Sep 24, 2008
Atom expulsion shapes nanodevices
Nanodevices can be shaped using a highly focused electron beam. So say researchers at the University of Illinois at Urbana Champaign in the US who have developed a technique called electron-beam expulsion of atoms (EBESA) to etch nanoholes as small as 2.5 nm across in multiwalled carbon nanotubes and niobium nanowires. If improved, the method might be used make single-electron and field-effect transistors, photon and chemical detectors and even superconducting-based solid-state qubits.
The technique, also dubbed Angstrom-fabrication by the researchers, will allow them to fabricate electron devices, like transistors, with single-atom precision. If the electron beam from a transmission electron microscope (TEM) is focused to the smallest achievable spot size (currently about the diameter of a single carbon atom) on a thin substrate, single atoms could controllably be removed one by one, explains team leader Alexey Bezryadin.
"Although our experiments have not yet reached this high resolution, we have demonstrated nanoholes with a diameter of 2.5 nm, etched through carbon nanotubes and niobium nanowires," he told nanotechweb.org. The axis of the hole is perpendicular to the nanotube's axis.
The researchers' technique involves using a high voltage to accelerate the electrons in a TEM to elevated energies of 200 keV. The high-energy electrons expel atoms in the sample as they collide with them. "Another great thing is that a modern TEM allows us to focus such high-energy electrons into a spot that is less than 1 Angstrom (0.1 nm) in diameter," added Bezryadin. To compare, the distance between neighbouring atoms in graphene is 1.4 Angstroms.
Removing atoms from a sample using such electronic impacts is normally avoided because it damages the sample. "However, by focusing the beam, we can achieve nanoscale etching with electrons similar to how sand blasting works with sand," explained graduate student Thomas Aref. "The technique is possible at scales small enough to engineer nanoscale objects such as carbon nanotubes and niobium nanowires."
At present, the technique can be used to create prototype devices in the lab with a resolution of 2–3 nm. Further improvement should eventually lead to targeted and controlled removal of single atoms from graphene, for example.
The team now plans to perform electrical measurements on its nanoengineered objects to determine how the modifications affect their transport properties. "For instance, a modified nanotube might have properties similar to those of a single electron transistor", commented Bezryadin.
The electron-beam piercing of nanoholes was done by Mikas Remeika, then an undergraduate in Bezryadin's lab at UIUC. Remeika is now a graduate student at the University of California, San Diego. His colleague Aref etched nanoholes and other structures through multiwalled carbon nanotubes.
The researchers reported their work in Journal of Applied Physics.
About the author
Belle Dumé is contributing editor at nanotechweb.org