The discovery of two-dimensional materials became possible due to the mechanical cleavage technique. Despite its simplicity, the as-cleaved materials demonstrated surprising macro-continuity, high crystalline quality and extraordinary mechanical and electrical properties that triggered global research interest. Here such cleavage processes and associated mechanical behaviours are investigated by a direct in situ transmission electron microscopy probing technique, using atomically thin molybdenum disulphide layers as a model material. Our technique demonstrates layer number selective cleavage, from a monolayer to double layer and up to 23 atomic layers. In situ observations combined with molecular dynamics simulations reveal unique layer-dependent bending behaviours, from spontaneous rippling (<5 atomic layers) to homogeneous curving (similar to 10 layers) and finally to kinking (20 or more layers), depending on the competition of strain energy and interfacial energy.RI Koskinen, Pekka/C-3964-2008; Tang, Dai-Ming/B-6641-2011OI Koskinen, Pekka/0000-0001-7711-3562;
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