Fig. 3: Step-by-step structural selectivity improvement of as-grown ReS₂ by substrate engineering.

a–d AFM images showing bare MgO(001) (a), and monolayer ReS₂ grown on reconstructed MgO(11l) (b), MgO(12l)^R (c), and MgO(12l)^S (d), respectively. Regions to the left of the white dashed lines in (b–d) are monolayer ReS₂-covered MgO surfaces with steps along \(\left[\bar{1}10\right],\left[\bar{2}10\right]\), and \(\left[\bar{1}20\right]\) marked by white, blue, and red arrows, respectively. Areas to the right of the white lines are bare substrates. The atomic models corresponding to each surface are displayed below the AFM images. Large spheres in different shades of orange represent O atoms in different layers of the MgO substrate, while small gray spheres represent Mg atoms. Scale bars: 200 nm. Insets in (b–d) are the nanometer-scale AFM images showing the reconstructed steps and kinks on the corresponding substrates after annealing. The white and yellow dashed lines mark the steps and kinks, respectively. Scale bar: 30 nm. e Optical image showing ReS₂ domains grown on MgO(001) with two preferred orientations of 0° and 90°. Scale bar: 50 µm. The inset is an angle-resolved Raman intensity mapping of mode V for ReS₂ domains grown on MgO(001). Scale bar: 5 μm. f Low-energy electron diffraction (LEED) patterns of ReS₂ grown on MgO(001). g In-plane orientation histograms of ReS₂ domains grown on MgO(001) and MgO(11l), respectively. 100 domains were counted on each substrate. h Linearly polarized Raman spectra of R-ReS₂ and S-ReS₂ for a laser wavelength of 532 nm. Inset is an optical image showing that the b-axis of ReS₂ (black arrow) is rotated 120° with respect to the polarization direction of the incident laser (green arrow) during characterization. Scale bar: 5 μm. i, j Polarization dependence of the intensities of Raman modes III (red) and V (blue) at 151 cm⁻¹ and 212 cm⁻¹ for R-ReS₂ (i) and S-ReS₂ (j), respectively. The b-axis of ReS₂ is marked by the black arrow. k, l Statistical distributions of the normalized intensity of Raman mode III (I_III/I_V) for ReS₂ domains grown on MgO(12l)^R and MgO(12l)^S (insets), respectively. The measurement condition was the same as that in (h). Each substrate counted ≈ 200 domains.