carnasciali, Maria Maddalena
    Journal of Alloys and Compounds (2008), 454 (1-2), 374-378CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
    Raman spectra of cubic Sc2O3, Y2O3 and C-type rare earth oxides are presented. Cubic Nd2O3 is prepd. by thermal decompn. of its hydroxide. The decompn. mechanism happens in 2 steps. At the end of this process cubic Nd2O3 form. All Raman spectra are characterized by the presence of a very strong band in the range 320-420 cm-1, depending on the lattice parameter of each oxide. Frequency of this line decreases increasing the cell const., meaning that the structure becomes less rigid. A simply approach is used to give a consistent evaluation of the force const. of each oxide. The position of the low wave no. bands depends mainly on the cationic mass, meaning that these bands are not due to stretching vibrations, but rather to bending vibrations.
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    Chikalla, T. D.; McNeilly, C. E.; Roberts, F. P. Polymorphic Modifications of Pm2O3. J. Am. Ceram. Soc. 1972, 55 (8), 428– 429,  DOI: 10.1111/j.1151-2916.1972.tb11329.x
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    Polymorphic modifications of promethium oxide (Pm2O3)
    Chikalla, T. D.; McNeilly, C. E.; Roberts, F. P.
    Journal of the American Ceramic Society (1972), 55 (8), 428-9CODEN: JACTAW; ISSN:0002-7820.
    Pm2O3 did not crystallize when heated in air for 1 week at 500 and 600° but formed cubic crystals at 650-750°, monoclinic (B) crystals ≥800°, and hexagonal crystals (A) at 1600-1800°. Monoclinic Pm2O3 had a structure isomorphous with Nd2O3, based on x-ray diffraction data. Upper phase-transformation temps. for Pm2O3 contg. 2% Sm were B ↹ A (1740 ± 20°), A ↹ H (2135 ± 20°), H ↹ X (2225 ± 20°), and X ↹ melt transition (2320 ± 20°), where H and X are high-temp. polymorphs analogous to those of Sm2O3.
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    Eyring, L.; Gschneidner, K. A.; Lander, G. H. Handbook on the Physics and Chemistry of Rare Earths; Elsevier: 2002.
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    Tang, M.; Lu, P.; Valdez, J. A.; Sickafus, K. E. Ion-Irradiation-Induced Phase Transformation in Rare Earth Sesquioxides (Dy2O3,Er2O3,Lu2O3). J. Appl. Phys. 2006, 99 (6), 063514  DOI: 10.1063/1.2184433
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    Ion-irradiation-induced phase transformation in rare earth sesquioxides (Dy2O3, Er2O3, Lu2O3)
    Tang, M.; Lu, P.; Valdez, J. A.; Sickafus, K. E.
    Journal of Applied Physics (2006), 99 (6), 063514/1-063514/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Polycryst. pellets of cubic C-type rare earth structure (Ia3) Dy2O3, Er2O3, and Lu2O3 were irradiated at cryogenic temp. (120 K) with 300 keV Kr2+ to a max. fluence of 1 × 1020 Kr/m2. Irradiated specimens were examd. using grazing incidence x-ray diffraction and TEM. Ion irradn. leads to different radiation effects in these 3 materials. First, Dy2O3 begins to transform to a monoclinic B-type rare earth structure (C2/m) at a peak dose of ∼5 displacements per atom (dpa), (corresponding to a fluence of 2 × 1019 Kr/m2). This transformation is nearly complete at a peak dose of 25 dpa (a fluence of 1 × 1020 Kr/m2). Er2O3 also transforms to the B-type structure, but the transformation starts at a higher irradn. dose of ∼15-20 dpa [a fluence of about (6-8)×1019 Kr/m2]. Lu2O3 was found to maintain the C-type structure even at the highest irradn. dose of 25 dpa (a fluence of 1 × 1020 Kr/m2). No C-to-B transformation was obsd. in Lu2O3. The irradn. dose dependence of the C-to-B phase transformation obsd. in Dy2O3, Er2O3, and Lu2O3 is closely related to the temp. dependence of the C-to-B phase transformation found in phase diagrams for these 3 materials.
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    Foex, M.; Traverse, J. Investigation about Crystalline Transformation in Rare Earths Sesquioxides at High Temperatures. Rev. Int. High Temp. Refract. 1966, 3, 429
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    Investigations about crystalline transformation in rare earths sesquioxides at high temperatures
    Foex, Marc; Traverse, Jean P.
    Revue Internationale des Hautes Temperatures et des Refractaires (1966), 3 (4), 429-53CODEN: RIHTAV; ISSN:0035-3434.
    The title transformations were studied by D.T.A. and x-ray diffraction at ≤2400°. New cryst. forms of rare earth sesquioxides were detected and identified, but they could not be obtained at room temp., even with very strong quenching. In addn. to the classical varieties, A (hexagonal), B (monoclinic), and C (cubic), 2 new forms are found, 1 hexagonal, different from A, which is exhibited by all the sesquioxides, Lu2O3 excepted, and another cubic form, different from C, which exists at the beginning of the series, i.e. from La2O3 to Ga2O3.
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    Yusa, H.; Tsuchiya, T.; Sata, N.; Ohishi, Y. High-Pressure Phase Transition to the Gd2S3 Structure in Sc2O3: A New Trend in Dense Structures in Sesquioxides. Inorg. Chem. 2009, 48 (16), 7537– 7543,  DOI: 10.1021/ic9001253
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    High-Pressure Phase Transition to the Gd2S3 structure in Sc2O3: A New Trend in Dense Structures in Sesquioxides
    Yusa, Hitoshi; Tsuchiya, Taku; Sata, Nagayoshi; Ohishi, Yasuo
    Inorganic Chemistry (2009), 48 (16), 7537-7543CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    In situ x-ray diffraction expts. using a laser-heated diamond anvil cell revealed a novel dense phase with the Gd2S3 structure stabilizing in Sc2O3 at pressures over 19 GPa. Although no phase transformation was induced during room-temp. compression up to 31 GPa, the C rare earth sesquioxide structure transformed into the B rare earth sesquioxide structure at 10 GPa after laser annealing and subsequently into the Gd2S3 structure at 19 GPa. Neither the A rare earth sesquioxide structure nor the U2S3 structure was found in Sc2O3. Static d. functional lattice energy calcns. demonstrated that the C structure prefers Gd2S3 over U2S3 as the post phase. Sc2O3 is the 2nd sesquioxide, after In2O3, to crystallize into a Gd2S3 structure at high pressures and high temps.
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    Liu, D.; Lei, W.; Li, Y.; Ma, Y.; Hao, J.; Chen, X.; Jin, Y.; Liu, D.; Yu, S.; Cui, Q.; Zou, G. High-Pressure Structural Transitions of Sc2O3 by X-Ray Diffraction, Raman Spectra, and Ab Initio Calculations. Inorg. Chem. 2009, 48 (17), 8251– 8256,  DOI: 10.1021/ic900889v
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    High-Pressure Structural Transitions of Sc2O3 by X-ray Diffraction, Raman Spectra, and Ab Initio Calculations
    Liu, Dan; Lei, Weiwei; Li, Yinwei; Ma, Yanming; Hao, Jian; Chen, Xiaohui; Jin, Yunxia; Liu, Dedi; Yu, Shidan; Cui, Qiliang; Zou, Guangtian
    Inorganic Chemistry (2009), 48 (17), 8251-8256CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The high-pressure behavior of scandium oxide (Sc2O3) has been investigated by angle-dispersive synchrotron powder X-ray diffraction and Raman spectroscopy techniques in a diamond anvil cell up to 46.2 and 42 GPa. An irreversible structural transformation of Sc2O3 from the cubic phase to a monoclinic high-pressure phase was obsd. at 36 GPa. Subsequent ab initio calcns. for Sc2O3 predicted the phase transition from the cubic to monoclinic phase but at a much lower pressure. The same calcns. predicted a second phase transition at 77 GPa from the monoclinic to hexagonal phase.
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    Ovsyannikov, S. V.; Bykova, E.; Bykov, M.; Wenz, M. D.; Pakhomova, A. S.; Glazyrin, K.; Liermann, H.-P.; Dubrovinsky, L. Structural and Vibrational Properties of Single Crystals of Scandia, Sc2O3 under High Pressure. J. Appl. Phys. 2015, 118 (16), 165901,  DOI: 10.1063/1.4933391
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    Structural and vibrational properties of single crystals of Scandia, Sc2O3 under high pressure
    Ovsyannikov, Sergey V.; Bykova, Elena; Bykov, Maxim; Wenz, Michelle D.; Pakhomova, Anna S.; Glazyrin, Konstantin; Liermann, Hanns-Peter; Dubrovinsky, Leonid
    Journal of Applied Physics (Melville, NY, United States) (2015), 118 (16), 165901/1-165901/8CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    The authors report the results of single-crystal x-ray diffraction and Raman spectroscopy studies of Sc oxide, Sc2O3, at ambient temp. under high pressure up to 55 and 28 GPa, resp. Both x-ray diffraction and Raman studies indicated a phase transition from the cubic bixbyite phase (so-called C-Res phase) to a monoclinic C2/m phase (so-called B-Res phase) at pressures around 25-28 GPa. The transition was accompanied by a significant volumetric drop by ∼6.7%. The Raman spectroscopy detected a minor crossover around 10-12 GPa, which manifested in the appearance of new and disappearance of some Raman modes, as well as in softening of one Raman mode. The authors found the bulk modulus values of the both C-Res and B-Res phases as B0 = 198.2(3) and 171.2(1) GPa (for fixed B' = 4), resp. Thus, the denser high-pressure lattice of Sc2O3 is much softer than the original lattice. Possible mechanisms that might be responsible for the pronounced elastic softening in the monoclinic high-pressure phase in this simple oxide with an ultra-wide band gap are discussed. (c) 2015 American Institute of Physics.
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    Huber, G.; Payne, S. A.; Chase, L. L.; Krupke, W. F. Optical Spectroscopy of Cr3+ in ScF3 and Sc2O3. J. Lumin. 1988, 39 (5), 259– 268,  DOI: 10.1016/0022-2313(88)90024-5
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    Optical spectroscopy of chromium(3+) in scandium trifluoride and scandium sesquioxide
    Huber, G.; Payne, Stephen A.; Chase, L. L.; Krupke, William F.
    Journal of Luminescence (1988), 39 (5), 259-68CODEN: JLUMA8; ISSN:0022-2313.
    Absorption, excitation, and emission spectra, and the emission decay measurements are described for Cr3+ in ScF3 and Sc2O3. Both of these materials have 6-fold coordinated metal sites that are available for substitution by Cr3+. From the anal. of the broadband spectra and the zero-phonon lines, information was derived about the a1g, eg and the odd-parity intensity-enabling coordinates. The quenching behavior of the Cr3+ luminescence was explored. The effect is discussed of the next nearest neighbor cations on the crystal field splitting of Cr3+.
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    Kück, S.; Fornasiero, L.; Mix, E.; Huber, G. Spectroscopic Properties of Cr-Doped Sc2O3. J. Lumin. 2000, 87–89, 1122– 1125,  DOI: 10.1016/S0022-2313(99)00563-3
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    Brik, M. G.; Avram, N. M. Crystal Field Analysis and Electron-Phonon Coupling in Sc2O3:Cr3+. Z. Für Naturforschung A 2004, 59 (11), 799– 803,  DOI: 10.1515/zna-2004-1113
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    Fang, M.-H.; Chen, K.-C.; Majewska, N.; Leśniewski, T.; Mahlik, S.; Leniec, G.; Kaczmarek, S. M.; Yang, C.-W.; Lu, K.-M.; Sheu, H.-S.; Liu, R.-S. Hidden Structural Evolution and Bond Valence Control in Near-Infrared Phosphors for Light-Emitting Diodes. ACS Energy Lett. 2021, 6 (1), 109– 114,  DOI: 10.1021/acsenergylett.0c02373
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    Hidden Structural Evolution and Bond Valence Control in Near-Infrared Phosphors for Light-Emitting Diodes
    Fang, Mu-Huai; Chen, Kuan-Chun; Majewska, Natalia; Lesniewski, Tadeusz; Mahlik, Sebastian; Leniec, Grzegorz; Kaczmarek, Slawomir M.; Yang, Chia-Wei; Lu, Kuang-Mao; Sheu, Hwo-Shuenn; Liu, Ru-Shi
    ACS Energy Letters (2021), 6 (1), 109-114CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    We aim to conduct a complete study on the unexpected structure evolution behavior in Cr3+-doped phosphors. A series of Ga2-xScxO3:Cr3+ phosphors are successfully synthesized and confirmed through structural studies, while the lattice parameters change unexpectedly. The unique partial substitution (~ 87%) of Sc3+ in the octahedral site is demonstrated via Rietveld refinement. Therefore, the bond valence sum calcn. explains the reason for this particular Sc3+ concn. The photoluminescent bandwidth and electron-lattice coupling energy initially increase and then decrease, implying an inhomogeneous broadening effect. Time-resolved spectra and ESR are utilized to further examine the subtle change in the microstructures and the second coordination sphere effect of Cr3+. Ga1.594Sc0.4O3:0.006Cr3+ exhibits high internal quantum efficiency (99%) and high phosphor-converted light-emitting diode output power (66.09 mW), demonstrating its capability as an outstanding IR phosphor. This work will motivate further research on unexpected partial substitution during the solid soln. process.
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    Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr. A 1976, 32 (5), 751– 767,  DOI: 10.1107/S0567739476001551
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    Baur, W. H. The Geometry of Polyhedral Distortions. Predictive Relationships for the Phosphate Group. Acta Crystallogr. B 1974, 30 (5), 1195– 1215,  DOI: 10.1107/S0567740874004560
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    Geometry of polyhedral distortions. Predictive relations for the phosphate group
    Baur, Werner H.
    Acta Crystallographica, Section B: Structural Crystallography and Crystal Chemistry (1974), 30 (5), 1195-215CODEN: ACBCAR; ISSN:0567-7408.
    The shape of 211 phosphate tetrahedra have been studied. Their dimensions are known precisely from published x-ray and neutron-diffraction investigations. Results: (1) The site symmetry of the P atoms was found in 85% of the cases to be 1. Other obsd. symmetries are m, 2, 3, 4‾, mm, 42‾m, 222, and 23. (2) The deviations from regular symmetry, 43‾m, are pronounced. The distortions can be measured by defining for every phosphate group 3 distortion indices: DI(TO) = (Σ|TOi-TOm|)/4TOm, DI(OTO) = (Σ|O-TOi-OTOm|)/6OTOm and DI(OO) = (Σ|OOi-OOm|)/ 6OOm, where TO is the distance P-O, OTO the angle O-P-O, OO the distance O-O, the subscripts m refer to the mean and i to individual values. The av. distortion indices for all groups are: DI(TO) = 0.021, DI(OTO) = 0.028, DI(OO) = 0.012. This means that distortions are more pronounced in the P-O distances than in O-O, and thus, the phosphate group can be viewed, to a 1st approxn., as a rigid regular arrangement of O atoms, with the P atoms displaced from their centroid. (3) Mean P-O distances of the phosphate groups vary from 1.506 to 1.572 Å. (4) Individual P-O distances are correlated with the bond strengths received by the individual O atoms. (5) The individual O-P-O angles are strongly correlated with the av. of the P-O distances on the sides of the angle, and with the opposite O-O distance. The various correlations are strong enough to formulate 6 equations which are useful for predictive purposes: the mean P-O distances can be calculated from the coordination numbers and thedistortion indices; the individual P-O distances follow from the bond strength distribution; the bond angles are based on the normalized bond lengths; the mean O-O distances are a function of the mean P-O distances and the distortion indices; the lengths of shared tetrahedral edges depend on the no. of shared edges per tetrahedron. Some of the correlations are affected by the presence of shared edges in the phosphate groups and differ for varioussubpopulations of the sample (ortho-, di-, ring-, poly-, acid or org. phosphates). The calcd. shapes can be used as input to computer simulation of crystal structures.
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    Robinson, K.; Gibbs, G. V.; Ribbe, P. H. Quadratic Elongation: A Quantitative Measure of Distortion in Coordination Polyhedra. Science 1971, 172 (3983), 567– 570,  DOI: 10.1126/science.172.3983.567
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    Quadratic elongation: a quantitative measure of distortion in [mineral] coordination polyhedra
    Robinson, Keith; Gibbs, G. V.; Ribbe, P. H.
    Science (Washington, DC, United States) (1971), 172 (3983), 567-70CODEN: SCIEAS; ISSN:0036-8075.
    Quadratic elongation and the variance of bond angles are linearly correlated for distorted octahedral and tetrahedral coordination complexes, both of which show variations in bond length and bond angle. The quadratic elongation is dimensionless, giving a quant. measure of polyhedral distortion which is independent of the effective size of the polyhedron.
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    Zhuang, Y.; Tanabe, S.; Qiu, J. Wavelength Tailorability of Broadband Near-Infrared Luminescence in Cr4+-Activated Transparent Glass-Ceramics. J. Am. Ceram. Soc. 2014, 97 (11), 3519– 3523,  DOI: 10.1111/jace.13128
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    Wavelength tailorability of broadband near-infrared luminescence in Cr4+-activated transparent glass-ceramics
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    Red-emitting Mn(4+)-doped fluorides are a promising class of materials to improve the color rendering and luminous efficacy of white light-emitting diodes (w-LEDs). For w-LEDs, the luminescence quenching temperature is very important, but surprisingly no systematic research has been conducted to understand the mechanism for thermal quenching in Mn(4+)-doped fluorides. Furthermore, concentration quenching of the Mn(4+) luminescence can be an issue but detailed investigations are lacking. In this work, we study thermal quenching and concentration quenching in Mn(4+)-doped fluorides by measuring luminescence spectra and decay curves of K2TiF6:Mn(4+) between 4 and 600 K and for Mn(4+) concentrations from 0.01% to 15.7%. Temperature-dependent measurements on K2TiF6:Mn(4+) and other Mn(4+)-doped phosphors show that quenching occurs through thermally activated crossover between the (4)T2 excited state and (4)A2 ground state. The quenching temperature can be optimized by designing host lattices in which Mn(4+) has a high (4)T2 state energy. Concentration-dependent studies reveal that concentration quenching effects are limited in K2TiF6:Mn(4+) up to 5% Mn(4+). This is important, as high Mn(4+) concentrations are required for sufficient absorption of blue LED light in the parity-forbidden Mn(4+)d-d transitions. At even higher Mn(4+) concentrations (>10%), the quantum efficiency decreases, mostly due to direct energy transfer to quenching sites (defects and impurity ions). Optimization of the synthesis to reduce quenchers is crucial for developing more efficient highly absorbing Mn(4+) phosphors. The present systematic study provides detailed insights into temperature and concentration quenching of Mn(4+) emission and can be used to realize superior narrow-band red Mn(4+) phosphors for w-LEDs.
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    Near-IR (NIR) phosphor-converted light-emitting diodes (pc-LEDs) are a highly efficient perspective NIR light source, with application hindered by a narrow emission band. In this work, we broaden the emission of a new series of NIR phosphors by controlling crystal structure disorder through cation cosubstitution. By substituting Ga3+ with (Al0.68In0.32)3+, we create a Ga2-x(Al0.68In0.32)xO3:Cr3+ phosphor series in which the av. crystal size is maintained, while cation disorder varies. The increased deviation of the cation radii in the substitution leads to increased electron-phonon coupling, with a resulting emission spectrum covering the 650-1000 nm range with a 30% increase in the emission full width at half-max. (FWHM) and a relatively high internal quantum efficiency of ~ 80%. A transition from the β phase to the α phase, which differs in structure from the undoped parent, is created by the application of high pressure and possesses ultra-broad-band emission and an FWHM of ~ 190 nm. This work shows that the emission bandwidth can be controlled through disorder and its influence on the Stokes shift, as captured by the effective Huang-Rhys factor.
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  1. Jin Yang, Jianhao Zha, Qinan Mao, Heyi Yang, Lang Pei, Jiasong Zhong. Achieving Thermally Stable YAl3–xGax(BO3)4:Cr3+ Near-Infrared Emitting Phosphors via Chemical Substitution for Plant Growth LEDs. Inorganic Chemistry 2024, Article ASAP.
  2. Longbing Shang, Lingkun Zhang, QianShan Quan, Jun Wen, Chong-Geng Ma, Chang-Kui Duan. Mechanistic Insights into Dual NIR Emission from Cr-Doped Sc2O3 via First-Principles Calculations. Inorganic Chemistry 2024, Article ASAP.
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  • Abstract

