Molecular motor: an Archimedes' screw made of one molecule

One of the oldest machines in history has just been rebuilt from a single molecule. Scientists in Israel, Denmark, Germany and China have made a spiral-shaped molecule that spins like a tiny Archimedes’ screw when an electric current flows through it, and as it spins, it lets up to 100 times more current through, according to their study in Nature.

Why it matters: electronics keep getting smaller, and the ultimate limit is a single molecule. A part that turns current into motion, and motion back into more current, is a brand-new kind of building block for that tiny world: a switch, a motor and a sensor all in one.

An ancient pump, shrunk hundreds of millions of times

The Archimedes’ screw is more than 2,000 years old: a big spiral inside a tube that lifts water uphill when you turn it. The new machine borrows that shape. Its heart is a helix-shaped molecule, a bit like a spiral staircase, built by Professor Michael Pittelkow’s group at the University of Copenhagen.

It is only a few nanometres long, a few millionths of a millimetre. If you lined up ten thousand of them, they still wouldn’t cross the width of a human hair.

How you make a molecule spin

At the Weizmann Institute of Science in Israel, Professor Oren Tal and Dr. Anil Kumar Singh hooked single molecules up to electricity. Each end of the spiral grips a gold electrode through a sulphur atom, and the two gold tips are sharpened down to a single atom. By moving the tips, the team could gently stretch or squeeze the molecule.

Then they sent a current through it. Electrons rushing along the spiral hand over some of their spin (physicists call it angular momentum) to the molecule and push it round, the way wind turns a wind turbine, the University of Augsburg explains. Above a certain voltage, the wobbling turns into steady rotation.

A red Archimedes screw turns inside a steep steel trough, its low end splashing in a turquoise pool as it carries water up to a red motor at the top. — The full-size idea: a turning spiral that lifts water uphill · Illustration by DIGITAL WOXTER TECHNEWS.

Flip the battery, flip the spin

Two details make this special. First, the direction is under control: reverse the voltage and the molecule turns the other way. Its handedness matters too, just like right- and left-handed screws. Chemeurope.com reports that this is the first time a current-driven molecular motor on metal electrodes could be reversed like that.

Second, the spinning changes the electricity itself. While the molecule rotates, its conductance (how easily current flows) can jump by up to two orders of magnitude, about 100 times. Supercomputer simulations by Professor Fabian Pauly’s group in Augsburg show why: the positions that conduct best are ones the molecule avoids when it is sitting still. “It is only the rotation that makes these positions accessible,” says Augsburg researcher Léon Martin.

So current drives the rotation, and the rotation boosts the current. Theorists at the Niels Bohr Institute in Copenhagen and Huazhong University of Science and Technology in Wuhan built a simpler model that reproduces this loop.

What it is, and what it isn’t

This is a lab proof of principle: one molecule at a time, held between two needle-sharp tips in a carefully controlled experiment. Electric molecular motors aren’t new either: the first ones, reported in 2011, sat on a surface and were driven by the tip of a scanning tunnelling microscope. What is new is a motor that works while wired between two metal contacts, like a component in a circuit.

Nobody is putting these into phones. The uses the team mentions, from molecular electronics to spintronics (electronics that also use the electron’s spin) and microfluidics (handling tiny drops of liquid), are possibilities for the future, not products.

What happens next

The researchers see a new operating principle for extremely small machines, and a fresh tool for studying physics one atom at a time. If you enjoyed this, molecules are busy in quantum tech too: one OLED-like molecule now works as a qubit that talks in light, and five silicon qubits now run on a single wire.