Strategies for broadening the emission spectra of Cr³⁺-doped near-infrared emitting phosphors

Abstract

Cr³⁺-activated near-infrared (NIR) phosphors have significant application potential in food detection and analysis of biological fluids and tissues components owing to the large spectrum overlap between Cr³⁺ emission and the absorption bands of water and organic groups. To realize real-time, convenient and multi-component detection, a wideband NIR phosphor with a large bandwidth (FWHM > 200 nm) is urgently required to effectively cover the absorption band of the target analyte. This review simply offers a retrospect of the researches on existing Cr³⁺-doped wideband phosphors and summarizes the key strategies, including crystal field regulation, lattice site engineering, Cr³⁺–Cr³⁺ pairs, Cr³⁺/Cr⁴⁺ double fluorescence centers, energy transfer process and the new emission centers resulting from the lattice distortion caused by Cr³⁺ doping, for broadening the Cr³⁺ bandwidth. The feature of the Cr³⁺ emission is also discussed using the Tanabe–Sugano energy level diagram and configuration coordinate model. In addition, the existing problems and future prospects of Cr³⁺-activated wideband emission phosphors are elucidated, providing a reference to broaden the bandwidth of Cr³⁺ for the development of efficient and stable wideband near-infrared phosphors.

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1. Introduction

Near-infrared (NIR) light refers to the electromagnetic waves in the wavelength range between 700 and 2500 nm, which is between the visible (400–700 nm) and mid-infrared light (2.5–25 μm).^1,2 In particular, NIR light between 700–1700 nm has been extensively researched and applied in the fields of food quality inspection,³ component analysis,⁴ biological imaging,⁵ health monitoring,⁶ night vision,⁷ and plant growth,⁸ owing to their advantages of strong penetrating ability, characteristic absorption by certain molecules, low tissue damage and invisibility to the human eye. For NIR spectroscopy detection technology, a broadband NIR light source is a pre-requisite to cover the absorption regions of water, fat, carbohydrate, sugar, hemoglobin, and other organic elements, enabling real-time, convenient and multi-component detection.^9,10 The traditional NIR sources of tungsten halogen lamps and incandescent bulbs have the drawbacks of a large size, low efficiency, high working energy consumption and short serving time.^11,12 The AlGaAs-based LED chips and solid-state laser diodes can effectively overcome the shortcomings of halogen lamps and incandescent bulbs, but their narrow emission bandwidth (<50 nm) still hinders their applications in the NIR spectroscopy detection field.¹³ However, the development of phosphor-converted light emitting diodes (pc-LEDs) made it possible to obtain broadband NIR sources with high efficiency, energy saving, and long service time, with miniaturization features by coating wideband emission NIR phosphors on blue-light chips.^14,15 While fabricating NIR pc-LEDs, the selected NIR emitting phosphor play an important role in determining their emission features; thus, it is urgent to explore NIR emitting phosphors with high efficiency, thermal stability, large bandwidth and well match with the blue-light chip.

It is well-known that a phosphor is composed of a host and an activator, where the activator is the core of the phosphor and directly determines its emission mechanism.^16,17 The existing activators with NIR emission are roughly classified into four types depending upon their electronic transition: the 4f–4f electron transition (such as those exhibited by rare earth ions of Yb³⁺, Ho³⁺, Nd³⁺, Sm³⁺, Er³⁺, Yb³⁺, Pr³⁺, Dy³⁺ and Tm³⁺ (ref. 18 and 19)); the 4f–5d electron transition (those exhibited by Eu²⁺ and Ce³⁺ (ref. 20 and 21)); the ns²-nsnp electron transition (typically exhibited by Bi³⁺/Bi²⁺ ion^22,23); and the d–d electron transition (those exhibited by Mn²⁺, Ni²⁺ and Cr³⁺ (ref. 24–26)). Among them, rare earth ions usually display sharp-line emission and narrow absorption bands owing to the parity forbidden f–f transition.²⁷ The f–d transition of parity Eu²⁺ and Ce³⁺ ions usually possess a large emission bandwidth and wide absorption band, whereas the deep red or NIR emission of Eu²⁺ and Ce³⁺ is hard to realize because it is difficult to find a host that can offer strong crystal field environments for Eu²⁺ and Ce³⁺.²⁸ The efficiency of Bi³⁺/Bi²⁺ at long wavelengths is low, and the absorption band remains in the ultraviolet region, which is mismatched with the blue light chip. The emission properties of Ni²⁺ and Mn²⁺ can be easily adjusted by changing the crystal field of the activators, whereas the two ions face challenges such as low emission and low absorption efficiency.^29,30 In contrast, Cr³⁺ usually displays a controlled emission feature from deep-red to NIR light and an adjustable bandwidth from sharp-line to broadband emission by changing the crystal field on Cr³⁺. Meanwhile, Cr³⁺ also displays a wideband absorption property in the visible regions, and the main absorption band from the ⁴A₂ → ⁴T₂ transition matches well with the blue light chip.^31,32 This unique feature makes Cr³⁺ an ideal activator for wideband NIR phosphors, which makes it possible to obtain a broadband NIR emitting pc-LED source by developing Cr³⁺-activated broadband NIR phosphors.

In previous studies, Cr³⁺ was primarily used as an activator in laser gain medium for tunable laser applications, such as Al₂O₃:Cr³⁺, Y₃Al₅O₁₂:Cr³⁺, BeAl₂O₄:Cr³⁺, β-Ga₂O₃:Cr³⁺ laser crystals.^33–36 However, with the rapid development of NIR phosphors and NIR spectroscopy techniques, a growing number of Cr³⁺-activated NIR phosphors with tunable emission peaks and bandwidths have emerged, drawing increasing interest from researchers across various application fields. In particular, broadband Cr³⁺-doped NIR-emitting phosphors have attracted significant attention due to their promising potential in spectral analysis applications.³⁷ Various efficient wideband NIR phosphors based on Cr³⁺ have been developed, such as LaMgGa₁₁O₁₉:Cr³⁺, NaScGe₂O₆:Cr³⁺, Sr₂ScSbO₆:Cr³⁺, Ca₂LuZr₂Al₃O₁₂:Cr³⁺, and ScBO₃:Cr³⁺.^38–42 The bandwidth of most Cr³⁺-activated phosphors is typically less than 200 nm, which still limits their further application in the NIR spectroscopy detection field. In this case, it is necessary to search for broadband and efficient Cr³⁺-activated NIR materials to realize real-time, convenient and multi-component inspection.

In this review, many Cr³⁺-activated phosphors with wideband emission are discussed, and strategies to broaden the Cr³⁺ emission spectrum are summarized by analyzing existing broadband phosphor realization methods. The spectrum-broadening strategies of Cr³⁺ involve crystal field regulation, lattice site engineering, Cr³⁺–Cr³⁺ pairs, Cr³⁺/Cr⁴⁺ double fluorescence centers, energy transfer processes and the construction of new Cr³⁺ emission centers. The existing problems associated with different spectral broadening methods were also discussed to better explore highly efficient and stable broadband NIR phosphors. Finally, the present challenges and future outlooks of Cr³⁺-activated broadband NIR luminescent materials are discussed.

