Morphological and Optical Properties of RE-Doped ZnO Thin Films Fabricated Using Nanostructured Microclusters Grown by Electrospinning–Calcination

Abstract

In this study, we report the fabrication and multi-technique characterization of pure and rare-earth (RE)-doped ZnO thin films using nanostructured microclusters synthesized via electrospinning followed by calcination. Lanthanum (La), erbium (Er), and samarium (Sm) were each incorporated at five concentrations (0.1–5 at.%) into ZnO, and the resulting powders were drop-cast as thin films on glass substrates. This approach enables the transfer of pre-engineered nanoscale morphologies into the final thin-film architecture. The morphological analysis by scanning electron microscopy (SEM) revealed a predominance of spherical nanoparticles and nanorods, with distinct variations in size and aspect ratio depending on dopant type and concentration. X-ray diffraction (XRD) and Rietveld analysis confirmed the wurtzite ZnO structure with increasing evidence of secondary phase formation at high dopant levels (e.g., Er₂O₃, Sm₂O₃, and La(OH)₃). Raman spectroscopy showed peak shifts, broadening, and defect-related vibrational modes induced by RE incorporation, in agreement with the lattice strain and crystallinity variations observed in XRD. Elemental mapping (EDX) confirmed uniform dopant distribution. Optical transmittance exceeded 70% for all films, with Tauc analysis revealing slight bandgap narrowing (Eg = 2.93–2.97 eV) compared to pure ZnO. This study demonstrates that rare-earth doping via electrospun nanocluster precursors is a viable route to engineer ZnO thin films with tunable structural and optical properties. Despite current limitations in film-substrate adhesion, the method offers a promising pathway for future transparent optoelectronic, sensing, or UV detection applications, where further interface engineering could unlock their full potential.

1. Introduction

Zinc oxide (ZnO) is a wide-bandgap semiconductor (Eg ~3.3 eV) with a hexagonal wurtzite structure, attracting growing attention for its multifunctionality in optoelectronic, sensing, and transparent device applications [1,2]. Its inherent advantages—such as high exciton binding energy, chemical stability, and abundant availability—have prompted extensive research into tailoring its properties via doping and nanostructuring [3]. Among the dopants explored, rare-earth elements (REs) such as lanthanum (La), erbium (Er), and samarium (Sm) offer distinctive opportunities for property modulation, owing to their large ionic radii, multiple oxidation states, and capability to induce localized states within the ZnO band structure [4,5,6].

Recent advances in bottom-up synthesis techniques, including electrospinning, have enabled the preparation of ZnO-based nanostructures with controlled morphology and enhanced defect chemistry [7,8,9,10,11,12,13,14,15,16,17,18,19]. However, few studies have investigated the integration of such nanostructured precursors into thin films for functional applications. In this work, we explore a hybrid synthesis route in which RE-doped ZnO nanostructured microclusters—obtained by electrospinning followed by calcination [12]—are dispersed in isopropanol and deposited as thin films via drop-casting. This method offers the advantage of transferring pre-defined nanoscale architectures and doping profiles directly into film form, potentially overcoming limitations associated with in situ doping during vapor-phase deposition [20]. Transferring pre-defined nanoscale architectures and doping profiles directly into film form offers several advantages, particularly in the fields of electronics, photonics, and materials science. This approach allows for precise control over the material properties and functionalities at the nanoscale, which is crucial for the development of advanced devices. By integrating these architectures and doping profiles into films, it is possible to enhance device performance, scalability, and versatility. Some specific advantages of this approach are underline by recent publications such as the following. Doping ZnO with rare-earth elements can significantly enhance its optical properties, such as photoluminescence, by facilitating efficient energy transfer from the ZnO host to the rare-earth ions. This is particularly beneficial for applications in optoelectronics and photonics, where improved luminescence is desired [4,6]. The incorporation of rare-earth elements like lanthanum into ZnO films can lead to a reduction in the bandgap, thereby improving the material’s electrical conductivity and making it more suitable for applications in dye-sensitized solar cells [21]. Doped ZnO films exhibit improved thermoluminescence properties, which are advantageous for radiation dosimetry applications. The presence of rare-earth elements enhances the material’s sensitivity and stability under radiation exposure [22]. The ability to maintain the hexagonal wurtzite structure of ZnO even after doping with rare-earth elements ensures that the material retains its desirable mechanical and chemical stability, which is crucial for long-term device performance [9,21,23]. Doped ZnO films can be tailored for a wide range of applications, including transparent conducting oxides for flexible electronics, gas sensors, and bio-imaging. The versatility of ZnO, combined with the tunable properties introduced by doping, makes it a highly attractive material for various technological applications [4,14,15]. The direct transfer of nanoscale architectures into film form allows for the integration of ZnO-based materials into existing device architectures, facilitating the development of advanced optoelectronic devices with enhanced performance. While the advantages of transferring doped ZnO nanoscale architectures into film form are significant, it is important to consider potential challenges. For instance, achieving uniform doping and maintaining the desired structural properties across large areas can be technically challenging. Additionally, the choice of dopants and their concentrations must be carefully optimized to avoid adverse effects on the material’s properties. Despite these challenges, the potential benefits of this approach make it a promising way for advancing the performance of ZnO-based materials in various applications. The added value of this work lies in the unique combination of material design and processing route. The rare-earth (RE)-doped ZnO films reported here are obtained from nanostructured microclusters synthesized via electrospinning followed by calcination, a morphology–composition configuration that, to the best of our knowledge, has not been reported before in thin film form. The present study focuses on translating these pre-formed nanostructured powders—previously optimized for structural and functional properties—into films/coatings while preserving their intrinsic 3D microcluster architecture and associated properties. This powder-to-film transfer is non-trivial, as most studies on RE-doped ZnO films rely on direct in situ growth methods that result in different morphologies and property sets. Our approach opens opportunities for optoelectronic, photocatalytic, and EMI shielding applications where such high-surface-area, porous, yet transparent architectures are beneficial. We investigate the influence of RE dopant type (La, Er, and Sm) and concentration (0.1–5 at.%) on the morphology, structural parameters, vibrational modes, and optical bandgap of ZnO thin films. Detailed characterization using SEM, EDX, XRD with Rietveld refinement, Raman spectroscopy, and UV-Vis spectroscopy provides insight into dopant-induced modifications and phase evolution. The findings highlight key structure–property correlations and suggest promising applications in UV photodetectors, transparent sensors, and future flexible electronics, pending optimization of film–substrate adhesion through optically inert interfacial layers.

