Controlled colloidal synthesis and structure–property–stability relationships in lead-free CsMBr₃ (M = Mn, Fe, Ni) perovskite nanocrystals

Abstract

Transition-metal halide perovskites of the form CsMBr₃ (M = Mn, Fe, Ni) are emerging as attractive lead-free alternatives for optoelectronic applications, offering composition-tunable electronic structures, strong light–matter interactions, and enhanced environmental stability compared with hybrid lead halide perovskites. Despite this promise, the reliable synthesis of phase-pure, highly crystalline CsMBr₃ nanocrystals (NCs) remains challenging due to their pronounced sensitivity to reaction temperature, precursor reactivity, and surface-ligand chemistry. Here, we report a reproducible and optimized colloidal hot-injection strategy for the synthesis of CsMnBr₃, CsFeBr₃, and CsNiBr₃ NCs, and systematically elucidate the influence of reaction temperature and growth time on their structural, morphological, and optical properties. X-ray diffraction confirms the formation of phase-pure hexagonal CsMBr₃ (P6₃/mmc) across all compositions under optimized conditions. Transmission electron microscopy reveals uniform nanocrystal morphologies with narrow size distributions and well-resolved lattice fringes, indicative of high crystallinity. Optical studies, including UV-vis absorption, steady-state photoluminescence, and time-resolved photoluminescence, demonstrate composition-dependent absorption characteristics and exciton recombination dynamics, reflecting the role of the transition metal cation in tuning optoelectronic behaviour. Time-dependent UV-vis and XRD analyses further reveal notable ambient stability over one month, with structural and optical degradation becoming evident only after prolonged exposure beyond 46 days, demonstrating stability far superior to that of equivalent untreated or unencapsulated CsPbBr₃ perovskite NCs. X-ray photoelectron spectroscopy confirms correct metal incorporation and stable metal-halide bonding environments, while thermogravimetric analysis uncovers distinct, composition-dependent thermal decomposition pathways. Collectively, this work establishes a clear structure–property–stability relationship for CsMBr₃ nanocrystals and provides a robust synthetic framework for advancing next-generation, lead-free perovskite nanomaterials for optoelectronic applications.

Influence of transition-metal d-electron configuration on the optical properties, exciton dynamics, and stability of phase-pure hexagonal CsMBr₃ (M = Mn, Fe, Ni) nanocrystals.

1. Introduction

Fully inorganic cesium halide perovskites (CsMX₃) have emerged as one of the most promising material systems for next-generation optoelectronic applications, including light-emitting diodes, solar cells, photodetectors, lasers, and spintronic devices.^1,2 Their purely inorganic lattice provides enhanced thermal and structural robustness compared to hybrid perovskites, while still offering the strong light–matter interaction and solution processability characteristic of the broader halide perovskite family. This combination of stability and desirable optoelectronic behaviour has driven significant interest in Cs-based perovskites as practical candidates for durable device architectures.²

A key advantage of the CsMX₃ perovskite lattice lies in its compositional tunability. Substituting the metal (M) site enables precise modification of electronic structure, optical response, and magnetic behaviour.^3–7 In particular, transition-metal-based perovskites incorporating ions such as Mn²⁺, Fe²⁺, or Ni²⁺ introduce rich d-electron physics that can strongly influence optical emission pathways, magnetic properties, and structural dynamics.⁸ These emerging materials offer an attractive route toward multifunctional and environmentally friendly perovskites, where optical and magnetic behaviours can be engineered within a single nanocrystal system. For instance, CsMnBr₃ nanocrystals (NCs) have already demonstrated promising red emission, relatively high PLQY, and rapid radiative decay, highlighting the potential of transition-metal perovskites as viable lead-free alternatives.^9,10

