We know that graphene is the strongest material as it has high tensile strength. Then why does it exist as flakes? Why we cannot make a complete sheet out of graphene?
Yes, Graphene is 2D as well as 3D sheet of carbon atoms which is single atomic layer thick
https://www.comsol.com/blogs/should-we-model-graphene-as-a-2d-sheet-or-thin-3d-volume/
@SKalaiselvan sir, actually I wanted to know if it is a 2d structure( or a 3D structure with exceptionally less thickness) it must be available like a sheet(like 1 piece of paper), rather it is found as flakes why? Is the planar covalent bond not strong enough to hold all the hexagonal molecules in planar form(even though it is known as the strongest material on earth)?
Dear Soumyajit Samal ;
you can form fairly big 2D sheets of graphene, but you have to consider the fact that these kind of (CVD) grown graphene are pretty fragile mechanically. it is "strong" for a one atom thick material but with the forces needed to manipulate it breaks really easily.
zakaria
Consider your question from another perspective. Gold is a very malleable metal, but does thin gold leaves have enough strength?
Metal's strength rises from metallic bond, and their point defects (alloy atoms), line defects (dislocation) and plane defects (Grain boundary). Metallic bond is quite weaker than covalent bond that hold graphene, and so graphene can have much higher strength than steel. However, when a sheet is disturbed by, say, air/grass pressure due to molecular flux or electromagnetic force, it tends to bend. While the bond angles between carbon atoms of graphene can stretch or bend little by little elastically much more than the limit permissible by ordinary polycrystalline pure metal, plastic deformation of graphene "sheet as intact" is not permissible, in the sense that, the broken and rearranged bond would no longer represent a true sheet. Like macroscopic flexure of beams or plates, concave face of the bend is compressed and convex face is expanded. Since layer-to-layer attraction of graphene (VDW) is much lower than intralayer bonding (unlike multi-atom thick metal sheet- metallic bond every direction), it flakes off during bending, and single layer graphene sheet fails to bend keeping oun sheet identity intact.
A macroscopic example is flaking of mica.
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