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    Figure 1

    Figure 1. (a) XRD patterns of the SGOC with x = 0–0.2. (b) Crystal structure of Sc2O3. (c) Coordinated environment of Sc1 and Sc2 sites in Sc2O3. (d) Rietveld refinement of the SGOC with x = 0. (e) Lattice parameters of SGOC with x = 0–0.2. The asterisk symbol in (a) indicates the diffraction peaks from the Ga1.17Sc0.83O3 impurity phase.

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    Figure 2

    Figure 2. Room temperature (a) PLE spectra upon Cr3+ and Cr4+ observation at 800 and 1300 nm, respectively, and (b) normalized PL spectra upon excitation at 473 (Cr3+) and 980 nm (direct Cr4+ excitation). (c) Unnormalized room temperature PL spectra upon excitation at 473 nm. (d) Arbitral emission intensity of Cr3+ (dots) and Cr4+ (diamonds) emission. (e,f) Evidence of the energy transfer mechanism.

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    Figure 3

    Figure 3. Cr K-edge XANES spectra and (b) Cr K-edge k2-weighted Fourier transforms of EXAFS spectra.

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    Figure 4

    Figure 4. Temperature dependence of the luminescence of Cr3+ and Cr4+ in SGOC. The temperature-dependent emission spectra are (a) upon excitation at 473 nm for Cr3+ and (b) at 980 nm for Cr4+ for x = 0.10. (c) Temperature dependence of fwhm for x = 0 and 0.10. (d) Total emission intensity of Cr3+ and Cr4+ for x = 0 and 0.10. The purple circles and green dots represent Cr4+ in x = 0 and x = 0.10, respectively. The cyan circles and red dots represent Cr3+ in x = 0 and x = 0.10, respectively. Solid lines represent the fitting to the experimental data using formulas (4 and 5). Configuration coordinate diagrams for the (e) Cr3+ and (f) Cr4+ ions for x = 0.10.

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    Figure 5

    Figure 5. Pressure dependence of the luminescence of Cr3+ and Cr4+ in SGOC, x = 0.10. (a) Normalized RT emission spectra at different pressure for Cr3+ emission excited with 473 nm. (b) Cr3+ luminescence after a phase transition. (c) Normalized RT emission spectra at different pressures for Cr4+ emission excited with 980 nm. (d) Pressure-dependent emission shift in the energy scale. (e) Schematic pressure dependence of the selected energy levels of the Cr3+ and Cr4+ ions before phase transition. Black solid and dashed gray lines are related to the energy of the 4T2 level reduced by Sℏω and 2Sℏω, respectively.

