Tailoring spintronic—optoelectronic functionality in MnSO4 doped ZnTe nanocomposites via lattice strain, defect engineering, and room temperature magnetic ordering

MnSO₄ doped ZnTe nanocomposites were synthesized using high-purity (99.99%) reagents obtained from E-Merck. A mixture containing 0.2 mol% manganese(II) sulfate (MnSO₄), 0.2 mol% zinc telluride (ZnTe), and 6.0 mL of polyethylene glycol (PEG-400, used as a surfactant and growth modifier) was prepared in a glass flask, resulting in a semi-transparent blackish solution. This mixture was subjected to thermal treatment at 190 °C for 9 h. Within 10 min of heating, a visible color transition to bluish dark brown occurred, indicating the formation of MnSO₄ doped ZnTe nanoparticles. After completion of the reaction, the solution was allowed to cool naturally, leading to nanocomposite precipitation. The precipitate, characterized by a greenish black, was collected via centrifugation and thoroughly washed with ethanol to remove residuals.

X-ray diffraction (XRD) patterns were recorded using a Rigaku D/max-RB diffractometer with Ni-filtered CuK_α radiation (λ = 1.5406 Å), operated at 40 kV and 30 mA over a 2θ range of 10° to 90°. Raman spectra, both resonant and non-resonant, were obtained using a Renishaw RM 2000 spectrometer with 632.8 nm and 325 nm laser sources. Surface morphology was assessed using a field emission scanning electron microscope (FESEM, Philips CM12) operated at 20 kV. High-resolution transmission electron microscopy (HRTEM) was conducted using a HITACHI H-8100 instrument at 200 kV, where samples were prepared by dropping an ultrafine dispersion onto carbon-coated copper grids and drying at ambient conditions. Selected area electron diffraction (SAED) patterns were recorded in conjunction with HRTEM. Photoluminescence properties were evaluated using a Fluorolog fluorescence lifetime spectrometer to analyze UV-induced luminescent behavior.

The magnetic behavior of MnSO₄ doped ZnTe nanocomposites was examined using a vibrating sample magnetometer (VSM). Samples were secured in inverted gel caps and positioned centrally between oppositely wound pickup coils. The measurement setup involved a vibration frequency of 10 Hz, an amplitude of 1.5 mm, and an applied magnetic field strength of 1 T. Hysteresis loops were recorded across a field range from −3 T to + 3 T at cryogenic (4 K) and ambient (300 K) temperatures. Additionally, zero-field cooling (ZFC) protocols were employed using a 1 T applied field to investigate magnetic transitions post-magnetization.

MnSO₄ doped ZnTe can be characterized and analyzed by X-ray diffraction (XRD) and crystallographic modeling, which provide a comprehensive overview of phase composition, crystal quality, and lattice strain. In Fig. 1a, you can see the XRD pattern of pure MnSO₄, which is indexed to the standard JCPDS file 65–3413. There is evidence of a well-defined monoclinic structure in MnSO₄ from the observed diffraction peaks, which correspond to planes (200), (100), (002), and (211). By comparing these peaks to their ZnTe inheritance, it’s possible to determine how the structure of ZnTe changes over time. A crystallographic model of MnSO₄ is shown in Fig. 1b, emphasizing coordination geometry between Mn²⁺ (purple), S⁶⁺ (yellow), and O²⁻ (red). Among the lattice structures, there is an intricate and stable network formed by MnSO₆ octahedra and SO₄ tetrahedra (e.g., sulfate removed as byproduct during thermal synthesis) detailed details were provided in the supplementary information (SI). The absence of sulfate-related peaks in the XRD and Raman spectra confirms that SO4²⁻ is not incorporated in the final lattice. Instead, it is eliminated during the thermal process and washing, leaving Mn²⁺ substitutionally incorporated into ZnTe. Interestingly, PEG-400, used as a growth modifier, is removed after synthesis as evidenced by the absence of PEG-specific vibrational signatures, indicating it does not form a permanent coating on the particles. An XRD pattern showing MnSO₄ doped ZnTe is displayed in Fig. 1c, aligned with JCPDS file no. 15–0746. When MnSO₄ is incorporated, significant structural changes are observed in the diffraction pattern. An asterisk denotes the cubic phase of pure ZnTe, which is obvious from its characteristic reflections. The appearance of additional peaks in the diffraction pattern (211), (002), (103), (202), and (205) indicates MnSO₄ has been successfully incorporated into the host lattice. MnSO₄ may have reacted with ZnTe matrix to produce doped or composite phases with modified crystallographic symmetry, as suggested by these newly emerging peaks. Further, the apparent peak shifts and intensity variations may also be associated with local distortions, and the formation of intermediate phases may be caused by differences in ionic radii between Mn²⁺ and Zn²⁺. A combination of sharper and more intense peaks within ZnTe and reflections from the MnSO₄ matrix, which coexist with ZnTe, suggests that MnSO₄ is actually integrated within ZnTe rather than merely forming a physical mixture.

