Essentially, they say, the tool is an electron version of the laser “optical tweezers” that have become a standard tool in biology, physics and chemistry for manipulating tiny particles. Except that electron beams could offer a thousand-fold improvement in sensitivity and resolution.
Optical tweezers were first described in 1986 by a research team at Bell Labs. The general idea is that under the right conditions, a tightly focused laser beam will exert a small but useful force on tiny particles. Not pushing them away, which you might expect, but rather drawing them towards the centre of the beam. Biochemists, for example, routinely use the effect to manipulate individual cells or liposomes under a microscope.
If you just consider the physics, says NIST metallurgist Vladimir Oleshko, you might expect that a beam of focused electrons — such as that created by a transmission electron microscope (TEM) — could do the same thing. However that is never been seen, in part because electrons are much fussier to work with. They can not penetrate far through air, for example, so electron microscopes use vacuum chambers to hold specimens.
So Oleshko and James Howe, UVA materials scientist, were surprised when, in the course of another experiment, they found themselves watching an electron tweezer at work. They were using an electron microscope to study, in detail, what happens when a metal alloy melts or freezes. They were observing a small particle — a few hundred microns wide — of an aluminum-silicon alloy held just at a transition point where it was partially molten, a liquid shell surrounding a core of still solid metal. In such a small sample, the electron beam can excite plasmons, a kind of quantised wave in the alloy’s electrons, that reveals a lot about what happens at the liquid-solid boundary of a crystallising metal. “Scientifically, it is interesting to see how the electrons behave,” says Howe, “but from a technological point of view, you can make better metals if you understand, in detail, how they go from liquid to solid.”
“This effect of electron tweezers was unexpected because the general purpose of this experiment was to study melting and crystallisation,” Oleshko explains. “We can generate this sphere inside the liquid shell easily; you can tell from the image that it is still crystalline. But we saw that when we move or tilt the beam — or move the microscope stage under the beam — the solid particle follows it, like it was glued to the beam.”
Potentially, Oleshko says, electron tweezers could be a versatile and valuable tool, adding very fine manipulation to wide and growing lists of uses for electron microscopy in materials science.
COMPAMED.de; Source: National Institute of Standards and Technology (NIST)