Molecular sandwiches

A team of researchers created a way to sandwich single-atom-thick sheets of 2D crystals between intercalating layers of a different material, to form what is known as superlattices, or Van der Waals heterostructures. These materials are used to make semiconductors that are finely tunable, thus of great technological promise. 

Traditionally, the formation of such semiconductor superlattices was carried out either through a top-down approach; tedious exfoliating and restacking of individual molecular layers of the different materials, or a bottom-up approach; chemical vapour deposition of one layer on top of the next. Both techniques have proved to be laborious, and inefficient, according to the researchers. 

The team, including scientists from the USA, China and Saudi Arabia, applied an electric voltage to reduce black phosphorous to isolate phosphorene monolayers and interleave layers of a quaternary ammonium compound between them. 

By using laser spectroscopy, they were able to monitor the changes in the electronic and optical properties of the material as the electrochemical intercalation took place. The material produced in this way demonstrated superior electronic properties and stability.

By using different ammonium compounds of different sizes and symmetries for the electrochemical process, the interlayer distance between the phosphorene layers can be finely adjusted, according to the findings in Nature.¹

Alternatively, the scientists showed that their method is also extrapolatable to different 2D atomic crystals, such as Molybdenum Disulfide. Combining the two together gives rise to a wide array of possible superlattices, fine-tuned to the desired semiconductor properties.

“This approach offers a general pathway to a vast library of superlattice structures between distinct atomic layers and molecular layers, and open up a new dimension to tailor and tame the electronic/optical properties of 2D materials. It could open up exciting opportunities for novel electronic and optoelectronic devices, including transistors and light emitting devices,” says Duan Xianfeng, who lead the study.

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