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  • References

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      Fang, Mu-Huai; Li, Tian-Yin; Huang, Wen-Tse; Cheng, Chiao-Ling; Bao, Zhen; Majewska, Natalia; Mahlik, Sebastian; Yang, Chia-Wei; Lu, Kuang-Mao; Leniec, Grzegorz; Kaczmarek, Slawomir M.; Sheu, Hwo-Shuenn; Liu, Ru-Shi
      ACS Energy Letters (2021), 6 (2), 659-664CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Mini-light-emitting diodes (mini-LEDs) are regarded as a promising light source for future high-end electronic products. Phosphors with a small size and high efficiency have been reported to achieve this goal. Here, we demonstrate that an easily synthesized Ga2O3:Cr3+-embedded mesoporous silica nanoparticle (GOC@MSN) is an outstanding nanophosphor with a superb internal quantum efficiency (91.4%) and a good thermal stability. Structural studies have detd. the mesostructure and intermol. transfer of free electrons. Meanwhile, spectral studies have demonstrated detailed luminescent and thermal properties. A mini-LED package using a GOC@MSN nanophosphor and covering 650-900 nm exhibits its potential for practical applications. This work provides insight into the space-limited ship-in-a-bottle synthesis method for achieving a high quantum efficiency in nanosized phosphors and motivates further research on luminescent materials that use mini-LEDs.
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    2. 2
      Fang, M.-H.; Huang, P.-Y.; Bao, Z.; Majewska, N.; Leśniewski, T.; Mahlik, S.; Grinberg, M.; Leniec, G.; Kaczmarek, S. M.; Yang, C.-W.; Lu, K.-M.; Sheu, H.-S.; Liu, R.-S. Penetrating Biological Tissue Using Light-Emitting Diodes with a Highly Efficient Near-Infrared ScBO3:Cr3+ Phosphor. Chem. Mater. 2020, 32 (5), 2166– 2171,  DOI: 10.1021/acs.chemmater.0c00101
      2
      Penetrating Biological Tissue Using Light-Emitting Diodes with a Highly Efficient Near-Infrared ScBO3:Cr3+ Phosphor
      Fang, Mu-Huai; Huang, Po-Yuan; Bao, Zhen; Majewska, Natalia; Lesniewski, Tadeusz; Mahlik, Sebastian; Grinberg, Marek; Leniec, Grzegorz; Kaczmarek, Slawomir M.; Yang, Chia-Wei; Lu, Kuang-Mao; Sheu, Hwo-Shuenn; Liu, Ru-Shi
      Chemistry of Materials (2020), 32 (5), 2166-2171CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Recently, IR light-emitting diodes (LEDs) have attracted considerable interest in the research field worldwide. IR phosphors, the basic materials utilized in LEDs, have become a research hotspot as well. Here, we introduce the high-quantum-efficiency IR ScBO3:Cr3+ phosphor, which provides a spectral range of emission from 700 to 1000 nm with a peak max. at 800 nm. ESR spectroscopy, with high element selectivity, was used to elucidate the unusual small peak in the photoluminescence spectrum. Phonon structure and electron-lattice interaction were well obsd. and discussed via temp.-dependent measurements. Moreover, the high quantum efficiency of 72.8% was achieved. To evaluate their potential practical application, phosphor-converted LED packages were designed, which revealed high stability and high output power of 39.11 mW. Furthermore, the fabricated IR LED demonstrated a remarkable ability to penetrate biol. tissues. This study provides insights into the luminescent properties and the practical applications of IR LEDs.
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    3. 3
      Yang, H.; Li, R.; Zhang, Y.; Yu, M.; Wang, Z.; Liu, X.; You, W.; Tu, D.; Sun, Z.; Zhang, R.; Chen, X.; Wang, Q. Colloidal Alloyed Quantum Dots with Enhanced Photoluminescence Quantum Yield in the NIR-II Window. J. Am. Chem. Soc. 2021, 143 (6), 2601– 2607,  DOI: 10.1021/jacs.0c13071
      3
      Colloidal Alloyed Quantum Dots with Enhanced Photoluminescence Quantum Yield in the NIR-II Window
      Yang, Hongchao; Li, Renfu; Zhang, Yejun; Yu, Mengxuan; Wang, Zan; Liu, Xi; You, Wenwu; Tu, Datao; Sun, Ziqiang; Zhang, Rong; Chen, Xueyuan; Wang, Qiangbin
      Journal of the American Chemical Society (2021), 143 (6), 2601-2607CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Semiconductor quantum dots (QDs) with photoluminescence (PL) emission at 900-1700 nm (denoted as the second near-IR window, NIR-II) exhibit much-depressed photon absorption and scattering, which has stimulated extensive researches in biomedical imaging and NIR devices. However, it is very challenging to develop NIR-II QDs with a high photoluminescence quantum yield (PLQY) and excellent biocompatibility. Herein, we designed and synthesized an alloyed silver gold selenide (AgAuSe) QD with a bright emission from 820 to 1170 nm and achieved a record abs. PLQY of 65.3% at 978 nm emission among NIR-II QDs without a toxic element and a long lifetime of 4.58μs. It is proved that the high PLQY and long lifetime are mainly attributed to the prevented nonradiative transition of excitons, probably resulted from suppressing cation vacancies and crystal defects from the high mobility of Ag ions by alloying Au atoms. These high-PLQY QDs with nontoxic heavy metal exhibit great application potential in bioimaging, light emitting diodes (LEDs), and photovoltaic devices.
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    4. 4
      Zhou, Y.; Li, C.; Wang, Y. Crystal-Field Engineering Control of an Ultraviolet–Visible-Responsive Near-Infrared-Emitting Phosphor and Its Applications in Plant Growth, Night Vision, and NIR Spectroscopy Detection. Adv. Opt. Mater. 2022, 10 (8), 2102246  DOI: 10.1002/adom.202102246
      4
      Crystal-Field Engineering Control of an Ultraviolet-Visible-Responsive Near-Infrared-Emitting Phosphor and Its Applications in Plant Growth, Night Vision, and NIR Spectroscopy Detection
      Zhou, Yunpeng; Li, Cancan; Wang, Yuhua
      Advanced Optical Materials (2022), 10 (8), 2102246CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Broadband near-IR (NIR) phosphor-converted light-emitting diodes (pc-LEDs) have many prospective applications in areas related to food inspection, health management, plant growth, and night vision, because NIR phosphors are crit. components of the corresponding devices. Given the wide-ranging applicability of the material, this study is designed to comprehensively evaluate a series of novel NIR phosphors: BaMgGaxAl10-xO17:Cr3+ (x = 0, 1, 2, 3). The ultra-low-temp. (4 K) fluorescence spectroscopy and decay curve results confirm that Cr3+ occupies the four lattice sites of Al3+ in the matrix. Addnl., the introduction of Ga3+ increases the full-width at half-max. (FWHM) of the NIR spectrum from 100 to 300 nm without affecting the fluorescence efficiency; it also considerably increases spectral coverage. An expt. to evaluate the applicability of the novel NIR phosphors reveals that the sunlight-converting film prepd. by applying the material combination of BaMgGa3Al7O17:0.02Cr3+ is able to effectively promote the growth of Chlorella. In addn., the NIR pc-LED with a BaMgGa3Al7O17:0.02Cr3+ basis and 395-nm LED chip has been proven to be an excellent candidate for applications in NIR spectral detection and night-vision technol.
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    5. 5
      Dang, P.; Wei, Y.; Liu, D.; Li, G.; Lin, J. Recent Advances in Chromium-Doped Near-Infrared Luminescent Materials: Fundamentals, Optimization Strategies, and Applications. Adv. Opt. Mater. 2023, 11 (3), 2201739  DOI: 10.1002/adom.202201739
      5
      Recent Advances in Chromium-Doped Near-Infrared Luminescent Materials: Fundamentals, Optimization Strategies, and Applications
      Dang, Peipei; Wei, Yi; Liu, Dongjie; Li, Guogang; Lin, Jun
      Advanced Optical Materials (2023), 11 (3), 2201739CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Development of chromium-doped luminescent materials is pertinent to many emerging applications, ranging from agriculture, food industry to noninvasive health monitoring. The fundamental importance of chromium-activated luminescent materials in the field of optics and biomedicine makes the rapid development of novel materials and relevant applications. Herein, the recent advances on the luminescence principle and photoluminescence (PL) optimization for Cr3+-activated luminescent materials together with their potential applications are reviewed. The different types of most recently developed Cr3+-doped luminescent materials and the design principles are systematically summarized. The assocns. between crystal structure and near-IR (NIR) PL properties, as well as performance-evaluating parameters are introduced with the examples of known NIR emitting phosphors, which will be helpful to explore future NIR luminescent materials. Based on crystal field control, site engineering, and electron-phonon coupling, several efficient strategies for optimizing luminescence performances including bandwidth, thermal stability, and quantum efficiency of Cr3+-doped NIR luminescent materials are proposed. Then, potential applications in the fields of food anal., night vision, information encryption, and optical sensors are surveyed. Finally, the challenges of promising Cr3+-doped luminescent materials are proposed.
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    6. 6
      Rajendran, V.; Fang, M.-H.; Huang, W.-T.; Majewska, N.; Lesniewski, T.; Mahlik, S.; Leniec, G.; Kaczmarek, S. M.; Pang, W. K.; Peterson, V. K.; Lu, K.-M.; Chang, H.; Liu, R.-S. Chromium Ion Pair Luminescence: A Strategy in Broadband Near-Infrared Light-Emitting Diode Design. J. Am. Chem. Soc. 2021, 143 (45), 19058– 19066,  DOI: 10.1021/jacs.1c08334
      6
      Chromium Ion Pair Luminescence: A Strategy in Broadband Near-Infrared Light-Emitting Diode Design
      Rajendran, Veeramani; Fang, Mu-Huai; Huang, Wen-Tse; Majewska, Natalia; Lesniewski, Tadeusz; Mahlik, Sebastian; Leniec, Grzegorz; Kaczmarek, Slawomir M.; Pang, Wei Kong; Peterson, Vanessa K.; Lu, Kuang-Mao; Chang, Ho; Liu, Ru-Shi
      Journal of the American Chemical Society (2021), 143 (45), 19058-19066CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Portable near-IR (NIR) light sources are in high demand for applications in spectroscopy, night vision, bioimaging, and many others. Typical phosphor designs feature isolated Cr3+ ion centers, and it is challenging to design broadband NIR phosphors based on Cr3+-Cr3+ pairs. Here, we explore the solid-soln. series SrAl11.88-xGaxO19:0.12Cr3+ (x = 0, 2, 4, 6, 8, 10, and 12) as phosphors featuring Cr3+-Cr3+ pairs and evaluate structure-property relations within the series. We establish the incorporation of Ga within the magentoplumbite-type structure at five distinct crystallog. sites and evaluate the effect of this incorporation on the Cr3+-Cr3+ ion pair proximity. Electron paramagnetic measurements reveal the presence of both isolated Cr3+ and Cr3+-Cr3+ pairs, resulting in NIR luminescence at approx. 650-1050 nm. Unexpectedly, the origin of broadband NIR luminescence with a peak within the range 740-820 nm is related to the Cr3+-Cr3+ ion pair. We demonstrate the application of the SrAl5.88Ga6O19:0.12Cr3+ phosphor, which possesses an internal quantum efficiency of ~ 85%, a radiant flux of ~ 95 mW, and zero thermal quenching up to 500 K. This work provides a further understanding of spectral shifts in phosphor solid solns. and in particular the application of the magentoplumbites as promising next-generation NIR phosphor host systems.
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    7. 7
      Yao, L.; Shao, Q.; Shi, M.; Shang, T.; Dong, Y.; Liang, C.; He, J.; Jiang, J. Efficient Ultra-Broadband Ga4GeO8:Cr3+ Phosphors with Tunable Peak Wavelengths from 835 to 980 nm for NIR Pc-LED Application. Adv. Opt. Mater. 2022, 10 (4), 2102229  DOI: 10.1002/adom.202102229
      7
      Efficient Ultra-Broadband Ga4GeO8:Cr3+ Phosphors with Tunable Peak Wavelengths from 835 to 980 nm for NIR pc-LED Application
      Yao, Leqi; Shao, Qiyue; Shi, Meiling; Shang, Tianqi; Dong, Yan; Liang, Chao; He, Jinhua; Jiang, Jianqing
      Advanced Optical Materials (2022), 10 (4), 2102229CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Near-IR (NIR) phosphor-converted light-emitting diodes (pc-LEDs) hold great potential for applications ranging from night vision to non-destructive detection. However, it remains a long-standing challenge to develop NIR phosphors simultaneously with longer-wavelength broadband emissions and higher efficiency. Herein, ultra-broadband Ga4GeO8:Cr3+ (GGO:Cr3+) phosphors are developed, with the NIR emission covering 700-1300 nm. Furthermore, tunable emission bands peaking from 835 to 980 nm are achieved simply by varying the Cr3+ concn. Particularly, emission maxima (λmax) of GGO:xCr3+ shift from 850 to 900 nm without intensity loss when increasing x values between 0.02 and 0.10. An internal quantum yield of 60% is achieved for GGO:0.02Cr3+ (λmax ≈ 850 nm, full width at half max. (FWHM) ≈215 nm). The origin of tunable ultra-broadband emissions of GGO:Cr3+ is revealed on the basis of structural and time-resolved spectroscopic anal. The pc-LED fabricated by GGO:0.02Cr3+ exhibits a max. NIR output power of ≈56 mW at 400 mA drive current, and its application in high-penetration quality anal. of fruits is also demonstrated. The results indicate that GGO:Cr3+ phosphors have high promise for practical applications in NIR pc-LED devices.
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    8. 8
      Chen, K.-C.; Fang, M.-H.; Huang, W.-T.; Kamiński, M.; Majewska, N.; Leśniewski, T.; Mahlik, S.; Leniec, G.; Kaczmarek, S. M.; Yang, C.-W.; Lu, K.-M.; Sheu, H.-S.; Liu, R.-S. Chemical and Mechanical Pressure-Induced Photoluminescence Tuning via Structural Evolution and Hydrostatic Pressure. Chem. Mater. 2021, 33 (10), 3832– 3840,  DOI: 10.1021/acs.chemmater.1c01041
      8
      Chemical and Mechanical Pressure-Induced Photoluminescence Tuning via Structural Evolution and Hydrostatic Pressure
      Chen, Kuan-Chun; Fang, Mu-Huai; Huang, Wen-Tse; Kaminski, Mikolaj; Majewska, Natalia; Lesniewski, Tadeusz; Mahlik, Sebastian; Leniec, Grzegorz; Kaczmarek, Slawomir M.; Yang, Chia-Wei; Lu, Kuang-Mao; Sheu, Hwo-Shuenn; Liu, Ru-Shi
      Chemistry of Materials (2021), 33 (10), 3832-3840CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      A chem. and mech. pressure-induced photoluminescence tuning method was developed through the structural evolution and hydrostatic pressure involving phase transition. A series of Ga1.98-xAlxO3:0.02Cr3+ phosphors were synthesized. Structural evolution reveals a crystal phase change with the incorporation of Al ions. The luminescent anal. shows the broad-to-sharp emission process with a high internal quantum efficiency value (>90%). The high-pressure study reveals the emission from the exchange-coupled Cr3+ pairs and the phase transition under high pressure. ESR indicates the distortion in the microstructures of the emission center. Finally, an ultra-broadband phosphor-converted light-emitting diode is achieved by utilizing the mixt. of Ga1.18Al0.8O3:0.02Cr3+ and Ga1.18Sc0.8O3:0.02Cr3+ phosphors with a bandwidth of 209 nm and an output power of 119 mW. This study provides insights into the effect of chem. and mech. pressure on the Cr3+-doped materials and the development of high-quality near-IR luminescent materials.
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    9. 9
      Liu, D.; Li, G.; Dang, P.; Zhang, Q.; Wei, Y.; Qiu, L.; Molokeev, M. S.; Lian, H.; Shang, M.; Lin, J. Highly Efficient Fe3+-Doped A2BB′O6 (A = Sr2+, Ca2+; B, B′ = In3+, Sb5+, Sn4+) Broadband near-Infrared-Emitting Phosphors for Spectroscopic Analysis. Light Sci. Appl. 2022, 11 (1), 112,  DOI: 10.1038/s41377-022-00803-x
      9
      Highly efficient Fe3+-doped A2BB'O6 (A = Sr2+, Ca2+; B, B' = In3+, Sb5+, Sn4+) broadband near-infrared-emitting phosphors for spectroscopic analysis
      Liu, Dongjie; Li, Guogang; Dang, Peipei; Zhang, Qianqian; Wei, Yi; Qiu, Lei; Molokeev, Maxim S.; Lian, Hongzhou; Shang, Mengmeng; Lin, Jun
      Light: Science & Applications (2022), 11 (1), 112CODEN: LSAIAZ; ISSN:2047-7538. (Nature Portfolio)
      Abstr.: Near-IR (NIR)-emitting phosphor-converted light-emitting diodes have attracted widespread attention in various applications based on NIR spectroscopy. Except for typical Cr3+-activated NIR-emitting phosphors, next-generation Cr3+-free NIR-emitting phosphors with high efficiency and tunable optical properties are highly desired to enrich the types of NIR luminescent materials for different application fields. Here, we report the Fe3+-activated Sr2-yCay(InSb)1-zSn2zO6 phosphors that exhibit unprecedented long-wavelength NIR emission. The overall emission tuning from 885 to 1005 nm with broadened full-width at half max. from 108 to 146 nm was realized through a crystallog. site engineering strategy. The NIR emission was significantly enhanced after complete Ca2+ incorporation owing to the substitution-induced lower symmetry of the Fe3+ sites. The Ca2InSbO6:Fe3+ phosphor peaking at 935 nm showed an ultra-high internal quantum efficiency of 87. The as-synthesized emission-tunable phosphors demonstrated great potential for NIR spectroscopy detection. This work initiates the development of efficient Fe3+-activated broadband NIR-emitting phosphors and opens up a new avenue for designing NIR-emitting phosphor materials.
