Transition-Metal-Doped Halide Perovskites for Near-Infrared Emissions: Beyond Oxides
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Transition-Metal-Doped Halide Perovskites for Near-Infrared Emissions: Beyond Oxides
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ACS Energy Letters

Cite this: ACS Energy Lett. 2026, 11, 4, 3024–3037
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https://doi.org/10.1021/acsenergylett.6c00105
Published March 16, 2026

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Abstract

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Transition-metal-doped oxide-based phosphors led to near-infrared (NIR) light-emitting diodes and lasers. Mostly, the first transition series (3d) metal ions, like Cr3+, are doped. Interestingly, heavier transition-metal ions, like Mo3+, Re4+, and Os4+, in their low oxidation states, can be doped in halide perovskites, unlike the oxide hosts that stabilize the higher oxidation state. Consequently, doping heavier transition metal ions in halide hosts opens up new avenues to tailor NIR-I/II emissions for both narrow and broad-band d–d transitions. Furthermore, characteristics such as (i) tunability of crystal field parameters by halide ions, (ii) host emissions, and (iii) colloidal nanocrystal synthesis add new NIR functionalities to transition-metal-doped halide perovskites compared to oxides. Here we provide insights on recent progresses on the NIR emitting transition-metal-doped halides, connecting synthetic materials chemistry, spectroscopy and device fabrication. The present challenges and future opportunities are discussed.

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Published as part of ACS Energy Letters special issue “The Evolving Landscape of Energy Research: Insights from Leading Researchers”.

Phosphors that are commercialized, such as BaMgAl10O17:Eu2+ (BAM:Eu2+), β-SiAlON:Eu2+, Y3Al5O12:Ce3+ (YAG:Ce3+), and Y2O3:Eu3+, emit the gamut from blue to red region of light and have revolutionized solid state lighting. (1) On the other hand, Nd:YAG (2) and ruby (Al2O3:Cr3+) (3) highlight the importance of phosphors in lasing applications. Therefore, the development of phosphors that show photoluminescence (PL) in the far red or near-infrared (NIR) window has a long history. Typically, NIR luminescence in the majority of phosphors originates from f–f electronic transitions of trivalent lanthanides (Ln3+) or d–d transitions of transition metal ions like Cr3+, Mn5+, Mo3+, and so on. The inner-core f–f transition of Ln3+ is largely insensitive to the surrounding lattice and, therefore, has its own pros and cons, as discussed in multiple review articles. (4,5) The difference is the PL originating from d–d transitions can be controlled by the surrounding host lattice. (6−10) So, a transition metal doped into oxide hosts can have significantly different NIR PL properties compared to the doping in halide perovskites. What are the major conceptual differences between NIR-emitting transition metal doping in oxides versus halide perovskites? Here, we present our perspective on this question by combining the recent advances on NIR-emitting transition-metal-doped halide perovskites with seminal reports published from 1960 to 2000.
Both oxide- and halide-based hosts possess the required diversity in chemical compositions and crystal structures, allowing for the doping of transition metal ions into the host lattice. In general, oxide-based phosphors show higher thermal and air/moisture stability. So, the oxides will be preferred over halide perovskites unless the halides show certain properties that are beyond the scope of oxides. For example, transition metal doping in halide perovskites leads to (i) stabilization of low oxidation states of heavier transition metals like Mo3+, (ii) NIR emission from host lattice, (iii) halide ion dependent tuneability of the crystal field and Racah parameter, and (iv) low-temperature colloidal synthesis of nanocrystals with potential as a solution-processed device. These material design aspects of transition-metal-doped halide perovskites lead to novel NIR emission properties and applications compared to the well-known features of oxide based NIR phosphors. Figure 1 highlights those unique aspects, which will be discussed one-by-one in this article.

Figure 1

Figure 1. Schematic highlights the scope of transition metal doping in halide perovskites (and related structures) for NIR emission, which is typically beyond the scope of oxide-based NIR phosphors.

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Low Oxidation States of Dopants and Spin-Flip Transition

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Transition metal ions are known to possess multiple oxidation states depending on the surrounding medium. A change in oxidation state changes the number of d-electrons and its coordination environment and, therefore, drastically changes the nature of d–d electronic transitions. For example, the d–d transitions of Mn2+ (d5 configuration), Mn4+ (d3 configuration), and Mn5+ (d2 configuration) yield broad yellow-orange (peak ∼ 580 nm), (11) narrow red (peak ∼ 622 nm), (12) and narrow NIR (peak ∼ 1181 nm) (13) emissions, respectively. Therefore, controlling the oxidation states of transition metal dopants is a key strategy to control the NIR emission properties. Different oxide-based hosts typically stabilize the higher oxidation states and are less suitable for stabilizing lower oxidation states, particularly for heavier 4d and 5d series of transition metals such as molybdenum, tungsten, and rhenium. For example, Mo5+/6+ can be stabilized in oxide hosts, but not Mo3+. In contrast, Mo3+ can be stabilized in a halide perovskite host.

Therefore, controlling the oxidation states of transition metal dopants is a key strategy to control the NIR emission properties.

Mo3+ Doping

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Early works of Reinberg, Paulusz, Güdel, Gamelin, and others during 1970–2002 showed strong Mo3+ emission ∼ 1100 nm from Mo3+ doped into different halide hosts. (14−19) But in all those cases, the emissions were measured below 200 K at vacuum or in a highly acidic environment. In ambient air and at room temperature, the Mo3+ emission quenched completely. Most likely, the oxophilic nature of molybdenum alters its oxidation state and the coordination environment, forming oxychlorides. Very recently, we (part of the present authors) reported a hydrothermal synthesis of ambient-stable Mo3+-doped Cs2NaInCl6. (20) The structure is schematically shown in Figure 2a. Different characterization techniques, including X-ray absorption fine structure (XAFS) spectroscopy, confirmed Mo3+ dopants substitute the In3+ sites in the Cs2NaInCl6. Figure 2b shows the first-shell (Cl) coordination environment around Mo3+ (red open symbols) and In3+ (blue open symbols), in terms of coordination number (CN), bond length (R), and local order (σ2). A consistent CN ≈ 6 with very small σ2 values confirms the [MoCl6]3– octahedral units in Mo3+-doped Cs2NaInCl6. It is noteworthy that undoped Mo3+-based halide perovskite hosts such as Cs2NaMoCl6 and (CH3NH3)2NaMoCl(6–x)Brx are also reported recently, signifying that the low oxidation state of Mo3+ can be stabilized in a halide perovskite and related structure. (21,22) The stability of Mo3+-halide bonds, instead of forming oxyhalides or oxides, is interesting and requires further understanding.

Figure 2

Figure 2. (a) Schematic representation of Mo3+-doped Cs2NaInCl6 double perovskite cubic lattice with the space group Fm3m. (b) Description of the Cl environment around Mo (red open symbols) and In (blue open symbols) for Mo3+-doped Cs2NaInCl6 with different dopant concentrations: coordination number (CN, top panel), bond length (R, middle panel), and local order (σ2, bottom panel). (c) d–d vs f–f transition-based NIR-II PL spectra from Ln3+- (Ln = Yb, Er) and Mo3+-doped Cs2NaInCl6. (d) Left panel: Tanabe-Sugano diagram for a d3 ion in an octahedral coordination; middle panel: schematic diagram comparing the orbital interaction between the metal (M) dxy ligand (L) px when the metal ion (d3: Cr3+, Mo3+, Re4+) is in the ground state (GS: 4A2g) and in an excited state (ES: 2Eg); right panel: schematic potential energy surface diagram, indicating that the ground state 4A2g, and excited states 2Eg/2T1g and 2T2g have the same t2g3eg0 electronic configuration and, therefore, have very similar M–L bond lengths. Reproduced with permission from ref (20). Copyright 2025 Wiley.

