Photoluminescence Properties of Mn²⁺-Doped CdS/ZnS Core/Shell Nanocrystals with Different Core Sizes and Shell Thicknesses

Abstract

The spectral line width of doped semiconductor nanocrystals (NCs) is a critical parameter governing their optical performance, yet isolating the contributions of static structural disorder and dynamic electron–phonon interactions remains a fundamental challenge. In this study, we systematically synthesized CdS:Mn/ZnS core/shell NCs with precisely controlled core-to-shell ratios to elucidate the mechanisms governing the Mn²⁺ photoluminescence (PL) spectral width. Using a time-gated photon-counting technique, we successfully isolated the long-lived Mn²⁺ PL and analyzed its temperature and shell-thickness dependencies. Low-temperature (15 K) measurements revealed that the static inhomogeneous broadening, dictated by the local coordination environment of the Mn²⁺ ions at the core/shell interface, is intrinsically independent of the core/shell ratio. In contrast, at room temperature, significant spectral broadening was observed in NCs with thinner shells. This phenomenon is attributed to the strong coupling between the highly localized Mn 3d electrons and broadly distributed surface phonon modes. Crucially, we demonstrate that a robust inorganic ZnS shell acts not only as a surface passivant but also as a physical shield that effectively decouples the Mn²⁺ ions from these surface phonons, thereby suppressing the line broadening. These findings provide a comprehensive physical chemistry perspective on the interplay between dopant localization and surface phonon coupling, offering critical design principles for achieving a high spectral purity in doped nanomaterials.

Introduction

The deliberate introduction of transition metal impurities into semiconductor nanocrystals (NCs) is a cornerstone of modern materials chemistry, enabling the precise engineering of their optical, electronic, and magnetic properties. When the dimensions of the host lattice are reduced to the quantum confinement regime, the resultant spatial confinement of charge carriers significantly amplifies their interaction with the localized states of dopant ions. Among the various dopants, divalent manganese (Mn²⁺) is uniquely prominent due to the strong exchange interactions between the host charge carriers and its highly localized 3d electrons. These interactions give rise to exceptional magneto-optical functionalities, such as giant Zeeman splitting and highly efficient, broad orange/yellow photoluminescence (PL). − Consequently, Mn²⁺-doped NCs have emerged as promising candidates for advanced applications spanning spintronics, , bioimaging, , and next-generation optoelectronic devices. −

To date, extensive research has been devoted to maximizing the efficiency of energy transfer from the host excitons to the Mn²⁺ centers. , It is well established that the energy transfer dynamics are critically dependent on the structural parameters of the NCs, such as the core size, shell thickness, and the precise radial location of the dopants. , Recently, we have demonstrated that the energy transfer dynamics from stable excitons to Mn²⁺ ions strongly depend on both the dopant concentration and their radial position within the shell. For instance, in core/shell architectures, tailoring the core/shell ratio effectively modulates the spatial overlap between the exciton wavefunctions and the localized 3d orbitals. However, while the influence of these structural variables on PL intensity and energy transfer efficiency is relatively well-documented, their impact on the spectral line widtha parameter vital for color purity in display and sensing applications , remains poorly understood.

The spectral width of the Mn²⁺ PL is intrinsically governed by the local crystal field and the strength of electron–phonon interactions. − Because the 3d electrons are deeply localized within the Mn²⁺ atoms, they are highly sensitive to their immediate microenvironment. In nanoscale systems, the high surface-to-volume ratio introduces structural heterogeneity and a high density of low-energy, broadly distributed surface phonon modes (interface phonons). Despite their potential importance, the specific role of these surface phonons in modulating the homogeneous line width of doped NCs and how the inorganic shell architecture mediates this interaction have not been systematically unraveled. A fundamental challenge lies in decoupling the static inhomogeneous broadening, arising from the distribution of local Mn²⁺ environments, from the dynamic homogeneous broadening driven by electron–phonon coupling.