Despite their promise, obtaining phase-pure CsMBr₃ nanocrystals remains challenging. Transition-metal bromides readily undergo halide-metal exchange and tend to form competing phases such as Cs₂MBr₅, Cs₂MBr_4.^8,11–13 Their nucleation behaviour is highly sensitive to ligand chemistry, precursor stoichiometry, reaction temperature, and growth time.¹³ Small deviations in these parameters can lead to mixed phases, broad size distributions, or reduced crystallinity. Moreover, like many halide-based nanocrystals, transition-metal perovskites suffer degradation upon exposure to moisture, oxygen, heat, or light, resulting in structural distortion, phase transitions, and diminished photoluminescence.^13,14

A systematic understanding of how synthesis parameters influence structural, morphological, optical, and stability characteristics is therefore essential for advancing CsMBr₃-based materials. Although the hot-injection method offers rapid nucleation and controlled growth, its optimization for transition-metal halide perovskites remains underexplored. In this work, we report an optimized hot-injection synthesis of CsMBr₃ NCs and conduct a comprehensive analysis of their structural, optical, and stability properties. By systematically tuning reaction temperature and growth time, we identify the conditions necessary to obtain phase-pure CsMnBr₃, CsFeBr₃, and CsNiBr₃ NCs. Our results provide insight into how the d-electron character of the metal center influences the structure–property relationships within this lead-free perovskite family. Through this comprehensive study, we aim to establish a reliable synthesis guideline and contribute to the development of stable, environmentally friendly transition-metal perovskite nanomaterials for future optoelectronic and multifunctional applications.

2. Experimental section

2.1. Materials

Cesium carbonate (Cs₂CO₃, 99%, Sigma-Aldrich), manganese(ii) bromide (MnBr₂, 98%, Sigma-Aldrich), nickel(ii) bromide (NiBr₂, 98%, Sigma-Aldrich), iron(ii) bromide (FeBr₂, 98%, Sigma-Aldrich), oleylamine (OLA, 70%, technical grade, Sigma-Aldrich), oleic acid (OA, 70%, technical grade, Sigma-Aldrich), 1-octadecene (ODE, 90%, technical grade, Sigma-Aldrich), toluene (99%, ACE Chemicals), and hexane (99%, ACE Chemicals) were used as received without further purification.

2.2. Synthesis of Cs-oleate precursor

Cs-oleate was prepared by loading Cs₂CO₃ (0.2035 g) into a 100 mL three-neck round-bottom flask containing ODE (10 mL) and oleic acid (0.63 mL). The mixture was heated to 150–160 °C under nitrogen flow using a Schlenk line and maintained for 60 min until all Cs₂CO₃ had completely reacted with OA. The formation of Cs-oleate was indicated by a colour change from colourless to a yellow-gold solution.

2.3. Synthesis of CsMBr₃ (M = Mn, Fe, Ni) NCs

The CsMBr₃ NCs were synthesized using a colloidal hot-injection approach under an inert nitrogen atmosphere on a Schlenk line. Typically, MX₂ (1.88 mmol) was loaded into a 100 mL three-neck round-bottom flask containing ODE (15 mL), and a 1 : 1 mixture of OA and OLA (2 mL each), to stabilize the colloidal solution. The mixture was heated at 150–160 °C under N₂ gas flow for 60 minutes to ensure complete dissolution of the metal bromide salts. After dissolution, the reaction temperature was increased to the optimized formation temperature for each composition: 270 °C for CsMnBr₃, 200 °C for CsFeBr₃, and 250 °C for CsNiBr₃. A preheated Cs-oleate solution (4 mL at 170 °C) was then rapidly injected into the MX₂ solution, initiating the product formation. The reaction mixture was allowed to stand for 1–10 minutes, depending on the metal precursor used. After which it was immediately quenched using an ice-water bath for 30 s to suppress further growth and prevent secondary phase formation. For CsMnBr₃ and CsFeBr_3, a 1 min reaction time was optimal, while CsNiBr₃ required a slightly longer time, 8–10 min.

