Weaving atomically thin seams of light with in-plane heterostructures

August 27, 2022

(Nanowerk News) Researchers at Tokyo Metropolitan University have developed a way to produce high-quality monolayers of a selection of different transition metaldicacogenides that are joined together in an atomically thin seam.

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

The team reported their findings in Advanced Functional Materials (“Chiral and Efficient Electroluminescence from In-plane Heterostructure of Transition Metal Dichalcogenide Monolayers”).

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

One of these options involves the application of in-plane heterostructures, where ultrathin layers of different materials are patterned on surfaces to produce boundaries. In-plane heterostructures grown on a surface (left) Regions of tungsten disulfide and tungsten diselenide observed by light 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” (moving voids in semiconductor materials) recombine to produce light. The efficiency, functionality and scope of applications 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 a large amount of research to control their structure to nanoscale.

A team of researchers led by Associate Professor Yasumitsu Miyata of Tokyo Metropolitan University, Assistant Professor Jiang Pu, and Professor Taishi Takenobu of Nagoya University have been investigating the use of a class of materials known as transition metal dicacogenides. (TMDC), a family of substances containing an element from group 16 of the periodic table and a transition metal.

They have been using a technique known as chemical vapor deposition to controllably deposit elements onto surfaces to create atomically thin monolayers; much of his work has had to do with how such monolayers can be varied to create patterns with different regions made of different TMDCs. Light emission from the boundary between two different TMDCs (left) Light microscope image of an in-plane heterostructure with two connected electrodes. (right) Once a voltage is applied, light is seen to be 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 redesigned their growth chamber so that the different materials could be moved closer to the substrate in a set sequence; they also introduced additives to change the vaporization temperature of each component, which made it possible to optimize the 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.” Furthermore, by adding an ion gel, a mixture of an ionic liquid (a fluid of positive and negative ions at room temperature) and a polymer, a voltage could be applied across the seams to produce electroluminescence, the same basic phenomenon that underlies the LEDs. Ion gel layer and heterostructure in the TMDC plane The positive and negative ions in the ionic liquid are mobile even when the polymer network keeps the gel rigid. When a voltage is applied, the 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 ability to customize its configuration and the high quality of its interfaces makes it possible to explore a wide range of permutations, including different degrees of “misfit” or tension between different TMDCs.

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

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