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

Abstract

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.

Unveiling the distinctive mechanical and thermal properties of γ-GeSe

“…Total thermal conductivity ( ) is the sum of the lattice thermal conductivity ( ) and electronic thermal conductivity ( ) [ 21 ]. We assumed that is inversely proportional to temperature above the Debye temperature [ 32 , 33 , 63 ]. From previous measurement, γ-GeSe shows high p -type doping with cm − 3 and shows significant contribution to [ 27 ].…”

Hidden structures: a driving factor to achieve low thermal conductivity and high thermoelectric performance

“…Numerous theoretical studies have predicted that MVB is the origin of phonon anomalies in many of the classical (and some potential) thermoelectric materials from the chalcogenide family. 400–402 MVB was also shown to improve the solubility of Ag (from 0.5% to >7%) in SnTe by AgSbTe 2 -alloying. 394 Finally, in a notable work by Wu et al 403 the strong carrier scattering at the grain boundaries of the champion thermoelectric material PbTe was traced down to the breakdown of MVB owing to the extreme Peierls’ distortion present therein.…”

Single Atomic Catalyst of Hydrogen Evolution Supported by Semimetallic γ-GeSe

“…Interestingly, bulk γ-GeSe was measured to have a higher electronic conductivity than that of graphite, rendering great possibilities in battery and thermoelectric applications. Many studies have been conducted to explore the unique structure on transport properties, , thermal conductivity, , high-performance thermoelectric, , ferroelectric, magnetic properties, photocatalytic activity, defect formation, strain-engineering behaviors, , and heterostructures. − …”