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This figure illustrates a strain and crystallite size analysis through the Williamson—Hall (W–H) method, which plots βcosθ as a function of 4sinθ. Based on the linear fit of the experimental data (red squares), we can see that doped material exhibits significant lattice strain, microstrain-induced broadening is signified by the positive slope, while crystallite size is represented by the inverse intercept. As shown by the high coefficient of determination (R² = 0.97935 and adjusted R² = 0.96831), the linear fitting supports the assertion that the peak broadening is not solely caused by crystallite size but also by the strain caused by the MnSO₄. The linear model’s robustness is further demonstrated by its residual sum of squares (2.867) and low standard errors. ZnTe’s crystal structure and microstructure are substantially altered by MnSO₄ doping, according to the structural analysis. It implies a strong interfacial interaction and substitutional incorporation associated with MnSO₄ inducing lattice distortion, altering peak positions, and enhancing crystalline order. In advanced optoelectronic or spintronic applications, where fine control over crystallinity and defects is crucial, this novel MnSO₄ doped ZnTe system exhibits promising potential, as it displays a tailored microstructure and induces strain effects [12].

In Fig. 2a, we present the Raman spectrum of MnSO₄ doped ZnTe which demonstrates a significant alteration in the structure of zinc telluride caused by manganese incorporation. By deconvoluting the spectrum, distinct MnSO₄ doping characteristics can be observed alongside ZnTe’s characteristic vibrational modes. Asymmetric phonon modes dominate ZnTe’s spectrum, including the fundamental transverse optical (TO) and longitudinal optical (LO). It is evident that the TO phonon, which is usually observed near 177 cm⁻¹ in undoped ZnTe, has shifted down to a lower wavenumber, probably in the 170—175 cm⁻¹ range, which indicating a reduction in lattice effective force constants. Due to the mass difference between Mn²⁺ ions substituting for Zn²⁺ ions and Sn²⁺ ions that have been replaced by Mn²⁺ ions, as well as local strain fields and weakened bonds, this softening occurs. In addition to the shift and asymmetric broadening to the low frequency side, the LO phonon is also observed near 210 cm⁻¹. There is clear evidence that this characteristic asymmetry has been triggered by an interaction between discrete LO phonons and a continuum of electronic states introduced by the MnSO₄ doping, possibly including acceptor levels or impurities [13]. In addition to this, the marked broadening of both TO and LO peaks implies an increase in phonon damping due to increased scattering and local structural disorder within the lattice from Mn ions [14].

