The ability to observe the behavior of individual molecules has long been a goal of chemists and physicists. Now, researchers at the University of California San Diego have developed a technique called infrared-integrated scanning tunneling microscopy (IRiSTM) that allows them to essentially “hear” the vibrational signature of a single molecule. This breakthrough, published on , in Science, brings scientists closer to controlling chemical reactions at the most fundamental level.
Molecules aren’t static objects; their atoms are constantly in motion, stretching, bending, and twisting. These vibrations occur at frequencies within the infrared region of the electromagnetic spectrum. Infrared spectroscopy, a well-established technique, exploits this phenomenon by measuring how light excites these vibrations. As Michelle Franklin of UC San Diego explains, this process is often described as listening to a molecule’s “voice.” Each molecule possesses a unique vibrational “fingerprint” that reveals its chemical structure and its immediate surroundings.
However, traditional infrared spectroscopy has a significant limitation: it requires a large number of molecules to generate a detectable signal. The “voices” of individual molecules are incredibly faint, lost in the collective chorus of billions. IRiSTM overcomes this hurdle by combining infrared excitation with scanning tunneling microscopy (STM). STM is a technique renowned for its ability to image surfaces at the atomic scale by measuring the quantum tunneling of electrons between a sharp metal tip and the surface itself.
The innovation lies in integrating infrared light into the STM process. By shining infrared light onto a molecule while simultaneously scanning its surface with the STM tip, the researchers can selectively excite specific vibrational modes within the molecule. This excitation alters the molecule’s electronic structure, which the STM tip can then detect. The IRiSTM technique allows scientists to pinpoint which vibrations are active in a single molecule, providing an unprecedented level of detail.
Shaowei Li, the lead researcher on the project, and his team at UC San Diego demonstrated the capability of IRiSTM by successfully identifying the vibrational modes of individual molecules on a surface. The study, led by Li along with Kangkai Liang, Zihao Wang, Weike Quan, Yueqing Shi, Hao Zhou, Liya Bi, Zhiyuan Yin, Nathan Romero and Mark Young, represents a significant step forward in single-molecule spectroscopy.
The implications of this research are far-reaching. Chemists have long envisioned the possibility of controlling chemical reactions by precisely depositing energy into specific bonds within a molecule. This would allow them to steer molecules along desired reaction pathways, potentially leading to the development of new materials and more efficient chemical processes. Single-molecule infrared spectroscopy, and specifically IRiSTM, brings this dream closer to reality.
While the technique is still in its early stages of development, the researchers believe it has the potential to revolutionize several fields. Beyond controlling chemical reactions, IRiSTM could be used to study the dynamics of molecules in real-time, providing insights into the fundamental processes that govern chemical behavior. It could also be applied to the development of new sensors and devices that rely on the precise manipulation of molecules.
The development builds on existing advancements in infrared nanoscopy. A report highlighted improvements in mid-infrared nanoscopy, achieving 30 times clearer images by utilizing light from a microscope. This earlier work demonstrates the ongoing progress in refining techniques to visualize and analyze materials at the nanoscale using infrared light.
Understanding how molecules vibrate is also connected to other areas of physics. For example, recent research has focused on using terahertz microscopy to reveal the motion of electrons in superconducting materials. Like infrared spectroscopy, terahertz microscopy utilizes electromagnetic radiation to probe the properties of matter, albeit at different frequencies. These complementary techniques provide a comprehensive picture of the behavior of matter at the atomic and molecular levels.
The ability to capture a molecule’s “voice,” as described by researchers, isn’t about audible sound. Molecules vibrate at frequencies far beyond the range of human hearing. Instead, it’s a metaphor for the unique infrared signature that each molecule emits, a signature that now, thanks to IRiSTM, can be detected and analyzed with unprecedented precision. This advancement promises to unlock new possibilities in chemistry, physics, and materials science, paving the way for a deeper understanding of the molecular world.
