Thermoelectric materials can be used to convert waste heat into electrical energy, which is considered to be a cleaner form of energy that reduces carbon dioxide and greenhouse gas emissions. In this work, we study the thermoelectric properties of penta-PdX₂ (X = S, Se, Te) using first-principles calculations together with Boltzmann transport theory. The dynamic and thermal stability of penta-PdX₂ is confirmed based on phonon dispersion and ab initio molecular dynamics (AIMD) simulations. The transport coefficients, such as thermal conductivity (k_l), power factor (PF) and thermoelectric figure of merit (ZT), exhibit strong anisotropy in both x and y directions. The lower thermal conductivity of penta-PdX₂ can be attributed to the lower group velocity and larger phase space. The thermal conductivity of penta-PdX₂ along the same direction (x or y direction) adheres to the trend of PdS₂ > PdSe₂ > PdTe₂. The anisotropy of PF and k_l of penta-PdX₂ monolayers inevitably leads to anisotropy of ZT. The largest ZT values of penta-PdX₂ (X = S, Se) for p-type are 0.85 and 1.18, respectively, while the maximum ZT value of penta-PdTe₂ reaches 2.42. The predicted ZT values of penta-PdX₂ are larger than those of the commercial TE material Bi₂Te₃ (about 0.8) and some other transition metal di-chalcogenides, indicating that penta-PdX₂ (X = S, Se, Te) monolayers are potential anisotropic thermoelectric materials.

Planar borophene, the truly 2D monolayer boron, has been independently successfully grown on Ag(1 1 1) by two groups (2016 Nat. Chem . 8 563 and 2015 Science 350 1513), which has received widespreading attentions. The superconducting property has not been unambiguously observed, which is unexpected because light element boron should have strong electron–phonon coupling. To resolve this puzzle, we show that the superconducting transition temperature T _c of β ₁₂ borophene is effectively suppressed by the substrate-induced tensile strain and electron doping via first principles calculations. The biaxial tensile strain of 2% induced by Ag(1 1 1) significantly reduces T _c from 14 K to 2.95 K; electron doping of 0.1 e ⁻ per boron atom further shrinks T _c to 0.09 K. We also predict that the superconducting transition temperature in β ₁₂ can be enhanced to 22.82 K with proper compressive strain (−1%) and 18.97 K with hole doping (0.1 h ⁺ per boron). Further studies indicate that the variation of T _c is closely related to the density of states of σ bands near the Fermi surface. Our results help to explain the challenges to experimentally probe superconductivity in substrate-supported borophene.

Single-layer transition-metal dichalcogenides (TMDs) such as MoS₂ and MoSe₂ exhibit unique electronic band structures ideal for hosting many exotic spin-orbital orderings. It has been widely accepted that Rashba spin splitting (RSS) is linearly proportional to the external field in heterostructure interfaces or to the potential gradient in polar materials. Surprisingly, an extraordinary nonlinear dependence of RSS is found in semiconducting TMD monolayers under a gate field. In contrast to small and constant RSS in polar materials, the potential gradient in non-polar TMDs gradually increases with the gate bias, resulting in nonlinear RSS with a Rashba coefficient an order-of-magnitude larger than the linear one. Most strikingly, under a large gate field MoSe₂ demonstrates the largest anisotropic spin splitting among all known semiconductors to our knowledge. Based on the k·p model via symmetry analysis, we identify that the third-order contributions are responsible for the large nonlinear Rashba splitting. The gate tunable spin splitting found in semiconducting pristine TMD monolayers promises future spintronics applications in that spin polarized electrons can be generated by external gating in an experimentally accessible way.

The Car-Parrinello molecular dynamics simulations were employed to investigate thermal decomposition of the solid nitromethane. It is found that it undergoes chemical decomposition at about 2200 K under ambient pressure. The initiation of reactions involves both proton transfer and commonly known C–N bond cleavage. About 75 species and 100 elementary reactions were observed with the final products being ${\mathrm{H}}_2\mathrm{O}$, ${\mathrm{CO}}_2$, ${\mathrm{N}}_2$, and CNCNC. It represents the first complete simulation of solid-phase explosive reactions reported to date, which is of far-reaching implication for design and development of new energetic materials.