2. Materials and Methods

2.1. Synthesis of RE-Doped ZnO Nanostructured Microclusters

Nanostructured microclusters of pure ZnO and RE-doped ZnO (La:ZnO, Er:ZnO, and Sm:ZnO) were synthesized via electrospinning followed by thermal treatment [12]. Precursor solutions were prepared by dissolving zinc acetate dihydrate in a polyvinylpyrrolidone (PVP)–ethanol system, followed by the addition of calculated amounts of La(NO₃)₃·6H₂O, Er(NO₃)₃·5H₂O, or Sm(NO₃)₃·6H₂O to yield target dopant concentrations of 0.1, 0.5, 1, 2.5, and 5 at.% RE relative to Zn²⁺. All the precursor materials and solvents were provided by Sigma Aldrich (St. Louis, MO, USA) with analytical-grade purity (≥99%). The mixtures were stirred until complete homogenization and loaded into a syringe equipped with a stainless-steel needle for electrospinning.

The electrospinning process was carried out at an applied voltage of 20 kV, a feed rate of 3 mL/h, and a tip-to-collector distance of 15 cm. A rotating drum collector was used to ensure uniform deposition of the nanofibers. The as-spun mats were subsequently calcined at 700 °C for 2 h in air to obtain RE:ZnO nanostructured microclusters composed of aggregated nanocrystals and nanorods.

2.2. Preparation of Thin Films by Drop-Casting

To fabricate thin films, 5 mg of each RE:ZnO nanostructured powder was dispersed in 5 mL of isopropanol and subjected to ultrasonication for 30 min to ensure homogeneous suspension. A fixed volume of the resulting dispersion was drop-cast (21 drops) onto cleaned 2 × 2 cm glass substrates and allowed to dry at ambient conditions. This procedure produced continuous but weakly adherent thin films due to the absence of binding agents or sintering. Future improvements are anticipated through the use of optically inactive adhesion promoters.

2.3. Characterization Techniques

The morphology of the thin films was examined using scanning electron microscopy (SEM, Nova NanoSEM 630, FEI, Hillsboro, OR, USA) operated at 10 kV acceleration voltage and working distance from 5 to 6 mm according to the samples needs. Particle and nanorod size distributions were extracted using ImageJ v1.54p software (National Institutes of Health (NIH), Bethesda, MD, USA) from at least 200 features per sample. Elemental composition and dopant distribution were investigated by energy-dispersive X-ray spectroscopy (EDX) and compositional mapping. Structural properties were assessed via X-ray diffraction (XRD, grazing incidence mode at 0.5°, 40 kV, 75 mA) using a SmartLab, (Rigaku Corporation, Tokyo, Japan) diffractometer. Rietveld refinement was applied to determine average crystallite size and lattice strain. The Rietveld refinement was performed using the Whole Powder Pattern Fitting (Rietveld) method integrated in PDXL: Integrated X-ray powder diffraction commercial software developed by Rigaku corporation. Raman spectroscopy was performed using a WITec Alpha300 R (WITec GmbH, Ulm, Germany) microscope with a 532 nm laser to evaluate phonon modes, lattice disorder, and defect states. UV-Vis optical transmittance and absorbance spectra were recorded in the 300–900 nm range using a Ossila (Ossila Ltd., Sheffield, UK) spectrophotometer. The optical bandgap (Eg) was calculated from Tauc plots assuming a direct allowed transition.

3. Results and Discussion

3.1. Morphological Analysis of RE:ZnO Thin Films

The surface morphology of pure and RE-doped ZnO (La:ZnO, Er:ZnO, Sm:ZnO) thin films was analyzed using SEM. Representative micrographs revealed nanostructured surfaces composed primarily of spherical nanoparticles and, in several cases, elongated nanorod-like features. The size, distribution, and relative abundance of these morphological features were strongly dependent on both the type and concentration of the RE dopant, as one can see in Figure 1a–c.

In the case of La:ZnO films, both spherical nanoparticles and nanorods were observed across all concentrations (0.1–5 at.%). As the La concentration increased, the average diameter of the nanoparticles exhibited a non-monotonic trend, peaking at 1 at.% (86.6 ± 21.1 nm) and slightly decreasing at higher concentrations. Meanwhile, the nanorods showed increased polydispersity, with average lengths ranging from 327.5 ± 81.8 nm (0.5% La) to 442.3 ± 237.5 nm (5% La). These trends suggest that La³⁺ incorporation promotes anisotropic growth at higher concentrations, potentially due to its large ionic radius (1.17 Å), which induces local strain and affects crystallite growth dynamics.

