Tianshan Zhao

Robust ferromagnetism in monolayer chromium nitride.

Design and synthesis of two-dimensional (2D) materials with robust ferromagnetism and biocompatibility is highly desirable due to their potential applications in spintronics and biodevices. However, the hotly pursued 2D sheets including pristine graphene, monolayer BN, and layered transition metal dichalcogenides are nonmagnetic or weakly magnetic. Using biomimetic particle swarm optimization (PSO) technique combined with ab initio calculations we predict the existence of a 2D structure, a monolayer of rocksalt-structured CrN (100) surface, which is both ferromagnetic and biocompatible. Its dynamic, thermal and magnetic stabilities are confirmed by carrying out a variety of state-of-the-art theoretical calculations. Analyses of its band structure and density of states reveal that this material is half-metallic, and the origin of the ferromagnetism is due to p-d exchange interaction between the Cr and N atoms. We demonstrate that the displayed ferromagnetism is robust against thermal and mechanical perturbations. The corresponding Curie temperature is about 675 K which is higher than that of most previously studied 2D monolayers.

All-metal clusters that mimic the chemistry of halogens.

Owing to their s(2)p(5) electronic configuration, halogen atoms are highly electronegative and constitute the anionic components of salts. Whereas clusters that contain no halogen atoms, such as AlH(4), mimic the chemistry of halogens and readily form salts (e.g., Na(+)(AlH(4))(-)), clusters that are solely composed of metal atoms and yet behave in the same manner as a halogen are rare. Because coinage-metal atoms (Cu, Ag, and Au) only have one valence electron in their outermost electronic shell, as in H, we examined the possibility that, on interacting with Al, in particular as AlX(4) (X=Cu, Ag, Au), these metal atoms may exhibit halogen-like properties. By using density functional theory, we show that AlAu(4) not only mimics the chemistry of halogens, but also, with a vertical detachment energy (VDE) of 3.98 eV in its anionic form, is a superhalogen. Similarly, analogous to XHX superhalogens (X=F, Cl, Br), XAuX species with VDEs of 4.65, 4.50, and 4.34 eV in their anionic form, respectively, also form superhalogens. In addition, Au can also form hyperhalogens, a recently discovered species that show electron affinities (EAs) that are even higher than those of their corresponding superhalogen building blocks. For example, the VDEs of M(AlAu(4))(2)(-) (M=Na and K) and anionic (FAuF)Au(FAuF) range from 4.06 to 5.70 eV. Au-based superhalogen anions, such as AlAu(4)(-) and AuF(2)(-), have the additional advantage that they exhibit wider optical absorption ranges than their H-based analogues, AlH(4)(-) and HF(2)(-). Because of the catalytic properties and the biocompatibility of Au, Au-based superhalogens may be multifunctional. However, similar studies that were carried out for Cu and Ag atoms have shown that, unlike AlAu(4), AlX(4) (X=Cu, Ag) clusters are not superhalogens, a property that can be attributed to the large EA of the Au atom.

Colossal Stability of Gas-Phase Trianions: Super-Pnictogens

Multiply charged negative ions are ubiquitous in nature. They are stable as crystals because of charge compensating cations; while in solutions, solvent molecules protect them. However, they are rarely stable in the gas phase because of strong electrostatic repulsion between the extra electrons. Therefore, understanding their stability without the influence of the environment has been of great interest to scientists for decades. While much of the past work has focused on dianions, work on triply charged negative ions is sparse and the search for the smallest trianion that is stable against spontaneous electron emission or fragmentation continues. Stability of BeB11 (X)123- (X=CN, SCN, BO) trianions is demonstrated in the gas phase, with BeB11 (CN)123- exhibiting colossal stability against electron emission by 2.65 eV and against its neutral adduct by 15.85 eV. The unusual stability of these trianions opens the door to a new class of super-pnictogens with potential applications in aluminum-ion batteries.