Physicists in the US claim to have made the world's thinnest resonators using sheets of carbon as little as one atomic layer thick. Paul McEuen and colleagues at Cornell University suspended graphene sheets over micrometre-sized trenches to create devices that vibrate at their own natural frequencies. According to McEuen, the resonators could be used to create highly-sensitive mass and force detectors (Science 315 490).
First made in 2004, graphene sheets are tiny flakes of graphite that can be as little as one atomic layer thick. They are the darling of nanotechnologists because they are easy to make and are very good electrical conductors — thereby providing an ideal system for exploring the often bizarre properties of two-dimensional electrons. However, researchers have been slower to exploit the novel structural properties of graphene, including its remarkable strength and resilience at atomic-scale thicknesses.
Working with physicists at Pomona College in California, the Cornell team made about 30 different resonators using flakes of graphene ranging from one atomic layer to 50 nm thick. The graphene flakes were dropped across trenches 1 to 5 micrometres wide, adhering to the tops thanks to the van der Waals attraction. In most instances the graphene spanned the trench to form a “doubly-clamped beam”. However, in some cases the graphene failed to span the trench and instead attached to one side forming a diving-board like structure called a “cantilever beam”.
The devices had resonant frequencies in the 1-170 MHz range depending upon the thickness of the beam, its tension and whether it is doubly-clamped or cantilever. The resonators were excited by using a pulsed laser, or by applying an electrical signal between the graphene and an electrode at the bottom of the trench. Vibrations were detected by observing the deflection of a laser that was reflected from the graphene surface.
McEuen told Physics Web that the very low mass of the resonators could be exploited to measure the mass of molecules to a high degree of accuracy. When a molecule adheres to a resonator, the increase in mass would change the resonant frequency. This change could be detected and used to calculate the mass of the molecule. The spring-like nature of the resonator could also be used to detect minute forces, says McEuen.
Although the resonators are about ten times thinner than other materials used to detect mass, McEuen cautions that the “quality” of the resonators – or their ability to continue vibrating after being struck – is not as good. As a result, they currently offer little benefit over existing mass detectors. McEuen said that the poor quality of the resonators is a mystery that his team is currently investigating.
