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AFM captures changing structures of charged molecules | Investigation

An image that shows the porphine structure.

The precise image of the individual molecules has been taken to a new level, with a demonstration that the electrons can be delivered one by one and the resulting changes in the molecular structure that are perceived in the images.one

The technique is reported by Leo Gross and his colleagues at the IBM research labs in Zürich, Switzerland. The findings are "impressive and convincing," says Daniel Ebeling of the Justus Liebig University in Giessen, Germany, who was not involved in the research. He adds that the technique "can provide fundamental information that goes far beyond simply improving the resolution of the image, and should allow us to increase our understanding of the reaction processes on the surface."

Gross's team has previously shown that probing molecules adsorbed to surfaces with an atomic force microscope (AFM), at the tip of which is attached a molecule of carbon monoxide (CO), can deliver images with an incredibly high resolution high. Previously, they have used the method to obtain images of individual molecules that undergo chemical reactions and charge transfer processes.

To obtain images of the charged states, the researchers placed the molecules on an insulating surface, using very thin films of sodium chloride deposited in the copper. They cooled the system to 5K to suppress the movements and vibrations of the molecules, and used a tip of AFM terminated in CO to obtain images of several organic molecules in ultra high vacuum.

Gross and his colleagues could transfer the charge of one electron at a time to individual molecules by applying a voltage to the tip. They measured how the force at the tip varied with voltage, a method called Kelvin probe force microscopy. The staggered steps in the force as the voltage varied indicated a change in the state of charge.

Among the molecules that Gross's team has loaded is azobenzene, an organic molecule commonly used as a photomechanical molecular switch because it can be transformed between cis Y trans isomers They found that in neutral trans Point out that the two benzene rings at each end of the molecule are tilted off the plane in the same direction, but the addition of an electron causes one ring to tilt to the other side, introducing a turn. The calculations of the molecular structure using the functional density theory (DFT) predict the same behavior.

The researchers also loaded pentacene molecules: linear chains of five fused benzene rings. They could solve the slight changes in the length of the links in different rings, since a charge of two electrons alters the bonding pattern, the different carbon-carbon bonds change their bonding orders, usually somewhere between one or two, in different locations.

An image showing azobenzene, pentacene, TCNQ, different porfinein loading states

Meanwhile, in the case of the tetracyanoquinodimethane molecule (TCNQ), a strong electron acceptor that is often used in molecular electronics, the charge of the neutral molecule changed it from a conformation that rises above the surface to a flat , and alter the union within molecule. This change in orientation as a function of the state of charge has not been seen before, and Gross says that the effect "could be useful as a molecular change in a device based on the jump of a single electron".

By loading a ring of porphyrin, the key molecular group in chlorophyll, where light absorption causes charge transfer, the IBM team observed changes in the binding due to the effects of conjugation, which again agreed with the calculations of the DFT. Such an agreement is reassuring, but it does not make the experiments redundant. DFT is based on several assumptions and approximations, and so, although "often it is correct, we can not trust it," says Gross. IBM team member Nikolaj Moll, a specialist in such calculations, says that "comparing experiments with DFT results can help find the limitations of the theory and improve it."

The ability to analyze the binding order in these experiments with charged molecules is impressive, says Yoshiaki Sugimoto, of the University of Tokyo in Japan, a specialist in molecular imaging. "The combination of charge state control and submolecular resolution images paves the way for accurate characterization of charged molecules," he says.

Gross says the technique could also help synthesize new molecules and molecular devices by manipulating atoms. His team has used the AFM method previously to make a molecule that is difficult to synthesize by traditional means, and recently demonstrated that, by creating states of molecules with multiple charges, they could form and break bonds.two "The identification of radical sites such as those of the pentacene and the resolution of structural changes in the charge will help us better understand and predict these new synthetic routes," he says.

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