Quantum chains in graphene nanoribbons

A material that consists of atoms of a single element, but has completely different properties depending on the atomic arrangement - this may sound strange, but is actually reality with graphene nano-ribbons. The ribbons, which are only a few carbon atoms wide and exactly one atom thick, have very different electronic properties depending on their shape and width: conductor, semiconductor or insulator.

An international research team led by Empa's laboratory has now succeeded in precisely adjusting the properties of the ribbons by specifically varying their shape. The particular feature of this technology is that not only can the "usual" electronic properties mentioned above be varied - it can also be used to generate specific local quantum states.

So what's behind it? If the width of a narrow graphene nanoribbon changes, in this case from seven to nine atoms, a special zone is created at the transition: because the electronic properties of the two areas differ in a special, so-called topological way, a "protected" and thus very robust new quantum state is created in the transition zone. This local electronic quantum state can now be used as a basic component to produce tailor-made semiconductors, metals or insulators - and possibly even as a component in quantum computers.

The Empa researchers under the lead of Oliver Groning were able to show that if these ribbons are built with regularly alternating zones of different widths, a chain of interlinked quantum states with its own electronic structure is created by the numerous transitions.

The exciting thing is that the electronic properties of the chain change depending on the width of the different segments. This allows them to be finely adjusted - from conductors to semiconductors with different bandgaps. This principle can be applied to many different types of transition zones - for example, from seven to eleven atoms.

"The importance of this development is also underlined by the fact that a research group at the Uni-versity of California, Berkeley, came to similar results independently of us," said Groning. The work of the US research team has been published in the same issue of Nature.

On the way to nanoelectronics
Based on these novel quantum chains, precise nano-transistors could be manufactured in the future - a fundamental step on the way to nanoelectronics. Whether the switching distance between the "1" state and the "0" state of the nanotransistor is actually large enough depends on the bandgap of the semiconductor - and with the new method this can be set almost at will.

In reality, however, this is not quite as simple: for the chain to have the desired electronic properties, each of the several hundred or even thousands of atoms must be in the right place. "This is based on complex, interdisciplinary research, " says Empa researcher Groning.

"Researchers from different disciplines in Dubendorf, Mainz, Dresden, and Troy (USA) worked together - from theoretical understanding and specific knowledge of how precursor molecules have to be built and how structures on surfaces can be selectively grown to structural and electronic analysis using a scanning tunneling microscope."

An excursion into the quantum realm
Ultrasmall transistors - and thus the next step in the further miniaturization of electronic circuits - are the obvious application possibilities here: although they are technically challenging, electronics based on nano-transistors actually work fundamentally the same as today's microelectronics. The semiconducting nanoribbons produced by the Empa researchers would allow transistors with a channel cross-section 1,000 times smaller than typically manufactured today. However, further possibilities can also be imagined, for example in the field of spintronics or even quantum informatics.

This is because the electronic quantum states at junctions of graphene nanoribbons of different widths can also carry a magnetic moment. This could make it possible to process information not by charge as was previously customary, but by the so-called spin - in the figurative sense the "direction of rota-tion" of the state.

And the development could even go one step further. "We have observed that topological end states occur at the ends of certain quantum chains. This offers the possibility of using them as elements of so-called qubits - the complex, interlocked states in a quantum computer," ex-plains Oliver Groning.

Today and tomorrow, however, no quantum computer is built from nanoribbons - there is still a lot of research needed, says Groning: "The possibility of flexibly adjusting the electronic properties through the targeted combination of individual quantum states represents a major leap for us in the production of new materials for ultra-miniaturized transistors." The fact that these materials are stable under environmental conditions plays an important role in the development of future applications.

"The further-reaching potential of the chains to create local quantum states and link them together in a targeted manner is also fascinating," Groning continues.

"Whether this potential can actually be ex-ploited for future quantum computers remains to be seen, however. It is not enough to create localized topological states in the nanoribbons - these would also have to be coupled with other materials such as superconductors in such a way that the conditions for qubits are actually met."

Research paper

Related Links
Swiss Federal Laboratories for Materials Science and Technology (EMPA)
Carbon Worlds - where graphite, diamond, amorphous, fullerenes meet

Tweet

Thanks for being there;
We need your help. The SpaceDaily news network continues to grow but revenues have never been harder to maintain.

With the rise of Ad Blockers, and Facebook - our traditional revenue sources via quality network advertising continues to decline. And unlike so many other news sites, we don't have a paywall - with those annoying usernames and passwords.

Our news coverage takes time and effort to publish 365 days a year.

If you find our news sites informative and useful then please consider becoming a regular supporter or for now make a one off contribution.

SpaceDaily Monthly Supporter
$5+ Billed Monthly

paypal only

SpaceDaily Contributor
$5 Billed Once

credit card or paypal

What makes diamonds blue
Washington DC (SPX) Aug 07, 2018
Blue diamonds - like the world-famous Hope Diamond at the National Museum of Natural History - formed up to four times deeper in the Earth's mantle than most other diamonds, according to new work published on the cover of Nature. "These so-called type IIb diamonds are tremendously valuable, making them hard to get access to for scientific research purposes," explained lead author Evan Smith of the Gemological Institute of America, adding, "and it is very rare to find one that contains inclusions, ... read more