Long-wavelength near-infrared emission in chromium-activated LiZnNbO₄ spinel crystals and valence-converting enhancement via Er³⁺ ion heterotopic doping

Abstract

Realizing efficient long-wavelength near-infrared (NIR) emission of Cr³⁺ ions is still a challenge in spinel-based phosphors due to the limitations of strong crystal fields. Here, we propose three design strategies for obtaining weak crystal fields in spinel-type crystals, and report for the first time designed Cr-activated LiZnNbO₄ (LZNO) crystals with unique weak crystal fields and application of an NIR enhancement strategy via heterotopic and heterovalent doping with trivalent rare earth ions. Under irradiation with 468 nm blue light, the phosphor presents ultra-wideband NIR emission centered at 800 nm covering the region of 650–1300 nm, which is attributed to the larger radius, high valence state of cations and the low symmetry of octahedral sites in the LZNO spinel. Supported by XRD refinement, XPS analysis, and density functional theory (DFT) calculation results, it was shown that when Er³⁺ ions are designedly doped with the Zn²⁺ sites of the spinel crystals, effective promotion of the valence state transformation of Cr⁴⁺ to Cr³⁺ in the LZNO:Cr³⁺ system is achieved by defect charge compensation, and this enhances the NIR emission by nearly 3 times. The results of this work not only enrich the material family of Cr³⁺-activated NIR emitting phosphors, but also offer a novel and simple method for improving the luminescence efficiency of Cr³⁺-activated phosphors.

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1 Introduction

NIR light sources have received increasing attention due to their wide range of applications in night vision, plant growth, biosensing, quantitative composition analysis and nondestructive testing.^1–4 In recent years, the emergence of NIR-emitting phosphor-converted light-emitting diodes (NIR pc-LEDs) has greatly expanded the development and utilization of NIR technology in practical applications due to their low cost, long lifespan, miniaturization, highly efficient luminosity and fast response characteristics.^5,6 The key to the production of efficient NIR pc-LEDs is the preparation of NIR luminescent phosphors that can be effectively pumped by blue light. Therefore, researchers have made significant efforts to explore a large number of NIR inorganic luminescent phosphors doped with active ions, including transition metal ions (e.g., Cr³⁺, Cr⁴⁺, Fe³⁺, Ni²⁺, and Mn⁴⁺), Bi²⁺ ions and lanthanide ions (e.g., Pr³⁺, Nd³⁺, Eu²⁺, Tm³⁺, Yb³⁺ and Er³⁺).^7–13 Among them, the transition metal ion Cr³⁺ can demonstrate broad absorption capacity that is well-aligned with blue LED chips and it can emit photons ranging from 650 to 1300 nm when located in a weak octahedral crystal field.^14–17 This enhances its potential for use in the creation of NIR light source devices, addressing the crucial issue of emerging phosphorus powder-coated NIR pc-LEDs. However, just like every coin has two sides, long-wavelength emission is frequently associated with low luminous efficiency. Many reported Cr³⁺ phosphors still exhibit low photoluminescence efficiency and insufficient full width at half maximum (FWHM). This greatly hinders the practical application development of NIR pc-LEDs.^18–20 Therefore, research work on developing new Cr³⁺-activated materials, as well as methods that can effectively improve their luminescence efficiency, are still urgently needed.

Spinel materials are considered as excellent hosts for photoluminescence because of their ability to adjust crystal structures and their suitability for ion doping.²¹ However, most of the octahedral sites of spinel phosphors typically exhibit strong crystal fields, leading to the emission of the doped Cr³⁺ ion being located at about 700 nm (several Cr³⁺-activated spinel-type NIR phosphors are listed in Table S1†).²² This NIR emission property greatly limits the practical application of spinel phosphors in some scenarios like food analysis^23,24 and biological imaging.²⁵ However, until now, from the perspective of materials selection and design, there is little understanding of obtaining long-wavelength broadband NIR in Cr³⁺-activated spinel phosphors.

The long wavelength emission of Cr³⁺ ions in octahedral sites is most likely due to weak crystal fields. This phenomenon is frequently observed in Cr³⁺-doped garnet-structured phosphors featuring local low-symmetry unit cells.^26–29 On the basis of these findings, we recognize that this feature is possibly applicable in achieving Cr ion long-wavelength broadband emission in spinel phorhpors. Based on controlling the redshift of Cr ion emission wavelength in garnets, we propose the following design principles for achieving Cr ion long-wavelength broadband emission: 1. (high valence state) the higher the valence state of the cation, the larger the volume of its coordination polyhedron, which can weaken the crystal field around Cr; 2. (large radius) the larger radius of the cation always corresponds with longer bond lengths and greater metal–ligand distances, which can decrease the crystal field; and 3. (low symmetry) the large spatial structure and low symmetry are more likely to cause polyhedral distortion, thereby causing uneven distribution of electric fields and intensifying the splitting of Cr³⁺ sub-energy levels.

According to the above given criteria, we initially considered designing phosphors with niobate materials, as they contain Nb⁵⁺ ions with high valence and larger ionic radius. Then the LiZnNbO₄ compound was selected because it can manifest the behavior of a superstructure or existence of empty interstitial space.³⁰ This feature can easily result in local distortion in the doping process, as well as low local symmetry. In addition, it is also expected to provide suitable octahedral sites for Cr³⁺. Therefore, these structural characteristics indicate that the compound has significant potential for producing long-wavelength broadband NIR emission activated by Cr³⁺.