2. Emission property of Cr³⁺

2.1. 3d electron orbits and crystal field theory

As shown in Fig. 1a, the 3d electron state of Cr³⁺ with d³ configuration has a fivefold energy degenerate state at the free state, where the fivefold degenerate orbitals all belong to the d orbit and were named the d_yz, d_xz, d_xy, d_z² and d_x²−y².⁴³ When Cr³⁺ is located in the six-coordination environment with O_h symmetry, the electrostatic interaction of the coordination atoms allows the d_yz, d_xz, d_xy, d_z² and d_x²−y² degenerate orbits to be split into a two-ford degenerate level of e_g and a three-ford degenerate level of t_2g, where the former is constructed by d_z² and d_x²−y² degenerate orbits and the latter is constituted by the d_yz, d_xz, and d_xy degenerate orbits.⁴⁴ The electron clouds of the d_yz, d_xz, and d_xy orbitals do not point to the coordination atoms, whereas the electron clouds of the d_z² and d_x²−y² orbitals point to the coordination atoms. Thus, the interaction of the d_z² and d_x²−y² orbitals with the coordination atoms is stronger than that of the d_yz, d_xz, and d_xy orbitals with the coordination atoms, which makes the energy of the e_g level higher than that of the t_2g level.⁴⁵ To keep the total energy of the system constant, the e_g and t_2g levels move about 6 Dq and −4 Dq energy from the original energy position, making the energy of the e_g level higher than that of the t_2g levels. The parameters D and q were analyzed using the following equations:⁴⁶where R is the distance between the center atom and coordination atoms, z is the valence of the anion, e is the electron charge, and r is the radial distance between the d orbital electron and nucleus. When the symmetry of the coordination environment of Cr³⁺ decreases from O_h to D_h symmetry through the form of tetragonal, trigonal or orthogonal distortion (in Fig. 1b), the degeneracy of e_g and t_2g levels will be further reduced, and the energy level splitting is more obvious due to the Jahn–Teller effect,⁴⁷ resulting in the enhancement of absorption and emission due to the broken forbidden d–d transition.

Fig. 1 3d electron cloud shape and the energy splitting of Cr³⁺ in an octahedral environment (a) and (b) elimination of energy level degeneracy induced by the octahedral distortion.

In the case of the radiation transition of Cr³⁺ originating from the d–d transition, the coordination environment of Cr³⁺ is sensitive. The emission energy of Cr³⁺ is usually related to the strength of the crystal field splitting degree; a large crystal field splitting degree usually generates short wavelength emission. The crystal field splitting degree (Dq) was calculated using eqn (3):⁴⁸

The above equation indicates that the crystal field strength (Dq) is inversely proportional to the bond length between Cr³⁺ and the coordination atom. The Dq gradually weakened with increasing bond length, causing the photoluminescence spectrum to exhibit a red shift.

2.2. Cr³⁺ emission in strong and weak crystal fields

The NIR emission of Cr³⁺ usually involves the ²E → ⁴A₂ and ⁴T₂ → ⁴A₂ electron transitions, where the former is related to sharp-line emission and the latter corresponds to wideband emission.⁴⁹ The relative energy locations of the ²E and ⁴T₂ levels directly determine the emission features of Cr³⁺. To further understand the inter-electronic transition property of Cr³⁺, the Tanabe–Sugano (T–S) diagram (in Fig. 2a) was introduced to research the relationship between ²E and ⁴T₂ energy level under the influence of the crystal field strength and the electrostatic interaction,⁵⁰ in which the ratio (Dq/B) between crystal field strength and the electrostatic interaction is taken as a quantitative parameter to study energy level splitting of Cr³⁺ at the specific crystal environment.⁵¹ The B symbol of the Racah parameters represents the Coulomb repulsion strength between the electrons.⁵² When Dq/B = 0, corresponding to the free state Cr³⁺, the three 3d electrons of the free Cr³⁺ ion form the ⁴F, ⁴P, ²P, ²D, ²F, ²G, ²H energy states, with the ⁴F state having the lowest energy among them.⁵³ Because the NIR emission of Cr³⁺ involves only ⁴F, ⁴P and ²G energy states, the energy splitting of the above three energy levels is mainly considered in the Tanabe–Sugano diagram. When Cr³⁺ is in an octahedral coordination environment (Dq/B > 0), the ⁴F state splits into three energy levels: ⁴T₁, ⁴T₂, and ⁴A₂. The ²G state is split into four energy levels: ²A₁, ²T₂, ²T₁, and ²E. If the ²E and ⁴A₂ levels possess the same electron configuration structure as, t_2g³, the ²E → ⁴A₂ transition leads to sharp-line emission. The ⁴T₂ level possesses a different electron configuration structure of t_2g²e_g in comparison with that of ²E, which lets the ⁴T₂ → ⁴A₂ transition display a wideband peak emission.⁵⁴

Fig. 2 Tanabe–Sugano diagram of Cr³⁺ in an octahedron environment (a) and energy level diagrams of Cr³⁺ (b) in strong and weak crystal fields, and the corresponding spectrum shape.

In the T–S diagram, the intersection point (Dq/B = 2.3) of the ⁴T₂ and ²E levels is commonly regarded as the boundary between the strong crystal field (Dq/B > 2.3) and the weak crystal field (Dq/B < 2.3).⁵⁵ The Dq/B critical value between the strong and weak crystal fields depends on the host materials because the Racah parameters B/C value is influenced by the host materials. When B/C = 5, the critical value of Dq/B is 2.3. Liu's recent research implies that the Dq/B critical value changes from 2.86 to 1.59 as the B/C value changes from 3.0 to 7.0.⁵⁶ Overall, Dq/B = 2.3 is still a widely applicable critical value. As shown in Fig. 2b, when Cr³⁺ was located in a strong crystal field environment, the ²E level was the lowest excited state. Under the excitation of external light, the electrons at the ground state of ⁴A₂ are excited to the higher energy excited states of ⁴T₂ and ⁴T₁, then relaxed to the lowest ²E excited state and radiate back to the ⁴A₂ level with the sharp-line emission originating from the spin-forbidden ²E → ⁴A₂(F) transition of the Cr³⁺ ion. On the contrary, the ⁴T₂(F) level is deemed the lowest excited level when the Cr³⁺ is seated in a weak crystal field environment. Under light excitation, the electrons at ⁴A₂(F) level are excited to the higher energy excited states of ⁴T₁(F) and ²E levels, relax to the lowest ⁴T₂(F) excited state and radiate back to the ⁴A₂ level with a broadband emission originating from the spin-allowed ⁴T₂ → ⁴A₂(F) transition of the Cr³⁺ ion. In the case of the ⁴T₂(F) level being strongly coupled with the host lattice, whereas that of ²E is not, so the ⁴T₂ → ⁴A₂(F) transition is thus more sensitive to the change in the crystal field strength on Cr³⁺ in comparison with that of the ²E → ⁴A₂(F) transition. In some special cases, the emission spectrum of Cr³⁺ consists of the sharp-line emission from ²E → ⁴A₂(F) and broadband emission from ⁴T₂ → ⁴A₂(F) transitions, in which the crystal field on Cr³⁺ is regarded as the critical field.⁵⁷ The Dq/B value of the critical field is usually difficult to determine, and it can only be determined using experimental phenomena.

