2D graphene nanosheets developed using Scotch-tape method
A team of experimentalists and theorists from the International Centre for Young Scientists, International Centre for Materials Nanoarchitectonics and Surface Physics and Structure Unit of the National Institute for Materials Science, National University of Science and Technology "MISiS" (Moscow, Russia), Rice University (USA) and University of Jyvaskyla (Finland) led by Daiming Tang and Dmitri Golberg, has for the first time succeeded in complete understanding of physics, kinetics and energetics behind the regarded "Scotch-tape" technique using molybdenum disulphide (MoS2) atomic layers as a model material.
The simplest mechanical cleavage technique using a primitive "Scotch" tape has resulted in the discovery of graphenes and is being used for assembling graphenes and other 2D graphene-like structures toward their use in novel high-performance nanoelectronic devices.
The simplicity of this method has initiated a booming research on 2D materials. However, the atomistic processes behind the micromechanical cleavage have still been poorly understood.
The researchers developed a direct in situ probing technique in a high-resolution transmission electron microscope (HRTEM) to investigate the mechanical cleavage processes and associated mechanical behaviours. By precisely manipulating an ultra-sharp metal probe to contact the pre-existing crystalline steps of the MoS2 single crystals, atomically thin flakes were delicately peeled off, selectively ranging from a single, double to more than 20 atomic layers. The team found that the mechanical behaviours are strongly dependent on the number of layers. Combination of in situ HRTEM and molecular dynamics simulations reveal a transformation of bending behaviour from spontaneous rippling (< 5 atomic layers) to homogeneous curving (about 10 layers), and finally to kinking (20 or more layers).
By considering the force balance near the contact point, the specific surface energy of a MoS2 monoatomic layer was calculated to be about 0.11N/m. This is the first time that this fundamentally important property has directly been measured.
After initial isolation from the mother crystal, the MoS2 monolayer could be readily restacked onto the surface of the crystal, demonstrating the possibility of van der Waals epitaxy. MoS2 atomic layers could be bent to ultimate small radii reversibly without fracture. Such ultra-reversibility and extreme flexibility proves that they could be mechanically robust candidates for the advanced flexible electronic devices even under extreme folding conditions.
Nanomechanical cleavage of molybdenum disulphide atomic layers. (left) Schematics of the experimental setup inside HRTEM. (centre) TEM image of a sharply etched tungsten nanoprobe in contact with the MoS2 single crystal deliberately placed with (0002) basal atomic planes viewed edge-on. (right) HRTEM image of a cleaved MoS2 atomic monolayer.
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