Osaka University achieves measurement of electron distortion within a single molecule using light with a resolution of less than 1 nm

A research team led by Professor Yasuhiro Sugawara and Doctoral Student Tatsuya Yamamoto (at the time of this research) from the Graduate School of Engineering at Osaka University, Professor Hajime Ishihara from the Graduate School of Engineering Science at the same university, Associate Professor Nobuhiko Yokoshi from the Graduate School of Engineering at Osaka Metropolitan University, and Researcher Hidemasa Yamane from the Osaka Research Institute of Industrial Science and Technology, has succeeded for the first time in the world in imaging the complex electron distortion within a single molecule. This achievement was made possible using a photoinduced force microscope that measures the force (optical pressure) generated by light irradiation, achieving a resolution of less than 1 nm. The results were published in the online bulletin of ACS Nano.

The electron distribution of a molecule is influenced not only by its isolated environment but also by neighboring molecules and substrates. Information on energy and charge transfer in real environments is crucial for designing desirable molecular functions. Although scanning near-field optical microscopy has been employed to observe near-field optical fields, visualizing these factors with spatial resolution beyond the molecular scale has been unattainable.

The research team observed pentacene bilayers through a combination of photoinduced force microscopy and Kelvin probe force microscopy and theoretically analyzed the data. Consequently, they successfully observed the electron cloud distortion inside a single molecule where charge transfer occurred with a spatial resolution of 0.6 nm and imaged multipolar excitations within a single molecule that were previously unobservable.

The 2D mapping shows that the photoinduced force is stronger at the edges of the molecule and weaker in the center. According to the results of theoretical calculations, when no charge transfer occurs, the vertical component of the dipole is strongly excited at the molecular center. When charge transfer occurs, the electron cloud undergoes complex distortions, revealing that the vertical component of the dipole is strongly excited at the molecular edges and canceled out at the center.

The multipolar excited states that emerge with charge transfer typically absorb little or no light, rendering the molecules transparent and invisible through ordinary optical measurements. This discovery, unattainable with conventional methods, was made by combining microscopy techniques that visualize both the proximity optical response of the molecule and the charge transfer between neighboring environments.

The newly developed technology enables real-space visualization of the optical response of molecules affected by charge transfer on a molecular scale. It provides a method for designing molecular functions based on optical responses at each stage of assembling molecules layer by layer. Thus, it is expected to be a fundamental technology for developing innovative photocatalytic and solar cell materials.

Sugawara stated, "The scientific exploration of the interaction between matter and light at the atomic level is a treasure trove of academic research challenges. Therefore, this technology holds high potential for discovering new physical phenomena and functionalities beyond conventional understanding. Such discoveries are expected to lead to the creation of novel materials and devices based on new concepts."

Journal Information
Publication: ACS Nano
Title: Optical Imaging of a Single Molecule with Subnanometer Resolution by Photoinduced Force Microscopy
DOI: 10.1021/acsnano.3c10924