3. Strategies for broadening the Cr³⁺ emission spectrum

3.1. Crystal field regulation

In the case of the transitions of ²E → ⁴A₂ with sharp-line emission or ⁴T₂ → ⁴A₂ with wideband emission, they play a dominant role in the emission spectrum of Cr³⁺ in strong (Dq/B > 2.3) or weak (Dq/B < 2.3) crystal fields, which provide a reasonable way to broaden the bandwidth of Cr³⁺ by regulating the crystal field strength. As a classical crystal structure, the garnet-type structure with A₃B₂C₃O₁₂ formula (A = Sc³⁺, Y³⁺, Lu³⁺, Gd³⁺, and La³⁺ ions; Al³⁺, Ga³⁺, Sc³⁺, In³⁺, Mg²⁺, Y³⁺, Hf⁴⁺, and Zr⁴⁺; C = Al³⁺, Ga³⁺, Si⁴⁺, Ge⁴⁺, and Sn⁴⁺) possesses complex cation sites for various cations to substitute,^58–60 which provides much more possibilities to realize the luminescence modulation of Cr³⁺ in compounds with garnet-type structure. In Cr³⁺-activated Ln₃Sc₂Ga₃O₁₂ (Ln = Lu, Y, Gd, and La) materials,⁶¹ the crystal field strength (CFS) for Cr³⁺ gradually decreased from 2.57 to 2.27 with the A site ion changing from Lu³⁺ to La³⁺, whose emission peak red-shifted from 722 to 818 nm and the full width at the half maximum (FWHM) value also enlarged from 73 to 145 nm (in Fig. 3a). A similar phenomenon of peak red-shift and FWHM expansion was also observed in Cr³⁺ Ln₃In₂Ga₃O₁₂ (A = Lu, Y, Gd, and La) materials when the A site ion with a large radius changed to low crystal field strength.⁶² Although the FWHM of the emission showed an increasing trend upon introducing large radius ions to weaken the CFS of Cr³⁺, the FWHM value of the spectrum was still limited because the crystal field strength (CFS) of Cr³⁺ was not sufficiently small. To further regulate the crystal field strength on Cr³⁺, the co-substitution method was adopted to increase the distortion degree and volume of the CrO₆ octahedron to weaken the (CFS) on Cr³⁺. The co-substitution of [YO₆]–[AlO₆] by [CaO₆]–[SiO₆] in Y₃Al₅O₁₂ allowed the FWHM of Cr³⁺ to exhibit significant expansion from 40 to 160 nm with a weakened Dq/B value from 2.62 to 2.38.⁶³ The co-substitution of [YO₆]–[GaO₆] by [MgO₆]–[GeO₆] in Y₃Ga₅O₁₂ enlarged the FWHM of Cr³⁺ from 84 to 190 nm with the Dq/B value decreasing from 2.71 to 2.44.⁶⁴ The co-substitution (in Fig. 3b) of [GdO₆]–[GaO₆] by [ZnO₆]–[GeO₆] in Gd₃Ga₅O₁₂ also reduced the Dq/B value from 2.71 to 2.44 and effectively widened the FWHM of Cr³⁺ from 105 to 211 nm.⁶⁵ The considerable FWHM expansion implied that the group co-substitution method is an effective way to expand the emission spectrum of Cr³⁺ in garnet-type materials. In the case of the group co-substitution method effectively regulating the FWHM of Cr³⁺ emission, plenty of Cr³⁺-activated garnet NIR phosphors with large FWHMs have been developed, such as the Cr³⁺-doped Ca₂LuGa₃Ge₂O₁₂, Ca₃MgSn_0.5Zr_0.5Ge₃O₁₂, Na₂CaZr₂Ge₃O₁₂, Lu₂CaMg₂Ge₃O₁₂ and so on phosphors.^66–69

Fig. 3 PL spectra (a) of Cr³⁺-doped garnet structure phosphor (copyright 2017, Wiley)⁶¹ and the Ge⁴⁺ and Zn²⁺ co-doped Gd₃Ga₅O₁₂ (b) together with the relevant spectrum change (copyright 2022 Elsevier).⁶⁵ Energy splitting and spectrum broadening (c) of Cr³⁺ in Ge⁴⁺ and Mg²⁺ co-doped Ga₂O₃ phosphors (copyright 2022, the American Chemical Society).⁷⁰ Emission spectrum and energy level (d) of Cr³⁺-doped Li(Sc/In)(Ge/Si)O₄ phosphors (copyright 2023, the American Chemical Society).⁷¹ Variation in the crystal field strength (Dq/B) and PL spectrum shape (e) of LiScSi₂O₆:Cr³⁺ and LiScGe₂O₆:Cr³⁺ materials under compression and decompression states (copyright ₂024, the Royal Society of Chemistry).⁷²

Except for the Cr³⁺-doped garnet materials, the Cr³⁺-activated Ga₂O₃ phosphor displayed a more evident feature of tunable emission FWHM through simple cation doping at the Ga³⁺ site to modulate the CFS. The introduction of In³⁺ or Sc³⁺ into Ga₂O₃:Cr³⁺ reduced the Dq/B value from 2.65 to 2.37 or 2.49 and expanded the FWHM values from 122 to 157 or 160 nm,^73,74 whose photoluminescence quantum yield (PLQY) severally up to 99% and 88% when the Sc³⁺ and In³⁺ doping concentrations reached 40%. In addition, the co-substitution of [GaO₆]–[GaO₆] by [GeO₆]–[MgO₆] (in Fig. 3c) was also applied to obtain a wider emission band, for which the FWHM of Cr³⁺ also displayed continuously adjustable features from 122 to 186 nm and the samples maintained a high PLQY of approximately 60%.⁷⁰ The pristine garnet and A₂O₃-type structures all produce a strong crystal field circumstance for Cr³⁺, which makes it feasible to realize FWHM modulation of Cr³⁺ by decreasing the crystal field strength through large-radius ion doping.

To further verify that the crystal field regulation strategy is also applicable to Cr³⁺-activated NIR phosphors with weak crystal fields, the wideband NIR phosphor LiScGeO₄:Cr³⁺ with a large FWHM (325 nm) and small Dq/B (0.949) was selected as a study object.⁷¹ As shown in Fig. 3d, with Sc³⁺ and Ge³⁺ gradually substituted by In³⁺ and Si⁴⁺, the FWHM of Cr³⁺ enlarged from 325 to 351 nm and Dq/B declined from 0.949 to 0.466, indicating that the crystal field regulation possesses a certain universality in both weak and strong CFS materials. In addition, pressure-dependent spectral tests were conducted to directly observe the effect of the crystal field on Cr³⁺.⁷² As shown in Fig. 3e, the LiScSi₂O₆:Cr³⁺ and LiScGe₂O₆:Cr³⁺ samples exhibited convertible wideband and narrow band emission, severally originating from ⁴T₂ → ⁴A₂ to ²E →⁴A₂ transitions during the structure compression and decompression process, for which the ²E →⁴A₂ transition gradually dominated the emission spectrum with the pressure enhancing from 0 to 30Gpa and the emission band transferred from wideband feature (FWHM ∼ 146/156 nm) to sharp-line because of the increasing Dq/B from 2.10/2.26 to 3.89/4.07 for the LiScSi₂O₆:Cr³⁺ and LiScGe₂O₆:Cr³⁺ samples. The above results imply that crystal field regulation is a convenient and effective strategy for regulating the emission band of Cr³⁺.