In this work, we present a comprehensive investigation into the mechanisms dictating the PL spectral width in CdS:Mn/ZnS core-shell NCs. By systematically tuning the core-to-shell precursor ratio during micellar synthesis, we precisely controlled the localization of Mn²⁺ ions relative to that of the NC surface. Utilizing time-gated spectroscopy, we isolated the Mn²⁺ PL and conducted comparative analyses at cryogenic (15 K) and room temperature. In order to distinguish clearly between static structural disorder and dynamic phonon coupling, we compared room temperature, where dynamic phonon interactions are maximally excited, with 15 K, an extreme temperature at which phonon interactions are completely frozen and static inhomogeneities dominate. This approach allowed us to successfully decouple the temperature-independent structural disorder from highly temperature-dependent phonon interactions. We demonstrate that while the static local environment is invariant with shell thickness, the dynamic coupling between Mn²⁺ d-electrons and surface phonons is the primary driver of spectral broadening at room temperature. Crucially, we reveal that a thick ZnS shell acts as an essential physical shield, isolating the localized Mn²⁺ centers from surface phonon modes and thereby suppressing the line broadening. Our findings provide deep physical chemistry insights into the interplay between dopant localization and dynamic lattice interactions, offering robust design principles for optimizing the spectral purity of doped nanomaterials.

Experimental Section

Sample Preparation

The reverse micelle method was employed to synthesize CdS:Mn/ZnS, CdS:Mn/CdS, and ZnS:Mn/ZnS NCs. − The NC samples were prepared using the same chemical synthesis protocol as that in a previous study. , Two identical mixtures of heptane and sodium bis­(2-ethylhexyl) sulfosuccinate (AOT) were prepared. An aqueous solution of cadmium acetate and manganese acetate was added to one mixture, while an aqueous solution of sodium sulfide was added to the other. These two mixtures resulted in micellar solutions, where the aqueous solutions were encapsulated within reverse micelles formed by the surfactant (AOT) in the oil (heptane). By addition of one micellar solution containing Cd and Mn²⁺ ions to the other micellar solution containing S ions, nanometer-sized water pools were formed within the reverse micelles, allowing the growth of CdS:Mn NC cores. The molar ratio of Mn²⁺ ions to Cd ions was maintained at 0.03. To overcoat the CdS:Mn NCs with a ZnS shell layer, aqueous solutions of zinc acetate and sodium sulfide were added to the micellar solution containing the CdS:Mn NC cores. The addition of water to the micellar solution increased the size of the micelles, leading to the growth of a ZnS shell layer on the surface of the CdS:Mn NCs and forming CdS:Mn/ZnS core/shell NCs. To synthesize CdS:Mn/CdS NCs, zinc acetate in the shell growth process was replaced with cadmium acetate. To synthesize ZnS:Mn/ZnS NCs, cadmium acetate in the core growth process was replaced with zinc acetate.

The size of the reverse micelles, which is directly influenced by the molar ratio W ([H₂O]/[AOT]), was manipulated to control the size of the NC core and the thickness of the shell layer. In this study, while maintaining a constant sum of W _c (W value during core growth) and W _s (W value during shell growth) at 7, the ratio of W _c to W _s was systematically varied from 1:6 to 6:1. By doing so, we investigated the correlation between the controlled core and shell dimensions and their resulting optical properties.

In the field of colloidal semiconductor NCs, estimating the core size from the first exciton absorption peak wavelength using the effective mass approximation (EMA) and established sizing curves is a standard, highly reliable, and nondestructive methodology. Experimentally, while our synthesized CdS/ZnS NCs remain stably dispersed in solution, they undergo severe irreversible aggregation upon drying on transmission electron microscopy (TEM) grids. This phenomenon is considered to originate from the inherent properties of AOT, which acts as a nonvolatile, highly viscous surfactant. During solvent evaporation, the nanocrystals become inevitably entrapped within the residual AOT matrix, leading to a dense agglomeration on the TEM grid. Consequently, the observed aggregation is purely a sample preparation artifact induced by drying in the presence of an excess surfactant. This drying-induced aggregation prevented clear identification of individual particle boundaries and accurate size distribution analysis via high-resolution TEM. Therefore, the core size was measured in its native solution state by using optical absorption spectroscopy. In liquid suspensions, NCs are thermodynamically stabilized within isolated reverse micelle water pools and remain uniformly dispersed without precipitation for several months. Furthermore, the absence of any significant absorption below the absorption onset point indicates that the colloidal solution is optically transparent and devoid of aggregates that would otherwise cause scattering. In addition, although nanoparticle aggregation in solution generally leads to spectral broadening, a distinct first exciton absorption peak was detected. The present findings provide evidence that the nanoparticles in solution are monodispersed.

