"Imagine if you could construct, from the ground up, an artificial environment for electrons in an ultraclean system. Then, instead of using imperfect real-world materials to investigate electron physics, you could simply design the experiment you need", team member Jonah Waissman told nanotechweb.org.

"This is what we have achieved, using a remarkable material – the carbon nanotube. Using our technique, we are able to make completely clean nanotube devices in which we can precisely control electrons."

Making complex devices with pristine nanotubes

Until now, there were only three main ways to make nanotube devices – by fabricating circuits on the top of tubes, growing the tubes on top of circuits or mechanically transferring nanotubes onto circuits. All of these techniques can be only used to make relatively simple devices though.

"Our new technique goes further by splitting device fabrication into two independent parts," says Waissman. "In the first step, we grow nanotubes on a chip, and on another chip we fabricate the electrical circuit. Making these two parts separately allows us to make each of them as perfect as possible. Then we can combine them to form a fully-functional, complex device."

The researchers, led by Shahal Ilani, began by making a silicon chip with many parallel nanotubes grown suspended across trenches that are 100 μm wide. Next, they had to squeeze the complex circuit into a structure small enough to fit into these trenches. To do this they fabricated the circuit on a separate chip, on top of a narrow and tall silicon pillar.

"We bring these two components close to each other using a specially-designed scanning probe microscope (SPM) that allows us to control the positions of the two chips with nanoscale accuracy," explains Waissman. "We then move the electrical circuit, at the edge of the silicon pillar, into a predefined trench until it gently touches a single nanotube."

Ilani and colleagues are then able to measure how clean the nanotube is within the same SPM setup. "If it looks dirty, we detach it from the circuit and find another tube," says Waissman. "If it looks good, we can cut the nanotube at two well-defined points by passing large electrical current through adjacent pairs of contacts at the edges of the circuit. This process separates the nanotube from the nanotube chip, leaving only the short nanotube segment that ultimately forms the active part of the device on the circuit."

Precision control

Since the researchers can continually measure the properties of the nanotubes during the entire fabrication process, and can detach and reattach nanotubes at will, they are free to choose which nanotubes to use and where they ultimately lie in a device. "This precision control also means that we can put multiple nanotubes at different positions in a circuit with nanoscale accuracy," adds Waissman.

Using this technology the researchers succeeded in constructing novel devices consisting of a nanotube suspended over seven gates and with multiple nanotubes combined into a single functional circuit. They also showed that they could use a "piano" of gates to shift individual electrons at will from point to point on the suspended nanotube and even shape their wavefunctions along the tube. "The electrons behave in a ‘clean’ way," explains Ilani, "and they only respond to the potential generated by the gates."

The new circuits open the way to hitherto "unimaginable" experiments, says Waissman. "With such pristine electron systems, we will be able, for example, to generate and explore fundamental new phases of matter of interacting electrons. We will also be able to make almost faultless nanomechanical systems from the suspended nanotubes that will interact with electrons to produce new electromechanical phenomena."

"Finally, being able to localize electrons along the circuits will allow us to realize chains of electron spins for quantum information processing."

Excellent charge detectors

And if that is not enough: the researchers have also found that their clean devices can make for excellent charge detectors. Indeed, they have already started to use their nanotube circuits to image on the nanoscale the physics in other interesting materials, they reveal.

"Our system stands as a milestone on the route to ‘quantum simulation’ – the ability to construct quantum systems from the ground up," explains Waissman. "Other experiments in this vein typically involve weakly-interacting bosonic components, like cold atoms or photons, but our set-up is unique because it provides us with a clean and tuneable system for manipulating electrons (which are fermions) that strongly interact with other."

The present research is published in Nature Nanotechnology doi:10.1038/nnano.2013.143.

Further reading

Wigner molecule appears in carbon nanotube (Aug 2013)
Putting a new spin on nanotubes (Mar 2008)