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2009-08-28 05:06:30
By Jason Palmer
Science and technology reporter, BBC News
The detailed chemical structure of a single molecule has been imaged for the
first time, say researchers.
The physical shape of single carbon nanotubes has been outlined before, using
similar techniques - but the new method even shows up chemical bonds.
Understanding molecular structure on this scale could help in the design of
many things on the molecular scale, particularly electronics or even drugs.
The IBM researchers report their findings in the journal Science.
It is the same group that in July reported the feat of measuring the charge on
a single atom.
Fine tuning
In both cases, a team from IBM Research Zurich used what is known as an atomic
force microscope or AFM.
Their version of the device acts like a tiny tuning fork, with one of the
prongs of the fork passing incredibly close to the sample and the other farther
away.
When the fork is set vibrating, the prong nearest the sample will experience a
minuscule shift in the frequency of its vibration, simply because it is getting
close to the molecule.
Comparing the frequencies of the two prongs gives a measure of just how close
the nearer prong is, effectively mapping out the molecule's structure.
The measurement requires extremes of precision. In order to avoid the effects
of stray gas molecules bounding around, or the general atomic-scale jiggling
that room-temperature objects experience, the whole setup has to be kept under
high vacuum and at blisteringly cold temperatures.
However, the tip of the AFM's prong is not well-defined and isn't necessarily
sharp on the scale of single atoms. The effect of this bluntness is to blur the
instrument's images.
The researchers have now hit on the idea of deliberately picking up just one
small molecule - made of one atom of carbon and one of oxygen - with the AFM
tip, forming the sharpest, most well-defined tip possible.
Their measurement of a pentacene molecule using this carbon monoxide tip shows
the bonds between the carbon atoms in five linked rings, and even suggests the
bonds to the hydrogen atoms at the molecule's periphery.
Tip of the iceberg
Lead author of the research Leo Gross told BBC News that the group is aiming to
combine their ability to measure individual charges with the new technique,
characterising molecules at a truly unprecedented level of detail.
That will help in particular in the field of "molecular electronics", a
potential future for electronics in which individual molecules serve as
switches and transistors.
Although the approach can trace out the ethereal bonds that connect atoms, it
cannot distinguish between atoms of different types.
The team aims to use the new technique in tandem with a similar one known as
scanning tunnelling microscopy - in which a tiny voltage is passed through the
sample - to determine if the two methods in combination can deduce the nature
of each atom in the AFM images.
That would help the entire field of chemistry, in particular the synthetic
chemistry used for drug design.
The results are of wide interest to others who study the nano-world with
similar instruments. For them, implementing the same approach is as simple as
picking up one of these carbon monoxide molecules with their AFM before taking
a measurement.