On the other hand, it is important to mention the dominant factor that affects the luminescence of Cr³⁺-doped phosphors. It is common in the high-temperature synthesis of phosphors that the self-oxidation process of Cr³⁺-doped materials sintered in air will, in general, partially form Cr⁴⁺ ions.³¹ Due to the low and unsatisfactory actual formation rate of Cr³⁺, a significant amount of luminescence “killer” Cr⁴⁺ with intense absorption in the NIR region can function as a non-radiative relaxation center, capturing energy and significantly impacting the luminescence efficiency of the phosphor.³² Although it can be partially prevented by the reducing atmosphere, this method is not suitable for all hosts. As a result, to address the issue of the low luminescence efficiency of Cr³⁺ due to these mentioned factors, researchers have carried out thorough research and suggested several effective methods. One uses energy transfer^33–35 of suitable sensitizers, such as Er³⁺ ions, to boost their luminescence effectiveness. However, due to the limitation of forbidden f–f transitions, the enhancement effect of rare earth ions is still far from satisfactory in practical applications. Therefore, exploring a suitable method that can inhibit the generation of Cr⁴⁺ simply and effectively will provide new insight into studying Cr³⁺-activated NIR phosphors.

In this work, to address the challenges associated with exploiting new Cr³⁺-activated spinel materials with long-wavelength NIR emission while devising a novel strategy to improve the luminescence efficiency, we exploited the NIR emission of a Cr-doped LiZnNbO₄ spinel system, as well as enhancing the formation of Cr³⁺ ions by heterotopic charge compensation of Er³⁺ ions for the first time. Based on optical and structural analyses and DFT calculations, it is shown that the octahedral sites of the LZNO spinel exhibit special weak crystal fields, and Cr³⁺-ion dopants present NIR emission centered at 800 nm as supposed. Moreover, when the Er³⁺ ion is selected for designed heterotopic doping at the Zn²⁺ site, Cr⁴⁺ luminescence assassins, which occupy the octahedral sites of Nb⁵⁺ ions, are transformed into beneficial Cr³⁺ emission centers by defect charge compensation. The universality experiments indicated that the strategy of heterotopic and heterovalent doping of trivalent rare earth ions offers a simple but efficient way to enhance the NIR luminescence efficiency of Cr³⁺ phosphors. Finally, under the excitation of a 470 nm blue laser, the quantum efficiency of LiZn(Er)NbO₄:Cr³⁺ is enhanced from 8.3% to 17.6%. The use of pc-LEDs in combination with commercial blue-light chips demonstrated significant potential in biological organ imaging and night-vision applications.

2 Experimental section

2.1 Materials and preparation

Powder samples containing LZNO:xCr³⁺ (x = 0–0.03) and LiZn_1−y(Er)_yNbO₄:0.01Cr³⁺ (y = 0–0.02) compositions were synthesized by high-temperature solid-phase reactions. Raw materials of Li₂CO₃ (99.9%), ZnO (99.99%), Nb₂O₅ (99.99%), Cr₂O₃ (99.99%) and Er₂O₃ (99.99%) were weighed stoichiometrically (all raw materials are purchased from Aladdin reagents, https://www.aladdinsci.com), mixed and ground in an agate mortar for at least 30 min to create a well-homogenized powder. Then, the mixture was placed into an alumina crucible and sintered under an air atmosphere at 1100 °C for 4 h until it naturally cooled to room temperature. After cooling to room temperature naturally, the samples were ground into powders, which are the final target phosphors for further measurements. NIR pc-LEDs were fabricated using the optimal composition of LiZn_0.986Er_0.014NbO₄:0.01Cr³⁺ (λ_em = 800 nm) with high-power blue-LED chips (5 W, 470 nm, San'an Optoelectronics Co., Ltd). The phosphor was evenly mixed with epoxy resin in a weight ratio of 1 : 1 and then applied to the chips.

2.2 Characterization

X-ray diffraction (XRD) patterns were obtained using a Bruker D8 X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å) at 40 kV and 40 mA. The general structure analysis system (GSAS) program was used to perform the Rietveld refinements of XRD profiles. The morphological properties were characterized using a scanning electron microscope (SEM, SU8000, Hitachi) equipped with an energy dispersive X-ray spectroscopy (EDS) system. A UV-3600 spectrometer was utilized for recording the diffuse reflectance (DR) spectra. Photoluminescence spectra at room temperature (RT) and different temperatures were recorded using an FLS-980 spectrometer. The NIR photoluminescence quantum yield (PLQY) values were measured using a Quantaurus-QY Plus instrument (C13534-12, Hamamatsu Photonics) and the luminescence decay time was measured using an integrated sphere on an Edinburgh FLS1000 spectrophotometer. X-ray photoelectron spectra (XPS) were recorded using an ESCALAB 250 photoelectron spectrometer (ThermoFisher Scientific, USA) with Al Kα (1486.6 eV) as the X-ray source. Electroluminescence (EL) spectra and the performances of the fabricated NIR pc-LED devices were measured using an integrating sphere (Labsphere) and the data were collected using a high accuracy array spectroradiometer (HAAS-2000, Everfine). Visible images and NIR images were obtained using a modified visible/infrared camera (Canon M100).

