2, University of Illinois at Chicago, Chicago, Illinois, United States
Transition metal dichalcogenides (TMDs), a major group of 2-dim materials beyond graphene, have shown extraordinary properties as candidates for future electronics. When the dimensions of materials reduce from bulk to 2-dim, the thermal expansion coefficient (TEC) dramatically increases due to the lack of the constraint from inter-layer interactions. The subsequent larger TEC mismatch becomes a significant problem in the design of the electronic nano-device. The knowledge of the thermal expansion of TMDs is one of the most important aspects of modern device design with such materials. Particularly, an effective method to control the thermal expansion coefficient of TMDs and an understanding of the related mechanism are needed.
In this contribution, we will introduce alloy engineering to tune the thermal expansion coefficients of monolayer Mo1-xWxS2 and study the interplay between thermal expansion and local defects using a combination of scanning transmission electron microscope (STEM), electron energy loss spectroscopy (EELS) and first-principles calculations. More specifically, we will measure thermal expansion coefficient based on the plasmon energy shift in a range of temperatures and first-principles modeling of the low-loss EELS signals. With this combination, we have previously measured the thermal expansion coefficients of monolayer MoS2 and WS2, which shows a considerable mismatch with each other. Free-standing Mo1-xWxS2 flakes are then prepared and we measure the local thermal expansion of flakes in different concentrations over a range of temperatures. It is important to note that the TEC can be tuned through controlling the doping concentration. Atomic-resolution Z-contrast images and the corresponding nanometer-scale thermal expansion maps will be used to explore the influence of local strains and defects on thermal expansion.
Acknowledgement: This work was supported by the National Science Foundation EFRI 2-DARE Grant 1542864. The acquisition of UIC JEOL JEM ARM200CF is supported by an MRI-R2 grant from the National Science Foundation (Grant No. DMR-0959470). This work made use of instruments in the Electron Microscopy Service and the High Performance Computing Clusters at Research Resources Center, UIC.