Deciphering the crystal structure evolution from 3D non-van der Waals solids to 2D nanosheets

Abstract

Two-dimensional (2D) materials research has predominantly focused on quantum-confinement effects like energy discretisation and momentum-space broadening, regardless of whether they are derived from layered, van der Waals (vdW) materials or 3D-bonded, non-vdW solids. Fragmentation of bulk non-vdW materials to realise 2D nanosheets perturbs the interatomic forces and the atomic coordination, inducing intrinsic strain in the resulting 2D nanosheets. The subsequent strain-relaxation can lead to distinct atomic arrangements within the nanosheets, altering their macroscopic properties compared to the bulk counterpart. This work investigates the largely unexplored impact of fragmentation-induced strain on the crystal structure of free-standing 2D nanosheets derived from non-vdW solids, using orthorhombic sulfur (cyclo-S₈) as a model system. Using first-principles density functional theory (DFT) calculations, we predict two 2D allotropes of sulfur (designated “sulfurene”); a metastable α-sulfurene (α-S) phase, with a three-atom-layer 1T-MoS₂-like structure and a stable tetragonal β-sulfurene (β-S) phase. The allotropes are further experimentally realised via shear-assisted fracturing of elemental, bulk sulfur (orthorhombic cyclo-S₈) in an aqueous medium. Our findings demonstrate that fragmentation-induced structural modifications dominate the macroscopic characteristics of low-dimensional systems derived from non-vdW solids, opening avenues for discovering unprecedented optoelectronic functionalities.