IBM pushes AFM to image molecular structure


Researchers at IBM in Zurich, Switzerland, have captured the "anatomy" of a molecule using noncontact atomic-force microscopy (AFM), peering through the surrounding electron cloud to capture images "with unprecedented resolution."

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Figure 1.Ball-and-stick model of the pentacene molecule: five linearly fused hexagonal rings of benzene, comprised of 22 carbon atoms (inner gray balls) and to which are bound 14 hydrogen atoms (outer white balls). The entire molecule is 1.4nm in length; spacing between neighboring carbon atoms is 0.14nm. (Image courtesy of IBM Research/Zurich)

The method, a longtime goal of surface microscopy, involves an AFM operated in an ultrahigh vacuum at very low temperatures (-268°C), to image the chemical structure of individual pentacene molecules (1.4nm in length). Key was using an "atomically sharp" tip apex to measure the forces between the tip and sample. Also, picking up single atoms and molecules showed that the foremost tip atom/molecule governs the AFM contrast and resolution. Terminating the AFM tip with a carbon monoxide (CO) molecule was shown to yield optimum contrast at a height of ~0.5nm, notes IBM scientist Leo Gross, in a statement. Another key: deriving a complete 3D force map of the molecule, enabled by the AFM's mechanical and thermal stability.

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Figure 2. The inner structure of a pentacene molecule imaged with an atomic force microscope. Pixels correspond to actual data points. (Image courtesy of IBM Research/Zurich

Corroborating the results using first-principles density functional theory calculations, the researchers also figured out what caused the atomic contrast: Pauli repulsion between the CO and the pentacene molecule, explains IBM scientist Nikolaj Moll (referring to a quantum mechanical force that prevents two identical electrons from coming too close together). van der Waals and electrostatic forces, the scientists determined, "only add a diffuse attractive background."

The AFM's imagery, seen here compared to a diagram, is striking: hexagonal shapes of the five carbon rings and carbon atoms are clearly resolved, and hydrogen atoms also can be discerned. (IBM has posted more pictures on Flickr, and even a video on YouTube.)

Most significantly, says IBM, this atomic-scale imaging, combined with similar experiments earlier this summer that measured the charge state of single atoms, help better understand charge distribution at the atomic scale, pointing a way to create molecular-scale devices and networks.

The work was done in collaboration with Peter Liljeroth of Utrecht University, and published in the Aug. 28 issue of the journal Science. — J.M.

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