Weaving atomically thin light seams with in-plane heterostructures

Aug 27, 2022

(Nanowork News) Researchers at Tokyo Metropolitan University have developed a way to produce high-quality monolayers from a selection of different transition metal dichalcogenides that meet across an atomically thin seam.

By coating this layer with an ion gel, a mixture of an ionic liquid and a polymer, they could induce light emission along the seam. The light was also found to be naturally circularly polarized, a product of the adjustable voltage across the boundary. Monolayers of tungsten diselenide and tungsten disulfide combine across an atomically thin seam in an in-plane heterostructure. (Image: Tokyo Metropolitan University)

The team reported three findings in Advanced functional materials (“Efficient and chiral electroluminescence of in-plane heterostructure of transition metal dichalcogenide monolayers”).

Light-emitting diodes (LEDs) have become ubiquitous due to their revolutionary impact on almost all forms of lighting. But as our needs diversify and performance demands increase, there is still a clear need for even more energy efficient solutions.

One such option involves the application of in-plane heterostructures, where ultra-thin layers of different materials are patterned on surfaces to produce boundaries. In-plane heterostructures grown on a surface (left) Tungsten disulfide and tungsten diselenide regions observed using optical microscopy. (right) Scanning transmission electron microscopy (STEM) image of the boundary between the two different TMDCs. (Image: Tokyo Metropolitan University)

In the case of LEDs, this is where electrons and “holes” (mobile cavities in semiconductor materials) recombine to produce light. The efficiency, functionality and scope of such structures are determined not only by the materials used, but also by the dimensions and nature of the boundaries, which has led to much research into controlling their structure at the nanoscale.

A team of researchers led by associate professor Yasumitsu Miyata of Tokyo Metropolitan University, assistant professor Jiang Pu and professor Taishi Takenobu of the University of Nagoya has investigated the use of a class of materials known as transition metal dichalcogenides (TMDCs), a family of substances containing a group 16 element from the periodic table and a transition metal.

They have used a technique known as chemical vapor deposition to deposit elements on surfaces in a controlled manner to create atomically thin monolayers; much of their work has dealt with how to vary such monolayers to create patterns with different regions made from different TMDCs. Light emission from the boundary between two different TMDCs (left) Optical microscope image of an in-plane heterostructure with two electrodes attached. (right) Once a voltage is applied, light is emitted from the interface between the two different TMDCs. (Image: Tokyo Metropolitan University)

Now the same team has managed to significantly refine this technology. They have redesigned their growth chamber so that different materials can be brought closer to the substrate in a fixed order; they also introduced additives to alter the evaporation temperature of each component, enabling optimized conditions for the growth of high-quality crystalline layers.

As a result, they managed to use four different TMDCs to create six different types of sharp, atomically thin “seams”. In addition, by adding an ion gel, a mixture of an ionic liquid (a liquid of positive and negative ions at room temperature), and a polymer, a voltage can be applied across the seams to produce electroluminescence, the same fundamental phenomenon underlying LEDs. Ion gel layer and the in-plane TMDC heterostructure Positive and negative ions in the ionic liquid are mobile even while the polymer network keeps the gel rigid. When a voltage is applied, ions migrate and induce the transport of electrons and holes, which in turn recombine at the interface to create light. (Image: Tokyo Metropolitan University)

The adaptability of their setup and the high quality of their interfaces allow exploring a wide range of permutations, including varying degrees of “misfit” or voltage between different TMDCs.

Interestingly, the team found that the boundary between a monolayer of tungsten diselenide and tungsten disulfide produced a “handed” form of light known as circularly polarized light, a direct product of the stress on the seam. This new degree of nanoscale control opens up a world of possibilities for how their new structures can be applied to real-world devices, particularly in the field of quantum optoelectronics.

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