# Tailoring trap depth distributions in ZnGa2O4: Cr3+ nanoparticles for optimized persistent luminescence across various functional temperature ranges

Celina Matuszewska, Nadia Touati, Laurent Binet, Ferdaous Ben Romdhane, Bruno Viana, Corinne Chanéac

<figure><img src="image_1.png" /></figure>

## ► To cite this version:

Celina Matuszewska, Nadia Touati, Laurent Binet, Ferdaous Ben Romdhane, Bruno Viana, et al. Tailoring trap depth distributions in ZnGa2O4: Cr3+ nanoparticles for optimized persistent luminescence across various functional temperature ranges. Journal of Luminescence, 2026, 289, pp.121521. 10.1016/j.jlumin.2025.121521. hal-05352926

HAL Id: hal-05352926

https://hal.sorbonne-universite.fr/hal-05352926v1

Submitted on 7 Nov 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Full Length Article

# Tailoring trap depth distributions in ZnGa₂O₄: Cr³⁺ nanoparticles for optimized persistent luminescence across various functional temperature ranges

Celina Matuszewska<sup>a,*</sup>, Nadia Touati<sup>c</sup>, Laurent Binet<sup>c</sup>, Ferdaous Ben Romdhane<sup>b</sup>, Bruno Viana<sup>c</sup>, Corinne Chaneac<sup>a,**</sup>

ARTICLE INFO

Keywords:

Nanoparticles

Microwave synthesis

persistent luminescence

Thermoluminescence

Luminescent thermometry

Cryopreservation

ABSTRACT

Persistent phosphors have attracted significant attention for their potential application in safety signage, road markings, data storage, anti-counterfeiting technologies, and biomedical fields. However, despite the significant advancements in room-temperature persistent phosphors and deep-trap materials for information storage, development of low-temperature persistent phosphors with strong afterglow emission remains a considerable challenge. In this study, we successfully modified the trap depth distribution in ZnGa₂O₄: Cr³⁺ nanoparticles (ZGO:Cr NPs) by carefully controlling the synthesis conditions in order to develop nanoscale persistent phosphors tailored for various temperature-dependent applications. ZnGa₂O₄: Cr³⁺ nanoparticles were synthesized via a rapid, facile, and environmentally friendly microwave-assisted hydrothermal method. A subsequent thermal treatment at temperatures of 500 and 700 °C was employed to further modify their structure and, consequently, their optical properties. Notably, the ZGO:Cr NPs without any subsequent calcination (ZGO MW) exhibit strong and long-lasting luminescence at cryogenic temperatures (15–200 K) and they show great potential for applications requiring continuous cooling at liquid nitrogen temperatures, such as the cryopreservation of biological agents, viruses, and tissues. In contrast, ZGO:Cr NPs calcined at 700 °C (ZGO 700) demonstrate stabilization of deeper traps, with activation energies near room temperature, which shows promise for conventional persistent luminescence applications requiring small particle sizes (<10 nm). Meanwhile, ZGO:Cr NPs calcined at 500 °C (ZGO 500) display a broad distribution of traps and a remarkably wide operational range (15–400 K), due to the coexistence of multiple Cr³⁺ environments, making it highly suitable for applications requiring stable persistent luminescence behaviour at very wide range of temperatures.

1. Introduction

Persistent luminescence (PersL) materials have attracted considerable attention in recent years due to their remarkable capability to emit light long after the excitation source has been stopped – spanning from several seconds to tens of hours. While persistent phosphors were initially used in emergency signage, decorations, and toys, their potential applications continue to expand. These materials are currently being explored for use in road markings [1], data storage [2], anti-counterfeiting technologies [3], AC LEDs [4], and biomedical

applications [5], including *in vivo* bioimaging [6,7], nanothermometry [8], and photodynamic cancer therapy (PDT) [9]. The phenomenon of persistent luminescence is primarily governed by the presence of structural defects within the host crystal lattice, forming intermediate energy levels within the host band gap. The additional energy levels, commonly referred to as trap levels, enable storage of charge carriers, while their position relative to the conduction band, namely trap depth, largely determines the functional temperature range of the material [10, 11]. Consequently, precise control over trap depths allows fine-tuning persistent luminescence performance of the material for particular

<footnote><sup>a</sup> Sorbonne Université, CNRS, Chimie de la Matière Condensée de Paris, LCMCP, F-75005, Paris, France</footnote>

<footnote><sup>b</sup> Sorbonne Université, CNRS, Fédération de Chimie et Matériaux de Paris-Centre, FCMat, Paris, 75005, France</footnote>

<footnote><sup>c</sup> IRCP, PSL, Chimie ParisTech, 11, rue Pierre et Marie Curie, 75231, Paris, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.
E-mail addresses: celina.matuszewska@sorbonne-universite.fr (C. Matuszewska), corinne.chaneac@sorbonne-universite.fr (C. Chaneac).</footnote>
applications [12,13].

In spite of the significant progress made in room-temperature (RT) persistent phosphors and deep-trap materials for information storage, development of persistent phosphors which are highly functional at low temperatures remains a considerable challenge [13]. A primary limitation lies in the thermal detrapping process, which becomes extremely slow at low temperatures for the materials with the traps of the ‘optimal’ energy (0.6–0.7 eV) due to insufficient thermal energy, leading to negligible release rate of the trapped charge carriers. Moreover, to date, only limited number of studies reported the successful synthesis of materials exhibiting well-stabilised and controlled electron traps with low trap depths. This situation can be attributed to two main constraints: firstly, very few host matrices can effectively form and sustain very shallow traps, and secondly, research studies on persistent phosphors have mainly focused on ambient-temperature applications, leading to the frequent underestimation of the potential of low-energy trap systems. Overcoming these limitations, could unlock significant potential of these materials for applications requiring sustained luminescence at cryogenic temperatures, such as the cryopreservation of biological agents, viruses, and tissues.

In general, the synthesis of persistent luminescence materials heavily relies on high-temperature treatment, which plays a critical role in both the crystallization process and the stabilization of defects, responsible for charge trapping. Traditional solid-state synthesis of metal oxide-based materials results in micrometric-sized particles with highly ordered crystalline structure, and abundance of well-defined defects yielding efficient RT persistent luminescence. In contrast, soft chemistry synthesis methods enable the control over particle size and morphology at the expense of less-efficient persistent luminescence performance [14]. It is primarily due to defected crystal structure, increasing the rate of non-radiative transitions and not contributing to RT PersL mechanism. As a result, high-temperature thermal treatments are required to diminish the influence of these defects and restore the desired structural and luminescent properties at RT. Taking advantage of the defects generated by soft-chemistry synthesis, along with their controlled evolution through carefully optimized post-calcination, is a major challenge for adapting persistent phosphors to low-temperature environments [12].