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The ambient-stable Mo3+-doped Cs2NaInCl6 shows narrow NIR emissions of ∼ 1095 nm due to d–d electronic transition of Mo3+, as shown in Figure 2c. The optimized 3.8% (estimated by inductively coupled plasma mass spectrometry) Mo3+ doping yielded the most intense emission, and a further increase in the dopant concentration decreased the emission intensity because of concentration quenching. Typically, d-electrons interacts with the surrounding ligands, and therefore, d–d electronic transitions are significantly broader than the inner core f–f electronic transitions. Interestingly, the Mo3+ d–d emission is as sharp as the f–f emissions of lanthanides like Yb3+ or Er3+ (Figure 2c).
This ultranarrow d–d emission is explained by Intra-Configurational-Spin-Flip (ICSF) transition, (23,24) using the Tanabe-Sugano diagram of the d3 electronic configuration in an octahedral crystal field, as shown in Figure 2d (left side). The Tanabe-Sugano diagram helps to understand the electronic d–d transitions by correlating electronic energy (E) as a function of crystal field strength (Dq) normalized by the electron–electron repulsion term, represented by the Racha parameter B. In a stronger crystal field (Dq/B > 2.3), the first excited state is the degenerate 2Eg/2T1g, along with the ground state 4A2g. The ∼1095 nm emissions of Mo3+ originate from the spin-forbidden 2Eg/2T1g → 4A2g, d–d transition. Importantly, the electronic configuration of the 2Eg/2T1g excited states and the ground state 4A2g remains the same t2g3eg0, with only a difference in the flipping of the spin of one electron (see the middle part of Figure 2d). Therefore, the 2Eg/2T1g → 4A2g transition is an ICSF t2g3eg0 → t2g3eg0 transition, where the electronic distribution remains unchanged in the t2g orbitals (dxy, dyz, dzx) before and after the transition. Therefore, the interaction of the metal d-orbital and ligand orbitals remains nearly unchanged in both ground and excited states, retaining the same metal–ligand (Mo–Cl) bond lengths for both states, as shown by the potential energy surface diagram in Figure 2d (right side). It is also noteworthy that the interaction between the metal t2g orbital (dxy, dyz, dzx) and ligand p-orbitals is geometrically not favored, with almost no contribution to σ-bonding. Because of the above reasons, the energy difference in the unit Racah parameter B, i.e., E/B, between 2Eg/2T1g and 4A2g remains largely unchanged in the strong-field region of the Tanabe-Sugano diagram (Figure 2d), yielding the ultranarrow ICSF emissions, where lattice vibrations do not contribute much to the spectral line broadening. But, still, the line width, nonradiative losses, and temperature dependency can get affected by factors such as host metal–halide bond covalency, phonon energies, and spin–orbit coupling.
The same ICSF 2Eg/2T1g → 4A2g transition is well-known for Ruby (Cr3+-doped Al2O3) that occurs around 700 nm, (3) because of 3d3 electrons of Cr3+ in an octahedral field with Dq/B ∼ 2.7, as shown in the Tanabe-Suagano diagram (Figure 2d). Compared to the 3d orbitals of Cr3+, the 4d orbitals of Mo3+ are more diffused with larger Dq and smaller B. Consequently, two differences are observed, (i) the ICSF 2Eg/2T1g → 4A2g emission of Mo3+ shifts to a longer wavelength of ∼1095 nm compared to ∼700 nm for Cr3+, and (ii) the Tanabe-Suagano diagram shows that the second excited state for Mo3+ is another ICSF 2T1g, whereas the second excited state is 4T2g for Cr3+. So, for Mo3+, there is a possibility of a second ICSF emission for the 2T2g → 4A2g transition. Indeed, at lower temperatures, Mo3+-doped Cs2NaInCl6 show the 2T2g → 4A2g ICSF emission ∼700 nm, in addition to the 2Eg/2T1g → 4A2g ICSF emission ∼1095 nm (see Figure 3a). This indicates the domination of nonradiative relaxation of 2T2g excited state at room temperature. The mechanistic insights on nonradiative relaxation of electrons from higher excited d-states to the lowest excited state 2Eg/2T1g needs to be studied further using ultrafast spectroscopic techniques like transient absorption.

Figure 3

Figure 3. (a) Temperature-dependent (7–300 K) PL spectra of 3.8% Mo3+-doped Cs2NaInCl6 in the visible to NIR region under 438 nm excitation. Reproduced with permission from ref (20). Copyright 2025 Wiley. (b) PL and PL excitation spectra of Cs2HfCl6:Re4+ at room temperature. Reproduced with permission from ref (26). Copyright 2025 Elsevier. (c) PL spectrum of Cs2HfCl6:Os4+ crystals at 2 K. Since the spectrum was collected in a broad range from visible to NIR-II (∼580–2100 nm), the relative detector response is also shown. Reproduced with permission from ref (14). Copyright 1971 American Physical Society.

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The transition from the lowest excited state to the ground state, namely, 2Eg/2T1g → 4A2g ICSF emission of ∼1095 nm, also shows interesting temperature dependence (Figure 3a). At 7 K, the spectrum shows a zero phonon line (ZPL) at 1081 nm, along with the Stokes line at 1095 nm. An increase in temperature increases the population in the higher vibrational states, resulting in an anti-Stokes line at 1068 nm. This observation of ZPL, Stokes and anti-Stokes lines along with other vibronic peaks have been shown in prior literature. (16,19,20) However, the extent of energy splitting between the 2Eg and 2T1g states and its influence on the emission spectrum are not yet clear. The PL lifetime of the 2Eg/2T1g → 4A2g ICSF of Mo3+-doped Cs2NaInCl6 increases from 9 ms at room temperature to 54 ms at 7 K, indicating suppression of nonradiative decays at lower temperatures. (19,20) The long lifetime is consistent with both spin and Laporte forbidden transitions of [MoCl6]3– octahedra. Notably, the emission intensity slightly increases with increasing temperature, which is opposite to the usual trend of thermal (nonradiative) quenching of luminescence at higher temperatures. This unusual temperature-induced enhancement in emission intensity is probably due to the asymmetric vibration-driven relaxation of the Laporte rule at higher temperatures.

Re4+, Os4+, and V3+ Doping

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After 3d3 (Cr3+), 4d3 (Mo3+), one would also like to explore 5d3 doping. In this regard, we could not find a report of W3+ (5d3) doping, which is placed below Cr and Mo in the same group of the periodic table. But interestingly, 5d3 doping in the octahedral environment was achieved by Re4+ doping, such as Re4+-doped vacancy ordered perovskites like Cs2ZrCl6 and Cs2HfCl6. (25,26) The Re4+ PL arises from [ReCl6]2– in Re4+-doped Cs2HfCl6, reported by Fan et al., (27) as shown in Figure 3b. The PL spectrum of Re4+-doped Cs2HfCl6 shows two ICSF emission peaks at ∼726 due to 2T2g → 4A2g and ∼1342 nm due to 2T1g → 4A2g. Expectedly, both the ICSF emission peaks of Re4+ are shifted toward longer wavelengths compared to that of Mo3+, since the 5d orbitals of Re4+ are more diffused compared to the 4d orbitals of Mo3+. Furthermore, the spin–orbit interaction increases for heavier metal ions, fine-tuning the spectral features for Re4+, which has been discussed elsewhere. (14) In addition to the two ICSF emissions, another incomplete PL feature is observed at ∼1600 nm for Re4+-doped Cs2HfCl6 (Figure 3b), which was assigned as spin-allowed 2T2g → 2T1g transition between two excited states of Re4+. (26)
Not only d3 systems, Güdel and coworkers (14) showed multiple PL peaks from Cs2HfCl6:Os4+ at 2 K, as shown in Figure 3c. Os4+ has a 5d4 outermost configuration and an expectedly high crystal field strength (Dq/B > 6) causing the first and second excited state terms 1T2g/1Eg and 1A1g, whereas the ground state is 3T1g. So, Os4+ also shows multiple ICSF emissions due to 1T2g → 3T1g and 1A1g → 3T1g transitions. However, as a result of strong spin orbit coupling, Os4+ shows multiple emissions at 591 nm (16910 cm–1), 714 nm (14000 cm–1), and 833 nm (12005 cm–1) due to the transition from 1A1g → 3T1g. Similarly, PL lines near 1000 nm (10000 cm–1), 1316 nm (7600 cm–1) and 1818 nm (5500 cm–1) are the result of 1T2g → 3T1g ICSF transition (Figure 3c). Notably, unlike d3 systems like Re4+ and Mo3+, PL emission from Os4+ d4 systems has not been achieved yet at room temperature. Similarly, Güdel and coworkers also reported NIR PL from the 3d2 system, such as V3+-doped Cs2NaYCl6, Cs2NaYBr6, and their temperature-dependent studies. (28) For the trivalent vanadium ion, the excited state is either 1T2g or 3T1g, depending on the temperature and crystal field (halide counterpart). Therefore, tuning of sharp (1T2g → 3T1g) to broad band (3T2g → 3T1g) NIR-II PL has been seen in the range of 1000–1250 nm. Overall, more investigations are required connecting NIR photoluminescence with the local structure of a few dopants like Os4+ and V3+.

