Search

Structural, electronic and optical absorption spectrum properties of lead free organometal halide perovskite CH3NH3BiX3 (X = I2Te, I2S, I2Se) as a new absorber for solar cells are studied in comparison to the well-known methylammonium lead iodide perovskite (CH3NH3PbI3) by using first principle calculations. CH3NH3PbI3 is known to be one of the major breakthrough in absorbing layers for solar cell material researches. These properties allowed this material to break records in the photoelectric conversion reaching 22% in Power Conversion Efficiency (PCE) in a short period of time, but this material have some drawbacks, putting obstacles for its commercialization. First one is the instability and the second one is the toxicity of lead. It was the reason why, the choice of Bi was the most appropriate to use in CH3NH3BiI2Te, CH3NH3BiI2S and CH3NH3BiI2Se as it is a non-toxic element and also for the fact that it does not induce a big difference in the electronic structure, being close to lead in the periodic table. Furthermore, for its high optical absorption in the visible obtained on CH3NH3BiI2Te, CH3NH3BiI2S and CH3NH3BiI2Se compounds. These advantages may suggest that halide perovskite CH3NH3BiX3 (X = I2Te, I2S, I2Se) based bismuth could be a promising alternative to CH3NH3PbI3. These results may provide a basis for future applications in photovoltaic domain.

In this work, the aim was to conduct a numerical study of the structural, electronic, magnetic, magnetocaloric, and transport-related properties of the antiperovskite compound AlCMn3, by using several theoretical methods such as: ab initio calculations, Monte Carlo simulations, and mean field theory. This material exhibits a second-order ferromagnetic-paramagnetic phase transition around TC = 287 K. The crystal field and the parameters of the exchange coupling interactions have been investigated using ab initio methods. A previous theoretical study based on the magnetoelastic and magnetoelectronic couplings was investigated in order to calculate the magnetocaloric effect (MCE) of AlCMn3, but the results that were obtained using this method (namely a magnetic entropy change of -ΔSm = 35 J/Kg. K) did not match the experimental value of -ΔSm = 3.28 J/Kg.K under the same applied magnetic field of 4.5T, which is why we resorted to using the Monte Carlo simulation. This new approach improved MCE-related results as they ended up matching the experimental data, contrary to the magnetoelastic and magnetoelectronic couplings method. This suggests that AlCMn3 may be considered as a potentially viable material for magnetic refrigeration at near-room temperature due to: (i) the large full width at half peak of its −ΔSm (T) curve, (ii) the absence of any hysteresis loss, (iii) its near-room temperature working conditions and (iv) the low cost and innocuous nature of its raw materials.

As it is known, the high surface area, permanent porosity, tunable pore sizes, rigidity, structural flexibility and thermal stability make MOFs and especially Zn-MOF-5 as promising materials. But the small enthalpy change (heat of adsorption) and its low desorption temperature hinders its use. Heat of adsorption varies with the type of metal, framework topology, surface area, and pore size, and hence the extent of hydrogen adsorption in MOF materials can be influenced by manipulating these parameters. Therefore, a substitution approach is presented in the present work to improve the thermodynamic properties of MOF-5. DFT method is used to investigate the effect of the substitution of Zn by Cd and Mg on structural, thermodynamic and hydrogen storage properties of Zn-MOF-5-connector. Results of calculation show that the formation energy for Zn-connector is greater than that of Cd and Mg connectors which enhances its stability and therefore increases the decomposition temperature to 191.79 and 175.81 K for Mg and Cd connector, respectively. In addition, the adsorption and diffusion of H2 are investigated and discussed. Furthermore, a significant increase in the gravimetric and volumetric capacities from 1.57 wt% and 12.26 g L-1. H2 for Zn based connector to 6.37 wt% and 40.16 g L-1. H2 for Mg-connector is observed.

The full-potential linearized augmented plane wave method (FP-LAPW) based on the density functional theory (DFT) and Boltzmann's Transport theory, are employed to investigate theoretically the electronic structure, optical and electrical properties of vanadium -doped wurtzite ZnO with different concentrations (3.125%, 6.25%, 12.5%, 25%). The FP-LAPW based on the new potential approximation known as the Tran-Blaha modified Becke–Johnson exchange potential approximation (mBJ) was also applied with the primary goal of improving the electronic structure description specially the band gap energy. The calculated band structure and density of states (DOS) exhibit a band gap of pure ZnO (3.3 eV) closer to the experimental one. As well, our results indicate that the average transmittance in the 400–1000 nm wavelength region was 93%. We found that Zn96.875V3.125O is the optimized composition of the V doped ZnO, which has the highest conductivity (3.2 × 103 (Ωcm)−1) and transmittance. The high transmittance and electrical conductivity indicate that hexagonal V:ZnO system is a potential as material for solar energy applications.

Using Monte Carlo method, we study the magnetic proprieties of La 2 FeCoO 6 double perovskite. First, we elaborate the ground state phase diagrams in different planes: ( R 1 , R 2 ), ( R 1 , d ), ( R 2 , d ), ( R 1 , h ), ( R 2 , h ) and ( h , d ), where, the reduced parameters R 1 = J Fe – Fe / J Fe – Co , R 2 = J Co – Co / J Fe – Co , d = Δ / J Fe – Co and h = H / J Fe – Co represent the reduced Fe–Fe, Co–Co exchange coupling interactions, the reduced crystal field and the reduced external magnetic field, respectively. We show that the only stable configurations in these planes are: (±2, ±1/2), (±1, ±1/2) and (0, ±1/2). Then, we investigate the thermal behavior of the magnetizations and the susceptibilities of such a system for the ferromagnetic case. Moreover, we reveal that the increasing exchange Fe–Fe coupling effect increases the critical temperature. We also discuss the hysteresis cycle loops.