In this study, we introduce the microwave-assisted hydrothermal synthesis of ZnGa₂O₄:0.5 % Cr³⁺ nanoparticles as a fast, direct and eco-friendly approach for producing high-performance PersL materials with tunable trap characteristics [15,16]. ZnGa₂O₄: Cr³⁺ is a well-known luminescent and persistent luminescence material, composed of a cubic spinel matrix of ZnGa₂O₄, exhibiting *Fd*3*m* space group symmetry, which hosts Cr³⁺ ions, substituting Ga³⁺ ions in their octahedrally-coordinated sites. ZnGa₂O₄: Cr³⁺ was proposed for this work, as this material can crystallize in the cubic spinel phase at very low temperatures, which allowed us to study its structure and the properties after crystallization solely during the microwave stage. Unlike conventional synthesis techniques, microwave-driven process enables full crystallization in a single, low-temperature synthesis step (125–200 °C for 15–60 min), considerably reducing the energy consumption and synthesis time [8,15]. To further refine the material’s structure and optical performance, a follow-up thermal treatment at 500 and 700 °C was employed. By post-synthesis calcination we successfully tailored the trap depth distributions in ZGO:Cr nanoparticles, thereby optimizing their thermal operational range in continuity with our preliminary work [17].

## 2. Experimental

ZnGa₂O₄: 0.5 % Cr³⁺ nanoparticles were synthesized by microwave-assisted hydrothermal method. In order to obtain bigger amount of product in one batch (>2g), the multimode microwave reactor from Anton Paar (Synthos 3000) was used. The starting materials for the

synthesis were: ZnCl₂ (Sigma Aldrich, >98 %), GaCl₃ (ABCR Chemicals, 99.999 %), and CrCl₃·6H₂O (Sigma Aldrich, >98 %). The stock solutions of starting materials were initially prepared in Milli-Q water with the concentrations of: 0.7 M, 1.4 M and 0.009 M, for ZnCl₂, GaCl₃, and CrCl₃, respectively.

To ensure the uniformity of the samples, the first stage of the synthesis was performed in one beaker, and the resulting material was separated just before the microwave heating step. Stoichiometric amounts of ZnCl₂, GaCl₃, and CrCl₃ stock solutions were used according to the chemical formula of ZnGa₂O₄: 0.5 % Cr³⁺, in which the dopant concentration refers to the molar percentage of Cr³⁺ ions substituting Ga³⁺ ions. Thus, 12 ml of ZnCl₂, 11.94 ml of GaCl₃, 6 ml of CrCl₃ stock solutions, and 36 ml of Milli-Q water were mixed together and stirred vigorously for a few minutes. Subsequently, the pH of the solution was gradually adjusted to 8 by slow addition of 4 M NaOH, a value at which the formation of stable metal hydroxides incorporating all the metal ions occurs. The resulting white suspension of hydroxides was diluted with Milli-Q water to reach 90 ml of the final volume. Straight after stirring, the suspension was divided into three equal parts of 30 ml and transferred to the microwave Teflon liners. The synthesis was performed at 200 °C for 1 h, with 10 min heating time to reach 200 °C and 600 W maximum power.

After the synthesis, the resulting nanoparticles were washed five times in water, by centrifugation at 15,000 rpm and dispersion using tip sonicator (Qsonica, 1 min, amplification 40 %), and dried in vacuum at RT overnight. The samples were then separately ground in an agate mortar with a pestle. One part of them was left for further experiments, while two others were additionally calcined at 500, and 700 °C for 5 h. Following the calcination step, the samples were thoroughly ground again in agate mortar. The respective samples will be referred to within the paper as ZGO MW, ZGO 500, and ZGO 700.

Powder X-ray diffraction patterns (XRD) were obtained using Bruker D8 Advance diffractometer with Cu-Kα radiation (λ = 1.5406 Å) at 30 kV and 40 mA, 0.04° 2θ step size (0.005° 2θ step size for Rietveld refinement) and 1 s step time in the range of 10–90° 2θ. Transmission electron microscopy (TEM) images were performed using Technai G2 Spirit operating at 120 kV. High resolution TEM (HR-TEM) images were performed using JEOL 2100 PLUS microscope operating at 200 kV with resolution of 1.9 Å. Carbon coated copper TEM grid were used (carbon film 300 mesh) for both TEM and HRTEM imaging and they were prepared in the following manner. 5 mg of powder sample was ground thoroughly with 1 ml of HNO₃ (pH = 3) for a few minutes, transferred to centrifuge tube and diluted to 5 ml. They were sonicated using tip sonicator (Qsonica, 2 min, amplification 40 %) and centrifuged at 24,000 rpm for 45 min. Afterwards, the samples were washed with water few times and redispersed in Milli-Q water to reach the concentration of 1 mg/ml. The solutions were then diluted to the concentrations of 0.1 mg/ml, sonicated for 2 min and one drop of each solution was placed on the TEM grid. The grids were left for drying overnight under cover at RT. Diffuse Reflectance spectra (DRS) were measured utilising CARY 5000 UV-Visible-NIR Spectrometer from Agilent Technologies, equipped with internal DRA-900 integrating sphere for the measurements in reflection mode. Electronic Paramagnetic Resonance (EPR) spectra were performed using X-Band with Bruker ELEXSYS E500 spectrometer equipped with a Bruker SHQE resonator, operating at 9.271 and 9.274 GHz at RT, depending on the measurement.

Photoluminescence emission and excitation spectra were recorded on powder samples using Eclipse Spectrofluorimeter (Agilent Technologies) in a phosphorescence mode. Persistent luminescence (PersL) spectra, decays and Thermoluminescence (TL) glow curves were recorded using pellets prepared by grinding 30 mg of powder ZGO sample with 90 mg of KCl, transferring to 10 mm pellet press die, and pressing at 3.5 T. Persistent luminescence spectra and decays were registered following 3 min of irradiation with UV LED (λ<sub>exc</sub> = 275 nm) at RT (unless stated otherwise). The signal was collected directly after switching off the excitation source using Pixis 100 CCD camera coupled to Acton
SpectraPro monochromator from Princeton Instruments. HRPL spectra were recorded using Princeton Instruments CCD PI-max4 camera coupled to Princeton Instruments Acton SpectraPro 2300 monochromator with 1200 grooves per mm grating, and the excitation was provided by the pulsed laser. TL glow curves were recorded between 10 K and 470 K, with 10 K min⁻¹ heating rate, following 3 min of

irradiation with UV LED ($\lambda_{exc}$ = 275 nm) at 10 K and time for the decay of PersL at this temperature (15 min–1 h, depending on the sample). The temperature during TL measurements and PersL decays at various temperatures was controlled by closed cycle cryostat from Advanced Research Systems equipped with Lakeshore temperature controller.

