University of Science & Technology Bannu

Excitons in recently discovered two-dimensional magnetic semiconductors have emerged as a promising vehicle for optoelectronic and spin-photonic applications. To exploit novel possibilities magnetic degrees of freedom offer, insight into the interplay of magnetism, lattice and optical excitations becomes essential. We consider Chromium Sulphur Bromide, which has two kinds of excitons, XB at 1.8 eV and XA at 1.38 eV. Here we show, through a combination of many body perturbation theory and experiment, that XB is an order of magnitude more sensitive to magnetic and lattice perturbations than XA. We trace the difference to the latter being localised (Frenkel-like), while the former is delocalised (Wannier-Mott-like) – a coexistence rarely seen in two-dimensional materials. This finding is supported by the strong temperature and magnetic field (up to 85 Tesla) dependent shifts in optical response for XB (much smaller for XA), and we show it is related to XB’s tendency for delocalisation (in-plane and out-of-plane) and enhanced coupling with Ag phonon modes. This study shows that CrSBr hosts Frenkel-like and Wannier-Mott-like excitons whose distinct spatial character explains their contrasting sensitivity to magnetic order and lattice vibrations, challenging the standard dichotomy in describing excitons.

Perovskite oxides have been extensively studied for optoelectronic applications because of their flexible structure. In this work utilizing the FP-LAPW method in the Wien2K code to examine the double perovskites Na2R-eXO6 (X = Al, In) for efficient energy harvesting devices. The structures were relaxed using Perdew-Burke-Ernzerhof (PBE) method's Generalized Gradient Approximation (GGA) and the Tran-Blaha modified Becke-Johnson (TB-mBJ) scheme was employed for accurate electronic properties. Both compounds are structurally and thermodynamically stable, and thermal stability of both compounds was demonstrated through AIMD simulations. These compounds exhibit mechanical stability and the absence of negative frequencies in the phonon dispersion curves confirms their dynamically stability. Analysis of the band structure revealed that the Na2ReXO6 (X = Al, In) perovskites exhibit direct bandgap of 1.243 eV and 1.741 eV respectively. Additionally, significant dielectric response and high absorption coefficients of Na2ReXO6 (X = Al, In) make these compounds optically active in the visible spectrum suggesting their potential for energy harvesting.

Halide perovskites have garnered significant interest as advanced materials for optoelectronic applications and energy harvesting devices, owing to their adjustable bandgaps, elevated absorption coefficients, and exceptional charge transport characteristics. Among them, double perovskites of the less explored formula, specifically A3BX6-type halide double perovskites, remain relatively underrepresented in the literature. In this work, the structural, mechanical, thermodynamic, electronic, optical, and thermoelectrical properties of the lead-free halide perovskites A3GaI6 (A = Cs, K, Rb) were systematically investigated using first-principles calculations within the WIEN2k framework employing GGA-PBE, TB-mBJ, and TB-mBJ+SOC functionals. Structural stability was confirmed through Goldschmidt tolerance factors, negative formation energies, convex-hull analysis, and elastic constants. The calculated direct band gaps based on both functionals (TB-mBJ/TB-mBJ+SOC) of 2.06/1.88 eV (Cs3GaI6), 1.83/1.65 eV (K3GaI6), and 1.94/1.76 eV (Rb3GaI6) indicate strong optical absorption in the visible to near-infrared region. Carrier-density and Bader-charge analyses reveal that the Ga-I framework governs electronic transport, while the A-site cations tune the charge distribution, with K3GaI6 and Rb3GaI6 exhibiting higher carrier densities and stronger charge transfer than Cs3GaI6. Among the studied compounds, K3GaI6 possesses the most suitable band gap (∼1.65 eV), lower carrier effective masses, higher carrier mobilities, and a larger static dielectric constant, indicating efficient charge separation and transport, and thus superior photovoltaic potential. Based on DFT-derived parameters, SCAPS-1D simulations of sixteen n-i-p device architectures based on K3GaI6 yield power conversion efficiencies ranging from 19.48% to 22.48%, with the AZO/STO/K3GaI6/Zn2P2 configuration showing the best performance due to favorable band alignment and transport-layer properties. After optimization, the efficiency reaches 27.19%, highlighting K3GaI6 as a highly promising lead-free absorber for high-performance perovskite solar cells. This work establishes a direct link between material properties and device performance and provides a solid theoretical foundation for the experimental realization of A3GaI6-based optoelectronic and energy-harvesting applications.

We investigated the structural, hydrogen storage, electronic, optical, mechanical, and thermodynamic properties of QBaAlH6 (Q = Na, K, Rb, Cs) utilizing density functional theory (DFT). All investigated hydrides were found to crystallize in a stable cubic phase, adopting the F 43 m space group (No. 216). The absence of imaginary phonon modes confirms the dynamic stability of the hydrides. Ab initio molecular dynamic (AIMD) simulations further confirmed thermal stability with no structural deformation. The calculated gravimetric hydrogen storage capacities for NaBaAlH6, KBaAlH6, RbBaAlH6, and CsBaAlH6 are 3.13, 2.89, 2.36, and 1.99 wt%, respectively. Among the four compounds, NaBaAlH6 shows the highest hydrogen storage capacity (3.13 wt%), making it a potential candidate for storage applications. Electronic properties demonstrate that all the studied materials are semiconductors at X-Gamma high symmetry points. Optical analyses reveal strong UV absorption and a notable redshift in the absorption edge caused by band gap variations. The mechanical indicators-Cauchy pressure, Pugh's ratio, and Poisson's ratio-suggest that all the studied materials are brittle, with NaBaAlH6 exhibiting the greatest stiffness.

This study theoretically investigates the properties of RbSrX3 halide perovskites to advance their technological applications, focusing on thermoelectric, mechanical, and optoelectronic characteristics via first-principles calculations. Structural stability at the ground state was confirmed using equation of state of Birch-Murnaghan and further by formation energy and tolerance factor. The structural analysis reveals a systematic increase in lattice parameters and unit cell volumes with the substitution of larger halide ions, consistent with ionic size effects. Analysis of electronic spectra shows that RbSrCl3, RbSrBr3, and RbSrI3 exhibit indirect bandgaps of 7.68, 6.52, and 4.96 eV, respectively, which increase to 9.50, 8.09, and 6.21 eV with the inclusion of spin-orbit coupling, indicating their potential in ultraviolet (UV) photodetectors, insulating optical coatings. Mechanical analysis show their stability, anisotropy, and ductile behavior. Optical properties analysis, including refractive index, optical conductivity, dielectric constants, and absorption characteristics, suggests that these compounds are well-suited for optoelectronic and UV sensor technologies. Using BoltzTrap code, the transport properties were evaluated, showing n-type conductivity due to negative Seebeck coefficients. The calculated thermoelectric efficiency (ZT) scores attained 1.25, 0.76, and 0.57 under a temperature of 950 K, indicating significant thermoelectric performance. This work identifies RbSrX3 and related materials as strong contenders for use in UV photodetectors, insulating optical coatings, radiation-resistant optoelectronic systems, and potential use in high-temperature thermoelectric devices, providing a foundation for future experimental studies.