Using noncontact atomic force microscopy researchers at IBM, with the help of folks at the Utrecht University, managed to view the structure of pentacene ( a polycyclic aromatic hydrocarbon molecule) down to the resolution of individual atoms.
With their AFM, the IBM scientists, for the first time ever, were able to look through the electron cloud and see the atomic backbone of an individual molecule. While not a direct technological comparison, this is reminiscent of x-rays that pass through soft tissue to enable clear images of bones.
The AFM uses a sharp metal tip to measure the tiny forces between the tip and the sample, such as a molecule, to create an image. In the present experiments, the molecule investigated was pentacene. Pentacene is an oblong organic molecule consisting of 22 carbon atoms and 14 hydrogen atoms measuring 1.4 nanometers in length. The spacing between neighboring carbon atoms is only 0.14 nanometers—roughly 1 million times smaller then the diameter of a grain of sand. In the experimental image, the hexagonal shapes of the five carbon rings as well as the carbon atoms in the molecule are clearly resolved. Even the positions of the hydrogen atoms of the molecule can be deduced from the image.
“The key to achieving atomic resolution was an atomically sharp and defined tip apex as well as the very high stability of the system,” said IBM scientist Leo Gross. To image the chemical structure of a molecule with an AFM, it is necessary to operate in very close proximity to the molecule. The range, where chemical interactions give significant contributions to the forces, is less than a nanometer. To achieve this, the IBM scientists were required to increase the sensitivity of the tip and overcome a major limitation: Similar to the way two magnets would attract or repel each other when getting close, the molecules would easily be displaced by or attach to the tip when the tip was approached too closely—rendering further measurements impossible.
Gross added, “We prepared our tip by deliberately picking up single atoms and molecules and showed that it is the foremost tip atom or molecule that governs the contrast and resolution of our AFM measurements.” A tip terminated with a carbon monoxide (CO) molecule yielded the optimum contrast at a tip height of approximately 0.5 nanometers above the molecule being imaged and—acting like a powerful magnifying glass—resolved the individual atoms within the pentacene molecule, revealing its exact atomic-scale chemical structure.
Furthermore, the scientists were able to derive a complete three-dimensional force map of the molecule investigated. “To obtain a complete force map the microscope needed to be highly stable, both mechanically and thermally, to ensure that both the tip of the AFM and the molecule remained unaltered during the more than 20 hours of data acquisition,” says Fabian Mohn, who is working on his Ph.D. thesis at IBM Research – Zurich.
Press release: IBM Scientists First to Image the “Anatomy” of a Molecule …
Abstract in Science: The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy
(hat tip: Gizmodo)