Er:ZnO thin films displayed a predominantly spherical morphology without significant rod-like structures, even at higher dopant levels. Nanoparticle size increased progressively with Er content, reaching an average diameter of 133.1 ± 52.5 nm at 5 at.%. This trend indicates a tendency toward particle coalescence or aggregation with increasing Er³⁺ content, likely due to the dopant’s ability to induce localized strain fields that favor grain growth.

Sm:ZnO films exhibited the most complex morphology, with both nanospheres and high-aspect-ratio nanorods present at all concentrations. At 2.5 at.% Sm, nanorods with an average length exceeding 600 nm and widths around 50 nm were observed, reaching up to ~940 nm in length at 5 at.% Sm. These results suggest a dopant-induced directional growth mechanism possibly enhanced by Sm³⁺ acting as a morphological promoter under thermal processing. The formation of elongated nanostructures at high Sm levels is of particular interest for anisotropic applications such as photodetectors, etc. More information about particle size evaluation is included in Appendix A,

Table A1. Dimensional distribution and grain size evaluation of La:ZnO thin films’ surface grains.

Sample Films	Nanoparticles	Nanorods (nm)
		Length	Width
0.1% La:ZnO
0.5% La:ZnO
1% La:ZnO
2.5% La:ZnO
5% La:ZnO

,

Table A2. Particle size evaluation for La:ZnO thin films’ surface grains.

Sample Film	NPS			NRDs—Length			NRDs—Width
	Min–Max (nm)	FWHM (nm)	Mean ± SD (nm)	Min–Max (nm)	FWHM (nm)	Mean ± SD (nm)	Min–Max (nm)	FWHM (nm)	Mean ± SD (nm)
0.1%La:ZnO	25–109	34–75	54 ± 21	194–850	227–490	350 ± 139	50–86	60–79	70 ± 8
0.5%La:ZnO	45–134	62–98	81 ± 16	202–638	242–415	328 ± 81	45–98	60–85	72 ± 11
1%La: ZnO	38–147	64–110	87 ± 21	200–700	273–472	370 ± 95	35–70	43–61	52 ± 7
2.5%La:ZnO	37–127	50–90	71 ± 18	205–660	262–470	367 ± 93	41–117	58–96	68 ± 11
5%La: ZnO	36–110	50–88	70 ± 18	225–1309	230–652	442 ± 138	68–93	57–110	80 ± 11

,

Table A3. Dimensional distribution and grain size evaluation of Er:ZnO thin films’ surface grains.

Sample Films	Nanoparticles	Nanorods (nm)
		Length	Width
0.1% Er:ZnO
0.5% Er:ZnO
1% Er:ZnO
2.5% Er:ZnO
5% Er:ZnO

,

Table A4. Particle size evaluation for Er:ZnO thin films’ surface grains.

Sample Film	NPS			NRDs—Length			NRDs—Width
	Min–Max (nm)	FWHM (nm)	Mean ± SD (nm)	Min–Max (nm)	FWHM (nm)	Mean ± SD (nm)	Min–Max (nm)	FWHM (nm)	Mean ± SD (nm)
0.1% Er:ZnO	24–160	43–90	70 ± 22	110–324	132–232	182 ± 44	27–102	40–70	54 ± 12
0.5% Er:ZnO	38–135	52–107	79 ± 24	270–763	308–493	400 ± 81	22–82	32–58	46 ± 12
1% Er: ZnO	43–204	75–145	110 ± 30	269–701	340–530	433 ± 85	23–93	41–68	54 ± 13
2.5% Er:ZnO	50–230	75–160	118 ± 40	373–778	434–618	526 ± 84	31–90	42–73	57 ± 13
5% Er: ZnO	62–283	66–173	123 ± 52	176–652	227–448	337 ± 106	35–81	42–67	55 ± 12

,

Table A5. Dimensional distribution and grain size evaluation of Sm:ZnO thin films’ surface grains.

Sample Films	Nanoparticles	Nanorods (nm)
		Length	Width
0.1% Sm:ZnO
0.5% Sm:ZnO
1% Sm:ZnO
2.5% Sm:ZnO
5% Sm:ZnO

and

Table A6. Particle size evaluation for Sm:ZnO thin films’ surface grains.

Sample Film	NPS			NRDs—Length			NRDs—Width
	Min–Max (nm)	FWHM (nm)	Mean ± SD (nm)	Min–Max (nm)	FWHM (nm)	Mean ± SD (nm)	Min–Max (nm)	FWHM (nm)	Mean ± SD (nm)
0.1% Sm:ZnO	23–75	32–53	42 ± 10	281–494	220–358	292 ± 70	22–108	46–80	61 ± 17
0.5% Sm:ZnO	27–87	37–60	48 ± 11	193–635	240–390	319 ± 72	41–73	55–93	72 ± 19
1% Sm: ZnO	32–105	42–73	57 ± 14	216–578	282–442	361 ± 80	44–110	54–81	66 ± 12
2.5% Sm:ZnO	24–111	38–63	50 ± 11	360–1442	418–804	611 ± 190	33–100	40–66	53 ± 13
5% Sm: ZnO	51–188	60–108	84 ± 22	301–744	365–564	499 ± 98	42–128	60–95	78 ± 16

.

In all cases, the films appeared continuous at the microscale, but a modest adhesion to the glass substrate was noted, attributed to the absence of sintering agents or interfacial layers. Future work will focus on introducing optically inert additives to improve adhesion without compromising transparency or functionality.

Overall, the morphology of RE:ZnO films is tunable by both dopant selection and concentration. The trends suggest that La promotes nanorod growth, Er favors particle coarsening, and Sm induces significant anisotropy in both particle and rod formation, providing a versatile platform for tailoring surface structure for targeted optoelectronic applications.