2.4. Isolation and purification of CsMBr₃ nanocrystals

The crude reaction mixture was centrifuged at 8000 rpm for 10 min. The precipitate was washed three times using a 1 : 1 (v/v) mixture of toluene and hexane to remove unreacted precursors and excess ligands. The purified nanocrystals were dried in an oven at 100 °C overnight.

2.5. Materials characterization

The crystalline structure of the samples was examined using a Bruker D2 Phaser X-ray diffractometer equipped with a Cu Kα radiation source (λ = 1.541 Å) operated at 30 kV and 10 mA. Diffraction patterns were collected over a 2θ range of 10–90° with a step time of 5 s at ambient temperature. Morphological and microstructural analyses were conducted using a JEOL JEM-2100 transmission electron microscope (TEM) operated at 200 kV. For TEM characterization, the nanocrystals were dispersed in toluene, drop-cast onto lacey carbon-coated copper grids, and dried at room temperature. Surface functional groups and ligand coordination were studied using a Thermo Scientific Nicolet iS20 FTIR spectrometer. Optical absorption spectra were recorded with a Specord 210 Plus (Analytik Jena) UV-vis spectrophotometer using nanocrystal dispersions in toluene loaded into a 1 cm path-length quartz cuvette. Photoluminescence (PL) measurements were performed on powdered samples using a HORIBA QM8000 spectrofluorometer equipped with a PPD-850 detector. X-ray photoelectron spectroscopy (XPS) was carried out using a Thermo Scientific ESCALAB 250Xi spectrometer with a monochromatic Al Kα source (1486.7 eV). Measurements were acquired using an X-ray power of 300 W, a 900 µm spot size, pass energies of 100 eV (survey) and 20 eV (high-resolution), and an operating pressure below 10⁻⁸ mbar. Samples were analysed as received without further treatment. Thermal stability was evaluated using a PerkinElmer STA6000 simultaneous thermogravimetric analyser.

3. Results and discussion

3.1. Structural and morphological characterization

X-ray diffraction (XRD) analysis was carried out to determine the crystalline phase, structural purity, and long-range ordering of the synthesized CsMBr₃ (M = Mn, Fe, Ni) nanocrystals. Fig. 1 shows the experimental diffraction patterns for the three compositions, with their standard reference patterns (red lines). All samples exhibit sharp, well-defined diffraction peaks, confirming the successful formation of the crystalline CsMBr₃ phases under the optimized hot-injection conditions. The observed diffraction reflections match well with the reported hexagonal CsMBr₃ structure (space group P6₃/mmc), indicating the formation of a phase-pure material for each material.^8,10,11 The absence of impurity peaks indicates that the Cs-oleate injection efficiently converted MBr₂ into the target perovskite phases under optimized conditions. TEM and HRTEM imaging revealed well-defined particle morphology and visible lattice fringes, confirming good crystallinity. The measured lattice fringe spacings of 0.29 nm for CsMnBr₃, 0.27 nm for CsFeBr₃, and 0.28 nm for CsNiBr₃ match well with the interplanar spacings derived from (201) XRD peak position.¹³ This confirms that the nanocrystals are phase pure. The observed HRTEM fringes arise from the same crystallographic planes identified in XRD, and there is strong structural coherence from the atomic level (HRTEM) to the bulk nanocrystal domain (XRD). Particle size distributions further reflected narrow size dispersity attributable to rapid nucleation under hot-injection conditions.

Fig. 1. Powder X-ray diffractograms, TEM, particle size distribution curve, HRTEM, and line intensity profile of CsMnBr₃ (a–c), CsFeBr₃ (d–f), and CsNiBr₃ (g–i). The reference bars (red lines at the bottom) of the XRD data.

