We review layered and ultrathin electrides with exciting properties like high electrical mobility, high carrier concentrations, and low work functions.

The ability to alter distances between atoms is among the most important tools in materials design. Despite this importance, controlling the interlayer distance in stacks of 2D materials remains a challenge. Here we show from firstprinciples that stacking electrenesa new class of electrondonating 2D materialswith other 2D materials provides this control. The resulting donor−acceptor heterostructures have interlayer distances 1 Å less than van der Waals layered materials but 1 Å more than covalent or ionic bonds. This yields a class of quasi-bonds that exhibit characteristics of both ordinary chemical bonds and van der Waals interactions. We show how quasi-bonds have tunable polarities and strengths and that these bonds can be understood by drawing on familiar concepts from molecular orbital theory. We also demonstrate several useful properties of 2D donor−acceptor heterostructures, including superlubricity, ultralow work functions, and greatly improved voltages for lithium-ion batteries

Despite the interest in MXenes in the last decade, all of the MXenes reported have a random mixture of surface terminations (-O, -OH, -F). In addition, restacked 3D films have turbostratic disorder and often contain ions, solvent, and other species in between their layers. Here we report Y 2 CF 2 , a layered crystal with a unit cell isostructural to a MXene, in which layers are capped only by fluoride anions. We directly synthesize the 3D crystal through a high-temperature solid-state reaction, which affords the 3D crystal in high yield and purity and ensures that only fluoride ions terminate the layers.We characterize the crystal structure and electronic properties using a combination of experimental and computational techniques. We find that relatively strong electrostatic 1 arXiv:1909.01490v1 [cond-mat.mtrl-sci] 3 Sep 2019 interactions bind the layers together into a 3D crystal and that the lack of orbital overlap between layers gives rise to a description of Y 2 CF 2 as slabs of MXene-like sheets electrically insulated from one another. Therefore, we consider Y 2 CF 2 as a pure 3D crystalline stack of MXene-like sheets. In addition, Y 2 CF 2 is the first transition metal carbide fluoride experimentally synthesized. We hope this work inspires further exploration of transition metal carbide fluorides, which are potentially a large and useful class of compositions.

Electrides are exotic materials that typically have electrons present in well-defined lattice sites rather than within atoms. Although all known electrides have an electropositive metal cation adjacent to the electride site, the effect of cation electronegativity on the properties of electrides is not yet known. Here, we examine trivalent metal carbides with varying degrees of electronegativity and experimentally synthesize Sc2C. Our studies identify the material as a two-dimensional (2D) electride, even though Sc is more electronegative than any metal previously found adjacent to an electride site. Further, by exploring Sc2C and Al2C computationally, we find that higher electronegativity of the cation drives greater hybridization between metal and electride orbitals, which opens a band gap in these materials. Sc2C is the first 2D electride semiconductor, and we propose a design rule that cation electronegativity drives the change in its band structure.

2,2'-Bipyridine-terminated poly(dimethylsiloxane)s (bpyPDMS) with number average molecular weights, M, of 3300, 6100, 26 200, and 50 000 g mol were synthesized. When mixed with Fe(BF) at low concentrations, red solutions formed with UV-vis spectra that match those of iron(ii) tris(2,2'-bipyridine) (Fe(bpy)). Upon solvent evaporation, Fe(bpy) crosslinked PDMS networks (bpyPDMS/Fe(ii)) formed, and were studied using oscillating shear rheometry. The shear storage moduli (0.084 to 2.6 MPa) were found to be inversely proportional to the M of the PDMS, though the storage moduli at low molecular weights greatly exceeded the storage moduli of comparable covalently crosslinked PDMS networks. The shear storage moduli exhibited the characteristic rubbery plateau up to ∼135 °C. Films of bpyPDMS/Fe(ii) coated onto electrodes were found to be electrochemically active, especially so when the PDMS M is low. The Fe(bpy) crosslinks can be reversibly oxidized over ∼500 nm away from the electrode surface in the presence of a suitable electrolyte.