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Abstract
xM-CdS (M = Ni, Co) and Sn1-xFexS (0 ≤ x ≤ 0.2) were synthesized using different methods (solid mixture with thermolysis for M-CdS and thermal evaporation for Sn1-xFexS) under nitrogen. The prepared samples were fully characterized applying various techniques: XRD (x-ray diffraction), FTIT (Fourier-transform infrared spectroscopy), TEM, SEM, and EDS (Transmission and Scanning Electron Microscopy and Energy Dispersive x-ray spectroscopy), XPS (X-ray Photoelectron Spectroscopy), The DFT calculations were performed to predict the changes in the materials characteristics: cell parameter, band structure, density of states, spin distribution, and optical properties The experimental results were correlated with DFT calculations.
For xM-CdS (pure CdS and M = Ni or Co) (x= 0.1), we can summery the results as:
Computational details: the electronic properties of undoped and Ni or Co-doped CdS systems, a 3×3×1 supercell was constructed from a wurtzite structure of space group of P63mc using a 3x3x5 Gamma center k-point mesh. a grid cutoff energy of 400 eV the optimization of geometry by using (PBE) functional within (GGA), and to overcome shortcoming in GGA, we introduced the (HSE) hybrid functional to calculate electronic properties.
X-ray diffraction confirmed a singular hexagonal wurtzite structure for both pure CdS and Cd0.9Ni0.1S and Cd0.9Co0.1S samples. The theoretical predictions obtained through DFT volume optimization show a comparable decrease in lattice parameters. This shrinkage aligns with Rietveld refinement that confirmed the incorporation of M (Ni or Co) ions, primarily substituting Cd ions and revealed a reduction in unit cell parameters, attributed to the difference in ionic radii between Cd and Ni/Co, further supports.
The incorporation of the dopants Ni and Co into the CdS lattice was confirmed by the FTIR and EDS analysis. New IR vibrational bands emerged in the lower wavelength region for doped samples, indicative of the stretching modes of Ni–S bonds and Co-S. The EDS spectrum confirmed the inclusion of Ni and Co ions and the obtained percentages for elements are in good agreement with the nominal values.
DFT DOS calculations, using HSE06 hybrid function, revealed a reduction in the energy bandgap from 2.133 eV for pure CdS to 1.965 eV for Ni-doped CdS at. The UV diffuse reflectance analysis also confirmed this trend where a notable reduction in the optical bandgap upon Ni incorporation from 2.26 eV for pure CdS and 1.98 eV for Ni-doped CdS. Similarly, Co-doping is anticipated to decrease the band gap. Experimentally, Co-doped CdS exhibited a band gap of approximately 2.15 eV, aligning with DFT predictions of 2.123 eV.
The photoluminescence intensity decreases with Ni/Co-CdS, accompanied by new weak bands at 550 nm (green), 620 nm, and 670 nm (red) for Ni-doped CdS, and at 540 nm (green) and 483 nm (blue) for Co-doped CdS. These changes result from new energy states introduced by Ni and Co doping in the CdS valence band (VB) and the bottom of the conduction band (CB) as observed band structure DFT calculations. Hyper-deep defect states at (-5, -6 eV) for Ni doping and (-5.5 eV) for Co doping below the Fermi level are also detected, attributed to the occupied Co, Ni (d states).
Co-doped CdS exhibits weak ferromagnetic properties (Coercivity= 9.9 G, saturation magnetization= 12x 10-3 emu/g, remnant magnetization= 87.61 x10-6 emu/g), whereas pure CdS is fully diamagnetic. This confirms DFT volume-energy optimization and spin configuration. CdS is diamagnetic due to electron spin pairing, while doping CdS aims to create Diluted Magnetic Semiconductors (DMS) with a high spin configuration, with three unpaired electrons per Co ion, as confirmed in DOS calculations.
DOS and Band structure calculations reveal that the non-magnetic nature of CdS aligns with its perfect electronic state symmetry (spin-up and spin-down). When Ni ions replace Cd, this balance is disrupted, leading to slight polarization at the top of the valence band (VB) due to p-d hybridization between Ni or Co and S ions creating shift between spin up and down This induces magnetism, transforming the material into a ferromagnetic state (Fig. 4.9a).