To investigate the influence of reaction temperature and time on phase formation, syntheses were carried out at multiple temperatures (180, 190, 250, and 270 °C). The reaction temperature was found to play a decisive role in phase formation. Lower temperatures (180–190 °C) led to the formation of alternative phases such as Cs₂MnBr₅ and Cs₂FeBr₅. In the case of CsNiBr₃, lower temperatures and shorter reaction times resulted in detectable impurity peaks that could be associated with unreacted NiBr₂ or CsBr, indicating incomplete reaction, or mixture of phases (Fig. S1). As the reaction temperature increases (250–260 °C) for CsNiBr₃, the XRD patterns display sharp reflections corresponding to the hexagonal CsNiBr₃ structure, with no detectable impurity peaks, confirming the phase-pure nature of the products. In addition, the TEM images (Fig. 2d–f) clearly show the effect of reaction temperature on particle morphology and size. Nanocrystals synthesized at the lower temperature (190 °C) exhibit much larger and less uniform particles, indicating incomplete nucleation and uncontrolled secondary growth. In contrast, samples prepared under optimized conditions produce smaller, more uniform nanocrystals, consistent with rapid nucleation and well-controlled growth. These results demonstrate that both temperature and reaction time critically influence crystal phase and purity, and that optimized conditions are essential for obtaining pure CsMBr₃ nanocrystals with composition-dependent structures.

Fig. 2. (a–c) XRD patterns, (d–f) TEM images, with particle size distribution curve for CsMBr₃ (M = Mn, Fe, Ni, respectively) synthesied at 190 °C.

To understand the structural stability of these synthesized materials that are exposed to the ambient environment, time-dependent XRD patterns were recorded on different days, as seen in Fig. 3. XRD analysis over 30 days shows that CsMBr₃ nanocrystals retain their structural integrity under ambient conditions. Notable degradation and peak suppression occurred after 46 days, suggesting that additional surface passivation or encapsulation strategies may be necessary to extend the lifetime of these materials.

Fig. 3. Time-dependent P-XRD pattern of (a) CsMnBr₃, (b) CsNiBr_3, and (c) CsFeBr₃ at different days.

3.2. Optical properties

The optical behaviour of these CsMBr₃ (M = Mn, Fe, Ni) nanocrystals was investigated using UV-vis absorption spectroscopy, steady-state photoluminescence (PL), and time-resolved photoluminescence (TRPL). These complementary techniques provide insights into electronic transitions, optical bandgaps, and exciton recombination dynamics across the series.

3.2.1. UV-vis absorption and electronic structure

The UV-vis absorption spectra of the three CsMBr₃ compositions are presented in Fig. 4, revealing strong and broad absorption from 300 to 600 nm, and clear differences attributable to the d-electron configurations of Mn²⁺, Fe²⁺, and Ni²⁺: The CsMnBr₃ NCs show a broad absorption edge and a weak shoulder consistent with 6A₁ → 4T₁ transitions, in line with its spin-forbidden d–d absorption profile. The CsFeBr₃ also exhibited multiple absorption features across the visible region, reflecting the rich crystal–field transitions of Fe²⁺/Fe³⁺ and the greater density of sub-band states in Fe-based bromide lattices. CsNiBr₃ shows a strong, structured absorption tail extending into the visible range, consistent with Ni²⁺ octahedral d–d transitions and the presence of deep defect levels. Stability monitoring over a month revealed negligible changes in absorption onset, demonstrating that the materials maintain their structural and UV-vis absorption characteristics under ambient storage for a month. Furthermore, all nanocrystals show remarkable ambient stability up to 30 days, based on UV-vis and XRD analyses. Band gaps extracted from Tauc plots in Table 1 remained stable for 30 days, confirming optical robustness. The optical band gaps were determined using Tauc plots, assuming a direct allowed electronic transition.

Fig. 4. UV-vis spectra of (a) CsMnBr₃ and (b) CsFeBr_3, and (c) CsNiBr₃ at different days.

Table 1. Optical band gap energies of all the synthesized perovskites.