2.3 Calculation details

The crystal structures of LZNO, LZNO-Cr and LZNO-Cr-Er were optimized using the Vienna ab initio simulation package, which is a plane-wave pseudopotential total energy package based on density functional theory (DFT).³⁶ The atomic positions and cell parameters were relaxed with an electronic convergence criterion of 1 × 10^–5 eV and an atomic convergence criterion of 0.01 eV Å⁻¹. A cutoff energy of 520 eV was used for the basis set of plane waves and a 2 × 2 × 1 Γ-centered Monkhorst–Pack k-point grid was used to sample the first Brillouin zone. The Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional was employed for structural optimization, density of states and charge density calculations, whereas the subsequent electronic properties were determined using the HSE06 hybrid functional.³⁷

3 Results and discussion

3.1 Phase, crystal structure and morphology

Fig. 1a shows the components of LZNO where it adopts a non-centrosymmetric tetragonal crystal structure with the P4₁22 space group. Usually, asymmetric crystals can result in an uneven distribution of electric fields, which leads to smaller crystal fields.³⁸ In addition, the structure comprises two crystallographically independent octahedral units, LiO₆ and NbO₆, which are linked through shared edges and angles. The ZnO₄ tetrahedra are linked to two octahedra only through shared angles, resulting in a large spatial gap. The Li–O and Nb–O bond lengths in the two octahedra are 2.173 and 2.016 Å, respectively, indicating a greater distance between the metal and the ligand. On the other hand, as the radius of Cr³⁺ (CN = 6, r = 0.615 Å) is closer to that of Nb⁵⁺ (CN = 6, r = 0.64 Å), when the Cr³⁺ ion is doped with LZNO, it is easier to enter the Nb⁵⁺ position with octahedral coordination as a luminescent center. Therefore, the analysis results indicated that the LZNO crystal structure is in line with our proposed three criteria, namely 1. high valence state, 2. large radius, and 3. low symmetry.

Fig. 1 (a) Crystal structure schematic illustration of LZNO. (b) XRD patterns of LZNO:x%Cr³⁺ (x = 0, 0.5, 1, 2 and 3) at room temperature. SEM images of (c) LZNO:0.01Cr³⁺ and (d) LZNO:0.01Cr³⁺,0.014%Er³⁺. (e) EDS mapping of LZNO:0.01Cr³⁺,0.014%Er³⁺.

Fig. 1b shows the diffraction profiles of all the samples and they agree with those of the tetragonal spinel LiZnNbO₄ phase [ICDD no. 23-1206], indicating that the high-purity target product was successfully prepared. The card number was matched using Jade software. Additionally, it should be noted that a previous article reported the ICSD number of the LiZnNbO₄ compound as 85735.³⁹ To further determine the target phase and crystal structure, the XRD data were analyzed by adopting the Rietveld refinement, as shown in Fig. 2. The confidence factors R_wp and χ² are shown in Table S3† and they indicate reliable refinement results for all the samples. The obtained data indicate good phase purity of the synthesized products and yield their structural parameters.

Fig. 2 Rietveld refinement of LZNO:x%Cr³⁺ with (a) x = 0, (b) x = 0.5, (c) x = 1, (d) x = 2, and (e) x = 3. (f) Changes in the cell parameters (a, b, and c) and unit cell volume in LZNO:x%Cr³⁺ with increasing x.

The SEM images of the two representative LZNO:1%Cr³⁺ and LZNO:1%Cr³⁺,1.4%Er³⁺ samples are displayed in Fig. 1c and d. Both samples exhibit good crystallinity and a dense microstructure. It can be seen that the samples contain individual irregular block structures with smooth surfaces, and the particle size is in the range of 20–30 μm. Co-doping with Er³⁺ did not cause significant morphological changes. Moreover, the EDS mapping of the LZNO:1%Cr³⁺,1.4%Er³⁺ sample was also conducted on a randomly selected plate, as shown in Fig. 1e. This further proves the homogeneous distribution of constituent elements in the grain. Notably, the Li element was ionized by the electron beam in the chamber and could not be observed in the EDS mapping spectrum due to the experimental limitations.

3.2 Site occupation and theoretical calculations

In order to clearly determine the site occupancy of Cr ions in the LZNO crystals, the refined cell parameters with increasing doping concentration of Cr³⁺ are plotted, as shown in Fig. 2f. This shows that parameters a, b, c and V initially increase and then decrease, as the concentration of Cr³⁺ increases. This non-linear trend indicates that Cr ions have successively occupied different sites. As is well known, Cr³⁺ prefers to occupy the octahedrally coordinated environment due to its luminescence nature. Therefore, the luminescence properties of Cr³⁺ are typically observed and discussed in the context of a six-coordinated environment, specifically the octahedral site.⁴⁰ Considering the crystal structure of LZNO containing two crystallographically independent octahedral sites (NbO₆ and LiO₆ octahedra), it is meaningful to determine the site occupation preference of Cr³⁺. As stated above, the radius of Cr³⁺ is closer to that of Nb⁵⁺, and it is easier to enter the Nb⁵⁺ sites with octahedral coordination. Nevertheless, taking into account that the substitution of Nb⁵⁺ with Cr³⁺ will result in an excess of free electrons, it probably leads to the formation of defects and charge imbalance. Thus, the Cr³⁺ ion can easily be oxidized into the higher valence state of Cr⁴⁺ (CN = 6, r = 0.55 Å). This is because that when Cr⁴⁺ replaces Nb⁵⁺, it is beneficial for maintaining the stability of the system towards charge balance.

