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Rare Mineral Yields Light-Emitting Nano Triangles

UNIVERSITY PARK, Pa., Feb. 6, 2013 — A rare mineral formed into single sheets has yielded triangular structures with unusual light-emitting properties that could be used in optical technologies such as light detectors or lasers.

Creating single, one-atom-thick layers, or monolayers, is of interest to scientists because the chemical properties of minerals and other substances are known to change depending on their atomic thickness, which could enable applications of multilayered materials of various thicknesses. In previous research, scientists at Penn State reached the similar goal of creating monolayers of graphene, although the process was difficult.

“The technique these researchers used was tedious, but it worked,” said Mauricio Terrones, a professor of physics and of materials science and engineering at Penn State University. “They basically removed, or exfoliated, the graphene, layer by layer, with Scotch tape, until they got down to a single atom of thickness.”

Triangular single layers of tungsten disulfide have been synthesized by Penn State researchers. The edges of the triangles exhibit extraordinary photoluminescence, while the interior area does not. The photoluminescent signal disappears as the number of layers increases. Courtesy of Terrones lab, Penn State University.

Terrones and colleagues have now used a controlled thermal reduction-sulfurization method called chemical vapor deposition to accomplish the same feat with a rare mineral called tungstenite. After depositing crystals of tungsten oxide less than 1 nm in height, they passed the crystals through sulfur vapor at 850 °C. This process yielded single sheets of a one-atom-thick structure called tungsten disulfide — a honeycomb pattern of triangles consisting of tungsten atoms bonded with sulfur atoms. This monolayer has unusual light-emitting, or photoluminescent, properties.

“One of the most exciting properties of the tungsten disulfide monolayer is its photoluminescence,” Terrones said. Photoluminescence occurs when a substance absorbs light at one wavelength and re-emits that light at a different wavelength. It also occurs in certain bioluminescent animals such as angler fish and fireflies.

“One interesting discovery from our work is the fact that we see the strongest photoluminescence at the edges of the triangles, right where the chemistry of the atoms changes, with much less photoluminescence occurring in the center of the triangles,” he said.

The monolayer also luminesces at room temperature, so “no special temperature requirements are needed for the material to exhibit this property,” Terrones said.

"The images of the photoluminescence are beautiful; the triangles light up all around their edges like little holiday ornaments — holiday ornaments with potentially transformative, long-term applications in nano-optics," said Vincent H. Crespi, co-author and distinguished professor of physics, chemistry and materials science and engineering at Penn State.

The structure has many potential applications in the fields of optical light detection, LED production and even laser technology. The scientists plan to use the chemical vapor deposition technology to produce layered materials using monolayers of other materials.

Results of the research appeared in Nano Letters (doi: 10.1021/nl3026357). 

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