Owing to their appealing photophysical and mechanical properties, two-dimensional nanostructures of semiconducting transition-metal dichalcogenides (2D-TMCs), such as MoS₂, WS₂, MoSe₂, and WSe₂ in the mono- to few-layer regime, are considered very promising nanomaterials for several applications, ranging from optoelectronics, energy storage/conversion to catalysis. However, the synthesis of free-standing, easy-to-manipulate 2D-TMCs with controllable thickness and lateral dimensions is challenging, because their poor chemical and mechanical stability promotes a tendency to collapse into useless bulk-like aggregates or micrometer-size structures. To address these difficulties, colloidal methods are currently being regarded as powerful alternative routes to the scalable synthesis of solution-processable 2D-TMCs.  

Here we report a solution-phase synthesis of colloidally stable 2D-WS₂ crystalline nanosheets via a sulfidation process. In our approach, preformed colloidal nonstoichiometric tungsten oxide (WO_3-x) nanocrystals, used as starting sacrificial templates, are induced to react with a suitable sulfur precursor in a hot surfactant medium to yield 2D-WS₂ nanosheets. The advantage of this method relies in the fine control over the chemical and structural conversion of WO_3-x nanocrystal precursors, which is achievable by tuning the sulfidation conditions during the entire reaction course. We have monitored the progress of the transformation, identifying the reaction pathways through which the WO_3-x nanocrystals are gradually converted to a mixed-phase WO_3-x/WS₂ nanostructure intermediates and finally into 2D-WS₂ nanosheets. Each reaction step has been characterized by a combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning TEM, and optical absorption analyses.  

Colloidal WS₂ nanosheets are subsequently deposited by layer-by-layer self-assembly technique to obtain smooth and homogeneous multiflake WS₂ thin-films, as confirmed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements. We studied the conduction properties of WS₂ thin films by 4-probes and temperature-dependent I-V measurements. We report conductivity values as high as 2.6*10^-1 S cm^-1 for an as cast film. The increase of the conductivity as the temperature increases confirms the semiconducting behaviour of the WS₂ films.  

In conclusion, we demonstrate that sulfidation solution approach is a powerful way for the synthesis of colloidally stable 2D-WS₂ nanostructures, which can be used as building blocks for the fabrication of highly efficient solution-processed optoelectronic devices.