A structure prediction method for layered materials based on two-dimensional (2D) particle swarm optimization algorithm is developed. The relaxation of atoms in the perpendicular direction within a given range is allowed. Additional techniques including structural similarity determination, symmetry constraint enforcement, and discretization of structure constructions based on space gridding are implemented and demonstrated to significantly improve the global structural search efficiency. Our method is successful in predicting the structures of known 2D materials, including single layer and multi-layer graphene, 2D boron nitride (BN) compounds, and some quasi-2D group 6 metals(VIB) chalcogenides. Furthermore, by use of this method, we predict a new family of monolayered boron nitride structures with different chemical compositions. The first-principles electronic structure calculations reveal that the band gap of these N-rich BN systems can be tuned from 5.40 eV to 2.20 eV by adjusting the composition.

The structure prediction at the atomic level is emerging as a state-of-the-art approach to accelerate the functionality-driven discovery of materials. By combining the global swarm optimization algorithm with first-principles thermodynamic calculations, it exploits the power of current supercomputer architectures to robustly predict the ground state and metastable structures of materials with only the given knowledge of chemical composition. In this Review, we provide an overview of the basic theory and main features of our as-developed CALYPSO structure prediction method, as well as its versatile applications to design of a broad range of materials including those of three-dimensional bulks, two-dimensional reconstructed surfaces and layers, and isolated clusters/nanoparticles or molecules with a variety of functional properties. The current challenges faced by structure prediction for materials discovery and future developments of CALYPSO to overcome them are also discussed.

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Atomically thin diamond, also called diamane, is a twodimensional carbon allotrope and has attracted considerable scientific interest because of its potential physical properties. However, the successful synthesis of a pristine diamane has up until now not been achieved. We demonstrate the realization of a pristine diamane through diamondization of mechanically exfoliated few-layer graphene via compression. Resistance, optical absorption, and X-ray diffraction measurements reveal that hexagonal diamane (h-diamane) with a bandgap of 2.8 ± 0.3 eV forms by compressing trilayer and thicker graphene to above 20 GPa at room temperature and can be preserved upon decompression to ∼1.0 GPa. Theoretical calculations indicate that a (−2110)-oriented h-diamane is energetically stable and has a lower enthalpy than its few-layer graphene precursor above the transition pressure. Compared to gapless graphene, semiconducting h-diamane offers exciting possibilities for carbon-based electronic devices.

ABSTRACT:The polymeric phase of nitrogen connected by the lower (than three) order N-N bonds has been long sought after for the potential application as high energy density materials. Here we report a hitherto unknown polymeric N 2 H phase discovered in the high-pressure hydronitrogen system by first-principle structure search method based on particle swarm optimization algorithm. This polymeric hydronitrogen consists of quasi-one-dimensional infinite armchair-like polymeric N chains, where H atoms bond with two adjacent N located at one side of armchair edge. It is energetically stable against decomposition above ~33 GPa, and shows novel metallic feature as the result of pressure-enhanced charge transfer and delocalization of π electrons within the infinite nitrogen chains. The high energy density (~4.40 KJ/g), high nitrogen content (96.6%), as well as relatively low stabilization pressure, make it a possible candidate for high energy density applications. It also has lattice dynamical stability down to the ambient pressure, allowing for the possibility of kinetic stability with respect to variations of external conditions. Experimental synthesis of this novel phase is called for.