Measurement Method

Absorption spectra were measured using light from a tungsten lamp, passed through a 25 cm monochromator as the light source. The transmitted light from the samples was detected by a Si photodiode with a lock-in amplifier. PL spectra were measured using a He–Cd laser (325 nm) as excitation sources. The PL signal was dispersed by a 25 cm single-grating monochromator and detected by a photomultiplier. To record the spectra of the long-lived Mn²⁺ PL, a time-gated photon-counting method was used. The excitation source was chopped at 330 Hz, and photon counts were recorded within two distinct gate windows: a 3 ms gate synchronized with the excitation pulse (0–3 ms, time-integrated spectrum) and a 1.5 ms delayed gate (1.5–3 ms, delayed spectrum) to isolate the long-lived component. In our previous study, the PL lifetime of Mn²⁺ PL in CdS/ZnS NCs was determined to be approximately 4.1 ms at 15 K and 1.6 ms at room temperature, whereas defect-related background PL decayed rapidly within less than 100 μs. Based on these decay characteristics, applying a time delay of 1.5 ms in our time-gated photon-counting measurements ensures that the defect PL is completely quenched, allowing pure extraction of the Mn²⁺ d-d transition spectra without spectral contamination. The experiments conducted at room temperature were carried out in an atmosphere of air, with the NCs dispersed in a solution. The experiments at 15 K were performed inside a closed-cycle He gas cryostat, with samples cast onto quartz substrates.

Results and Discussion

Figure shows the room-temperature optical absorption spectra of the synthesized CdS:Mn/ZnS core-shell NCs. These samples were prepared by varying the precursor ratio of the core to the shell (W _c:W _s) from 1:6 to 6:1, while maintaining a constant total W value (W _c + W _s = 7). In the figure, each spectrum is labeled with its corresponding W _c value. A well-defined excitonic absorption peak originating from the CdS core is clearly observed in each spectrum. Notably, this peak systematically shifts toward lower energies as W _c increases. This trend is attributed to the quantum confinement effect, indicating a progressive increase in the NC core size with larger W _c. To quantify the core dimensions, we determined the precise excitonic transition energies from the second derivative of the absorption spectra. Using these values, the average core radii were estimated on the basis of the EMA model. This analysis confirms that the core radius increases systematically from approximately 1.6 to 2.4 nm as W _c is varied from 1 to 6.

1.

Room-temperature optical absorption spectra of CdS:Mn/ZnS core/shell NCs with various core/shell ratios. The systematic redshift of the excitonic peak with increasing W _c indicates the progressive growth of the NC core size.

Figure shows the room-temperature PL spectra of CdS:Mn/ZnS core/shell NCs with various core/shell ratios, labeled with their respective W _c values. The solid black and dashed red lines represent the time-integrated and delayed PL spectra, respectively, both normalized to the peak intensity of the Mn²⁺ PL. Because the Mn²⁺ PL exhibits a slow decay on the millisecond time scale, the delayed PL measurement allows for the effective isolation of the Mn²⁺ PL from the short-lived PL originating from surface defects. , For samples with W _c = 1 and 2, a broad Mn²⁺ PL peak centered at approximately 2.05 eV is observed, which appears almost identical in both the time-integrated and delayed spectra. This indicates the formation of high-quality NCs where the Mn²⁺ PL is dominant and the surface-defect-related PL is effectively suppressed. In contrast, for W _c = 3 and 4, while the Mn²⁺ PL remains clearly visible in the delayed spectra, an additional peak attributed to surface defects begins to emerge around 1.7 eV in the time-integrated spectra. As the core size increases further, the contribution from surface defects intensifies, becoming observable even in delayed PL spectra. Finally, at W _c = 6, the Mn²⁺ PL is almost entirely quenched, and both spectra are dominated by the surface defect PL centered at 1.7 eV.

2.

Time-integrated (solid black lines) and delayed (dashed red lines) PL spectra of CdS:Mn/ZnS NCs measured at room temperature for various W _c values. The spectra are normalized to the peak intensity of the Mn²⁺ PL.

Figure shows the integrated intensity of the Mn²⁺ PL (derived from the delayed spectra) as a function of the core size. The core sizes were determined from the excitonic absorption peaks using the effective mass approximation, with the corresponding W _c values also indicated. It is observed that the Mn²⁺ PL intensity tends to increase as the core size decreases to approximately 1.8 nm. In contrast, for core sizes smaller than 1.8 nm, the Mn²⁺ PL intensity exhibits a decreasing trend. Consequently, the maximum Mn²⁺ PL is observed at a core size of ∼1.8 nm.

3.

