Not to pick up electrons, but tweezers made of electrons. A
recent paper by researchers from NIST and the University of Virginia (UVA)
demonstrates that the beams produced by modern electron microscopes can be used
not just to look at nanoscale objects, but to move them around, position them,
and perhaps even assemble them.
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 center 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’s never been seen, in part because electrons are much
fussier to work with. They can’t penetrate far through
air, for example, so electron microscopes use vacuum chambers to hold
specimens.
So Oleshko and his colleague, UVA materials scientist James
Howe, 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 quantized wave in the alloy’s electrons, that
reveals a lot about what happens at the liquid-solid boundary of a
crystallizing metal. “Scientifically, it’s 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 crystallization,”
Oleshko explains. “We can generate this sphere inside the liquid shell easily;
you can tell from the image that it’s 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. “Of course, this is
challenging because it requires a vacuum,” he says, “but electron probes can be
very fine, three orders of magnitude smaller than photon beams—close to the
size of single atoms. We could manipulate very small quantities, even single atoms,
in a very precise way.”