According to the above analysis and taking into account the characteristic emission of Cr³⁺ in the PL spectrum (Fig. 3a and b), we can consider that, at a low Cr³⁺ doping level, Cr³⁺ primarily replaces Zn²⁺ (CN = 4, r = 0.6 Å), and any remaining Cr³⁺ will enter the Nb⁵⁺ site simultaneously to maintain the electrical neutrality of the system. Then, as the Cr-doping content increases, it becomes easier to form Cr⁴⁺ to stably exist at Nb⁵⁺ sites while maintaining the system's electrical neutrality. Undoubtedly, replacing Nb⁵⁺ with Cr⁴⁺ will cause lattice contraction, resulting in a decrease in cell parameters. Therefore, although we only considered replacing Nb⁵⁺ with Cr³⁺/⁴⁺ from the initial experimental observation, we believe that in the actual synthesis process, there is a portion of Cr³⁺ ions that will actively replace the Zn²⁺ ions to maintain a certain charge balance. This could result in a reduction in the creation of efficient luminescent sites. As is well known, the electronic structure of host materials is closely related to the luminescence performance of phosphors. However, the electronic structure of LZNO has never been reported in previous studies. Therefore, to better understand LZNO crystals, their electronic structure was calculated and analyzed using the DFT method and VASP software. The detailed information and descriptions are presented in the ESI (Fig. S1 and S2†).

Fig. 3 (a) DR spectra of LZNO:x%Cr³⁺ (x = 0, 0.5, 1, 2 and 3); the inset is an absorption spectrum of LZNO calculated using the Kubelka–Munk equation. (b) PLE and (c) PL spectra of LZNO:x%Cr³⁺ (x = 0, 0.5, 1, 2 and 3). (d) Excitation spectra of LZNO:x%Cr³⁺ at different monitoring wavelengths. (e) Emission spectra of LZNO:0.01Cr³⁺ measured at 273, 173 and 80 K. (f) PL decay curves of LZNO:x%Cr³⁺ (x = 0, 0.5, 1, 2 and 3) monitored at 800 nm.

3.3 Photoluminescence

The DR spectra of LZNO:x%Cr³⁺ (x = 0.5, 1, 2 and 3) were recorded as shown in Fig. 3a. The absorption spectrum of the undoped sample is consistent with the calculated electronic structure of LZNO crystals (Fig. S2†). This indicates an indirect bandgap with allowed transitions, which results in an optical bandgap of 2.81 eV. The Cr³⁺-substituted samples exhibit intense absorption at around 463 and 653 nm in the DR spectrum, corresponding to the ⁴A₂ → ⁴T₁ (4F) and ⁴A₂ → ⁴T₂ (4F) Cr³⁺ electronic transitions, respectively. The optical bandgap (E_g) of LZNO can be evaluated by fitting the data with eqn (1):^41,42

[hνα]^1/n = A(hν − E_g) (1)

where hν, α, E_g, and A refer to the photon energy, absorption coefficient, bandgap, and proportionality constant, respectively. The value of the exponent n is determined by the properties of electronic transition. The n values for the direct allowed transition, direct forbidden transition, indirect allowed transition, and indirect forbidden transition are 1/2, 3/2, 2 and 3, respectively.^43–45 The obtained bandgap of 3.12 eV (Fig. 3a) is in good agreement with our theoretical calculations. Fig. S3a† shows the band gap changes of LZNO after doping it with different concentrations of Cr³⁺. It can be clearly seen that as the Cr³⁺ doping concentration increases from 0.5% to 3%, the band gap of LZNO gradually decreases from 3.08 eV to 2.33 eV. According to Fig. S1,† Cr³⁺ doping will form impurity energy levels near the top of the valence band of the LZNO host, leading to a gradual decrease in the bandgap width.

Significantly, we did not detect any distinctive absorption peaks associated with Cr⁴⁺, as mentioned earlier. This is an intriguing phenomenon, as Cr⁴⁺ seems to be playing a game of “hide and seek” with us. To coax out this “naughty mouse,” we conducted tests on the DR spectrum of LZNO highly doped with Cr³⁺ (Fig. S3†). When the Cr³⁺ doping concentration reaches or exceeds 4%, a new absorption peak emerges at 575 nm, unmistakably associated with the characteristic ³A₂ → ³T₁ (³F) absorption transition of Cr⁴⁺.⁴⁶ Cr⁴⁺ typically undergoes specific energy level transitions only in a tetrahedral coordination environment, whereas the Cr⁴⁺ in LZNO remains stable in an NbO₆ octahedral coordination environment, making it undetectable. As the doping level of Cr³⁺ increases, a larger amount of Cr⁴⁺ is more readily produced in the system. Consequently, to maintain charge balance, Cr⁴⁺ will actively occupy the ZnO₄ tetrahedral site, leaving it with no means of escape.

Then, to verify Cr³⁺ as the luminescent ion in the LZNO host, both the excitation spectra (Fig. 3b) and the emission spectra (Fig. 3c) of LZNO with Cr concentrations from 0.5% to 3% were investigated. The excitation spectrum covers a wide region of 250–750 nm, and the peaks centered at 325, 468 and 650 nm correspond to the ⁴A₂ → ⁴T₁ (4P), ⁴A₂ → ⁴T₁ (4F) and ⁴A₂ → ⁴T₂ (4F) spin-allowed transitions of Cr³⁺ in a weak octahedral crystal field.^47,48 Under 468 nm excitation, the phosphor exhibits an ultra-broadband emission spectrum centered at 800 nm with an FWHM of about 200 nm, which is attributed to the spin-allowed ⁴T₂ → ⁴A₂ (4F) transition of the octahedrally coordinated Cr³⁺ ions.⁴⁹ Concurrently, the emission intensity of Cr³⁺ is less affected by the doping concentration, which may also be related to the limited luminescence efficiency of Cr³⁺ owing to the increased content of non-emission Cr⁴⁺.

