Thanks to specialised microscopes, we have long been
able to see the beauty of single atoms. But strange though it might
seem, imaging larger molecules at the same level of detail has not
been possible – atoms are robust enough to withstand existing
tools, but the structures of molecules are not. Now researchers at
IBM have come up with a way to do it.
The earliest pictures of individual atoms were captured in the
1970s by blasting a target – typically a chunk of metal – with a
beam of electrons, a technique known as transmission electron
microscopy (TEM).
Later refinements of this technique, such as the TEAM project at
the Lawrence Berkeley National Laboratory in California achieved
resolutions of less than the radius of a single hydrogen atom. But
while this method works for atoms in a lattice or thin layer, the
electron bombardment destroys the arrangement of atoms in
molecules.
Other techniques use a tiny stylus-like scanning probe to
explore the atom-scale world. One method uses such a probe to
measure the charge density associated with individual atoms – a
technique called scanning tunnelling microscopy (STM).
Another, called atomic force microscopy (AFM), measures the
attractive force between atoms in the probe and the target. The
image is created by bumping the probe over the atoms of the
molecule – much in the way we might feel our way around in a dark
bedroom.
Both methods build up a picture of a target's surface and should
be suitable for imaging individual molecules. But they have not
been able to approach the detail of TEM.
Sticky problem
Leo Gross and his colleagues at IBM in Zurich, Switzerland,
modified the AFM technique to make the most detailed image yet of
pentacene, an organic molecule consisting of five benzene rings
(see picture).
The molecule is very fragile, but the researchers were able to
capture the details of the hexagonal carbon rings and deduce the
positions of the surrounding hydrogen atoms.
One key breakthrough was finding a way to stop the microscope's
tip from sticking to the fragile pentacene molecule because of
attraction due to electrostatic and van der Waals forces – van der
Waals is a weak force that operates only at an intermolecular
level.
The team achieved this by fixing a single carbon monoxide
molecule to the end of the probe so that only one atom of
relatively inactive oxygen came into contact with the
pentacene.
Although van der Waals force attracted the tip to its target, a
quantum-mechanical effect called the Pauli exclusion principle
pushed back. This happens because electrons in the same quantum
state cannot approach each other too closely. As the electrons
around the pentacene and carbon monoxide molecules are in the same
state, a small repulsive force operates between them.
Repulsive pictures
The researchers measured the repulsive force the probe
encountered at each point, and from this they could construct a
"force map" of the molecule. The level of detail available depends
on the size of the probe: the smaller the tip, the better the
picture.
The image is "astonishing", says Oscar Custance of Japan's
National Institute for Materials Science in Tsukuba. In 2007, his
team used AFM to distinguish individual atoms on a silicon surface,
but he acknowledges that the IBM team has surpassed this
achievement. "This is the highest resolution I have ever seen," he
says.
The IBM researchers believe their technique may open the door to
super-powerful computers whose components are built with precisely
positioned atoms and molecules. The work may also provide insights
into the actions of catalysts in reactions, allowing researchers to
understand what is happening at the atomic level, says Gross.
Journal reference: Science, DOI: 10.1126/science.1176210