In-plane Aligned Colloidal 2D WS 2 Nanoflakes for Solution-Processable Thin Films with High Planar Conductivity

Two-dimensional transition-metal dichalcolgenides (2D-TMDs) are among the most intriguing materials for next-generation electronic and optoelectronic devices. Albeit still at the embryonic stage, building thin films by manipulating and stacking preformed 2D nanosheets is now emerging as a practical and cost-effective bottom-up paradigm to obtain excellent electrical properties over large areas. Herein, we exploit the ultrathin morphology and outstanding solution stability of 2D WS 2 colloidal nanocrystals to make thin films of TMDs assembled on a millimetre scale by a layer-by-layer deposition approach. We found that a room-temperature surface treatment with a superacid, performed with the precise scope of removing the native insulating surfactants, promotes in-plane assembly of the colloidal WS 2 nanoflakes into stacks parallel to the substrate, along with healing of sulphur vacancies in the lattice that are detrimental to electrical conductivity. The as-obtained 2D WS 2 thin films, characterized by a smooth and compact morphology, feature a high planar conductivity of up to 1 μS, comparable to the values reported for epitaxially grown WS 2 monolayers, and enable photocurrent generation upon light irradiation over a wide range of visible to near-infrared frequencies.