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

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