In order to confirm whether the crystal field environment of Cr³⁺ in the LZNO host meets our standards as a weak field, we performed quantitative calculations. The crystal field strength can be characterized by crystal field splitting (D_q) and the Racah parameter (B) calculated using eqn (2)–(4):⁵⁰

10D_q = E(⁴T₂) = E(⁴A₂ → ⁴T₂) (2)

The calculated D_q and B parameters equal to 1459.8 and 722.7 cm⁻¹ yield a D_q/B ratio of 2.02 upon examination of the data. The corresponding Tanabe–Sugano energy level diagram of the Cr³⁺ ion in octahedral coordination is shown in Fig. S4.† This indicates that the [NbO₆] octahedra in the LZNO host indeed provide a relatively weak crystal field environment for Cr³⁺. The principles we proposed in this work may offer practical solutions for designing long-wavelength broadband NIR emitting spinel materials and even other material systems activated by Cr³⁺ in weak crystal fields.

In addition, we observed that the Cr³⁺ emission peak in the LZNO host is asymmetric. Since the LiO₆ octahedral site is also suitable as a luminescent site for Cr³⁺, it is necessary to analyze and determine the actual luminescent center of Cr³⁺. The excitation spectra, which were normalized and monitored at different wavelengths (λ_em = 750, 810, 890, and 990 nm, Fig. 3d), show a significant overlap, clearly demonstrating that there are no multiple crystallographically independent emission sites. Meanwhile, the low-temperature-dependent spectrum of LZNO:1%Cr³⁺ is displayed in Fig. 3e. As the temperature gradually decreases, the emission peak undergoes a slight blue shift and becomes narrower, which is attributed to the reduced electroacoustic coupling between Cr³⁺ and the environment. However, no new emission peaks appeared, further indicating that the transition emission process of Cr³⁺ in the host is independent and singular with no multiple luminescent centers. Conversely, the asymmetric emission spectra may be attributed to local structural distortion around the Cr³⁺ cation, which exacerbates the low symmetry of the crystal field and results in uneven peak broadening.

Fig. 3f shows the RT photoluminescence decay curve of LZNO:Cr³⁺ obtained by exciting the phosphor at 468 nm while monitoring at 800 nm. All the decay curves can be accurately fitted by a single-exponential function. These findings further corroborate the dominance of a single type of emission center in this phosphor. As the concentration of Cr³⁺ increases from 0.5% to 3%, the decay curves exhibit a reduction in lifetime, dropping from 16.7 to 10.1 μs. This is due to the concentration quenching effect of Cr³⁺ and the combined effect of Cr⁴⁺ as the energy transfer center. Hence, in LZNO:1%Cr³⁺ under 468 nm excitation, the QY value is determined to be 8.3% (Fig. S5†) and it is relatively low. Therefore, it is imperative to suppress the generation of Cr⁴⁺ and stabilize the valence state of Cr³⁺.

3.4 NIR enhancement by heterotopic doping of Er³⁺

As we know, the luminescence properties of Cr³⁺-activated phosphors will be suppressed because of the formation of Cr⁴⁺, especially during inequivalent substitution of Cr³⁺ for Nb⁵⁺ in the doping process. Doping charge compensator ions into phosphors is a common method to improve inequivalent substitution. In order to avoid competition with Cr ions entering the LZNO host lattice, we designed charge compensation by doping rare earth ions of Er³⁺ at the Zn²⁺ sites. The XRD patterns and Rietveld refinement results of co-doped Er³⁺ are shown in Fig. S6 and S7.† While maintaining purity, the trend of the overall expansion of crystal cell parameters indicates the successful replacement of Zn²⁺ by Er³⁺. It should be noted that the radius difference between Er³⁺ (CN = 6, r = 0.89 Å) and Zn²⁺ (CN = 4, r = 0.6 Å) is significant, but this is a comparison under different coordination environments. We know that when Er³⁺ enters the four-coordinated Zn site, its radius will inevitably decrease due to the change in the coordination environment from six to four. This process will also force the Cr ions originally intended to enter the Zn²⁺ sites to enter the Nb⁵⁺ sites instead as potentially effective luminescent centers.

