Ternary pentagonal BXN (X = C, Si, Ge, and Sn) sheets with high piezoelectricity.

The discovery of new and stable two-dimensional pentagonal materials with piezoelectric properties is essential for technological advancement. Inspired by recently reported piezoelectric materials penta-BCN and penta-BSiN, we proposed penta-BGeN and penta-BSnN as new members of the penta-family based on first-principles calculations. Comprehensive analyses indicated that both penta-BGeN and penta-BSnN are thermodynamically, dynamically, mechanically, and thermally stable. In terms of mechanical stability, the elastic constant decreased as lower elements in group 4A of the periodic table were used. Therefore, penta-BGeN and penta-BSnN are softer than penta-BCN and penta-BSiN. In terms of piezoelectric properties, piezoelectric stress and strain tensors increase following the same pattern. In group 4A, penta-BSnN had the highest intrinsic piezoelectricity, especially the e 22 piezoelectric stress. Typically, the piezoelectric strain d ij coefficient increases with material softness; penta-BSnN possessed the highest d ij . Thus, due to its inherent piezoelectricity, penta-BSnN has tremendous potential as a nanoscale piezoelectric material.

Theoretically proposed stable polymorph of two-dimensional pentagonal ß-PdPSe.

The theoretical discovery of new and stable 2D penta materials based on first-principles calculations has stimulated technological advances due to the anticipated exotic properties of such structures, which include the α and ß phases of penta-NiPS. Inspired by the similarity between the theoretically proposed penta-NiPS and the experimentally synthesized (α phase) of penta-PdPSe, we propose herein the ß phase of penta-PdPSe as a new penta-2D material. Comprehensive analyses indicated that ß phase penta-PdPSe is thermodynamically, dynamically, mechanically, and thermally stable, similar to its NiPS analogue. It was found that ß penta-PdPSe is a wide band gap semiconductor with an indirect band gap of 1.58 eV, significantly lower than 2.15 eV for the α phase. Moreover, the two polymorphs of penta-PdPSe are soft materials with 2D Young's modului of Ea = 151 N m-1 and Eb = 123 N m-1 for the ß phase, compared with Ea = 155 N m-1 and Eb = 113 N m-1 for the α phase. The calculated absorption coefficient showed that ß phase penta-PdPSe is acceptable for electronic and optical nanodevices.

Doping Limits of Phosphorus, Arsenic, and Antimony in CdTe.

Low p-type doping is a limiting factor to increase CdTe thin-film solar-cell efficiency toward the theoretical Shockley-Queisser limit of 33%. Previous calculations predict relatively high ionization energies for group-V acceptors (P, As, and Sb), and they are plagued by self-compensation, forming AX centers, severely limiting hole concentration. However, recent experiments on CdTe single crystals indicate a much more favorable scenario, where P, As, and Sb behave as shallow acceptors. Using hybrid functional calculations, we solve this puzzle by showing that the ionization energies significantly decrease with the supercell size. When including the effects of spin-orbit coupling and extrapolating the results to the dilute limit, we find these impurities behave as hydrogenic-like shallow acceptors, and AX centers are unstable and do not limit p-type doping. We address the differences between our results and previous theoretical predictions and show that our ionization energies predict hole concentrations that agree with recent temperature-dependent Hall measurements.