3.2. Structural Analysis: XRD and Rietveld Refinement

X-ray diffraction (XRD) analysis in grazing incidence geometry (0.5°) was employed to assess the crystal structure and phase composition of the RE:ZnO thin films. All samples primarily exhibited the hexagonal wurtzite ZnO phase (JCPDS No. 36-1451), characterized by strong reflections corresponding to the (100), (002), and (101) planes, confirming the preservation of the ZnO crystalline lattice after drop-casting of electrospun–calcined powders, as observed from Figure 2. In order to clarify the successful transfer of the powders to films/coatings, XRDs of initial powders were added to the manuscript in Appendix A,

Table A10. XRD spectra of powders (blue lines) and obtained films on microscope slide glass from the respective powders (black lines).

. The XRDs were recorded in powder form (blue line) and final material as a film on glass substrate (black line). One can note the presence of specific diffraction peaks of wurtzite ZnO before and after drop casting, without any shift. For Sm and La, after drop-casting, no additional diffraction peaks can be observed. For Er-doped ZnO thin films at 1% and 2.5%, we observe two very small diffraction peaks, at ~44° and ~39°, respectively, present only in the films, which could be ascribed to accidental metal contamination possibly from the glass substrates consisting of common microscope slides glass.

As it can be observed from the XRD spectra, in La:ZnO films, the diffraction peaks remained sharp and well-defined for low La concentrations (≤1 at.%), suggesting good crystallinity. However, at 2.5 and 5 at.% La, additional peaks were observed, which matched the orthorhombic La(OH)₃ phase (JCPDS No. 36-1481), indicating the formation of secondary phases at high dopant loadings. This can be attributed to the limited solubility of La³⁺ in ZnO and its tendency to segregate at grain boundaries when its concentration exceeds the substitutional capacity of the host lattice. For Er:ZnO, the XRD patterns revealed pure ZnO signatures at 0.1 and 0.5 at.% Er, while higher dopant levels led to the emergence of weak peaks at 2θ ≈ 29.2°, 48.9°, and 58.1°, corresponding to the (222), (400), and (622) planes of cubic Er₂O₃ (JCPDS No. 08-0050). This indicates that Er³⁺ incorporation is possible up to a certain threshold, beyond which phase segregation occurs. Similarly, Sm:ZnO films displayed only ZnO peaks up to 1 at.% Sm, whereas small additional reflections appeared at 2.5 and 5 at.% dopant concentration. These were attributed to cubic Sm₂O₃ (JCPDS No. 15-9813), with main reflections at 2θ ≈ 27.7°, 32.8°, and 45.1°. The tendency for phase separation at higher Sm content further supports the limited solubility of RE ions in ZnO.

Rietveld refinement of the XRD data provided quantitative insight into crystallite size (τ) and microstrain (ε) evolution with doping (

Table 1. Crystallite size and microstrain determined by Rietveld refinement for RE:ZnO thin films.

Sample	τ_ZnO (nm)	τ_Secondary (nm)	ε_ZnO (%)	ε_Secondary (%)
ZnO (undoped)	23.3	—	+0.06	—
0.1% La:ZnO	23.9	—	+0.18	—
1% La:ZnO	24.8	—	+0.24	—
2.5% La:ZnO	24.4	12.5	+0.20	+0.32
5% La:ZnO	22.9	14.9	+0.16	+0.29
0.1% Er:ZnO	21.0	—	+0.09	—
0.5% Er:ZnO	23.2	—	+0.21	—
1% Er:ZnO	20.8	—	+0.16	—
2.5% Er:ZnO	23.2	8.9	+0.25	+0.36
5% Er:ZnO	21.2	9.2	+0.21	+0.28
0.1% Sm:ZnO	22.0	—	+0.16	—
0.5% Sm:ZnO	22.7	—	+0.23	—
1% Sm:ZnO	21.2	—	+0.20	—
2.5% Sm:ZnO	21.9	9.2	+0.15	+0.26
5% Sm:ZnO	22.5	12.3	+0.19	+0.24

). Rietveld refinement is based on the least squared method of the theoretical profile against the experimental one in which each diffraction peak may be described by a symmetric, or nearly symmetric, pseudo-Voigt function. It requires the reasonable initial approximation of many free parameters, including peak shape, unit cell dimensions, or coordinates of all atoms in the crystal structure. In particular, such an analysis is important taking into account that the size of the crystalline domains (called mean crystallite size) and the lattice strain govern the structural defect formation at the boundaries of crystallites. More details were reported in some previous works [19].

In all RE:ZnO systems, a reduction in τ and an increase in ε were observed upon doping, consistent with substitutional stress and lattice distortion induced by RE³⁺ ions. For La:ZnO, τ slightly increased with dopant level up to 1 at.% (from 23.3 nm to 24.8 nm), followed by the emergence of La(OH)₃ domains at higher levels, which had distinct crystallite sizes (~12–15 nm). In contrast, Er:ZnO and Sm:ZnO films exhibited a consistent reduction in crystallite size τ and simultaneous growth of secondary phases with very small crystallites (τ ≈ 8–10 nm), supporting the formation of strain-inducing inclusions.

These structural findings align with the Raman results (Section 3.3) and confirm that rare-earth doping influences ZnO lattice integrity in a concentration-dependent manner. Moderate doping up to 1% preserves the wurtzite phase and introduces beneficial strain, while larger doping concentrations (2.5, 5%) promote the formation of RE-oxide inclusions, which may impact charge transport, optical transparency, and phonon behavior.

These results confirm that the electrospun-derived RE:ZnO thin films maintain structural integrity up to moderate doping levels and that phase separation at higher RE content introduces new functionalities but may compromise crystallinity.

