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 MoS2, 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 A1g-E2g 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.

Dual Roles of MoO3 Thin Film in Improving the Performance of Copper Bismuth Oxide Photocathode for Solar Water Splitting

Promoting the hole extraction from the photocathode semiconductor is crucial to not only enhance the charge separation and suppress the charge recombination but also to protect the oxidation of the photocathode semiconductor by the photogenerated holes. Here, we use a very thin MoO3 film as a hole buffer layer between conductive substrate fluorine-doped tin oxide and the p-type semiconductor CuBi2O4. Through comprehensive photoelectrochemical characterizations, we find that the insertion of a hole buffer layer MoO3 not only accelerates the hole traction from the CuBi2O4 photocathode but also blocks the backward transfer of photogenerated electrons. This optimized charge transfer behavior contributes to the improved photoelectrochemical performance. Based on our results, some interesting designs on CuBi2O4 photocathode are given at the end that will be potentially working as effective photocathodes.