Capturing anharmonic and anisotropic natures in the thermotics and mechanics of Bi₂Te₃ thermoelectric material through an accurate and efficient potential

Abstract

Force-field-(FF)-based molecular simulation is essential but challenging in the theoretical research of complex thermoelectric (TE) materials. As they are general and crucial in TE semiconductors, the structural natures of anharmonicity and anisotropy can help us understand the inherent relation between thermal and mechanical behavior, and therefore the reliability of FF studies can be assessed. In this paper, given prior knowledge of the structural, mechanical and thermal properties as well as the limitations and necessary approximations of the FF method, a feasible and detailed FF modeling scheme and simulation has been designed for Bi2Te3, which is a typical high-performance TE material. Using the complementary approach combining quasi-harmonic lattice and molecular dynamics, the obtained potential is systematically confirmed to be accurate and efficient for the prediction of anharmonic and anisotropic behavior in thermotics and mechanics over a wide temperature range compared with the present Bi2Te3 models. This reveals that the intrinsic anisotropy and anharmonicity can measure the asymmetry of crystal lattices and the interatomic force in the current state. In addition, the significant distinction of temperature-dependent anharmonic effects in different directions of Bi2Te3 stems from its layered hierarchical structure, in which weak van der Waals bonding will probably be the key structural factor in comprehensively improving performance for mass production and wearable application. This prior-knowledge-based FF study is also suggested as a bridge between the theoretical understanding of micro-mechanisms and the experimental measurement of TE material properties, leading to a general framework of molecular simulation for other complex energy materials.

Order-Tuned Deformability of Bismuth Telluride Semiconductors: An Energy-Dissipation Strategy for Large Fracture Strain

“…The shear deformation of the Bi 2 Te 3 crystal is simulated using large-scale atomic/molecular massively parallel simulator (LAMMPS) codes with our well-established potential energy model . All the MD configuration models are cubic with a side length of 60 Å (e.g., 6 nm) to demonstrate the significant contribution of surface effects in the nanosized models.…”

“…Bi 2 Te 3 has a layered hierarchical bonding structure (HBS) − with van der Waals (vdW) force between repetitive Te1-Bi-Te2-Bi-Te1 quint substructures along the [0001] axis. Owing to the strength difference (i.e., bond hierarchy) compared with Bi-Te1 and Bi-Te2 covalent bonds, weak vdW force shows the leading role in the structural evolution of deformed Bi 2 Te 3 crystal as well as the responsibility for the failure modes under external stimuli. ,− The crystal/grain deformability governed by the slipping process in vdW-coupled Te1 atomic layers contributes greatly to the mechanical performance of the Bi 2 Te 3 semiconductor (including the localization problem) at room temperature because other mechanisms of structural evolution such as grain boundary slippage and dynamic recrystallization tend to be activated at higher temperatures. − On the other hand, vdW in this covalent crystal belongs to the sacrificial bond (SB) that is characterized by the dynamic reversibility as well as the energy-dissipation mechanism, which accounts for strong and tough attributes of self-healing materials. − Moreover, our previous work has indicated the synergy between vdW SB and strain-induced defects during slipping. , When suffering shear loads, the lattice order will change through the synergetic evolution with spontaneous redistribution of the driving force (e.g., energy and stress), leading to alternating local deformations on multilayers and enhanced deformability of this layered HBS. Therefore, Bi 2 Te 3 crystal inherently exhibits an energy-dissipation manner that is related to order changes during the substructure evolution of bonds and defects in the slipping process.…”

Synergetic Evolution of Sacrificial Bonds and Strain-Induced Defects Facilitating Large Deformation of the Bi2Te3 Semiconductor

“…The Bi 2 Te 3 shear deformation at room temperature is simulated using MD codes LAMMPS 41 with our newly developed potential model that has been confirmed to be a reasonable abstraction of the HBS to capture the anharmonicity and anisotropy in mechanics and thermotics. 28 For the simulation of the ideal crystal, periodic boundary conditions are applied in all the three directions in a Cartesian representation, which is named the PPP model, where "P" stands for periodicity. To illuminate the role of surfaces, particularly the finite-size effect of the observed ultrafine grains (e.g.…”

“…24 Other than the introduction of various defects and external conditions, the welltailored deformability should be related to the layered HBS especially VdW. According to our previous researches, though weak VdW is mainly responsible for large deformation and fracture of Bi2Te3 lattice [25][26][27][28] as the cumulative disadvantage of structure softening with applied loads found in other TE semiconductors, [29][30][31][32][33] it can also be greatly strengthened to form a new covalent Te1-Te1 bond and triple the shear strength via nanotwinning. 34 Moreover, inspired by some peculiar behaviors dominated by weak but reversible dynamic bonding, namely the dislocation-controlled deformation of extraordinarily ductile α-Ag2S semiconductor 15,35 and the reversible interlayer separation of flexible MoS2 sheet, 36 this intramolecular dispersion force should belong to the concept of sacrificial bonds (SB) concerned in artificial polymeric and natural materials with excellent mechanical properties.…”

“…Other than the introduction of various defects and external conditions, the well-tailored deformability should be related to the layered HBS, especially VdW forces. According to our previous research studies, although weak VdW forces are mainly responsible for large deformation and fracture of the Bi 2 Te 3 lattice − as the cumulative disadvantage of structure softening with applied loads found in other TE semiconductors, − it can also be greatly strengthened to form a new covalent Te1–Te1 bond and triple the shear strength via nanotwinning . Moreover, inspired by some peculiar behaviors dominated by weak but reversible dynamic bonding, namely, the dislocation-controlled deformation of the extraordinarily ductile α-Ag 2 S semiconductor , and the reversible interlayer separation of the flexible MoS 2 sheet, this intramolecular dispersion force should belong to the concept of sacrificial bonds (SBs) concerned in artificial polymeric and natural materials with excellent mechanical properties. − Because of the nature of adaptability and self-healing, the SBs in these hierarchical structures can break and reform dynamically during deformation, providing a microscopic, reversible, energy-dissipation mechanism to strengthen/toughen the materials. , Because both SBs and strain-induced defects involve VdW breakage in the Bi 2 Te 3 HBS, is there any sophisticated interaction between them accounting for the stated improvement of mechanical performance? , …”