Research progress on Fe³⁺-activated near-infrared phosphor

3.1 Why choose Fe³⁺ as the activator and the luminescent properties of Fe³⁺ ions?

Fe³⁺ is a non-rare earth element and one of the most abundant elements on Earth. The cost-effective extraction of Fe³⁺ from iron, along with its stable supply and wide availability of iron resources compared to other rare earth elements, makes Fe³⁺ more sustainable and reduces dependence on rare elements, thus saving energy. One of the important advantages of Fe³⁺ is its non-toxicity, which is crucial in many applications. The use of safe and non-toxic substances is particularly important in various fields, especially in medicine and biosciences, where Fe³⁺ dopants fulfill this requirement and are widely used in applications such as biomedical imaging, drug delivery, and biosensing. Additionally, Fe³⁺ possesses a unique energy level structure and tends to exhibit NIR emission in octahedral environments (which will be discussed in detail in the next section). These advantages make Fe³⁺ an ideal dopant for NIR applications.

As a typical ion with a 3d⁵ electron configuration, Fe³⁺ ion has five electrons in its outermost shell, with an electron configuration of 1s²2s²2p⁶3s²3p⁶3d⁵. The 3d electrons, being in the outermost shell, are significantly influenced by the surrounding crystal field. Therefore, the choice of matrix directly affects the luminescent properties of Fe³⁺ ions. It is known that Fe³⁺ ions can achieve NIR emission in an octahedral coordination environment. The energy level splitting of Fe³⁺ ions in an octahedral coordination environment is shown in the Tanabe–Sugano (T–S) diagram (Figure 2): the ground state term of this configuration is ⁶A₁ (⁶S), the free electron term ⁴G splits into ⁴T₁ and ⁴T₂, and the degenerate ⁴A₁/⁴E and ⁴D splits into ⁴E and ⁴T₂. The ⁶A₁ (⁶S) term is a horizontal line, and the ⁴A₁/⁴E and ⁴E terms are also horizontal, so their energy is independent of the crystal field. The transitions from the ground state to these three states should produce sharp peaks, while transitions to stronger ligand fields such as ⁴T₁ and ⁴T₂ will result in broader bands. Similar to the 3d³ electron configuration of Cr³⁺ ion, it can be observed from the T–S energy level diagram in Figure 2 that the emission wavelength of Fe³⁺ is strongly influenced by the crystal field strength. The values of octahedral crystal field parameters D _q, Racah parameters B and C, as well as the crystal field strength D _q/B, for Fe³⁺-doped fluorescent powders can be estimated using the following equation [47]:

Figure 2

T–S energy-level diagram of the 3d⁵ electronic configuration in the octahedral field as well as the PLE and PL spectra of Fe³⁺ doping NIR phosphor [47].

Generally, the excitation of Fe³⁺-doped NIR fluorescent powders originates from ⁶A₁(⁶S) → ⁴E(⁴D), ⁶A₁(⁶S) → ⁴T₂(⁴D) and ⁶A₁(⁶S) → ⁴T₂(⁴G) transitions; the emission is due to the Fe^{3+ 4}T₁(⁴G) → ⁶A₁(⁶S) of electronic transition. From the T–S diagram (Figure 2), it can be observed that the weaker the crystal field strength, the smaller the peak wavelength of the emitted light [47].

The d–d transitions of Fe³⁺ in octahedral sites are subject to the strict restriction of the Laporte selection rule. This limitation presents a challenge in achieving high luminescence efficiency for long-wavelength NIR emission in Fe³⁺. The Laporte selection rule is a principle used to predict the spectroscopic activity of transition metal ions. According to this rule, charge transfer transitions are forbidden when the initial and final orbitals have the same symmetry. In the case of octahedral coordination, this means that d-electrons at the center of the octahedron cannot undergo transitions between the same spin orbitals, thus limiting the spectroscopic properties of octahedral complexes. Therefore, to achieve high luminescence efficiency for long-wavelength NIR emission in Fe³⁺, it is necessary to modify the structure of octahedral complexes by altering the ligand environment or introducing additional modifications. This may involve selecting appropriate ligands, tuning the crystal field strength, or introducing external alkaline earth metals, among other approaches. While this task presents challenges, it also offers numerous opportunities and prospects for applications. Achieving high luminescence efficiency for long-wavelength NIR emission in Fe³⁺ will contribute to the development of efficient NIR emissive materials, which are of significant importance in fields such as optoelectronics, biomedical imaging, and energy conversion. Consequently, researchers have been exploring various methods to overcome the limitations imposed by the Laporte selection rule in order to achieve high luminescence efficiency for long-wavelength NIR emission in Fe³⁺. The outcomes of these efforts will provide us with more choices of NIR emissive materials and drive further advancements and innovations in related fields.

