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Posted: Sep 9th, 2010
A novel single electron pump based on a carbon nanotube
(Nanowerk Spotlight) Electron pumps are devices that can transfer a certain number of electrons during each pumping cycle. Besides being of fundamental interest to physicists, single-electron pumps have a potential for practical application in metrology, acting as an accurate frequency-current converter.
The general goal of this field is to build a current standard based on the electrical charge of a single electron in order to achieve high accuracy for current measurement. A device called single-electron transistor (SET) can confine charges down to single electron level and hence is applicable for quantized current generation (current that consists of only one or a few electrons).
Experimentally, this concept was mostly tested on devices made by III-V semiconductor heterostructures or metallic nanoparticles. In principle, the smaller the device is, the better the performance will be. However, the size of the devices made by either semiconductor heterostructures or metal particles is limited by current lithography technologies.
Most implementations of single electron pumps employ a very small region weakly coupled to electron reservoirs, usually described in metallic systems as an island. The island is sufficiently small that the addition or removal of a single electron makes an appreciable energetic difference. Researchers have proposed to modify this structure by replacing the normal-metal island with an individual single-walled carbon nanotube (SWCNT), which already possess nanoscale dimensions with diameters of about 1 nm.
Attempts to generate quantized current in SWCNTs have been made with various methods over the past few years, but were not very successful in obtaining a high degree of current quantization, especially when trying to generate current with more than one electron (read more: "Quantum dot nanodevices with carbon nanotubes").
A research team in Germany has now demonstrated the feasibility of using a single molecule – in this case, a single-walled carbon nanotube – for the generation of quantized electric current.
"With the concept of using superconducting electrodes and the natural nanosize of carbon nanotubes, we show that high degree of current quantization in SWCNTs, even with current with multiple electrons – 4 electrons – is possible," Chen-Wei Liang, a Postdoc researcher now at the University of Illinois at Urbana-Champaign, tells Nanowerk.
Scheme of the device. The SWCNT is contacted by superconducting Ti/Nb leads. (Image: Dr. Viktor Siegle, Max Planck Institute for Solid State Research)
Reporting their findings in the August 31, 2010 online issue of Nano Letters ("A Molecular Quantized Charge Pump"), first-authored by Viktor Siegle and Liang, the researchers demonstrate a single electron pump device that consists of a single-walled carbon nanotube with two niobium leads (similar devices using metallic nanoparticles were previously demonstrated by Pekola and his team in 2008).
"While it has been known that the smallness of natural objects like single molecules or carbon nanotubes has the potential for quantized current generation, successful experiments that demonstrate their feasibility for high-precision multiple-electron pumping have not been reported previously" says Liang. "Our work confirms such a potential experimentally."
A drawback of the devices demonstrated by the team is that they suffered seriously from noises that might be caused by defects in the CNTs, imperfect superconductor-CNT contacts, or the surrounding environment.
The team feels confident that several methods should help improve the device performance, e.g. reducing the defects of CNTs or covering CNTs with a protecting layer of insulator.
Liang points out that, in addition to the generation of quantized charge current, this technique could be applied to the study on other fundamental physical phenomena.
"For example" he says, "it has been shown that CNTs can transport electron spin and Cooper pairs over a long distance (micrometer scale). One could use our technique to design other innovative CNT-based devices to generate current of 'single electron spin' or 'single Cooper pair'. If that was possible, then one could study their dynamics in nanoelectronics because essentially these devices clock the particles (charge, spin, Cooper pair) that travel through the nanotube."
This technology shares the same challenge faced by all molecular electronics: How to gain control of single molecules, and massively fabricate devices with high reproducible properties. While the controllability of CNTs has been largely improved, it is still far behind when comparing with the conventional semiconductor devices.