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.

Diodes are a basic component of electrical circuits to control the flow of charge, and geometric diodes (GDs) are a special class that can operate using ballistic or quasi-ballistic transport in conjunction with geometric asymmetry to direct charge carriers preferentially in one direction, enabling an electron ratcheting effect. Nanomaterials present a unique platform for the development of GDs, and silicon nanowire (NW)-based GDs�cylindrically symmetric but translationally asymmetric three-dimensional nanostructures�have recently been demonstrated functioning at room temperature. These devices can theoretically achieve a near zero-bias turn-on voltage and rectify up to THz frequencies. Here, we synthesize silicon NW GDs and fabricate single-NW devices from which significant changes in diode performance are observed from relatively minor changes in geometry. To elucidate the interplay between geometry and ballistic behavior, we develop a Monte Carlo simulation that describes the quasi-ballistic behavior of electrons within a three-dimensional NW GD. We examine the effects of doping level, temperature, and geometry on charge carrier transport, revealing the relationships between charge carrier mean free path (MFP), specular reflection at surfaces, and geometry on GD performance. As expected, geometry strongly influences performance by directing or blocking charge carrier passage through the nanostructure. Interestingly, we find that the blocking effect is at least as important as the directing effect. Moreover, within certain geometric limits, the diode behavior is less sensitive to the MFP than might be initially expected because of the short relevant length scales and importance of the blocking effect. The results provide guidelines for the future design of NW GDs and enable the prediction and interpretation of trends in experimental results. An improved understanding of quasi-ballistic transport is crucial to guiding future experiments toward realizing THz rectification for applications in high-speed data transfer and long-wavelength energy harvesting.

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

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.