Materials	Optical band gap energy (eV)
	Fresh	After 4 days	After 15 days	After 30 days	After 46 days
CsMnBr3	3.15	3.15	3.15	3.16	3.56
CsNiBr3	3.13	3.13	3.13	3.14	3.38
CsFeBr3	2.36	2.36	2.36	2.37	2.40

3.2.2. Light-emission and exciton dynamics

Fig. 5a–c show the photoluminescence spectra of all the synthesized perovskite materials. All three compositions display emission peaks in the 500–700 nm range, which may originate from crystal-field-influenced electronic transitions, surface-related emissive states, and defect-assisted recombination processes arising from the small nanocrystal sizes and their high surface-to-volume ratios. While CsMnBr₃ is a well-established emissive transition-metal halide perovskite, CsFeBr₃ and CsNiBr₃ are comparatively less explored and are not generally regarded as strong emitters. The comparatively weaker and broader emission features observed for these compositions may therefore reflect contributions from localized electronic states and surface-related recombination pathways. The dominant emission band (fitted peak 2) is attributed to radiative recombination through crystal-field-influenced d–d transitions within the [MBr₆] octahedral framework, whereas the shoulder peak appearing at shorter wavelengths could be associated with higher-energy emissive states arising from surface-related trap states, shallow defect levels effects. To quantify PL lifetime and understand the exciton-recombination dynamics, time-resolved PL measurements were carried out using TCSPC following 469 nm excitation. The decay curves in Fig. 5d were well fitted using a bi-exponential model, indicating the coexistence of short-lived (τ₁) and long-lived (τ₂) decay components (Table 2). The fitted parameters show small standard errors (Table 2), confirming the good fitting quality, the reliability, and reproducibility of the extracted PL lifetimes. Among the samples, CsFeBr₃ demonstrated a long lifetime compared to others. Longer lifetimes are indicative of reduced non-radiative recombination, which is beneficial for solar cell applications where prolonged charge carrier lifetimes directly correlate with higher efficiency. The composition-dependent exciton dynamics observed across the CsMBr₃ series can be directly linked to the distinct d-electron configurations of the transition-metal cations. Mn²⁺ (d⁵) undergoes crystal-field-split and spin-forbidden d–d transitions, resulting in comparatively faster recombination dynamics. In contrast, Fe²⁺ (d⁶), with partially filled d-orbitals, promotes stronger metal-halide hybridization and reduced non-radiative losses, leading to the longest average carrier lifetime. Ni²⁺ (d⁸) introduces more localized d-states, giving rise to deeper emissive levels and an extended slow-decay component. These results demonstrate that d-electron configuration is a key design parameter governing exciton recombination and optical performance in transition-metal-based, lead-free perovskite nanocrystals.

Fig. 5. (a–c) PL spectra of CsMBr₃ (M = Mn, Ni, Fe) nanocrystals under 469 nm excitation wavelength. (d) PL decay curves for CsMBr₃ (M = Mn, Fe, Ni) at emission wavelengths of 585, 640, and 552 nm, respectively.

Table 2. Fitted parameters of the PL decay curves with their associated standard errors for synthesized CsMBr₃ NCs.

Material	A 1	τ 1 (ns)	A 2	τ 2 (ns)	τ average (ns)
CsMnBr3	338 ± 2.27	0.122 ± 0.002	128 ± 2.23	0.729 ± 0.007	0.78 ± 0.02
CsFeBr3	502 ± 2.12	0.157 ± 0.001	86.8 ± 1.02	2.04 ± 0.030	2.15 ± 0.02
CsNiBr3	186 ± 1.12	0.11 ± 0.001	32.3 ± 0.31	2.27 ± 0.031	1.80 ± 0.02