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Structure and characterization of ZnGa₂O₄: 0.5 % Cr³⁺ (ZGO:Cr) calcined at various temperatures: a) schematic protocol of ZnGa₂O₄: 0.5 % Cr³⁺ synthesis; b) XRD patterns of ZGO:Cr samples along with standard pattern of ZnGa₂O₄ (PDF 038–1240); c) evolution of the full width at half maximum (FWHM) of the (311) XRD peak; d) evolution of the NPs size; e) transmission electron microscopy (TEM) of ZGO MW; f) high resolution TEM (HRTEM) of ZGO MW and fast Fourier transform (FFT) as an inset with marked characteristic reflections of ZnGa₂O₄; g) size distribution of ZGO MW; h) TEM of ZGO 500; i) HRTEM of ZGO 500 with FFT as an inset; j) size distribution of ZGO 500; k) TEM of ZGO 700; l) HRTEM of ZGO 700 with FFT as an inset; m) size distribution of ZGO 700.</figcaption></figure>
### 3. Results and discussion

The synthesis procedure of ZnGa₂O₄: 0.5 % Cr³⁺ including the mixing of metal cations, pH adjustment, microwave treatment, and calcination steps is schematically depicted in Fig. 1a. X-ray diffraction (XRD) analysis was conducted to verify the success of the synthesis and assess the purity of the samples, with the corresponding powder XRD patterns presented in Fig. 1b. All observed diffraction peaks exhibit excellent agreement with the standard pattern of ZnGa₂O₄ (PDF 038–1240), with no further identified diffraction peaks. Zinc gallate adopts a cubic spinel structure, characterized by the Fd3m space group symmetry, with Ga³⁺ ions occupying octahedral sites, and Zn²⁺ ions residing in tetrahedral sites. Despite the fact that zinc gallate is known to form a normal spinel structure, a phenomenon of partial inversion has been reported, where Ga³⁺ ions are found in tetrahedral sites, and Zn²⁺ ions occupy octahedral sites. The degree of the inversion was reported to be around 3 % [18–20]. XRD results confirm the formation of a pure cubic spinel phase of ZGO nanoparticles, even for the sample solely obtained after microwave-assisted hydrothermal synthesis. The additional calcination step induced a slight rise in the particle size, from 5.7 nm for ZGO MW to 9.1 nm for ZGO 700. Detailed comparison of the size of nanoparticles derived by various methods, including Rietveld refinement, Scherer equation, and TEM size distribution is presented in Table SI 1. The values of the cell parameters are derived from Rietveld refinement (see Table SI 1) suggesting slight expansion of the unit cell with calcination temperature. However, as the peak positions exhibit no shift from ZGO MW to ZGO 700 sample (with 2θ step of 0.005), the observed minimal variations are solely a result of the fitting of XRD patterns, which, for broadened peaks, remains challenging.

The nanoparticles morphology and size distribution were further explored using TEM imaging. Lower-magnification images (see Fig. 1c–f,i), indicate that the morphology of the samples is independent of the synthesis conditions, consistently exhibiting quasi-spherical nanoparticles with a pronounced tendency for aggregation. Imaging at high resolution (see Fig. 1d–g,j) was conducted to assess the crystallinity of the nanoparticles as a function of the applied calcination temperature. High-quality HRTEM images clearly depicting crystallographic lattice fringes endorse the high degree of crystallinity across all the studied samples with each nanoparticle exhibiting a single crystallographic domain. Furthermore, insets displaying the results of the Fast Fourier Transform (FFT) of HRTEM images provide additional confirmation of the cubic spinel phase of the nanoparticles, as evidenced by characteristic reflections of (111), (220), (311), (222), and (331) corresponding to the respective d spacing of 0.4808, 0.2946, 0.2513, 0.2406, and 0.1912 nm. The size distribution derived from TEM images (see Fig. 1g–j,m) offers further insights into the size variation between the samples, showing broader size distribution with the increase of the calcination temperature, along with the increase of the mean nanoparticles size. To conclude, while the size increases slightly as a consequence of calcination step and this behaviour was revealed through XRD peaks broadening and TEM-derived size distribution, the differences among the samples in terms of their structure, morphology and crystallinity remain negligible.

Consecutively, optical properties of ZGO:Cr NPs were thoroughly investigated at RT by measurements of photoluminescence (PL) emission and excitation (PLE) spectra, high-resolution PL spectra, as well as persistent luminescence (PersL) spectra and decays profiles. PL spectra measured upon 565 nm excitation are presented in Fig. 2 a. PL spectra

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Optical spectroscopic characterization of ZGO:Cr samples at room temperature (RT): a) steady-state photoluminescence emission (PL) and excitation (PLE) spectra measured at λ<sub>exc</sub> = 565 nm, and λ<sub>exc</sub> = 697 nm, respectively; b) high resolution PL spectra recorded at λ<sub>exc</sub> = 565 nm; c) persistent luminescence (PersL) spectra recorded after 3 min of irradiation at λ<sub>exc</sub> = 275 nm; d) PersL decays recorded after 3 min of irradiation at λ<sub>exc</sub> = 275 nm.</figcaption></figure>
exhibit a narrow emission band with maximum at 696 nm attributed to the $ ^{2}E \rightarrow ^{4}T_{2} $ spin-forbidden electronic transition of $Cr^{3+}$ ions in an octahedral symmetry [20]. The evolution of the shape of PL spectra across the studied samples indicate changes in the local environment around $Cr^{3+}$ ions. Notably, PL spectra evolve from undefined and unstructured bands in the case of ZGO MW, to bands with well-determined shapes and clear peaks separation for ZGO 700. While for all samples, the most intense line corresponds to the $N_{2}$ line at 696 nm, the increase of the calcination temperature results in the rise of the intensity of the Stokes phonon side bands (S-PSB), and R lines.