Debate on [MX6]2– Octahedra vs [MOX5]2– Distorted C4v Centers (M: Mo, W, V, etc.)

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In the year 1962, Gray et al. reported the electronic transition of (NH4)2MoOCl5, with a d1 electronic configuration of Mo5+, a C4v symmetry of [MoOCl5]2–. (29) Subsequently, multiple reports of salts of oxomolybdenum(IV) cations like [MoOCl-(CN-t-Bu)4]+, or Mo(V) compound [(18-Crown-6)K][MoOCl4(H2O)], or dioxo complexes of Re(V), and Os(VI), are reported. (30−33) In all these samples, metal–oxygen bonds are there along with the metal–halide bonds and emit broad NIR emissions, because of the d–d transitions of the metal ions in the C4v symmetry or similar symmetry lower than the octahedral symmetry. Unfortunately, the NIR emissions quenched drastically at room temperature, probably because of the high-frequency vibrational overtones of the organic bonds and/or degradation of the samples.
Interestingly, Liu et al. reported all-inorganic Cs2MoCl6, showing intense NIR (900–1300 nm) emission with a high PL quantum yield (PLQY) of 26% in ambient conditions at room temperature. (34) They assigned the emission as a self-trapped exciton (STE) emission. Later on, Mondal et al. (part of the present authors) assigned the emission as a d–d transition of the molybdenum ion. Based on single crystal X-ray diffraction (XRD) data, Liu et al. showed the formation of a vacancy ordered (0D) perovskite structure (see Figure 4a, top part) of Cs2MoCl6, where each of the [MoCl6]2– units are electronically isolated from each other, and the charge neutrality is maintained by Cs+ ions through ionic interactions. Mondal et al. reproduced the single crystal XRD analysis of Liu et al., suggesting that the d–d transition of [MoCl6]2– is responsible for the NIR emission. This means that the Tanabe-Sugano diagram (see Figure 4a, bottom part) of the d2 configuration in the octahedral field of [MoCl6]2– should be employed to explain the electronic absorption and emission properties of Cs2MoCl6.

Figure 4

Figure 4. (a) Crystal structure of Cs2MoCl6 containing a [MoCl6]2– unit with an octahedral symmetry (top) followed by the Tanabe-Sugano diagram as reference for showing possible electronic transitions of Mo4+ (4d2) in an octahedral local symmetry (bottom). The crystal structure was obtained using a CIF file reported in ref (42). (b) Crystal structure of Cs2MoOxCl6–x containing the [MoOCl6]2– unit with reduced C4v symmetry (top) followed by possible electronic transitions of Mo5+ (4d1) (bottom). The crystal structure was obtained using the CIF file reported in ref (35).

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Contrarily, Morgan et al. suggested that the sample is not Cs2MoCl6; instead, it is Cs2MoOxCl6–x. (35) The single crystal XRD of the sample obtained by all three groups is very similar, but Morgan et al. showed that the same data can be fitted considering both the Cs2MoCl6 and Cs2MoOxCl6–x (Figure 4b, top panel) with the same vacancy ordered perovskite structure. To resolve the ambiguity, Raman spectroscopy came up as an important tool. Apart from the expected Raman modes associated with the metal halide framework, (36) an additional peak was observed in the 800–900 cm–1 region. This higher energy peak is characteristic of molybdenum–oxo bonds, suggesting the formation of Cs2MoOxCl6–x. (35) The composition “x” can vary by ∼1, with the oxidation state of Mo being predominantly +5, but can vary between +4 and +6. Consequently, Morgan et al. proposed that the broad NIR emission of Cs2MoOxCl6–x originates from the d–d transition of Mo5+ (d1) in C4v symmetry of [MoOCl5]2–, adapting the electronic states reported by Gray et al., (29) and other reports, (33) as shown in (Figure 4b, bottom panel).
Both [MoCl6]2– in Cs2MoCl6 and [MoOCl5]2– in Cs2MoOxCl6–x remain electronically isolated because of the vacancy ordered 0D crystal structure. In other words, if molybdenum is doped in similar vacancy ordered perovskites like Cs2ZrCl6, then also one would expect the formation of [MoCl6]2– or [MoOCl5]2– units, yielding the NIR emission. Indeed, there are multiple reports appeared for doping molybdenum and tungsten in Cs2TeCl6, (37) Cs2HfCl6, (38) and Cs2Zr(Cl,Br)6. (39−41) These doped samples show broad NIR emission. Some authors assigned the broad emission to M4+ in [MCl6]2–, (37) and some assigned the emission to M5+ in [MOCl5], (2−35,42) where M = Mo and W. In the doped system, the dopant centers [MCl6]2– or [MOCl5]2– are about 100 times dilute, and typical characterization techniques are governed by the host material. For example, at a dilute doping level of [MOCl5]2–, the intensity of the metal-oxo Raman peak around 800–900 cm–1 might diminish completely. In such low levels of doping, probably in an in-depth XAFS study and advanced Raman spectroscopy on a series of samples is required to distinguish between [MCl6]2– octahedra, distorted [MCl6]2– with lower symmetry like C4v, or [MOCl5]2– in C4v symmetry or some other symmetry. At present, this clarity is not there, which complicates the assignment of d–d electronic transitions to understand NIR luminescence in these samples.

In such low levels of doping, probably in an in-depth XAFS study and advanced Raman spectroscopy on a series of samples is required to distinguish between [MCl6]2– octahedra, distorted [MCl6]2– with lower symmetry like C4v, or [MOCl5]2– in C4v symmetry or some other symmetry.