For SnxFe1-xS (0 ≤ x ≤ 0.2), only experimental characterization was performed and revealed the following conclusions:
X-ray diffraction analysis revealed a single SnS phase with an orthorhombic structure, space group Pbnm, for compositions up to x = 0.1, In contrast, at x = 0.15 and 0.2 compositions, a minor phase of iron dichloride tetrahydrate (FeCl2·4H2O) appeared, comprising 4.8% and 11.1% of the phases, respectively. The lattice parameters showed minimal impact, attributed to the small difference in ionic radii between Sn2+ and Fe2+ ions.
High- Particle morphology and size distribution in Sn0.95Fe0.05S were assessed using HRTEM images and SAED patterns. Varied particle sizes were observed, leading to a SAED pattern with diffuse rings. HRTEM images showed agglomerated particles, some with uniform spherical morphology and with an average particle size of 12.4 nm. The SAED pattern highlighted nanoparticles with distinct atomic planes, including one with a d-spacing of 0.56 nm for the (200) plane. The polycrystalline nature was evident from faint rings indicating fine particles and bright dots resembling single crystals.
XPS analyses investigated chemical compositions and oxidation states. The high-resolution Sn 3d spectrum showed main peaks at 486.7 eV (Sn 3d5/2) and 495.1 eV (Sn 3d3/2), with subpeaks at 486.0 and 494.5 eV Sn2+ with sulfur bonding and 487.1 and 495.5 eV with adsorbed oxygen. The Fe 2p spectrum (710–730 eV) overlapped with Sn 3p, revealing six peaks confirming Fe2+ presence. The O 1s spectrum displayed peaks at 530.2 eV (Sn–O bonds) and 531.8 eV (O = C and/or adsorbed OH). The S 2p spectrum showed peaks at 161.27 and 164.8 eV, attributed to metal-sulfur bonding and higher sulfur oxidation states (〖SO〗_4^(2-)).
The Excitation spectra show peaks at 229 and 289 nm for pure SnS and shift and intensify with Fe content, displaying peaks at 223, 277, and 228, 283 nm for x = 0.05 and 0.1. The emission band for x = 0.1 exhibits subpeaks associated with electronic transitions from shallow trap states (VSn, ISn, and VS). Photoluminescence intensity increases with Fe doping, indicating enhanced electron-hole recombination rates.
The UV–Vis diffuse reflectance spectra of nano Sn1−xFexS (x = 0.0, 0.025, 0.05, 0.075, 0.1) show Absorbance decreases notably with increasing wavelength. Doping enhances absorbance at x = 0.025 and 0.1 Fe content but reduces it for other Fe ratios, likely due to surface defects and vacancies arising from doping. Optical band gap value was Pristine SnS (1.12 eV) exhibits altered band gap values with Fe ion doping, ranging from a maximum of 1.17 eV (for 2.5% Fe) to a minimum of 1.1 eV (for 7.5% Fe). Additionally.
The extinction coefficient (k) measures energy absorption, influenced by structural defects and free electron density. In the visible range, k rises with photon wavelength, possibly due to increased absorption from free carriers. With Fe doping in SnS, k increases with 2.5% or 10% Fe but decreases with 5% or 7.5% Fe. The n values show anomalous dispersion up to λ≈ 250 nm, with SnS n-values increasing with 2.5% Fe doping but decreasing with other Fe ratios.
The dielectric constant (εr and εi) and volume energy loss functions (SELF and VELF) analysis reveals εr decreases with wavelength, except for the 2.5% Fe-doped sample. εr changes upon SnS doping with Fe ions suggest variations in capacitance and response times. In the visible range, εi and SELF values increase with photon wavelength, especially with 2.5% or 10% Fe doping, while decreasing with other Fe ratios. Both VELF and SELF values generally increase with Fe doping, except for the 2.5% Fe-doped sample. These changes may stem from alterations in defects and electronic structure.