To confirm the role of heterotopic doping of Er³⁺ in the increase of Cr³⁺ ions, the DR spectra and NIR luminescence performances of 1%Cr doped samples with varying concentrations of Er³⁺ were investigated. The introduction of Er³⁺ significantly enhances the light absorption of Cr³⁺ ions, indicating that the content of Cr³⁺ in the system may be increased (Fig. 4a). The additional absorption peaks at 520, 970 and 1455–1640 nm are attributed to electronic transitions from the ground state ⁴I_15/2 of Er³⁺ to its ²H_11/2, ⁴I_11/2 and ⁴I_13/2 excited states, respectively. The PL and PLE spectra of the Li_1−xEr_xZnNbO₄:0.01Cr³⁺ samples are shown in Fig. 4b and c. The peak shapes of the PLE and PL spectra are similar to those of single-doped Cr³⁺, and there is no significant positional deviation. As the doping concentration of Er³⁺ reaches 1.4%, the integrated intensity of the Cr³⁺ emission band (650–1300 nm) is 2.9 times that of the original sample. The subsequent decrease in intensity is attributed to concentration quenching. The IQE (17.6%) of the optimal 1.4%Er³⁺ co-doped sample is more than two times that compared to the sample doped solely with 1%Cr³⁺ (8.3%). We also noticed that as the concentration of Er³⁺ increases, a new concave peak appears at 808 nm in the PL spectrum. Fig. S9† shows the low-temperature-dependent spectrum of the 1.4%Er³⁺ co-doped sample, analyzing the reason for the formation of this concave peak. At lower temperatures, the concave peak becomes more noticeable. When Er³⁺ is in an excited state, there is a higher likelihood of reabsorbing the photons it emits, particularly at lower temperatures. Therefore, the concave peak at 808 nm is due to the self-absorption of the ⁴I_9/2 level of Er³⁺. The decay curve of co-doped Er³⁺ is presented in Fig. 4d. Co-doping with Er³⁺ leads to a reduction in the lifetime of Cr³⁺, suggesting that the localized defect energy level created by Er³⁺ at the bottom of the conduction band has a trapping effect for electrons.

Fig. 4 (a) DR spectra of LZNO:1%Cr³⁺,x%Er³⁺ (x = 0–1.8). (b) PLE and (c) PL spectra of representative LZNO:1%Cr³⁺,x%Er³⁺ (x = 0–1.8). (d) PL decay curves of LZNO:0.01Cr³⁺ and LZNO:0.01Cr³⁺,0.014Er³⁺ monitored at 800 nm. (e) 3D mapping surface plot of the emission spectra of the LZNO:0.01Cr³⁺,0.014%Er³⁺ phosphor excited at 468 nm as a function of temperature. (f) Normalized integrated intensity of the emission spectra as a function of temperature.

Due to the significant substitution radius of Er³⁺, it may lead to a modification in the rigidity of the crystal structure, subsequently impacting the near-infrared luminescence of Cr³⁺. The fluorescence thermal stability, which can indirectly reflect the structural rigidity of LiZn_1−xEr_xNbO₄:0.01Cr³⁺ (x = 0 and 0.014), was examined through temperature-dependent emission spectra recorded from 298 to 473 K under 468 nm excitation, as shown in Fig. S10.† The 3D mapping surface for the emission spectrum of LiZn_99.86Er_0.014NbO₄:0.01Cr³⁺ is plotted in Fig. 4e. When heated to 423 K, its integrated emission intensity remains at about 12% of the RT (298 K) value. Fig. 4f shows the temperature variation curves of LZNO:0.01Cr³⁺ and LiZn_0.986Er_0.014NbO₄:0.01Cr³⁺ phosphors, indicating that Er³⁺ doping has almost no effect on the structural rigidity of LZNO crystals, and thus, the enhancement of Cr³⁺ luminescence is independent of crystal rigidity. More detailed analysis regarding the fluorescence thermal stability can be found in Fig. S11–S13 in the ESI.†

3.5 Underlying mechanism of NIR enhancement by trivalent rare earth ions

Spectral analysis and thermal stability studies have indicated that the increase in the NIR emission of Cr³⁺ in the LZNO system through co-doping with Er³⁺ is not attributed to energy transfer behavior or structural changes, suggesting a different underlying mechanism. As mentioned above, Cr³⁺ and Cr⁴⁺ can coexist in the LZNO:Cr system due to the matching of the radius and valence state. Therefore, our initial focus was on observing the changes in the Cr valence state in the system before and after Er³⁺ doping. As shown in Fig. 5a and b, the high-resolution XPS spectra of the Cr 2p doublet in LZNO:0.01Cr³⁺ before and after Er³⁺ doping were analyzed. Four Gaussian components can be resolved in the two peaks of Cr. Specifically, the peaks at 586.17 and 577.65 eV are attributed to Cr 2p_1/2 and Cr 2p_3/2 components related to Cr⁴⁺ and they can be used to determine the Cr⁴⁺/Cr³⁺ ratio. Obviously, the XPS peaks at 585.07and 575.58 eV are attributed to the Cr 2p_1/2 and Cr 2p_3/2 of Cr³⁺.

Fig. 5 (a) and (b) High-resolution Cr 2p XPS core energy level spectra of LZNO:0.01Cr³⁺ and LZNO:0.01Cr³⁺,0.014%Er³⁺. (c) Deformation of the charge density comparison of Er³⁺-doped LZNO:Cr before and after doping. (d) The variation pattern of the optical electronegativity of trivalent lanthanide ions with the number of 4f orbital electrons.

It is found that, on co-doping with Er³⁺, the peak area associated with Cr⁴⁺ is notably decreased and the peak area related to Cr³⁺ is significantly increased. This observation contrasts with the behavior of the LZNO phosphor doped solely with Cr³⁺. It is evident that the addition of Er³⁺ effectively enhances the valence state transition from Cr⁴⁺ to Cr³⁺ in the LZNO system. Fig. S3b† shows the band gap variation of LZNO after Er³⁺ co-doping. Compared to Cr³⁺ single doping, 1.4%Er³⁺ co-doping resulted in a decrease in the band gap of LZNO from 2.62 eV to 2.45 eV. According to Fig. S1,† Er³⁺ doping will form impurity energy levels near the bottom of the conduction band in the LZNO host, thereby reducing the bandgap width. Moreover, due to Er³⁺ co-doping, it promotes the conversion of Cr⁴⁺ to Cr³⁺ in the system. Therefore, the change in the valence state of Cr leads to the broadening of the Cr³⁺ impurity energy level, as shown in Fig. S1c,† ultimately resulting in a decrease in the bandgap width. The decrease in the bandgap is consistent with the behavior of Cr³⁺ single doping, which may indicate that the concentration of Cr³⁺ ions in the system indeed increased.

