Thermal Transport In Layered Tin Disulfide Nanofilms

Tin disulfide(SnS₂)is a layered n-type semiconductor material that has shown interesting physical properties and great potentials in applications such as photodetectors,sensors,field-effect transistors,battery,etc.According to the density functional theory,SnS₂ has an ultra-low lattice thermal conductivity so its application in thermoelectric conversion field has attracted more attention.The understanding of thermal transport in layered SnS₂ is essential to optimize the thermal management,energy transport and conversion of these devices.However,related studies on the thermal transport properties of SnS₂ are far less than studies on its other physical properties,such as electrical and optical properties.There are still a lot of unknowns about the thermal transport properties of SnS₂ nanofilms,especially on the thermal transport of SnS₂ nanofilms grown by chemical vapor deposition.In this dissertation,the in-plane thermal conductivities of layered single-crystalline and polycrystalline SnS₂ nanofilms with different thicknesses were measured by the Raman optothermal technique at room temperature.We extracted the thermal conductivities of single-crystalline SnS₂ nanofilms with 45,77,85 and 171nm-thickness to be 9.2 Wm^-1K^-1,8.7 Wm^-1K^-1,8.3 Wm^-1K^-1and 11.8 Wm^-1K^-1,and the thermal conductivities of polycrystalline SnS₂ with 60,76,and 98 nm-thickness are 5.5 Wm^-1K^-1,5.9 Wm^-1K^-1,and 4.8 Wm^-1K^-1,respectively.The thermal conductivity of the exfoliated single-crystalline SnS₂ nanofilm is about twice that of the CVD-grown polycrystalline SnS₂ with similar thickness.In addition,to gain insights into the difference and mechanism of the thermal properties between exfoliated and CVD-grownSnS₂ nanofilms,by high-resolution transmission electron microscopy measurement to observe the microstructure of SnS₂nanofilms,we grained the average grain size of 250 nm for CVD-grown polycrystalline SnS₂.By finite element simulation,we found the grain boundary thermal conductance of polycrystalline SnS₂ is 60 MWm^-2K^-1 and the effect of average grain size and uniformity of grain size distribution for the thermal conductivity of polycrystalline SnS₂.The non-uniform distribution didn't change the thermal conductivity of polycrystalline SnS₂ at the same average grain size,but the heat flow distribution changed.As the average grain size decreases,the thermal conductivity of polycrystalline SnS₂ reduces sharply,which implies that phonon grain boundary scattering is dominant in polycrystalline SnS₂ with a small grain size.Model predicts that the in-plane thermal conductivity of polycrystalline SnS₂will be less than 1 Wm^-1K^-1,when the average grain size reduces to 15 nm,which shows a possibility of adjusting the thermal conductivity of polycrystalline SnS₂ by changing average grain size.The research results of this dissertation provide significant experimental data for the energy transport and thermal management of electronic/optoelectronic devices based onSnS₂,and offer an effective guidance for further research on thermoelectric conversion and other applications of polycrystalline SnS₂.