Controlling Nanoscale Thermal Expansion of Monolayer Transition Metal Dichalcogenides by Alloy Engineering*

Two dimensional materials, such as transition metal dichalcogenides (TMDs), graphene, boron nitride (BN) are seen as promising materials for future high power/high frequency electronics. However, the large difference in the thermal expansion coefficient (TEC) between many of these 2D materials could impose a serious challenge for the design of monolayer-materials-based nano-devices. To address this challenge, alloy engineering of TMDs is used to tailor their TECs. Here, in-situ heating experiments in a scanning transmission electron microscope are combined with electron energy-loss spectroscopy and first-principles modeling of monolayer Mo_1-xW_xS₂ with different alloying concentrations to determine the TEC. Significant changes in the TEC are seen as a function of chemical composition in Mo_1-xW_xS₂, with the smallest TEC being reported for a configuration with the highest entropy. This study provides key insights into understanding of nanoscale phenomena that control TEC values of 2D materials.