Furthermore, to clarify the specific impact of Er³⁺ on promoting the conversion of Cr⁴⁺ to Cr³⁺, the change in charge density after Er³⁺ doping to LZNO:Cr was calculated. As depicted in Fig. 5c, Cr³⁺ tends to shift electrons towards the surrounding O²⁻ anions. However, when Er³⁺ replaces the Zn²⁺ cation, Er³⁺ significantly redistributes the charge density around Cr and this results in an increase in its electron cloud, indicating that Cr⁴⁺ in the system will gain electrons and transform into Cr³⁺. Therefore, we can conclude that the underlying mechanism of doping Er³⁺ that promotes the conversion of Cr⁴⁺ to Cr³⁺ is mainly through charge compensation. Then, a model is proposed that the heterotopic and heterovalent doping of Er³⁺ leads to defect charge compensation. This process can be explained by the following equation:

V^··_O → 2h⁺ + O_o^x (6)

Cr³⁺ + h⁺ → Cr⁴⁺ (7)

V′′_Zn → 2e⁺ + V_Zn^x (10)

O′′_i → 2e⁺ + O_o^x (11)

Cr⁴⁺ + h⁺ → Cr³⁺ (12)

When Cr³⁺ is doped into the LZNO host at high temperature (1100 °C), Cr³⁺ replaces Nb⁵⁺ to form Cr′′_Nb defects because of valence mismatch. At the same time, to maintain charge balance, an equal amount of is formed (eqn (5)). Positive charges in oxygen vacancies can be released under thermal disturbance, causing Cr³⁺ to self-oxidize to Cr⁴⁺ (eqn (6) and (7)). Similarly, when Er³⁺ is introduced to replace Zn²⁺, negative zinc vacancy defects or interstitial oxygen defects will be generated to balance the charge (eqn (8) and (9)). Afterwards, electrons in these negatively charged defects can be released through thermal stimulation (eqn (10) and (11)). Due to the appropriate electronegativity of Er³⁺, the constructed defect positions are suitable to form efficient electron transfer channels with Cr⁴⁺ within adjacent Nb⁵⁺ sites, ultimately allowing Cr⁴⁺ to capture electrons and reduce to Cr³⁺ (eqn (12)). Fig. S14a, b and c† show the high-resolution XPS spectra of the Zn 2p and O 1s core energy levels of LZNO:1%Cr³⁺ before and after Er³⁺ compensation. The diminished Zn signal, indicating an increase in Zn vacancies,⁵¹ and the increased interstitial oxygen signal (chemically adsorbed oxygen peaks at 532 eV with adsorbed states that can be molecular, atomic, or interstitial oxygen^52–55) in Er³⁺-doped phosphors provide reliable evidence for our model.

The schematic diagram of the defect charge compensation mechanism model for Er³⁺ heterotopic and heterovalent doping is depicted in Fig. 6, and it can help us to vividly understand the valence conversion process of Cr ions. In the depicted scene, a mischievous demon is using fireball magic to attack Cr³⁺ ions. This process occurs during high-temperature sintering, where O₂ provides Cr³⁺ ions with positive charges to facilitate the formation of Cr⁴⁺. In order to effectively suppress the transformation process of Cr ions, we introduced Er³⁺ ions into the Zn²⁺ sites. This action is akin to embedding an energetic magic stone in the core defense center, enabling the release of lightning magic through its own mechanism (zinc vacancy) and attracting elf fairies (interstitial oxygen) capable of employing frost magic. Together, these measures safeguard Cr³⁺ ions from the fireball assaults of the mischievous demon.

Fig. 6 Schematic diagram of the Cr valence-converting mechanism model via defect charge compensation for Er³⁺ heterotopic and heterovalent doping.

In order to confirm the reliability and universality of the proposed strategy, namely, the valence conversion of Cr⁴⁺ to Cr³⁺ induced by co-doped lanthanum ions of appropriate electronegativity, the relationship between the electronegativity of trivalent rare earth ions and the NIR luminescence of Cr³⁺ ions is further investigated. Fig. 5d shows the variation of the optical electronegativity of trivalent lanthanide ions with the number of 4f orbital electrons. As shown in Fig. S15,† besides Er³⁺, both Dy³⁺ and Tm³⁺ can boost the NIR luminescence of Cr³⁺. However, Nd³⁺ co-doping decreases the luminescence of Cr³⁺ ions. We proposed that this may be attributed to the variation in electronegativity among different rare earth ions, resulting in different defect constructions and charge compensation abilities. The electronegativity values of Dy³⁺, Er³⁺, and Tm³⁺ are very similar and the number of 4f electrons is also close. Although the electronegativity of Nd³⁺ is similar to that of Dy³⁺, Er³⁺, and Tm³⁺, the difference in the number of 4f electrons is too large. Electronegativity indicates the ion's ability to attract electrons, while the variation in the number of 4f electrons indicates the difference in the chemical stability of lanthanide ions. Based on the above analysis, we predict that when Eu³⁺/Yb³⁺ is doped into LZNO, the ion will not stabilize the Cr³⁺ ion due to partial/full 4f orbitals, as it is not possible to construct suitable defects to effectively compensate for electrons to Cr⁴⁺. The luminescence quenching of Cr³⁺, as shown in Fig. S16,† indeed proves our idea. In summary, the fundamental reason for co-doping trivalent rare earth ions to facilitate the conversion of Cr⁴⁺ to Cr³⁺ is closely linked to the electronegativity values of the lanthanide ions and the number of their 4f electrons.

