An Indian Institute of Technology Madras (IIT Madras) faculty member is part of an international research team that has made a breakthrough in fundamental research at the intersection of physics, chemistry and nanotechnology.

The research focuses on how molecular motion and electrical transport (the movement of electric charge) affect each other. This could help researchers develop nanoscale components whose electrical response can be controlled by molecular motion.

The researchers demonstrated a mechanism in which molecular motion and electrical conduction influence each other at the atomic scale. They showed a molecular machine in which an electric current drives the directional rotation of a single molecule, and this rotation, in turn, can increase electrical current through the molecule.

The research involved collaborators from five countries – India, Israel, Germany, Denmark and China.

Molecular machines are structures that perform specific functions in response to stimuli such as light, chemicals or electricity. Biological systems use molecular machines to perform essential cellular functions. Scientists are studying molecular machines for their potential applications in nanotechnology and molecular electronics. A major challenge has been to understand how an electrical current can drive controlled molecular motion and how that motion, in turn, changes the electrical current.

The study shows how electrical current can drive molecular motion and how molecular rotation can, in turn, enhance electrical conduction at the atomic scale, advancing our understanding of molecular machines and their potential role in future nanoscale technologies.

The study has been published in Nature, an international scientific journal as one of the world’s leading research journals. Dr. Anil Kumar Singh from IIT Madras is a corresponding author and the principal experimental contributor to the study.

Elaborating on the importance of this research, Dr. Anil Kumar Singh, Assistant Professor in the Atomic-Scale Quantum Devices Laboratory, Department of Physics, IIT Madras, said, “An exciting aspect of this work is that molecular motion is not merely a consequence of electrical transport; it can also facilitate the passage of electrons through the molecule. As the molecule rotates, it accesses configurations with higher electrical conductance that are not readily accessible when the molecule is at rest. By demonstrating this feedback between rotation and electrical conduction in a single-molecule junction, we have taken a step towards understanding how mechanical motion and electronic function can be coupled at the atomic scale.”

Highlighting the current status of this research, Dr. Anil Kumar Singh added, “This is a fundamental proof-of-principle demonstration, rather than a technology ready for practical use. The experiments were performed at cryogenic temperatures, and further research will be needed to establish reliable operation under practical conditions. Nevertheless, the findings provide a foundation for exploring future molecular-scale switches, current-modulating elements and systems designed to promote directional transport near surfaces. These remain potential applications rather than demonstrated outcomes.”

‘ARCHIMEDEAN SCREW’

Inspired by the Archimedean screw, a device traditionally used to lift water that can also operate in reverse as a turbine, the researchers investigated whether a similar relationship between electrical current and rotation could be demonstrated in a single molecule.

The team demonstrated a molecular rotor based on helicene, a molecule with a spiral structure resembling a tiny screw. Using a technique known as a mechanically controllable break junction, the researchers suspended a single helicene molecule between two gold electrodes. When a voltage was applied, the electrical current drove the molecule through a cyclic sequence of configurations.

A distinctive finding was the directionality of this motion. In all 122 cases in which the direction of the transitions could be determined, the researchers observed the same cyclic sequence. Reversing the voltage polarity reversed the sequence, indicating that the direction of molecular rotation could be controlled electrically.

The researchers also observed a sharp increase in electrical current beyond a threshold voltage. Experimental measurements, supported by theoretical calculations, indicated that the molecule transitions from moving stepwise between preferred orientations to continuous rotation. During this rotation, the molecule passes through configurations that allow electrons to flow more readily, increasing the current through the intact molecular junction by up to approximately 100-fold.

The onset of this enhancement could also be tuned by gently stretching or compressing the junction. This changes the effective pitch of the helical molecule and influences the conditions under which rotation occurs.

NANOSCALE TECHNOLOGIES

The findings offer new insights into the interaction between electrical energy, molecular motion and momentum transfer at the atomic scale. In the longer term, this understanding could inform the design of nanoscale electronic components whose electrical properties can be controlled through molecular motion. Potential applications include molecular-scale switches, current-modulating elements and systems that enable directional transport near surfaces.

Future investigations could explore whether molecular rotors can be controlled using circularly polarised light or spin-polarised electrical currents, potentially opening new avenues for the optical and spin-dependent control of molecular motion.

INTERNATIONAL COLLABORATION

During his research tenure at the Weizmann Institute of Science in Israel, Dr. Anil Kumar Singh designed and performed the single-molecule break-junction experiments, analysed the experimental data and prepared the associated figures. The work brought together researchers from the Weizmann Institute of Science and partner institutions in Germany, Denmark and China, combining experimental measurements, molecular synthesis, quantum-mechanical calculations and theoretical modelling.

The experiments were conducted at a base temperature of 4.2 kelvin (approximately minus 269 degrees Celsius), with sub-ångström control over the electrode separation. The paper lists Anil Kumar Singh, Fabian Pauly, Michael Pittelkow, Jing-Tao Lü, Per Hedegård and Oren Tal as corresponding authors.

The research provides a scientific foundation for Dr. Anil Kumar Singh’s independent research programme at IIT Madras, where he is pursuing work in experimental quantum transport and molecular electronics.