Based on the high-resolution PL (HRPL) spectra (see Fig. 2 b), different lines can be clearly distinguished only for ZGO 700 sample, including $R_{1}$ and $R_{2}$ lines at 689 nm and 687 nm, $N_{1}$ and $N_{2}$ lines at 691 nm and 696 nm, as well as S-PSB lines at 709 nm and 715 nm. R lines are attributed to the $Cr^{3+}$ ions in an ideal octahedral environment with an axial symmetry. In contrast to R lines, N lines correspond to $Cr^{3+}$ cations in the distorted environment, due to the presence of defects, located mainly in the first neighbour cationic position, such as $Cr^{3+}$-$Cr^{3+}$ pairs (two neighbouring $Cr^{3+}$ ions), or $Cr^{3+}$ clusters ($n_{7}$ line) [20,21]. Moreover, detailed studies on the defects governing persistent luminescence in $ZnGa_{2}O_{4}$: $Cr^{3+}$ reported that anti-site defects ($Ga_{2}Zn$, $N'_{Ga}$) are responsible for observed NIR persistent luminescence, in particular after prior irradiation with lower energetic light from the Vis region [22–24]. The evolution of the PL and HRPL spectra with increase of the calcination temperature clearly indicates the decrease in the local disorder and increase in the symmetry around $Cr^{3+}$ ions.

The change in the crystal field strength of matrix was further studied by PLE spectra, in which a blue shift of band due to $ ^{4}A_{2} \rightarrow ^{4}T_{2} $ electronic transition is observed from 574 nm for ZGO MW to 564 nm for ZGO 700. The shift of the PLE band toward lower wavelengths suggest an increase of the crystal field strength affecting $Cr^{3+}$ ions, leading consequently to the increase of the $ ^{4}T_{2} $ energy level [25]. Further evaluation of the matrix crystal field strength were performed by calculations of the 10Dq/B parameter. The results of these calculations are presented in Table SI 2 along with all utilised equations. The obtained values of the 10Dq/B parameters show more complex changes in the crystal field among the studied samples, as 10Dq/B equals to 28.6 for ZGO MW, followed by increase to 33.5 for ZGO 500, and decrease to 31.5 for ZGO 700. Notably, calcined samples exhibit higher crystal field strength compared to the sample without calcination, while the differences in the 10Dq/B value between them might be due to relatively small shift of the peaks along with the slightly bigger error for the determination of the peak maximum for the broad bands.

Persistent luminescence spectra (see Fig. 2 c) recorded after irradiation for 3 min at 275 nm are naturally broader and much less defined compared to their PL counterparts, but the position of the emission maximum remains the same, namely at 696 nm. The broadening of the PersL spectra is the result of the superposition of different lines observed in the PL spectra. Decays of persistent luminescence intensity (see Fig. 2 d), recorded at RT for 1200 s after prior 3-min irradiation at 275 nm, show an improvement of persistent luminescence intensity and duration of afterglow after calcination. This experiment demonstrates that high level of local disorder in the $ZnGa_{2}O_{4}$ affecting $Cr^{3+}$ ions prevents creation of trap levels at energy depth inducing persistent luminescence at RT, and thus the calcination step is essential for the reorganization in the material leading to optimized PersL properties.

Owing to the fact that ZGO samples exhibit increase in the crystal field strength with the increase of the calcination temperature, their abilities for the temperature sensing has been investigated in the following part of the paper. The unique properties of transition metal (TM) ions for luminescence thermometry applications have been widely reported in the literature in recent years. The luminescence properties of TM ions mainly depend on their 3d electrons, which form the most outer electron shell, making them particularly susceptible to the environmental changes, such as temperature or pressure. Therefore, their

luminescence based on the d-d intraconfigurational transitions might be utilised for rapid and accurate temperature sensing [26].

PL spectra at different temperatures were recorded for ZGO NPs under 275 nm excitation between 15 K and 400 K, with a 10 K step. The evolution of the PL spectra of ZGO 500 sample with increase of temperature is presented in Fig. 3 a. The corresponding PL spectra of ZGO MW and ZGO 700 samples are included in the SI (see Fig. SI 2 a,c). Apart from the $Cr^{3+}$ band, the PL spectra recorded at low temperatures also consist of a broad band with lower intensity centred at around 480 nm, which is attributed to the ZGO matrix emission [27–29]. With material engineering though cationic substitutions, this emission could be used for photocatalytic applications [30–32]. The intensity of this band in the ZGO:Cr drastically decreases with temperature, making it undetectable at RT. Thus, this matrix emission was not exploited for nanothermometry applications. While analysing the temperature evolution of the $Cr^{3+}$ bands, several changes can be observed, including the decrease of the luminescence intensity, and the broadening of the emission band due to appearance of the contribution from the $ ^{4}T_{2} \rightarrow ^{4}A_{2} $ electronic transition in addition to the $ ^{2}E \rightarrow ^{4}A_{2} $ electronic transition with the rise of temperature [33].

In the luminescence thermometry, there are multitude of parameters that can be utilised for the determination of temperature, such as intensity of the single emission band, luminescence intensity ratio (LIR) of two bands, broadening of the emission peak, or changes in the excited state lifetimes. The approach based on intensity of the single band is the least reliable method, due to the number of factors affecting the intensity of the measured emission apart from the influence of temperature. On the other hand, utilising luminescence intensity ratio and lifetimes variation provide the most effective solution to make the temperature parameter independent of the measurement conditions. For the lifetimes measurements however some expensive apparatus for pulse modes generation and detection are required [26,34,35]. On that account, in the case of ZGO:Cr NPs, two approaches were combined, namely the luminescence intensity ratio along with the peak broadening. The LIR was calculated using two $Cr^{3+}$ emission lines: the most intense $N_{2}$ line at 696 nm, and $R_{2}$ line at 680 nm (see Equation (1)).

$$ \text{LIR} = \frac{\text{I}(\text{N}_{2})}{\text{I}(\text{R}_{2})} = \frac{\text{I}(696 \text{ nm})}{\text{I}(680 \text{ nm})} \qquad (\text{Eq. 1}) $$

Their intensity is affected by the temperature changes in a different manner. The former declines rapidly with temperature, while the latter increases, mainly owing to the appearance of the $ ^{4}T_{2} \rightarrow ^{4}A_{2} $ broad band. Such choice of the wavelengths for the LIR calculation maximizes the relative intensity changes following the increase of temperature, resulting in the enhanced sensitivity of the nanothermometer. Notably, the $R_{2}$ line is clearly identified only for ZGO 700, thus, in the case of ZGO MW and ZGO 500 intensity at the same wavelength as $R_{2}$ line for ZGO 700 was used (680 nm). Moreover, the wavelengths of $N_{2}$ used for different samples are subject to $ \pm 1 $ nm changes, due to slight shifts of the line positions.