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There are prominent new vibrational modes present in the spectrum that do not appear in ZnTe in its pristine form. It appears in the range associated with Te-related vibrations or disorder-activated modes between 120 and 140 cm⁻¹. As the vibrations show a significant intensity and a possible slight shift relative to tellurium precipitation modes, they might be the product of Mn-induced local vibrational modes (LVM’s) or disorder-activated acoustic phonons. A distinct and relatively sharp peak is also present near 250 cm⁻¹. Based on the alignment of this position with the characteristic MnTe-like LO phonon mode, Mn substitution onto Zn sites is highly likely to lead to the formation of Mn-Te vibrational units within the crystal lattice. In addition to the presence of a broad feature around 300 cm⁻¹, a second order scattering process, such as a 2TO(L) or TO + TA phonon combination, may also alter the intensity profile due to doping effects. According to Fig. 2b, those intrinsic spectral components have been effectively isolated and fit to a cumulative peak, confirming that the response consists of multiple peaks. This spectroscopic signature of successful Mn incorporation into the ZnTe lattice is complemented by the observation of phonon softening, asymmetric broadenings, Fano line shapes of the LO mode, and new Mn-related and disorder-activated peaks [15]. Moreover, it alters fundamental vibrational properties essential for tailoring the material’s electronic and optoelectronic properties, causing substantial local structural distortions, modifying charge distributions, and altering fundamental vibrational properties.

Based on scanning electron microscopy analysis of pure manganese sulfate crystals, Fig. 3a–i illustrates distinctive morphological characteristics. There are clearly defined geometric habits characterized by irregular polyhedral aggregates and prismatic structures, indicating that MnSO₄’s crystal system is orthorhombic. The primary particles demonstrate significant size dispersion, ranging from sub-micron platelets to euhedral crystallites surpassing 10 mm in size, with the magnification scale bar confirming nanoscale surfaces. An anisotropic crystallization kinetics may have occurred during the synthesis of intergrown tabular crystal assemblies showing sharp interfacial angles of approximately 60°—80°. The topology analysis of manganese sulfate hydrate reveals terraced microstructures with distinct step edges and cleavage planes. This is consistent with a layered, ionic lattice structure. It is significant to note that higher-magnification fields demonstrate nanotextured surfaces with dendritic growth fronts and localized dislocation patterns, indicating non-classical crystallization pathways, as shown in Fig. 3(b, c & f). As a result of high sample purity and the absence of amorphous phases or secondary precipitates, micrometer-scale fractures along crystallographic planes confirm the inherent brittleness of the material. As particles accumulate face-to-face, mesoscale architectures with internally porous pore structures develop. Crystal facets exhibit substantial variation in surface roughness parameters, with some planes showing atomically smooth terraces and others displaying dissolution pits and growth striations. Heterogeneity likely arises from differences in surface energies between crystallographic orientations, which are mediated by solvents in the crystallization process.

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In this study, microstructure suggests a diffusion-controlled crystallization mechanism, where rapid initial nucleation results in nanocrystalline building blocks that are subsequently oriented to form hierarchical architectures. Lattice matching between adjacent crystals consistently shows 76°-78° interfacial angles between adjacent crystals. It might be possible that minor morphological deviations in peripheral regions, as shown in Fig. 3d and e, are the result of localized variations in the state of hydration or a layer of surface hydration. As a result of these structural characteristics, a variety of studies investigating modifies manganese sulfates can be conducted, including catalytic, energy storage, and functional material applications whose reactivity is governed by surface topology and crystal faceting.

