Strain engineering of the transition metal dichalcogenide chalcogen-alloy WSSe

Alloying has been a powerful and practical strategy to widen the palette of physical properties available to semiconductor materials. Thanks to recent advances in the synthesis of van der Waals semiconductors, this strategy can be extended to monolayers (MLs) of transition metal dichalcogenides (TMDs). Due to their extraordinary flexibility and robustness, strain is another powerful means to engineer the electronic properties of two-dimensional (2D) TMDs. In this article, we combine these two approaches in an exemplary metal dichalcogenide chalcogen-alloy, WSSe. Highly strained WSSe MLs are obtained through the formation of micro-domes filled with high-pressure hydrogen. Such structures are achieved by hydrogen-ion irradiation of the bulk material, a technique successfully employed in TMDs and h-BN. Atomic force microscopy studies of the WSSe ML domes show that the dome morphology can be reproduced in terms of the average of the elastic parameters and adhesion energy of the end compounds WSe2 and WS2. Micro-photoluminescence measurements of the WSSe domes demonstrate that the exceedingly high strains (ε∼4%) achieved in the domes trigger a direct-to-indirect exciton transition, similarly to WSe2 and WS2. Our findings heighten the prospects of 2D alloys as strain- and composition-engineerable materials for flexible optoelectronics.