J. Eur. Opt. Society-Rapid Publ. 22, 2( 2026) 13
microstrain, optical band gap, Urbach energy, extinction coefficient, refractive index, and dielectric complex evolve with Mn content is critical for optimizing these materials for targeted optoelectronic device applications. In this context, both Wenting D. and Congshan Z. [ 16 ] and Yanfei C. et al. [ 17 ] have studied the direct absorption band gap( Eg) of Bi 2 O 3, finding consistent results that bulk Bi 2 O 3 is a direct semiconductor with a band gap around 2.85 eV. When synthesized as Stearic acid-coated nanoparticles, a blue shift occurs, increasing the band gap – orange Bi 2 O 3 nanoparticles exhibit a band gap near 3.33 eV( 0.48 eV blue shift), and wine-red ones near 3.28 eV( 0.43 eV blue shift). This increase is mainly attributed to the quantum confinement effect, which alters the electronic structure due to reduced particle size and surface effects. Although Yanfei C. et al. focused on band gap and luminescence, their results corroborate these band gap trends. These findings highlight the significant impact of nanoparticle size and surface modification on Bi 2 O 3’ s optical and electronic properties, underscoring the importance of studying how Mn doping similarly influences these parameters to enhance optoelectronic performance.
Therefore, the present work focuses on systematically characterizing Bi 2-x Mn x O 3 thin films with varying Mn contents( x = 0, 0.025, 0.050, 0.075, and 0.1). We investigate the structural modifications using X-ray diffraction( XRD). Detailed optical properties are analyzed based on UV-Vis-NIR spectroscopy measurements( absorbance, transmittance, reflectance) to determine the optical parameters( absorption coefficient – optical band gap – Urbach energy – refractive index – extinction coefficient) and dispersion parameters via the Wemple-DiDomenico model. Furthermore, the dielectric properties( e 1, e 2, Tand) are evaluated to understand the films’ response to electric fields. The aim is to establish clear correlations between the Mn doping level and the resulting physical properties, providing invaluable insights for the fabrication of highperformance Bi 2 O 3-based optoelectronic devices.
2 Experimental methods
For this investigation, all necessary chemical reagents, including isopropanol, citric acetate, and bismuth oxide, were procured from Sigma Aldrich. Bi 2-x Mn x O 3 nanoparticles, with“ x” values ranging from 0 to 0.1, were synthesized via a sol-gel combustion technique. This process commenced with the dissolution of manganese nitrate, bismuth oxide, and citric acid in 50 ml of isopropanol within a beaker. The blend was continuously stirred at normal temperature to ensure stoichiometric proportions of citric acid and metal cations. Subsequent heating to 150 ° C with a magnetic stirrer facilitated solvent evaporation and gel formation. Further heating ignited the gel, producing a significant flame, and the resulting material was then heated to 600 ° C before being finely ground. The gel-combustion method was selected for its simplicity, cost-effectiveness, and high efficiency, aligning with circular economy principles through sustainable resource utilization [ 18 ]. To prepare films, the synthesized Bi 2 O 3: Mn powder was combined with a Chitosan solution at optimized concentrations. Uniform and high-performing films were achieved by a 1-hour dip-coating period at an extraction rate of 40 mm / min. post-deposition, the films underwent annealing at 300 ° C in a nitrogen atmosphere, with a controlled temperature ramp of 0.2 ° C / min, to preserve structural integrity and mitigate thermal stress. The film thickness of the materials which calculated according to the ref. [ 19 ]. The crystalline structures of both pure Bi 2 O 3 and Bi 2 O 3: Mn powders were meticulously analyzed using a LAScientific X-ray diffraction( XRD) machine equipped with Cu Ka radiation( k = 1.54 Å). Microstructural characterization was performed using a Supra( Ziess) FE-SEM( Field emission scanning electron microscope) operating at 15.0 kV. Finally, the optical bandgap and transmission spectra of the prepared samples were investigated utilizing a Jasco V670 instrument.
3 Results and discussion
Figure 1a exhibits the XRD analysis plotted as intensity( arbitrary units, a. u.) versus the diffraction angle 2h in degrees of Bi 2-x Mn x O 3. All samples exhibit well-defined diffraction peaks, indicating crystalline structures. The peak positions and intensities vary slightly across the samples, reflecting changes in composition. The primary diffraction peaks are located at specific 2h values, which correspond to characteristic reflections from the crystal lattice. The most prominent peaks appear around 2h = 28 °, 34 °, 46 °, and 57 °, and confirmed Bi 2 O 3 is a monoclinic phase with COD No. 96-152-6459 [ 20 ]. As the Mn concentration decreases( from Bi 1. 90 Mn 0. 10 O 3 to Bi 2 O 3), the overall peak intensities tend to decrease. The Bi 2 O 3 sample( black curve) shows the lowest peak intensities, suggesting a less crystalline or more amorphous structure compared to the other samples. The patterns for Bi 1. 90 Mn 0. 10 O 3 to Bi 1. 975 Mn 0. 025 O 3 are consistent with perovskite-like structures, as evidenced by the sharp and well-defined peaks. The Bi 2 O 3 sample displays broadened diffraction peaks, suggestive of diminished crystallinity or the potential presence of secondary crystalline phases. Increasing Mn content( x in Bi 2-x Mn x O 3) leads to subtle shifts in peak positions, likely due to lattice parameter variations caused by Mn substitution. The peak broadening observed in the Bi 2 O 3 sample suggests that the absence of Mn results in a less ordered crystal structure. The elemental analysis confirmed that the pure material was Bi 2 O 3 and the doping with Mn was successful, as shown in Figure S1( a – f).
The peak positions in the XRD patterns were used to further analyze the structural evolution caused by Mn doping. A systematic shift of the diffraction peaks to lower 2h values was observed with increasing Mn content( x) in Bi 2-x Mn x O 3, as illustrated in Figure 1b. According to Bragg’ s law, a decrease in the diffraction angle( 2h) corresponds to an increase in the interplanar spacing( d). This lattice expansion suggests that the incorporated Mn ions are effectively modifying the host lattice. The expansion could be due to the ionic radius difference between the host cation( Bi ³ +) and the dopant, or it may be driven by the