Ultrabroad NIR Emission Combining Host and Dopant Emission

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Halide perovskites, particularly lower-dimensional ones, show ultrafast structural distortion in the excited state, resulting in STE emission at room temperature. (43) Therefore, halide perovskites allow the combination of luminescence from both the host and the dopant, giving an opportunity to design broadband single emitters. A few compositions, such as Cs2(AgNa)BiCl6, (44−46) and Cu2AgBiI6, (47) show STE emissions in the NIR-I region. In Cs2(AgNa)BiCl6, the STE formation involves lattice distortion between adjacent [BiCl6]3–-[AgCl6]5– octahedra, driven by strong electron–phonon coupling. (44,48) The STE emission is due to strong carrier localization, where the electron is confined within a single [BiCl6]3– octahedron and the hole is distributed over neighboring [AgCl6]5– units. [NaCl6]5– octahedra act as spatial barriers for the hole, further enhancing electron–hole overlap. Zhang et al. (44) reported an optimal composition of Cs2Na0.95Ag0.05BiCl6 with STE emission maxima at 700 nm, large fwhm of 270 nm (∼0.68 eV), and a high PLQY of 51%.
To increase the fwhm further with an emission spectrum spreading across both NIR-I and -II regions, Saikia et al. doped Mo and W in Cs2Na0.95Ag0.05BiCl6. (49) The bottom panel in Figure 5a shows the broad emission with a peak ∼680 nm, from the undoped sample. The middle and top panels of Figure 5a show addition of new PL emissions with peaks at 950 and 940 nm, after introducing 2.5% W and 2.8% Mo dopants in Cs2Na0.95Ag0.05BiCl6, respectively. Both the host STE emission and the dopant emissions have the same band edge excitation at 350 nm. Importantly, the host and dopant emission overlap with each other, resulting in an ultrabroadband NIR emission, with a combined PLQY of 42% and 31% for 2.5% W- and 2.8% Mo-doped Cs2Na0.95Ag0.05BiCl6, respectively.

Figure 5

Figure 5. (a) PL spectra of undoped, 2.5% W-doped and 2.8% Mo-doped Cs2Na0.95Ag0.05BiCl6 under the excitation of 350 nm, showing unprecedented spectral widths of 434 and 468 nm, respectively. The spectra were collected using two detectors and then combined at 720 nm. (b) fwhm of various reported broadband NIR emitters is compared with that of 2.5% W-doped and 2.8% Mo-doped Cs2Na0.95Ag0.05BiCl6. The composition of various NIR emitters is mentioned in the figure with serial numbers that correspond to a table reported in the Supporting Information of ref (46). The position of the tringles indicates the PL peak position(s) of the broadband NIR PL, while the solid and dashed lines show the emission wavelength range coverage with continuity and discontinuity, respectively. The colors of the triangle indicate various dopants, as stated at the top of the figure. Different PL peak for a single dopant due to occupation of two different lattice sites is indicated by tringle of same color with dashed and solid edges. (c) Schematics summarizing the excitation and emission pathways for ultrabroadband NIR emissions from W-doped and Mo-doped Cs2Na0.95Ag0.05BiCl6. (d) PL and PL excitation of 2.5% W-doped and 2.8% Mo-doped Cs2Na0.95Ag0.05BiCl6 at 7 K. Reproduced with permission from ref (49). Copyright 2025 Wiley.

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The dual overlapping emission of 2.5% W-doped Cs2Na0.95Ag0.05BiCl6 were fitted with Gaussian functions, yielding the fwhm of host STE emissions as 265 nm and dopant emission as 169 nm. (49) The resulting fwhm combining both emissions is 434 nm (898 meV). Similarly, the combined fwhm obtained for 2.8% Mo-doped Cs2Na0.95Ag0.05BiCl6 is 468 nm (983 meV). For a comparison, Figure 5b presents fwhm and peak positions for different kinds of NIR phosphors with broad emissions, reported in the prior literature. Clearly, the fwhm 2.5% W- and 2.8% Mo-doped Cs2Na0.95Ag0.05BiCl6 is the highest reported so far, for any NIR phosphor. Such ultrabroadband NIR light source, in terms of compact, cost-effective, and lightweight pc-LEDs, might find applications in different spectroscopic and noninvasive imaging based sensing applications.
Figure 5c explains the mechanism of single excitation of the host leading to both STE emission and d-d transition emission of the dopants. In the previous subsection, we discussed the difficulty in distinguishing between [MCl6]2– and [MOCl5]2– units (M = W and Mo) in the doped crystals. In the case of 2.5% W- and 2.8% Mo-doped Cs2Na0.95Ag0.05BiCl6, XAFS data confirmed the absence of perfect [MCl6]2– octahedra. (49) Consequently, either a highly distorted [MCl6]2– unit with a lower symmetry like C4v, or [MOCl5]2– units or both kinds are likely to present in the samples. Therefore, in Figure 5c, we assumed a d–d transition similar to that in C4v symmetry. However, we note that the local structure and d–d transition mechanism need further verification. Furthermore, the dopant NIR emission can be observed by exciting the d-electrons as well in addition to the band edge excitation. Figure 5d shows the vibronic fine structure of PL excitation and the emission (de-excitation) of the d-electrons, at 7 K.
On the other hand, the broad NIR PL of [MOCl5]2– units has recently been combined with sharp NIR luminescence of trivalent lanthanides (Er, Ho, Tm, Yb). Mo has been explored to sensitize the NIR PL of lanthanides and achieve tunable NIR emitters, including Cs2NaInCl6:Mo4+/Ln3+ (Ln: Ho, Tm, Yb), (50) Mo4+/Er3+ codoped Cs2ZrCl6, (41) and Mo4+/Er3+ codoped Cs2NaBiCl6. (51) Although these compositions are successful from a material design perspective, further structural and spectroscopic investigations, particularly regarding transition metal–lanthanide interactions, will be helpful.

Halide Ion Dependent Tunable NIR Emission

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The nature of d–d transitions is expected to change based on the coordinating ligands. If the coordinating ligand changes from O2– to halides (Cl–, Br– or I–), then the crystal field energy and Racah parameters are expected to change. Consequently, the properties of d-electron absorption and emission change. However, such tunability of NIR emission of d-electrons remained less explored for different halide ligands. (28) One of the most studied NIR emitting transition metal ions in both oxides and halide host is Cr3+ with 3d3 electronic configuration, in an octahedral (Oh) environment. Figure 2d shows the corresponding Tanabe-Sugano diagram. Notably, the first excited state of Cr3+ ion could be either 4T2g or 2Eg/2T1g, depending on the value of crystal field strength measured as Dq/B. At weak field, i.e., Dq/B value in the left side of the dashed line in Figure 2d, 4T2g → 4A2g transition gives a broad NIR PL, which is spin-allowed and Laporte forbidden. At a strong field, Dq/B > 2.3, the emission originates from the ICSF 2Eg/2T1g → 4A2g transition. A huge number of reports in oxide (52−55) and fluorides, (56) including the pioneer work by T. H. Maiman, showing lasing in ruby (Cr3+ doped Al2O3), (3) could be seen as tuning the Cr3+ NIR emission by manipulation of crystal field around it.
NIR-I PL originating from 2% Cr3+ doped Ga1.98–x(Al0.68In0.32)xO3 with x = 0–0.8 is shown in Figure 6a. Chang et al. (57) showed that the NIR PL could be tuned by alloying the Ga3+-lattice site with Al3+ and In3+. A series of compositions could tune broad NIR PL peaks with a peak from 722 to 784 nm. In fact, when x is small, the spectrum is a combination of two PL arising from the sharp ICSF 2Eg/2T1g → 4A2g, and a broad spin-allowed 4T2g → 4A2g transitions. Substitution of larger In3+ for Ga3+ decreases the crystal field strength, causing domination of a broad spin-allowed transition and a red shift in the NIR PL peak.

Figure 6

Figure 6. (a) NIR PL of Ga1.98–x(Al0.68In0.32)xO3:2% Cr3+ (x = 0–0.8) at room temperature. Reproduced with permission from ref (57). Copyright 2022 American Chemical Society. (b) Normalized absorption (violet), PL emission (orange), and PL excitation (blue) spectra of Cs2AgInCl6:1.3% Cr3+ nanocrystals. Reproduced with permission from ref (58). Copyright 2022 American Chemical Society. (c) Low-temperature (4 K) PL spectra of CrX3 (X = Cl, Br, I) showing broad PL due to 4T2g → 4A2g, and corresponding Yb3+-doped CrX3 showing sharp PL due to 2F5/2 → 2F7/2 transition of Yb3+. The absence of lattice PL in the latter indicates efficient ET from Cr3+ to Yb3+. Reproduced with permission from ref (59). Copyright 2023 American Chemical Society. (d, e) NIR to visible upconversion (UC) photoluminescence of Re4+-doped Cs2NaYCl6 and Cs2NaYBr6 at 15 K respectively. Reproduced with permission from ref (17). Copyright 1998 American Chemical Society.