3.3. Raman Spectroscopy

Raman spectroscopy was employed to evaluate the vibrational properties and lattice integrity of the RE:ZnO thin films. Raman spectra for all samples (Figure 3) revealed the characteristic modes of the wurtzite ZnO structure, with modifications induced by RE doping. The key modes identified include the E₂ (low), E₂ (high), A₁(LO), and E₁(LO) phonons, whose positions, intensities, and linewidths were sensitive to the dopant type and concentration.

In undoped ZnO, strong E₂ (high) (~437 cm⁻¹) and E₂ (low) (~99 cm⁻¹) peaks were observed, indicative of high crystalline quality and low defect density. The E₂ (high) mode is particularly sensitive to lattice order and is considered a fingerprint of wurtzite ZnO [24].

La:ZnO films exhibited a progressive shift and broadening of the E₂ (high) and E₁(LO) (~583 cm⁻¹) modes with increasing La content. At low concentrations (0.1–0.5 at.%), the E₂ (high) mode intensified slightly, suggesting a modest increase in crystalline order. However, at higher La levels (≥1 at.%), the E₂ (high) mode broadened and decreased in intensity, accompanied by the emergence of additional peaks around 391 cm⁻¹ and above 1100 cm⁻¹, which are attributed to local vibrational modes induced by La³⁺ or La-related secondary phases (La(OH)₃). The increased intensity and broadening of A₁(LO) and E₁(LO) modes point to enhanced defect formation and carrier concentration, in agreement with XRD and UV-Vis results.

Er:ZnO samples displayed a distinct Raman signature, with new peaks appearing between 150 and 300 cm⁻¹ and between 700 and 800 cm⁻¹ as Er concentration increased. These bands are assigned to Er–O and Er–Zn vibrational modes or to Er₂O₃ nanodomains, corroborating the secondary phase formation observed in XRD. A systematic red-shift and significant broadening of the E₂ (high) and E₁(LO) modes were recorded as Er content increased from 0.1% to 5%, suggesting increased lattice strain and phonon–defect coupling. At 5 at.% Er, the E₂ (high) mode was severely damped, consistent with reduced ZnO crystallinity and partial amorphization induced by dopant clustering or phase segregation.

Sm:ZnO spectra showed similar trends, with clear Raman features corresponding to ZnO preserved at low doping levels (0.1–1%) but increasingly distorted at 2.5–5 at.%. New peaks at ~490 cm⁻¹ and ~678 cm⁻¹ emerged, attributed to Sm₂O₃ vibrational modes. The E₂ (high) mode became progressively weaker and broader, while E₁(LO) and multifonon bands (~1100–1200 cm⁻¹) intensified with Sm content, implying higher defect densities and potential oxygen vacancies or Zn interstitials. Notably, the presence of long nanorods in Sm:ZnO correlated with more pronounced Raman mode anisotropy, indicating the influence of morphology on phonon confinement effects.

These observayions are summarized in

Table 2. Extracted Raman spectral features of pure and RE-doped ZnO thin films (E₂ (high) phonon position, FWHM trend, and dopant-induced modes).

Sample	E2 (high) Position (cm−1)	FWHM (Qualitative)	Defect/Secondary Modes
Pure ZnO	437.0	Sharp	None
La:ZnO 0.1%	436.8	Slightly broadened	Weak ~580
La:ZnO 0.5%	436.2	Moderate	580
La:ZnO 1%	435.6	Moderate	580, ~330
La:ZnO 2.5%	435.0	Broad	580, ~330
La:ZnO 5%	434.8	Broad	580, ~330
Er:ZnO 0.1%	436.7	Slightly broadened	580
Er:ZnO 0.5%	435.9	Moderate	580, ~655
Er:ZnO 1%	435.0	Broad	580, 655, 330
Er:ZnO 2.5%	434.2	Very broad	580, 655, 330
Er:ZnO 5%	434.0	Very broad	580, 655, 330
Sm:ZnO 0.1%	436.6	Slightly broadened	Weak ~580
Sm:ZnO 0.5%	435.6	Moderate	580, ~610
Sm:ZnO 1%	434.9	Moderate	580, 610, 330
Sm:ZnO 2.5%	434.2	Broad	580, 610, 330
Sm:ZnO 5%	434.0	Very broad	580, 610, 330

.

The Raman results confirm that RE doping modifies the phonon landscape of ZnO through lattice distortion, defect generation, and local symmetry breaking. These effects are minimal at ≤1 at.% doping but become dominant at ≥2.5 at.%, where secondary phases are detected, as can be observed in Figure 4. Among the REs investigated, La introduces the most subtle modifications (until phase segregation), while Er and Sm induce stronger vibrational disorder and spectral complexity at comparable concentrations. These findings are consistent with the XRD (Section 3.2) and optical (Section 3.5) analyses, reinforcing the structure–property correlations governed by dopant nature, concentration, and ionic radius. For Sm:ZnO coatings, E₂ (low) downshifts and LO modes strengthen with concentration, reflecting local mass/strain perturbations and anisotropy of the rod-rich surface. GI-XRD confirms retention of the wurtzite ZnO phase with only weak Sm₂O₃ reflections at ≥2.5 at.%, i.e., long-range crystallinity remains largely intact. The stronger Raman disorder signature therefore reflects local fields and defect–phonon coupling rather than a large loss of ZnO phase fraction.

3.4. Elemental Composition and Dopant Distribution (EDX Analysis)

The incorporation of rare-earth dopants into the ZnO matrix was confirmed through energy-dispersive X-ray spectroscopy (EDX) performed on selected RE:ZnO thin films. Elemental quantification and mapping were used to validate both the presence and the homogeneity of La, Er, and Sm within the films deposited on glass substrates and are presented in

Table A7. Elemental composition and dopant distribution of La:ZnO thin films.