MnSO₄ doped ZnTe samples were examined by scanning electron microscopy (SEM) micrographs in Fig. 4, showing their morphological characteristics at different magnifications, providing valuable insights into the grain structure, morphology, and aggregation behaviors. In Fig. 4a through e, particles are tightly packed into clusters with irregular surfaces and flake-like topologies, indicating polycrystalline architecture. Observed in Fig. 4b and d, these images exhibit layers and distortions that may be due to Mn²⁺ substitution occurring within the ZnTe lattice at Zn²⁺ sites. A surface turned uneven, and the surface-to-volume (s/v) ratio increased as a result of this ionic substitution, which is highly desirable in catalytic and optoelectronic applications. This could induce localized strain and anisotropic growth during synthesis, leading to an uneven surface with an enhanced surface-to-volume ratio. It is important to note that these micrographs show agglomerated formations which are likely to be associated with strong interparticle interactions, probably owing to van-der-Waals forces or residual synthesis byproducts acting as binders. Additionally, Fig. 4e, captured at lower magnification, illustrates how the particles are assembled into a sponge-like porous network, which could play a significant role in charge transport and ion diffusion in electronics and energy storage. By observing the distinct concentric diffraction rings punctuated by bright spots in Fig. 4f, the SAED pattern further confirms the polycrystalline nature of the sample. It is possible that a partial ordered nanocrystalline framework is implied by these traits, suggesting the existence of multiple crystalline domains with preferential orientations. It appears that despite the morphological irregularities, the crystallographic rings are well-defined and sharp. The results of the SEM investigation indicate that MnSO₄ doping significantly changes meso- and microstructural attributes of ZnTe in addition to its effect on the atomic structure of the material. Rugged, coarse particles with high surface roughness as well as agglomerated assemblies suggest altered nucleation and growth kinetics of the particles during synthesis—which could be related to Mn²⁺ incorporation, which may play a structure-directing role [16‐18]. With these dual effects on crystallinity and morphology, the electronic band structure modulation, defect-induced carrier transport, and surface reactivity of MnSO₄ doped ZnTe will be significantly improved, leading to more efficient optoelectronics, catalytic, and energy applications. A combination of morphological data and diffraction evidence allows us to understand how MnSO₄ based doping dynamically alters ZnTe semiconductor physicochemical properties. In spite of this, comparing the pure ZnTe morphology pattern could be more useful for understanding the current phenomenon. We examined the surface morphology of MnSO₄ doped ZnTe samples using SEM, as shown in Fig. 4(a–e). Microstructures with irregular shapes and a rough surface texture can be seen in the images which consist of densely packed grains with irregular shapes. Particles are estimated to have a size distribution ranging from 200 to 800 nm, with an average grain size of 500 nm, suggesting moderate grain coalescence during growth. There is a significant influence of MnSO₄ on the nucleation and crystallization processes, resulting in heterogeneous grain growth due to the rough surface and nonuniform edges. As opposed to this, the morphology of pure ZnTe has been well documented. According to Minegishi et al. (APL Mater. 8, 041101 (2020), ZnTe films prepared by a single-step cross-sectional deposition consist of grains between 100 and 1000 nm. Whereas films deposited in two steps exhibit grains of 500—1000 nm and have a stepped surface structure reflecting crystal habit [19]. Based on our results, MnSO₄ doped ZnTe is somewhat smaller and rougher than previously reported materials, suggesting that Mn incorporation alters surface energy and restricts grain coarsening.

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The average particle size of the MnSO₄ doped ZnTe granular particles is about ~ 100 nm and they exhibit a spherical shape as shown in Fig. 5a–f. It is difficult to discern any difference between the spherical morphology and that after doping with zinc telluride precursors other than the appearance of a few tiny particles. While Figs. 5a and b depict metal oxides produced during synthesis, Figs. 5a–c show a roughened surface after PEG coating, confirming the construction of nanoshells. Figure 5d, e displays mesoporous silica hollow spheres that are embedded in zinc telluride nanoparticles of about ~ 40—90 nm and exhibit yolk-shell architecture. A yolk-shell nanostructure demonstrated excellent thermal stability even when calcined at temperatures above 900 °C. Upon calcining MnSO₄ doped ZnTe granular particles directly, yolk-shell nanostructured materials formed severe aggregation as shown in Fig. 1a–c, indicating that, nanoshells play a significant role in their dispersibility and stability. Further, the TEM image revealed large and uniform particle sizes of MnSO₄ loaded on the ZnTe support, which indicates severe aggregation of the particles. Due to this yolk-shell architecture, catalysts were designed with sintering properties. Further, the shell and core of the shell are uniformly filled with O. The outer shell contains Zn-Te, whereas the inner core contains MnSO₄. This egg-shell architecture of MnSO₄ doped ZnTe nanocomposites is in agreement with previous studies.

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