In order to make such ultrasmall structures, researchers need to manipulate both the architecture and the electronic properties of very small volumes of matter. This can be achieved mechanically using atomic force microscopy, in which single atoms are positioned in a desired geometry, or chemically through self-assembly methods.
Now, however, a new type of device in which electron transport is manipulated by both electrical and mechanical means has been built by Dominik Scheible, Artur Erbe and Robert Blick at the Center for NanoScience at Ludwig-Maximilians University in Munich (D Scheible et al. 2002 New Journal of Physics 4 86.1-86.7). This represents the next step towards one of the ultimate goals in miniaturized electronic components - the first nanomechanically assisted single-electron transistor.
Transport barriers
When trying to manipulate the electronic properties of nanostructures, a phenomenon known as the Coulomb blockade becomes important. As electric charge is quantized in units of the electron charge, this means that any exchange of charge between conductors involves an integer number of elementary charges. Charge exchange normally takes place by the quantum tunnelling of electrons through insulating barriers.
This is important in nanometre-size conductors because the electrostatic charging energy required to allow a single electron to tunnel through a barrier can be much greater than the energy provided by thermal fluctuations or by a battery. Therefore, at low temperatures and low bias voltages, charge fluctuation - and therefore electron transport - is blocked and current can flow only if the bias voltage overcomes a certain threshold value.
The strength of this Coulomb blockade can be tuned electrostatically using an external electrode or gate, the voltage on which controls the threshold energy for single-electron transport. A number of devices that make use of the effect have been invented, including sensitive charge meters, primary thermometers and single-electron transistors (see "Nanoelectromechanical systems face the future").
Transport breakthroughs
The idea behind the Munich team's development is to use a controllable mechanical device to transfer electrons one-by-one between two electrodes. The electrons are shuttled by a small, isolated, metallic island located on the tip of an oscillating silicon cantilever (see figure). The cantilever is part of a system of nanomechanical coupled resonators, designed much like a conventional tuning fork to minimize energy loss. This construction makes it possible to drive the shuttle mechanically with minimal destructive interference to the dynamics of the shuttle itself.
The team achieved this by using a clever design to minimize the mechanical coupling between the driving part of the device - either a magnetomotively driven beam resonator or a capacitively coupled remote cantilever - and the cantilever that is being driven. An important feature of this device is that the normally weak coupling between the driving and driven components rises sharply at certain frequencies due to the phenomenon of mechanical resonance. In fact, the electric current across the mechanically oscillating structure increases exponentially with the input power.
The resonance-type sensitivity to the actuation frequency proves that the measured electric current is mechanically assisted. However, the exponential sensitivity of the current to the energy stored in the vibrating nanomechanical system indicates that the ideal shuttle-current regime has yet to be achieved. In the ideal regime the current is determined by just two factors: the number n of electrons allowed on the island by the Coulomb-blockade conditions and the vibration frequency f. It does not depend on the tunnelling rates. The researchers use a simple formula for the shuttle current, I=gnef, where e is the electron charge and g is a "transport factor" that accounts for the fact that electrons travelling on and off the shuttle may not be fast enough to reach the ideal regime. There are therefore two parameters, g and n, that determine the current, which means that a measurement of the current only enables the product gn to be calculated. Additional measurements are therefore required to find the number n of shuttled electrons, and hence if single electrons are indeed being controlled.
The Munich team plans to carry out such measurements in the near future. If a current with a step-like dependence on bias voltage is observed, then this would demonstrate the staircase behaviour typical of Coulomb-blockade systems. In this regime, the height of the current steps determines the contribution to the shuttle current caused by the transfer of a single electron. Direct proof of the shuttling of single electrons would be obtained if the current steps smooth out as the temperature rises. The team believes that future work of this type will definitely prove that its device is indeed the first nanomechanically assisted single-electron transistor (NEM-SET). This seems entirely likely.
Applying shuttling
As well as the possible practical applications of NEM-SET devices, they could also be important in fundamental physics. Studies of the mechanically controlled transport of single electrons will enhance our understanding of the physics of electromechanical coupling on the nanometre scale. Such physics is relevant in understanding the basic properties of nanocomposite materials such as certain self-assembled metal-organic structures. Here, intrinsic shuttling may occur as a result of electromechanical instabilities, as discussed by the present authors and colleagues in 1998. We have also proposed that shuttling could offer a mechanism for mechanical intervention in other types of transport phenomena as well. It could provide mechanical assistance to superconducting charge transfer (the shuttling of so-called Cooper pairs) and also to the shuttling of magnetization between nanomagnets.
This new device is a promising tool for studying the physics of mechanically assisted coupling on the nanometre scale.