Effect of a Stoichiometric Trefoil-like Rotational Reconstruction on the Electronic and Optical Properties of WS2 and WSe2 Monolayers

Structural defects provide an effective means of tailoring the electronic and optical properties of two-dimensional transition-metal dichalcogenides. In this work, the structural, electronic, and polarization-resolved optical properties of pristine WS2 and WSe2 monolayers and their stoichiometric trefoil-like rotational reconstructions were investigated using density functional theory. The reconstruction was generated through a local 60° bond rotation without removing or adding atoms and therefore differs fundamentally from the experimentally observed chalcogen-deficient T1(3DV) defect. Electronic-structure calculations were performed using the PBE-GGA + U and SOGGA approaches, with noncollinear spin–orbit coupling included in the latter, while the optical response was evaluated within the independent-particle Kubo-Greenwood formalism. Structural relaxation revealed two distinct regimes. In WS2, the reconstruction produced a largely compensated redistribution of W–S bond lengths, with the mean distance remaining nearly unchanged, whereas WSe2 exhibited a net expansion of the W–Se coordination network and substantially stronger out-of-plane buckling. The reconstruction introduced additional W 5d–chalcogen p states near the band edges and markedly reduced the electronic band gaps. For WS2, the gap decreased from 1.87 to 1.55 eV within DFT + U and from 1.96 to 1.46 eV within SOGGA. A substantially stronger response was obtained for WSe2, for which the corresponding gaps decreased from 1.533 to 0.751 eV and from 1.657 to 0.645 eV. The reconstructed monolayers exhibited pronounced optical anisotropy, red-shifted absorption edges, spectral broadening, and additional low-energy in-plane optical transitions. These effects were particularly strong in WSe2, where the calculated optical response extended into the near-infrared region. The results demonstrate that the electronic and optical response to a stoichiometric trefoil-like reconstruction is strongly chalcogen-dependent and is governed by the interplay among local geometrical distortion, W 5d–chalcogen p hybridization, and spin–orbit coupling.