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    10. 10
      Song, P.; Qiao, B.; Song, D.; Cao, J.; Shen, Z.; Xu, Z.; Zhao, S.; Wageh, S.; Al-Ghamdi, A. Modifying the Crystal Field of CsPbCl3:Mn2+ Nanocrystals by Co-Doping to Enhance Its Red Emission by a Hundredfold. ACS Appl. Mater. Interfaces 2020, 12 (27), 30711– 30719,  DOI: 10.1021/acsami.0c07655
      10
      Modifying the Crystal Field of CsPbCl3:Mn2+ Nanocrystals by Co-doping to Enhance Its Red Emission by a Hundredfold
      Song, Pengjie; Qiao, Bo; Song, Dandan; Cao, Jingyue; Shen, Zhaohui; Xu, Zheng; Zhao, Suling; Wageh, Swelm; Al-Ghamdi, Ahmed
      ACS Applied Materials & Interfaces (2020), 12 (27), 30711-30719CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      CsPbCl3:Mn2+ is a practical soln. for obtaining red-orange light inorg. perovskite nanocrystals since CsPbI3 is unstable. Increasing the concn. of Mn2+ is an effective way to enhance the orange-red emission of CsPbCl3:Mn2+. However, the relationship between emission intensity of the Mn2+ dopant and the concn. of Mn2+ is very chaotic in different studies. As a transition metal ion, the electronic states of Mn2+ are very sensitive to the crystal field environment. Here, the crystal field of the CsPbCl3:Mn2+ nanocrystals was adjusted by co-doping other cations, and the concn. of Mn2+ remained unchanged. Addnl., the crystal field strength of different samples was calcd. Compared with the CsPbCl3:Mn2+ nanocrystals, the red-orange peak in the fluorescence spectrum of CsPbCl3:Mn2+, Er3+ nanocrystals was red shifted from 580 to 600 nm and enhanced by 100 times successfully. The same expt. was carried out on CsPbCl3:Mn2+ nanoplatelets at the same time to confirm the changed crystal field around Mn2+. The effect of co-doping cations on the luminescence properties of Mn2+ is similar to that in nanocubes, and the mechanism was analyzed in detail.
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    11. 11
      Leśniewski, T. Evolution of the Full Energy Structure of Mn 4+ in Fluoride Phosphors under High Pressure Conditions. Phys. Chem. Chem. Phys. 2023, 25 (20), 14449– 14462,  DOI: 10.1039/D3CP01045G
      11
      Evolution of the full energy structure of Mn4+ in fluoride phosphors under high pressure conditions
      Lesniewski, Tadeusz
      Physical Chemistry Chemical Physics (2023), 25 (20), 14449-14462CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      This paper analyzes the photoluminescence excitation and emission spectra of fluoride phosphors doped with Mn4+: KNaSiF6:Mn4+, Rb2GeF6:Mn4+, and Na3HTiF8:Mn4+ under high pressure conditions. From the optical spectra, the pressure-dependent energies of optically active 4T2, 4T1, and 2E crystal field subterms of Mn4+ have been detd. in the 0-30 GPa pressure range. A strong blueshift of the 4T2 and 4T1 subterms was found, as expected from the Tanabe-Sugano diagram for Mn4+ (d3). At the same time, the 2E emitting state exhibited a red shift under pressure - an effect opposite to the prediction of the Tanabe-Sugano diagram. This is a manifestation of the pressure-driven nephelauxetic effect, governed by pressure induced changes of Racah parameters, which demonstrates the necessity of taking into account the Racah parameters for a correct description of Mn4+ emission under pressure. The high pressure exptl. data allowed to det. the pressure dependence of crystal field strength parameter Dq and Racah parameters B and C. Finally, obtaining the pressure dependence of Dq and Racah parameters allowed to calc. the full energy structure of the d3 configuration of Mn4+ in KNaSiF6, Rb2GeF6, and Na3HTiF8 in the pressure range of 0-30 GPa. The calcns. reproduced the red shift of the 2E emitting state under pressure, as well as gave the pressure shift direction and magnitude for all crystal field subterms of Mn4+ up to 50 000 cm-1 (i.e. the equiv. of the Tanabe-Sugano diagram for high-pressure expts.). The approach presented in this paper can be easily extended for calcg. the energy structure of materials doped with isoelectronic Cr3+ as well as other transition metal ions.
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    12. 12
      Satpathy, A.; Huang, W.-T.; Chan, M.-H.; Su, T.-Y.; Kamiński, M.; Majewska, N.; Mahlik, S.; Leniec, G.; Kaczmarek, S. M.; Hsiao, M.; Liu, R.-S. Near-Infrared I/II Nanophosphors with Cr3+/Ni2+ Energy Transfer for Bioimaging. Adv. Opt. Mater. 2023, 11 (15), 2300321  DOI: 10.1002/adom.202300321
      12
      Near-Infrared I/II Nanophosphors with Cr3+/Ni2+ Energy Transfer for Bioimaging
      Satpathy, Aishwarya; Huang, Wen-Tse; Chan, Ming-Hsien; Su, Ting-Yi; Kaminski, Mikolaj; Majewska, Natalia; Mahlik, Sebastian; Leniec, Grzegorz; Kaczmarek, Slawomir M.; Hsiao, Michael; Liu, Ru-Shi
      Advanced Optical Materials (2023), 11 (15), 2300321CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In the biomedical field, the use of fluorescence imaging in the second near-IR (NIR-II) region is growing rapidly because it imparts the advantages of reduced autofluorescence and low photon scattering. The advantage of reduced scattering is that it increases penetration depth in vivo and improves imaging clarity. Herein, this work uses mesoporous silica, a biocompatible template that can be easily modified, functionalized, and loaded with drugs for use in several bioapplications. The ZnGa2O4 spinel oxide system is integrated into mesoporous silica and different concns. of Cr3+ and Ni2+ are loaded in octahedral sites to obtain the highest emission intensity in the NIR-II region at 1285 nm via energy transfer from Cr3+ to Ni2+. Given that only a few nanophosphor systems with emission in the NIR-II region are available, this work attempts to establish emission in the NIR-II and NIR-I regions to obtain images in vivo with an increased penetration depth to 5 mm and improved clarity for bioimaging purposes. This system will open the door for biomedical research on other NIR-II nanophosphors.
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    13. 13
      Rajendran, V.; Chen, K.-C.; Huang, W.-T.; Kamiński, M.; Grzegorczyk, M.; Mahlik, S.; Leniec, G.; Lu, K.-M.; Wei, D.-H.; Chang, H. Unraveling Luminescent Energy Transfer Pathways: Futuristic Approach of Miniature Shortwave Infrared Light-Emitting Diode Design. ACS Energy Lett. 2023, 8 (5), 2395– 2400,  DOI: 10.1021/acsenergylett.3c00680
      13
      Unraveling Luminescent Energy Transfer Pathways: Futuristic Approach of Miniature Shortwave Infrared Light-Emitting Diode Design
      Rajendran, Veeramani; Chen, Kuan-Chun; Huang, Wen-Tse; Kaminski, Mikolaj; Grzegorczyk, Maciej; Mahlik, Sebastian; Leniec, Grzegorz; Lu, Kuang-Mao; Wei, Da-Hua; Chang, Ho; Liu, Ru-Shi
      ACS Energy Letters (2023), 8 (5), 2395-2400CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Phosphor-converted shortwave IR phosphor light-emitting diodes (pc-SWIR LEDs, 900-1700 nm) are promising next-generation portable light sources for spectroscopy, security, optical communication, and medical applications. A typical design strategy involves energy transfer from Cr3+ to Ni2+, and thus, energy transfer from Cr3+-Cr3+ pairs to Ni2+ ions is important but challenging. Here, we report a Sr1-xLaxAl5.92Cr0.08Ga6-xO19:xNi2+ (x = 0-0.09) series for the SWIR emissions range of 900-1600 nm due to an energy transfer from Cr3+ and Cr3+-Cr3+ pair to Ni2+. Short-range structural studies using ESR and magnetometry measurements reveal that Ni2+ ions likely exist as isolated Ni2+ ions and Cr3+-Ni2+ pairs rather than forming Ni2+-Ni2+ pairs. The fabricated prototype SWIR pc-LED delivers a radiant flux of 12.43 mW under a 350 mA driving current. This work provides insights into the codopant strategy for energy transfer and the design of promising next-generation SWIR phosphors.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3sXosFaqtLs%253D&md5=425b45843a995a39a8d8f192895dba74
    14. 14
      Fang, M.-H.; Hsueh, H.-P.; Vasudevan, T.; Huang, W.-T.; Bao, Z.; Majewska, N.; Mahlik, S.; Sheu, H.-S.; Liu, R.-S. Dual-Emission Eu-Doped Ca2–xSrxPN3 Nitridophosphate Phosphors Prepared by Hot Isostatic Press. J. Mater. Chem. C 2021, 9 (26), 8158– 8162,  DOI: 10.1039/D1TC01207J
      14
      Dual-emission Eu-doped Ca2-xSrxPN3 nitridophosphate phosphors prepared by hot isostatic press
      Fang, Mu-Huai; Hsueh, Hsi-Ping; Vasudevan, Thangaraji; Huang, Wen-Tse; Bao, Zhen; Majewska, Natalia; Mahlik, Sebastian; Sheu, Hwo-Shuenn; Liu, Ru-Shi
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2021), 9 (26), 8158-8162CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      Nitridophosphates are regarded as an emerging compd. class with applications in phosphor-converted light-emitting diodes. In this work, we aim to develop a series of Ca2-xSrxPN3:Eu2+ nitridophosphate phosphors through a solid-soln. strategy involving a hot isostatic press. Unexpected dual emissions in the red and IR regions are obsd., different from previous research results. The unique change in red and near-IR emission with the incorporation of Sr2+ is revealed and explained in detail through the structural anal. of the locally coordinated environment. Thermal properties including temp.-dependent photoluminescence and decay, as well as electron-lattice relaxation properties, are analyzed in detail. Furthermore, photobiomodulation based on the dual emission of Ca2-xSrxPN3:Eu2+ in the red and near-IR regions is proposed, demonstrating their potential practical applications. This study provides insight into the anal. of nitridophosphate phosphors and the development of novel luminescent materials.
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    15. 15
      Wang, H.; Xu, Y.; Pang, T.; Chen, B.; Xin, F.; Xing, M.; Tian, M.; Fu, Y.; Luo, X.; Tian, Y. Engineering Er3+-Sensitized Nanocrystals to Enhance NIR II-Responsive Upconversion Luminescence. Nanoscale 2022, 14 (3), 962– 968,  DOI: 10.1039/D1NR06945D
      15
      Engineering Er3+-sensitized nanocrystals to enhance NIR II-responsive upconversion luminescence
      Wang, Hong; Xu, Yang; Pang, Tao; Chen, Baojiu; Xin, Fangyun; Xing, Mingming; Tian, Meng; Fu, Yao; Luo, Xixian; Tian, Ying
      Nanoscale (2022), 14 (3), 962-968CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      An Er3+-sensitized system with a high response to 1550 nm radiation in the second near-IR window (NIR II) has been considered for a new class of potential candidates for applications in bio-imaging and advanced anti-counterfeiting, yet the achievement of highly efficient upconversion emission still remains a challenge. Here, we constructed a novel Er3+-sensitized core-shell-shell upconversion nanostructure with a Yb3+-enriched core as the emitting layer. This designed nanostructure allows the Yb3+ emitting layer to more efficiently trap and lock excitation energy by combining the interfacial energy transfer (IET) from the shell (Er3+) to the core (Yb3+), high activator Yb3+ content, and minimized energy back-transfer. As a result, the NIR II emission at 1000 nm is remarkably enhanced with a high quantum yield (QY) of 11.5%. Based on this trap and lock-in effect of the excitation energy in the Yb3+-enriched core, highly efficient 1550 nm-responsive visible and NIR upconversion emissions are also achieved by co-doping with other activator ions (e.g., Ho3+ and Tm3+). Our research provides a new functional design for improving NIR II-responsive upconversion luminescence, which is significant for developing practical applications.
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    16. 16
      Shafei, R.; Maganas, D.; Strobel, P. J.; Schmidt, P. J.; Schnick, W.; Neese, F. Electronic and Optical Properties of Eu2+-Activated Narrow-Band Phosphors for Phosphor-Converted Light-Emitting Diode Applications: Insights from a Theoretical Spectroscopy Perspective. J. Am. Chem. Soc. 2022, 144 (18), 8038– 8053,  DOI: 10.1021/jacs.2c00218
      16
      Electronic and Optical Properties of Eu2+-Activated Narrow-Band Phosphors for Phosphor-Converted Light-Emitting Diode Applications: Insights from a Theoretical Spectroscopy Perspective
      Shafei, Rami; Maganas, Dimitrios; Strobel, Philipp Jean; Schmidt, Peter J.; Schnick, Wolfgang; Neese, Frank
      Journal of the American Chemical Society (2022), 144 (18), 8038-8053CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      In this work, we present a computational protocol that is able to predict the exptl. absorption and emission spectral shapes of Eu2+-doped phosphors. The protocol is based on time-dependent d. functional theory and operates in conjunction with an excited-state dynamics approach. It is demonstrated that across the study set consisting of representative examples of nitride, oxo-nitride, and oxide Eu2+-doped phosphors, the energy distribution and the band shape of the emission spectrum are related to the nature of the 4f-5d transitions that are probed in the absorption process. Since the 4f orbitals are very nearly nonbonding, the decisive quantity is the covalency of the 5d acceptor orbitals that become populated in the electronically excited state that leads to emission. The stronger the (anti) bonding interaction between the lanthanide and the ligands is in the excited state, the larger will be the excited state distortion. Consequently, the corresponding emission will get broader due to the vibronic progression that is induced by the structural distortion. In addn., the energy sepn. of the absorption bands that are dominated by states with valence 4f-5d and a metal to ligand charge transfer character defines a measure for the thermal quenching of the studied Eu2+-doped phosphors. Based on this anal., simple descriptors are identified that show a strong correlation with the energy position and bandwidth of the exptl. emission bands without the need for elaborate calcns. Overall, we believe that this study serves as an important ref. for designing new Eu2+-doped phosphors with desired photoluminescence properties.
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    17. 17
      Qiao, J.; Zhou, G.; Zhou, Y.; Zhang, Q.; Xia, Z. Divalent Europium-Doped near-Infrared-Emitting Phosphor for Light-Emitting Diodes. Nat. Commun. 2019, 10 (1), 5267,  DOI: 10.1038/s41467-019-13293-0
      17
      Divalent europium-doped near-infrared-emitting phosphor for light-emitting diodes
      Qiao Jianwei; Zhou Guojun; Zhou Yayun; Zhang Qinyuan; Xia Zhiguo; Qiao Jianwei; Zhou Guojun; Xia Zhiguo
      Nature communications (2019), 10 (1), 5267 ISSN:.
      Near-infrared luminescent materials exhibit unique photophysical properties that make them crucial components in photonic, optoelectronic and biological applications. As broadband near infrared phosphors activated by transition metal elements are already widely reported, there is a challenge for next-generation materials discovery by introducing rare earth activators with 4f-5d transition. Here, we report an unprecedented phosphor K3LuSi2O7:Eu(2+) that gives an emission band centered at 740 nm with a full-width at half maximum of 160 nm upon 460 nm blue light excitation. Combined structural and spectral characterizations reveal a selective site occupation of divalent europium in LuO6 and K2O6 polyhedrons with small coordination numbers, leading to the unexpected near infrared emission. The fabricated phosphor-converted light-emitting diodes have great potential as a non-visible light source. Our work provides the design principle of near infrared emission in divalent europium-doped inorganic solid-state materials and could inspire future studies to further explore near-infrared light-emitting diodes.
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    18. 18
      Wang, Y.; Shang, M.; Huang, S.; Sun, Y.; Zhu, Y.; Xing, X.; Dang, P.; Lin, J. Continuous Ultra-Broadband Near-Infrared Sc2O3-Based Nanophosphor Realized by Spectral Bridge of Cr3+-Yb3+-Cr4+ for Multiple Optical Applications. Adv. Opt. Mater. 2023, 11 (19), 2300517  DOI: 10.1002/adom.202300517
      18
      Continuous Ultra-Broadband Near-Infrared Sc2O3-Based Nanophosphor Realized by Spectral Bridge of Cr3+-Yb3+-Cr4+ for Multiple Optical Applications
      Wang, Yining; Shang, Mengmeng; Huang, Shuai; Sun, Yixin; Zhu, Yiying; Xing, Xiaole; Dang, Peipei; Lin, Jun
      Advanced Optical Materials (2023), 11 (19), 2300517CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Cr3+-activated near-IR (NIR) phosphors are one of the most influential candidates for the new generation of intelligent NIR phosphor-converted light-emitting diode (pc-LED) light sources. For spectral anal., the broader the spectrum, the more useful information it contains. Herein, ultra-broadband Cr-activated Sc2O3 nanophosphors, which exhibit NIR ultra-broadband from 650 to 1600 nm, are developed by the sol-gel method. This ultra-broadband emission originates from the presence of Cr3+ and Cr4+, two luminescent centers confirmed by diffuse reflection spectra, ESR, time-resolved photoluminescence spectra, and temp.-dependent emission spectra. The nanophosphor exhibits excellent temp.-sensing performance with SA = 4.102% K-1 and SR = 1.855% K-1. Co-doping Yb3+ into Sc2O3 nanophosphors builds the Cr3+-Yb3+-Cr4+ model to bridge the spectral gap located at 1000 nm, forming a continuous NIR ultra-broadband spectrum ranging from 650 to 1600 nm for the first time. Moreover, the introduction of Yb3+ improves the thermal stability of nanophosphors from 29.49% to 50.65% at 150°C. The NIR transmission spectra of water, ethanol, and peanut oil demonstrate that Sc2O3:Cr3+-Yb3+-Cr4+ NIR nanophosphors favor potential applications in spectral anal. The construction of a Cr3+-Yb3+-Cr4+ spectral bridge for realizing NIR ultra-broadband phosphors based on cheap blue LEDs provides novel and effective insights.