Integrated Mn²⁺ PL intensity (derived from the delayed spectra) as a function of core size. The maximum intensity at approximately 1.8 nm results from the interplay between enhanced excitonic confinement and effective surface passivation.

The increase in the Mn²⁺ PL intensity with decreasing core size is attributed to the enhanced excitonic confinement within the smaller core. This enhancement increases the spatial overlap between the exciton and the localized d-electrons of the Mn²⁺ ions, thereby increasing the probability of energy transfer through an exchange interaction. Furthermore, as the core size decreases, the ZnS shell layer becomes thicker (to maintain the total NC size), which provides a more robust protective effect. This likely leads to a more effective suppression of nonradiative energy loss via surface defects, thereby improving the PL efficiency of the Mn²⁺ ions.

On the other hand, the decrease in the Mn²⁺ PL intensity for extremely small cores can be explained by the increased surface-to-volume ratio of the core. Since defects are more likely to reside at the core/shell interface, the efficiency of energy transfer to these nonradiative centers increases, which in turn reduces the energy transfer efficiency to the Mn²⁺ ions. Additionally, the quantum confinement effect shifts the exciton energy levels as the core size changes, potentially altering the resonance condition with the Mn²⁺ ion energy levels. The maximum Mn²⁺ PL is likely achieved when these resonance and confinement conditions are optimally satisfied. In practice, the interplay between these competing factors determines the core size dependence of the Mn²⁺ PL intensity, resulting in an optimal core/shell ratio for achieving the highest PL intensity.

As shown in Figure , Mn²⁺ PL was barely observed at room temperature for the NC samples prepared with W _c = 5 and 6. In contrast, at low temperatures, clear Mn²⁺ PL was observed even in these samples because nonradiative centers were effectively deactivated. Figure shows the delayed PL spectrum of the CdS:Mn/ZnS NCs prepared with W _c = 5, measured at 15 K (solid blue line). For comparison, the delayed PL spectrum of the sample with W _c = 3 is also shown (dashed red line). A distinct difference in the Mn²⁺ PL peak energies is observed between the two samples. To investigate the origin of this spectral shift, we prepared reference samples: CdS:Mn NCs overcoated with CdS (CdS:Mn/CdS) and ZnS:Mn NCs overcoated with ZnS (ZnS:Mn/ZnS). Their delayed PL spectra at 15 K are plotted in Figure as open and solid circles, respectively. The peak energy of Mn²⁺ PL is sensitive to the crystal field of its local environment, resulting in different peak positions in the CdS and ZnS host matrices.

4.

Delayed PL spectra of CdS:Mn/ZnS NCs prepared with W _c = 5 (solid blue line) and W _c = 3 (dashed red line) measured at 15 K. For comparison, the delayed PL spectra of Mn²⁺-doped CdS (open circles) and Mn²⁺-doped ZnS (solid circles) reference samples are also shown. The peak position of the W _c = 3 sample relative to the references confirms that the Mn²⁺ ions are localized at the core/shell interface.

The Mn²⁺ PL spectrum of the CdS:Mn/ZnS NCs with W _c = 5 almost perfectly coincides with that of the CdS:Mn/CdS NCs. On the other hand, the Mn²⁺ PL from the NCs with W _c = 3 exhibits a peak at an energy intermediate between those of the CdS:Mn/CdS and ZnS:Mn/ZnS NCs. This finding suggests that in the W _c = 3 sample, the Mn²⁺ ions are located at the CdS/ZnS core/shell interface, where they experience an intermediate crystal field from both the CdS and ZnS lattices. The assignment of Mn²⁺ ions localizing at the core/shell interface is supported by an interface model established through electron paramagnetic resonance and luminescence dynamics observations in our previous study, which utilized samples prepared by using the same chemical synthesis protocol as in the present work. In contrast, for the W _c = 5 sample, the Mn²⁺ ions are affected solely by the crystal field originating from CdS. This indicates that the deposited ZnS shell layer is insufficient to cover the Mn²⁺ ions precipitated on the CdS core surface, leaving them exposed. Collectively, these results imply that in the CdS:Mn cores, the Mn²⁺ ions are primarily precipitated on the core surface, as illustrated in Figure a.

5.

Schematic illustrations of the Mn²⁺ localization model: (a) Mn²⁺ ions precipitated on the CdS core surface, (b) Mn²⁺ ions effectively encapsulated at the interface by a thick ZnS shell (W _c < 4), and (c) Mn²⁺ ions left exposed on the surface due to an insufficient shell (W _c > 5).

