2023-07

Ukrainian Journal of Physical Optics (Issue : 3) (Volume : 24)

We study structural, electronic and optical properties of inorganic lead-free halide perovskites RbGeX3 (X = Cl, Br and I) under hydrostatic pressure, which could facilitate development of new optoelectronic and solar-cell technologies. ab initio first-principles calculations are employed based on the generalized gradient approximation within the framework of density functional theory. We demonstrate that the bandgap of our perovskites decreases with increasing pressure. At a given pressure, the bandgap becomes narrower when the halogen atom is changed from Cl to I. We also examine the density of states and demonstrate that the energy levels near the Fermi level change significantly under pressure. The optical properties are calculated using the density functional perturbation theory and the Kramers–Kronig relation. The optical parameters such as the real and imaginary parts of the dielectric function, the refractive index and the absorption coefficient are calculated under different pressures.

2023-04

Passer journal of basic and applied science (Issue : 1) (Volume : 5)

The current research uses density functional theory (DFT) approximations in conjunction with the plane wave-pseudopotential method to investigate structural, electronic, and optical properties of Pb-free cubic perovskite RbGeX3 (X= I, Br and Cl) materials. More specifically, Norm-conserving pseudopotential has been employed to describe the ion and valence electrons interaction, and Perdew-Burke-Ernzerhof (PBE) flavor is used to represent the exchange-correlation part of the energy of the GGA approximation. Our calculated lattice constants are 5.95, 5.55, and 5.29 Å for RbGeX3 (where X=I, Br, and Cl), respectively, and they are of are in good agreement with available empirical and other values. The band structure shows the direct band gap nature of the three compounds under research here and our values of the band gap energy E_g are in good agreement with the other available results. Materials under research show response to the electromagnetic radiation starting from the infrared region to the very high energies (~33 eV). The RbGeI3 has the lowest E_g value at the low region energies and the highest optical response peaks but RbGeCl3 has the highest optical response peaks at energies located near ~20 eV. Our results show that these materials are good candidates for photo electronic applications including solar cells.

2021-08

Materials Science in Semiconductor Processing (Volume : 131)

A self-consistent calculation within Density Functional Theory (DFT) using norm-conserving pseudopotential plane-wave and the precise hybrid functional HSE06 were performed. The stability, elastic, originatemechanical, electronic, and optical properties of the cubic perovskites LiGeX3 (X = Br Cl, and I) are investigated. The behaviour of the stability and elastic characteristics under hydrostatic pressure is also presented and studied. Band structure analysis using PBE, GW-Approximation, and HSE06, show that LiGeX3 (X = Br Cl, and I) are direct bandgap semiconductors. The structures are most stable at the computed relaxed lattice parameters (a=b=c=5.880,5.470and5.190Ao)for the LiGeI3, LiGeBr3, and LiGeCl3, respectively. The Pressure dependence of cubic perovskite elastic moduli, bulk modulus B, shear modulus/constant (G,Cs), Young's modulus E, the Poisson's ratio σ, Vickers hardness Hv, Lame's constants (λ,μ), Cauchy pressure C12−C44, the Anisotropy factor A, Kleinman parameter ζ, and the P-wave modulus Pw, elastic wave velocities v, Debye temperature θD is presented. The optical properties including the static refractive index and dielectric constant are found to be related to the direct bandgaps, proportionally. The refractive index, extinction coefficient, complex dielectric function, energy loss function, optical conductivity, reflectivity, and absorption coefficient for 0–25eVincident photon energies are also presented.

2020-03

Science Journal of University of Zakho (Issue : 1) (Volume : 8)

Plane waves with norm conserving pseudopotentials (PW-PP) method in conjunction with density functional theory (DFT) frame work have been used to investigate structural, electronic and optical properties of lead-halide cubic perovskite CsPbX3 (X=Br, Cl and I). The generalized gradient approximation (GGA), specifically Perdew-Burke-Ernzerhof (PBE) flavor, has been chosen to treat the exchange correlation term of Kohn-Sham equation. Structural parameters are comparable with other theoretical and experimental studies. In spite of good agreement of our band gap values with other theoretical works, however, they were not comparable when compared to the experimental values due to the well-known problem of Eg value underestimation of DFT. To update the value, we have used GW method as a self-consistent quasiparticle method on energies and wave functions and indeed they have been improved. Optical properties have been calculated using density functional perturbation theory (DFPT). Our results show that CsPbX3 (X=Br, Cl, I) has maximum response to the electromagnetic spectrum at low energies (visible region) but minimum response at high energies.

