J. Eur. Opt. Society-Rapid Publ. 22, 2( 2026) 15
Figure 2. FESEM photos of( a) Bi 2 O 3,( b) Bi 1. 95 Mn 0. 05 O 3, and( c) Bi 1. 9 Mn 0. 1 O 3. The scale bars show a length of 3 lm for reference.
reveal molecular behavior [ 21, 22 ]. Figure 3 illustrates the absorbance-wavelength relationship for pure Bi 2-x Mn x O 3 thin films( x = 0) and those doped with different Mn contents( x = 0, 0.025, 0.05, 0.1). Higher Mn concentrations rise absorbance, suggesting improved light absorption due to new electronic states within the material’ s bandgap. Additionally, absorbance peak amplitudes rise with increased Mn content, indicating stronger or more probable electronic transitions. Substituting Mn for Bi may introduce defects or traps in the crystal lattice, facilitating these transitions. Incorporating Mn could alter the Bi 2 O 3 lattice’ s electronic structure by modifying valence and conduction bands or introducing localized states, enhancing absorption properties. A narrowed bandgap may shift the absorption spectra toward the red. Elevated Mn concentrations could enhance photocatalytic properties, making the material more effective for applications like photocatalysis or solar energy harvesting, as the enhanced absorbance in the visible spectrum suggests improved light-harvesting capabilities [ 23, 24 ].
Understanding and designing optical devices necessitates understanding thin film transmittance and reflectance. Transmittance indicates the amount of light passing through the film, while reflectance shows the portion reflected. These properties, influenced by the film’ s thickness and material composition, can be tailored to control light interaction at specific wavelengths. Figures 4a and 4b illustrate the wavelength-dependent transmittance and reflectance, respectively. In Figure 4a, increasing Mn concentration in Bi 2 O 3 thin films reduces transmittance, likely due to enhanced light absorption or scattering caused by Mn. This suggests that Mn introduces new energy levels or defects in the Bi 2 O 3 structure, increasing absorption and reducing light transmission. Higher Mn levels also amplify certain electronic transition peaks, possibly due to localized states that enhance transition probabilities [ 25 ]. In Figure 4b, reflectance rises with raised Mn content, likely due to changes in the material’ s refractive index from doping. Higher Mn content may increase scattering at internal or external surfaces, leading to greater light reflection. These optical property changes enable the customization of thin films for applications like photocatalytic systems, where precise light control is essential, or optoelectronic devices, where tailored absorption and reflection are critical. Such modifications highlight the potential of Mn-doped Bi 2 O 3 films for advanced optical and energy-related technologies.
Figure
3. Wavelength dependence of absorbance for the Bi 2-x Mn x O 3 thin films.
Analyzing the absorption coefficient( a) of a material is crucial for elucidating its composition and structure, which is vital for developing optical devices such as lenses, solar cells, and filters. The absorption coefficient is calculated based on the material’ s transmittance( T) andreflectance( R) values, as outlined in references [ 26, 27 ].
"
a ¼ 1
d ln ð1 � RÞ 2 þ ð1 � RÞ 4 þ 4R 2 T 2 1 = 2#
: ð3Þ
2T
Where d is the thickness of the thin films. Figure 5, which displays the absorption coefficient versus the energy of photons, shows that increasing the Mn content enhances the ability of the material to absorb the light. This improvement is likely due to Mn doping creating new energy levels within the bandgap, facilitating easier electronic transitions upon light exposure. As expected for semiconductor materials, the absorption coefficient generally increases with higher photon energy. Additionally, a sharp peak observed in Figure 5 points to resonant transitions. This peak’ s intensity slightly increases as the Mn content rises( from x = 0to x = 0.1), suggesting a higher density of available electronic states at greater Mn concentrations, which enhances the probability of photon absorption at those specific energies. The peaks also shift and broaden with increased Mn doping, which is related to the changes in the Bi 2 O 3 electronic structure induced by the doping. A possible explanation involves