The LIR values exhibit a monotonous asymptotic-like decrease with temperature, while the $S_{R}$ value firstly increases until 120 K, reaching the maximum intensity of 1.5 % K$^{-1}$, and then decreases. It is generally considered that luminescence thermometer should be characterised with sensitivity value exceeding 1 % K$^{-1}$ to be suitable for real-life applications [36,37]. In the case of thermometer based on luminescence of ZGO 500 NPs, sensitivity values higher than 1 % K$^{-1}$ are observed in a wide range of temperatures from 65 to 223 K. The temperature dependence of LIR and $S_{R}$ values for ZGO MW and ZGO 700 samples is demonstrated in Fig. SI 2 b,d, while the comparison of the thermal sensing performance is depicted in Fig. SI 3. Despite the changes in the crystal field strength between samples heat-treated at different temperatures, their applicability as nanothermometers is not significantly different. The resulting LIR and $S_{R}$ values are almost identical for ZGO MW and ZGO 500 samples, regarding both $S_{R}$ maximum value and the applicability range. In the case of ZGO 700 sample, $S_{R}$ maximum is
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Temperature dependence of ZGO:Cr NPs photoluminescence: a) ZGO 500 PL emission spectra ($\lambda_{exc}$ = 275 nm) recorded in 15–400 K range with 10 K step; b) temperature evolution of luminescence intensity ratio LIR and relative sensitivity $S_R$.</figcaption></figure>

slightly lower, reaching 1.4 % K⁻¹, and the applicability range is shifted to lower values (53–191 K). To sum up, all investigated samples demonstrate very promising thermal sensing performance, regardless of their calcination temperature, and their applicability range suggests, that they are most suitable for low-temperature applications.

Following the studies of PL properties, persistent luminescence performance of ZGO:Cr NPs as a function of temperature was investigated. PersL decay measurements were performed using a close-cycle 10 K cryostat, in the temperature range of 15 K–300 K, with a step size of 50 K (except the initial interval after the first measurement). At each temperature, PersL signal was recorded for 30 min after following 3 min irradiation at $\lambda_{exc}$ = 275 nm. The respective PersL decay curves of ZGO

MW, ZGO 500, and ZGO 700 are presented in Fig. 4a and b,c.

The significant difference is evident between the samples calcined at varying temperatures. The sample subjected solely to microwave treatment, namely ZGO MW, exhibits a very short duration of PersL at RT, as previously demonstrated in Fig. 2 d. However, both the intensity and duration of resulting PersL substantially improves as the temperature of the measurement (and charging) decreases. The most prolonged and intense PersL for this sample was recorded at the two lowest measurements temperatures namely 15 and 50 K. At these temperatures, the signal persists for over an hour after ceasing the excitation light, highlighting the remarkable performance of the synthesized NPs. In contrast, the sample after calcination at 700 °C (ZGO 700) reveals the opposite

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Optical spectroscopic characterization of ZGO:Cr samples at different temperatures: PersL decays of a) ZGO MW; b) ZGO 500; c) ZGO 700 recorded after 3 min of irradiation at $\lambda_{exc}$ = 275 nm at 15–300 K; d) thermoluminescence (TL) glow curves of ZGO MW, ZGO 500, and ZGO 700 recorded after 3 min of irradiation at $\lambda_{exc}$ = 275 nm from 10 K to 470 K with 10 K min⁻¹ heating rate.</figcaption></figure>
trend. For this sample, the intensity along with the duration of PersL are elevated at higher measurement temperatures, with the optimal performance observed at 250 and 300 K. The sample thermally treated at the mid-range calcination temperature, namely ZGO 500, shows intermediate behaviour between that of ZGO MW and ZGO 700. Notably, the PersL decay curves recorded for ZGO 500 remain relatively consistent across the studies temperature range, contrary to the significant temperature-dependant variations observed in the two other samples. Although its performance does not surpass that of ZGO MW at low temperatures or ZGO 700 at room temperature, ZGO 500 exhibits stable and reasonably strong PersL across all tested temperatures.

The measurements of PersL decay curves clearly indicate a substantial difference in the trap levels governing persistent luminescence properties of the ZGO:Cr NPs. To validate this observation, thermoluminescence (TL) studies were conducted for all three samples. The TL glow curves of ZGO MW, ZGO 500, and ZGO 700 were recorded following 3 min of irradiation at $\lambda_{exc} = 275$ nm, over the temperature range of 10–470 K, using a heating rate of 10 K min$^{-1}$. The obtained results are depicted in Fig. 4 d. The considerably broad TL glow curves obtained for all the samples reveal the presence of trap depth distributions, rather than discrete trap levels [10]. However, the shape of the TL glow curves and the position of the peak maximum $T_{max}$ demonstrate extensive differences among the studied samples. Notably, $T_{max}$ shifts towards higher temperatures with the increase of the calcination temperature, the lowest $T_{max}$ was observed at 132 K for ZGO MW, followed by ZGO 500 at 202 K, and the highest for ZGO 700 at 259 K. This trend clearly indicates that the trap depth increases with the calcination temperature. The corresponding trap depths, estimated using Equation (2), are approximately 0.26 eV for ZGO MW, 0.40 eV for ZGO 500, and 0.52 eV for ZGO 700.

$$E_t = \frac{T_{max}}{500} = 23kT_{max} \qquad (\text{Eq. 2})$$

The peak position method given by Urbach [38] is a simple and straightforward approach for calculating trap depth values. Recently, an alternative method has been proposed that utilizes the peak maximum together with the heating rate [39]; however, its applicability appears to be limited to TL glow curves obtained at heating rates between 1 and 20 K s$^{-1}$. Given that the heating rate employed in this study is significantly lower, namely 0.167 K s$^{-1}$ (10 K min$^{-1}$), the newly proposed method is not considered appropriate for the present analysis.

The results of the TL measurements are consistent with the PersL measurements at different temperatures, and taken together they clearly demonstrate the most optimal thermal applicability range of each material. The ZGO 700 sample exhibits a trap depth distribution that closely aligns with those of conventional phosphors, making it highly suitable for temperatures close to room temperature. The ZGO MW sample features a narrower trap distribution, with the highest traps density localised at around 0.26 eV with respect to the conduction band (CB), resulting in a persistent phosphor particularly adapted for applications at extremely low temperatures. Finally, ZGO 500 shows the highest density of traps around 0.40 eV. Although this suggests a preference for low-temperature applications, the broadest trap distribution indicates that this material may be the most versatile among the three analysed compounds in terms of its operational temperature range.

The previous results of the TL analysis clearly demonstrate that thermal treatment following the microwave synthesis has a direct effect on the trap depth values that govern the PersL behaviour of the ZGO:Cr NPs, thereby affecting their operational temperature range. Consequently, the observed variations in the trap depth distributions across different samples can be attributed to the existence of distinct intrinsic defects acting as electron traps within these materials. To enhance the understanding of the relationship between material properties, further investigation is necessary. For this purpose, electron paramagnetic resonance (EPR) is proposed as a suitable analytical approach in the following part of the paper.