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    19. 19
      Meng, M.; Zhang, T.; Wang, J.; Cheng, Z.; Liu, Y.; Qiao, X.; Wen, J.; Resch-Genger, U.; Long, W.; Ou, J. NaYF4:Yb3+/Tm3+@NaYF4:Yb3+ Upconversion Nanoparticles for Optical Temperature Monitoring and Self-Heating in Photothermal Therapy. ACS Appl. Nano Mater. 2023, 6 (1), 759– 771,  DOI: 10.1021/acsanm.2c05110
      19
      NaYF4:Yb3+/Tm3+@NaYF4:Yb3+ Upconversion Nanoparticles for Optical Temperature Monitoring and Self-Heating in Photothermal Therapy
      Meng, Mingzhou; Zhang, Tianmei; Wang, Jiaoyu; Cheng, Zhenlong; Liu, Yuanli; Qiao, Xin; Wen, Jian; Resch-Genger, Ute; Long, Wen; Ou, Jun
      ACS Applied Nano Materials (2023), 6 (1), 759-771CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
      The core-shell NaYF4:Yb3+/Tm3+@NaYF4:Yb3+ upconversion nanoparticles were successfully prepd. by a solvothermal method, and a layer of mesoporous silica (mSiO2) was successfully coated on the periphery of the core-shell nanoparticles to transform their surface from lipophilic to hydrophilic, further expanding their applications in biol. tissues. The phys. phase, morphol., structure, and fluorescence properties were characterized by X-ray diffraction (XRD), field emission transmission electron microscopy (TEM), Fourier IR spectroscopy (FT-IR), ζ potential anal., and fluorescence spectroscopy. It was found that the material has a hexagonal structure with good hydrophilicity and emits intense fluorescence under 980 nm pump laser excitation. The non-contact temp. sensing performance of nanoparticles was evaluated by analyzing the upconversion fluorescence of Tm3+ (1G4 → 3F4 and 3F3 → 3H6) in the temp. range of 284-344 K. The abs. and relative sensitivities were found to be 0.0067 K-1 and 1.08% K-1, resp., with high-temp. measurement reliability and good temp. cycling performance. More importantly, its temp. measurement in phosphate-buffered saline (PBS) soln. is accurate. In addn., the temp. of the cells can be increased by adjusting the laser power d. and laser irradn. time. Therefore, an optical temp. sensing platform was built to realize the application of real-time monitoring of cancer cell temp. and the dual function of photothermal therapy.
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    20. 20
      Qiao, J.; Zhang, S.; Zhou, X.; Chen, W.; Gautier, R.; Xia, Z. Near-Infrared Light-Emitting Diodes Utilizing a Europium-Activated Calcium Oxide Phosphor with External Quantum Efficiency of up to 54.7%. Adv. Mater. 2022, 34 (26), 2201887  DOI: 10.1002/adma.202201887
      20
      Near-Infrared Light-Emitting Diodes utilizing a Europium-Activated Calcium Oxide Phosphor with External Quantum Efficiency of up to 54.7%
      Qiao, Jianwei; Zhang, Shuai; Zhou, Xinquan; Chen, Weibin; Gautier, Romain; Xia, Zhiguo
      Advanced Materials (Weinheim, Germany) (2022), 34 (26), 2201887CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Near-IR (NIR) luminescence materials with broadband emissions are necessary for the development of light-emitting diodes (LEDs) based light sources. However, most known NIR-emitting materials are limited by their low external quantum efficiency. This work demonstrates how the photoluminescence quantum efficiency of europium-activated calcium oxide (CaO:Eu) NIR phosphor can be significantly improved and stabilized at operating temps. of LEDs. A carbon paper wrapping technol. is innovatively developed and used during the solid-state sintering to promote the redn. of Eu3+ into Eu2+. In parallel, the oxygen vacancies in the CaO lattice are repaired utilizing GeO2 decompn. Through this process, a record-high external quantum efficiency of 54.7% at 740 nm is obtained with a thermal stability greatly improved from 57% to 90% at 125°C. The as-fabricated NIR-LEDs reach record photoelec. efficiency (100 [email protected]%) and output power (100 mA @ 319.5 mW). This discovery of high-performance phosphors will open new research avenues for broadband NIR LED light sources in a variety of photonics applications.
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    21. 21
      Li, Y.; Xiong, P.; Lou, B.; Swart, H. C.; Ma, C.; Xia, Z. Near-Infrared Emission in Perovskite Oxides MZrO3:Bi (M = Ba, Sr) by Dopants’ Selective Site Occupancy. J. Lumin. 2023, 256, 119659  DOI: 10.1016/j.jlumin.2022.119659
      21
      Near-infrared emission in perovskite oxides MZrO3:Bi (M = Ba, Sr) by dopants' selective site occupancy
      Li, Yuanyuan; Xiong, Puxian; Lou, Bibo; Swart, Hendrik C.; Ma, Chonggeng; Xia, Zhiguo
      Journal of Luminescence (2023), 256 (), 119659CODEN: JLUMA8; ISSN:0022-2313. (Elsevier B.V.)
      Near-IR (NIR) emitting materials are essential for spectroscopic anal., biol. imaging and special lighting sources. Herein, broadband NIR emission is achieved in the Bi3+ activated perovskite oxides MZrO3 (M = Ba,Sr), and the luminescence mechanism was revealed using the combination of the exptl. and first-principles studies. The valence state of the Bi3+ ions was confirmed to be trivalent in the XPS spectra and the formation energy calcns. The possibility of NIR emission from lower state bismuth is excluded as the redn. atm. treatment, and therefore the selective occupancy of Bi3+ in the Zr4+ and M2+ sites enables the luminescence discrepancy in different atmospheres, where the Bi3+ on M2+ sites accounts for visible emission while Bi3+ on Zr4+ sites shows NIR emission. For the visible emission of Bi3+, the existence of Ba2+ vacancy shows remarkable influence on the emission spectra. This work not only provides a novel method to develop Bi-activated NIR emitting materials, but also helps to understand the intrinsic NIR luminescence mechanism of Bi3+ ions in inorg. compds.
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    22. 22
      Su, B.; Geng, S.; Xiao, Z.; Xia, Z. Highly Distorted Antimony(III) Chloride [Sb2Cl8]2– Dimers for Near-Infrared Luminescence up to 1070 nm. Angew. Chem., Int. Ed. 2022, 61 (33), e202208881  DOI: 10.1002/anie.202208881
      22
      Highly Distorted Antimony(III) Chloride [Sb2Cl8]2- Dimers for Near-Infrared Luminescence up to 1070 nm
      Su, Binbin; Geng, Shining; Xiao, Zewen; Xia, Zhiguo
      Angewandte Chemie, International Edition (2022), 61 (33), e202208881CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Zero-dimensional (0D) hybrid metal halides with unique compositional and structural tunability appear as an emerging class of luminescent materials, but near-IR (NIR) emitters therein are largely unexplored to date. This study presents three novel 0D hybrid antimony chlorines with edge-sharing [Sb2Cl8]2- dimers, showing unusual room-temp. broadband NIR emission with the max. emission wavelength up to 1070 nm. Photoluminescence studies and d. functional theory calcn. demonstrate that the emissions originate from the highly localized excitons, and that the confined [Sb2Cl8]2- dimers in these structures show low symmetry and a large degree of structural freedom. These hybrid antimony chlorines with [Sb2Cl8]2- dimers expand the range of new NIR materials in 0D metal halides.
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    23. 23
      Hsu, J.-Y.; Chung, R.-J.; Majewska, N.; Kreft, D.; Sheu, H.-S.; Lee, J.-F.; Mahlik, S.; Fang, M.-H. Probing Local Structural Changes by Sharp Luminescent Infrared Nanophosphor for Application in Light-Emitting Diodes. Chem. Mater. 2022, 34 (24), 11093– 11100,  DOI: 10.1021/acs.chemmater.2c03224
      23
      Probing Local Structural Changes by Sharp Luminescent Infrared Nanophosphor for Application in Light-Emitting Diodes
      Hsu, Jia-Yu; Chung, Ren-Jei; Majewska, Natalia; Kreft, Dominik; Sheu, Hwo-Shuenn; Lee, Jyh-Fu; Mahlik, Sebastian; Fang, Mu-Huai
      Chemistry of Materials (2022), 34 (24), 11093-11100CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Cr3+-doped IR phosphors are promising candidates for next-generation phosphor-converted IR light-emitting diodes (LEDs) because they can, in principle, tune and convert the luminescence spectra from an LED chip. However, most studies focus on broad-band Cr3+-doped phosphors, and the control mechanism of Cr3+-doped phosphors with sharp line emissions remains ambiguous. Here, we report LiGa5(1-x)Al5xO8:Cr3+ phosphors with sharp line emissions. The luminescence anal. reveals the subtle change of the local structure around Cr3+, which cannot be well resolved by X-ray diffraction. The deviation between the temp.-dependent photoluminescence and decay profile is introduced as well. Furthermore, the morphologies of LiGa5(1-x)Al5xO8:Cr3+ phosphors with high aluminum concn. demonstrate their great potential for mini-LED applications. Finally, an LED package is constructed, and it reveals the potential for angiog. applications. This study opens up a new understanding and perspective for Cr3+-doped sharp emission phosphors and reveals their potential for LED applications.
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    24. 24
      Majewska, N.; Muñoz, A.; Liu, R.-S.; Mahlik, S. Influence of Chemical and Mechanical Pressure on the Luminescence Properties of Near-Infrared Phosphors. Chem. Mater. 2023, 35 (12), 4680– 4690,  DOI: 10.1021/acs.chemmater.3c00203
      24
      Influence of Chemical and Mechanical Pressure on the Luminescence Properties of Near-Infrared Phosphors
      Majewska, Natalia; Munoz, Alfonso; Liu, Ru-Shi; Mahlik, Sebastian
      Chemistry of Materials (2023), 35 (12), 4680-4690CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The authors aim to compare the changes in the luminescence properties of Ga2O3:Cr3+ modified by Al or Sc ion substitution (chem. pressure) and hydrostatic pressure. The same behavior for Ga2-xAlxO3:Cr3+ and different behavior for Ga2-xScxO3:Cr3+ in terms of the optical properties under chem. and mech. pressure were found. Al substitution, which does not affect the chem. bond angles in the Cr3+ local environment and changes the crystal vol., like mech. pressure does are considered. As confirmed by Raman spectroscopy, the Sc ions cause lattice distortion and influence the chem. bond lengths and angles in the Cr3+ local environment. The energy structure diagrams of all levels of the d3 configuration of the Cr3+ ion as a function of pressure are calcd. by considering the pressure dependence of the Racah parameters. The energy structure diagrams presented in the paper show a decrease in the energy of the 2E, 2T1, and 2T2 excited levels with an increase in Dq. This does not align with the behavior predicted for these excited levels by the std. Tanabe-Sugano diagram. It seems correct that a high-pressure expt. involving Cr3+ and other transition metals should be interpreted using the method and diagrams presented herein.
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    25. 25
      Liu, B.-M.; Guo, X.-X.; Cao, L.-Y.; Huang, L.; Zou, R.; Zhou, Z.; Wang, J. A High-Efficiency Blue-LED-Excitable NIR-II-Emitting MgO:Cr3+,Ni2+ Phosphor for Future Broadband Light Source toward Multifunctional NIR Spectroscopy Applications. Chem. Eng. J. 2023, 452, 139313  DOI: 10.1016/j.cej.2022.139313
      25
      A High-efficiency blue-LED-excitable NIR-II-emitting MgO:Cr3+,Ni2+ phosphor for future broadband light source toward multifunctional NIR spectroscopy applications
      Liu, Bo-Mei; Guo, Xiao-Xuan; Cao, Lu-Yu; Huang, Lin; Zou, Rui; Zhou, Zhi; Wang, Jing
      Chemical Engineering Journal (Amsterdam, Netherlands) (2023), 452 (Part_2), 139313CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
      The development of phosphor-converted light-emitting diodes (pc-LEDs) in the second near-IR window (NIR-II, 1000-1700 nm) represents an important, newly emerging, and dynamic field in NIR spectroscopy. Unfortunately, lacking efficient NIR-II phosphors that can be excited by com. blue LED chips impedes NIR spectroscopy applications. Herein, a donor-acceptor strategy is developed by introducing Cr3+ sensitizers to greatly enhance the blue-light excitation efficiency of NIR-II-emitting MgO:Cr3+,Ni2+ phosphor. Consequently, a high-efficiency blue-light-excitable MgO:Cr3+,Ni2+ phosphor that gives a NIR-II emission at 1335 nm with a full width at half-max. (FWHM) of 235 nm, is demonstrated. The product has a high internal quantum efficiency of 92.7 % and excellent thermal stability, maintaining 83.0 % of the room temp. emission intensity at 150 °C. Excitingly, the fabricated NIR-II pc-LED device shows a high NIR-II optical power (27.4 mW@350 mA). The performances of the achieved NIR-II pc-LED are almost the best results until now. Addnl., multifunctional applications including nondestructive detection and anti-counterfeiting of the NIR-II light source are demonstrated. These results are pretty crucial for the further development of NIR-II spectroscopy and imaging technol.
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    26. 26
      Yan, Y.; Shang, M.; Huang, S.; Wang, Y.; Sun, Y.; Dang, P.; Lin, J. Photoluminescence Properties of AScSi2O6:Cr3+ (A = Na and Li) Phosphors with High Efficiency and Thermal Stability for Near-Infrared Phosphor-Converted Light-Emitting Diode Light Sources. ACS Appl. Mater. Interfaces 2022, 14 (6), 8179– 8190,  DOI: 10.1021/acsami.1c23940
      26
      Photoluminescence Properties of AScSi2O6:Cr3+ (A = Na and Li) Phosphors with High Efficiency and Thermal Stability for Near-Infrared Phosphor-Converted Light-Emitting Diode Light Sources
      Yan, Yu; Shang, Mengmeng; Huang, Shuai; Wang, Yining; Sun, Yixin; Dang, Peipei; Lin, Jun
      ACS Applied Materials & Interfaces (2022), 14 (6), 8179-8190CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Near-IR (NIR) phosphors are fascinating photoluminescence materials with applications in phosphor-converted light-emitting diodes (pc-LEDs) for night vision lighting, which are still restricted by low efficiency and thermal stability in the current research stage. In this work, AScSi2O6 (A = Na/Li) are chosen as hosts due to a larger band gap and a single octahedral site for Cr3+ doping. The NIR-emitting Cr3+-activated AScSi2O6:Cr3+ phosphors were successfully prepd. by a common high-temp. solid-state method. X-ray diffraction and Rietveld refinement confirm that the Cr3+ prefers to enter the Sc3+-octahedral lattice site in the AScSi2O6 structure. Under blue light excitation, AScSi2O6:Cr3+ phosphors exhibit broadband NIR emission from 700 to 1100 nm with a full width at half-max. of ~ 150 nm owing to the 4T2 → 4A2 electron transition of Cr3+. The photoluminescence properties were enhanced by adjusting the fluxes and sintering conditions, and highly efficient LiScSi2O6:Cr3+ NIR phosphors with external quantum efficiencies of 33.4% were obtained. Moreover, the optimized LiScSi2O6:Cr3+ exhibits excellent thermal stability (75% at 150°C) with an activation energy of 0.33 eV. Importantly, the fabricated NIR pc-LED with the highly efficient LiScSi2O6:Cr3+ phosphor demonstrates brighter NIR light and a higher luminous efficacy than the NaScSi2O6:Cr3+ phosphor in night vision.
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    27. 27
      Jiang, H.; Chen, L.; Zheng, G.; Luo, Z.; Wu, X.; Liu, Z.; Li, R.; Liu, Y.; Sun, P.; Jiang, J. Ultra-Efficient GAGG:Cr3+ Ceramic Phosphor-Converted Laser Diode: A Promising High-Power Compact Near-Infrared Light Source Enabling Clear Imaging. Adv. Opt. Mater. 2022, 10 (11), 2102741  DOI: 10.1002/adom.202102741
      27
      Ultra-efficient Aluminum gadolinium gallium oxide:chromium ceramic phosphor-converted laser diode: a promising high-power compact near-infrared light source enabling clear imaging
      Jiang, Hangjie; Chen, Liyan; Zheng, Guojun; Luo, Zhaohua; Wu, Xianhui; Liu, Zehua; Li, Ruiyang; Liu, Yongfu; Sun, Peng; Jiang, Jun
      Advanced Optical Materials (2022), 10 (11), 2102741CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Near-IR (NIR) light is widely used in fields such as org. components detection and biol. imaging, due to its strong tissue penetration and non-destructive characteristics. To improve the detection sensitivity and spatial resoln. of bioimaging, it is important to obtain a new NIR light source with good thermal stability, high efficiency, and ultra-high optical output power, which currently remains a great challenge. In view of this, a Gd3Al2Ga3O12:Cr3+ (GAGG:Cr3+) NIR ceramic phosphor is proposed in this study. By optimizing the prepn. process of GAGG:Cr3+ ceramic phosphor and the concn. of Cr3+ ions, high external quantum efficiency (EQE = 70.1%) and excellent thermal stability (96.8%@150°C) are achieved. After coupling the GAGG:Cr3+ ceramic phosphor with a blue light-emitting diode (LED), the electro-optical conversion efficiency of the NIR ceramic phosphor converted LED (NIR-cpc-LED) reaches 31%@20 mA, which is higher than the previously reported values. Moreover, when coupled with a blue laser diode (LD), the optical output power of the NIR-cpc-LD reaches as high as 1652.6 [email protected] W. Therefore, GAGG:Cr3+ NIR-cpc-LD not only shows a great prospect in practical applications, but also provides a new soln. for high-power compact NIR light sources.
