This study evaluated the electronic structure and optical properties of Fe-doped, Ni-doped, and (Fe,Ni)-co-doped Cu₂ZnSnS₄ through the first-principles pseudopotential plane-wave method based on density functional theory. The results indicated that Fe single-doping and Ni single-doping Cu₂ZnSnS₄ can reduce the charge transfer number of adjacent S atoms, enhancing covalent bonding in Fe–S and Ni–S bonds and reducing the bond length, lattice constants a and c, and unit cell volume v. The formation energies for Fe-doping, Ni-doping, and (Fe,Ni)-co-doping were 1.0 eV, 0.58 eV, and 0.78 eV, respectively. Both Fe and Ni-doping introduced 3d electrons near the Fermi level, resulting in new impurity levels and a gradual decrease in the band gap of Cu₂ZnSnS₄ from 0.16 eV. The conduction band density of Cu₂ZnSnS₄ was primarilycontributed by Sn 5s, Sn 5p, and a portion of S 3p orbital electrons, whereas the valence band density mainly stemmed from Cu 3d, Sn 5p, and S 3p orbital electrons. Fe and Ni-doping also partly contributed to the 3d layer electrons. In the case of (Fe,Ni)-co-doping, the maximum static dielectric constant was 100.49, and the dielectric peak shifted toward the low-energy direction in the presence of both Fe and Ni. Within the visible light range, Fe-doping, Ni-doping, and (Fe,Ni)-co-doping in Cu₂ZnSnS₄ exhibited absorption coefficients greater than 10⁴ cm⁻¹, with the maximum absorption coefficient being 1.6 × 10⁵ cm⁻¹ in the case of (Fe,Ni)-co-doping. In the energy range from 1.5 to 6.3 eV, the reflectivity of Cu₂ZnSnS₄ doped with Fe, Ni, or both was lower than 30%. Notably, a high conductivity peak at 1.9 eV indicated that Cu₂ZnSnS₄ possesses good photoconductivity in the visible range. Fe-doping and Ni-doping resulted in a slight shift of the conductance peak position towardthe low-energy direction, accompanied by an increase in the peak value.

Borophene is a new type of two-dimensional material with a series of unique and diversified properties. However, most of the research is still in its infancy and has not been studied in depth. Especially in the field of semiconductor optoelectronics, there is no related research on the modulation of photoelectric properties of borophene. In this work, we focus on the effect of doping on the photoelectric properties of borophene by using the first-principles pseudopotential plane wave method. We calculate the geometric structure, electronic structure, Mulliken population analysis, and optical properties of impurity (X=Al, Ga) doped α-sheet borophene. The results show that α-sheet borophene is an indirect band gap semiconductor with 1.396eV. The band gap becomes wider after Al and Ga doping, and the band gap values are 1.437eV and 1.422eV, respectively. Due to the orbital hybridization between a small number of Al-3p electrons and Ga-4p state electrons and a large number of B 2p state electrons near the Fermi level, the band gap of borophene changes and the peak value of the electron density of states reduces after doping. Mulliken population analysis shows that the B₀-B bond is mainly covalent bond, but there is also a small amount of ionic bond. However, when the impurity X is doped, the charge transfer between X and B atoms increases significantly, and the population of the corresponding X-B bonds decreases, indicating that the covalent bond strength of the chemical bonds in the doped system is weakened, and the chemical bonds have significant directionality. The calculation of optical properties shows that the static dielectric constant of the borophene material increases, and the appearance of a new dielectric peak indicates that the doping of Al and Ga can enhance the ability of borophene to store electromagnetic energy. After doping, the peak reflectivity decreases and the static refractive index n₀ increases, which also fills the gap in the absorption of red light and infrared light by borophene materials. The research results provide a basis for the development of borophene materials in the field of infrared detection devices. The above results indicate that doping can modulate the photoelectric properties of α-sheet borophene.

Herein, the electronic structure and optical properties of GaAs doped with rare-earth elements (Sc, Y, La, Ce, and Pr) were evaluated using the first-principles method. Results showed that the lattice constants and cell volume of GaAs increased after doping. Band structure calculations indicated that the lowest conduction band and highest valence band were evident at the G-point, demonstrating that rare-earth-element doping did not alter the material type of GaAs, which remained a direct-bandgap semiconductor. The bandgap of Sc-doped GaAs increased, whereas those of Y-, La-, Ce-, and Pr-doped GaAs decreased. Moreover, the density of energy levels increased. Doping with Ce and Pr introduced impurity levels, and the Fermi level shifted into the conduction band. Investigation of the optical properties revealed that the static dielectric constant increased with La doping but decreased with Y, La, Ce, and Pr doping. The variation trends of the extinction coefficients of the doped samples were consistent with that of undoped GaAs: the extinction coefficient shifted to a low-energy region. In addition, a slight redshift occurred in the absorption spectrum. The absorption peak also diminished owing to rare-earth-element doping. We concluded that Ce- and Pr-doped GaAs can form metal alloys with different compositions. Such doping may provide a new class of materials for use in optoelectronic devices.