Sample	Element	Weight%	Atomic%
0.1% La-doped ZnO	Ok	15	14.8
	Znk	84.84	58.13
	LaL	0.23	0.08
0.5% La-doped ZnO	Ok	14	41.80
	Znk	83.88	58.49
	LaL	1.75	0.57
1% La-doped ZnO	Ok	17	45.35
	Znk	80.48	53.74
	LaL	2.90	0.91
2.5% La-doped ZnO	Ok	15	42.34
	Znk	77.66	55.08
	LaL	7.73	2.58
5% La-doped ZnO	Ok	18	50.22
	Znk	60.57	42.54
	LaL	21.92	7.24

,

Table A8. Elemental composition and dopant distribution of Er:ZnO thin films.

Sample	Element	Weight%	Atomic%
0.1% Er:ZnO	Ok	19.22	49.35
	ErL	0.32	0.08
	Znk	80.47	50.58
0.5% Er:ZnO	Ok	16.36	44.64
	ErL	1.20	0.31
	Znk	82.44	55.05
1% Er:ZnO	Ok	14.35	41.25
	ErL	3.52	0.97
	Znk	82.13	57.78
2.5% Er:ZnO	Ok	13.87	41.17
	ErL	8.41	2.38
	Znk	77.72	56.45
5%Er:ZnO	Ok	17.72	50.17
	ErL	17.00	4.60
	Znk	65.28	45.23

and

Table A9. Elemental composition and dopant distribution of Sm:ZnO thin films.

Sample	Element	Weight%	Atomic%
0.1% Sm-doped ZnO	Ok	10.51	32.96
	SmL	2.41	0.60
	Znk	87.08	66.44
0.5% Sm-doped ZnO	Ok	11.72	35.56
	SmL	2.60	0.84
	Znk	85.68	35.56
1% Sm-doped ZnO	Ok	11.86	35.98
	SmL	3.42	1.11
	Znk	84.72	62.92
2.5% Sm-doped ZnO	Ok	13.03	39.17
	SmL	7.56	2.42
	Znk	79.41	58.41
5% Sm-doped ZnO	Ok	13.65	41.52
	SmL	13.78	4.46
	Znk	72.57	54.02

in Appendix A.

For all three RE systems, EDX spectra revealed clear peaks corresponding to Zn, O, and the respective dopant element (La, Er, or Sm). The detected atomic percentages closely matched the nominal doping levels (0.1–5 at.%), confirming the accuracy and reproducibility of the synthesis and deposition process. Importantly, compositional mapping images demonstrated a uniform spatial distribution of RE dopants across the film surface, with no significant clustering or phase separation observable at the micron scale.

La:ZnO, Er:ZnO, and Sm:ZnO films were analyzed in detail.

Table 3. Elemental composition of La:ZnO, Er:ZnO, and Sm:ZnO thin films determined by EDX analysis (all the values have ±5% errors).

Sample	Element	Wt.%	At.%	Sample	Element	Wt.%	At.%	Sample	Element	Wt.%	At.%
0.1% La:ZnO	Ok	15	14.8	0.1% Er:ZnO	Ok	19.22	49.35	0.1% Sm: ZnO	Ok	10.51	32.96
	Znk	84.84	58.13		ErL	0.32	0.08		SmL	2.41	0.60
	LaL	0.23	0.08		Znk	80.47	50.58		Znk	87.08	66.44
0.5% La:ZnO	Ok	14	41.80	0.5% Er:ZnO	Ok	16.36	44.64	0.5% Sm: ZnO	Ok	11.72	35.56
	Znk	83.88	58.49		ErL	1.20	0.31		SmL	2.60	0.84
	LaL	1.75	0.57		Znk	82.44	55.05		Znk	85.68	35.56
1% La:ZnO	Ok	17	45.35	1% Er:ZnO	Ok	14.35	41.25	1% Sm: ZnO	Ok	11.86	35.98
	Znk	80.48	53.74		ErL	3.52	0.97		SmL	3.42	1.11
	LaL	2.90	0.91		Znk	82.13	57.78		Znk	84.72	62.92
2.5% La:ZnO	Ok	15	42.34	2.5% Er:ZnO	Ok	13.87	41.17	2.5% Sm: ZnO	Ok	13.03	39.17
	Znk	77.66	55.08		ErL	8.41	2.38		SmL	7.56	2.42
	LaL	7.73	2.58		Znk	77.72	56.45		Znk	79.41	58.41
5% La:ZnO	Ok	18	50.22	5% Er:ZnO	Ok	17.72	50.17	5% Sm: ZnO	Ok	13.65	41.52
	Znk	60.57	42.54		ErL	17.00	4.60		SmL	13.78	4.46
	LaL	21.92	7.24		Znk	65.28	45.23		Znk	72.57	54.02

summarizes the elemental composition obtained from point analysis on the film surface. At low concentrations (0.1–1 at.%), Sm was successfully detected with atomic fractions between 0.6% and 1.1%, aligning well with target values. At 2.5% and 5% Sm, an increase in both absolute and relative Sm content was observed, accompanied by a slight decrease in Zn content, suggesting partial substitution or surface segregation. These trends are consistent with the XRD-detected emergence of Sm₂O₃ at higher doping levels.

