Metavalent Bonding Induced Phonon Transport Anomaly in 2D γ-MX (M = Ge, Sn, Pb; X = S, Se, Te) Monolayers

High Resolution Image Download MS PowerPoint Slide Exploring thermoelectric materials with captivating chemical bonding and low lattice thermal conductivity (κ l ) is crucial in thermoelectric research. Here, we propose two-dimensional (2D) γ-monochalcogenide (MX; M = Ge, Sn, Pb, and X = S, Se, Te) monolayers as potential thermoelectric materials. The exfoliation energies of γ-MX monolayers fall in the 12–16 meV/Å 2 range, less than graphene (21 meV/Å 2 ), suggesting their feasible realization through mechanical or chemical exfoliation. The electronic band structures exhibit an indirect band gap ranging from 0.41 eV for γ-PbTe to 0.86 eV for γ-PbS. The metavalent bonding in these monolayers results in an unusual trend in κ l with atomic mass. Among studied monolayers, the γ-PbTe monolayer exhibits an ultralow room temperature κ l of 0.49 W/mK, which accounts for its increased anharmonicity and higher scattering rates from acoustic and low-lying optical phonons. Despite having a higher mass than γ-PbS, γ-PbSe demonstrates enhanced κ l values. Similar anomalous behavior is observed in the case of γ-GeTe (8.57 W/mK), γ-GeSe (3.10 W/mK), γ-SnTe (4.30 W/mK), and γ-SnSe (3.31 W/mK) monolayers. The enhanced κ l in tellurides is attributed to the high bond stiffness, low charge transfer between M and X atoms, and widening of the phonon gap, which significantly reduces the phonon scattering. The above anomalies observed in κ l originate in the unique bonding mechanism (metavalent bonding), which suppresses the conventional reductive effect of mass to define κ l . Our findings elucidate the distinct thermal transport properties of 2D materials and provide valuable insights for designing efficient thermoelectric materials.