We have measured the room temperature magnetization in Al-substituted magnesium oxide, Mg(Al)O nanoparticles with Al fractions of up to 5 at. %. All samples, including undoped MgO nanoparticles, exhibit weak room temperature ferromagnetism, with the saturation magnetization reaching a maximum of 0.023 emu/g at 2 at. % of Al. X-ray photoelectron spectroscopy identifies the presence of oxygen vacancies in both doped and undoped MgO nanoparticles, with the vacancy concentration increasing upon vacuum annealing of Mg(Al)O, resulting in two-fold enhancement of the saturation magnetization for 2 at. % Al-doped MgO. Our results suggest that the oxygen vacancies are largely responsible for the weak room temperature ferromagnetism in MgO.

Transition-metal dichalcogenides (TMDs) such as ${\mathrm{MoS}}_2$, ${\mathrm{MoSe}}_2$, and ${\mathrm{WSe}}_2$ have emerged as promising two-dimensional semiconductors. Many anticipated applications of these materials require both $p$-type and $n$-type TMDs with long-term doping stability. Here, we report on the synthesis of substitutionally doped ${\mathrm{WSe}}_2$ crystals using Nb and Re as $p$- and $n$-type dopants, respectively. Hall coefficient and gate-dependent transport measurements reveal drastically different doping properties between nominally 0.5% Nb- and 0.5% Re-doped ${\mathrm{WSe}}_2$. While 0.5% Nb-doped ${\mathrm{WSe}}_2$ (${\mathrm{WSe}}_2:\mathrm{Nb}$) is degenerately hole doped with a nearly temperature-independent carrier density of approximately ${10}^{19}{\mathrm{cm}}^{-3}$, electrons in 0.5% Re-doped ${\mathrm{WSe}}_2$ (${\mathrm{WSe}}_2:\mathrm{Re}$) are largely trapped in localized states below the mobility edge and exhibit thermally activated behavior. Charge transport in both ${\mathrm{WSe}}_2:\mathrm{Nb}$ and ${\mathrm{WSe}}_2:\mathrm{Re}$ is found to be limited by Coulomb scattering from ionized impurities. Furthermore, we fabricate vertical van der Waals–junction diodes consisting of multilayers of ${\mathrm{WSe}}_2:\mathrm{Nb}$ and ${\mathrm{WSe}}_2:\mathrm{Re}$. Finally, we demonstrate reverse rectifying behavior as a direct proof of band-to-band tunneling in our ${\mathrm{WSe}}_2:\mathrm{Nb}/{\mathrm{WSe}}_2:\mathrm{Re}$ diodes.

Ca₁₂Al₁₄O₃₃ (C12A7 or Mayenite) is a material whose caged clathrate structure and occluded anionic species leads to significant functionality. The creation of occluded anionic vacancies leads to the injection of localized electrons at the center of the cage, converting the wide band gap insulator to a semi- or metallic conducting material. The conversion to the electride historically requires the synthesis of oxy-C12A7, consolidation, and then reduction to introduce anionic vacancies. This report develops and characterizes an electride formation procedure from three starting points: unconsolidated oxy-C12A7, heterogenous solid-state reactants (CaCO₃ and Al₂O₃), and homogenous non-carbonaceous polymer assisted sol-gel reactants. Electride-C12A7 formation is observed in a vacuum furnace where the reactants are in direct contact with a carbon source. Process time and temperature-dependent structural characterization provides insight into the source of high temperature C12A7 stability, the mechanism of anionic vacancy formation, and the magnitude of ultimate conductivity that cannot be explained by current reduction theories. A new theory is presented where mixed O- and C-occupied cages lead to high temperature stability, oxidation of C species creates anionic vacancies, and an equilibrium between the reducing power of the electride-C12A7 and of the C species leads to the ultimate conductivity achieved by the process. This represents a shift in understanding of the carbonaceous reduction process and the first report of high purity electride-C12A7 formation from heterogenous solid-state reactants and homogenous non-carbonaceous polymer assisted sol-gel reactants.

A room temperature amorphous ferromagnetic oxide semiconductor can substantially reduce the cost and complexity associated with utilizing crystalline materials for spintronic devices. We report a new material (Fe_0.66Dy_0.24Tb_0.1)₃O_7-x (FDTO), which shows semiconducting behavior with reasonable electrical conductivity (~500 mOhm-cm), an optical band-gap (2.4 eV), and a large enough magnetic moment (~200 emu/cc), all of which can be tuned by varying the oxygen content during deposition. Magnetoelectric devices were made by integrating ultrathin FDTO with multiferroic BiFeO₃. A strong enhancement in the magnetic coercive field of FDTO grown on BiFeO₃ validated a large exchange coupling between them. Additionally, FDTO served as an excellent top electrode for ferroelectric switching in BiFeO₃ with no sign of degradation after ~10¹⁰ switching cycles. RT magneto-electric coupling was demonstrated by modulating the resistance states of spin-valve structures using electric fields.