3.2 Typical luminescent properties and research progress of Fe³⁺-activated NIR phosphor

Recently reported Fe³⁺-doped phosphors have emission wavelengths exceeding 800 nm. They have great potential for various applications, such as CaAl₁₂O₁₉:Fe³⁺ (808 nm) [48], SrAl₁₂O₁₉:Fe³⁺ (811 nm) [48], and CaGa₂O₄:Fe³⁺ (809 nm) [49]. It offers a flexible design for new Fe³⁺-doped NIR persistent phosphors through cation substitution and local crystal field modification. Table 1 summarizes some NIR phosphors doped with Fe³⁺. In 2022, Wang reported unprecedented long-wavelength NIR emission phosphors of Fe³⁺-activated Sr_2−y Ca _y (InSb)_1−z Sn_2z O₆ [50]. Overall emission tuning from 885 to 1,005 nm and full-width at half-maximum (FWHM) broadening from 108 to 146 nm were achieved through crystallographic site engineering strategies. NIR emission was significantly enhanced after complete Ca²⁺ incorporation due to the reduced symmetry induced by substitution. Ca₂InSbO₆:Fe³⁺ phosphor exhibited a peak at 935 nm with an ultra-high internal quantum efficiencies (IQE) of 87%. The synthesized tunable emission phosphors showed enormous potential for NIR spectral detection. This work initiated the development of efficient Fe³⁺-activated broadband NIR emission phosphors and opened up a new pathway for the design of NIR emitting phosphor materials. In 2022, Zhang et al. successfully synthesized Fe³⁺-activated NaScSi₂O₆ phosphor [51]. Upon excitation at 300 nm UV light, a wide NIR emission band at 900 nm with an FWHM of 135 nm and an IQE of 13.3% was observed. The luminescence was related to the 3d–3d transitions of Fe³⁺ in octahedral coordination, confirming the effectiveness of Fe³⁺ activators in achieving efficient NIR emission. In 2022, Xiang et al. reported a series of environmentally friendly and low-cost Fe³⁺-activated ZnGa₂O₄ phosphors [52]. The Fe³⁺-doped phosphors exhibited a broad emission range from 650 to 850 nm, with a maximum emission peak at 720 nm and an FWHM of 70 nm. The emission intensity remained at 71% even at an elevated temperature of 423 K. The NIR pc-LED made of this phosphor exhibited strong NIR emission. In 2023, Li et al. reported a novel Fe³⁺-activated NIR persistent phosphor composed of Mg₂SnO₄ [53]. Fe³⁺ ions occupied both tetrahedral and octahedral sites, and due to the energy level alignment, electrons released from traps preferentially returned to the excited energy level of Fe³⁺ in the tetrahedral sites via tunneling, resulting in efficient NIR persistent emission at a peak wavelength of 789 nm with an FWHM of 140 nm. This phosphor demonstrated a record-breaking persistence duration of over 31 h in Fe³⁺-based phosphors for night vision applications, serving as a self-sustainable light source. This work not only provided a new type of efficient Fe³⁺-doped NIR persistent phosphor for technological applications but also established practical guidelines for the rational tuning of persistent emission. Based on the analysis and summary of the above literature, it can be observed that a disadvantage of Fe³⁺-doped NIR phosphors is the inability to be efficiently excited by blue light, which limits their applications to some extent. In 2023, Cheng et al. synthesized Fe³⁺-activated NaAl₅O₈ NIR phosphors [54]. Under excitation at 346 nm, NaAl₅O₈:Fe³⁺ phosphors exhibited NIR emission with a main peak at 754 nm. Emission spectra were measured in the temperature range of 303–463 K, revealing that the emission intensity at 423 K remained at 83.1% of that at 303 K, indicating excellent thermal stability of the sample. The luminescent behavior of the sample was studied after immersing it in deionized water for 0–10 h. These findings suggest that NaAl₅O₈:Fe³⁺ samples hold promise for applications in high-temperature or humid environments. Furthermore, the use of NaAl₅O₈:Fe³⁺ in the preparation of anti-counterfeit ink demonstrated its potential in the field of anti-counterfeiting. In 2023, Yang et al. synthesized Fe³⁺-activated Li₂ZnAO₄ (A = Si, Ge) phosphors through solid-state reactions, with Fe³⁺ occupying the tetrahedral sites of Zn²⁺ [38]. Under excitation by 300 nm ultraviolet light, broad NIR emission bands were observed at 750 nm (Li₂ZnSiO₄:Fe³⁺) and 777 nm (Li₂ZnGeO₄:Fe³⁺), with IQE of 62.70% (Li₂ZnSiO₄:Fe³⁺) and 30.57% (Li₂ZnGeO₄:Fe³⁺). The thermal stability at 373 K was improved from 35.43 to 49.79% through cation tuning. The combination of activation energy, electron-phonon coupling, and Debye temperature explained the enhanced thermal stability of Li₂ZnGeO₄:Fe³⁺ phosphors. Additionally, the synthesized phosphors exhibited sensitive and selective detection of Cu²⁺ ions. In 2023, the research team led by Quanlin Liu utilized the structural confinement effect in Sr₉Ga(PO₄)₇ (SGP) to selectively control energy transfer pathways, suppressing luminescence concentration and thermal quenching effects. They found that in Fe³⁺-doped SGP compounds, the relatively large Fe³⁺-Fe³⁺ distances hindered energy transfer between Fe³⁺ ions, resulting in weakened concentration quenching. The Sr₉Ga_0.8(PO₄)₇:0.2Fe³⁺ (SGP:0.2Fe³⁺) phosphor exhibited the highest NIR luminescence intensity. Additionally, they introduced trivalent Yb³⁺ to investigate its influence on the luminescence of Fe³⁺-doped phosphor systems. Co-doping Yb³⁺ into SGP:0.2Fe³⁺ resulted in much shorter Fe³⁺–Yb³⁺ distances compared to Fe³⁺–Fe³⁺, facilitating rapid energy transfer from the quenching center Fe³⁺ to the thermally stable center Yb³⁺. The thermal stability of SGP:0.2Fe³⁺ ,0.07Yb³⁺ was greatly enhanced compared to SGP:0.2Fe³⁺. This study provided a method to enhance NIR luminescence by utilizing structural confinement to control energy transfer pathways and suppress concentration and thermal quenching effects. Finally, they demonstrated the potential applications of SGP:0.2Fe³⁺ and SGP:0.2Fe³⁺ ,0.07Yb³⁺ phosphors in the fields of night vision and optical thermometry [47].

