Multiscale Investigation of the Structural, Electrical and Photoluminescence Properties of MoS₂ Obtained by MoO₃ Sulfurization

In this paper, we report a multiscale investigation of the compositional, morphological, structural, electrical, and optical emission properties of 2H-MoS₂ obtained by sulfurization at 800 °C of very thin MoO₃ films (with thickness ranging from ~2.8 nm to ~4.2 nm) on a SiO₂/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 MoO₃ present in the final film. High-resolution TEM/STEM analyses revealed the formation of few (i.e., 2–3 layers) of MoS₂ nearly aligned with the SiO₂ surface in the case of the thinnest (~2.8 nm) MoO₃ film, whereas multilayers of MoS₂ 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 MoS₂ 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 MoS₂ on SiO₂, where the p-type doping is probably due to the presence of residual MoO₃. Nanoscale resolution current mapping by C-AFM showed local inhomogeneities in the conductivity of the few-layer MoS₂, which are well correlated to the lateral changes in the strain detected by Raman. Finally, characteristic spectroscopic signatures of the defects/disorder in MoS₂ films produced by sulfurization were identified by a comparative analysis of Raman and photoluminescence (PL) spectra with CVD grown MoS₂ flakes. View Full-Text