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Synthetic two-dimensional (2D) materials without layered bulk allotropes are approaching a new frontier of materials flatland, one with properties richer than those of graphene-like materials. This is the case even as only a few chemical elements and blends have shown synthetic 2D forms. While hydrogen and metals are earth-abundant and form numerous compounds, rarely are 2D materials with only robust metal-hydrogen bonds. Here, a large new family of 2D materials is found from metal hydrides by high-throughput computational search augmented with first-principles calculations. There are 110 thermally and dynamically stable 2D materials that range from metallic materials to wide-gap semiconductors. A subgroup of these materials even varies from topological insulators to nodal-loop semimetals as well as from antiferromagnetic semiconductors to ferromagnetic half-metals. Unexpectedly, these monolayers resemble graphene in an ability to form weak interlayer interaction due to the variable multicenter bonding of hydrogen that eliminates the otherwise prevalent dangling bonds, rather than the covalent bonds between stacked layers as in previously reported synthetic 2D materials. This feature will favor potential experimental synthesis of these metal hydride monolayers.

Graphene-based electric power generation that converts mechanical energy of flow of ionic droplets over the device surface into electricity has emerged as a promising candidate for blue-energy network. Yet the lack of a microscopic understanding of the underlying mechanism has prevented ability to optimize and control the performance of such devices. This requires information on interfacial structure and charging behavior at the molecular level. Here, we use sum-frequency vibrational spectroscopy to study the roles of solvated ions, graphene, surface moiety on substrate and water molecules at the aqueous solution/graphene/polymer interface. We discover that the surface dipole layer of the neutral polymer is responsible for ion attraction toward and adsorption at the graphene surface that leads to electricity generation in graphene. Graphene itself does not attract ions and only acts as a conducting sheet for the induced carrier transport. Replacing the polymer by an organic ferroelectric substrate could allow switching of the electricity generation with long durability. Our microscopic understanding of the electricity generation process paves the way for the rational design of scalable and more efficient droplet-motion-based energy transducer devices.

We report a two-dimensional tetragonal Titanium Carbide (TiC) monolayer sheet with distinguished structure and properties based on comprehensive first-principles calculations. The TiC sheet exhibits a novel zigzag-shaped buckling structure with all atoms being quasiplanar tetracoordinate, as favored by strong in-plane C2p-Ti3d bonding and synergetic out-of-plane electronic delocalization. This unique structure endows the sheet with high kinetic stability and anisotropic mechanical properties. Moreover, the TiC sheet displays orientation-dependent electronic properties derived from its special rectangular symmetry, with indirect band gap of ~0.2 eV and substantial ferromagnetism along its edges, thus promising for wide applications in nanoelectronics.

Cubic boron nitride (c-BN) possesses a number of extreme properties rivaling or surpassing those of diamond. Especially, owing to the high chemical stability, c-BN is desired for fabricating electronic devices that can stand up to harsh environments. However, realization of c-BN-based functional devices is still a challenging task due largely to the subtlety in the preparation of high-quality c-BN films with uniform thickness and controllable properties. Here, we present a simple synthetic strategy by surface fluorination of few-layered hexagonal boron nitride (h-BN) sheets to produce thermodynamically favorable F-terminated c-BN nanofilms with an embedded N-N bond layer and strong inbuilt electric polarization. Due to these specific features, the fluorinated c-BN nanofilms have controllable band gap by thickness or inbuilt and applied electric fields. Especially, the produced nanofilms can be tuned into substantial ferromagnetism through electron doping within a reasonable level. The electron-doping-induced deformation ratio of the c-BN nanofilms is found to be 1 order of magnitude higher than those of carbon nanotubes and graphene. At sufficient high doping levels, the nanofilm can be cleaved peculiarly along the N-N bond layer into diamond-like BN films. As the proposed synthesis strategy of the fluorinated c-BN nanofilms is well within the reach of current technologies, our results represent an extremely cost-effective approach for producing high-quality c-BN nanofilms with tunable electronic, magnetic, and electromechanical properties for versatile applications.

We find through first-principles calculations that fluorine atoms topologically adsorbed on boron nitride nanotubes induce long-ranged ferromagnetic spin ordering along the tube, offering strong spin polarization around the Fermi level. The spin polarization and magnetic moment increase significantly with decreasing tube radius, even giving rise to half-metal when the tube diameter is reduced to 3.3 A, while in a flat boron nitride sheet with the same topological fluorine arrangement the magnetic moment nearly disappears. This radius-dependent behavior is then developed into a local curvature modulation procedure to efficiently enhance or quench the ferromagnetic ordering, which enables the F-BNNTs to function as piezomagnetic nanotubes. These findings suggest a new route to facilitate the design of tunable spin devices.