IIT Mandi team achieves crack-free WS2 monolayer

Researchers at the Indian Institute of Technology, Mandi have developed a process to achieve crack-free Tungsten Disulphide (WS₂) monolayer by investigating the role of the growth substrate, thermal stress and defects. The result provides a platform to mitigate defects in chemical vapor deposition (CVD) grown 2D materials for pragmatic applications, such as flexible electronics, neuromorphic devices, and sensors.

The findings of this study were published in the ACS Applied Materials Interface. The study was led by Viswanath Balakrishnan, associate professor, School of Engineering, IIT Mandi and co-authored by his students Divya Verma, Pawan Kumar and Ms Deepa Thakur. At the same time, density functional theory (DFT) simulations were done in collaboration with Chandra Singh, University of Toronto and Sankha Mukherjee, IIT Kharagpur. The project was funded by MHRD-STARS, India and Natural Sciences and Engineering Research Council of Canada (NSERC).

Atomically thin WS₂ monolayers are widely known because of its direct band gap, high mobility values and ability to provide a platform for heterogeneous integration of multiple functionalities. But the device fabrication of 2D materials using these mono layers is limited due to the lack of large-scale, crack-free growth of mono layers. However, IIT Mandi researchers have demonstrated that crack generation can be avoided despite defects and voids in a WS₂ monolayer if CVD growth is performed on a suitable substrate such as sapphire.

Highlighting this research, Balakrishnan, associate, said, "Our team has carried out a systematic investigation to describe severe cracking in WS₂ monolayer grown on different substrates by relating the thermal mismatch stress, substrate-monolayer binding energy, and the presence of defect-mediated voids formation. We have demonstrated that the crack generation can be avoided in spite of defects and voids in a WS₂ monolayer, if CVD growth is performed on a suitable substrate and reported that the WS₂ monolayer grown on sapphire substrate consisted of voids without any visible crack as it develops compressive residual stress in the WS₂ monolayer."

The growth of WS₂ is also crucial as the growth temperature, growth substrate, and defect formation influence its mechanical stability.

Explaining the research, Divya Verma said, “WS₂ monolayer grown on sapphire substrate can have voids or defects, yet it will not fracture as it develops compressive residual stress in the WS₂ monolayer. This will be crucial for large-scale, crack-free growth of WS₂ monolayer where defect engineering is possible to enhance functionalities without the compromise in mechanical reliability.”

The team’s focus is mainly to develop CVD processed large-scale 2D materials and demonstrate transistor, memristor and photodetector functions. However, prototyping such devices and proceeding towards heterogeneous integration will be done in collaboration with the experts from other IITs, IISc Bangalore in near future.