In Situ Tem Investigations On The Controlled Phase Transformation Of Vertically Aligned Ws2 At Designated Locations On An Atomic Scale

Revealing the structural transformation and chemical stability of two-dimensional vertical WS2 layers and understanding their underlying mechanism are particularly significant for their electrochemical and catalytic applications. In this letter, through in situ heating treatment and controlled electron beam irradiation in a transmission electron microscope, the transformation from vertically aligned WS2 layers into cubic W particles at predefined locations has been revealed in real time. During the phase transformation, pristine WS2 becomes thin and some nanoscale voids appear between the newly formed W particles because of the evaporation of S atoms from the WS2 lattice framework and the migration of W atoms. Meanwhile, it is found that the density of vertically aligned WS2 has a great influence on the transformation mechanism. Vertically aligned WS2 layers with high density are directly converted to cubic W nanocrystals, while vertically aligned WS2 layers with low density are first transformed into horizontal WS2 and/or amorphous W and then are transformed into cubic W nanocrystals. Furthermore, by controlling the location of the electron beam irradiation, the formation of W nanocrystals with the designed pattern has been achieved. This study provides useful insights for a better understanding of the basic atomic structure of general two-dimensional layered crystal materials.