Poly(triazine imide) (PTI), a crystalline g-C₃N₄, hosting two-dimensional nanoporous structure with an electron density gap of 0.34 nm, is highly promising for high-temperature hydrogen sieving because of its high chemical and thermal robustness. Currently, layered PTI is synthesized in potentially unsafe vacuum ampules in milligram quantities. Here, we demonstrate a scalable and safe ambient pressure synthesis route leading to several grams of layered PTI platelets in a single batch with 70% yield with respect to the precursor. Solvent exfoliation under anhydrous conditions led to single-layer PTI nanosheets evidenced by the observation of triangular g-C₃N₄ nanopores. Gas permeation studies confirm that PTI nanopores can sieve He and H₂ from larger molecules. Last, high-temperature H₂ sieving from PTI nanosheet–based membranes, prepared by the scalable filter coating technique, is demonstrated with H₂ permeance reaching 1500 gas permeation units, with H₂/CO₂, H₂/N₂, and H₂/CH₄ selectivities reaching 10, 50, and 60, respectively, at 250°C.

Thermal transport is a key feature for the operation of phase change memory devices which rest on a fast and reversible transformation between the crystalline and amorphous phases of chalcogenide alloys upon Joule heating. In this paper we report on the ab initio calculations of bulk thermal conductivity of the prototypical phase change compounds ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ and GeTe in their crystalline form. The related ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ compound is also investigated for the sake of comparison. Thermal conductivity is obtained from the solution of the Boltzmann transport equation with phonon scattering rates computed within density functional perturbation theory. The calculations show that the large spread in the experimental data on the lattice thermal conductivity of GeTe is due to a variable content of Ge vacancies which at concentrations realized experimentally can halve the bulk thermal conductivity with respect to the ideal crystal. We show that the very low thermal conductivity of hexagonal ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ of about 0.45 ${\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ measured experimentally is also resulting from disorder in the form of a random distribution of Ge/Sb atoms in one sublattice.

Two-dimensional (2D) materials are among the most promising candidates for beyond-silicon electronic, optoelectronic, and quantum computing applications. Recently, their recognized importance sparked a push to discover and characterize novel 2D materials. Within a few years, the number of experimentally exfoliated or synthesized 2D materials went from a few to more than a hundred, with the number of theoretically predicted compounds reaching a few thousand. In 2018 we first contributed to this effort with the identification of 1825 compounds that are either easily (1036) or potentially (789) exfoliable from experimentally known 3D compounds. Here, we report on a major expansion of this 2D portfolio thanks to the extension of the screening protocol to an additional experimental database (MPDS) as well as the updated versions of the two databases (ICSD and COD) used in our previous work. This expansion leads to the discovery of an additional 1252 monolayers, bringing the total to 3077 compounds and, notably, almost doubling the number of easily exfoliable materials to 2004. We optimize the structural properties of all these monolayers and explore their electronic structure with a particular emphasis on those rare large-bandgap 2D materials that could be precious in isolating 2D field-effect-transistor channels. Finally, for each material containing up to 6 atoms per unit cell, we identify the best candidates to form commensurate heterostructures, balancing requirements on supercell size and minimal strain.

A prominent surface acoustic wave dominates the vibrational dynamics of Bi ₂ Te ₃ (111). Theoretical calculations reveal the crucial role of vdW interactions.