The increase in O content observed with dopant level may reflect oxygen-rich RE-oxide inclusions (e.g., Sm₂O₃) or the formation of hydroxides, such as La(OH)₃ in La:ZnO, as also evidenced by XRD. Similar compositional validation was obtained for La:ZnO and Er:ZnO films, with atomic percentages matching the nominal values and confirming the successful incorporation of RE dopants in all cases. The EDX analysis supports the key findings from the morphological and structural investigations, namely that RE dopants are effectively introduced into the ZnO matrix up to moderate concentrations, with uniform distribution. At higher doping levels, partial phase segregation occurs, especially for Er and Sm, leading to the formation of RE-rich nanodomains that may influence charge transport and optical absorption.

3.5. Optical Properties: UV-Vis Absorption and Bandgap Analysis

The optical transmittance and absorption behavior of the RE:ZnO thin films were analyzed by UV-Vis spectroscopy in the wavelength range of 300–900 nm. All samples exhibited high optical transparency, with average transmittance values around 70% in the visible region as it can be observed in Figure 5, confirming their potential applicability in transparent optoelectronic devices.

The influence of rare-earth (RE) dopants—La, Er, and Sm—on the optical bandgap of ZnO thin films was systematically evaluated by comparing Eg values at matched concentrations (0.1%, 0.5%, 1%, 2.5%, and 5%). All three RE dopants induced bandgap narrowing compared to undoped ZnO (Eg = 3.341 eV), but their efficiency and behavior varied depending on the dopant type, as can be observed from

Table 4. Optical bandgap (Eg) values for RE:ZnO thin films.

Dopant	Concentration (at.%)	Eg (eV)
None	0.0	3.34
La	0.1	2.96
La	0.5	2.95
La	1.0	2.95
La	2.5	2.96
La	5.0	2.93
Er	0.1	2.96
Er	0.5	2.94
Er	1.0	2.96
Er	2.5	2.97
Er	5.0	2.94
Sm	0.1	2.97
Sm	0.5	2.97
Sm	1.0	2.97
Sm	2.5	2.97
Sm	5.0	2.97

.

To evaluate the effect of doping on the electronic structure, the optical bandgap (Eg) was extracted from the absorption spectra using the Tauc method for direct allowed transitions:

(αhν)² = A(hν − Eg)

(1)

where α is the absorption coefficient, hν the photon energy, and A a constant. Eg was determined by extrapolating the linear portion of the Tauc plots to the energy axis (i.e., where (αhν)² = 0). All the Tauc plots are presented in Appendix B.

La:ZnO films showed gradual bandgap narrowing with increasing dopant content. Eg decreased from 2.964 eV (0.1% La) to 2.930 eV (5% La), which can be attributed to the formation of localized states near the conduction band minimum and the increase in free carrier density induced by La³⁺ incorporation. The relatively consistent Eg values suggest that the La dopant acts primarily by introducing shallow donor states, without disrupting the overall band structure at low-to-moderate concentrations. Er:ZnO exhibited a more complex trend, with Eg values ranging from 2.940 to 2.970 eV. The maximum Eg of 2.970 eV was observed at 2.5% Er, likely due to quantum confinement or Burstein–Moss effects, while slight bandgap narrowing occurred at higher doping (5%), possibly due to the formation of Er₂O₃ and associated defect states. These fluctuations reflect a balance between two competing mechanisms: band filling (leading to apparent Eg widening) and defect-state formation (causing bandgap narrowing). Sm:ZnO samples showed the most stable optical response, with Eg values tightly clustered between 2.965 and 2.971 eV across all concentrations. This suggests that Sm³⁺ ions exert a moderate influence on the electronic structure of ZnO and that their incorporation does not substantially perturb the band edges. However, at the highest Sm content (5%), the slightly lower Eg (2.965 eV) may result from structural disorder and the onset of Sm₂O₃ phase formation, in agreement with the XRD and Raman findings.

These results confirm that RE doping enables fine-tuning of the ZnO optical bandgap without severely compromising transparency. While bandgap modulation is modest in magnitude, it is directionally consistent with the changes in crystallinity, microstrain, and defect levels discussed in Section 3.2 and Section 3.3.

Overall, the observed bandgap values (2.93–2.97 eV) are lower than those typically reported for RE:ZnO films deposited by other techniques (e.g., spray pyrolysis and sol–gel), which may be attributed to the unique nanostructured morphology and defect chemistry imparted by the electrospun–calcined microcluster precursors. These characteristics could be advantageous for light absorption enhancement and photoconductive behavior in UV or visible-light optoelectronic devices.

3.6. Comparative Discussion and Application Relevance

The comprehensive analysis of RE:ZnO thin films synthesized using nanostructured microclusters derived from electrospinning–calcination highlights both the versatility and the tunability offered by La³⁺, Er³⁺, and Sm³⁺ doping.

Table 5. Summary of main observations for the RE:ZnO-doped thin films.

Property/Sample	Pure ZnO	La:ZnO	Er:ZnO	Sm:ZnO
Morphology	Nanoparticles, uniform	Nanoparticles + nanorods at all concentrations; longest at 5%	Only spherical particles; increased coalescence with concentration	Nanoparticles + pronounced nanorods at ≥2.5%
Particle Size (nm)	~50–60	54.4 → 69.4 (max at 1%: 86.6)	66.9 → 133.1 (monotonic increase)	42.3 → 84.1 (sharp increase at 5%)
Nanorod Length (nm)	None	327.5 → 442.3	Not observed	291.8 → 938.6
Secondary Phases	None	La(OH)3 at ≥2.5%	Er2O3 at ≥1%	Sm2O3 at ≥2.5%
Crystallite Size (nm)	23.3	~24.8 (ZnO), ~15 (La(OH)3)	~21 (ZnO), ~9 (Er2O3)	~22 (ZnO), ~12 (Sm2O3)
Microstrain (%)	+0.06	+0.24 (ZnO)/+0.29 (secondary)	+0.21/+0.28	+0.19/+0.24
Raman E2 (high) peak	Strong, sharp	Broadened and weakened at ≥1%	Shifted, suppressed at ≥2.5%	Slightly broadened; defect-related modes appear at ≥2.5%
New Raman modes	None	La-induced at ~391 and ~1150 cm−1	Er–O and Er2O3 bands at 150–800 cm−1	Sm2O3 modes at 490, 678 cm−1
Eg (eV)	~2.97	2.96 → 2.93 (decrease with conc.)	2.96 → 2.97 (non-monotonic)	2.97 → 2.97 (stable)
Transparency (Vis)	~70%	~70%	~70%	~70%
EDX Distribution of dopant	—	Uniform	Uniform	Uniform
Film Adhesion	Moderate	Moderate	Moderate	Moderate

