As well as having remarkable electronic properties, graphene sheets (tiny flakes of graphite that can be as small as just one atom thick) are extremely stiff and strong too. This means that the material is very robust – a single atomic sheet can be made into a free-hanging structure that does not break easily. The sheets can also be used to make very high frequency devices that are extremely sensitive to added mass thanks to their low density.
"Although graphene shares these advantages with carbon nanotubes, which have also been used to make highly sensitive mass detectors, it has the added bonus of being a 2D sheet that we can 'carve' into the shapes we want," explained team leader James Hone. "This gives us more control over the properties of the finished resonators."
The Columbia team made its devices by placing monolayer graphene flakes onto silicon/silica substrates, then patterning metal electrodes and etching away the silica to produce suspended graphene. The portion of each electrode that is in contact with the graphene is also suspended, which makes electrical readout easier later on.
The devices vibrate at megahertz frequencies, with a peak around 65 MHz that depends on the device geometry. The frequency can also be adjusted with a DC voltage applied to the gate, which introduces tension to the sheet. When an object is placed on the device, the frequency changes. This change is detected with the electrodes, and used to calculate the mass of the molecule.
"Our measurements indicate that the devices should be sensitive to around 1 zeptogram (10–21g), which is about two gold atoms, at low temperatures" Hone told nanotechweb.org. "They also show that the response is not as simple as expected because placing material on the graphene changes both the mass of the sheet and its tension – a new phenomenon that has never been seen before."
The team is now experimenting with different geometries for the devices and looking at various readout techniques that will improve their performance.
The work was published in Nature Nanotechnology.