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 (1) over bar 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<(2)over bar>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 (1) over bar 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 E-2g(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.