Chromium (III) ions employed as dopants in zinc gallate matrix, are particularly susceptible to changes in the local crystal field, and, similarly to optical spectroscopy, these changes exhibit a pronounced effect on the EPR spectra of $Cr^{3+}$. The formation of anti-site defects, widely regarded as the primary contributors to effective PersL in $ZnGa_2O_4$: $Cr^{3+}$, modifies the axial component of the crystal field, leading to changes in symmetry and accordingly, lifting the degeneracy of $^2E$ and $^4A_2$ levels. Under these conditions, the zero-field splitting (ZFS) parameter comprises an axial component D and rhombic (non-axial) component E. The relationship between these two components follows $0 \le E/D \le 1/3$ [23,40]. The simulated EPR spectra of bulk $ZnGa_2O_4$: $Cr^{3+}$ for the two limiting cases, namely $E/D = 0$ and $E/D = 1/3$ are presented in Fig. SI 4. The former corresponds to $Cr^{3+}$ ion in a normal octahedral site with axial symmetry, while the latter represents a $Cr^{3+}$ ion in an octahedral site lacking axial symmetry, due to the presence of neighbouring antisite defects. Both of the spectra can be divided into three main sections, composed of characteristic features, as specified in SI (Fig. SI 4).

The EPR spectra of ZGO MW, ZGO 500 and ZGO 700 are presented in Fig. 5, and substantial differences between the samples are readily apparent. The EPR spectrum of ZGO 700 sample exhibits a signal characteristic of an ideal octahedral environment ($E/D = 0$) at approximately 175 mT, along with two lower-intensity features, located at 130 and 320 mT, attributed to distorted environment ($E/D = 1/3$). Additionally, the characteristic doublet feature at around 800 mT appears as a broad and low-intensity single peak, indicating that even $Cr^{3+}$ normal sites experience a weak crystal field disorder. On the contrary, the EPR spectrum of ZGO MW sample is dominated by a strong and very broad signal, which likely arises from a highly disordered environment, potentially due to a high concentration of defects in the crystal structure. Finally, the spectrum of ZGO 500 sample demonstrates a combination of the different $Cr^{3+}$ environments identified in both ZGO MW and ZGO 700 structures. Specifically, a decrease in the intensity of broad signal associated with a highly-disordered environment is observed compared to ZGO MW, alongside the emergence of a well-defined signal at 175 mT. It inclines a notable increase in the population of $Cr^{3+}$ ions situated in an ideal octahedral environment, although this proportion is significantly lower compared to ZGO 700 sample.

EPR results provide a valuable insight into the local crystallographic structure of the ZGO:Cr NPs synthesized under different conditions. A direct correlation with the TL results is also evident. The intense and broad EPR signal observed for ZGO MW corresponds to a high

<figure><img src="image_7.png" /><figcaption>Fig. 5. Electron Paramagnetic Resonance (EPR) spectra recorded for ZGO MW, ZGO 500, and ZGO 700 samples.</figcaption></figure>
concentration of shallow traps, which allow intense and long PersL at cryogenic temperatures. The ZGO 500 sample, which exhibits an extensive range of Cr³⁺ environments, from highly distorted to the ideal octahedral symmetry, demonstrates the broadest distribution of trap depths, spanning from very shallow traps to those that can be thermally emptied only above RT. Such varied crystallographic environments surrounding Cr³⁺ ions accounts for PersL with uniform duration and intensity across wide temperature range (15–300 K). In contrast, nanoparticles calcined at the highest temperature, namely ZGO 700, exhibit a more well defined local structure, with the majority of Cr³⁺ ions residing in the perfect octahedral symmetry of Ga³⁺ sites. Nevertheless, the EPR spectrum of this sample still reveals signatures of antisite defects, indicating that the full symmetry at this calcination temperature was not achieved. This observation correlates well with the PL spectra, in which N₂ line at 696 nm remains the strongest emission line.

EPR confirms that the degree of local disorder in the structure of zinc gallate nanoparticles is the parameter that changes strikingly across the studies samples, primarily influenced by additional heat treatment applied following the microwave-assisted hydrothermal synthesis. Furthermore, a pronounced contrast is evident between the results obtained from XRD and TEM versus those from TL and EPR measurements. While XRD and TEM measurements show minimal structural differences between the samples, the combined PL, TL and EPR studies reveal the real and substantially more complex structures of these materials. This study clearly demonstrates that a wide range of characterization techniques focused on the long-distance order and averaged structural features, such as TEM and XRD, are insufficient to accurately predict the optical properties of these materials, especially regarding their persistent luminescence performance, which is particularly sensitive to local order.

## 4. Conclusions

To conclude, this study demonstrates the successful synthesis of Cr³⁺-doped zinc gallate nanoparticles via a rapid, facile, and environmentally friendly microwave-assisted hydrothermal method. Using a soft chemistry approach for material synthesis, spinel crystalline nanoparticles were obtained. Subsequent thermal treatments at 500 and 700 °C were applied to further tailor their optical properties, without significantly modifying their crystalline structure or particle size. The resulting nanoparticles were thoroughly characterized using XRD, TEM, EPR, PL, TL, and PersL techniques to investigate their multiscale structural organization and its relationship to persistent luminescence performance, as well as their potential for nanothermometry applications.

By adjusting the calcination protocol, we have effectively modified the trap depth distributions in ZGO:Cr nanoparticles, leading to tuning their optimal temperature operation range. Notably, the ZGO MW sample exhibits intense and long-lasting luminescence at extremely low temperatures (15–200 K), attributed to shallow traps resulting from a highly distorted crystal structure. This nanomaterial presents high potential for cryogenic applications such as cryopreservation. In contrast, ZGO 700 demonstrates formation of deeper traps, with activation energies near room temperature, corresponding to Cr³⁺ ions in an ideal octahedral environment along with minor population of Cr³⁺ ions under the influence of antisite defects. ZGO 700 sample can be incorporated for conventional persistent luminescence applications requiring small particle sizes (<10 nm). Meanwhile, ZGO 500 displays a broad distribution of traps and a remarkably wide operational range (15–400 K), due to the existence of multiple Cr³⁺ environments, from highly distorted to ideal octahedral symmetry. Thus, this material is highly suitable for applications requiring stable persistent luminescence behaviour at very wide range of temperatures. Additionally, the potential of these materials for luminescence thermometry applications was evaluated. All investigated samples demonstrate high thermal sensing performance, regardless of their calcination temperature, with relative sensitivity exceeding 1 %

K⁻¹. Their applicability range suggests that they are most suitable for low-temperature applications.