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Compared to oxides, Cr3+ experiences a weaker crystal field with lower Dq/B when doped in the lattice of the halide perovskite. Consequently, Figure 6b shows lower energy of the Cr3+4T2g → 4A2g NIR PL with a peak at 998 nm for Cs2AgInCl6:1.3% Cr3+. (58) The PL excitation (PLE) spectra of Cs2AgInCl6:1.3% Cr3+ include three excitation bands around 300–400, 500–630, and 730–900 nm, and are assigned to the 4A2g → 4T1g (P), 4A2g → 4T1g (F), and 4A2g → 4T2g (F) transitions of Cr3+ ions, respectively (see Figure 2d). These excitation bands are also red-shifted compared to the that of Cr3+ doped in typical oxide hosts. The narrow ICSF 2Eg/2T1g → 4A2g emission (Ruby laser emission) has not yet been reported for Cr3+-doped compounds in any chloride host.
The Cr3+ emission is supposed to be pushed further toward longer wavelengths in the NIR-II region, for bromide- and iodide-based perovskite hosts. However, until now there is no report of Cr3+ NIR-II PL in bromide/iodide perovskites. Interestingly, Snoeren et al. (59) reported similar tuning of Cr3+ broad NIR PL in van der Waals lattice, CrX3, where X = Cl–, Br– to I–, as shown Figure 6c. The peak maxima of the Cr3+ NIR emission shifts from 1.43 eV (867 nm) to 1.35 eV (919 nm) to 1.10 eV (1127 nm) for the decreasing crystal field of chloride to bromide to iodide. This brings the credibility of designing halide based hosts extending the Cr3+ dopant emissions to the NIR-II regions. It is noteworthy that, there are few oxide phosphors such as Al1–xNbO4:xCr3+ (x = 0 and 0.01), (60) and Ba2ScSbO6:Cr3+, (61) where NIR-II PL of Cr3+ could be achieved through multiple cations mixing in the host compositions. However, further tuning toward longer wavelengths will be challenging because of the stronger crystal field of O2–. The issue might be addressed by properly designed halide hosts for not only doping Cr3+, but also for other transition metals as a generic strategy to achieve NIR-II PL.
Notably, upconversion (UC) PL of several transition metal ions, Os4+, Mo3+, V3+, and Ni2+, was previously studied by Güdel and co-workers in different halide hosts such as Cs2ZrCl6 and Cs2NaYX6 (X = Cl, Br). (17,62,63) Majority of those studies were at cryogenic temperature and often in an inert environment. The effect of halide ion for tuning the UC PL of Re4+-doped in Cs2NaYX6 (X = Cl and Br) is shown in Figure 6d,e. (17) In both compositions, the UC PL featuring multiple peaks is due to 2T2g → 4A2g ICSF transition, coupled with spin–orbit coupling. Notably, the UC PL peaks at 699 nm (∼14300 cm–1) shows a redshift by nearly 300 cm–1 when halide was changed from Cl– to Br–.

Colloidal Nanocrystals

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Applications such as solution-phase printing of micro-LEDs or thin film-based electroluminescence are challenging using the bulk crystals discussed above. For such applications, colloidal nanocrystals (NCs) are often preferred. Interestingly, nanocrystals of different halide perovskite hosts are well-studied. (64−70) Also, transition metals like Mn2+, Fe3+ emitting dopant emission in the visible region and lanthanide doping in perovskite nanocrystals are well studied. (4,71) Colloidal nanocrystals of lanthanide-doped NaYF4 (72,73) and undoped Cs2ZrF6 nanocrystals (74) are reported. In the backdrop of this existing literature, it is natural to attempt doping transition metals in a halide-based perovskite and related host nanocrystals for NIR emission. However, reports of transition metal doping in NIR-emitting colloidal halide-based nanocrystals are rather limited. For such a colloidal synthesis, the commonly known high boiling solvent, octadecane (ODE), and surface passivating ligands, oleylamine (OLA) and oleic acid (OA), were used under a N2 environment, as shown in Figure 7a. However, the choice of precursors and reaction conditions becomes crucial depending on transition metal ions.

Figure 7

Figure 7. (a) Hot-injection synthesis setup for colloidal nanocrystals. (b) Precursors and reaction conditions used to prepare (top panel) Cr3+-doped Cs2NaxAg1–xInCl6 (x = 0–1) and (bottom panel) Cs2MoCl6 nanocrystals. (c, d) TEM images of Cr3+-doped Cs2NaInCl6 nanocrystals. Inset images of (c) and (d) show high resolution TEM and size distribution of nanocrystals. (e) Room temperature PL of Cr3+-doped Cs2NaxAg1–xInCl6 (x = 0 −1) colloidal nanocrystals under different excitations. (c)–(e) were reproduced with permission from ref (58). Copyright 2022 American Chemical Society. (f–g) TEM and high-resolution TEM images of Cs2MoCl6 nanocrystals. Reproduced with permission from ref (77). Copyright 2024 American Chemical Society.

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Zhang et al. reported doping of Cr3+ in colloidal Cs2NaxAg1–xInCl6 perovskite nanocrystals, achieving tunable broad NIR PL in the solution phase. (58) They used acetate precursor and benzoyl chloride as nucleating agents and relatively low injection temperature, as expected with acetate precursors (Figure 7b (top)). Similar synthesis was already known for the undoped Cs2NaxAg1–xInCl6 perovskite NCs. (75) Transmission electron microscopy (TEM) images in Figure 7c,d show an average size of ∼18.5 nm of Cr3+-doped Cs2NaInCl6 NCs. The series of Cr3+-doped Cs2NaxAg1–xInCl6 alloyed nanocrystals shows broad band NIR emission, as shown in Figure 7e. The PL peak position as well as the excitation band peak systematically vary with composition x = 0 to 1. The Cr3+-doped Cs2NaInCl6 nanocrystals exhibit a reasonably good PLQY of 19.7%. We also note that there is a recent report of doping lanthanides in Na3CrF6 nanocrystals emitting NIR radiation. (76)
Doping heavier transition metals (4d/5d) in the colloidal synthesis of halide perovskite nanocrystals has not been reported yet. Discovering dopant precursors with desired reactivity and oxidation state for colloidal synthesis still remains a challenge. In this regard, Kong et al. reported Cs2MoCl6 nanocrystals (also refer to Figure 4 and related discussion). (77) They employed a similar hot-injection method (Figure 7a), using Mo6+ precursors, 12-molybdophosphoric acid hydrate (H3PO4·12MoO3·xH2O), which was reduced during the reaction to achieve Cs2MoCl6 perovskite NCs (see Figure 7b, bottom). The nucleating source was chlorotrimethylsilane (TMS-Cl). Figure 7f,g confirms the nanocrystal formation but with a broader size distribution. The Cs2MoCl6 perovskite nanocrystals show weak broadband NIR PL at ∼965 nm.

Potential Applications

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Though there are old reports of NIR emissions from transition metal doped halide perovskites, the emissions were weak and often studied at the cryogenic temperatures. (16) It is only in the last few years that intense and stable NIR emissions from transition metal doped halide perovskites have been reported in ambient conditions. Consequently, exploring their application has started rather recently, showcasing their potentials.

