Light-Powered Nanomotors Perform as Ultrasmall Energy Converters

Researchers at the University of Texas (UT) have designed a solid-state, optical nanomotor that uses light to power functional devices at the nanoscale — without the challenges that hinder nanomotors operating in liquid environments. According to the research team, its light-driven nanomotor, which is less than 100 nm in width, is the first such device to operate in a solid state.

With their capability to operate on-chip in an ambient environment, the tiny, light-driven motors could have many possible uses. The spinning motion of the motors could pick up dust and other particles, making them useful for air quality measurements. The nanomotors could also serve as the engines that propel nanodevices such as drug delivery mechanisms in the human body, and they could power tiny drones and other miniature vehicles for performing tasks including measurement and surveillance.

The opto-thermocapillary nanomotor rotates on a solid substrate when illuminated with light. The solid state of the nanomotor is possible because of the researchers’ design incorporation of a thin layer of phase change material on the solid substrate.

When the thin film is exposed to light, it undergoes a local, reversible change from a solid to a quasi-liquid phase. This phase change reduces the friction force of the nanomotor and drives the rotation.

Graphical depiction of spinning nanomotors; the nanomotor rotates on a solid substrate when illuminated with light. According to its developers at the University of Texas, it could potentially be used in place of batteries to generate mechanical motion and power. As fuel-free, gear-free engines for converting light into mechanical energy, they could power a range of solid-state micro- and nano-electro-mechanical systems. Courtesy of the University of Texas at Austin.

Nanomotors in liquid environments must contend with the effects of Brownian motion, a phenomenon that occurs when water molecules push the light-driven nanomotors off their spin. The smaller the motor, the stronger the Brownian motion becomes. When the motor is removed from a liquid solution, Brownian motion is suppressed — and one of the biggest hurdles restricting the application of nanomotors is overcome.

The UT team demonstrated an orbital rotation of 80-nm gold nanoparticles around a laser beam on a solid substrate by optically controlling the nanomotor’s particle-substrate interactions and thermocapillary actuation.

According to the researchers, nanomotors constitute the middle ground in scale between molecular machines at the smaller end and micro-engines at the larger end. It is known that nanomotors mimic biological structures. In nature, biological “motors” drive the division and movement of cells. The combined effort of these bio-motors helps give organisms mobility. Additionally, they are part of the growing field of miniature power sources.

Yet before these tiny motors can be made more viable, researchers need a better understanding of the fundamental science on which they are based, team members said.

The researchers will continue to improve the optical nanomotors, enhancing their performance by making them more stable and easier to control. These refinements will enable the nanomotors to convert light to mechanical energy at higher rates.

Additionally, the light-driven nanomotors could potentially be used in place of batteries to generate mechanical motion and power. As fuel-free, gear-free engines for converting light into mechanical energy, they could power a range of solid-state micro- and nano-electro-mechanical systems.

The research was published in ACS Nano (www.doi.org/10.1021/acsnano.1c09800).