Decoding Phonon Transport Mechanism for Ultralow Thermal Conductivity in 2D Chalcogenides for Next Generation Thermoelectrics

Abstract

Waste heat is abundant in various sectors, including fossil fuel combustion from heavy industries and automobiles, causing global warming and climate change. Efficient waste heat recovery could potentially meet up to 30% of global electricity demand. Thermoelectric generators, based on the Seebeck effect, offer a promising approach to sustainable energy by converting waste heat into electricity. Thermoelectric generators offer several advantages, including generating electricity without chemical reactions, operating across a wide temperature range, minimal maintenance, and long lifespans with no moving parts. Their applications extend beyond waste heat recovery in heavy industries and automobiles to solar and geothermal power plants, chip cooling, and space stations. The electricity generated from waste heat can also be supplied to remote areas where conventional power grids are impractical. TEGs can also be integrated into sensors, smartwatches, fitness trackers, and implantable medical devices, utilizing the body’s temperature gradient to generate power. Thermoelectric power generation can play a significant role in achieving the UAE’s Energy Strategy 2050 goal of generating approximately 44% of energy from clean sources.
Chalcogenides have been the most extensively studied and commercially utilized class of thermoelectric materials since the mid-20th century. Their tunable band gaps, ranging from narrow to wide, enable excellent thermal transport properties. However, the complex interdependence of thermoelectric parameters (Seebeck coefficients S, electrical conductivity σ, and electronic thermal conductivity κe) constrains their thermoelectric efficiency, while lattice thermal conductivity κl is a relatively independent parameter. This thesis employs first-principles calculations integrated with semiclassical Boltzmann transport theory to explore the impact of phonon dynamics, lattice configurations, and nanostructuring on the thermoelectric performance of various two-dimensional (2D) layered chalcogenides.
The group IV-VI monochalcogenide (γ-MX; M = Ge, Sn, Pb; X = S, Se, Te) monolayers crystallize in a double-layer honeycomb lattice and exhibit anomalous κl behavior. Despite their higher atomic masses, an anomalous trend in κl is observed in γ-GeTe and γ-SnTe, with a higher κl compared to their selenide counterparts due to a wide phonon gap and metavalent bonding. Among the studied monolayers, γ-PbTe demonstrates the lowest room-temperature κl of 0.49 W/mK, attributed to significant anharmonicity, reduced phonon group velocities, and enhanced phonon scattering due to the coupling between the acoustic and low-lying optical phonons. Beyond the conventional lattice systems, MX monolayers are predicted to be dynamically stable in an unconventional square-octagonal (so) lattice. The thermal transport properties of so-SnX (X=S, Se) monolayers are calculated. The room-temperature κl of so-SnS (4.01 W/mK) and so-SnSe (1.47 W/mK) monolayers are approximately 63% and 77% lower than those of their hexagonal counterparts. This can be attributed to the absence of a phonon gap, lower Debye temperature, topologically similar band crossing between acoustic and optical phonon modes, and weaker bonding.
The unconventional so lattice is further extended to the dichalcogenide family, where the so-MoS2 monolayer is characterized by Dirac cone-like features in the electronic band structure and topological band crossing regions in the phonon band structure. These band crossings cause enhancement of the anharmonic phase space for three-phonon scattering by 1000 times, compared to the pristine hexagonal MoS2 monolayer, resulting in a room temperature κl of 18.67 W/mK, sevenfold less than that of the hexagonal MoS2 monolayer. This thesis explores the influence of lattice distortions in the form of rippling, which can be experimentally realized by bulged substrates on the κl of MoS2 monolayers. The rippled MoS2 monolayer shows an ultralow room-temperature κl of 1.44 W/mK (a 100-fold reduction from pristine MoS2) along the armchair direction at a rippling (r) of 1.5 Å. This value further decreases to 0.44 W/mK with an optimal power factor of 0.68 mW/mK2 and a record-high ZT of 1.42 at 1000 K, resulting in 16% thermoelectric efficiency at a temperature gradient of 700 K. This enhancement in thermoelectric performance is due to ripples in the MoS2 lattice, which close the phonon gap, enhancing phonon scattering and phase space.
Additionally, the effect of quantum confinement is investigated through a comparative study of bulk and monolayer TlPt2Se3, a representative of the ternary chalcogenide family. A maximum p-type ZT of 0.64 at 600 K is obtained for the monolayer, 32 times higher than its bulk counterpart at the same doping level. The calculated κl of TlPt2Se3 monolayer is 1.92 W/mK at 600 K, 13% lower than the bulk, underscoring the impact of low-dimensionality on phonon transport. A 10-fold reduction in the sample size of monolayer TlPt2Se3 reduces the room temperature κl by 53%, reaching a value of 2 W/mK, coinciding with the calculated value at 600 K. This emphasizes the role of nanostructuring and increased boundary scattering. The findings in the thesis hold significant potential in advancing thermoelectric research for revolutionizing sustainable energy conversion and provide valuable insights to guide the development of next-generation thermoelectric generators.

Date of Award	2025
Original language	American English
Supervisor	Nirpendra Singh (Supervisor)