‘Sodium-Scooter’ Delivers Tellurium to MoS₂

- IBS researchers use a fast and efficient catalyst to scoot tellurium in place of sulfur,
and convert 2D MoS₂ to MoTe₂ at low temperature -

Korean food delivery system is renowned to be fast and efficient. Scooters speed through the city to bring orders timely to your doorstep. Researchers from the Center for Integrated Nanostructure Physics, within the Institute for Basic Science (IBS, South Korea) have developed a low-temperature reaction, where a “chemical scooter delivery” can be used as a metaphor. A “sodium-scooter”, namely Na₂Te, transports tellurium to molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) monolayers. With the help of the scooter, sulfur atoms were replaced with tellurium. The process occurs at 525 °C, about 300 °C lower than previously achievable. This study, published in Nature Communications, is expected to facilitate the exploration of new properties in these 2D materials.

“We call it scooter, because it delivers quickly. MoTe₂ molecules decompose to molybdenum and tellurium at high temperatures, but the scooter anchors telluride to MoS₂ and acts as a catalyst that lowers the activation temperature of the reaction,” explains YUN Seok Joon, the first author of the study.

Using this approach, the research team prepared semiconducting molybdenum ditelluride (2H-MoTe₂), metallic 1T’-MoTe₂, MoS₂−_xTe_x and WS₂−_xTe_x alloys. The conversion which began at the edges and grain boundaries of MoS₂, was complete. 2H-MoTe₂ formed near the edge and is favored at low temperatures, while 1T’-MoTe₂ at high temperatures. In this method, the team could produce a diode with 2H-MoTe₂ on the edge and 2H-MoS₂ on the inner part.

The resulting materials had different bandgap (1.1 eV), and higher degree of valley polarization (~37%) than MoS₂.

▲ Figure 1: New tellurization process for MoS₂ developed in this study. a) A NaOH substrate and a MoS₂ sample face each other and create MoTe₂. b) Most likely, NaOH in the presence of Te vapors converts to Na₂Te, which is scooted to the MoS₂ substrate to exchange Te with S atoms. c) The Na-scooter reduced the energy required for the reaction.

◀ Figure 2: Tellurization rate in relation to temperature and Na content. Below the dissociation temperature of MoTe₂ (700 °C), IBS scientists found out that semiconducting molybdenum ditelluride (2H-MoTe₂) is favored at low temperatures, and its metallic version (1T’-MoTe₂) at higher temperatures.

▲ Figure 3: Cartoonized version of the Na-scooter (modified from Freepik.com)

Letizia Diamante