3.6 Performance of NIR pc-LEDs

Finally, the actual application of the LiZn(Er)NbO₄:Cr³⁺ phosphor was evaluated. Fig. 7a shows the NIR pc-LED device prototype fabricated by combining the as-prepared optimal LiZn_0.986Er_0.014NbO₄:0.01Cr³⁺ phosphor and a 470 nm blue InGaN chip. Fig. 7b shows the electroluminescence (EL) spectra of the NIR pc-LED device under different forward bias currents (20–280 mA), while the NIR output power and photoelectric efficiency as a function of current are shown in Fig. S17.† With the increase in driving current, the NIR output power increased gradually from 0.21 to 1.66 mW, while the NIR photoelectric conversion efficiency decreased gradually from 0.41% to 0.21%. The continuous decrease in the current-dependent photoelectric efficiency may be attributed to the low absorption of the phosphor and thermal quenching resulting from the high-temperature working environment. Under strong currents, an apple can be clearly captured by an NIR camera under the light of the NIR pc-LEDs, as shown in Fig. 7c. Fig. 7d shows the particular use of our NIR phosphor in imaging a biological object. Placing the palm in front of the NIR pc-LEDs enabled the light to pass through easily, capturing the detailed blood vessel distribution. Under visible light, an ordinary camera was unable to reveal any clear details. While its photoelectric conversion efficiency properties still require improvement, these findings still emphasize the potential for real-world applications in night-vision and bio-tissue imaging technology.

Fig. 7 (a) Working and nonworking states of the LED device. (b) Electroluminescence spectra of the prepared NIR pc-LED device that combines a 470 nm InGaN blue LED chip with the LZNO:0.01Cr³⁺,0.014Er³⁺ NIR phosphor under a forward bias of 20–280 mA. Photographs of (c) an apple and (d) the palm captured under daylight using a visible light camera and under NIR pc-LED light using an NIR camera.

4 Conclusion

In summary, according to the possible standards (1. high valence state; 2. large radius; and 3. low symmetry) proposed for achieving long-wavelength broadband emission of Cr ions in spinel-type crystals, we have successfully developed novel Cr-activated LiZnNbO₄ spinel phosphors with excellent long-wavelength NIR emission performance. Under the excitation of 468 nm blue light, LZNO:Cr³⁺ shows an ultra-wide and long-wavelength NIR emission at 800 nm and the designed LiZnNbO₄:Cr³⁺ was comprehensively investigated. In addition, a compensation strategy for defect charges involving the heterotopic doping of trivalent rare earth ions has been implemented to enhance the NIR luminescence of Cr³⁺ ions for the first time, which is closely linked to the electronegativity of lanthanide ions and the number of 4f electrons. Experimental evidence indicates that this strategy effectively promotes the valence state transformation of Cr⁴⁺ to Cr³⁺ in the LZNO:Cr³⁺ system, where 1.4%Er³⁺ co-doping increases the IQE from 8.3% to 17.8%. Finally, the LiZn(Er)NbO₄:Cr³⁺ phosphor was incorporated into a prototype NIR pc-LED device to demonstrate its practical application. This study not only broadens the range of spinel NIR luminescent materials activated by Cr³⁺, but also explores the inherent relationship between rare earth ion heterotopic doping and Cr valence conversion. The design principles of Cr³⁺-activated long-wavelength spinel phosphors and the NIR emission enhancement strategy proposed in this study may offer a roadmap for the efficient creation of long-wavelength NIR emitting phosphors activated by Cr³⁺, and also presents a new approach for enhancing the luminescence efficiency.

Author contributions

Wen Song: conceptualization, methodology, data curation, and writing – original draft. Kaiwen Zhang: software and investigation. Xiaoyi Dong: data curation and resources. Liang Xu: data curation and resources. Yongjin Li, Rui Hu, and Zhiguo Song: project administration, funding acquisition, writing – review and editing, and supervision. Zhaoyi Yin: data curation and resources. Zhengwen Yang: data curation and resources. Jianbei Qiu: data curation and resources.

Data availability

All data supporting the findings of this study are available within the paper and its ESI.†

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 12264023), the National Natural Science Foundation of High and Foreign Experts Introduction Plan (No. G2022039008L), the Yunnan Major Scientific and Technological Projects (No. 202202AG050016), the Yunnan Fundamental Research Projects (No. 202301AT070459), the Analysis and Testing Foundation of Kunming University of Science and Technology (No. 2021T20200124) and the Yunnan Xing Dian Youth Talent Support Program (No. XDYC-QNRC-2022-0591). We also thank the Shiyanjia Lab (https://www.shiyanjia.com) for the XRD and XPS analyses and the Shenzhen Shuidi Scientific Research Service Co., Ltd for the testing of the PLQYs and NIR pc-LEDs.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01168f

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