An Insight into Chemistry and Structure of Colloidal 2D-WS₂ Nanoflakes: Combined XPS and XRD Study

The surface and structural characterization techniques of three atom-thick bi-dimensional 2D-WS₂ colloidal nanocrystals cross the limit of bulk investigation, offering the possibility of simultaneous phase identification, structural-to-morphological evaluation, and surface chemical description. In the present study, we report a rational understanding based on X-ray photoelectron spectroscopy (XPS) and structural inspection of two kinds of dimensionally controllable 2D-WS₂ colloidal nanoflakes (NFLs) generated with a surfactant assisted non-hydrolytic route. The qualitative and quantitative determination of 1T' and 2H phases based on W 4f XPS signal components, together with the presence of two kinds of sulfur ions, S₂^2- and S^2-, based on S 2p signal and related to the formation of WS₂ and WO_xS_y in a mixed oxygen-sulfur environment, are carefully reported and discussed for both nanocrystals breeds. The XPS results are used as an input for detailed X-ray Diffraction (XRD) analysis allowing for a clear discrimination of NFLs crystal habit, and an estimation of the exact number of atomic monolayers composing the 2D-WS₂ nanocrystalline samples.