As technology and industrialization quickly advance, global energy demand increases immensely, burdening energy systems to keep up. Renewable energy is a sustainable method to remedy the demand, in particular using solar energy, which is incredibly abundant and its implementation is rapidly increasing worldwide. In addition, optoelectronics have also yielded applications in thermophotovoltaics and photodetectors, with research utilizing mechanical flexibility, optical anisotropy, and exploring magnetic semiconductors in advancing the field of optoelectronics. Despite the popularity of lead (Pb) based perovskites as absorbers in optoelectronics, their toxicity and phase instability have moved interest towards Pb-free perovskites. This research performs a preliminary theoretical exploration of CsMnBrxI3-x (x = 0, 1, 2, and 3) to assess their potential as candidate materials for optoelectronic applications. The Kohn-Sham framework of density functional theory (DFT) is used with spin-dependent generalized gradient approximations (SGGA) and Perdew–Burke–Ernzerhof (PBE) functional with linear combination of atomic orbitals (LCAO) basis set. The mixed halide compounds relaxed to lower-symmetry lattice structures. Although all examined compounds are generally soft and ductile, CsMnBrI2 shear is soft. All bandgaps are direct and close to one electron volt, with the exception of the far narrower direct bandgap of CsMnBrI2. The optical properties of the four samples exhibit anisotropy in the perovskites, with CsMnBr2I as the most isotropic, but all exhibit high absorption and low reflectivity.
A systematic first-principles study of the crystal structure, chemical bonding and elastic properties of alkali metal hydroxides MOH (M = Li, Na, K, Rb, Cs) in various polymorphic modifications was performed. Calculations were carried out within the density functional theory using the CRYSTAL code. For the first time, complete elastic constant tensors for all stable phases are reported; uniaxial compression moduli, Young's, shear, and bulk moduli, Poisson ratio, Vickers hardness, Debye temperature, and minimum thermal conductivity were calculated. Correlations between elastic characteristics and crystallochemical parameters are established: strong O···H hydrogen bonds (ICOHP energy > 40 kJ/mol) provide a shear modulus enhancement up to 33 GPa and reduce anisotropy; phases with isolated square (OH)4 rings exhibit an anomalously low shear stiffness (5–15 GPa) and extreme anisotropy. Along the Li→Cs series, bulk modulus, shear modulus, Debye temperature and thermal conductivity systematically decrease, while ductility increases. The equation of state parameters (B0 = 21–52 GPa, B0′ = 2.3–9.2) reveal the compression anisotropy related to the layered structure. Since the experimental data on elastic properties of alkali metal hydroxides are unavailable, the results are predictive in nature and are of interest for materials science and geophysics.
In structural engineering, creating heterostructures of two dimensional (2D) van der Waals materials is an effective way to tune physical properties for potential application in nano-electronics, spin electronics and renewable energy sources. In the present work, we have studied the GeS2 monolayer and Janus MoXY((X ≠ Y) = S, Se, Te) monolayers individually and the corresponding GeS2 and Janus MoXY ((X ≠ Y) = S, Se, Te) van der Waals (vdW) heterostructures by DFT calculations. The optimized band structure, band gap, bond length and lattice constant values for the GeS2 and Janus MoXY monolayers are in excellent agreement with previous published values. The desirable lattice mismatch enables these monolayers to form vdW heterostructures. These vdW heterostructures are stable with particular stacking, based on binding energies, interlayer distances, ab initio molecular dynamics simulations (AIMD) and phonon dispersion. All of the investigated vdW heterostructures exhibit an indirect bandgap character and have a type-II band alignment. Type-II band alignments are critical for light amplification and detection because they prevent electron hole recombination. GeS2 and van der Waals (vdW) heterostructures with Janus MoXY X≠Y (where X,Y= S, Se, Te), the lowest-energy optical transitions primarily governed by excitonic effects are examined to better understand their optical behavior by studying their absorption spectra. Here, these heterostructures exhibit moderate light absorption in the visible and ultraviolet (UV) ranges, suggesting their suitability for photovoltaic, photo-thermoelectric and sensing applications. In order to minimize energy losses in the form of heat, the GeS2-based heterostructures also exhibit excellent thermoelectric performance. At 300 K, GeS2/MoSSe achieves a figure of merit ZT = 0.96 for p-type and 0.98 for n-type. At 800 K, it excels in p-type (ZT = 0.84), while GeS2/MoSTe is optimal for n-type material (ZT = 0.48). Hence, these materials have strong potential for energy conversion and thermoelectric applications.