3.2. Multi crystal sites engineering

Considering that the emission property of Cr³⁺ is sensitive to coordinating environments like the Eu²⁺,⁷⁵ the occupancy of Cr³⁺ at multiple octahedron sites with different coordinating environments is beneficial for increasing the FWHM of the emission band, which makes it possible to obtain wideband NIR phosphors by doping the Cr³⁺ into a host rich in octahedron groups. The double perovskite structure materials with formula A₂B_IB_IIO₆ (A = Ca, Sr, Ba, La, and Gd; B_I = Ga, In, Sc, Y, and Mg; B_II = Ti, Zr, Hf, Ge, Sn, Sb, Nb, and W) with two different coordinating environments octahedrons are considered as the suitable host for Cr³⁺ to obtain wideband emission phosphors.^76,77 The double perovskite materials (in Fig. 4a) such as La₂MgZrO₆ (MgO₆/ZrO₆), Ca₂InNbO₆ (InO₆/NbO₆), BaLaMgNbO₆ (InO₆/NbO₆), Sr₂GaTaO₆ (GaO₆/TaO₆), Ca₂InTaO₆ (InO₆/TaO₆) and Sr₂ScSbO₆ (SbO₆/ScO₆) all existed the two occupation sites for Cr³⁺ to substitute,^78–83 which made the Cr³⁺ displayed wideband emission with FWHM value larger than 200 nm. In addition, hosts with an ABO₄ (A = Ga, Sc, and In; B = Ta and Nb) structure compromised two probable substitution sites are also suitable hosts for Cr³⁺,⁸⁴ which allows Cr³⁺ to give large FWHM ranging from 125 to 231 nm (in Fig. 4b). Wideband NIR emission was also obtained by doping Cr³⁺ into materials with two different octahedron sites, such as the Li₂Mg₃TiO₆, La₃Ga₅SnO₁₄, La₃Ga₅GeO₁₄, Sr₃Ga₂Ge₄O₁₄, Li₃Sc₂(PO₄)₃ and so on.^85–89

Fig. 4 PL and PLE spectra of La₂MgZrO₆:Cr³⁺ (a) (copyright 2019, the American Chemical Society)⁷⁸ and PL spectrum of InNbO₄:Cr³⁺ phosphor (b) (copyright 2022, Wiley).⁸⁴ PL spectrum of Mg₃Ga₂GeO₈:Cr³⁺ (c) with three distinct octahedrons for Cr³⁺ (copyright 2019, Elsevier)⁹⁰ and the four suitable octahedrons in Mg₇Ga₂GeO₁₂ for Cr³⁺ to be entered (d) (copyright 2021, Elsevier).⁹¹ PL spectrum of Mg₂SiO₄:Cr³⁺ and the site occupation diagram of Cr³⁺ and Cr⁴⁺ (e) (copyright 2022, the American Chemical Society).⁹² Schematic of the site occupation tendency of Cr³⁺ at high and low concentrations (f) (copyright 2021, Elsevier).⁹³

In consideration of materials with double substitution sites that have great prospects in realizing Cr³⁺-activated wideband NIR phosphors, materials with three or more lattice sites for Cr³⁺ were also developed to further expand the emission band of Cr³⁺. As shown in Fig. 4c, in the Cr³⁺-activated Mg₃Ga₂GeO₈ phosphor,⁹⁰ there were three distinct Mg/GaO₆ octahedrons (Mg1/Ga1O₆, Mg2/Ga2O₆, Mg3/Ga3O₆) for Cr³⁺ to enter, whose spectrum range covered 650–1200 nm with a FWHM of 244 nm. The Mg₇Ga₂GeO₁₂ even possessed four different Mg/CaO₆ octahedrons (Mg1/Ga1O₆, Mg2/Ga2O₆, Mg3/Ga3O₆, Mg4/Ga4O₆) for Cr³⁺ (in Fig. 4d),⁹¹ which also displayed large FWHM values of 226 nm.

In some special structures, the tetrahedron and octahedron centers were also considered suitable substitution sites because of the coexistence of Cr³⁺ and Cr⁴⁺. In the Mg₂SiO₄:Cr³⁺ phosphor (in Fig. 4e), there were two kinds of MgO₆ octahedrons (Mg₁O₆ and Mg₂O₆) for Cr³⁺ and one SiO₄ tetrahedron for Cr⁴⁺,⁹² which endowed the NIR phosphor possessing an ultra-wide emission band ranging from 600 to 1400 nm with a FWHM of 419 nm. Furthermore, some studies on Cr³⁺-activated garnet-type phosphors have suggested that Cr³⁺ can simultaneously enter the tetrahedron, octahedron and dodecahedron sites to realize wideband NIR emission. For instance, in the Y₂Mg₂Al₂Si₂O₁₂ host, Cr³⁺ ions were found to enter the [Al/SiO₄] tetrahedron, [Mg/AlO₆] octahedron and [Y/MgO₈] dodecahedron, resulting in a phosphor with an FWHM of 165 nm and PLQY of approximately 86%.⁹⁴ Thus, it can be inferred that searching for materials with two or more substitution sites is indeed an effective way to broaden the Cr³⁺ bandwidth.

It was observed that the FWHM of the emission spectrum of Cr³⁺ varied with changing doping concentration in Cr³⁺-activated phosphors with two or more occupation sites, which is due to the different CFS and the occupation proportion at different occupation sites (in Fig. 4f). Usually, the occupation proportion of Cr³⁺ at more weak crystal field sites gradually increases with increasing Cr³⁺ doping concentration, which may be related to the fact that Cr³⁺ is more inclined to enter the distorted octahedron due to the more evident Jahn–Teller effect at high Cr³⁺ concentration doping.⁹³ It is accepted that Cr³⁺ at a distorted octahedron emits long wavelength emission,⁹⁵ which benefits the bandwidth expansion of Cr³⁺ under high-concentration doping conditions. Typically, the FWHM of the emission band increased from 9 to 92.6 nm with the enhancement of the Cr³⁺ concentration from 0.5 to 20% in the BaMgAl₁₀O₁₇:Cr³⁺ phosphor,⁹⁶ which, due to the long wavelength emission, gradually dominated the whole emission band at high Cr³⁺ concentration conditions. In addition, to achieve effective spectrum broadening for Cr³⁺, the host materials should provide many more weak crystal field sites for Cr³⁺. Although Li₃Mg₂NbO₆ possessed three probable Mg(1, 2, 3) sites for Cr³⁺, the Mg1 sites were located at the strong crystal field (Dq/B = 2.57), leading to the FWHM of Cr³⁺ being about 160 nm and narrower than that of Cr³⁺-doped phosphors with double weak crystal field sites.⁹⁷ Moreover, it is noticed that the similar crystal field strength of Cr³⁺ in lattice site-sharing materials makes it difficult to obtain effective spectrum broadening. Thus, the number of occupied sites for Cr³⁺ and the corresponding crystal field on Cr³⁺ should be considered simultaneously when applying the multi-crystal site strategy to achieve a wideband NIR phosphor based on Cr³⁺.

