Insight into the growth behaviors of MoS₂ nanograins influenced by step edges and atomic structure of the substrate

The step edges and intrinsic atomic structure of single-crystal substrate play a critical role in determining the growth pathways of transition metal dichalcogenide (TMD) grains, particularly whether the TMDs will grow into wafer-scale single-crystal or anisotropic nanoribbons. Hereby, we investigate the growth behaviours of the MoS₂ nanograins on (0001) and ( $1\overline{1}02$) sapphire substrates. On one hand, the step edges formed on the (0001) surface after thermal treatment are found to promote the macroscopic aggregation of MoS₂ nanograins and to form unidirectional large triangular islands along with the < $11\overline{2}0$> steps in the annealing process, while on the pristine (0001) surface, the MoS₂ nanograins grow into a random network-like pattern. Moreover, oxygen treatment on the substrate can further enhance the growth of MoS₂ nanograins. Transmission electron microscopy and fast Fourier transform patterns reveal that the substrate could modulate the orientation of MoS₂ nanograins during their growing process. On the other hand, the MoS₂ nanograins on the $(1\overline{1}02)$ surface could self-assemble into one-dimensional nanoribbons due to the strong structural anisotropy of the substrate. In addition, the ratio of Raman intensities for peaks that correspond to the ${\mathrm{E}}_{2\mathrm{g}}^1$ and A_1g phonon modes shows a linear relationship with the grain size due to the change of the “phonon confinement”. Moreover, new peaks located at 226 and 280 cm⁻¹ can be observed in the off-resonant and resonant Raman spectra for the MoS₂ nanograin samples, respectively, which can be attributed to the scatterings from the edges of as-fabricated MoS₂ nanostructures.