Phosphor-Converted LED (PC-LED) and NIR Imaging

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Liu at al. reported the NIR pc-LED fabrication using Cs2MCl6 (or Cs2MOxCl6–x, see Figure 4), M = Mo/W, vacancy ordered perovskite. (34) The pc-LED was fabricated by combining the sample with a polydimethylsiloxane (PDMS) polymer matrix and then coating the mixture on a UV-LED chip. From a panel of pc-LED, approximately 12 W NIR output power was achieved under ∼350 mA bias, as shown in Figure 8a. The maximum electrical to optical power conversion efficiency ηE–O was 13%. This is one of the highest NIR output powers achieved among transition-metal-based halide perovskites. Using the pc-LED panel, high-penetration NIR imaging was achieved by Liu at al. (34) Mondal et al. followed up the pc-LED fabrication using Cs2MoCl6 (or Cs2MoCl6–x) phosphor, and showed that the phosphor–polymer composites can be excited by using a 730 nm far-red-LED chip (see Figure 8b) instead of a UV-LED chip. (42,78) The NIR pc-LED with a far-red-LED chip has a significantly lower (∼1.5 V) turn-on voltage compared to pc-LED with UV or blue chips (>2.5 V). Such a design strategy, where the NIR phosphors can be excited with a low-energy far-red-LED chip, is desired to increase wall-plug efficiency of future NIR pc-LEDs.

Figure 8

Figure 8. (a) Output power for NIR emission with a peak at ∼980 nm as a function of applied current for a Cs2MCl6 (M = Mo/W) based pc-LED panel. The phosphor was coated with UV chips. Reproduced with permission from ref (34). Copyright 2022 Wiley. (b) Luminescence spectra as a function of biased voltages for a pc-LED fabricated by coating Cs2MoCl6 phosphor composite on a far-red (730 nm) LED chip. Reproduced with permission from ref (42). Copyright 2024 American Chemical Society. (c) Luminescence spectra of 2.5% W-doped Cs2Na0.95Ag0.4BiCl6 NIR pc-LEDs at different driving currents. (d) The composite 2.5% W-doped Cs2Na0.95Ag0.4BiCl6 and poly(lactic acid) forming a 3D printed structure over the UV-LED panel. The 3D printed phosphor composites yield better thermal stability for the prolonged operation of the NIR-pc-LED. (c, d) Reproduced with permission from ref (49). Copyright 2025 Wiley. (e) Luminescence spectra of the pc-LED fabricated by coating a polymer composite of 3.8% Mo3+-doped Cs2NaInCl6 on a blue LED chip. The schematics of fabrication and digital photographs of the devices are shown in the insets. Reproduced with permission from ref (20). Copyright 2025 Wiley. (f) pc-LED panel and NIR imaging for food freshness detection using 2% Cr3+-0.5%Bi3+-codoped Cs2Ag0.6Na0.4InCl6. Images of a pair of apples were collected by a visible camera and NIR camera under illumination from pc-LED. The NIR image could detect the early rotting stage of an apple, as shown with a red circle. Reproduced with permission from ref (79). Copyright 2023 Wiley.

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Figure 8c shows ultrabroad NIR luminescence spectra from a pc-LED fabricated using the 2.5% W-doped Cs2Na0.95Ag0.05BiCl6 double perovskite. (49) The combination of the host STE emission and dopant emission leads to an ultrabroad emission. The composite of the phosphor with a biodegradable polymer poly(lactic acid) (PLA) was coated on a UV-LED chip. The composites were found stable under different solvents of various polarities, and the pc-LED shows ∼ 6.2 mW output power under 475 mA driving current. The stability of the pc-LED panel can be improved further by making a 3D printed structure of the composite, as shown in Figure 8d. The 3D printed structure is placed over the UV-LED panel without touching the UV-LEDs. Consequently, the waste heat dissipated from UV-LED does not get transferred to the phosphor composite, significantly improving the thermal stability of the ultrabroad NIR pc-LEDs for prolonged operation. Such pc-LEDs might be tested as a compact and low-power ultrabroad NIR source for different diagnostic and sensing applications.
In contrast to ultrabroad, Figure 8e shows ultranarrow NIR luminescence spectra from pc-LEDs of 3.8% Mo3+-doped Cs2NaInCl6 double perovskite. (20) The pc-LEDs were fabricated by coating a composite of the phosphor and poly(methyl methacrylate) (PMMA) on top of a commercial UV-LED chip, as shown in the inset of Figure 8e. The inset also shows the digital images of the pc-LED under no bias captured by visible camera and under bias captured by NIR camera. This pc-LED shows nearly 49 mW output power at 300 mA, and reasonably stable performance for over 12 h of continuous operation. (20) Notably, it is the first demonstration of pc-LED from ICSF emission of Mo3+, and indeed with reasonably good device performance.
Another interesting pc-LED is fabricated by using 2% Cr3+-0.5%Bi3+-codoped Cs2Ag0.6Na0.4InCl6 perovskite, that could emit both white light and broad NIR, as shown in Figure 8f. (79) Bi3+ is responsible for the white light emission and d–d transition of Cr3+ yielded the NIR emission. The pc-LED panel was employed for dual-mode imaging for assessing the food freshness. The white light emission was used for the usual visual inspection of apple, whereas the NIR radiation can be absorbed by the vibrational overtones of O–H (water content), sensing the water content in the apple. Higher water content leads to a dark spot in the NIR image, indicating an early stage of rotting of fruits and vegetables. Such dual (white + NIR) emitting pc-LEDs might find applications in compact and cost-effective devices for detecting food freshness. Notably, though Cr3+-based broad NIR emitting pc-LED are well developed in oxides, (80) they are not known for imparting such dual luminescence enabled by halide perovskites.
pc-LEDs combining Mo broadband emission with lanthanide luminescence have shown promising performance. A comparison of transition-metal-halide-based NIR PL, and their pc-LED devices is provided in Table 1. Mo4+/Er3+ codoped Cs2NaBiCl6 exhibits a total NIR-II PLQY of 69%, out of which 35% PLQY is for the Er3+ emission at ∼ 1540 nm. When integrated with a commercial blue LED chip, the pc-LED delivered an output power of 80 mW at 300 mA, and the highest electrical to optical power conversion efficiency (ηE–O) of 13% at 20 mA. (51) The broadband Mo absorption allows commercial LED chips emitting at 410 nm (violet), 525 nm (green), and 640 nm (red), as the excitation source for pc-LEDs is made up of Mo4+/Er3+ codoped Cs2ZrCl6. (81) The tunable excitation opens up a future direction to improve parameters, including wall-plug efficiencies. However, the potentials for Os4+-doped Cs2HfCl6, Mo-doped Cs4ZnBi2Cl12 for NIR pc-LED applications are not yet explored. (14,82)
Table 1. Comparison of Heavier Transition Metal Halide-Based NIR Emissions and Their pc-LED Device Performancesa
compositionPL peak positions (nm)fwhm (nm)PLQY (%)pc-LED performance and applicationref
Mo3+-doped Cs2NaInCl6700, 10959 P = 49 mW at 300 mA; ηE–O = 7.5% and ηO–O = 13% at 10 mA (20)
Cs2MoCl6∼100020026P = 12 W at 350 mA; ηE–O ∼ 13% (34)
deep penetration NIR imaging
Cs2MCl6 (M = Mo, W)986, 965200, 170 P = 2 mW at 160 mA (42)
Mo/W-doped Cs2(AgNa)BiCl6700, 950265, 200/16940P = 6.2 mW at 475 mA (49)
3D printed pc-LED for NIR Imaging
Mo-doped Cs2MCl6 (M = Zr, Te)95020029pc-LED (37)
NIR Imaging
Mo-doped Cs2ZnCl4960206 P = 132 mW at 350 mA (81)
IQE = 78.7%
NIR imaging for food freshness detection
W-doped Cs2SnCl6965160 P = 10 mW at 400 mA (78)
Mo/Er-codoped Cs2NaBiCl6970, 1540211, 2069P = 80 mW at 300 mA (51)
ηE–O ∼ 13%
anticounterfeiting; nondestructive inspection
Re4+-doped Cs2MCl6 (M = Hf, Te)726, 1342, 160016, 80, 10092IQE = 91.9% (26,27)
deep penetration NIR imaging
Os4+-doped Cs2HfCl6591, 714, 833, 1000, 1316, 1818∼100–50 (at 2 K)   (14)
Cs2MoCl6 nanocrystals965190   (77)
a

The NIR emission arises from d–d transitions of 4dn and 5dn (n = 1–3) outermost electronic configuration of Mo3+/5+, W5+, and Re4+ ions, which are stable in ambient conditions. P = output power; ηE–O and ηO–O electrical to optical and optical to optical power conversion efficiency, respectively; IQE = internal quantum efficiency.

