Imaging the crystal edges of 2D molybdenum disulfide

Another step toward novel 2D ultrasmall and ultrafast electronic and photonic devices, replacing silicon
May 8, 2014

A new imaging technique allows rapid and all-optical determination of the crystal orientations of 2D semiconductor membranes at a large scale, providing the knowledge needed to use these materials in nanoelectronic devices (credit: Berkeley Lab)

Researchers with Lawrence Berkeley National Laboratory (Berkeley Lab) have recorded the first observations of a strong nonlinear optical resonance along the edges of a single layer of two-dimensional molybdenum disulfide, which may enable novel ultrasmall and ultrafast electronic and photonic devicesas well as a catalyst for the hydrogen evolution reaction in fuel cells, desulfurization, and other chemical reactions.

The research is described in the journal Science.


(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 1D edge states (credit: Berkeley Lab)

Emerging two-dimensional semiconductors are prized in the electronics industry for their superior energy efficiency and capacity to carry much higher current densities than silicon. Only a single molecule thick, they are well-suited for integrated optoelectronic devices.

Until recently, graphene has been the unchallenged superstar of 2D materials, but today there is considerable attention focused on 2D semiconducting crystals that consist of a single layer of transition metal atoms, such as molybdenum, tungsten or niobium, sandwiched between two layers of chalcogen atoms, such as sulfur or selenium.

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

Full realization of the vast potential of transition metal dichalcogenides will only come with a better understanding of the domain orientations of their crystal structures that give rise to their exceptional properties. Until now, however, experimental imaging of these three-atom-thick structures and their edges has been limited to scanning tunneling microscopy and transmission electron microscopy, technologies that are often difficult to use.

Mapping crystal orientation 

Nonlinear optics at the crystal edges and boundaries enabled the researchers to develop a new imaging technique based on second-harmonic generation (SHG) light emissions that can easily capture the crystal structures and grain orientations with an optical microscope.

For the SHG imaging of molybdenum disulfide, the researchers illuminated sample membranes that are only three atoms thick with ultrafast pulses of infrared light. The nonlinear optical properties of the samples yielded a strong SHG response in the form of visible light that is both tunable and coherent. The resulting SHG-generated images enabled the researchers to detect “structural discontinuities” or edges along the 2D crystals only a few atoms wide where the translational symmetry of the crystal was broken.

“By analyzing the polarized components of the SHG signals, we were able to map the crystal orientation of the molybdenum disulfide atomic membrane,” says Ziliang Ye, the co-lead author of the paper. “This allowed us to capture a complete map of the crystal grain structures, color-coded according to crystal orientation. We now have a real-time, non-invasive tool that allows us explore the structural, optical, and electronic properties of 2D atomic layers of transition metal dichalcogenides over a large area.”

This research was supported by the DOE Office of Science through the Energy Frontier Research Center program, and by the U.S. Air Force Office of Scientific Research Multidisciplinary University Research Initiative.

Abstract of Science paper

The translational symmetry breaking of a crystal at its surface may form two-dimensional (2D) electronic states. We observed one-dimensional nonlinear optical edge states of a single atomic membrane of molybdenum disulfide (MoS2), a transition metal dichalcogenide. The electronic structure changes at the edges of the 2D crystal result in strong resonant nonlinear optical susceptibilities, allowing direct optical imaging of the atomic edges and boundaries of a 2D material. Using the symmetry of the nonlinear optical responses, we developed a nonlinear optical imaging technique that allows rapid and all-optical determination of the crystal orientations of the 2D material at a large scale. Our technique provides a route toward understanding and making use of the emerging 2D materials and devices.