summarizes the main trends in structural, morphological, and optical properties, offering a direct comparison among the three dopants and pure ZnO.

These results demonstrate that the type and concentration of RE dopants directly influence the final thin film characteristics:

La³⁺ promotes nanorod growth and lattice distortion, with moderate bandgap narrowing, and the late formation of La(OH)₃. La:ZnO films offer tunable optical gaps with well-formed crystallites, making them promising for UV detectors and transparent thin-film transistors.

Er³⁺ favors particle growth and phase segregation at relatively low concentrations, resulting in larger grain sizes and a complex phononic signature. Er:ZnO films may be useful in photonic or luminescent applications where Er³⁺ f-levels can be exploited.

Sm³⁺ yields elongated nanorods and subtle Eg modulation, with well-preserved ZnO phonon modes at low doping. The strong anisotropy and Sm₂O₃ phase formation at high concentrations make Sm:ZnO attractive for polarization-sensitive devices or anisotropic sensors.

Compared to conventional ZnO thin films fabricated by sol–gel, spray pyrolysis, or CVD methods, the electrospun-derived RE:ZnO films present unique advantages: (i) tailored nanoscale morphology from pre-structured microclusters; (ii) moderate doping without harsh chemical precursors; and (iii) compatibility with low-temperature, drop-cast deposition, enabling integration on flexible substrates.

The key limitation remains the moderate adhesion to glass substrates, which currently restricts device integration. However, this drawback can be addressed in future work by introducing optically inert adhesion layers or sol–gel binders that preserve transparency while improving film robustness.

4. Conclusions

This work presents a novel approach to fabricating nanostructured ZnO and rare-earth-doped ZnO (La³⁺, Er³⁺, and Sm³⁺) thin films by integrating two scalable and cost-effective techniques: electrospinning–calcination and drop-casting. Unlike conventional vapor-phase or wet-chemical methods, this strategy leverages pre-engineered doped ZnO microclusters with tailored morphologies and controlled dopant content, enabling a unique transfer of nanoscale features into thin-film architecture without the need for in situ high-temperature synthesis or complex precursor chemistry.

A comprehensive comparative analysis across dopant types and concentrations (0.1–5 at.%) reveals distinct and dopant-specific effects on microstructure, crystallinity, vibrational modes, and optical properties. La doping favors nanorod growth and induces mild bandgap narrowing; Er enhances grain size and triggers secondary phase formation at lower concentrations; and Sm promotes anisotropic rod-like growth and introduces complex Raman activity. Despite moderate adhesion on glass substrates, all films demonstrate uniform RE distribution, high transparency (~70%), and bandgap values in the 2.93–2.97 eV range, reinforcing their applicability for transparent optoelectronic devices, UV detectors, and future flexible sensing platforms. The scientific originality of this study lies in the integration of electrospun nanostructured materials into thin films—a route largely unexplored in the context of doped ZnO—and in the systematic comparison of three chemically and structurally distinct rare-earth ions. Moreover, the synthesis methodology is adaptable, scalable, and compatible with flexible substrates, aligning well with current priorities in low-cost, solution-processed, and sustainable nanomaterial development. The added value of this approach lies in the use of electrospun nanostructured microclusters as precursors, which results in films combining fine-tuned morphology, controlled defect structure, and adjustable optical properties. Compared to films produced by conventional spray pyrolysis or sol–gel methods, these materials exhibit enhanced light–matter interaction potential, advantageous for devices requiring visible-light responsiveness such as photodetectors, solar cells, and electro-optical modulators.

In the current scientific landscape—where the demand for defect-engineered, nanostructured metal oxides with tailored functionalities is rapidly growing—this work provides timely and relevant insight into both the fabrication and fundamental understanding of RE:ZnO systems. It sets a strong foundation for further device-level integration and opens promising pathways toward multifunctional thin films engineered from electrospun nanomaterials. Nonetheless, the present films show limited adhesion to glass substrates, a factor that must be addressed before integration into robust device architectures. While their optoelectronic properties fulfill key functional requirements, mechanical durability can be improved in future work by introducing optically inactive binders or adhesion-promoting interlayers. Another limitation is the lack of in-device testing, which will be essential to translate the observed laboratory-scale properties into real-world performance metrics.

Future perspectives include the following:

(i)

Optimizing deposition parameters and substrate treatments to improve adhesion and film continuity;

(ii)

Extending the doping strategy to other rare-earth and transition-metal ions to further tune optical and electronic behavior;

(iii)

Integrating the films into prototype optoelectronic devices to assess their functional advantages over conventional ZnO;

(iv)

Performing in situ studies under operational conditions to better understand stability, defect dynamics, and performance evolution.

By addressing these aspects, the current findings establish a foundation for the rational design of nanostructured ZnO-based thin films with tailored properties for advanced optoelectronic, sensing, and photocatalytic applications.