A material that consists of atoms of a single element, but has completely different properties depend-ing 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 insu-lator. An international research team led by Empa's «nanotech@surfaces».
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 ar-eas 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 Gröning 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 Gröning. 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 Gröning. «Researchers from different disci-plines in Dübendorf, 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 semicon-ducting 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 Gröning.
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Nature