ITHACA, N.Y., Aug. 8, 2008 – The world’s thinnest balloon is just one atom thick. Developed by researchers at Cornell University, this balloon is made of a single layer of graphite and it is impermeable to even the tiniest airborne molecules, including helium. It has a range of applications in sensors, filters and imaging of materials at the atomic level.
Paul L. McEuen and colleagues noted that membranes are fundamental components of a wide variety of physical, chemical and biological systems, found in everything from cellular compartments to mechanical pressure sensing. Graphene, a single layer of graphite, is chemically stable and is an electrically conducting membrane. The researchers wanted to answer whether such an atomic membrane would be impermeable to gas molecules and easily incorporated into other devices.
Their data showed that graphene membranes were, in fact, impermeable to even the smallest gas molecules. These results show that single atomic sheets can be integrated with microfabricated structures to create a new class of atomic scale membrane-based devices.
According to McEuen, these graphene sealed microchambers will have many applications, including hypersensitive pressure, light and chemical sensors to filters able to produce ultrapure solutions.
The group demonstrated that by applying a pressure difference across the membrane, they could measure both the elastic constants and the mass of a single layer of graphene. This pressurized graphene membrane provides a unique separation barrier between two distinct regions that is only one atom thick.
Scientists have developed the world's thinnest balloon that is impermeable to even the smallest gas molecules. Above is a multilayer graphene membrane that could be used in various applications, including filters and sensors. Photo courtesy of Jonathan Alden.
Membranes are fundamental components of a wide variety of physical, chemical, and biological systems, used in everything from cellular compartmentalization to mechanical pressure sensing. They divide space into two regions, each capable of possessing different physical or chemical properties. A simple example is the stretched surface of a balloon, where a pressure difference across the balloon is balanced by the surface tension in the membrane. An interesting question is whether such an atomic membrane can be impermeable to atoms, molecules and ions.
The researchers have shown that these membranes are impermeable and can support pressure differences larger than one atmosphere. They used such pressure differences to tune the mechanical resonance frequency. This allowed them to measure the mass and elastic constants of graphene membranes and demonstrate that atomic layers of graphene have stiffness similar to bulk graphite.
These results show that single atomic sheets can be integrated with microfabricated structures to create a new class of atomic scale membrane-based devices.
After initial fabrication, the pressure inside the microchamber, pint, is atmospheric pressure. If the pressure external to the chamber, pext, is changed, then pint will equilibrate to pext on a time scale that ranges from minutes to days, depending on the gas species and the temperature.
To create a positive pressure difference they placed a sample in a pressure chamber with pext. After it is removed, a tapping mode atomic force microscope (AFM) image at ambient external pressure showed that the membrane bulges upward. Similarly, we they created a lower pressure in the chamber by storing the device under vacuum and then returning it to atmospheric pressure.
Over time, the internal and external pressures equilibrate. They characterize the equilibration process by monitoring the pressure change and using the ideal gas law to convert this to a leak rate.
The lack of dependence of the leak rate on the membrane thickness indicates that the leak is not through the graphene sheets, or through defects in these sheets. This suggests it is either through the glass walls of the microchamber or through the graphene-SiO2 sealed interface.
They concluded that the graphene layer is essentially perfect and for all intents and purposes, impermeable to all standard gases.
According to the researchers, these graphene sealed microchambers can act as compliant membrane sensors that probe pressures in small volumes and explore pressure changes associated with chemical reactions, phase transitions, and photon detection. In addition to these spectroscopic studies, graphene drumheads offer the opportunity to probe the permeability of gases through atomic vacancies in single layers of atoms and defects patterned in the graphene membrane can act as selective barriers for ultrafiltration.
The tensioned suspended graphene membranes also provide a platform for STM imaging of both graphene and graphene-fluid interfaces and offer a unique separation barrier between two distinct phases of matter that is only one atom thick.
For more information, visit: www.cornell.edu