Metavalent Bonding Induced Phonon Transport Anomaly in 2D γ-MX (M = Ge, Sn, Pb; X = S, Se, Te) Monolayers
Surabhi Suresh Nair, Muhammad Sajjad, Kanishka Biswas, Nirpendra Singh
Abstract
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.