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Charge Distribution in Single Molecule Studied Using Kelvin Probe Force Microscopy

Published on February 29, 2012 at 4:38 AM

By Cameron Chai

Researchers from IBM have successfully imaged the distribution of charges in a molecule using a type of atomic force microscopy, known as Kelvin probe force microscopy (KPFM), at ultra high vacuum and reduced temperature.

3D representation of the experimental Kelvin probe force microscopy image of a single naphthalocyanine molecule, recorded with a CO-functionalized tip. Courtesy of IBM Research - Zurich

This will allow the scientists to perform a detailed analysis on formation of bonds and molecular switching between molecules and atoms.

In this study, on a conductive sample, the tip of the scanning probe will be positioned. Due to the difference in the electrical potentials in the sample and the tip, an electric field shall be produced. By using a voltage, the electric potentials can be measured. A higher electrical field on the charged molecules region results in a higher KPFM signal. The regions with opposite charges produce a different contrast in the micrograph, either red or blue, due to the reversal of the electric field direction.

An organic molecule, naphthalocyanine, which is a symmetric cross-shaped molecule, has been used in this study. In naphthalocyanine, two hydrogen atoms placed in opposition from the centre with a size of 2 nm, proved useful for the study. By employing a voltage pulse, a controllable switching of hydrogen atoms can be achieved among two configurations. This tautomerization impacts the molecular charge distribution and the molecules will be redistributed as the hydrogen atoms reach their positions.

The images were generated for distribution of charges in both the states. The study was done for a long time at the submolecular level, which required a higher mechanically and thermally stable microscope with greater atomic accuracy. A greater resolution can be achieved by introducing a carbon monoxide molecule to the tip.

The study was carried out by Gerhard Meyer, Nikolaj Moll, Leo Gross and Fabian Mohn of IBM Research.

The paper was published online in the journal Nature Nanotechnology.

Source: http://www.ibm.com/

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