Ultrasensitive Microscope Reveals How Charging Changes Molecular Structures

New ability to image molecules under charging promises big changes for molecular electronics and organic photovoltaics

Illustration of molecules in different charged states.
Illustration: IBM
Colored atomic force microscopy images of the four different molecules studied (1st row: azobenzene, 2nd row: pentacene, 3rd row: TCNQ, 4th row: porphine), in the different charge states accessed (1st column in blue: positive, 2nd column in green: neutral, 3rd column in orange: negative; 4th column in red: doubly negative).
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All living systems depend on the charging and discharging of molecules to convert and transport energy. While science has revealed many of the fundamental mechanisms of how this occurs, one area has remained shrouded in mystery: How does a molecule’s structure change while charging? The answer could have implications for range of applications including molecular electronics and organic photovoltaics.

Now a team of researchers from IBM Research in Zurich, the University of Santiago de Compostela and ExxonMobil has reported in the journal Science the ability to image, with unprecedented resolution, the structural changes that occur to individual molecules upon charging.

This ability to peer into this previously unobserved phenomenon should reveal the molecular charge-function relationships and how they relate to biological systems converting and transporting energy. This understanding could play a critical role in the development of both organic electronic and photovoltaic devices.

“Molecular charge transition is at the heart of many important phenomena, such as photoconversion, energy and molecular transport, catalysis, chemical synthesis, molecular electronics, to name some,” said Leo Gross, research staff member at IBM Zurich and co-author of the research. “Improving our understanding of how the charging affects the structure and function of molecules will improve our understanding of these fundamental phenomena.”

This latest breakthrough is based on research going back 10 years when Gross and his colleagues developed a technique to resolve the structure of molecules with an atomic force microscope. AFMs map the surface of a material by recording the vertical displacement necessary to maintain a constant force on the cantilevered probe tip as it scans a sample's surface.

Over the years, Gross and his colleagues refined the technique so it could see the charge distribution inside a molecule, and then were able to get it to distinguish between individual bonds of a molecule.

The trick to these techniques was to functionalize the tip of the AFM probe with a single carbon monoxide (CO) molecule. Last year, Gross and his colleague Shadi Fatayer at IBM Zurich believed that the ultra-high resolution possible with the CO tips could be combined with controlling the charge of the molecule being imaged.

“The main hurdle was in combining two capabilities, the control and manipulation of the charge states of molecules and the imaging of molecules with atomic resolution,” said Fatayer.

The concern was that the functionalization of the tip would not be able to withstand the applied bias voltages used in the experiment. Despite these concerns, Fatayer explained that they were able to overcome the challenges in combining these two capabilities by using multi-layer insulating films, which avoid charge leakage and allow charge state control of molecules.

The researchers were able to control the charge-state by attaching single electrons from the AFM tip to the molecule, or vice-versa. This was achieved by applying a voltage between the tip and the molecule. “We know when an electron is attached or removed from the molecule by observing changes in the force signal,” said Fatayer.

The IBM researchers expect that this research could have an impact in the fundamental understanding of single-electron based and molecular devices. This field of molecular electronics promises a day when individual molecules become the building blocks of electronics.

Another important prospect of the research, according to Fatayer and Gross, would be its impact on organic photovoltaic devices. Organic photovoltaics have been a tantalizing solution for solar power because they are cheap to manufacture. However, organic solar cells have been notoriously poor compared to silicon solar cells at converting sunlight to energy efficiently.

The hope is that by revealing how the structural changes of molecules under charge impact the charge transition of molecules, engineers will be able to further optimize organic photovoltaics.

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