Researchers Probe Graphene Alternatives with SHG

Scientists have a new way to study the optical and electrical properties of 2-D metals that could compete with graphene in semiconductor and other applications.

A new SHG imaging technique allows rapid and all-optical determination of the crystal orientations of 2-D semiconductor membranes at a large scale, providing the knowledge needed to use these materials in nanoelectronic devices. Images courtesy of Lawrence Berkeley National Laboratory.

Using ultrafast IR pulses, researchers with the Lawrence Berkeley National Laboratory observed strong nonlinear optical resonance along the edges of a single layer of molybdenum disulfide (MoS₂).

“We … discovered extraordinary second-harmonic light-generation properties that may be used for the in situ monitoring of electronic changes and chemical reactions that occur at the one-dimensional atomic edges,” said Xiang Zhang, a Berkeley Lab faculty scientist.

MoS₂ and other transition metal dichalcogenides feature the same flat, hexagonal structure as graphene and many of the same electrical advantages but, unlike graphene, have direct energy bandgaps. This facilitates their application in transistors and other electronic devices, particularly light-emitting diodes.

(A) Optical image of a large area of monolayer MoS₂ and (B) an SHG image of the same area revealing grains and grain boundaries where translational symmetry is broken to form 1-D edge states.

Until now, observation of dichalcogenides had been limited to scanning tunneling and transition electron microscopes. Discovering the visible SHG properties of MoS₂ allowed researchers to easily capture the crystal structures and grain orientations with an optical microscope.

“Our nonlinear optical imaging technique is a noninvasive, fast, easy metrologic approach to the study of 2-D atomic materials,” said Xiaobo Yin, a former member of Zhang’s research group now at the University of Colorado, Boulder. “We don’t need to prepare the sample on any special substrate or vacuum environment, and the measurement won’t perturb the sample during the imaging process.”

Transition metal dichalcogenides could also serve as catalysts for hydrogen evolution reaction in fuel cells, desulfurization and other chemical reactions, researchers said.

The research was supported by the US Department of Energy Office of Science and by the US Air Force Office of Scientific Research Multidisciplinary University Research Initiative. It is published in Science (doi: 10.1126/science.1250564).