3.3 Some application areas and potential of Fe³⁺-activated NIR phosphor

The phosphor activated by Fe³⁺ ions distinguishes itself through its superior optical properties, which significantly amplify its application potential across diverse fields. These properties, encompassing high luminescence efficiency and exceptional stability, pave the way for its use in advanced imaging, plant growth, food analysis, biomedical imaging, and night vision. Next, let’s take the application in the field of anti-counterfeiting as an example to demonstrate. In 2023, Cheng et al. presented their findings on Fe³⁺-activated NaAl₅O₈ NIR phosphor, examining its phase and luminescent properties [54]. It was observed that the optical bandgap narrows as the Fe³⁺ doping concentration increases. The sample’s thermal stability was confirmed by maintaining 83.1% emission intensity at 423 K compared to 303 K, as measured across an emission spectrum range of 303–463 K. These research results not only demonstrate the potential applications of NaAl₅O₈:Fe³⁺ samples in high temperature or humid environments but also highlight their significance in the field of anti-counterfeiting, particularly in the context of anti-counterfeiting inks. Anti-counterfeiting inks are widely used in the protection of important documents such as currency, identification cards, and labels to prevent forgery and fraud. By utilizing NaAl₅O₈:Fe³⁺ as a phosphor material in anti-counterfeiting inks, enhanced anti-counterfeiting effects can be achieved. There are several advantages of incorporating NaAl₅O₈:Fe³⁺ in anti-counterfeiting inks. First, NaAl₅O₈:Fe³⁺ phosphor exhibits high stability and is resistant to high temperatures, allowing it to maintain its luminescent properties in environments with elevated temperatures. This is crucial during the manufacturing process of anti-counterfeiting inks, which often involves heating and curing. Second, NaAl₅O₈:Fe³⁺ phosphor demonstrates good stability in humid environments, preventing performance degradation associated with humidity and ensuring the ink’s reliability and durability in such conditions. Furthermore, the successful application of NaAl₅O₈:Fe³⁺ in anti-counterfeiting has been verified. Figure 3 illustrates the remarkable efficacy of NaAl₅O₈:Fe³⁺ when utilized as an anti-counterfeiting ink. The phosphor’s luminescent qualities are uniquely tailored, exhibiting distinct spectral features that can be precisely identified and authenticated. These tailored characteristics render the ink exceptionally conspicuous and swiftly verifiable, ensuring its integrity is effortlessly ascertainable with the employment of specialized detection apparatus or analytical instruments. This not only underscores the ink’s suitability for high-security applications but also enhances its practicality for widespread use in safeguarding valuable items from counterfeiting attempts. Hence, the successful utilization of NaAl₅O₈:Fe³⁺ phosphor as a constituent in anti-counterfeiting inks not only underscores its significance in the realm of anti-counterfeiting but also serves as a pivotal tool in the ongoing battle against counterfeit currency and forged identification cards. Furthermore, its implementation plays a crucial role in safeguarding brand integrity, fortifying fraud deterrents, and upholding consumer trust. These groundbreaking research findings provide compelling evidence to propel the advancement of anti-counterfeiting technologies, paving the way for new avenues of innovation and progress in combating counterfeit practices [54].

