In this study, using the density functional theory, the mechanical properties of methylammonium lead halide perovskites (CH 3 NH 3 PbX 3 , X = I, Br, Cl) were investigated. Young's modulus, bulk modulus, and shear modulus, Poisson's ratio, and many other parameters were calculated using the PBEsol and vdW approximations. Also, in this work, utilizing a new accuracy in calculating the elastic constants, the intense conflict between the previous theoretical results and the experimental data were fixed. Moreover, for the first time, through combination of the PBEsol and vdW methods, the effect of the interaction between methylammonium and PbX 3 scaffold on the mechanical properties of lead halide perovskites was well cleared. In continuation, using the PBEsol+vdW method, a phase transition appeared for the MAPbBr 3 and MAPbCl 3 structures, which proved more stability of MAPbBr 3 and MAPbCl 3 in comparison with MAPbI 3 . In what follows, by studying these materials under an applied strain beyond the harmonic region, the transition zone to the plastic area in the strain region of 5.5% and smaller was identified, and the small values of the aforementioned applied strains were found to be the reason for the instability of these materials at room temperature and above.
Optical absorption and reflectance measurements are performed to evaluate compositional and temperature dependences of band gap energies of Zn1−xCdxO alloys grown by molecular-beam epitaxy. The compositional dependence of the band gap energy, determined by taking into account excitonic contributions, is shown to follow the trend Eg(x)=3.37−2.82x+0.95x2. Incorporation of Cd was also shown to somewhat slow down thermal variation of the band gap energies, beneficial for future device applications.
Strong suppression of potential fluctuations in the band edges of GaNAs alloys due to postgrowth hydrogen treatment, which is accompanied by a reopening of the alloy band gap, is revealed from temperature-dependent photoluminescence (PL) and PL excitation measurements. The effect likely indicates preferential trapping of hydrogen near the lattice sites with the highest nitrogen content. A remarkable improvement in the radiative efficiency of the alloys at room temperature is also demonstrated and is ascribed to efficient hydrogen passivation of competing nonradiative centers. (C) 2003 American Institute of Physics
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