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    28. 28
      Back, M.; Ueda, J.; Brik, M. G.; Lesniewski, T.; Grinberg, M.; Tanabe, S. Revisiting Cr3+-Doped Bi2Ga4O9 Spectroscopy: Crystal Field Effect and Optical Thermometric Behavior of Near-Infrared-Emitting Singly-Activated Phosphors. ACS Appl. Mater. Interfaces 2018, 10 (48), 41512– 41524,  DOI: 10.1021/acsami.8b15607
      28
      Revisiting Cr3+-Doped Bi2Ga4O9 Spectroscopy: Crystal Field Effect and Optical Thermometric Behavior of Near-Infrared-Emitting Singly-Activated Phosphors
      Back, Michele; Ueda, Jumpei; Brik, Mikhail G.; Lesniewski, Tadeusz; Grinberg, Marek; Tanabe, Setsuhisa
      ACS Applied Materials & Interfaces (2018), 10 (48), 41512-41524CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The increasing interest in the development of ratiometric optical thermal sensors led to a wide variety of new systems with promising properties. Among them, singly-doped ratiometric thermometers were recently demonstrated to be particularly reliable. With the aim to discuss the development of an ideal optical thermal sensor, a combined exptl. and theor. insight into the spectroscopy of the Bi2Ga4O9:Cr3+ system is reported showing the importance of an insightful anal. in a wide temp. range. Low-temp. luminescence anal. (from 10 K) and the temp. dependence of the lifetime study, together with the crystal field anal. and the modeling of the thermal quenching process, allow the estn. of key parameters such as the Debye temp. (cutoff frequency), the Huang-Rhys parameter, and the energy barrier between 2Eg and 4T2g. Addnl., by considering the reliable class of singly-doped ratiometric thermometers based on a couple of excited states obeying the Boltzmann law, the important role played by the abs. sensitivity was discussed and the great potential of Cr3+ singly-activated systems was demonstrated. The results may provide new guidelines for the design of reliable optical thermometers with outstanding and robust performances.
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    29. 29
      Zhou, H.; Cai, H.; Zhao, J.; Song, Z.; Liu, Q. Crystallographic Control for Cr4+ Activators toward Efficient NIR-II Luminescence. Inorg. Chem. Front. 2022, 9 (9), 1912– 1919,  DOI: 10.1039/D2QI00217E
      29
      Crystallographic control for Cr4+ activators toward efficient NIR-II luminescence
      Zhou, Hao; Cai, Hao; Zhao, Jing; Song, Zhen; Liu, Quanlin
      Inorganic Chemistry Frontiers (2022), 9 (9), 1912-1919CODEN: ICFNAW; ISSN:2052-1553. (Royal Society of Chemistry)
      Broadband near-IR (NIR) emitting phosphors have attracted great interest due to their potential applications in non-destructive examn. and bioimaging. However, most of the reported broadband NIR phosphors emit in the NIR-I region with a wavelength shorter than 950 nm, while rare-earth activated NIR-II phosphors can hardly meet the requirements because of their sharp emission. Herein, we successfully synthesized the broadband NIR-II phosphor Li2ZnGeO4:Cr4+. By employing the all-tetrahedron-built matrix, all the Cr ions are stabilized in the tetravalent state due to crystallog. control. This phosphor shows wide absorption from the red to near-IR region. Under 646 nm excitation, it exhibits broadband NIR-II emission peaking at 1218 nm with an FWHM of 220 nm at room temp. We also demonstrated the potential applications of Li2ZnGeO4:Cr4+ as an NIR-II light source in non-destructive examn. and bioimaging. This work provides a new strategy for exploring broad-band NIR-II luminescent materials.
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    30. 30
      Wang, D.; Zhang, X.; Wang, X.; Leng, Z.; Yang, Q.; Ji, W.; Lin, H.; Zeng, F.; Li, C.; Su, Z. Investigation of the Structural and Luminescent Properties and the Chromium Ion Valence of Li2CaGeO4 Crystals Doped with Cr4+ Ions. Crystals 2020, 10 (11), 1019,  DOI: 10.3390/cryst10111019
      30
      Investigation of the structural and luminescent properties and the chromium ion valence of Li2CaGeO4 crystals doped with Cr4+ ions
      Wang, Dongmei; Zhang, Xiaowei; Wang, Xinyu; Leng, Zhuang; Yang, Qianqian; Ji, Wen; Lin, Hai; Zeng, Fanming; Li, Chun; Su, Zhongmin
      Crystals (2020), 10 (11), 1019CODEN: CRYSBC; ISSN:2073-4352. (MDPI AG)
      Herein, we report on the growth of Cr4+-Li2CaGeO4 crystals by the flux growth method from the flux of LiCl, as well as on the effect of doping Li2CaGeO4 with Cr4+ ions on the NIR region spectral properties and crystal structure. The results quantified the occupancy of Cr4+ in Ge4+ sites. The emission spectrum presented broad bands in the NIR region, i.e., 1000-1500 nm excited by 980 nm, with max. peaks at 1200 nm at room temp. caused by the transition of 3T2→3A2 in Cr4+ ions. The lifetime decreased with the Cr4+ ion doping concn., specifically from 14.038 to 12.224 ms. The chem. compn. and the valence state of chromium in Li2CaGeO4 were analyzed using XPS, which showed that the chromium in Li2CaGeO4 was tetravalent and no trivalent chromium was found. Therefore, the Cr4+-Li2CaGeO4 crystal has a great potential and future in optical applications.
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    31. 31
      Hömmerich, U.; Shen, Y.; Bray, K. High-Pressure Luminescence Studies of Cr4+-Doped Laser Materials. J. Lumin. 1997, 72–74, 139– 140,  DOI: 10.1016/S0022-2313(97)00053-7
      There is no corresponding record for this reference.
    32. 32
      Kuleshov, N. V.; Mikhailov, V. P.; Scherbitsky, V. G.; Minkov, B. I.; Glynn, T. J.; Sherlock, R. Luminescence Study of Cr4+-Doped Silicates. Opt. Mater. 1995, 4 (4), 507– 513,  DOI: 10.1016/0925-3467(94)00120-0
      32
      Luminescence study of Cr4+ -doped silicates
      Kuleshov, N.V.; Mikhailov, V.P.; Scherbitsky, V.G.; Minkov, B.I.; Glynn, T.J.; Sherlock, R.
      Optical Materials (Amsterdam, Netherlands) (1995), 4 (4), 507-13CODEN: OMATET; ISSN:0925-3467.
      The results of optical absorption, luminescence, and lifetime measurements on Cr-doped Y2SiO5 and Gd2SiO5 crystals are reported. The dominant absorption and emission bands in both crystals are assigned to transitions on the Cr4+ ion in distorted tetrahedral sites. An addnl. near IR emitting center was obsd. in Y2SiO5.
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    33. 33
      Shen, Y.; Riedener, T.; Bray, K. L. Effect of Pressure on Site-Symmetry Distortions of Mn5+ and Cr4+ in Y2SiO5. Phys. Rev. B 2000, 61 (14), 9277– 9286,  DOI: 10.1103/PhysRevB.61.9277
      33
      Effect of pressure on site-symmetry distortions of Mn5+ and Cr4+ in Y2SiO5
      Shen, Yongrong; Riedener, Toni; Bray, Kevin L.
      Physical Review B: Condensed Matter and Materials Physics (2000), 61 (14), 9277-9286CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      High pressure is used to study the effects of nontetrahedral site distortions on luminescence properties of Cr4+ and Mn5+ in Y2SiO5. Luminescence spectra and lifetime measurements of Mn5+:Y2SiO5 as a function of pressure ≤120 kbar indicated that the local Mn5+ site distortion increased strongly at .ltorsim.50 kbar and increased only slightly at higher pressure. A crystal field energy level calcn. that included spin-orbit coupling effects was completed to quant. model the luminescence and lifetime data in the context of a D2d distortion model. The model provided excellent agreement with the spectral and lifetime data and showed that the 3T1 state has a significant influence on the lifetime of 1E luminescence. Luminescence studies of Cr4+:Y2SiO5 revealed a red shift at .ltorsim.40 kbar followed by a blue shift at higher pressure. These results are consistent with a pronounced increase in the Cr4+ site distortion at .ltorsim.40 kbar that levels off at higher pressures. The Cr4+:Y2SiO5 results also support assignment of the ambient-pressure luminescence spectrum to a transition from orbital components of the 3T2 state to the 3A2 ground state. No evidence of a 3T2-1E electronic crossover in Cr4+:Y2SiO5 was obsd. at ≤120 kbar. The results indicate more generally that the luminescence of 3d2 systems is controlled by competing cubic crystal field and distortion effects. The ability of modest pressures to alter the relative importance of these effects suggests that cubic crystal field and distortions vary widely from host lattice to host lattice at ambient pressure and are responsible for the complicated optical properties of 3d2 ions.
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    34. 34
      Nanai, Y.; Ishida, R.; Urabe, Y.; Nishimura, S.; Fuchi, S. Octave-Spanning Broad Luminescence of Cr3+, Cr4+-Codoped Mg2SiO4 Phosphor for Ultra-Wideband near-Infrared LEDs. Jpn. J. Appl. Phys. 2019, 58, SFFD02,  DOI: 10.7567/1347-4065/ab0b9f
      There is no corresponding record for this reference.
    35. 35
      Shen, Y.; Bray, K. L. Influence of Low-Symmetry Distortions on the Luminescence of Cr4+-Doped Forsterite. Phys. Rev. Lett. 2000, 84 (17), 3990– 3993,  DOI: 10.1103/PhysRevLett.84.3990
      35
      Influence of Low-Symmetry Distortions on the Luminescence of Cr4+-Doped Forsterite
      Shen, Yongrong; Bray, Kevin L.
      Physical Review Letters (2000), 84 (17), 3990-3993CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      By using pressure to vary the extent of nontetrahedral distortions of the Cr4+ site in Mg2SiO 4, the authors reveal the important influence of the T13 state on the emission properties of the T23 state. T13- T23 mixing has a pronounced effect on the line shape and radiative decay rate of emission from the T23 state and the extent of mixing depends critically on the magnitude of nontetrahedral distortions. The results provide an explanation for the wide variation of Cr4+ emission properties in different host lattices at ambient pressure and indicate that the tailoring of asym. distortions of luminescent centers represents an effective new strategy for tuning the linewidth of spectral transitions.
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    36. 36
      Hazenkamp, M. F.; Güdel, H. U.; Atanasov, M.; Kesper, U.; Reinen, D. Optical Spectroscopy of Cr4+-Doped Ca2GeO4 and Mg2SiO4. Phys. Rev. B 1996, 53 (5), 2367– 2377,  DOI: 10.1103/PhysRevB.53.2367
      36
      Optical spectroscopy of Cr4+-doped Ca2GeO4 and Mg2SiO4
      Hazenkamp, M. F.; Guedel, H. U.; Atanasov, M.; Kesper, U.; Reinen, D.
      Physical Review B: Condensed Matter (1996), 53 (5), 2367-77CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      Polarized single crystal absorption spectra and luminescence spectra of Cr4+-doped Ca2GeO4 and Mg2SiO4 are presented and discussed. The absorption spectra are analyzed using the angular overlap model (AOM). The agreement between the exptl. and the AOM-calcd. energies of the ligand field states is satisfactory. The ligand field parameters 10Dq and B are 8950 cm-1 and 540 cm-1 for Ca2GeO4 and 10 100 cm-1 and 560 cm-1 for Mg2SiO4, resp.,. The 3A2 → 1A1 transition appears as a Fano antiresonance in the spectrum of Cr4+-doped Mg2SiO4. The luminescence spectra are assigned to a transition from an excited triplet state, which is definitely not mixed with 1E contributions in Ca2GeO4. The luminescence spectra are surprisingly sharp. Possible explanations are discussed. The luminescence of Cr4+-doped Ca2GeO4 is much less quenched at room temp. than in the Mg2SiO4 host. Finally, the spectroscopic properties of Cr4+-doped materials are compared with those of other 3d2 ions in tetroxo coordination.
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    37. 37
      Zhang, J.; Luo, L.; Zhao, W.; Zhang, W.; Hu, Z. Luminescence Properties and Spectral Modulation of the Ultra-Broadband, Highly Efficient near-Infrared Luminescent Material CaGa4O7:Cr3+, Cr4+, Yb3+. Opt. Mater. 2023, 135, 113388  DOI: 10.1016/j.optmat.2022.113388
      37
      Luminescence properties and spectral modulation of the ultra-broadband, highly efficient near-infrared luminescent material CaGa4O7:Cr3+, Cr4+, Yb3+
      Zhang, Jian; Luo, Li; Zhao, Weiren; Zhang, Wei; Hu, ZhengFa
      Optical Materials (Amsterdam, Netherlands) (2023), 135 (), 113388CODEN: OMATET; ISSN:0925-3467. (Elsevier B.V.)
      Recently, near-IR phosphors have received a lot of attention due to their wide application in the field of detection and anal., and it is urgent to develop efficient broadband NIR phosphors. In this paper, a conventional high-temp. solid-phase method was used to prep. a novel near-IR phosphor CaGa4O7:Cr3+, Cr4+, which provides two emission bands ranging from 600 to 1000 nm and 1100-1600 nm. Although there are only four and five coordination sites in the CaGa4O7 (CGO) lattice, the doping of the Cr3+ ions in the CGO causes the Ga-O tetrahedral distortion into an octahedron, providing a favorable environment for the emission of the Cr3+ ions. Under blue light excitation, Cr3+ emits luminescence in the NIR I region from 600 to 1000 nm with a half-height width of up to 148 nm. With increasing Cr ion concn., more and more Cr4+ preferentially occupy the four coordinated Ga3+ sites, and the intensity of the ultra-broad emission band in the NIR II region (1100 nm-1600 nm) increases with a full width at half max. (FWHM) of 262 nm. To further optimize the emission spectrum, Yb3+ was codoped into CGO: Cr3+, Cr4+, the energy transfers from Cr3+ to Yb3+ in CGO was found, thus filling the gap between 1000 and 1100 nm in the CGO emission spectrum and obtaining a more complete ultra-wide spectrum. Moreover, the luminescence mechanism, energy transfer efficiency, and quantum yield are also studied in detail, and the ability of CaGa4O7:Cr3+, Cr4+, and Yb3+ luminescent materials to achieve ultra-broadband efficient near-IR emission was demonstrated.
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    38. 38
      Ubaldini, A.; Carnasciali, M. M. Raman Characterisation of Powder of Cubic RE2O3 (RE = Nd, Gd, Dy, Tm, and Lu), Sc2O3 and Y2O3. J. Alloys Compd. 2008, 454 (1), 374– 378,  DOI: 10.1016/j.jallcom.2006.12.067
      38
      Raman characterisation of powder of cubic RE2O3 (RE=Nd, Gd, Dy, Tm, and Lu), Sc2O3 and Y2O3
      Ubaldini, Alberto; Carnasciali, Maria Maddalena
      Journal of Alloys and Compounds (2008), 454 (1-2), 374-378CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      Raman spectra of cubic Sc2O3, Y2O3 and C-type rare earth oxides are presented. Cubic Nd2O3 is prepd. by thermal decompn. of its hydroxide. The decompn. mechanism happens in 2 steps. At the end of this process cubic Nd2O3 form. All Raman spectra are characterized by the presence of a very strong band in the range 320-420 cm-1, depending on the lattice parameter of each oxide. Frequency of this line decreases increasing the cell const., meaning that the structure becomes less rigid. A simply approach is used to give a consistent evaluation of the force const. of each oxide. The position of the low wave no. bands depends mainly on the cationic mass, meaning that these bands are not due to stretching vibrations, but rather to bending vibrations.
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    39. 39
      Chikalla, T. D.; McNeilly, C. E.; Roberts, F. P. Polymorphic Modifications of Pm2O3. J. Am. Ceram. Soc. 1972, 55 (8), 428– 429,  DOI: 10.1111/j.1151-2916.1972.tb11329.x
      39
      Polymorphic modifications of promethium oxide (Pm2O3)
      Chikalla, T. D.; McNeilly, C. E.; Roberts, F. P.
      Journal of the American Ceramic Society (1972), 55 (8), 428-9CODEN: JACTAW; ISSN:0002-7820.
      Pm2O3 did not crystallize when heated in air for 1 week at 500 and 600° but formed cubic crystals at 650-750°, monoclinic (B) crystals ≥800°, and hexagonal crystals (A) at 1600-1800°. Monoclinic Pm2O3 had a structure isomorphous with Nd2O3, based on x-ray diffraction data. Upper phase-transformation temps. for Pm2O3 contg. 2% Sm were B ↹ A (1740 ± 20°), A ↹ H (2135 ± 20°), H ↹ X (2225 ± 20°), and X ↹ melt transition (2320 ± 20°), where H and X are high-temp. polymorphs analogous to those of Sm2O3.
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    40. 40
      Eyring, L.; Gschneidner, K. A.; Lander, G. H. Handbook on the Physics and Chemistry of Rare Earths; Elsevier: 2002.
      There is no corresponding record for this reference.
    41. 41
      Tang, M.; Lu, P.; Valdez, J. A.; Sickafus, K. E. Ion-Irradiation-Induced Phase Transformation in Rare Earth Sesquioxides (Dy2O3,Er2O3,Lu2O3). J. Appl. Phys. 2006, 99 (6), 063514  DOI: 10.1063/1.2184433
      41
      Ion-irradiation-induced phase transformation in rare earth sesquioxides (Dy2O3, Er2O3, Lu2O3)
      Tang, M.; Lu, P.; Valdez, J. A.; Sickafus, K. E.
