Robust 2nm Molecular Propeller Enables Unidirectional Rotation

August 26, 2019 by Paul Shepard

A team of scientists from Ohio University, Argonne National Laboratory, Universitié de Toulouse in France and Nara Institute of Science and Technology in Japan led by OHIO Professor of Physics Saw-Wai Hla and Prof. Gwenael Rapenne from Toulouse developed a molecular propeller that enables unidirectional rotations on a material surface when energized. The STM images above show a stationary (d) and rotating molecular propeller (e).

In nature, molecule propellers are vital in many biological applications ranging from the swimming bacteria to intracellular transport, but synthetic molecular propellers, like what has been developed, are able to operate in harsher environments and under a precise control. This new development is a multiple component molecular propeller specially designed to operate on solid surfaces.

This tiny propeller is composed of three components; a ratchet shape molecular gear as a base, a tri-blade propeller, and a ruthenium atom acting as an atomic ball bearing that connects the two. The size of the propeller is only about 2 nanometers (nm) wide and 1 nm tall.

“What is special about our propeller is its multi-component design that becomes chiral on the gold crystal surface, i.e. it forms right- or left-tilted gears,” said Hla. “This chirality dictates the rotational direction when energized.”

The paper “A chiral molecular propeller designed for unidirectional rotations on a surface” has been published in Nature Communications. In the abstract, the authors state:

“Synthetic molecular machines designed to operate on materials surfaces can convert energy into motion and they may be useful to incorporate into solid state devices. Here, we develop and characterize a multi-component molecular propeller that enables unidirectional rotations on a material surface when energized. Our propeller is composed of a rotator with three molecular blades linked via a ruthenium atom to a ratchet-shaped molecular gear. Upon adsorption on a gold crystal surface, the two-dimensional nature of the surface breaks the symmetry and left or right tilting of the molecular gear-teeth induces chirality.

“The molecular gear dictates the rotational direction of the propellers and step-wise rotations can be induced by applying an electric field or using inelastic tunneling electrons from a scanning tunneling microscope tip. By means of scanning tunneling microscope manipulation and imaging, the rotation steps of individual molecular propellers are directly visualized, which confirms the unidirectional rotations of both left- and right-handed molecular propellers into clockwise and anticlockwise directions respectively.”

Hla and his team have also been able to mechanically manipulate and record the molecule’s stepwise rotations. This enables them to understand the detail motions at the single molecule level, allowing a direct visualization of the rotation of the individual molecular propellers from images acquired at each rotation steps.

The rotation occurs by an applied electric field, by transfer of electron energy or by mechanical force with a scanning tunneling microscope tip. Through this supply of power, scientists can control the rotation and switch off the propeller by denying it any energy.