IBM Scientists Distinguish Individual Molecular Bonds

IBM announced that its researchers have found a way to differentiate chemical bonds in individual molecules, a discovery that could lead to further nanotechnology breakthroughs.

IBM announced that its 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).

According to Big Blue, 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 high-bandwidth wireless communication and electronic displays.

"We found two different contrast mechanisms to distinguish bonds," said IBM scientist Leo Gross, in a statement. "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. 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 Sept. 14 issue of Science magazine, IBM Research scientists imaged the bond order and length of individual carbon-carbon 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 Investigacion en Quimica Bioloxica e Materiais Moleculares (CIQUS) at the Universidade de Santiago de Compostela and Centre National de la Recherche Scientifique (CNRS).

The individual bonds between carbon atoms in such molecules differ subtly in their length and strength, IBM said in a press release about the breakthrough. All the important chemical, electronic and optical properties of such molecules are related to the differences of bonds in the polyaromatic systems, IBM said. 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 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, which is about one-hundredth of an atom's diameter, IBM said.

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, IBM said. Therefore, the scientists had to select and synthesize molecules in which perturbing background effects could be ruled out.

Scientists have been striving to "see" and manipulate atoms and molecules to extend human knowledge and push the frontiers of manufacturing capabilities to the nanometer regime. IBM has been a pioneer in nanoscience and nanotechnology ever since the development of the scanning tunneling microscope (STM) in 1981 by IBM Fellows Gerd Binnig and Heinrich Rohrer at IBM Research–Zurich.

For this invention, which made it possible to image individual atoms and later on to manipulate them, Binnig and Rohrer were awarded the Nobel Prize in Physics in 1986. The AFM, an offspring of the STM, was invented by Binnig in 1986. The STM is widely regarded as the instrument that opened the door to the nanoworld, IBM said.

A new facility for world-class collaborative nanoscale research, the Binnig and Rohrer Nanotechnology Center was opened last year on the campus of IBM Research–Zurich. The Center is part of a strategic partnership in nanotechnology with ETH Zurich, one of Europe's premier technical universities.