Effects of Van der Waals Bonding on the Compressive Mechanical Behavior of Bulk Bi2Te3: A Molecular Dynamics Study

Abstract

Along the c axis of the crystal lattice, Bi 2 Te 3 has periodic quintuple layers ''-Te1-Bi-Te2-Bi-Te1-'' which are connected by Van der Waals bonding. The weak bonding between Te1-Te1 layers substantially affects the mechanical properties of Bi 2 Te 3 . In the work discussed in this paper, the molecular dynamics method was used to study the mechanical properties of cuboid single-crystal of bulk Bi 2 Te 3 under compressive loads. The emphasis was on the effects of the Van der Waals bonding on the deformation and failure mechanism. The molecular dynamics simulation results revealed that Van der Waals bonding has a dominant effect on the mechanism of deformation, and fundamentally determines the ultimate stress and fracture strain. Furthermore, the compressive load along the a and c axes lead to quite different failure behavior, which can be distinguished by their specific effects on the deformation of the Van der Waals bonding. However, only models with the load along the a axis dramatically demonstrate the effect of strain rate on the stress-strain curves, in accordance with the poor structural stability.

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

“…Excluding the coulomb force, the Huang potential has four Morse terms and six angle terms with 24 parameters, in which all the interacting atoms have to be assigned (pre-bonded) in the calculated supercell of Bi 2 Te 3 and then maintained until possible breakage. Although the systematical verification by Huang et al and us [54,62,64,65,81] indicates that this FF model can only reasonably reproduce the anharmonicity and anisotropy of the thermal and mechanical properties of Bi 2 Te 3 with the exception that it does not take into account the selfhealing nature of van der Waals bonding and overestimates phonon modes higher than 2.5 THz (there is a gap between 2.5 and 2.9 THz in the calculated phonon density of states though it is not found in the experiment [82]), because of its quite complicated form, most researchers favor the simpler Qiu potential with only six Morse terms and 18 parameters. Furthermore, in the latter, it is much friendlier for all the interacting atoms to be automatically selected (paired) within the appropriate cut-off radius during the whole calculation, and while they are also different from the Huang potential, the Te1-Te1 and Bi-Bi Morse forms here involve the interaction of both the same as well as different layers in the Bi 2 Te 3 lattice.…”

“…The van der Waals force is surely a key factor in phonon transport or scattering, and may play a more interesting role in the mechanical properties of Bi 2 Te 3 . Our previous research [31,[64][65][66] indicates that this weak bonding causes a smaller elastic constant and tensile strength on the c-axis than that on the a-axis. This was taken into account in the flexible thermoelectric material design [49], while it can also be strengthened through nanotwinning, leading to a tripling of the ideal shear strength.…”

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

“…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? , …”

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

“…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.…”