Ultra low lattice thermal conductivity and exceptional thermoelectric conversion efficiency in rippled MoS2

Molybdenum disulfide (MoS2) holds significant potential as a semiconductor for next-generation flexible thermoelectric modules, but its high thermal conductivity and low figure of merit have limited its commercial viability. In this study, we report a breakthrough, achieving a record-high n-type (p-type) thermoelectric figure of merit of 1.42 (1.25) at 1000 K, coupled with a thermoelectric conversion efficiency of 16 % (14 %) (along armchair direction), outperforming commercially available thermoelectric modules. Our first-principles calculations on rippled monolayer MoS2 show a transition from a direct to indirect band gap semiconductor at a rippling amplitude (r) of 1.0 & Aring; and metal at r >= 3.0 & Aring;. The maximum n-type Seebeck coefficient of 0.66 mV/K (0.59 mV/K) achieved along the armchair direction, at r = 0.5 & Aring; (1.5 & Aring;), at 1000 K is notable in the case of flexible thermoelectric materials. A high electrical conductivity contributes to an optimal power factor of 0.68 mW/mK2 along the armchair direction. The phonon dispersion reveals the dynamic stability of the system up to r = 1.5 & Aring;. The forbidden gap between the acoustic and optical phonons branches reduces as r increases. An ultralow room temperature lattice thermal conductivity x l of 1.44 W/mK along the armchair direction is obtained at r = 1.5 & Aring;, which further reduces to 0.44 W/mK at 1000 K. The obtained value is 100-fold smaller than the room temperature x l of pristine monolayer MoS2 (144.60 W/ mK). Our findings reveal a noteworthy n-type figure of merit (ZT) of 0.45 at 300 K (r = 1.50 & Aring;) along the armchair direction, which is one order of magnitude more than the pristine monolayer MoS2. A significant thermoelectric conversion efficiency of 13 %, taking a temperature gradient of 700 K, is obtained, outperforming Bi2Te3-based thermoelectric materials. These results highlight the potential of lattice distortions, which can be induced using bulged substrates, to drastically reduce the lattice thermal conductivity of MoS2 and other 2D materials, opening new possibilities for strain-engineered flexible electronic devices.