Fig. 1: Terrace-step-kink strategy of substrate design.

a Schematic illustrating chirality of monolayer ReS₂ on the substrate, where R and S refer to rectus and sinister in Latin, respectively, representing two types of enantiomers. The structural differences between R- and S-ReS₂ on the substrate surface are marked by red symbols. b–d Schematics showing the substrate surface reconstruction from MgO(11l) to terrace facets (001) with bunched steps along \(\left[\bar{1}10\right]\) (b) and reconstruction from chiral surface R-(12l)/S-(21l) to terrace facets (001) with bunched steps along \(\left[\bar{1}10\right]\) and kinks along \(\left[\bar{1}00\right]\)/[010] (c and d). e–g Scheme of the designed terrace-step-kink strategy, which gradually increases the structural selectivity of the as-grown ReS₂ on the MgO substrate by sequentially introducing specific terraces, steps, and kinks to the reconstructed substrate surfaces to reduce their structural symmetry, thus eliminating energy degeneracy of monolayer ReS₂ single crystals with different orientations and chirality. Blue and red blocks represent ReS₂ domains in different chirality. The gray lines represent microstructures on the reconstructed substrate surfaces with the zoomed-in inset highlighting steps and kinks. h Binding energies (E_b) calculated using density functional theory (DFT) for ReS₂ single-crystalline domains with different rotation angles (θ) on MgO(001), θ represents the angle between the b-axis ([010]) of ReS₂ and MgO\(\left[\bar{1}10\right]\). i Formation energies (E_f) of three grain boundaries with different inter-grain twist angles (insets). j Binding energies (E_b) of monolayer single-crystalline ReS₂ domains oriented perpendicularly on reconstructed MgO(11l) composed of terrace(001) and step\(\left[\bar{1}10\right]\). The red lines in the top panel of the insets represent step\(\left[\bar{1}10\right]\) on the reconstructed MgO surface, while the blue rhombuses represent Re4 chains of monolayer ReS₂. k Binding energies (E_b) of R- and S-ReS₂ monolayer single crystals with the same b-axis orientation on the chiral reconstructed MgO(12l)^R surface. The red curves (top panels in the insets) represent the “step + kink” configuration on MgO.