Stacked Vertical MoS2 Layers Show Strong Optoelectrical Qualities

While molybdenum disulphide (MoS₂) has been used since the 1970s and 1980s as a solid lubricant in the aerospace industry and for high-performance mechanics, the material has attracted a great deal of interest from the scientific communities because of its properties as a 2D semiconductor material.

An international research team led by professor My Ali El Khakani of the Institut national de la recherche scientifique (INRS), in collaboration with professor Mustapha Jouiad’s team at the Université de Picardie Jules Verne (UPJV), has proposed a new way to grow MoS₂ films. The method bypasses traditional challenges associated with the material and could unlock innovations in optoelectronics and renewable energies.

“By proposing a new way of growing MoS₂ films with a vertically layered structure, we are paving the way for the synthesis of MoS₂ that is labelled as ‘3D’ but has exceptional ‘2D’ behavior,” said Driss Mouloua, currently a postdoctoral researcher at Commissariat à l’énergie atomique in France.

MoS₂ can strongly absorb light and transform it into electrical charges with high electron mobility, giving it the capacity for rapid signal transmission. This combination makes it particularly appealing for the development of optoelectronic applications such as photodetectors, photonic switches, next-generation solar cells, and LEDs.

Professor My Ali El Khakani of INRS (center) is pictured with his research team. The team, along with the team from UPJV, found that thick MoS₂ films exhibit strong optoelectronic properties when their layers are vertically aligned. Courtesy of INRS.

However, these properties depend on the way the monolayers of this 2D material are arranged in the films. Over time, scientists have developed manufacturing strategies to obtain two to five horizontally layered monolayers in order to take advantage of MoS₂’s optoelectronic properties.

With their most recent study, El Khakani’s team has shown that it is possible to synthesize relatively thick MoS₂ films that are made up of vertically aligned MoS₂ layers. To achieve this, the team used an approach based on pulsed-laser deposition (PLD).

By controlling the growth conditions of these thin PLD-MoS₂ films and studying their properties, the researchers have achieved relatively thick MoS₂ films, but their optoelectronic behavior resembles that of ultra-thin 2D MoS₂. The films are about 100 nm thick, equivalent to ~200 atomic monolayers of MoS₂.

In their observations, the researchers found that layers arranged more vertically meant better photodetection performance in the PLD-MoS₂ films. In this structure, the vertical MoS₂ monolayers can interact individually with light, thereby enhancing their capacity to absorb light and achieve a swift vertical transfer of the photocharges along the MoS₂ layers. This translates to an optoelectronic performance comparable to that of the 2D MoS₂ ultrathin films. Moreover, these 3D PLD-MoS₂ films can be scaled up to the wafer level while circumventing the difficulties associated with the synthesis of only few horizontal monolayers.

“Not only is this the first time that MoS₂ with vertically aligned layers has been achieved by using the PLD technique, but, even more importantly, we have succeeded in correlating directly the degree of vertical alignment of the monolayers with the photodetection performance of the MoS₂ films,” said El Khakani.

With this breakthrough, he added, a better understanding of quantum confinement phenomena can be cultivated, as well as improvements in the design of new optoelectronic devices based on 2D materials, such as MoS₂ or tungsten disulfide.

The research was published in Advanced Optical Materials (www.doi.org/10.1002/adom.202302966).