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.

Plasma electrolysis, where a solid electrode in an electrolytic cell is replaced by a plasma (or gas discharge), differs from conventional electrolysis by not being dictated by the surface characteristics of an electrode, but by the chemical species injected into the solution from the plasma. Reduction in a plasma cathode configuration occurs mostly by plasma-injected solvated electrons (e − aq ), which may engage in side reactions, such as the second order recombination of e − aq , that ultimately reduce the faradaic efficiency for the production of a desired product. In this work, we show that the depletion of reactants at the plasma-liquid interface due to insufficient transport can reduce the predicted faradaic efficiency for a plasma cathode at low concentrations. Measurements of the faradaic efficiency using the dissociative electron attachment to chloroacetate and the ferri/ferrocyanide redox couple confirm this behavior. The effect of other mechanisms on the faradaic efficiency, such as competing oxidation reactions with the hydroxyl radical, are also evaluated and found to be far less significant. Unlike conventional electrolysis, stirring the solution does not increase the faradaic efficiency, but increasing the species concentration does.

Although order is a defining characteristic of crystals, disordered structuresespecially near interfacesoften govern a material’s performance. For example, the interfaces in batteries, coatings, or catalysts exemplify systems in which disorder plays a critical role. Despite this importance, characterization of local structure in disordered materials remains a challenge. To solve this challenge, the electron pair distribution function (ePDF) method has given insight into the local structure of many complex samples because the technique can be applied to disordered materials containing as few as tens or hundreds of atoms. The ePDF method takes a transmission electron microscope (TEM) diffraction pattern and transforms this into information about the bond lengths present in a material. This information can then be used to create a 3D, atomic-scale model of local chemical structure. In this review, we introduce the theory behind ePDF, describe common methods for data acquisition, and highlight recent advances in the ePDF technique, including its combination with cryo-electron microscopy, ultrafast TEM, and precession electron diffraction. We also show how ePDF has been applied to important classes of materials, including 2D heterostructures, nanoscale interfaces, and materials fabricated by atomic layer deposition. Finally, we review how ePDF can be combined with other techniques to provide a comprehensive understanding of a material. Because ePDF can be performed on most TEMs, it is beginning to emerge as a routine method for the analysis of complex, disordered structures in technologically important materials.

Electrides are exotic materials that have electrons present in well-defined lattice sites. The existence of Y2C and Gd2C as 2D electrides inspired us to examine other trivalent metal carbides, including Sc2C and Al2C. It has been proposed that design rules for electride materials include the need for an electropositive cation adjacent to the electride site, but the effect of cation electronegativity on electronic structure in electride materials is not yet known. Here, we examine trivalent metal carbides with varying degrees of electronegativity and experimentally synthesize a 2D electride, Sc2C, containing the most electronegative metal yet found neighboring the electride site. Further, we find that higher electronegativity of the cation drives greater hybridization between metal and electride orbitals. Our calculations predict that Sc2C is a small band gap semiconductor with a band gap of 0.305 eV, with an experimental conductivity of 1.62 S/cm at room temperature. This is the first 2D electride material to exhibit semiconducting behavior, and we propose that electronegativity of the cation drives the change in band structure. Introduction:Challenges in energy storage, electronics, and catalysis motivate the search for exotic materials with extreme properties, and electrides-crystals with bare electrons trapped at stoichiometric concentrations 1-3 -offer some of the most exceptional. These electrons have been ejected from atomic orbitals to reside in vacant lattice sites and, because they are so weakly bound, are better electron donors than alkali metals [4][5][6] , can offer electrical conductivity that rivals silver, and can catalyze challenging reactions. These properties have led to the exploration of electrides in applications where electron-rich materials are needed: N2 and CO2 reduction 7,8 , battery electrodes 9,10 , and electron emitters [11][12][13] . Despite this progress, rules that might predict an electride's properties based on its structure or composition are underdeveloped. For example, unlike in conventional materials, it is unknown how to tune the band gap of semiconducting

The wave nature of electrons makes the quantification of charge fundamentally challenging. In complex materials like electrides, this challenge is amplified by the small charge and complex shape of electride wavefunctions. For these reasons, popular integration methods such as the Bader method usually fail to assign any charge to the electron in an electride. To address this challenge, we advance an algorithm that, like the Bader method, assigns charge to an atom through the creation of a dividing surface. Unlike Bader, the dividing surface is defined not by a minimum in charge density but by a minimum in the electron localization function (ELF). We apply this method, “BadELF”, to the quantification of oxidation state in both ionic compounds and electrides. For ionic compounds, we observe that Bader and BadELF perform similarly; on electrides only BadELF yields chemically meaningful charges. We conclude that the BadELF method provides a useful strategy to identify electrides and obtain new chemical insight about their most essential property: the quantity of electrons within them.