Overall, one of the main objectives of this manuscript was to demonstrate that rapid microwave synthesis followed by relatively low-temperature calcination can produce high-performance persistent luminescence nanomaterials. This approach takes advantage of the defects generated by the soft-chemistry route and their controlled evolution through post-calcination treatments. The resulting materials show strong potential for applications in fields such as cryogenic preservation, temperature sensing, and biomedical technologies, depending on the required operating temperature. This research is especially significant considering the current lack of persistent phosphors that show effective and well-controlled performance at low or extremely low temperatures.

## CRediT authorship contribution statement

**Celina Matuszewska:** Writing – original draft, Methodology, Formal analysis, Data curation. **Nadia Touati:** Methodology, Data curation. **Laurent Binet:** Writing – review & editing, Methodology, Formal analysis. **Ferdaous Ben Romdhane:** Data curation. **Bruno Viana:** Writing – review & editing, Validation, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization. **Corinne Chaneac:** Writing – review & editing, Validation, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Corinne Chaneac reports financial support was provided by Sorbonne University. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

This research was funded by Agence Nationale de la Recherche (ANR-22-CE09-0029-01 PLEaSe).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jlumin.2025.121521.

## Data availability

Data will be made available on request.

## References

[1] J. Nance, T.D. Sparks, From streetlights to phosphors: a review on the visibility of roadway markings, Prog. Org. Coating 148 (2020) 105749, https://doi.org/10.1016/j.porgcoat.2020.105749.

[2] Y. Zhuang, L. Wang, Y. Lv, T.L. Zhou, R.J. Xie, Optical data storage and multicolor emission readout on flexible films using deep-trap persistent luminescence materials, Adv. Funct. Mater. 28 (2018) 1705769, https://doi.org/10.1002/adfm.201705769.

[3] J. Zhang, Z. Wang, X. Huo, X. Meng, Y. Wang, H. Suo, P. Li, Anti-Counterfeiting application of persistent luminescence materials and its research progress, Laser Photon. Rev. 18 (2024) 2300751, https://doi.org/10.1002/lpor.202300751.

[4] W. Yuan, R. Pang, S. Wang, T. Tan, C. Li, C. Wang, H. Zhang, Enhanced blue-light excited cyan-emitting persistent luminescence of BaLu₂Al₂Ga₂SiO₁₂:Ce³⁺, Bi³⁺ phosphors for AC-LEDs via defect modulation, Light Sci. Appl. 11 (2022) 184, https://doi.org/10.1038/s41377-022-00868-8.

[5] J. Wang, Q. Ma, Y. Wang, H. Shen, Q. Yuan, Recent progress in biomedical applications of persistent luminescence nanoparticles, Nanoscale 9 (2017) 6204–6218, https://doi.org/10.1039/c7nr01488k.

[6] Q. le Masne de Chermont, C. Chaneac, J. Seguin, F. Pellé, S. Maitrejean, J.-P. Jolivet, D. Gourier, M. Bessodes, D. Scherman, Nanoprobes with near-infrared
persistent luminescence for in vivo imaging, Proc. Natl. Acad. Sci. 104 (2007) 9266-9271. https://www.pnas.org/cgidoi10.1073/pnas.0702427104.

[7] T. Maldiney, A. Bessière, J. Seguin, E. Teston, S.K. Sharma, B. Viana, A.J.J. Bos, P. Dorenbos, M. Bessodes, D. Gourier, D. Scherman, C. Richard, The in vivo activation of persistent nanophosphors for optical imaging of vascularization, tumours and grafted cells, Nat. Mater. 13 (2014) 418-426, https://doi.org/10.1038/nmat3908.

[8] E. Glais, M. Pellerin, V. Castaing, D. Alloyeau, N. Touati, B. Viana, C. Chanéac, Luminescence properties of ZnGa₂O₄: Cr³⁺, Bi³⁺ nanophosphors for thermometry applications, RSC Adv. 8 (2018) 41767-41774, https://doi.org/10.1039/c8ra08182d.

[9] A. Bessière, J.O. Durand, C. Noûs, Persistent luminescence materials for deep photodynamic therapy, Nanophotonics 10 (2021) 2999-3029, https://doi.org/10.1515/nanoph-2021-0254.

[10] K. Van Den Eeckhout, A.J.J. Bos, D. Poelman, P.F. Smet, Revealing trap depth distributions in persistent phosphors, Phys. Rev. B Condens. Matter 87 (2013) 045126, https://doi.org/10.1103/PhysRevB.87.045126.

[11] J. Ueda, How to design and analyze persistent phosphors? Bull. Chem. Soc. Jpn. 94 (2021) 2807-2821, https://doi.org/10.1246/bcsj.20210255.

[12] D. Poelman, D. Van Der Heggen, J. Du, E. Cosaert, P.F. Smet, Persistent phosphors for the future: fit for the right application, J. Appl. Phys. 128 (2020) 240903, https://doi.org/10.1063/5.0032972.

[13] J. Du, D. Poelman, Identifying near-infrared persistent luminescence in Cr³⁺-Doped magnesium gallogermanates featuring afterglow emission at extremely low temperature, Adv. Opt. Mater. 8 (2020) 1901848, https://doi.org/10.1002/adom.201901848.

[14] Y. Li, M. Gecevicius, J. Qiu, Long persistent phosphors - from fundamentals to applications, Chem. Soc. Rev. 45 (2016) 2090-2136, https://doi.org/10.1039/c5cs00582e.

[15] I. Bilecka, M. Niederberger, Microwave chemistry for inorganic nanomaterials synthesis, Nanoscale 2 (2010) 1358-1374, https://doi.org/10.1039/b9nr00377k.

[16] M.B. Gawande, S.N. Shelke, R. Zboril, R.S. Varma, Microwave-assisted chemistry: synthetic applications for rapid assembly of nanomaterials and organics, Acc. Chem. Res. 47 (2014) 1338-1348, https://doi.org/10.1021/ar400309b.

[17] C. Matuszewska, B. Viana, C. Chanéac, Tuning trap depths in ZnGa₂O₄: Cr³⁺ persistent luminescence nanoparticles for low-temperature applications, in: Optics, Photonics and Laser: Proceedings of the 8th International Conference, 2025, pp. 92-94.

[18] M. Allix, S. Chenu, E. Véron, T. Poumeyrol, E.A. Kouadri-Boudjelthia, S. Alahraché, F. Porcher, D. Massiot, F. Fayon, Considerable improvement of long-persistent luminescence in germanium and tin substituted ZnGa₂O₄, Chem. Mater. 25 (2013) 1600-1606, https://doi.org/10.1073/pnas.0702427104.

[19] A. De Vos, K. Lejaeghere, D.E.P. Vanpoucke, J.J. Joos, P.F. Smet, K. Hemelsoet, First-Principles Study of antisite defect configurations in ZnGa₂O₄:Cr persistent phosphors, Inorg. Chem. 55 (2016) 2402-2412, https://doi.org/10.1021/acs.inorgchem.5b02805.

