Multiscale Investigation of the Structural, Electrical and Photoluminescence Properties of MoS 2 Obtained by MoO 3 Sulfurization.

In this paper, we report a multiscale investigation of the compositional, morphological, structural, electrical, and optical emission properties of 2H-MoS 2 obtained by sulfurization at 800 °C of very thin MoO 3 films (with thickness ranging from ~2.8 nm to ~4.2 nm) on a SiO 2 /Si substrate. XPS analyses confirmed that the sulfurization was very effective in the reduction of the oxide to MoS 2, with only a small percentage of residual MoO 3 present in the final film. High-resolution TEM/STEM analyses revealed the formation of few (i.e., 2-3 layers) of MoS 2 nearly aligned with the SiO 2 surface in the case of the thinnest (~2.8 nm) MoO 3 film, whereas multilayers of MoS 2 partially standing up with respect to the substrate were observed for the ~4.2 nm one. Such different configurations indicate the prevalence of different mechanisms (i.e., vapour-solid surface reaction or S diffusion within the film) as a function of the thickness. The uniform thickness distribution of the few-layer and multilayer MoS 2 was confirmed by Raman mapping. Furthermore, the correlative plot of the characteristic A 1g -E 2g Raman modes revealed a compressive strain (ε ≈ -0.78 ± 0.18%) and the coexistence of n- and p-type doped areas in the few-layer MoS 2 on SiO 2 , where the p-type doping is probably due to the presence of residual MoO 3 . Nanoscale resolution current mapping by C-AFM showed local inhomogeneities in the conductivity of the few-layer MoS 2 , which are well correlated to the lateral changes in the strain detected by Raman. Finally, characteristic spectroscopic signatures of the defects/disorder in MoS 2 films produced by sulfurization were identified by a comparative analysis of Raman and photoluminescence (PL) spectra with CVD grown MoS 2 flakes.