Stacked 2D materials possess an array of tunable properties that are expected to be important for future applications in electronics and optics.
When some atomically thin—or 2D—materials are stacked like Lego bricks in different combinations with other ultrathin materials, new properties often emerge that are potentially useful for next-generation device applications. For example, tungsten disulfide (WS2) is a semiconductor that belongs to a family of 2D materials (transition-metal dichalcogenides, or TMDs) that have received an enormous amount of interest due to their many advantageous properties that can be tuned by mixing and matching them in stacks with other 2D materials.
In this work, single-layer WS2 was stacked on a thin flake of hexagonal boron nitride (h-BN), all on a base of titanium dioxide (TiO2). This heterostructure provided a stable, non-interacting platform that enabled a team of researchers to directly and accurately probe the WS2 electronic states and excitations, including the effects of interactions between the electrons themselves (many-body effects), at a level of detail not previously possible.
MAESTRO’s exquisite sensitivity
MAESTRO (Microscopic and Electronic Structure Observatory), a facility at ALS Beamline 7.0.2 that opened to scientists in 2016, can handle very small sample sizes, on the order of tens of microns, which is key to studying 2D materials. Scientists are continuing to push MAESTRO’s capabilities to study even smaller features—down to the nanoscale. The endstation also features the ability to fabricate and manipulate samples for x-ray studies while maintaining pristine conditions that protect them from contamination.
Image: Rendering of the atomic structure of a 2D layer of tungsten disulfide, or WS2 (blue and yellow), on top of layers of 2D boron nitride (silver and gold). Above that is a representation of the WS2 conduction band (pink-edged metallic surface) and valence bands (green- and blue-edged metallic surfaces). The results of this experiment suggest that the observed increase in valence-band splitting could be due to the presence of “trions,” exotic three-particle combinations of holes and electrons (red circles), in the conduction and valence bands. The background shows the raw WS2 electronic-structure data, as measured in the experiment.
Credit: Chris Jozwiak/Berkeley Lab