Graphene goes from strength to strength

CARBON

August 6, 2008

AFM tip deforming a graphene sheet. (Courtesy of James Hone.)

Predictions about the phenomenal strength of defect-free graphene appear to be well-founded, according to new experimental data from researchers at Columbia University [Lee et al., Science (2008) 321, 385].

Changgu Lee and colleagues used nanoindentation to measure the breaking strength and elastic properties of nano-sized flakes of graphene suspended over open wells.

They probed free-standing monolayers of graphene with a diamond-tipped atomic force microscope (AFM) to determine the forcedisplacement behavior of the material.

The measurements reveal a material with second- and third-order elastic stiffnesses of
340 Nm-1 and -690 Nm-1, breaking strength of 42 Nm-1, a Young’s modulus of 1 TPa, and an intrinsic strength of 130 GPa.

“This work demonstrates what has been predicted theoretically, namely that graphene is
the strongest material,” says Kostya Novoselov of the University of Manchester.

The fact that the breaking force tallies with predictions of the intrinsic strength of graphene
suggests that the film in the vicinity of the AFM tip is defect-free.

These unique mechanical properties have enabled a team of researchers from the University of California at Berkeley and the Lawrence Berkeley National Laboratory to push
transmission electron microscopy (TEM) toward the ultimate sensitivity [Meyer et al., Nature
(2008) 454, 319].

The researchers have used single layers of graphene as sample-support membranes to
enhance the signal-to-background ratio of light atoms such as C and H. Viewing such light atoms in the TEM is difficult because the low signal levels are drowned out by background signals from the substrate.

Graphene, however, is transparent to the electron beam. So when Alex Zettl and
colleagues use it as a TEM membrane, they are able to see individual C and H atoms.
As well as imaging individual adatoms directly, the researchers also captured C chains and
vacancies dancing across the surface of the membranes in real time.

However, the technique is limited to atoms that are present in the TEM chamber and settle onto the graphene membrane. Adatoms have to be bound to the graphene surface to be visible.

Cordelia Sealy