By using the pseudopotential plane-wave method of first principles based on density functional theory, the band structure, density of states and optical properties of Cu₂ZnSnS₄ under isotropic stress are calculated and analyzed. The results show that Cu₂ZnSnS₄ is a direct band gap semiconductor under isotropic stress, the lattice is tetragonal, and the band gap of Cu₂ZnSnS₄ is 0.16 eV at 0 GPa. Stretching the lattice causes the bottom of the conduction band of Cu₂ZnSnS₄ to move toward lower energies, while the top of the valence band remains unchanged and the band gap gradually narrows. Squeezing the lattice causes the bottom of the conduction band to move toward the high-energy direction, while the top of the valence band moves downward toward the low-energy direction, and the Cu₂ZnSnS₄ band gap becomes larger. The static permittivity, absorption coefficient, reflectivity, refractive index, electrical conductivity, and energy loss function all decrease when the lattice is stretched, and the above optical parameters increase when the lattice is compressed. When the lattice is stretched, the optical characteristic peaks such as the dielectric function shift to the lower-energy direction, while the optical characteristic peak position shifts to the higher-energy direction when the lattice is compressed.

Borophene is a novel two-dimensional material with abundant crystal structure and photoelectric properties. We focus on the effect of co-doping on the electronic structure and optical properties of borophene using the first-principles method. The results show that the structure of Al and Ga co-doped borophene is obviously distorted because Al and Ga atoms have formed bonds with a bond length of 2.378Å, and the two B atoms that bond together with Al and Ga are no longer formed bonds. However, it is also a two-dimensional planar structure after co-doping. After co-doping, the band gap width of the borophene system is narrowed from 1.409eV to 1.376eV, and the band gap is narrowed by 0.033eV. Mulliken population analysis shows an obvious charge transfer between Al-B and Ga-B atoms in the co-doped borophene. The calculation of optical properties shows that the static dielectric constant ε₁ (0) increases from 5.08 to 7.01, and ε₂ (ω) is larger than that of the undoped sample in the low-energy range. Thus, the co-doping of Al and Ga can enhance the electromagnetic energy storage capacity and the visible light absorption ability. Although the reflectance of borophene is reduced by co-doping (the peak of the reflectivity can be decreased from 71 to 61 at E=2.94eV), it still presents metallic reflection characteristics. The static refractive index n₀ can be increased from 2.25 to 2.65 by co-doping. The extinction coefficient shows strong band edge absorption at the low-energy range with an absorption edge of 0.85eV. The light loss is limited to a very narrow energy range of approximately 7.30eV, which indicates that borophene co-doped with Al and Ga can also be used as a light storage material. The optical conductivity reaches its maximum at E=1.78eV and 2.52eV, which correspond to the light irradiation with a wavelength of 698nm (red light) and 492nm (cyan light), respectively. The results show that the Al-Ga co-doped borophene is sensitive to cyan light and red light, so it can be used to make photosensitive devices. The results can hopefully fill the gap in the application of borophene in semiconductor photoelectric devices and provide a theoretical basis for its application.

The electronic structures and optical properties of Mn-doped, Co-doped, and (Mn,Co)-co-doped Cu₂ZnSnS₄ were calculated and analyzed using the first-principles pseudopotential plane-wave approach. The results indicate that doping with Mn or Co increases the bond population and decreases the bond length of the S-Mn and S-Co bonds, respectively, enhancing their covalent character. The undoped Cu₂ZnSnS₄ exhibits a bandgap of 0.16 eV, whereas doping with Mn or Co introduces impurity levels near the Fermi level, resulting in bandgap narrowing. Within the visible light spectrum, the static dielectric constant ε₁(0) reaches its maximum value of 67.7 under co-doping conditions, and the absorption coefficient also attains its maximum value of 6.7 × 10⁴ cm⁻¹ under co-doping. Doping with Mn and Co induces a redshift (shift towards lower energy) in both the absorption peaks and dielectric function peaks, concomitantly increasing the probability of photon-induced electronic transitions. Conversely, doping shifts the reflectivity peaks towards higher energies (blue-shift), with the most pronounced blue-shift occurring under co-doping; the strongest reflectivity peaks remain below 43%. A prominent conductivity peak is observed at 1.7 eV. Doping shifts this peak position towards lower energies, with the maximum peak intensity reaching 1.6. These findings collectively suggest that Mn and Co doping effectively modulate key optical properties of Cu₂ZnSnS₄, such as its band gap and absorption coefficient, constituting an effective strategy for enhancing its optoelectronic transport characteristics.