Rare-earth complex put in a spin by STM

Researchers at Ohio University, Argonne National Laboratory and the University of Illinois at Chicago have, for the first time, formed a charged rare-earth molecule on a metal surface and then used scanning tunneling microscopy (STM) to rotate it – both clockwise and counterclockwise – without affecting its charge.

Their work, reported in a paper in Nature Communications, opens a new window for research on the atomic-scale manipulation of materials important for future technologies, from quantum computing to consumer electronics.

"Rare earth elements are vital for high-technological applications including cell phones, HDTVs and more,” said team lead Saw-Wai Hla, who has dual appointments as a scientist at Argonne and professor of physics and astronomy at Ohio University. “This is the first-time formation of rare-earth complexes with positive and negative charges on a metal surface and also the first-time demonstration of atomic-level control over their rotation."

The experiment was conducted both at Argonne and Ohio University using two separate low-temperature STM systems. The environment for STM experiments requires a temperature of about 5K (-450°F) in an ultrahigh vacuum. The sample molecules are about 2nm in size.

"The same results were achieved in both locations, which ensures reproducibility," Hla said. The Ohio lab is operated by students of the Hla group associated with the Nanoscale & Quantum Phenomena Institute.

The rare-earth complexes assembled by the researchers comprised positively charged europium (Eu) base molecules with negatively charged triflate (CF₃SO₃^-) counterions on a gold surface. The complexes were rotated by applying an electric field emanating from the STM tip, using the counterion underneath as a pivot. The researchers were able to achieve 100% directional control over the rotation of these rare-earth complexes.

Eric Masson, professor of chemistry at Ohio University and one of the co-investigators of the project designed the rare-earth complexes, and his group at Ohio University synthesized them. Density functional theory calculations were performed by scientists at Argonne and the group of Anh Ngo, an associate professor of chemical engineering at the University of Illinois at Chicago, using Argonne’s BEBOP, the most powerful supercomputer in the US to date. These calculations unveiled only a negligible amount of charge transfer at the molecule-substrate interface, which means the complexes remained charged on the surface.

The chemical state of the Eu ion in the complexes adsorbed on the surface was determined by a nascent experimental method known as synchrotron X-rays scanning tunneling microscopy at the Advanced Photon Source in Argonne by Hla and his co-workers. This allowed them to confirm that the molecules on the gold surface are positively charged.

STM images showed the complex as a distorted triangular shape with three arms, while an STM movie acquired with a record number of 8000 spectroscopic frames proved that the counterion had been incorporated underneath. The Hla group also used STM manipulation to control the rotation of the complex, demonstrating clockwise and counterclockwise rotations at will.

"These findings may be useful for the development of nanomechanical devices where the individual units in the complex are designed to control, promote or restrict motion," Hla said. "We have demonstrated the rotation of charged rare-earth complexes on a metal surface, which now enables investigations one-complex-at-a-time for their electronic and structural as well as mechanical properties."

This story is adapted from material from Ohio University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.