NIR Electroluminescence LED?

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Doping heavier transition metal ions into halide perovskite hosts with a reasonable charge transport and solution processability is required to fabricate NIR electroluminescence (EL) LED devices. Recently, such efforts are being made for lanthanide doped halide perovskites. (83,84) For example, Yb3+-doped CsPb(Cl1–xBrx)3 shows the NIR EL peak at 990 nm due to Yb3+ f–f transition with external quantum efficiency (EQE) of 8.5%. This success gives the hope of achieving NIR EL from transition metal doped perovskites in the near future. The immediate challenge would be to prepare thin films of such NIR emitting transition metal doped halides. Strategies such as synthesizing colloidal nanocrystals for solution-processed thin films might be an interesting approach. Also, thermal evaporation and other film growth techniques might be employed, but maintaining a homogeneous composition with uniform doping will be challenging. Furthermore, the host materials in such films should show reasonable efficacy for charge injection and charge transport. Note that the majority of the host materials reported in Table 1 exhibit wide band gaps, and some of the hosts possess a 0D structure, which are not conducive for a good charge injection/transport. So doping transition metals in bromide and iodide based 3D perovskites will also be an interesting material design strategy to fabricate NIR EL devices of transition metal doped perovskites.

Doping heavier transition metal ions into halide perovskite hosts with a reasonable charge transport and solution processability is required to fabricate NIR electroluminescence (EL) LED devices.

Lasing Applications?

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The ICSF d–d transitions of 3d3 electrons of Cr3+ in the octahedral coordination of Cr3+-doped Al2O3 (ruby) yielded a Ruby laser at 694.3 nm. Recent progress discussed in Figures 2 and 3 show that Mo3+ (4d3 in octahedral) and Re4+ (5d3 in octahedral) show intense and stable ICSF emissions, with mechanisms similar to the Ruby laser, but at different wavelengths, 1095 and 1342 nm, respectively. This similarity is encouraging enough to conduct future studies exploring lasing properties of Mo3+- and Re4+-doped halide perovskites, developing lasers at different NIR wavelengths. The major material design challenge here is to grow large crystals of heavier transition metal-doped halides, such as Mo3+ ion-doped double perovskites. Such large crystals with polished surfaces reduce detrimental effects like light scattering and provide a long enough optical gain medium.
In conclusion, a series of 3d, 4d, and 5d transition metals, like Cr3+, V3+, Mo3+, Mo4+/5+, W4+/5+, Re4+ and Os4+, have been doped in halide perovskites and derivative structures, emitting both narrow and broad NIR-I/II emissions, because of the d–d electronic transitions. Doping heavier (4d/5d) metal ions, particularly at lower oxidation states, is a unique advantage for halide hosts compared to oxide hosts. For example, Mo3+ can be doped in Cs2NaInCl6, different from an oxide host that will tend to oxidize Mo to 6+. The 4d and 5d orbitals are more diffused, leading to higher crystal field splitting and lower Racah parameters compared to 3d orbitals. Additionally, the heavier ions have higher spin–orbit coupling. These inherent features bring out novel NIR emission properties of 4d/5d transition metal ion-doped halide perovskites compared to a typical 3d metal ion-doped oxide. Furthermore, some of the halide hosts have their own STE emission in the NIR region, which can be combined with a broad d-d transition of the dopant, resulting into a ultrabroad NIR emission (Figure 5a-b). Additionally, halide-based systems often could be synthesized at relatively low temperatures, enabling synthesis of colloidal nanocrystals. This solution processability offers the prospect of realizing thin-film-based NIR electroluminescence devices from transition metal doped perovskites. In recent years, good progress has been made in the fabrication of NIR pc-LEDs based on different transition metal doped (and also codoped) halide perovskites. The NIR pc-LEDs are pretty stable, suggesting their suitability as a light source for NIR imaging and sensing for various applications. The ultranarrow ICSF d–d emissions exhibited by Mo3+, Re4+ and Os4+ are worth exploring for NIR-to-visible UC PL and lasing applications. Continued efforts integrating rational material design, advanced structural probes, and spectroscopic insights are expected to unlock new structure–property relationships and push transition-metal luminescence toward technologically viable NIR optoelectronics.

Author Information

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  • Corresponding Authors
  • Author
    • Animesh Ghosh - Department of Chemistry, Indian Institute of Science Education and Research (IISER), Pune 411008, India
  • Notes
    The authors declare no competing financial interest.

Biographies

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Sajid Saikia received his B.Sc. degree in chemistry from Tezpur University. He completed his M.S. and Ph.D. dual degree from the Indian Institute of Science Education and Research (IISER) Pune (2025). His research interests focus on the design of halide perovskites for near-infrared luminescence, photophysical investigations and upconversion luminescence studies.

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Animesh Ghosh completed his M.Sc. from The University of Burdwan. Currently, he is pursuing his Ph.D. at IISER Pune, India. His research focuses on transition metal-based perovskites and their derivatives for near-infrared luminescence and potential application in devices.

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Angshuman Nag is a Professor of Chemistry at IISER Pune. He earned his Ph.D. from Indian Institute of Science Bengaluru (2009) and completed two postdoctoral terms. Subsequently, he joined IISER Pune in 2012. His recent research focuses on halide perovskite semiconductors and colloidal quantum dots for optoelectronic applications.

Acknowledgments

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We thank our collaborators including past and present members of research group, particularly, Dr. Barnali Mondal, Mr. Vijay Mani Tripathi, Dr. Srijita Banerjee and Dr. Habibul Arfin, for their research contributions and insightful discussion that helped enhance our understanding. S.S. thanks Prime Minister Research Fellowship (PMRF) and A.G. thanks University Grants Commission (UGC) for fellowships. A.N. acknowledges Anusandhan National Research Foundation (ANRF) (Swarnajayanti Fellowship, SB/SJF/2020-21/02), India.

References

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Cited By

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This article is cited by 2 publications.

  1. Sajid Saikia, Animesh Ghosh, Diogo Alves Gálico, Claudia Manuela Santos Calado, Muralee Murugesu, Angshuman Nag. Spin‐Flip Upconversion Luminescence and Tunable Downshifting Near‐Infrared Emissions via 4 d ‐4 f Interaction in Doped Halide Perovskite. Angewandte Chemie 2026, 11 https://doi.org/10.1002/ange.7420770
  2. Sajid Saikia, Animesh Ghosh, Diogo Alves Gálico, Claudia Manuela Santos Calado, Muralee Murugesu, Angshuman Nag. Spin‐Flip Upconversion Luminescence and Tunable Downshifting Near‐Infrared Emissions via 4 d ‐4 f Interaction in Doped Halide Perovskite. Angewandte Chemie International Edition 2026, 11 https://doi.org/10.1002/anie.7420770
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  • Abstract

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    Figure 1

    Figure 1. Schematic highlights the scope of transition metal doping in halide perovskites (and related structures) for NIR emission, which is typically beyond the scope of oxide-based NIR phosphors.