3.3. XPS analysis

The XPS elemental studies in Fig. 6a confirm the successful formation of the targeted CsMBr₃ (M = Ni, Mn, Fe) perovskites, with all expected elements, Cs, M, Br, and signals from the capping ligands clearly detected and no impurity peaks, indicating high phase purity. The high-resolution spectra in Fig. 6(b–f) provide deeper insight into the chemical environments of the constituent elements. The Cs 3d doublet in Fig. 6b appears at binding energies characteristic of Cs⁺, confirming its stable incorporation at the A-site of the perovskite lattice. Similarly, the Br 3d doublet in Fig. 6c exhibits binding energies consistent with metal-halide coordination, demonstrating intact [MBr₆] octahedra. For the transition metals, the Ni 2p, Mn 2p, and Fe 2p spectra in Fig. 6(d–f) show peak positions and splitting patterns matching their expected oxidation states (e.g., Ni²⁺, Mn²⁺, Fe²⁺), with no significant satellite features or shifted components, evidence that secondary phases or oxidation products are absent. The Fe 2p high-resolution XPS spectrum (Fig. 6e) shows binding energies and multiplet features consistent with Fe²⁺, with no detectable contributions from Fe³⁺ or other oxidized species, thereby directly confirming the +2 oxidation state of iron in CsFeBr₃. Overall, the survey and high-resolution spectra collectively confirm correct stoichiometry, well-preserved metal-halide bonding, and chemical stability, consistent with the long-term optical stability demonstrated in the month-long UV-vis and XRD measurements.

Fig. 6. XPS analysis of CsMBr₃ (M = Mn, Fe, Ni) nanocrystals. (a) Survey spectra confirming the presence of Cs, M, Br, and capping ligands without impurity peaks. (b and c) High-resolution Cs 3d and Br 3d spectra showing Cs⁺ and lattice Br⁻ environments. (d–f) High-resolution Mn 2p, Fe 2p, and Ni 2p spectra with binding energies consistent with Mn²⁺, Fe²⁺, and Ni²⁺, confirming correct metal incorporation and absence of secondary phases.

3.4. FTIR: surface ligand binding

The presence and coordination of surface capping ligands on the synthesized perovskite nanocrystals were confirmed using FTIR spectroscopy. Fig. 7 presents the FTIR spectra of the pure ligands, oleylamine (OLA) and oleic acid (OA) alongside those of the synthesized CsMBr₃ (M = Mn, Fe, Ni) perovskites. Pure OLA and OA exhibit strong absorption bands at 2917 and 2844 cm⁻¹, corresponding to the asymmetric and symmetric stretching vibrations of methylene (–CH₂–) groups. A weak band near 3007 cm⁻¹ is attributed to = C–H stretching adjacent to the C C bond of the long alkyl chain. OA further displays a prominent band at 1707 cm⁻¹, characteristic of the carbonyl (C O) stretching vibration of the carboxylic acid group, along with a weak feature at 2672 cm⁻¹ related to the O–H stretching of dimerized acid species. In contrast, OLA shows distinct amine-group vibrations: an NH₂ bending mode at 1458 cm⁻¹ and the C–N stretching band at 1058 cm⁻¹. Hence, the FTIR spectra of all synthesized perovskites retain these key ligand-associated absorption bands, confirming the successful binding of both OLA and OA to the nanocrystal surfaces. However, slight peak shifts, broadening, and reduced intensities are observed relative to the pure ligands. These spectral changes indicate chemical interactions between the ligand functional groups (–COOH/–NH₂) and the metal-bromide surface sites of the perovskites. This confirms that both capping agents are effectively coordinated to the surface of all synthesized CsMBr₃ nanocrystals, contributing to their stabilization and colloidal dispersibility.

Fig. 7. FTIR spectra of the pure oleic acid, oleylamine, and CsMBr₃ (M = Mn, Fe, Ni).