Figure 3

Security inks (a) and the patterns written on (b) stainless steel, (c) glass, (d) plastic, and (e) paper under room light and UV light [54].

Moreover, apart from its applications in anti-counterfeiting technology, NIR phosphors doped with Fe³⁺ have also demonstrated remarkable potential in the field of night vision. In 2023, Li et al. reported that Fe³⁺-doped NIR phosphor demonstrated a record-breaking duration of over 31 h, making it a self-sustainable light source for night vision applications, as shown in Figure 4 [57].

Figure 4

Schematic (a) and experimental setup (b) for the self-sustainable night vision imaging. (c) Photos of a toy car taken in the dark with and without NIR PersL illumination by visible and NIR cameras [57].

Phosphors doped with Fe³⁺ ions have broad prospects in applications such as luminescence thermometry and NIR stickers. In 2022, Zhang et al. conducted a study on Fe³⁺-activated NaScSi₂O₆ phosphor [51]. In their research, they excited the sample with 300 nm ultraviolet light and observed a broad NIR emission band at 900 nm, with an FWHM of 135 nm and an IQE of 13.3%. This emission behavior is closely related to the 3d–3d transitions of Fe³⁺ ions in octahedral sites, validating the effectiveness of Fe³⁺ as an activator for achieving efficient NIR emission. The research results also showed that the emission intensity of Fe³⁺ exhibited a linear quenching trend within the temperature range of 293–433 K. This indicates that Fe³⁺-doped phosphor can serve as a sensitive luminescent temperature sensing material, and the changes in its emission intensity can be used for real-time temperature measurement and monitoring. In addition, the feasibility of the synthesized phosphor material was also investigated in applications such as NIR luminescence thermometry and NIR stickers, as shown in Figure 5. This implies that Fe³⁺-doped phosphor has potential applications in non-contact temperature measurement technologies and patch-type infrared sensors. These applications contribute to achieving high-precision, high-sensitivity, and long-distance temperature measurements, providing more possibilities for industries such as industrial production, energy utilization, and medical diagnostics. In summary, Fe³⁺-doped phosphor exhibits tremendous potential in applications such as luminescence thermometry and NIR stickers. The compelling research findings surrounding Fe³⁺-doped phosphor offer robust evidence, bolstering the case for its continued development and expanded utilization within the realm of infrared luminescence. The intrinsic properties of this material, such as its efficient infrared emission and stable luminescent behavior under varying conditions, lay a solid foundation for breakthroughs in technological innovation. Moreover, these findings illuminate a path forward for application development in fields including, but not limited to, bioimaging, security, and optical communications. The promising results obtained thus far herald a new era of advancements, underscoring the transformative potential of Fe³⁺-doped phosphor in driving forward the frontiers of infrared luminescent applications [51].

Figure 5

(a) Scheme of the fabrication of an IR patch by the screen printing method. Photographs under natural light captured using a visible camera (b, left) and in the dark captured using a NIR camera under UV excitation light when it is off (c, middle) and on (d, right) [51].

Research has found that the majority of Fe³⁺-doped NIR fluorescent materials exhibit lower IQE values. It is noteworthy that Fe³⁺-doped NIR fluorescent materials have broad absorption bands. This broad band feature allows them to absorb more light energy, thereby enhancing the efficiency of light excitation. This is crucial for applications such as bioimaging and photovoltaics, as it can provide stronger excitation signals or higher energy conversion efficiency. Additionally, Fe³⁺-doped NIR fluorescent materials have relatively long lifetimes. The lifetime of fluorescent materials is critical for the stability and optical performance of fluorescent probes. A longer lifetime can reduce background noise in fluorescence bioimaging, improve imaging clarity and signal-to-noise ratio, and also benefit the stability and long-term use of photovoltaic devices.