[20] W. Mikenda, A. Preisinger, N-lines in the luminescence spectra of Cr³⁺-doped spinels (I) identification of N-lines, J. Lumin. 26 (1981) 53-66, https://doi.org/10.1016/0022-2313(81)90169-1.

[21] W. Mikenda, A. Preisinger, N-lines in the luminescence spectra of Cr³⁺-doped spinels (II) origins of N-lines, J. Lumin. 26 (1981) 67-83, https://doi.org/10.1016/0022-2313(81)90170-8.

[22] A. Bessière, S. Jacquart, K. Priolkar, A. Lecointre, B. Viana, D. Gourier, ZnGa₂O₄: Cr³⁺: a new red long-lasting phosphor with high brightness, Opt. Express 19 (2011) 10131-10137, https://doi.org/10.1364/OE.19.0101031.

[23] D. Gourier, A. Bessière, S.K. Sharma, L. Binet, B. Viana, N. Basavaraju, K. R. Priolkar, Origin of the visible light induced persistent luminescence of Cr³⁺-

doped zinc gallate, J. Phys. Chem. Solid. 75 (2014) 826-837, https://doi.org/10.1016/j.jpcs.2014.03.005.

[24] A. Bessière, S.K. Sharma, N. Basavaraju, K.R. Priolkar, L. Binet, B. Viana, A.J.J. Bos, T. Maldiney, C. Richard, D. Scherman, D. Gourier, Storage of visible light for long-lasting phosphorescence in chromium-doped zinc gallate, Chem. Mater. 26 (2014) 1365-1373, https://doi.org/10.1021/cm403050q.

[25] N. Basavaraju, K.R. Priolkar, D. Gourier, S.K. Sharma, A. Bessière, B. Viana, The importance of inversion disorder in the visible light induced persistent luminescence in Cr³⁺ doped AB₂O₄ (A = Zn or Mg and B = Ga or Al), Phys. Chem. Chem. Phys. 17 (2015) 1790-1799, https://doi.org/10.1039/c4cp03866e.

[26] L. Marciniak, K. Kniec, K. Elżbieciak-Piecka, K. Trejgis, J. Stefanska, M. Dramićanin, Luminescence thermometry with transition metal ions. A review, Coord. Chem. Rev. 469 (2022), https://doi.org/10.1016/j.ccr.2022.214671.

[27] Z. Gu, F. Liu, X. Li, J. Howe, J. Xu, Y. Zhao, Z. Pan, Red, green, and blue luminescence from ZnGa₂O₄ nanowire arrays, J. Phys. Chem. Lett. 1 (2010) 354-357, https://doi.org/10.1021/jz900213p.

[28] M.P. Dias, M.S. Batista, A. Pimentel, E. Fortunato, R. Martins, F.M. Costa, S. O. Pereira, J. Rodrigues, T. Monteiro, Revealing the nature of the Zn₂GeO₄ bluish-white emission in microwave-assisted hydrothermal synthesized nanorods, Mater. Chem. Phys. 334 (2025) 130463, https://doi.org/10.1016/j.matchemphys.2025.130463.

[29] C. Satya Kamal, S. Boddu, B. Vishwanadh, K.R. Rao, V. Sudarsan, R.K. Vatsa, Blue luminescence from ZnGa₂O₄: effect of lattice distortion and particle size, J. Lumin. 188 (2017) 429-435, https://doi.org/10.1016/j.jlumin.2017.04.056.

[30] A. Tuerdi, A. Abdukayum, Dual-functional persistent luminescent nanoparticles with enhanced persistent luminescence and photocatalytic activity, RSC Adv. 9 (2019) 17653-17657, https://doi.org/10.1039/c9ra02235j.

[31] W. Zhang, J. Zhang, Z. Chen, T. Wang, Photocatalytic degradation of methylene blue by ZnGa₂O₄ thin films, Catal. Commun. 10 (2009) 1781-1785, https://doi.org/10.1016/j.catcom.2009.06.004.

[32] C. Matuszewska, T.W. Pańczuk, P.A. Tanner, K.L. Wong, Persistent luminescence of zinc gallogermanates, J Mater Chem C Mater 9 (2021) 7200-7213, https://doi.org/10.1039/d1tc01527c.

[33] J. Ueda, M. Back, M.G. Brik, Y. Zhuang, M. Grinberg, S. Tanabe, Ratiometric optical thermometry using deep red luminescence from ⁴T₂ and ²E states of Cr³⁺ in ZnGa₂O₄ host, Opt. Mater. 85 (2018) 510-516, https://doi.org/10.1016/j.optmat.2018.09.013.

[34] C.D.S. Brites, P.P. Lima, N.J.O. Silva, A. Millán, V.S. Amaral, F. Palacio, L.D. Carlos, Thermometry at the nanoscale, Nanoscale 4 (2012) 4799-4829, https://doi.org/10.1039/c2nr30663h.

[35] D. Jaque, F. Vetrone, Luminescence nanothermometry, Nanoscale 4 (2012) 4301-4326, https://doi.org/10.1039/c2nr30764b.

[36] C.D.S. Brites, A. Millán, L.D. Carlos, Lanthanides in luminescent thermometry, in: Handbook on the Physics and Chemistry of Rare Earths, 2016, pp. 339-427, https://doi.org/10.1016/bs.hpcre.2016.03.005.

[37] C.D.S. Brites, P.P. Lima, N.J.O. Silva, A. Millán, V.S. Amaral, F. Palacio, L.D. Carlos, A luminescent molecular thermometer for long-term absolute temperature measurements at the nanoscale, Adv. Mater. 22 (2010) 4499-4504, https://doi.org/10.1002/adma.201001780.

[38] F. Urbach, Zur Lumineszenz der Alkalihalogenide, Sitzungsberichte Akad. Der Wiss. Wien 139 (1930) 363-372.

[39] S. Zhang, F. Zhao, S. Liu, Z. Song, Q. Liu, An improved method to evaluate trap depth from thermoluminescence, J. Rare Earths 43 (2025) 262-269, https://doi.org/10.1016/j.jre.2024.02.004.

[40] N. Basavaraju, K.R. Priolkar, A. Bessière, S.K. Sharma, D. Gourier, L. Binet, B. Viana, S. Emura, Controlling disorder in the ZnGa₂O₄:Cr³⁺ persistent phosphor by Mg²⁺ substitution, Phys. Chem. Chem. Phys. 19 (2017) 1369-1377, https://doi.org/10.1039/c6cp06443d.