Low thermal conductivity in 2H -polytype SnSe2

Tin diselenide ($\mathrm{Sn}{\mathrm{Se}}_{2}$) is a promising semiconducting material that exhibits diverse potential applications, including nanoscale electronics, resistive memories, sensors, and thermoelectric devices. Although $\mathrm{Sn}{\mathrm{Se}}_{2}$ offers low thermal conductivity (\ensuremath{\kappa}) and high-power factor (PF) compared with other two-dimensional (2D) materials, layer-dependent as well as phase-sensitive studies in particular polytypes are very limited. Here, we have probed $2H$-polytype $\mathrm{Sn}{\mathrm{Se}}_{2}$ to study the layer-dependent thermal conductivity by using the optothermal Raman technique. The $2H$ polytype of $\mathrm{Sn}{\mathrm{Se}}_{2}$ has been confirmed by Raman spectroscopy and second harmonic generation (SHG). We have achieved a low thermal conductivity value from 1.37 \ifmmode\pm\else\textpm\fi{} 0.04 to 2.81 \ifmmode\pm\else\textpm\fi{} 0.10 $\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ in $\mathrm{Sn}{\mathrm{Se}}_{2}$ for the layer thicknesses in the range of 3L--8L. The first-principles calculation has been performed to model the thermal conductivity for the $2H$ polytype from the monolayer (1L) up to the bulk limit. The calculated thermal conductivity value falls in the range of 1.29--2.87 $\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for 1L to the bulk range, which is consistent with our experimental findings. The remarkably low thermal conductivity in layered $2H$-polytype $\mathrm{Sn}{\mathrm{Se}}_{2}$ could be attributed to its inherent high anharmonicity and fragile interatomic bonding between the heavy elements Sn and Se. Moreover, the enhancement of phonon boundary scattering with decreasing thickness could be the possible cause of having an extremely low \ensuremath{\kappa} value in 2D $\mathrm{Sn}{\mathrm{Se}}_{2}$ nanoflakes. The origin of thickness-dependent low thermal conductivity in $2H\text{\ensuremath{-}}\mathrm{Sn}{\mathrm{Se}}_{2}$ is further supported by studies on phonon lifetime, group velocity, and electron localization function analysis. In this paper, we stipulate the groundwork for the finetuning of any other 2D system phase-dependent thermal conductivity.