Joined: 16 Mar 2004
|Posted: Fri Jan 09, 2009 12:32 pm Post subject: Atoms Shape Up with Precision Sculpting
|French physicists use STEM as a carving tool
Physicists in France have developed a new way to shape nanomaterials using a scanning electron microscope as a highly precise cutting tool. The technique allows materials like carbon nanotubes to be carved by removing individual atoms from chosen locations and could open the way to nanoengineering a variety of structures using a versatile top-down approach.
Electron microscopy uses high-energy electrons to obtain images of samples on the nanoscale. However, the electron beam also interacts with the sample and can destroy it. This is because energy is transferred from the electron beam to the sample, knocking atoms out of the material.
Now, Alberto Zobelli of the University Paris-Sud (Orsay) and colleagues have turned this problem to their advantage. "Our idea is to use a scanning transmission electron microscope (STEM) as a carving tool with nanometric precision, with the added bonus of imaging what we are doing at the same time," said Zobelli.
Previous research on modifying nanomaterials with an electron beam was not controlled enough to induce damage only where it was wanted and not elsewhere. To achieve this, the microscopy team in Orsay used theory to quantify the interaction between the material and the electron beam at different voltages.
"The work shows that the damage cross-section of individual atoms – that is, their chance of being knocked out of place – is not uniform," explained Zobelli. "Instead, it strongly depends on whether the atoms are on the sides or base of a nanotube, next to defects or in a perfect lattice."
Knowing such emission probabilities has allowed the researchers to design a new class of irradiation experiments using a dedicated STEM. The technique is similar to conventional ablation using a focused ion beam where the beam scans a limited zone on the surface of the sample and atoms are ejected from a small area. However, unlike ions, electron irradiation is much better suited to nanostructures as it produces lower sputtering rates. Moreover, the irradiated zone can be chosen with an extremely high spatial precision of just 0.5 nm.
The researchers successfully used their technique to locally change the diameter of single-walled organic and inorganic nanotubes. "We chopped out small notches just a few nanometres in length of the tube exactly where we wanted, leaving the rest of the tube undamaged," said Zobelli. "And we can reshape the tubes by repeating the procedure on different sections of the tube."
The team's calculations show that once an atom is removed from a nanotube, it becomes much easier to remove a neighbouring atom, and so on for a series of atoms in a line. "This means that adjacent atoms are sequentially removed under irradiation through a 'laddering mechanism'," explained Zobelli. "The tube then reconstructs, repairing any defects generated by the electron beam."
Being able to shape nanotubes in this controlled way could be technologically important, say the researchers. "Removing a line of atoms in a nanotube changes the chirality of the tube and thus its electronic character," they explained. "This means that nanotubes could be switched from being metal to semiconductor and vice versa – a good way to produce a Schottky diode at the nanoscale!"
The irradiation method might be extended to other nanosystems. "For example, we could cut graphene layers and produce nanoribbons of well-defined widths," explained Zobelli. "We might also be able to make size-limited quantum dots."
Last month, the lab at Paris-Sud was equipped with a new generation STEM mounted with a spherical corrector that produces previously unachievable probe sizes of just 0.1 nm, revealed Christian Colliex, microscopy group leader. "We now expect to open up new frontiers in electron irradiation with the possibility of 'seeing' the structure of generated defects in more detail. For certain samples, we might even be able to count the number of sputtered atoms."
And as if all that was not enough, the researchers say that they can tune the microscope and precisely control the effect it has on nanomaterials by quantifying the interaction between the electron beam and the host material.