3.5. Thermal stability analysis

Thermogravimetric analysis (TGA) in Fig. 8 reveals distinct mass-loss behaviours across the CsMBr₃ series. CsMnBr₃ exhibits a pronounced derivative peak at 338 °C with mass loss of 19.8%, which can be attributed to the removal of surface-bound organic ligands (oleylamine and oleic acid) together with the onset of lattice destabilization and partial halide release.^15–18 This relatively low decomposition temperature indicates early framework destabilization, consistent with weaker Mn–Br bonding and lower lattice cohesion.^17,19 A second major mass loss of 30.8% occurs at higher temperatures (780–800 °C), corresponding to the complete decomposition of the perovskite framework and the formation of thermally stable inorganic residues. Similarly, CsFeBr₃ displays a two-step thermal decomposition profile, characterized by derivative peaks at 305 °C and 792 °C. The first mass-loss event at 305 °C likely corresponds to initial lattice destabilization and partial halide release. The second, more pronounced mass loss at 792 °C could be attributed to the oxidative decomposition of Fe-containing residues in air, possibly leading to the formation of iron oxide phases and the further release of volatile bromine-containing species.²⁰ This multistage degradation behaviour suggests sequential structural transformations, positioning CsFeBr₃ as exhibiting intermediate thermal stability within the series.

Fig. 8. TGA-DTA curves of the (a) CsMnBr₃, (b) CsFeBr₃, and (c) CsNiBr₃.

In contrast, CsNiBr₃ shows a dominant derivative feature at 789 °C. Careful inspection of the TGA trace and the relatively small associated mass loss indicates that this high-temperature event corresponds to a secondary transformation of thermally robust Ni-containing residues, rather than the survival of the intact perovskite lattice up to this temperature. This behaviour reflects a distinct decomposition pathway, producing stable inorganic residues that persist at elevated temperatures, in line with previous reports on Cs-based halide perovskites.²⁰ Although ex situ XRD or evolved-gas analysis techniques were not employed to directly identify the decomposition products, the markedly different mass-loss profiles clearly demonstrate that CsMnBr₃, CsFeBr₃, and CsNiBr₃ undergo fundamentally different thermal-decomposition mechanisms. These results indicate that CsMnBr₃, CsFeBr₃, and CsNiBr₃ exhibit broadly similar thermal stability profiles, with all samples showing an initial mass-loss event in the 300–340 °C range, followed by major decomposition at elevated temperatures (780–800 °C). The observed differences are primarily associated with variations in decomposition pathways and residue formation. This highlights the role of B-site cation substitution in influencing degradation mechanisms within this class of materials.

4. Conclusion

We report the successful synthesis of phase-pure, highly crystalline CsMBr₃ (M = Mn, Fe, Ni) nanocrystals via an optimized colloidal hot-injection method, where reaction temperature and growth time critically govern phase purity and crystallinity. Structural, morphological, and optical analyses revealed clear composition-dependent properties driven by the transition-metal cation. Differences in photoluminescence and exciton-recombination dynamics were rationalized by the d-electron configurations of Mn²⁺ (d⁵), Fe²⁺ (d⁶), and Ni²⁺ (d⁸). Mn- and Ni-based perovskites exhibit faster or more localized recombination pathways, whereas CsFeBr₃ shows suppressed non-radiative recombination and the longest carrier lifetime, attributed to stronger metal-halide hybridization. These findings highlight transition-metal perovskites as promising candidates for lead-free optoelectronic applications and establish d-electron configuration as an important design parameter for future perovskite materials. All nanocrystals show remarkable ambient stability up to 30 days, with degradation beyond 46 days, and XPS/TGA analyses confirm proper metal incorporation, chemical robustness, and distinct decomposition pathways. Overall, this work provides critical insight into structure–property relationships in transition-metal perovskite nanocrystals and delivers a reproducible synthetic platform for future device integration.

Conflicts of interest

There are no conflicts to declare.

Data availability

All possible experimental and analysed results have been included in this manuscript and supplementary information (SI) explicitly. No other new data have been generated by any further experiments/analyses. Supplementary information: optimization studies of CsMBr₃ (M = Mn, fe, Ni). See DOI: https://doi.org/10.1039/d6ra02065h.