Every chemist’s dream – to snap an atomic-scale picture of
achemical before and after it reacts – has now come true,
thanks to a new technique developed by chemists and
physicists at the University of California, Berkeley.
Using a state-of-the-art atomic force microscope, the
scientists have taken the first atom-by-atom pictures,
including images of the chemical bonds between atoms,
clearly depicting how a molecule’s structure changed during a
reaction. Until now, scientists have only been able to infer
this
type of information from spectroscopic analysis.
Non-contact atomic force microscope (nc-AFM) images (center) of a molecule before and after a reaction improve immensely over images (top) from a scanning tunneling microscope and look just like the classic molecular structure diagrams (bottom).
“Even though I use
these
molecules on a day to
day basis, actually being
able to see these
pictures blew me away.
Wow!” said lead
researcher Felix
Fischer,
UC Berkeley assistant
professor of chemistry.
“This was what my
teachers used to say
that you would never be
able to actually see, and
now we have it here.”
The ability to image
molecular reactions in
this way will help not
only
chemistry students as they study chemical structures and
reactions, but will also show chemists for the first time the
products of their reactions and help them fine-tune the
reactions to get the products they want. Fischer, along with
collaborator Michael Crommie, a UC Berkeley professor of
physics, captured these images with the goal of building new
graphene nanostructures, a hot area of research today for
materials scientists because of their potential application in
next-generation computers.
“However, the implications go far beyond just graphene,”
Fischer said. “This technique will find application in the study
of heterogeneous catalysis, for example,” which is used
widely in the oil and chemical industries. Heterogeneous
catalysis involves the use of metal catalysts like platinum to
speed reactions, as in the catalytic converter of a car.
“To understand the chemistry that is actually happening on a
catalytic surface, we need a tool that is very selective and
tells us which bonds have actually formed and which ones
have been broken,” he added. “This technique is unique out
there right now for the accuracy with which it gives you
structural information. I think it’s groundbreaking.”
“The atomic force microscope gives us new information
about the chemical bond, which is incredibly useful for
understanding how different molecular structures connect up
and how you can convert from one shape into another
shape,” said Crommie. “This should help us to create new
engineered nanostructures, such as bonded networks of
atoms that have a particular shape and structure for use in
electronic devices. This points the way forward.”
Fischer and Crommie, along with other colleagues at UC
Berkeley, in Spain and at the Lawrence Berkeley National
Laboratory (LBNL), published their findings online May 30 in
the journal Science Express.
From shadow to snapshot
Traditionally, Fischer and other chemists conduct detailed
analyses to determine the products of a chemical reaction,
and even then, the actual three-dimensional arrangement of
atoms in these products can be ambiguous.
“In chemistry you throw stuff into a flask and something else
comes out, but you typically only get very indirect
information
about what you have,” Fischer said. “You have to deduce
that
by taking nuclear magnetic resonance, infrared or ultraviolet
spectra. It is more like a puzzle, putting all the information
together and then nailing down what the structure likely is. But
it is just a shadow. Here we actually have a technique at hand
where we can look at it and say this is exactly the molecule.
It’s like taking a snapshot of it.”
An atomic force microscope probes a molecule adsorbed onto a surface, using a carbon monoxide molecule at the tip for sensitivity.
Fischer is developing new
techniques for making
graphene nanostructures
that display unusual
quantum properties that
could make them useful in
nano-scale electronic
devices. The carbon atoms
are in a hexagonal
arrangement like chicken
wire. Rather than cutting up
a sheet of pure carbon – graphene – he hopes to place a b
unch of smaller molecules onto a surface and induce them to
zip together into desired architectures. The problem, he said,
is how to determine what has actually been made.
That’s when he approached Crommie, who uses atomic
force
microscopes to probe the surfaces of materials with atomic
resolution and even move atoms around individually on a
surface. Working together, they devised a way to chill the
reaction surface and molecules to the temperature of liquid
helium – about 4 Kelvin, or 270 degrees Celsius below zero –
which stops the molecules from jiggling around. They then
used a scanning tunneling microscope to locate all the
molecules on the surface, and zeroed in on several to probe
more finely with the atomic force microscope. To enhance
the spatial resolution of their microscope they put a single
carbon monoxide molecule on the tip, a technique called non-
contact AFM first used by Gerhard Meyer and collaborators
at IBM Zurich to image molecules several years ago.
After imaging the molecule – a “cyclic” structure with several
hexagonal rings of carbon that Fischer created especially for
this experiment – Fischer, Crommie and their colleagues
heated the surface until the molecule reacted, and then again
chilled the surface to 4 Kelvin and imaged the reaction
products.
“By doing this on a surface, you limit the reactivity but you
have the advantage that you can actually look at a single
molecule, give that molecule a name or number, and later
look at what it turns into in the products,” he said.
“Ultimately, we are trying to develop new surface chemistry
that allows us to build higher ordered architectures on
surfaces, and these might lead into applications such as
building electronic devices, data storage devices or logic
gates out of carbon materials.”
The research is coauthored by Dimas G. de Oteyza, Yen-
Chia Chen, Sebastian Wickenburg, Alexander Riss, Zahra
Pedramrazi and Hsin-Zon Tsai of UC Berkeley’s Department
of Physics; Patrick Gorman and Grisha Etkin of the
Department of Chemistry; and Duncan J. Mowbray and Angel
Rubio from research centers in San Sebastián, Spain.
Crommie, Fischer, Chen and Wickenburg also have
appointments at Lawrence Berkeley National Laboratory.
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