      Journal of Applied Physics (2006), 99 (6), 063514/1-063514/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Polycryst. pellets of cubic C-type rare earth structure (Ia3) Dy2O3, Er2O3, and Lu2O3 were irradiated at cryogenic temp. (120 K) with 300 keV Kr2+ to a max. fluence of 1 × 1020 Kr/m2. Irradiated specimens were examd. using grazing incidence x-ray diffraction and TEM. Ion irradn. leads to different radiation effects in these 3 materials. First, Dy2O3 begins to transform to a monoclinic B-type rare earth structure (C2/m) at a peak dose of ∼5 displacements per atom (dpa), (corresponding to a fluence of 2 × 1019 Kr/m2). This transformation is nearly complete at a peak dose of 25 dpa (a fluence of 1 × 1020 Kr/m2). Er2O3 also transforms to the B-type structure, but the transformation starts at a higher irradn. dose of ∼15-20 dpa [a fluence of about (6-8)×1019 Kr/m2]. Lu2O3 was found to maintain the C-type structure even at the highest irradn. dose of 25 dpa (a fluence of 1 × 1020 Kr/m2). No C-to-B transformation was obsd. in Lu2O3. The irradn. dose dependence of the C-to-B phase transformation obsd. in Dy2O3, Er2O3, and Lu2O3 is closely related to the temp. dependence of the C-to-B phase transformation found in phase diagrams for these 3 materials.
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    42. 42
      Foex, M.; Traverse, J. Investigation about Crystalline Transformation in Rare Earths Sesquioxides at High Temperatures. Rev. Int. High Temp. Refract. 1966, 3, 429
      42
      Investigations about crystalline transformation in rare earths sesquioxides at high temperatures
      Foex, Marc; Traverse, Jean P.
      Revue Internationale des Hautes Temperatures et des Refractaires (1966), 3 (4), 429-53CODEN: RIHTAV; ISSN:0035-3434.
      The title transformations were studied by D.T.A. and x-ray diffraction at ≤2400°. New cryst. forms of rare earth sesquioxides were detected and identified, but they could not be obtained at room temp., even with very strong quenching. In addn. to the classical varieties, A (hexagonal), B (monoclinic), and C (cubic), 2 new forms are found, 1 hexagonal, different from A, which is exhibited by all the sesquioxides, Lu2O3 excepted, and another cubic form, different from C, which exists at the beginning of the series, i.e. from La2O3 to Ga2O3.
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    43. 43
      Yusa, H.; Tsuchiya, T.; Sata, N.; Ohishi, Y. High-Pressure Phase Transition to the Gd2S3 Structure in Sc2O3: A New Trend in Dense Structures in Sesquioxides. Inorg. Chem. 2009, 48 (16), 7537– 7543,  DOI: 10.1021/ic9001253
      43
      High-Pressure Phase Transition to the Gd2S3 structure in Sc2O3: A New Trend in Dense Structures in Sesquioxides
      Yusa, Hitoshi; Tsuchiya, Taku; Sata, Nagayoshi; Ohishi, Yasuo
      Inorganic Chemistry (2009), 48 (16), 7537-7543CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      In situ x-ray diffraction expts. using a laser-heated diamond anvil cell revealed a novel dense phase with the Gd2S3 structure stabilizing in Sc2O3 at pressures over 19 GPa. Although no phase transformation was induced during room-temp. compression up to 31 GPa, the C rare earth sesquioxide structure transformed into the B rare earth sesquioxide structure at 10 GPa after laser annealing and subsequently into the Gd2S3 structure at 19 GPa. Neither the A rare earth sesquioxide structure nor the U2S3 structure was found in Sc2O3. Static d. functional lattice energy calcns. demonstrated that the C structure prefers Gd2S3 over U2S3 as the post phase. Sc2O3 is the 2nd sesquioxide, after In2O3, to crystallize into a Gd2S3 structure at high pressures and high temps.
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    44. 44
      Liu, D.; Lei, W.; Li, Y.; Ma, Y.; Hao, J.; Chen, X.; Jin, Y.; Liu, D.; Yu, S.; Cui, Q.; Zou, G. High-Pressure Structural Transitions of Sc2O3 by X-Ray Diffraction, Raman Spectra, and Ab Initio Calculations. Inorg. Chem. 2009, 48 (17), 8251– 8256,  DOI: 10.1021/ic900889v
      44
      High-Pressure Structural Transitions of Sc2O3 by X-ray Diffraction, Raman Spectra, and Ab Initio Calculations
      Liu, Dan; Lei, Weiwei; Li, Yinwei; Ma, Yanming; Hao, Jian; Chen, Xiaohui; Jin, Yunxia; Liu, Dedi; Yu, Shidan; Cui, Qiliang; Zou, Guangtian
      Inorganic Chemistry (2009), 48 (17), 8251-8256CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The high-pressure behavior of scandium oxide (Sc2O3) has been investigated by angle-dispersive synchrotron powder X-ray diffraction and Raman spectroscopy techniques in a diamond anvil cell up to 46.2 and 42 GPa. An irreversible structural transformation of Sc2O3 from the cubic phase to a monoclinic high-pressure phase was obsd. at 36 GPa. Subsequent ab initio calcns. for Sc2O3 predicted the phase transition from the cubic to monoclinic phase but at a much lower pressure. The same calcns. predicted a second phase transition at 77 GPa from the monoclinic to hexagonal phase.
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    45. 45
      Ovsyannikov, S. V.; Bykova, E.; Bykov, M.; Wenz, M. D.; Pakhomova, A. S.; Glazyrin, K.; Liermann, H.-P.; Dubrovinsky, L. Structural and Vibrational Properties of Single Crystals of Scandia, Sc2O3 under High Pressure. J. Appl. Phys. 2015, 118 (16), 165901,  DOI: 10.1063/1.4933391
      45
      Structural and vibrational properties of single crystals of Scandia, Sc2O3 under high pressure
      Ovsyannikov, Sergey V.; Bykova, Elena; Bykov, Maxim; Wenz, Michelle D.; Pakhomova, Anna S.; Glazyrin, Konstantin; Liermann, Hanns-Peter; Dubrovinsky, Leonid
      Journal of Applied Physics (Melville, NY, United States) (2015), 118 (16), 165901/1-165901/8CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      The authors report the results of single-crystal x-ray diffraction and Raman spectroscopy studies of Sc oxide, Sc2O3, at ambient temp. under high pressure up to 55 and 28 GPa, resp. Both x-ray diffraction and Raman studies indicated a phase transition from the cubic bixbyite phase (so-called C-Res phase) to a monoclinic C2/m phase (so-called B-Res phase) at pressures around 25-28 GPa. The transition was accompanied by a significant volumetric drop by ∼6.7%. The Raman spectroscopy detected a minor crossover around 10-12 GPa, which manifested in the appearance of new and disappearance of some Raman modes, as well as in softening of one Raman mode. The authors found the bulk modulus values of the both C-Res and B-Res phases as B0 = 198.2(3) and 171.2(1) GPa (for fixed B' = 4), resp. Thus, the denser high-pressure lattice of Sc2O3 is much softer than the original lattice. Possible mechanisms that might be responsible for the pronounced elastic softening in the monoclinic high-pressure phase in this simple oxide with an ultra-wide band gap are discussed. (c) 2015 American Institute of Physics.
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    46. 46
      Huber, G.; Payne, S. A.; Chase, L. L.; Krupke, W. F. Optical Spectroscopy of Cr3+ in ScF3 and Sc2O3. J. Lumin. 1988, 39 (5), 259– 268,  DOI: 10.1016/0022-2313(88)90024-5
      46
      Optical spectroscopy of chromium(3+) in scandium trifluoride and scandium sesquioxide
      Huber, G.; Payne, Stephen A.; Chase, L. L.; Krupke, William F.
      Journal of Luminescence (1988), 39 (5), 259-68CODEN: JLUMA8; ISSN:0022-2313.
      Absorption, excitation, and emission spectra, and the emission decay measurements are described for Cr3+ in ScF3 and Sc2O3. Both of these materials have 6-fold coordinated metal sites that are available for substitution by Cr3+. From the anal. of the broadband spectra and the zero-phonon lines, information was derived about the a1g, eg and the odd-parity intensity-enabling coordinates. The quenching behavior of the Cr3+ luminescence was explored. The effect is discussed of the next nearest neighbor cations on the crystal field splitting of Cr3+.
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    47. 47
      Kück, S.; Fornasiero, L.; Mix, E.; Huber, G. Spectroscopic Properties of Cr-Doped Sc2O3. J. Lumin. 2000, 87–89, 1122– 1125,  DOI: 10.1016/S0022-2313(99)00563-3
      There is no corresponding record for this reference.
    48. 48
      Brik, M. G.; Avram, N. M. Crystal Field Analysis and Electron-Phonon Coupling in Sc2O3:Cr3+. Z. Für Naturforschung A 2004, 59 (11), 799– 803,  DOI: 10.1515/zna-2004-1113
      There is no corresponding record for this reference.
    49. 49
      Fang, M.-H.; Chen, K.-C.; Majewska, N.; Leśniewski, T.; Mahlik, S.; Leniec, G.; Kaczmarek, S. M.; Yang, C.-W.; Lu, K.-M.; Sheu, H.-S.; Liu, R.-S. Hidden Structural Evolution and Bond Valence Control in Near-Infrared Phosphors for Light-Emitting Diodes. ACS Energy Lett. 2021, 6 (1), 109– 114,  DOI: 10.1021/acsenergylett.0c02373
      49
      Hidden Structural Evolution and Bond Valence Control in Near-Infrared Phosphors for Light-Emitting Diodes
      Fang, Mu-Huai; Chen, Kuan-Chun; Majewska, Natalia; Lesniewski, Tadeusz; Mahlik, Sebastian; Leniec, Grzegorz; Kaczmarek, Slawomir M.; Yang, Chia-Wei; Lu, Kuang-Mao; Sheu, Hwo-Shuenn; Liu, Ru-Shi
      ACS Energy Letters (2021), 6 (1), 109-114CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      We aim to conduct a complete study on the unexpected structure evolution behavior in Cr3+-doped phosphors. A series of Ga2-xScxO3:Cr3+ phosphors are successfully synthesized and confirmed through structural studies, while the lattice parameters change unexpectedly. The unique partial substitution (~ 87%) of Sc3+ in the octahedral site is demonstrated via Rietveld refinement. Therefore, the bond valence sum calcn. explains the reason for this particular Sc3+ concn. The photoluminescent bandwidth and electron-lattice coupling energy initially increase and then decrease, implying an inhomogeneous broadening effect. Time-resolved spectra and ESR are utilized to further examine the subtle change in the microstructures and the second coordination sphere effect of Cr3+. Ga1.594Sc0.4O3:0.006Cr3+ exhibits high internal quantum efficiency (99%) and high phosphor-converted light-emitting diode output power (66.09 mW), demonstrating its capability as an outstanding IR phosphor. This work will motivate further research on unexpected partial substitution during the solid soln. process.
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    50. 50
      Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr. A 1976, 32 (5), 751– 767,  DOI: 10.1107/S0567739476001551
      There is no corresponding record for this reference.
    51. 51
      Baur, W. H. The Geometry of Polyhedral Distortions. Predictive Relationships for the Phosphate Group. Acta Crystallogr. B 1974, 30 (5), 1195– 1215,  DOI: 10.1107/S0567740874004560
      51
      Geometry of polyhedral distortions. Predictive relations for the phosphate group
      Baur, Werner H.
      Acta Crystallographica, Section B: Structural Crystallography and Crystal Chemistry (1974), 30 (5), 1195-215CODEN: ACBCAR; ISSN:0567-7408.
      The shape of 211 phosphate tetrahedra have been studied. Their dimensions are known precisely from published x-ray and neutron-diffraction investigations. Results: (1) The site symmetry of the P atoms was found in 85% of the cases to be 1. Other obsd. symmetries are m, 2, 3, 4‾, mm, 42‾m, 222, and 23. (2) The deviations from regular symmetry, 43‾m, are pronounced. The distortions can be measured by defining for every phosphate group 3 distortion indices: DI(TO) = (Σ|TOi-TOm|)/4TOm, DI(OTO) = (Σ|O-TOi-OTOm|)/6OTOm and DI(OO) = (Σ|OOi-OOm|)/ 6OOm, where TO is the distance P-O, OTO the angle O-P-O, OO the distance O-O, the subscripts m refer to the mean and i to individual values. The av. distortion indices for all groups are: DI(TO) = 0.021, DI(OTO) = 0.028, DI(OO) = 0.012. This means that distortions are more pronounced in the P-O distances than in O-O, and thus, the phosphate group can be viewed, to a 1st approxn., as a rigid regular arrangement of O atoms, with the P atoms displaced from their centroid. (3) Mean P-O distances of the phosphate groups vary from 1.506 to 1.572 Å. (4) Individual P-O distances are correlated with the bond strengths received by the individual O atoms. (5) The individual O-P-O angles are strongly correlated with the av. of the P-O distances on the sides of the angle, and with the opposite O-O distance. The various correlations are strong enough to formulate 6 equations which are useful for predictive purposes: the mean P-O distances can be calculated from the coordination numbers and thedistortion indices; the individual P-O distances follow from the bond strength distribution; the bond angles are based on the normalized bond lengths; the mean O-O distances are a function of the mean P-O distances and the distortion indices; the lengths of shared tetrahedral edges depend on the no. of shared edges per tetrahedron. Some of the correlations are affected by the presence of shared edges in the phosphate groups and differ for varioussubpopulations of the sample (ortho-, di-, ring-, poly-, acid or org. phosphates). The calcd. shapes can be used as input to computer simulation of crystal structures.
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    52. 52
      Robinson, K.; Gibbs, G. V.; Ribbe, P. H. Quadratic Elongation: A Quantitative Measure of Distortion in Coordination Polyhedra. Science 1971, 172 (3983), 567– 570,  DOI: 10.1126/science.172.3983.567
      52
      Quadratic elongation: a quantitative measure of distortion in [mineral] coordination polyhedra
      Robinson, Keith; Gibbs, G. V.; Ribbe, P. H.
      Science (Washington, DC, United States) (1971), 172 (3983), 567-70CODEN: SCIEAS; ISSN:0036-8075.
      Quadratic elongation and the variance of bond angles are linearly correlated for distorted octahedral and tetrahedral coordination complexes, both of which show variations in bond length and bond angle. The quadratic elongation is dimensionless, giving a quant. measure of polyhedral distortion which is independent of the effective size of the polyhedron.
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    53. 53
      Zhuang, Y.; Tanabe, S.; Qiu, J. Wavelength Tailorability of Broadband Near-Infrared Luminescence in Cr4+-Activated Transparent Glass-Ceramics. J. Am. Ceram. Soc. 2014, 97 (11), 3519– 3523,  DOI: 10.1111/jace.13128
      53
      Wavelength tailorability of broadband near-infrared luminescence in Cr4+-activated transparent glass-ceramics
      Zhuang, Yixi; Tanabe, Setsuhisa; Qiu, Jianrong
      Journal of the American Ceramic Society (2014), 97 (11), 3519-3523CODEN: JACTAW; ISSN:0002-7820. (Wiley-Blackwell)
      Four Cr4+-activated transparent glass-ceramics contg. different species of silicate nano-crystals (Zn2SiO4, Mg2SiO4, Li2ZnSiO4, and Li2MgSiO4) were successfully prepd. Absorption spectra, photoluminescence spectra, lifetime decay curves, and quantum yield of these transparent glass-ceramics were measured. According to the crystal field strength of Cr4+-incorporated tetrahedral sites, the broadband near-IR (NIR) luminescence of Cr4+ can be tailored from 1130 to 1350 nm and the lifetime of Cr4+ luminescence can be prolonged from 6 to 100 μs. Quantum yield in the transparent glass-ceramics contg. Li2ZnSiO4 nano-crystals reached at 17%, which is the highest value of NIR luminescence in transition-metal-activated glass materials.
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    54. 54
      Zhang, R.; Read, G.; Lang, F.; Lancaster, T.; Blundell, S. J.; Hayward, M. A. La2SrCr2O7F2: A Ruddlesden–Popper Oxyfluoride Containing octahedrally Coordinated Cr4+ Centers. Inorg. Chem. 2016, 55 (6), 3169– 3174,  DOI: 10.1021/acs.inorgchem.6b00114
      54
      La2SrCr2O7F2: A Ruddlesden-Popper Oxyfluoride Containing Octahedrally Coordinated Cr4+ Centers
      Zhang, Ronghuan; Read, Gareth; Lang, Franz; Lancaster, Tom; Blundell, Stephen J.; Hayward, Michael A.
      Inorganic Chemistry (2016), 55 (6), 3169-3174CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The low-temp. fluorination of the n = 2 Ruddlesden-Popper phase La2SrCr2O7 yields La2SrCr2O7F2 via a topochem. fluorine insertion reaction. The structure-conserving nature of the fluorination reaction means that the chromium centers of the initial oxide phase retain an octahedral coordination environment in the fluorinated product, resulting in a material contg. an extended array of apex-linked Cr4+O6 units. Typically materials contg. networks of octahedrally coordinated Cr4+ centers can only be prepd. at high pressure; thus, the prepn. of La2SrCr2O7F2 demonstrates that low-temp. topochem. reactions offer an alternative synthesis route to materials of this type. Neutron diffraction, magnetization, and μ+SR data indicate that La2SrCr2O7F2 undergoes a transition to an antiferromagnetic state below TN ≈ 140 K. The structure-property relations of this phase and other Cr4+ oxide phases are discussed.
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    55. 55
      Tanabe, Y.; Sugano, S. On the Absorption Spectra of Complex Ions. I. J. Phys. Soc. Jpn. 1954, 9 (5), 753– 766,  DOI: 10.1143/JPSJ.9.753
      55
      The absorption spectra of complex ions. I