Magnetic nanoparticles of tin ferrites doped with transition metal (Sn0.5–TM0.5Fe2O4, TM = Ni; Co; Mn) have been synthesized by co-precipitation route. Particles sizes were found in the range 8–14 nm. These compounds exhibit superparamagnetic behavior, the blocking temperature was determined from the Field Cooled (FC) and Zero Field Cooled (ZFC) magnetization curves. The blocking temperature and the coercitive field of Co-doped tin nanoferrite are higher than those of Mn-doped and Ni-doped tin nanoferrites. While, the saturation magnetization of Mn-doped tin nanoferrite is higher than that of Co-doped and Ni-doped tin nanoferrites.

(Fe,Cu)-doped SnO 2 nanocrystals was synthesized using the co-precipitation method. Magnetic Properties Measurement System (MPMS) revealed that for simple doping, Fe-doped SnO 2 soft ferromagnetism at low temperature appears, while the ferromagnetic phase is stable at temperature higher than room temperature for Cu co-doping element. The ferromagnetism is significantly enhanced by the Cu addition to Fe-doped SnO 2 , according to the ZFC and FC magnetizations and the hysteresis loops. The evidences for the existence of superparamagnetism are characterized and high blocking temperature super-paramagnetism in (Fe,Cu)-doped SnO 2 nanocrystals was observed. Based on first-principles calculations, we have investigated electronic structures and magnetic properties of Fe-doped SnO 2 and (Fe,Cu)-doped SnO 2 with and without defect with LDA and LDA-SIC approximations. The results suggest that the oxygen vacancies (V O ) play a critical role in the activation of ferromagnetism in Fe doped SnO 2 . For (Fe,Cu)-doped SnO 2 the results exhibit that Cu strongly influences on the magnetic properties of these doped systems which are in good agreement with the experimental observations. Electronic structure show that the presence of Cu promote the ferromagnetic bound magnetic polaron interaction through the carriers introduce by d (Cu).

We report, in this work, a theoretical study on the electronic, optical and electrical properties of pure and Al doped ZnO with different concentrations. In fact, we investigate these properties using both First Principles calculations within TB-mBJ approximation and Boltzmann equations under the constant relaxation time approximation for charge carriers. It is found out that, the calculated lattice parameters and the optical band gap of pure ZnO are close to the experimental values and in a good agreement with the other theoretical studies. It is also observed that, the incorporations of Al in ZnO increase the optical band edge absorption which leads to a blue shift and no deep impurities levels are induced in the band gap as well. More precisely, these incorporations create shallow donor states around Fermi level in the conduction band minimum from mainly Al-3s orbital. Beside this, it is found that, the transmittance is decreased in the visible and IR regions, while it is significantly improved in UV region. Finally, our calculations show that the electrical conductivity is enhanced as a result of Al doping and it reaches almost the saturation for high concentration of Al. These features make Al doped ZnO a transparent conducting electrode for optoelectronic device applications.

In this work we use Monte Carlo (MC) simulations to study the magnetic properties of a double perovskite nano Sr 2 VMoO 6 , with crystal and magnetic fields. The crystal structure of Sr 2 FeMoO 6 is constituted of two magnetic cubic sub-lattices: The sub-lattice (I) is occupied by the V 3+ magnetic ions with a magnetic moment S = 1, while the sub-lattice (II) is occupied by the Mo 5+ magnetic ions with a magnetic moment σ = 1/2. A number of interesting phenomena such as the blocking temperature, the exchange interaction coupling between V 3+ and Mo 5+ ions, the size and temperature effects, at fixed values of the crystal field, have been investigated. On the other hand, the response of the magnetization to the field is illustrated in the hysteresis loops.

Using first principles calculations based on spin-polarized density functional theory, the magnetic and optical properties of GaN and Fe-doped GaN thin films with and without acceptor defect is studied. The band structure calculations show that the band gaps of GaN thin films with 2, 4 and 6 layers are larger than the one of the bulk with wurtzite structure and decreases with increasing the film thickness. In Fe doped GaN thin films, we show that layer of thickness and acceptor defect can switch the magnetic ordering from disorder local moment (DLM) to ferromagnetic (FM) order. Without acceptor defect Fe doped GaN exhibits spin glass phase in 4 layers form and ferromagnetic state for 2 layers form of the thin films, while it exhibits ferromagnetic phase with acceptor defect such as vacancies defect for 2 and 4 layers. In the FM ordering, the thin films is half-metallic and is therefore ideal for spin application. The different energy between ferromagnetic state and disorder local moment state was evaluated. Moreover, the optical absorption spectra obtained by ab initio calculations confirm the ferromagnetic stability based on the charge state of magnetic impurities.

The magnetic properties of diluted spinels Zn1−xMxFe2O4 (M = Mg, Ni) systems have been studied by mean field theory, probability law and high-temperature series expansions method in the range 0 ≤ x ≤ 1. The exchange interactions are calculated by the first and second theory, respectively. The magnetic phase diagrams of Zn1−xMxFe2O4 (M = Mg, Ni) are calculated by the high-temperature series expansions, combined with the Pade approximants method. The critical exponent associated with the magnetic susceptibility (γ) is deduced.