A nanographene molecule exhibiting carbon-carbon bonds of different length and bond order imaged by noncontact atomic force microscopy using a carbon monoxide functionalized tip. This molecule was synthesized at the Centre National de la Recherche Scientifique (CNRS) in Toulouse. Image: IBM Research – Zurich |
IBM
scientists have been able to differentiate the chemical bonds in
individual molecules for the first time using a technique known as
noncontact atomic force microscopy (AFM).
The
results push the exploration of using molecules and atoms at the
smallest scale and could be important for studying graphene devices,
which are currently being explored by both industry and academia for
applications including highbandwidth wireless communication and
electronic displays.
“We
found two different contrast mechanisms to distinguish bonds. The first
one is based on small differences in the force measured above the
bonds. We expected this kind of contrast but it was a challenge to
resolve,” said IBM scientist Leo Gross. “The second contrast mechanism
really came as a surprise: Bonds appeared with different lengths in AFM
measurements. With the help of ab initio calculations we found that the
tilting of the carbon monoxide molecule at the tip apex is the cause of
this contrast.”
As
reported in the cover story of the September 14 issue of Science
magazine, IBM Research scientists imaged the bond order and length of
individual carboncarbon bonds in C60, also known as a buckyball for its
football shape and two planar polycyclic aromatic hydrocarbons (PAHs),
which resemble small flakes of graphene. The PAHs were synthesized by
Centro de Investigación en Química Biolóxica e Materiais Moleculares
(CIQUS) at the Universidade de Santiago de Compostela and Centre
National de la Recherche Scientifique (CNRS) in Toulouse.
The
individual bonds between carbon atoms in such molecules differ subtly
in their length and strength. All the important chemical, electronic,
and optical properties of such molecules are related to the differences
of bonds in the polyaromatic systems. Now, for the first time, these
differences were detected for both individual molecules and bonds. This
can increase basic understanding at the level of individual molecules,
important for research on novel electronic devices, organic solar cells,
and organic light-emitting diodes (OLEDs). In particular, the
relaxation of bonds around defects in graphene as well as the changing
of bonds in chemical reactions and in excited states could potentially
be studied.
As in their earlier research The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy the IBM scientists used an atomic force microscope (AFM) with a tip
that is terminated with a single carbon monoxide (CO) molecule. This tip
oscillates with a tiny amplitude above the sample to measure the forces
between the tip and the sample, such as a molecule, to create an image.
The CO termination of the tip acts as a powerful magnifying glass to
reveal the atomic structure of the molecule, including its bonds. This
made it possible to distinguish individual bonds that differ only by 3
picometers or 3 × 10-12 m, which is about one-hundredth of an
atom’s diameter.
In
previous research the team succeeded in imaging the chemical structure
of a molecule, but not the subtle differences of the bonds.
Discriminating bond order is close to the current resolution limit of
the technique and often other effects obscure the contrast related to
bond order. Therefore the scientists had to select and synthesize
molecules in which perturbing background effects could be ruled out.
To
corroborate the experimental findings and gain further insight into the
exact nature of the contrast mechanisms, the team performed
first-principles density functional theory calculations. Thereby they
calculated the tilting of the CO molecule at the tip apex that occurs
during imaging. They found how this tilting yields a magnification and
the very sharp images of the bonds.
This
research was funded within the framework of several European projects
including ARTIST, HERODOT, CEMAS, the Spanish Ministry of Economy and
Competitiveness and the Regional Government of Galicia.
Bond-Order Discrimination by Atomic Force Microscopy
Source: IBM