2019-08

Ukrainian Journal of Physical Optics (Issue : 3) (Volume : 20)

In the recent years, caesium lead halides CsPbX3 with the halogen elements Cl, Br and I have gained much attention of researchers owing to their attractive optical properties. In the present work we discuss the changes in their structure, bandgap and optical properties that occur under hydrostatic pressures 1–10 GPa. The density functional theory based on the generalized gradient approximation within the Perdew–Burke–Ernzerhof approach for exchange-correlation energy is used for calculations, in conjunction with the augmented plane-wave pseudopotential method. Since the generalized gradient approximation underestimates the bandgap, we employ the GW method to improve the bandgap values. The optical properties are computed in the photon-energy range 0.1–3.6 eV, using the density functional perturbation theory. As the pressure increases, the Pb–halogen bonds become contracted, whereas the volume of the unit cell shrinks uniformly, with no phase or structure-type transformations. The bandgap decreases with increasing pressure and the corresponding decrease rate for CsPbI3 is less than that for CsPbBr3. In general, the optical response of the caesium lead halides increases with increasing pressure, while the peaks of maximums of the response functions are red-shifted.

2015-04

Journal of Modern Physics (Volume : 6)

During operating of the X-ray machines, if the protection of X-ray rooms is insufficient, not only the patient but also clinical staffs as well as public are exposed to high X-ray dosage and they are affected from X-ray related to the dose level. In the present survey, by testing the radiological leakage and scatter from X-rays machines in radiology departments of 7 randomly selected hospitals in Duhok governorate, the effects dose of X-ray to the both control panel area and the patients waiting or visiting area who are located near the radiography room, were measured. The dose was recorded for a range of peak kilovoltage (kVp) and mAs values to find efficiency of shielding materials (barriers) of radiography rooms for different X-rays level. The measurements were performed at one meter above the ground surface which was the same height of X-rays tube by using Gamma Scout dosimeter. From the measurement results, it was seen that the most hospitals barriers (doors and walls) were not appropriate to the standards except 2 hospitals. The maximum effective doses were measured in uncontrolled area of Khazer hospital which was 82.48 ± 0.73 mSv·yr-1 that was much more than the reference dose limits and in controlled area of Haval Banda Zaroka hospital which was 12.98 ± 0.16 mSv·yr-1. In result, the knowledge about the radiation dose affecting the radiologists and public in the selected hospitals was obtained, and by informing the radiologists and the hospitals managements, the necessary regulations would be planned.

2013-01

ijser (Issue : 1) (Volume : 4)

Pseudopotential plane wave method within local density approximation (LDA), generalized gradient approximation (GGA) frameworks and GW approximation (in conjunction with the ABINIT package) is used to investigate lattice constant parameter (ܽ) and band structure for zincblende BN and GaN. To be more specific, the Perdew-Wang92 (PW92) and Perdew-Burke-Ernzerhof (PBE) flavors have been employed for exchange-correlation term of LDA and GGA, respectively. Lattice constant (ܽ), band gap energy (ܧ (௚have been calculated using LDA and GGA approximations. The GGA results for (ܽ) are strongly agree with the experimental and much more accurate than LDA values. The values of ܽ in both LDA and GGA methods are well accurate when compared with the other theoretical works. The ܧ ௚values obtained are in a good agreement with other theoretical works especially for ZB-BN. On the other hand, the ܧ ௚values are not in good agreement with the experimental due to the well known band gap problem of density functional theory (DFT). To improve the ܧ ௚value, GW approximation has been used. It was found that the improvement is better for ZB-BN than ZB-GaN