Fig. 2: Growth and characterization of chirality-selected, unidirectional, single-crystalline monolayer ReS₂.

a Scanning electron microscopy (SEM) image of unidirectionally aligned spindle-like ReS₂ domains grown on MgO(12l)^S. The inset shows the angle-resolved Raman intensities of the vibrational mode of ReS₂ at 212 cm⁻¹ captured at three different locations in one domain. Scale bar: 20 µm. b Annular dark-field scanning transmission electron microscopy (ADF-STEM) image of monolayer ReS₂. The Re4 chains are represented by white rhombuses with crystallographic directions a [100] and b [010] labeled. Scale bar: 2 nm. c Selected area electron diffraction (SAED) patterns obtained at four different domains. The yellow hexagons mark the innermost diffraction spots, while the insets are the zoomed-in views of the red- and blue-boxed high-index diffraction spots. Scale bar: 2 nm⁻¹. d Circularly polarized Raman (CPR) spectra of as-grown ReS₂ in (a) (bottom) and the difference between the Raman intensities excited by right-handed (RCP) and left-handed (LCP) circularly polarized light (top). The wavelength of the polarized light is 532 nm. e Histogram showing the circular intensity difference (CID) distribution of Raman modes III (153 cm⁻¹), V (212 cm⁻¹), and VI (235 cm⁻¹) captured from ≈50 ReS₂ domains located at different regions on MgO(12l)^S. f Atomic force microscopy (AFM) image displaying a ReS₂ domain (left to the white dashed line) grown on bare MgO(12l)^S (right to the white dashed line). The reconstructed steps along MgO\(\left[\bar{1}20\right]\) (which are composed of step\(\left[\bar{1}10\right]\) and kink[010] at the atomic scale) are marked by the red arrow, while the b-axis of ReS₂ is labeled by the white arrow. Scale bar: 200 nm. g, Zoomed-in atomically resolved AFM image of the ReS₂ domain in (f). The Re rhombic lattice along the b-axis is marked by white lines. Scale bar: 2 nm. h Cross-sectional ADF-STEM image of the ReS₂/MgO interface viewed along MgO\(\left[\bar{1}10\right]\). Scale bar: 1 nm. i Schematic illustrating the epitaxial relationship of monolayer ReS₂ on reconstructed MgO(12l)^S. The orange and white spheres represent the first and second layers of O atoms of the MgO surface, respectively.