Unfortunately, the understanding of the luminescence mechanism and certain related luminescent properties of Fe³⁺-doped NIR phosphors is not yet sufficiently deep. Although some studies have revealed key luminescence mechanisms and band structures, further research is still needed to address some of the complexities involved. Second, the stability of Fe³⁺-doped NIR fluorescent materials needs to be improved. Some studies indicate that certain fluorescent materials undergo degradation or irreversible deactivation of fluorescence intensity under prolonged excitation or high-temperature environments. This instability limits the further promotion and application scope of the materials in practical use. Additionally, the selection of matrix materials and optimization of structural design remain challenging. The choice of matrix and novel structural design significantly impact the improvement of optical performance. It is necessary to consider the balance of ion concentration with fluorescence efficiency, lifetime, and light absorption intensity to obtain better optical performance.

In conclusion, Fe³⁺-doped NIR fluorescent materials have a wide range of optical performance advantages, including broad emission wavelength range, high fluorescence quantum yield, wide light absorption band, and relatively long lifetime. However, challenges regarding understanding the mechanism, stability, and structural design still need to be addressed. Future research will focus on overcoming these challenges and further improving and optimizing the optical performance of Fe³⁺-doped NIR fluorescent materials to achieve their widespread application in fields such as biomedical imaging and optoelectronics.

4.1 Outlook

Looking ahead, with the increasing demand for NIR fluorescent materials, Fe³⁺-doped NIR fluorescent materials are expected to find more applications in a wider range of fields. In the field of biomedical imaging, Fe³⁺-doped NIR fluorescent materials have great potential in tumor labeling, early cancer detection, and photodynamic therapy due to the deep penetration ability of NIR light and low tissue autofluorescence interference. In the field of biosensors, Fe³⁺-doped NIR fluorescent materials have high sensitivity and selectivity, enabling efficient, rapid, and accurate detection of biomolecules, providing strong support for disease diagnosis and drug screening. Additionally, Fe³⁺-doped NIR fluorescent materials have a wide range of application prospects in the field of photovoltaics, which can improve photovoltaic conversion efficiency and energy output, providing new possibilities for the development and utilization of renewable energy. In summary, significant progress has been made in the research of Fe³⁺-doped NIR phosphors due to their tunable luminescent properties and potential applications in the NIR spectral range, making them a hot research topic. Further research will help to understand its luminescence mechanism, improve its luminescence efficiency and stability, and expand its application prospects in the fields of biomedical imaging and optoelectronics.

4.2 Challenges

Although in recent years, Fe³⁺-doped NIR fluorescent materials have been widely studied and received extensive attention due to their unique optical properties and potential applications, significant progress has already been made in the fields of biomedical imaging, biosensors, and photovoltaics. However, there are still challenges that need to be addressed in current research. For example, some Fe³⁺-doped NIR phosphors still exhibit issues with long-term excitation and stability, which limits their further advancement in practical applications. Additionally, further research is needed to deepen our understanding of the Fe³⁺ doping mechanism and luminescent properties.

First, in the realm of preparation and synthesis, it is crucial to embark on further exploration and refinement of suitable synthesis methods and process conditions. As the current preparation methods possess certain complexities and challenges in controlling material parameters, it is imperative to undertake more research to discover efficient, controllable, and reproducible preparation techniques. Additionally, to bolster the efficiency and stability of fluorescence, a more comprehensive understanding of the Fe³⁺ doping mechanism and luminescent properties becomes indispensable in facilitating enhanced material design and synthesis.

Second, emphasis should be placed on material stability and long-term reliability. The challenges lie in mitigating light attenuation and degradation, which can adversely impact luminescence performance in prolonged excitation and high-temperature environments. To tackle this predicament, it is imperative to develop more robust materials, including enhancing the light damage threshold and augmenting their optical durability. These advancements will enhance stability and render them more suitable for practical applications.

Additionally, to further enhance luminescent performance, structural design improvements, crystal structure enhancement, and doping methods can be implemented. In particular, optimizing the doping concentration and dopants is an important research direction with the potential to achieve optimal control of material properties and luminescence efficiency. Furthermore, exploring novel synthesis techniques and the influence of external factors, such as temperature and pressure, on luminescent properties may offer additional avenues for improving material performance.

In summary, Fe³⁺-doped NIR fluorescent materials hold immense promise and potential for applications in biomedical imaging, biosensors, and photovoltaics. Nevertheless, certain challenges persist in terms of preparation methods, stability, and luminescent performance. Future research efforts will be directed toward overcoming these challenges, enhancing the performance and stability of Fe³⁺-doped NIR fluorescent materials, expanding their application prospects, and expediting their translation and utilization in the fields of biomedical imaging and optoelectronics.