An artist’s rendering of an array of pressurized graphene membranes. A CU-Boulder team recently discovered that graphene has surprisingly high adhesion properties, findings that may help lead to the development of new graphene-based mechanical devices like gas separation membranes. (Illustration courtesy Victor Tzen and Rex Tzen.)
graphene has surprisingly powerful adhesion qualities — are expected to help guide the development of graphene manufacturing and of graphene-based mechanical devices such as resonators and gas separation membranes, according to the CU-Boulder team. The experiments showed that the extreme flexibility of graphene allows it to conform to the topography of even the smoothest substrates.
As mechanical structures enter the nanoscale regime, the influence of van der Waals forces increases. Graphene is attractive for nanomechanical systems because its Young’s modulus and strength are both intrinsically high, but the mechanical behaviour of graphene is also strongly influenced by the van der Waals force. For example, this force clamps graphene samples to substrates, and also holds together the individual graphene sheets in multilayer samples. Here we use a pressurized blister test to directly measure the adhesion energy of graphene sheets with a silicon oxide substrate. We find an adhesion energy of 0.45 ± 0.02 J m−2 for monolayer graphene and 0.31 ± 0.03 J m−2 for samples containing two to five graphene sheets. These values are larger than the adhesion energies measured in typical micromechanical structures and are comparable to solid–liquid adhesion energies We attribute this to the extreme flexibility of graphene, which allows it to conform to the topography of even the smoothest substrates, thus making its interaction with the substrate more liquid-like than solid-like
“The real excitement for me is the possibility of creating new applications that exploit the remarkable flexibility and adhesive characteristics of graphene and devising unique experiments that can teach us more about the nanoscale properties of this amazing material,” Bunch said.
Not only does graphene have the highest electrical and thermal conductivity among all materials known, but this “wonder material” has been shown to be the thinnest, stiffest and strongest material in the world, as well as being impermeable to all standard gases. It’s newly discovered adhesion properties can now be added to the list of the material’s seemingly contradictory qualities, said Bunch.
The CU-Boulder team measured the adhesion energy of graphene sheets, ranging from one to five atomic layers, with a glass substrate, using a pressurized “blister test” to quantify the adhesion between graphene and glass plates.
Adhesion energy describes how “sticky” two things are when placed together. Scotch tape is one example of a material with high adhesion; the gecko lizard, which seemingly defies gravity by scaling up vertical walls using adhesion between its feet and the wall, is another. Adhesion also can play a detrimental role, as in suspended micromechanical structures where adhesion can cause device failure or prolong the development of a technology, said Bunch.
The CU research, the first direct experimental measurements of the adhesion of graphene nanostructures, showed that so-called “van der Waals forces” — the sum of the attractive or repulsive forces between molecules — clamp the graphene samples to the substrates and also hold together the individual graphene sheets in multilayer samples.
The researchers found the adhesion energies between graphene and the glass substrate were several orders of magnitude larger than adhesion energies in typical micromechanical structures, an interaction they described as more liquid-like than solid-like, said Bunch.