3.3. Cr³⁺–Cr³⁺ pair strategy

The Cr³⁺–Cr³⁺ pair is a distinctive and intriguing ion aggregation phenomenon for Cr³⁺ due to the structural aggregation in Cr³⁺-doped phosphors, which exhibits extended energy levels and richer emission states in comparison with individual Cr³⁺ due to the ion coupling interaction.⁹⁸ Usually, the criterion to prove the formation of ion pairs may include the following: (1) the distance between the two neighboring Cr atoms usually no more than 3 Å; (2) the electronic sequential resonance (EPR) signal peak of Cr³⁺ pairs are located at the position of g factor about 2.000, and the EPR signal peaks become more divided at low temperature; (3) the Cr³⁺ pair possesses the anti-ferromagnetic properties, so the change of its susceptibility with temperature follows the Curie–Weiss law. When Cr³⁺ is located in a strong crystal field environment, single Cr³⁺ generally displays strong, sharp-line emission from the ²E →⁴A₂ transition, but broadband emission from ⁴T₂ → ⁴A₂ transition is hard to observe.^99,100 The NIR phosphor SrAl_11.88−xGa_xO₁₉:0.12Cr³⁺ displayed wideband emission with lifetime of 1.1–2.4 ms though the host provided strong crystal field environment for Cr³⁺,¹⁰¹ in which SrAl₁₂O₁₉ and SrGa₁₂O₁₉ provided the Al/GaO₆–Al/GaO₆ common-sided octahedron (in Fig. 5a) for high concentration Cr³⁺ to form the Cr³⁺–Cr³⁺ pair to expand the emission bandwidth of Cr³⁺, and the ms level lifetime and nuclear magnetic resonance signal (g_eff ∼ 1.9785) directly manifested the formation of Cr³⁺–Cr³⁺ pairs (in Fig. 5b-c). Meanwhile, the formation of Cr³⁺–Cr³⁺ also endows NIR emission with a high quantum yield of 85%, suppressing the fluorescence quenching under heavy doping conditions. A similar bandwidth widening effect was also observed in Na(Al/Ga)₁₁O₁₇:Cr³⁺ and Mg_0.8Zn_0.2GaO₄:Cr³⁺ phosphors,^102,103 whose emission bands all gradually transformed from the narrowband line-spectrum into wideband emission, accompanied by the formation of a Cr³⁺–Cr³⁺ pair by enhancing the Cr³⁺ concentration. In addition, the above two Cr³⁺-activated phosphors possessed large FWHMs of more than 200 nm and high PLQY exceeding 80%, although the doped Cr³⁺ was located in a strong crystal field environment.

Fig. 5 Crystal structure (a) of SrM₁₂O₁₉ (M = Al and Ga) (copyright 2021, the American Chemical Society), (b) emission and excitation spectra analysis of SrGa_xAl_12−xO₁₉:Cr³⁺ (copyright 2021, the American Chemical Society), and (c) corresponding EPR characterization (copyright 2021, the American Chemical Society).¹⁰¹ (d) Concentration-dependent spectra of LaMgGa₁₁O₁₉:0.7Cr³⁺ (copyright 2023, Springer).¹⁰⁴ (e) In situ EPR spectra of LiGa₅O₈:xCr³⁺ (x = 0.26 and 0.30) samples with and without irradiation at room temperature (copyright 2024, Wiley); (f) fluorescence quantum yields of LiGa₅O₈:Cr³⁺ samples (copyright 2024, Wiley).¹⁰⁵

To further enlarge the bandwidth and expand the emission range to the NIR-II (1000–1800 nm) region for better NIR spectroscopy detection technology, NIR-II exhibits lower tissue absorption and scattering coefficient than NIR-I (750–950 nm) and leads to deeper penetration, lower auto-fluorescence, and a higher imaging signal-to-noise ratio.¹⁰⁶ According to Fig. 5d, the Cr³⁺–Cr³⁺ aggregation was introduced into LaMgGa₁₁O₁₉ to realize dual-band emission centered at 890 and 1200 nm, covering the NIR-I and NIR-II regions.¹⁰⁴ Under low doping conditions, the phosphor possessed only a single emission band located at the NIR-I (750–950 nm) regions, but the new emission band gradually occurred at about 1200 nm with enhancing the Cr³⁺ concentration due to the inter-valence charge transfer (IVCT) of Cr³⁺ + Cr³⁺→ Cr²⁺ + Cr⁴⁺ between neighboring Cr³⁺–Cr³⁺ pairs. LaMgGa₁₁O₁₉:0.7%Cr³⁺ possessed an ultra-wide NIR band with an FWHM of about 626 nm under excitation of the 440 nm light, which provided an effective way to acquire a wideband NIR phosphor covering the NIR-I and NIR-II regions. Furthermore, the IVCT of Cr³⁺ pairs with an emission peak at 1210 nm was also observed in the LiGa₅O₈:Cr³⁺ phosphor, and the in situ electron paramagnetic resonance (EPR) was applied to confirm the transition of Cr³⁺ to Cr²⁺/Cr⁴⁺ under the light excitation process.¹⁰⁵ As shown in Fig. 5e, there emerged three EPR signal peaks associated with the Cr³⁺–Cr³⁺ pairs interaction after light excitation, where the peaks with g_eff factors of 2.0000(2), 2.0438(2) and 1.9994(2), severally attributed to the Cr³⁺–Cr³⁺, Cr³⁺–Cr²⁺ and Cr³⁺–Cr⁴⁺ pairs. The EPR signals before and after light excitation directly demonstrate the rationality of the IVCT fluorescence mode.

Because the Cr³⁺ pairs are composed of adjacent Cr³⁺ with electronic interactions, the distance between the two neighboring Cr³⁺ and the corresponding octahedron connection form directly determines the probability of Cr ion-pair formation. The relevant research indicated that Cr³⁺ pairs are easily observed in corundum-type, magnetoplumbite-type and spinel-type compounds with face-shared or edge-shared octahedrons, where the distance of two adjacent lattice sites for Cr³⁺ doping is below about 3 Å.^107,108 Such structures include the corundum structure Cr₂O₃ and spinel structure Mg/Zn(GaCr)₂O₄ containing edge-shared octahedrons with Cr³⁺–Cr³⁺ distance about 2.650 Å and 2.931 Å,^109,110 the magnetoplumbite structure SrCr₉Ga₃0₁₉ containing face-shared octahedrons with Cr³⁺–Cr³⁺ distance about 2.634 Å. In this case, there is a high possibility of developing the Cr³⁺–Cr³⁺ pairs fluorescence materials based on a host rich in edge-shared and face-shared octahedrons. Generally, the heavy doping of Cr³⁺ limits the luminescence efficiency (in Fig. 5f) of NIR light due to the fluorescence quenching phenomenon caused by high Cr³⁺ concentrations. So, there is still a serious challenge to balance the bandwidth and the efficiency of the NIR phosphors.

