Oct 1, 2012
Nanostructure could help make electron cloak
Researchers at the Massachusetts Institute of Technology have put forward a way to make an "electron cloak" – or an object that is invisible to electrons. Inspired by cloaks that hide objects from light waves, the electron cloak would be made of a nanostructure that is around the same size as the wavelength of electrons themselves. The new approach might be exploited to make novel electronic devices and perhaps even help develop better thermoelectric materials for improved energy harvesting and conversion.
Researchers have already succeeded in making "invisibility cloaks" that hide objects from electromagnetic waves. Such cloaks are made from "metamaterials", which are artificial structures with special optical properties such as negative indices of refraction. These structures are arranged in such a way that incoming light waves flow smoothly around the cloak, meeting up on the other side as if the cloak was simply not there.
The same principle can be applied to sound waves – to make cloaks invisible to sound – and now even to electrons, according to new theory calculations by Gang Chen’s team at MIT. The researchers have put forward a practical design for the electron cloak – which would be made of core-shell nanoparticle structures embedded in a host semiconductor – that does not disturb the flow of electrons.
Electrons normally travel as waves over a certain distance before scattering destroys their wave phases. Over this so-called coherent transport length, the particles exhibit characteristic wave behaviour, such as amplitude superposition (or interference).
Reflecting electron waves
"In our electron cloak design, the core-shell nanoparticles essentially provide multiple interfaces where electron waves are reflected," explained team member Bolin Liao. "Through careful tuning of the interfaces, the multiple reflected waves from the interfaces can destructively interfere with each other and cancel the total reflection almost perfectly. The electron waves with the 'correct energy' can thus travel through the nanoparticle structure without being reflected, as if there was nothing in their way."
The nanoparticle structures are about the same size as the wavelength of electrons themselves – around 10 nm in this case.
Such electron cloaks may find use in applications where high electron mobility is required, such as in semiconductor electronics, says Chen. "We might also be able to design novel electronic switches that go from the visible ('open' structure) and invisible ('closed') states," he told nanotechweb.org. "What is more, the electron scattering versus energy profile of the structures, which varies greatly, could benefit applications that call for strong energy-dependent scattering mechanisms, like those at work in thermoelectric devices."
The team is now busy putting its theories into practise, by trying to make real core-shell nanoparticle electron cloaks. "We are also looking at extending our idea to lower-dimensional structures," added team member Mona Zebarjadi.
The current work is detailed in Physical Review Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org.