The localization of Mn²⁺ ions and their subsequent coverage by the ZnS shell, which depends on the W _c value, significantly influences the room-temperature Mn²⁺ PL intensity. Based on this model, in samples with W _c < 4, the Mn²⁺ ions on the CdS core surface are sufficiently encapsulated by the ZnS shell (Figure b), leading to clear Mn²⁺ PL even at room temperature, as shown in Figure . Conversely, in NC samples with W _c > 5, the ZnS shell is too thin to provide adequate coverage for the Mn²⁺ ions on the CdS core surface (Figure c). In this case, the nonradiative recombination centers on the CdS core surface are not sufficiently deactivated at room temperature, which explains the near absence of Mn²⁺ PL. In conclusion, for achieving bright Mn²⁺ PL at room temperature, it is crucial to fully encapsulate the Mn²⁺ ions precipitated on the CdS core surface with a robust ZnS shell layer, thereby confining them at the CdS/ZnS interface and deactivating the associated nonradiative centers.

Next, we discuss the spectral width of the Mn²⁺ PL. Figure shows the delayed PL spectra at 15 K (dashed lines) for samples with W _c = 2–4, and at room temperature (solid lines) for samples with W _c = 1–4. The Mn²⁺ PL peak energy remained almost constant regardless of the measurement temperature or W _c value. In contrast, variations were observed in the spectral width of the Mn²⁺ PL. Figure plots the full width at half maximum (FWHM) of the Mn²⁺ PL at 15 K and room temperature as a function of core size. Regarding the temperature dependence, increasing the temperature from 15 K to room temperature resulted in a spectral broadening of approximately 100 meV. Regarding the core size dependence, no significant change in the Mn²⁺ PL width was observed at 15 K. Conversely, at room temperature, the Mn²⁺ PL width exhibited a clear tendency to increase with increasing core size.

6.

Comparison of delayed PL spectra at room temperature (solid lines) and 15 K (dashed lines) for samples with various W _c values. The spectra highlight the temperature- and size-dependent changes in the Mn²⁺ PL spectral width.

7.

Core size dependence of the FWHM for the Mn²⁺ PL at room temperature (open circles) and 15 K (solid circles). The size-independent FWHM at 15 K versus the size-dependent broadening at room temperature highlights the dominant role of electron-surface-phonon coupling and the shielding effect of the ZnS shell.

Since the efficiency of Mn²⁺ ion uptake may depend on the core size, changes in the Mn²⁺ PL width may be due to fluctuations in the Mn²⁺ concentration. However, it has been demonstrated that the fundamental interfacial coordination of Mn²⁺ remains unchanged regardless of fluctuations in Mn²⁺ concentration and that fluctuations in Mn²⁺ concentration in this low-doping region do not cause significant changes in the PL line width or peak energy. Furthermore, in our experimental results, the PL line width at 15 K was found to be essentially identical across all samples regardless of the core size (W _c). This indicates that variations in the Mn²⁺ concentration between samples, if any, do not induce inhomogeneous line broadening. Therefore, the pronounced shell-thickness dependence of the PL line width observed at room temperature can be unambiguously attributed to the core/shell dimension ratio (shell thickness) rather than subtle fluctuations in Mn²⁺ concentration.