3.4. Cr³⁺/Cr⁴⁺ valence regulation strategy

As a 3d² configuration activator for NIR light, Cr⁴⁺ usually exhibits wideband emission ranging from 1100 to 1600 nm, originating from the spin-allowed ³T₂(³F) → ³A₂(³F) transition of Cr⁴⁺.^111,112 Unlike Cr³⁺, Cr⁴⁺ tends to enter tetrahedral sites whose ground state is ³T₂(³F) level and the excitation state are ³T₁(³P), ³T₁(³F), ¹E and ³T₂(³F) levels (in Fig. 6a). In theory, Cr⁴⁺ exists the spin-forbidden transition ¹E → ³A₂(³F) and spin allowable ³T₂(³F) →³A₂(³F) transition, where the two transitions happen at strong (Dq/B > 3.71) and weak crystal fields (Dq/B < 3.71) and lead to sharp-line emission and broadband NIR emission, respectively.^113,114 Although the sharp-line emission of Cr⁴⁺ is difficult to observe, the emission of Cr⁴⁺-activated phosphors exhibits wide coverage and a large FWHM. As shown in Fig. 6b, the Cr⁴⁺-activated LiAlO₂, NaAlO₂, and NaGaO₂ phosphors perform wideband NIR emission centered about 1200–1400 nm with FWHM larger than 200 nm, in which the Cr⁴⁺ enters the AlO₄ or GaO₄ tetrahedron in the host.¹¹⁵ Meanwhile, the emission peak gradually red-shifts from 1260 to 1420 nm with large radius cations replacing the host cations, which indicates that the wideband emission characteristics of Cr⁴⁺ are also as easy to regulate as those of Cr³⁺. Since Cr⁴⁺ possesses a large bandwidth and suitable emission peaks, it is a good choice to simultaneously co-dope the Cr³⁺ and Cr⁴⁺ in the host to construct ultra-wide NIR emission covering the NIR-I, NIR-II and NIR-III regions.

Fig. 6 Tanabe–Sugano diagram of Cr⁴⁺ at tetrahedrons (a) (copyright 2022, Wiley)¹¹³ and PL spectra of tCr⁴⁺-doped LiAlO₂, NaAlO₂ and NaGaO₂ phosphors (b) (copyright 2024, the American Chemical Society).¹¹⁵ PL spectra of Sc_2–2xGa_2xO₃:Cr³⁺, Cr⁴⁺ (c) (copyright 2023, the American Chemical Society),¹¹⁶ Sc₂O₃:Cr³⁺, Cr⁴⁺ and Sc₂O₃:Cr³⁺, Yb³⁺, Cr⁴⁺ (d) (copyright 2023, Wiley).¹¹⁷ NIR spectra (e) of the Mg₂GeO₄-based phosphors under low and high Cr doping conditions (copyright 2021, Elsevier).¹¹⁸

The existence of Cr³⁺ and Cr⁴⁺ makes the Mg₂SiO₄:Cr³⁺ phosphor possess a wide coverage area from 650 to 1400 nm, in which the Cr³⁺ and Cr⁴⁺ simultaneously enter the MgO₆ and SiO₄ octahedron and tetrahedron.¹¹⁶ The large difference in emission intensity between Cr³⁺ and Cr⁴⁺ makes it difficult to expand the FWHM of the NIR emission. The Cr³⁺, Cr⁴⁺ co-doped Sc_2–2xGa_2xO₃ exhibited two emission bands in the range of 700–1100 and 1100–1700 nm with approximate intensity (in Fig. 6c), where the Cr³⁺ enters the ScO₆ octahedron and the Cr⁴⁺ accesses the GaO₄. But there still is a valley in the 1000–1000 nm region because of the little overlap between the ⁴T₂ → ³A₂ PL band of Cr³⁺ and the ³T₂ → ³A₂ PL band of Cr⁴⁺, which still limits the effective expansion of the FWHM of NIR emission.¹¹⁹ To overcome the emission trough problem, Yb³⁺ was introduced into the Sc₂O₃:Cr³⁺, Cr⁴⁺ and CaGa₄O₇:Cr³⁺, Cr⁴⁺ phosphors as a bridge to connect the emission bands between Cr³⁺ and Cr⁴⁺.^117,120 In particular, for the Sc₂O₃:Cr³⁺, Cr⁴⁺, Yb³⁺ phosphor (in Fig. 6d), its emission band uniformly covers a range of 800–1600 nm.

There remains an inevitable problem of effectively controlling the valence of Cr in a single composition phosphor. Regarding this issue, Liu's group successfully realized controllable Cr valence in a Mg₂GeO₄ host (in Fig. 6e). The transformation of Cr³⁺ to Cr⁴⁺ or a mixed valence can be feasibly controlled by adjusting the Li⁺ content and sintering conditions.¹¹⁸ With excessive Li⁺, the doped Cr element tends to be Cr³⁺ and goes into the MgO₆ octahedron with a Li⁺ to maintain charge balance, which mainly displays the NIR emission of Cr³⁺ ranging from 650–1300 nm. While in the Li⁺ deficiency and high-temperature conditions, the doped Cr element is mainly Cr⁴⁺ and enters the GeO₄ tetrahedron, which performs the wideband emission of Cr⁴⁺ in the region of 1000–1600 nm. By controlling the amount of Li₂CO₃ in the raw materials, it is easy to obtain comparable dual emission from Cr³⁺ and Cr⁴⁺ to cover a super broad range from 650 to 1600 nm with an FWHM value of about 500 nm. The above results indicate that co-doping Cr³⁺ and Cr⁴⁺ is a promising way to broaden the Cr³⁺ bandwidth. In particular, the present study on Cr³⁺ and Cr⁴⁺ co-doping NIR phosphors are still relatively scarce. There are still great research possibilities and problems to be explored in Cr³⁺ and Cr⁴⁺ co-existence phosphors, such as the fluorescence thermal stability, light conversion efficiency, and controllability of the valence state of the activator.

3.5. Energy transfer strategy

The wideband absorption and tunable emission features of Cr³⁺ make it suitable for co-doping with rare earth (Ln) ions (Ce³⁺, Yb³⁺, Er³⁺, Ho³⁺), transition metal (TM) ions (Mn⁴⁺, Ni²⁺), and ns² configuration ions (Bi³⁺, Sb³⁺) to realize fluorescence modulation through the energy transfer (ET) process.^121–123 Thus, a similar strategy can be used to achieve spectrum broadening in Cr³⁺-activated NIR phosphors. The existence of co-doping of ions with Cr³⁺ mainly comprises two kinds: the Ln³⁺ ions (Yb³⁺, Nd³⁺) with NIR emission ranging from 800 to 1100 nm and the other is the TM ions (Ni²⁺) with broadband NIR emission in the 1100–1700 nm region.^124,125

