Transition metal dichalcogenides (TMDs) and 2-dim materials beyond graphene have shown excellent potential for future electronics. Controlling the heat flow across a hetero-structure will be crucial to developing high-speed electronic devices based on 2-dim materials. We have recently shown that the thermal expansion coefficient (TEC) dramatically increases in 2-dim materials when the thickness of the material shrinks from bulk to a few monolayers. Therefore, the TEC mismatch of 2-dim materials becomes an additional concern in designing electronic nano-devices. More specifically, we need to develop methods that enable us to control and tailor the TEC of TMDs through alloying or defect engineering.
In this contribution, I will employ transition metal alloying in TMDs to tune the TEC of monolayer Mo1-xWxS2 and study the interplay between thermal expansion and local defects using a combination of the scanning transmission electron microscope (STEM), electron energy loss spectroscopy (EELS) and first-principles DFT calculations. More specifically, we will measure the thermal expansion coefficient based on the plasmon energy shift as a function of temperature and combine this with first-principles modeling of the low-loss EELS signals. Using DFT calculations in the random phase approximation (RPA) we model the the plasmon peak shift as a function of lattice expansion. Combining the experimental and modeling data, we can now predict the TEC for WSe2.
Using this approach, we have determined the TEC of monolayer MoS2 and WS2 and found a significant mismatch between the two materials. To explore the influence of alloy engineering on the TEC, free-standing Mo0.7W0.3S2 2-dim materials are prepared. Finally, I will compare the TEC of alloyed Mo0.7W0.3S2 monolayer with that of MoS2/WS2 lateral heterointerfaces and explore the effects of strain or point defects on the local TEC using a combination of STEM imaging, EEL spectroscopy and DFT modeling.