Rice Scientists Drive Three-Wheeled, Single-Molecule Nanoroadsters with Light

A scanning tunneling microscope image shows two three-wheeled nanoroadsters created at Rice and tested at the University of Graz. The light-activated roadsters, next to their molecular models, reached a top speed of 23 nanometers per hour. Images by Alex Saywell/Leonhard Grill

Scientists at Rice University and at the University of Graz, Austria, have for the very first time observed the movement of three-wheeled, single-molecule “nanoroadsters” by driving these nanoroadsters with light.

Six years ago, the Rice lab of nanocar inventor and chemist James Tour synthesized light-driven nanocars. With the help of experimental physicists in Austria, they can now drive fleets of single-molecule vehicles at once.

The American Chemical Society journal ACS Nano features a report on this research.

It is exciting to see that motorized nanoroadsters can be propelled by their light-activated motors. These three-wheelers are the first example of light-powered nanovehicles being observed to propel across a surface by any method, let alone by scanning tunneling microscopy.

James Tour, Chemist, Rice University​

The researchers used light at particular wavelengths to move their nanoroadsters along a copper surface instead of driving them chemically or with the tip of a tunneling microscope, as will be done with various other vehicles in the forthcoming NanoCar Race in Toulouse, France. The rear-wheel molecular motors incorporated in the vehicles rotate in one direction when light strikes them. This rotation propels the vehicle just like a paddle wheel on water.

The team headed by Tour and Leonhard Grill, a professor at the University of Graz and formerly at the Fritz-Haber-Institute, Berlin, used wavelength-sensitive modified motors developed by Bernard Feringa, a Dutch scientist who shared this year’s Nobel Prize in chemistry for his molecular machine.

Remote control is key to the useful abilities of the car. “If we have to ‘wire’ the car to a power source, like an electron beam, we would lose a lot of the cars’ functionality,” Tour said. “Powering them with light frees them to be driven wherever one can shine a light — and eventually we hope they will carry cargo.”

Another benefit is the potential to activate fleets of nanocars at once.

This is precisely what we seek - to use a light to activate motors and have swarms of nanovehicles moving across the surface, made directional through electric field gradients. This would permit us the future prospect of using nanomachines like ants that work collectively to perform construction.

James Tour, Chemist, Rice University

According to Grill, remote control by light prevents the requirement for a local probe that is responsible for addressing the molecules one by one. “Additionally, no ‘fuel’ molecules are required that would contaminate the surface and modify the diffusion properties,” he said.

Modified Feringa’s motors have been used by Tour to power his lab’s nanosubmersibles. The motors are the back wheel in this case. Tour said that the three-wheeled configuration helps to simplify its application, as larger nanocars are extremely difficult to put onto an imaging surface and often get dissociated during deposition under vacuum, according to Grill.

Experiments conducted by lead author Alex Saywell of the Grill group on nanoroadsters developed at Rice demonstrated the requirement for a good balance of temperature and light in order to permit “enhanced diffusion” of the molecules present in a vacuum.

Grill stated that the utilization of light to drive nanomachines provides a fundamental advantage - the ability to selectively induce motion due to the motors’ sensitivity to wavelength. The roadsters’ movement was doubled by ultraviolet light at 266 nm in comparison to “control” roadster molecules without motors. It tripled at 355 nm.

A top speed of 23 nm per hour was reached by the roadsters made of 112 atoms.

A surface activation temperature of 161 K (-170°F) proved to be best for driving conditions. The roadsters stick to the surface when the temperature is too cold, and they randomly diffuse without help from the motor when the temperature is too warm.

We were surprised by the very clear correlation of the enhanced motion to the presence of the motor, the need for both heat and light to activate this motion — in perfect agreement with the concept of the Feringa motor — and the wavelength sensitivity that nicely fit our expectations from spectroscopy in solution.

Leonhard Grill, Professor, University of Graz

Co-authors include Rice alumni Víctor García-López and Pinn-Tsong Chiang, and Anne Bakker, Johannes Mielke, Takashi Kumagai and Martin Wolf of the Max Planck Society, Berlin. Saywell is currently the Marie Curie Research Fellow at University of Nottingham, United Kingdom. Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice.

The research was supported by the National Science Foundation, the Marie Curie Intra-European Fellowship and the German Science Foundation.

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