Regarding the Cr³⁺ and Ln³⁺ co-doping strategy, Yb³⁺ is usually considered a popular co-dopant to both broaden the FWHM and increase the thermal quenching resistance of the NIR emission owing to the simple and stable NIR emission levels (²F_5/2 → ²F_7/2) ranging from 900 to 1100 nm, the relatively large FWHM from 30 to 80 nm, and the relatively better thermal stability than Cr³⁺.¹²⁶ The Cr³⁺–Yb³⁺ ET strategy was initially adopted in YAl₃(BO₃)₄:Cr³⁺,Yb³⁺ phosphor to improve the silicon solar cell photovoltaic conversion efficiency using the effective absorption of Cr³ and emission of Yb³⁺ property.¹²⁷ With the development of wideband near-infrared light source research, numerous excellent studies have emerged on ultra-wide NIR phosphors (FWHM > 200 nm) using the Cr³⁺ and Yb³⁺ co-doping methods. Zhang et al. (in Fig. 7a) reported that the introduction of Yb³⁺ effectively increased the FWHM of Ca₂LuZr₂Al₃O₁₂:Cr³⁺ phosphors from 150 to 320 nm and maintained a good IQE of 69%.¹²⁸ A similar bandwidth gaining effect was realized in Cr³⁺, Yb³⁺ co-doped La₂MgHfO₆, Lu_0.2Sc_0.8BO₃, LiScP₂O₇, and Ca₄ZrGe₃O₁₂ phosphors,^129–132 which respectively displayed large emission bandwidths of 333, 303, 230 and 210 nm and excellent IQEs of 69,73, 75 and 74%. The high IQE and wideband emission features of the above NIR phosphors indicate that co-doping with Yb³⁺ is an effective way to broaden the emission bandwidth for improved NIR light source applications. Based on the Cr³⁺ and Yb³⁺ co-doped conditions, Ce³⁺ exhibiting red light emission was also induced in Ba₂SrSc₄O₉ to further expand the spectrum bandwidth (in Fig. 7b), whose emission band covers a wide region from 500 to 1200 nm.¹³³ There were three energy transfer paths in the tri-doped Ba₂SrSc₄O₉ of the Ce³⁺ → Yb³⁺, Ce³⁺ → Cr³⁺ and Ce³⁺ → Cr³⁺ → Yb³⁺(in Fig. 7c), which endowed the phosphor with tunable wideband emission and displayed large application potential in the component detection field.

Fig. 7 PL and PLE spectra of Ca₂LuZr₂Al₃O₁₂:Cr³⁺ and Ca₂LuZr₂Al₃O₁₂:Cr³⁺, Yb³⁺(a) (copyright 2020, Wiley).¹²⁸ PL spectra (b) and fluorescence mechanism (c) of Ba₂SrSc₄O₉:Ce³⁺,Yb³⁺ Cr³⁺ (copyright 2024, Wiley).¹³³ Energy transfer diagram (d) between Cr³⁺ and Ni²⁺ and concentration-dependent PL spectra (e) of MgGeOF₄:Cr³⁺,Ni²⁺ (copyright 2023, the Royal Society of Chemistry).¹³⁴

To achieve efficient broadband emission in Cr³⁺ and Yb³⁺ co-doping NIR emitting phosphors, the emission property of Cr³⁺ should satisfy the following conditions. First, Cr³⁺ is located at the weak (Dq/B < 2.3) crystal field or critical field coordination environment with wideband emission covering the 800–1000 nm region, which partially overlaps with the Yb³⁺ emission band. When the emission bands of Cr³⁺ and Yb³⁺ are distant or very close, Cr³⁺ just plays the sensitizer role to enhance Yb³⁺ emission, which can hardly realize emission band broadening.^135,136 Second, the ET process between Cr³⁺ and Yb³⁺ should be controlled to avoid the fluorescence quenching of Cr³⁺ induced by high Yb³⁺ doping concentrations. Lastly, Cr³⁺ should possess good emission intensity and thermal quenching (TQ) resistance to match the emission performance of Yb³⁺ because Yb³⁺ usually has good luminescence intensity and excellent fluorescence thermal stability resulting from the simple energy level of Yb³⁺. Among the existing Cr³⁺ and Yb³⁺ co-doped NIR light-emitting materials, phosphors based on garnet and double-perovskite structures are much easier to realize high efficiency, thermal stability and large FWHM NIR emission.¹³⁷ In this case, it is a good choice to develop broadband NIR phosphors based on a garnet- and double-perovskite structure host by simultaneously adopting Cr³⁺, Yb³⁺ co-doping and crystal field regulation strategies.

In addition to Cr³⁺, Ni²⁺ is another prospective activator to achieve broad band NIR emission originating from the ³T₂ → ³A₂ electron transition due to the ultra-wide emission region covering almost the whole shortwave infrared radiation region (SWIR 900–1700 nm) at the weak crystal field (Dq/B < 1.8).¹³⁸ Ni²⁺ activated Mg₃Ga₂GeO₈, MgGa₂O₄, and KMgF₃ phosphors all exhibited ultra-wide NIR emission with FWHM values up to 300 nm,^139–141 but their low efficiency and poor matching degree with blue light chips still hinder their applications in NIR light sources. In consideration of the Cr³⁺ possessing the high quantum yield and wide absorption band in UV and blue light regions, as well as the large overlap with the absorption band of Ni²⁺,^142,143 it is a promising way to acquire high efficiency and broadband NIR emitting phosphors through the Cr³⁺, Ni²⁺ co-doping strategy. The Cr³⁺ and Ni²⁺ co-doping LaZnGa₁₁O₁₉, Mg₂SnO₄, Li₂ZnSn₃O₈, and Mg₃Ga₂GeO₈ phosphors all displayed a wide coverage from 650 to 1600 nm region almost covering the whole deep red and SWIR regions,^144–147 which meanwhile possessed the enhanced quantum yield and good response to blue light chip through the efficient ET process from Cr³⁺ to Ni²⁺ (in Fig. 7d). The Cr³⁺/Ni²⁺ co-doping method allows the NIR phosphors to be used in multifunctional spectroscopy applications. The disconnected emission band (in Fig. 7e) of Cr³⁺ and Ni²⁺ leads to the evident emission trough between 1000 and 1200 nm, resulting from the main emission band of Ni²⁺ is usually after 1200 nm, which achieves the fluorescence enhancement of Ni²⁺ and expansion of the NIR light covering region, but no evident gaining in broadening the FWHM value.¹³⁴ To make up the emission trough, the following methods may be considered: the construction of the Cr³⁺ → Cr⁴⁺ → Ni²⁺ ET pathway to acquire the ultra-wide NIR emission by using the broadband emission around the 1200 nm Cr⁴⁺ as an ET bridge to enhance the emission intensity in the 1000–1400 nm regions; searching the suitable host fluorescent combined with the crystal field control strategy, which can endow the emission spectra of Cr³⁺ and Ni²⁺ possessing the natural overlap.

3.6. Cr³⁺ doping induces a new fluorescence center

It is generally accepted that Cr³⁺ emits light only in the six-coordination crystal environment, so materials rich in octahedral groups are usually considered as the fluorescence host for Cr³⁺ in reported research.^148,149 In some special cases, Cr³⁺ doping-induced local structure distortion produces a new CrO₆ octahedron, which enables materials without intrinsic octahedrons to achieve NIR emission of Cr³⁺. In SrGeO₃, SrGa₂Si₂O₈ and SrGa₄O₇ materials,^150–152 there was no intrinsic octahedron site for Cr³⁺ to substitute, but the Cr³⁺ doping let the GeO₄ or GaO₄ tetrahedron distorted into the octahedron (in Fig. 8a), resulting in the new generated CrO₆ fluorescence center severally giving wideband NIR emission with the FWHM about 214, 160 and 140 nm. In some other host materials with a single octahedron, the structure distortion caused by the doped Cr³⁺ also generated a new CrO₆ fluorescence center, which was coupled with the intrinsic CrO₆ luminescence center to further effectively extend the FWHM of Cr³⁺. In CaMgSi₂O₆:Cr³⁺ NIR phosphors,¹⁵³ the low-concentration Cr³⁺ displayed one emission center at 785 nm originating from the Cr³⁺ entering the intrinsic MgO₆ octahedron. The high concentration of Cr³⁺ led to a new fluorescence center at 980 nm due to the large structure distortion induced by Cr³⁺ (in Fig. 8b), which allowed the phosphor to display double band emission with the evidence of bandwidth broadening from 138 to 373 nm.