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    Figure 2

    Figure 2. (a) Schematic representation of Mo3+-doped Cs2NaInCl6 double perovskite cubic lattice with the space group Fm3m. (b) Description of the Cl environment around Mo (red open symbols) and In (blue open symbols) for Mo3+-doped Cs2NaInCl6 with different dopant concentrations: coordination number (CN, top panel), bond length (R, middle panel), and local order (σ2, bottom panel). (c) d–d vs f–f transition-based NIR-II PL spectra from Ln3+- (Ln = Yb, Er) and Mo3+-doped Cs2NaInCl6. (d) Left panel: Tanabe-Sugano diagram for a d3 ion in an octahedral coordination; middle panel: schematic diagram comparing the orbital interaction between the metal (M) dxy ligand (L) px when the metal ion (d3: Cr3+, Mo3+, Re4+) is in the ground state (GS: 4A2g) and in an excited state (ES: 2Eg); right panel: schematic potential energy surface diagram, indicating that the ground state 4A2g, and excited states 2Eg/2T1g and 2T2g have the same t2g3eg0 electronic configuration and, therefore, have very similar M–L bond lengths. Reproduced with permission from ref (20). Copyright 2025 Wiley.

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    Figure 3

    Figure 3. (a) Temperature-dependent (7–300 K) PL spectra of 3.8% Mo3+-doped Cs2NaInCl6 in the visible to NIR region under 438 nm excitation. Reproduced with permission from ref (20). Copyright 2025 Wiley. (b) PL and PL excitation spectra of Cs2HfCl6:Re4+ at room temperature. Reproduced with permission from ref (26). Copyright 2025 Elsevier. (c) PL spectrum of Cs2HfCl6:Os4+ crystals at 2 K. Since the spectrum was collected in a broad range from visible to NIR-II (∼580–2100 nm), the relative detector response is also shown. Reproduced with permission from ref (14). Copyright 1971 American Physical Society.

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    Figure 4

    Figure 4. (a) Crystal structure of Cs2MoCl6 containing a [MoCl6]2– unit with an octahedral symmetry (top) followed by the Tanabe-Sugano diagram as reference for showing possible electronic transitions of Mo4+ (4d2) in an octahedral local symmetry (bottom). The crystal structure was obtained using a CIF file reported in ref (42). (b) Crystal structure of Cs2MoOxCl6–x containing the [MoOCl6]2– unit with reduced C4v symmetry (top) followed by possible electronic transitions of Mo5+ (4d1) (bottom). The crystal structure was obtained using the CIF file reported in ref (35).

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    Figure 5

    Figure 5. (a) PL spectra of undoped, 2.5% W-doped and 2.8% Mo-doped Cs2Na0.95Ag0.05BiCl6 under the excitation of 350 nm, showing unprecedented spectral widths of 434 and 468 nm, respectively. The spectra were collected using two detectors and then combined at 720 nm. (b) fwhm of various reported broadband NIR emitters is compared with that of 2.5% W-doped and 2.8% Mo-doped Cs2Na0.95Ag0.05BiCl6. The composition of various NIR emitters is mentioned in the figure with serial numbers that correspond to a table reported in the Supporting Information of ref (46). The position of the tringles indicates the PL peak position(s) of the broadband NIR PL, while the solid and dashed lines show the emission wavelength range coverage with continuity and discontinuity, respectively. The colors of the triangle indicate various dopants, as stated at the top of the figure. Different PL peak for a single dopant due to occupation of two different lattice sites is indicated by tringle of same color with dashed and solid edges. (c) Schematics summarizing the excitation and emission pathways for ultrabroadband NIR emissions from W-doped and Mo-doped Cs2Na0.95Ag0.05BiCl6. (d) PL and PL excitation of 2.5% W-doped and 2.8% Mo-doped Cs2Na0.95Ag0.05BiCl6 at 7 K. Reproduced with permission from ref (49). Copyright 2025 Wiley.

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    Figure 6

    Figure 6. (a) NIR PL of Ga1.98–x(Al0.68In0.32)xO3:2% Cr3+ (x = 0–0.8) at room temperature. Reproduced with permission from ref (57). Copyright 2022 American Chemical Society. (b) Normalized absorption (violet), PL emission (orange), and PL excitation (blue) spectra of Cs2AgInCl6:1.3% Cr3+ nanocrystals. Reproduced with permission from ref (58). Copyright 2022 American Chemical Society. (c) Low-temperature (4 K) PL spectra of CrX3 (X = Cl, Br, I) showing broad PL due to 4T2g → 4A2g, and corresponding Yb3+-doped CrX3 showing sharp PL due to 2F5/2 → 2F7/2 transition of Yb3+. The absence of lattice PL in the latter indicates efficient ET from Cr3+ to Yb3+. Reproduced with permission from ref (59). Copyright 2023 American Chemical Society. (d, e) NIR to visible upconversion (UC) photoluminescence of Re4+-doped Cs2NaYCl6 and Cs2NaYBr6 at 15 K respectively. Reproduced with permission from ref (17). Copyright 1998 American Chemical Society.

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    Figure 7

    Figure 7. (a) Hot-injection synthesis setup for colloidal nanocrystals. (b) Precursors and reaction conditions used to prepare (top panel) Cr3+-doped Cs2NaxAg1–xInCl6 (x = 0–1) and (bottom panel) Cs2MoCl6 nanocrystals. (c, d) TEM images of Cr3+-doped Cs2NaInCl6 nanocrystals. Inset images of (c) and (d) show high resolution TEM and size distribution of nanocrystals. (e) Room temperature PL of Cr3+-doped Cs2NaxAg1–xInCl6 (x = 0 −1) colloidal nanocrystals under different excitations. (c)–(e) were reproduced with permission from ref (58). Copyright 2022 American Chemical Society. (f–g) TEM and high-resolution TEM images of Cs2MoCl6 nanocrystals. Reproduced with permission from ref (77). Copyright 2024 American Chemical Society.

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    Figure 8

    Figure 8. (a) Output power for NIR emission with a peak at ∼980 nm as a function of applied current for a Cs2MCl6 (M = Mo/W) based pc-LED panel. The phosphor was coated with UV chips. Reproduced with permission from ref (34). Copyright 2022 Wiley. (b) Luminescence spectra as a function of biased voltages for a pc-LED fabricated by coating Cs2MoCl6 phosphor composite on a far-red (730 nm) LED chip. Reproduced with permission from ref (42). Copyright 2024 American Chemical Society. (c) Luminescence spectra of 2.5% W-doped Cs2Na0.95Ag0.4BiCl6 NIR pc-LEDs at different driving currents. (d) The composite 2.5% W-doped Cs2Na0.95Ag0.4BiCl6 and poly(lactic acid) forming a 3D printed structure over the UV-LED panel. The 3D printed phosphor composites yield better thermal stability for the prolonged operation of the NIR-pc-LED. (c, d) Reproduced with permission from ref (49). Copyright 2025 Wiley. (e) Luminescence spectra of the pc-LED fabricated by coating a polymer composite of 3.8% Mo3+-doped Cs2NaInCl6 on a blue LED chip. The schematics of fabrication and digital photographs of the devices are shown in the insets. Reproduced with permission from ref (20). Copyright 2025 Wiley. (f) pc-LED panel and NIR imaging for food freshness detection using 2% Cr3+-0.5%Bi3+-codoped Cs2Ag0.6Na0.4InCl6. Images of a pair of apples were collected by a visible camera and NIR camera under illumination from pc-LED. The NIR image could detect the early rotting stage of an apple, as shown with a red circle. Reproduced with permission from ref (79). Copyright 2023 Wiley.

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