Graphine is a one atom thick crystal layer, a chemically stable and electrically conducting membrane exhibiting a variety of unique properties due to its novel molecular structure One of the big question still remaining unanswered was; can such membranes be impermeable to atoms, molecules and ions?

Researchers at Cornell University in the US have addressed this question for gases: They successfully used micron-scale graphene sheets to create the world's smallest balloons.

The team isolated graphene sheets by mechanical exfoliation, placing them across wells that had been created in silica substrates. Van der Waals forces held the sheets in place around their circumference, forming sealed microchambers nearly five microns on a side.

Both positive and negative pressure differentials were created across the atom-thick membranes by placing the microchambers under pressure or in vacuum and then allowing the pressure in the chambers to equilibrate over hours or days. The sheets were then imaged by atomic force microscopy, showing that they bulged inward or outward significantly (see image).

The approach was attempted with nitrogen, air, and helium as the high pressure gases, and with graphene thicknesses from just one to 75 layers. The team found that the timescale of the equilibration to ambient pressure was not dependent on the thickness of the graphene. Thus, any leakage was either through the glass or the interface.

The result is important, says lead author Scott Bunch, because it shows that “a single sheet of graphene is impermeable to helium gas atoms and therefore free of any significant vacancy over micron size areas.”

Measuring the size of the bulges above or below the substrate for given pressure differentials allowed estimates of the sheets' elasticity, which the researchers found to be more or less equal to that of graphite. That solves a longstanding question about the use of bulk elastic constants for nanoscale materials [Bunch, et. al., Nano Lett. (2008), DOI: 10.1021/nl801457b].

The work suggests graphene sheets are applicable as incredibly sensitive pressure sensors, and selectively patterning the sheets would make them ideal for ultrafiltration, the authors say. Graphene drumheads can also offer the opportunity to probe the permeability of gases through atomic vacancies in single layers of atoms.