      Tanabe, Yukito; Sugano, Satoru
      Journal of the Physical Society of Japan (1954), 9 (), 753-66CODEN: JUPSAU; ISSN:0031-9015.
      The energy matrix elements for the electron configuration dn in cubic field are calcd. in detail to det. the success of the cryst. field approximation in explaining the origin of both absorption bands and lines of the octahedral normal complex ions contg. the Fe-group elements. The calcns. are made by Racah's method and assume the generalized cryst. field model.
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    56. 56
      Tanabe, Y.; Sugano, S. On the Absorption Spectra of Complex Ions II. J. Phys. Soc. Jpn. 1954, 9 (5), 766– 779,  DOI: 10.1143/JPSJ.9.766
      56
      The absorption spectra of complex ions. II
      Tanabe, Yukito; Sugano, Satoru
      Journal of the Physical Society of Japan (1954), 9 (), 766-79CODEN: JUPSAU; ISSN:0031-9015.
      The secular determinants are solved for the energy levels, important in the absorption spectra of normal complex ions, with the cryst. field strength as a parameter. Racah's parameters B and C were detd. from the spectra of free ions. The f-values of transitions were estd. and compared with observed intensities. The ions of the type [MX6]++ (M = Cr, Mn, Fe, Co, Ni) show good agreement between calcd. and observed positions of lines and bands when the cryst. field parameter was adjusted. With the comparable trivalent ions smaller values of B and C are needed for close agreement. A possible explanation in terms of the cryst. field approximation is offered, especially in the cases of H2O and NH3 complexes of Co(III).
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    57. 57
      Henderson, B.; Imbusch, G. F. Optical Spectroscopy of Inorganic Solids; Monographs on the Physics and Chemistry of Materials; Oxford University Press: Oxford, NY, 2006.
      There is no corresponding record for this reference.
    58. 58
      Senden, T.; van Dijk-Moes, R. J. A.; Meijerink, A. Quenching of the Red Mn4+ Luminescence in Mn4+-Doped Fluoride LED Phosphors. Light Sci. Appl. 2018, 7 (1), 8,  DOI: 10.1038/s41377-018-0013-1
      58
      Quenching of the red Mn(4+) luminescence in Mn(4+)-doped fluoride LED phosphors
      Senden Tim; Meijerink Andries; van Dijk-Moes Relinde J A
      Light, science & applications (2018), 7 (), 8 ISSN:.
      Red-emitting Mn(4+)-doped fluorides are a promising class of materials to improve the color rendering and luminous efficacy of white light-emitting diodes (w-LEDs). For w-LEDs, the luminescence quenching temperature is very important, but surprisingly no systematic research has been conducted to understand the mechanism for thermal quenching in Mn(4+)-doped fluorides. Furthermore, concentration quenching of the Mn(4+) luminescence can be an issue but detailed investigations are lacking. In this work, we study thermal quenching and concentration quenching in Mn(4+)-doped fluorides by measuring luminescence spectra and decay curves of K2TiF6:Mn(4+) between 4 and 600 K and for Mn(4+) concentrations from 0.01% to 15.7%. Temperature-dependent measurements on K2TiF6:Mn(4+) and other Mn(4+)-doped phosphors show that quenching occurs through thermally activated crossover between the (4)T2 excited state and (4)A2 ground state. The quenching temperature can be optimized by designing host lattices in which Mn(4+) has a high (4)T2 state energy. Concentration-dependent studies reveal that concentration quenching effects are limited in K2TiF6:Mn(4+) up to 5% Mn(4+). This is important, as high Mn(4+) concentrations are required for sufficient absorption of blue LED light in the parity-forbidden Mn(4+)d-d transitions. At even higher Mn(4+) concentrations (>10%), the quantum efficiency decreases, mostly due to direct energy transfer to quenching sites (defects and impurity ions). Optimization of the synthesis to reduce quenchers is crucial for developing more efficient highly absorbing Mn(4+) phosphors. The present systematic study provides detailed insights into temperature and concentration quenching of Mn(4+) emission and can be used to realize superior narrow-band red Mn(4+) phosphors for w-LEDs.
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    59. 59
      Chang, C.-Y.; Majewska, N.; Chen, K.-C.; Huang, W.-T.; Leśniewski, T.; Leniec, G.; Kaczmarek, S. M.; Pang, W. K.; Peterson, V. K.; Cherng, D.-H.; Lu, K.-M.; Mahlik, S.; Liu, R.-S. Broadening Phosphor-Converted Light-Emitting Diode Emission: Controlling Disorder. Chem. Mater. 2022, 34 (22), 10190– 10199,  DOI: 10.1021/acs.chemmater.2c03045
      59
      Broadening Phosphor-Converted Light-Emitting Diode Emission: Controlling Disorder
      Chang, Chih-Yu; Majewska, Natalia; Chen, Kuan-Chun; Huang, Wen-Tse; Lesniewski, Tadeusz; Leniec, Grzegorz; Kaczmarek, Slawomir M.; Pang, Wei Kong; Peterson, Vanessa K.; Cherng, Ding-Hua; Lu, Kuang-Mao; Mahlik, Sebastian; Liu, Ru-Shi
      Chemistry of Materials (2022), 34 (22), 10190-10199CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Near-IR (NIR) phosphor-converted light-emitting diodes (pc-LEDs) are a highly efficient perspective NIR light source, with application hindered by a narrow emission band. In this work, we broaden the emission of a new series of NIR phosphors by controlling crystal structure disorder through cation cosubstitution. By substituting Ga3+ with (Al0.68In0.32)3+, we create a Ga2-x(Al0.68In0.32)xO3:Cr3+ phosphor series in which the av. crystal size is maintained, while cation disorder varies. The increased deviation of the cation radii in the substitution leads to increased electron-phonon coupling, with a resulting emission spectrum covering the 650-1000 nm range with a 30% increase in the emission full width at half-max. (FWHM) and a relatively high internal quantum efficiency of ~ 80%. A transition from the β phase to the α phase, which differs in structure from the undoped parent, is created by the application of high pressure and possesses ultra-broad-band emission and an FWHM of ~ 190 nm. This work shows that the emission bandwidth can be controlled through disorder and its influence on the Stokes shift, as captured by the effective Huang-Rhys factor.
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    60. 60
      Zhong, J.; Zhuo, Y.; Du, F.; Zhang, H.; Zhao, W.; You, S.; Brgoch, J. Efficient Broadband Near-Infrared Emission in the GaTaO4:Cr3+ Phosphor. Adv. Opt. Mater. 2022, 10 (2), 2101800  DOI: 10.1002/adom.202101800
      60
      Efficient Broadband Near-Infrared Emission in the GaTaO4:Cr3+ Phosphor
      Zhong, Jiyou; Zhuo, Ya; Du, Fu; Zhang, Hongshi; Zhao, Weiren; You, Shihai; Brgoch, Jakoah
      Advanced Optical Materials (2022), 10 (2), 2101800CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Efficient broadband near-IR (NIR) emitting materials with an emission peak centered above 830 nm are crucial for smart NIR spectroscopy-based technologies. However, the development of these materials remains a significant challenge. Herein, a series of design rules rooted in computational methods and empirical crystal-chem. anal. is applied to identify a new Cr3+-substituted phosphor. The compd. GaTaO4:Cr3+ emerged from this study is based on the material's high structural rigidity, suitable electronic environment, and relatively weak electron-phonon coupling. Irradiating this new phosphor with 460 nm blue light generates a broadband NIR emission (λem,max = 840 nm) covering the 700-1100 nm region of the electromagnetic spectrum with a full width at half max. of 140 nm. The phase has a high internal quantum yield of 91% and excellent thermal stability, maintaining 85% of the room temp. emission intensity at 100 °C. Fabricating a phosphor-converted light-emitting diode device shows that the new compd. generates an intense NIR emission (178 mW at 500 mA) with photoelec. efficiency of 6%. This work not only provides a new material that has the potential for next-generation high-power NIR applications but also highlights a set of design rules capable of developing highly efficient long-wavelength broadband NIR materials.
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    61. 61
      Galanciak, D.; Perlin, P.; Grinberg, M.; Suchocki, A. High Pressure Spectroscopy of LLGG Doped with Cr3+. J. Lumin. 1994, 60–61, 223– 226,  DOI: 10.1016/0022-2313(94)90135-X
      61
      High pressure spectroscopy of LLGG doped with Cr3+
      Galanciak, D.; Perlin, P.; Grinberg, M.; Suchocki, A.
      Journal of Luminescence (1994), 60-61 (), 223-6CODEN: JLUMA8; ISSN:0022-2313.
      In this contribution the authors present fluorescence spectra and fluorescence decays of LLGG:Cr3+ measured at pressures up to 152 kbar. The authors have obsd. a blue shift of the broad band (4T2 → 2A2) emission, with the pressure coeff. of 10 cm-1/kbar. At pressures above 100 kbar the R line related to the 2E → 4A2 transition is obsd. Pressure dependence of the fluorescence decays have been analyzed using an adiabatic model.
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    62. 62
      Shen, Y. R.; Grinberg, M.; Barzowska, J.; Bray, K. L.; Hanuza, J.; Dereń, P. J. The Effect of Pressure on Luminescence Properties of Cr3+ Ions in LiSc(WO4)2 Crystals─Part I: Pressure Dependent Emission Lineshape. J. Lumin. 2006, 116 (1), 1– 14,  DOI: 10.1016/j.jlumin.2005.02.010
      There is no corresponding record for this reference.
    63. 63
      Bray, K. L. High Pressure Probes of Electronic Structure and Luminescence Properties of Transition Metal and Lanthanide Systems. In Transition Metal and Rare Earth Compounds: Excited States, Transitions, Interactions I; Yersin, H., Ed.; Topics in Current Chemistry; Springer: Berlin, Heidelberg, 2001; pp 1– 94.
      There is no corresponding record for this reference.
    64. 64
      Grinberg, M.; Lesniewski, T. Nonradiative Processes and Luminescence Quenching in Mn4+ Doped Phosphors. J. Lumin. 2019, 214, 116574  DOI: 10.1016/j.jlumin.2019.116574
      64
      Non-radiative processes and luminescence quenching in Mn4+ doped phosphors
      Grinberg, Marek; Lesniewski, Tadeusz
      Journal of Luminescence (2019), 214 (), 116574CODEN: JLUMA8; ISSN:0022-2313. (Elsevier B.V.)
      We present the temp. dependence of the 2E→4A2 emission of Na2TiF6:Mn4+. Our results are discussed in the framework of the single configurational coordinate diagram (SCCD) model proposed by Struck and Fonger. We have critically analyzed the SCCD model and calcd. the vibrational overlap integrals directly from the Manneback recurrence formulas. We have considered the phys. origin of shift of the excited state parabolae in the configurational space as well as possible different slopes of the ground and excited electronic manifolds. We have compared our results with numerous results obtained for Mn4+ in other lattices available in the literature. We have found the high inconsistency between the exptl. results and values obtained under the SCCD model. Moreover, we have found strong correlation between the values of exptl. activation energies Enr and values of const. describing the probability of nonradiative process (i.e. frequency const.). We have considered interaction with lattice phonons described by power dependence defined by exponent α, related to dimension of phonon space and effective phonon energy ℏωeff. We found α ≈ 7.42 and ℏωeff = 315 cm -1 for Enr < 4000 cm-1 and α ≈ 19.73, and ℏωeff =1130 cm-1 for Enr > 4000 cm-1.
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    65. 65
      Sharma, S. K.; Bessière, A.; Basavaraju, N.; Priolkar, K. R.; Binet, L.; Viana, B.; Gourier, D. Interplay between Chromium Content and Lattice Disorder on Persistent Luminescence of ZnGa2O4:Cr3+ for in Vivo Imaging. J. Lumin. 2014, 155, 251– 256,  DOI: 10.1016/j.jlumin.2014.06.056
      65
      Interplay between chromium content and lattice disorder on persistent luminescence of ZnGa2O4:Cr3+ for in vivo imaging
      Sharma, Suchinder K.; Bessiere, Aurelie; Basavaraju, Neelima; Priolkar, Kaustubh R.; Binet, Laurent; Viana, Bruno; Gourier, Didier
      Journal of Luminescence (2014), 155 (), 251-256CODEN: JLUMA8; ISSN:0022-2313. (Elsevier B.V.)
      In the quest of bright and long persistent far-red/near-IR phosphors for in vivo optical imaging, the interest in the family of ZnGa2O4 spinel compds. doped with Cr3+ has been aroused in the most recent years. We show that the dopant concn. plays an important role in the total persistent luminescence output of the material. ZnGa2O4 doped with 0.25%, 0.50% and 0.75% Cr relative to (Ga+Cr) was prepd. by solid state synthesis. 0.50% Cr was found optimal to obtain the most intense persistent luminescence after matrix excitation with X-rays or localized excitation in Cr3+ absorption band with 550 nm wavelength. Up to 0.5% Cr content, persistent luminescence increases as a consequence of an increased no. of Cr3+ luminescent centers and assocd. defects. With 0.75% Cr content, a too large no. of defects locally concd. around Cr3+ ions are detrimental to the long-term persistent luminescence intensity. We supplement long lasting phosphorescence investigation with laser excited photoluminescence and thermally stimulated luminescence results.
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    66. 66
      Dolan, J. F.; Kappers, L. A.; Bartram, R. H. Pressure and Temperature Dependence of Chromium Photoluminescence in K2NaGaF6:Cr3+. Phys. Rev. B 1986, 33 (10), 7339– 7341,  DOI: 10.1103/PhysRevB.33.7339
      66
      Pressure and temperature dependence of chromium photoluminescence in chromium(3+)-doped dipotassium sodium hexafluorogallate(III)
      Dolan, J. F.; Kappers, L. A.; Bartram, R. H.
      Physical Review B: Condensed Matter and Materials Physics (1986), 33 (10), 7339-41CODEN: PRBMDO; ISSN:0163-1829.
      Photoluminescence spectra and lifetimes of K2NaGaF6:Cr3+, measured as functions of pressure and temp., reveal a pressure-induced transition from broad-band fluorescence to narrow-band phosphorescence.
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    67. 67
      Back, M.; Ueda, J.; Nambu, H.; Fujita, M.; Yamamoto, A.; Yoshida, H.; Tanaka, H.; Brik, M. G.; Tanabe, S. Boltzmann Thermometry in Cr3+-Doped Ga2O3 Polymorphs: The Structure Matters!. Adv. Opt. Mater. 2021, 9 (9), 2100033  DOI: 10.1002/adom.202100033
      67
      Boltzmann Thermometry in Cr3+-Doped Ga2O3 Polymorphs: The Structure Matters
      Back, Michele; Ueda, Jumpei; Nambu, Hiroshi; Fujita, Masami; Yamamoto, Akira; Yoshida, Hisao; Tanaka, Hiromitsu; Brik, Mikhail G.; Tanabe, Setsuhisa
      Advanced Optical Materials (2021), 9 (9), 2100033CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The performance of luminescent Cr3+-doped thermometers is strongly influenced by the locally surrounding ligand field. A universal relationship between the thermometric performance and structural/chem. parameters is highly desirable to drive the development of effective Cr3+-based thermal sensors avoiding trial-and-error procedures. In this view, as prototypes, the electronic structure and the thermometric performance of Cr3+-doped α-Ga2O3 and β-Ga2O3 polymorphs are compared. Combining a detailed theor. and spectroscopic investigation, the electronic configuration and the crystal field (CF) acting on the Cr3+ in α-Ga2O3 are described for the first time and compared with β-Ga2O3:Cr3+ polymorph to discuss the thermometric behavior. A linear relationship between the 4T2-2E energy gap (directly linked to the relative sensitivity) and the CF strength Dq is demonstrated for a wide variety of materials. This trend can be considered as a first step to set guiding principles to design effective Cr3+-based Boltzmann thermometers. In addn., as a proof of concept, particles of β-Ga2O3:Cr3+ thermometer are used to locally measure in operando thermal variations of Pt catalysts on β-Ga2O3:Cr3+ support during a catalytic reaction of C2H4 hydrogenation in a contactless and reliable mode, demonstrating their real potentials.
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    68. 68
      Syassen, K. Ruby under Pressure. High Press. Res. 2008, 28 (2), 75– 126,  DOI: 10.1080/08957950802235640
      68
      Ruby under pressure
      Syassen, K.
      High Pressure Research (2008), 28 (2), 75-126CODEN: HPRSEL; ISSN:0895-7959. (Taylor & Francis Ltd.)
      A review. The ruby luminescence method is widely used for pressure measurement in the diamond anvil cell and other optically transparent pressure cells. With this application in mind, we briefly review the ground-state phys. properties of corundum (α-Al2O3) with some emphasis on its behavior under high pressure, survey the effects of temp. and stress on the R-line luminescence of ruby (Cr-doped corundum), and address the recent efforts towards an improved calibration of the R-line shift under hydrostatic pressures beyond the 50 GPa mark.
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  • Supporting Information

    Supporting Information

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    • Characterization; crystal structure; X-ray diffraction; Rietveld refinement; photoluminescence; and decay time (PDF)


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