Fig. 8 Two GaO₄ tetrahedrons distorted into octahedrons (a) induced by Cr³⁺ in a SrGeO₃ host (copyright 2024, Elsevier).¹⁵⁰ PL and PLE spectra of CaMgSi₂O₆:Cr³⁺ phosphor, and MgO₆ octahedron distortion caused by Cr³⁺ (b) (copyright 2024, Elsevier).¹⁵³ PL spectrum of SrMgGe₂O₆:3%Cr³⁺ at 4 K and the structure of SrMgGe₂O₆:Cr³⁺(c) together with a schematic of Cr³⁺ entering the MgO₆ and CaO₆ groups (d).

No matter the tetrahedron distortion into octahedron or the enhanced octahedron distortion degree induced by Cr³⁺, there is no direct evidence to demonstrate the structure distortion from an experimental or theoretical perspective. To further prove the rationalization of Cr³⁺ induced structure distortion to construct a new fluorescence center, our latest work chose the Cr³⁺ doped Sr/CaMgGe₂O₆ phosphors as the research objects and observed a similar FWHM expanding phenomenon similar to that of the CaMgSi₂O₆:Cr³⁺ phosphor. As shown in Fig. 8c, the PL spectrum of CaMgGe₂O₆:Cr³⁺ at 4 K could be divided into two emission bands, indicating that the two fluorescence centers existed in the CaMgGe₂O₆:Cr³⁺ phosphors. While SrMgGe₂O₆ was constructed using CaO₈ dodecahedrons, MgO₆ octahedrons and GeO₄ tetrahedrons (in Fig. 8d), it only possesses one kind of intrinsic MgO₆ octahedron for Cr³⁺ to enter. To analyze the effect of Cr³⁺ on the CaMgGe₂O₆ host, a Cr atom was placed into the [GeO₄] tetrahedron, [MgO₆] octahedron and [CaO₈] dodecahedron to conduct the structure relaxation process using DFT calculation. After the structure relaxation process, the Cr atom in [GeO₄] tetrahedron and [MgO₆] octahedron were coupled severally to the surrounding four and six oxygen atoms to form stable [CrO₄] tetrahedrons and [CrO₆] octahedrons. When the Cr atom enters the [CaO₈] dodecahedron, the two more distant coordination oxygen atoms move away from the Cr center, and Cr interacts with the six nearby oxygen atoms to form the [CaO₆] octahedron. The charge density difference and partial density of state analysis were also applied to prove that Cr in the [CaO₈] group has a strong interaction with the six adjacent oxygen atoms. The first-principles calculation results surprisingly indicated that the Ca²⁺ substitution by Cr³⁺ caused the CaO₈ dodecahedron to be distorted into the CrO₆ octahedron due to the Jahn–Teller effect, which generated a new emission and greatly expanded the FWHM of the sample from 160 to 325 nm. Research on Cr³⁺-induced new fluorescence centers is still relatively lacking, and the relevant formation mechanisms of fluorescence centers are still difficult to determine experimentally. Thus, related work can be further studied to explore wideband emission NIR phosphors.

4. Conclusion and outlook

This review briefly analyzes the relationship between the emission properties of Cr³⁺ and the crystal field circumstances. Furthermore, the corresponding methods to broaden the emission spectrum of Cr³⁺ were summarized into six types based on published literature, which involved the aspects of crystal field regulation, the construction of Cr³⁺–Cr³⁺ pairs, multi-crystal site engineering, Cr³⁺/Cr⁴⁺ valence regulation, energy transfer and the new fluorescence center induced by Jahn–Teller effect of Cr³⁺. Existing investigations have developed several Cr³⁺-activated phosphors with wideband emission, high quantum yield and good stability, which display great application potential in NIR spectrum detection technology. There are still inescapability challenges and underlying opportunities for the development of excellent Cr³⁺-activated NIR phosphors in future work, including the following:

(1) The crystal field regulation method displayed an evident bandwidth regulation effect for Cr³⁺, whereas the degradation of crystal symmetry and the increase in crystal vacancy influenced the fluorescence thermal stability.

(2) Owing to the different occupation tendencies of Cr³⁺ at distinct crystal sites, the intensity and bandwidth of Cr³⁺ at multi-site crystals are hard to reach their maximum simultaneously. High-concentration doping usually enlarges the FWHM of Cr³⁺ but does harm its intensity and thermal stability, which inhibits the relevant application potential.

(3) The formation of Cr³⁺–Cr³⁺ pairs and a new fluorescence center induced by Cr³⁺ is closely related to the crystal structure of the host materials, but there is no evident reference or standard for searching suitable hosts, and the corresponding mechanism needs to be further studied.

(4) The energy transfer strategy between Cr³⁺ and Yb³⁺ or Ni²⁺ usually results in longer emission wavelengths and increased luminescence efficiency. However, the significant loss of Cr³⁺ luminescence is usually accompanied by the ET process, which also raises the problem that the maximum value of bandwidth and luminous intensity cannot be reached at the same time.

(5) Excellent fluorescence thermal stability is an important application premise for phosphors, so the thermal stability of Cr³⁺-activated wideband emission phosphors should also be considered. To realize high thermal stability NIR emission of Cr³⁺, strategies for selecting a high rigidity matrix, structural rigidity regulation, energy transfer and defect-assisted luminescence processes should be considered.

(6) Cr³⁺-activated NIR phosphors have been verified to possess great application potential in plant growth, food analysis, biomedical imaging, night vision, and other fields. Exploring new applications of NIR light is also important for future research.

In summary, Cr³⁺-activated NIR phosphors have attracted significant research and application attentions. With the unremitting efforts of the researchers, it is expected that the development of effective and stable NIR phosphors and their corresponding applications will extend to more extensive fields.

Author contributions

Chen Changheng: conceptualization, investigation, formal analysis and writing – original draft. Jiwen Chang: investigation and writing – review & editing. Renze Chen: formal analysis and writing – review & editing. Ruibo Gao: writing – review & editing. Yiqing Wang: formal analysis. Kexin Zhu: investigation. Jinmeng Xiang: funding acquisition and writing – review & editing. Chongfeng Guo: conceptualization, writing – review & editing, funding acquisition, methodology and supervision.

Data availability

No primary research results, software or code have been included, and no new data were generated or analyzed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 12374383, 12404462), Youth Innovation Team of Shaanxi Universities (No. 23JP167) and Natural Science Basic Research Program of Shaanxi (Program No. 2019JZ-32, 2024JC-YBQN-0027).

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