Unmasking the properties of 2D materials (Nanowerk News) It is now possible to grow large-area ultrathin sheets of molybdenum disulfide, a two-dimensional (2D) material promising the next generation of electronic and optoelectronic devices, thanks to a new twist on a standard method developed by A*STAR scientists ("Dispersive growth and laser-induced rippling of large-area singlelayer MoS2 nanosheets by CVD on c-plane sapphire substrate").

Molybdenum disulfide, one of a family of so-called semiconducting transitional metal dichalcogenides (TMDCs), has attracted considerable attention as a 2D material, thanks to its remarkable electronic and optoelectronic properties. But preparing large-area atomically thin layers of TMDCs is notoriously difficult, with conventional growth methods such as mechanical exfoliation and physical vapor deposition yielding single-layer films only a few micrometers in size.

By illuminating nanosheets with a laser, A*STAR researchers were able to induce ripple structures in molybdenum disulphide, similar to those in sand.

To overcome the limitation of such a useful material, Dongzhi Chi and Hongfei Liu of the A*STAR Institute of Materials Research and Engineering searched for a way to modify a standard fabrication technique, to grow high quality, millimeter-sized single-layer molybdenum disulfide nanosheets.

The growth mechanism of 2D films is still not fully understood and is a major hurdle for their large scale adoption in electronic applications, says Chi. Growing large-area 2D materials allows for large scale fabrication of integrated circuits using conventional semiconductor processing methods.

By modifying chemical vapor deposition  a manufacturing tool used in everything from sunglasses to potato chip bags and fundamental to the production of much of todays electronic devices  they were able to grow single-layer molybdenum disulfide nanosheets of greatly increased grain size.

Smaller grain sizes result in structural defects, so devices fabricated with such materials perform poorly, explains Chi. Larger grain sized 2D TMDCs, however, minimize these defects and lead to improved performance.

In a pressurized reaction chamber, powdered molybdenum trioxide and sulfur were vaporized. To create larger grain sizes, the researchers increased the temperature of the reaction chamber and used a silicon or quartz shadow mask, held over a sapphire substrate, to indirectly supply the molybdenum trioxide and sulfur vapors to the advancing molybdenum disulfide growth front on the substrate.

Ripples were introduced into the single-layer molybdenum disulfide nanosheets by illuminating them with a laser. These ripple structures are predicted to have a significant effect on the electronic, mechanical, and transport properties of single-layer molybdenum disulfide.

To compare the single-layer molybdenum disulfide nanosheets and their laser-induced ripple structures, the researchers used a number of characterization tools, including Raman scattering and photoluminescence spectroscopy as well as atomic-force microscopy.