Because the 3d electrons responsible for the Mn²⁺ PL are highly localized within the Mn atoms, the PL properties are predominantly governed by their local coordination environment. As established above, the Mn²⁺ ions reside at the core/shell interface. In samples with larger cores (and consequently thinner shells), the dopants are situated in close proximity to the outer surface of the NC. Near the surface, the localized 3d electrons are expected to interact strongly with local strain, structural disorder, and surface phonon modes (interface phonons) that feature lower energies and broader energy distributions than bulk LO phonons. Therefore, the total spectral line width Γ­(T) of Mn²⁺ PL is governed by Γ­(T) = Γ_inhom + Γ_hom(T), where Γ_inhom represents static inhomogeneous broadening (local strain, crystal field fluctuations, and size distribution), which is temperature-independent, and Γ_hom(T) represents dynamic homogeneous broadening caused by phonon coupling. At 15 K, the thermal energy (k _B T ≈ 1.3 meV) is far smaller than relevant lattice/surface phonon energies (tens of meV). Thus, phonon absorption/emission is completely frozen (Γ_hom ≈ 0), and the measured line width directly reflects Γ_inhom. Critically, our low-temperature (15 K) measurements, where the spectral width is dominated by static inhomogeneous broadening, show that the line width is independent of the shell thickness. This demonstrates that the static structural disorder of the Mn²⁺ coordination environment remains virtually unaffected by the core/shell ratio. If static local pressure or lattice strain caused by varying core/shell ratios was the primary driver of spectral broadening, , such strain variations should inevitably induce noticeable PL peak shifts and line width variations even at low temperatures (15 K) where lattice phonons are frozen. However, our experimental results clearly show that at 15 K, both the PL peak position and the line width are completely independent of the core/shell dimension ratio. Peak shifts were not observed across any of the samples. This critical observation effectively rules out static strain/pressure as the origin of the line broadening. Consequently, the distinct shell-thickness dependence of the PL broadening observed exclusively at room temperature cannot be attributed to variations in the static local environment or strain. Instead, it must originate from dynamic homogeneous broadening driven by changes in electron–phonon coupling with surface phonon modes. This is because, at room temperature (300 K), thermal excitation activates phonon modes, causing Γ_hom (300 K) to dominate. The pronounced broadening observed at room temperature for thinner shells must originate purely from dynamic coupling with surface phonon modes (dynamic homogeneous broadening). By decoupling these static and dynamic factors, we reveal that the room-temperature spectral broadening is mediated by coupling with surface phonons.

Such an assignment is consistent with other NC systems, where surface phonons have been theoretically and experimentally shown to act as a major source of spectral broadening. In Mn²⁺-doped core/shell systems, the distance from the dopant to the surface is known to significantly affect the emission line width. In contrast, for our NCs with smaller cores and thicker shells, the Mn²⁺ ions are deeply embedded within a robust inorganic ZnS matrix. The thick shell acts as a physical shield that effectively decouples the localized Mn²⁺ centers from surface phonon modes. Consequently, the dopants couple predominantly with bulk-like lattice phonons, suppressing line broadening. While Yang et al. discussed phonon-induced broadening, they did not address the dynamic shielding of surface phonons enabled by an inorganic shell. By systematically controlling the core-to-shell ratio, we have experimentally isolated and clarified this physical shielding effect for the first time. The spectral width of Mn²⁺ PL is determined by both local structural strain and electron–phonon coupling. Our temperature-dependent analysis reveals that the static local environment (strain) around Mn²⁺ centers changes negligibly with the core/shell ratio. Instead, a thicker inorganic ZnS shell acts as a physical shield that isolates Mn²⁺ ions from surface phonon modes, suppressing dynamic line broadening, and enhancing luminescence efficiency. In conclusion, the efficiency of the shell in isolating the Mn²⁺ ions from surface phonon interactions determines the room-temperature electron–phonon coupling strength, thereby dictating the ultimate spectral purity of the emission.

Conclusions

We synthesized CdS:Mn/ZnS core/shell NCs with precisely controlled core/shell ratios and investigated their optical properties. Analyses of absorption and delayed PL spectra revealed that the Mn²⁺ ions are localized at the core/shell interface. This indicates that as the core size decreases (and the shell thickness increases), the Mn²⁺ ions become increasingly isolated from the NC surface. The Mn²⁺ PL intensity exhibited a maximum at a core radius of approximately 1.8 nm, which originates from an optimal balance between enhanced excitonic confinement and the effective suppression of surface-related nonradiative losses.

By investigating the temperature and shell-thickness dependence of the PL spectra, we successfully distinguished the contributions of the static local environment from the dynamic electron–phonon coupling to the Mn²⁺ PL spectral width. Our low-temperature measurements demonstrated that the static inhomogeneous broadening of the Mn²⁺ environment remains independent of the core/shell ratio. In contrast, room-temperature results showed significant spectral broadening in samples with thin shells due to strong coupling with surface phonon modes. We demonstrated that increasing the shell thickness effectively provides a physical shield, isolating the Mn²⁺ ions from these surface phonons and thereby maintaining narrow bulk-like PL.

These findings provide a new perspective on the spectral broadening mechanisms in doped NCsmoving beyond conventional models of inhomogeneous broadening and bulk LO phonon coupling to highlight the critical role of surface phonon shielding by the shell. This work establishes that robust encapsulation within an inorganic shell is essential not only for enhancing PL intensity but also for achieving high spectral purity, offering a fundamental design guideline for high-performance doped nanomaterials.

The author declares no competing financial interest.