Carbon nanotubes and graphene are both made up of carbon and have unique properties. Graphene comprises an atom-thick layer of carbon atoms, while carbon nanotubes can be likened to a graphene sheet that has been rolled up to form a tube.
"If you stretch a graphene sheet from end to end the thin layer can oscillate at a basic frequency of getting on for a billion times a second," says researcher Anders Nordenfelt. "This is the same frequency range used by radios, mobile phones and computers."
Self Oscillations and Cooling of Carbon Based NEMS Devices (73 page thesis)
Possible to weigh DNA molecules
It is hoped that the limited size and weight of these new carbon materials could further reduce both the size and power consumption of our electronic circuits.
In addition to new applications in electronics, research is under way into how graphene can be used to weigh extremely small objects such as DNA molecules.
The high mechanical resonance frequencies mean that carbon nanotubes and graphene can pick up radio signals.
"The question is whether they can also be used to produce this type of signal in a controlled and effective way," says Anders Nordenfelt. "This assumes that they themselves are not driven by an oscillating signal that, in turn, needs to be produced by something else."
In his research Anders Nordenfelt carried out a mathematical analysis to demonstrate that it is possible to connect the nanowire with a fairly simple electronic circuit, and at the same time to apply a magnetic field and thus get the nanowire to self-oscillate mechanically.
"At the same time we're converting a direct current to an alternating current with the same frequency as the mechanical oscillation," says Anders Nordenfelt.
We investigate the electromechanical properties of a number of system geometries featuring a doubly clamped Carbon Nanotube or Graphene sheet with a deflection sensitive resistance and an electronic feedback in the form of a Lorentz force or an electrostatic attraction. The nanotube is subjected to a constant current- or voltage bias and it is shown that when the electromechanical coupling exceeds a certain critical value the system becomes unstable to self-excitations of the mechanical vibrations accompanied by oscillations in the voltage drop and current through the nanotube. The critical value typically depends on the quality factor and some function of the mechanical and electronic relaxation times. We discuss applications of the devices as active tunable radiofrequency oscillators and for cooling.
In this thesis we have explored a number of NEMS devices with the common feature that they are all capable of producing mechanical self oscillations. Some of these emerge as a result of a negative differential resistance but in most cases this is not required. Most of them result in a drop in average current, but there is at least one exception from this rule. Some are mathematically relatively simple while others are more complicated, but as we have seen, simplicity does not stand in opposition to versatility. On the contrary, even the simplest device demonstrates the entire range of functionality, all the way from cooling up to an S-shaped I-V characteristic.
A question that poses itself in light of these results is if there is a more general framework in which all different instances of electro-mechanical instability fit in. Knowlege of such a general theory could perhaps provide a tool to tailor the system geomtery so that it, for example, meets the requirements of a certain application and not merely come out as a ’lucky shot’. But this belongs to future research.
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