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 Å and metal at r ≥ 3.0 Å. The maximum n-type Seebeck coefficient of 0.66 mV/K (0.59 mV/K) achieved along the armchair direction, at r = 0.5 Å (1.5 Å), 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 Å. The forbidden gap between the acoustic and optical phonons branches reduces as r increases. An ultralow room temperature lattice thermal conductivity κ_l of 1.44 W/mK along the armchair direction is obtained at r = 1.5 Å, which further reduces to 0.44 W/mK at 1000 K. The obtained value is 100-fold smaller than the room temperature κ_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 Å) 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.