Mystery material stuns scientists

It's a UV light, semiconductor, sensor, superconductor, ferromagnet, optoelectronic device. Just add water.
December 18, 2015

How does water on the surface of this bizarre material control UV light emission and also its conductivity? (credit: Mohammad A. Islam et al./Nano Letters)

In a remarkable chance landmark discovery, a team of researchers at four universities has discovered a mysterious material that emits ultraviolet light and has insulating, electrical conducting, semiconducting, superconducting, and ferromagnetic properties — all controlled by surface water.

It happened while the researchers were studying a sample of lanthanum aluminate film on a strontinum titanate crystal. The sample mysteriously began to glow, emitting intense levels of ultraviolet light from its interior. After carefully reproducing the experimental conditions, they tracked down the unlikely switch that turns UV light on or off: surface water moisture.

The team of researchers from Drexel University, the University of Pennsylvania, the University of California at Berkeley, and Temple University also found that the interface between the materials’ two layers of electrical insulators also had an unusual electrical conducting state that, like UV, could also be altered by the water on the surface. On top of that, the material also exhibited superconducting, ferromagnetic ordering, and photoconductive properties.

Even weirder, “we can also make [the effects] stronger by increasing the distance between the molecules and surface and the buried interface, by using thicker films for example,” said Drexel College of Engineering Professor Jonathan E. Spanier.

Calling in the theorists

Puzzled, the researchers turned to their theory collaborators on the team: Penn’s Andrew M. Rappe, Fenggong Wang, and Diomedes Saldana-Grego.

“Dissociation of water fragments on the oxide surface releases electrons that move to the buried interface, cancelling out the ionic charges,” Wang said. “This puts all the light emission at the same energy, giving the observed sharp photoluminescence.”

According to Rappe, this is the first report of the introduction of molecules to the surface controlling the emission of light — of any color — from a buried solid-surface interface. “The mechanism of a molecule landing and reacting, called dissociative chemisorption, as a way of controlling the onset and suppression of light is unlike any other previously reported,” Saldana-Grego added.

The team recently published its findings in the American Chemical Society journal Nano Letters.

Multiple personality

“We suspect that the material could be used for simple devices like transistors and [chemical] sensors,” said Mohammad Islam, an assistant professor from the State University of New York at Oswego, who was on Spanier’s team when he was at Drexel.

“By strategically placing molecules on the surface, the UV light could be used to relay information — much the way computer memory uses a magnetic field to write and rewrite itself, but with the significant advantage of doing it without an electric current. The strength of the UV field also varies with the proximity of the water molecule; this suggests that the material could also be useful for detecting the presence of chemical agents.”

Abstract of Surface Chemically Switchable Ultraviolet Luminescence from Interfacial Two-Dimensional Electron Gas

We report intense, narrow line-width, surface chemisorption-activated and reversible ultraviolet (UV) photoluminescence from radiative recombination of the two-dimensional electron gas (2DEG) with photoexcited holes at LaAlO3/SrTiO3. The switchable luminescence arises from an electron transfer-driven modification of the electronic structure via H-chemisorption onto the AlO2-terminated surface of LaAlO3, at least 2 nm away from the interface. The control of the onset of emission and its intensity are functionalities that go beyond the luminescence of compound semiconductor quantum wells. Connections between reversible chemisorption, fast electron transfer, and quantum-well luminescence suggest a new model for surface chemically reconfigurable solid-state UV optoelectronics and molecular sensing.