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

Abstract

Bismuth telluride (Bi2Te3) based semiconductor is one of the typical inorganic thermoelectric (TE) materials with excellent energy conversion efficiency, but the intrinsic brittleness severely limits its mechanical performance for further application of long-term reliability and wearable devices. To understand the recent mechanical improvement of the ductile and flexible inorganic TE materials at atomic scales, here we use molecular dynamics simulations to intuitively illuminate the enhanced shear deformability and performance stability of brittle Bi2Te3 crystal through the tailored effects of surfaces. We reveal that the peculiar micro-behavior origins from the layered hierarchical bonding structure with weak but reversible Van der Waals force, namely a sacrificial bond, between Te1-Te1 adjacent layers. The synergetic evolution of local structures including sacrificial bonds and strain-induced defects tends to partly compensate for the mechanical degradation caused by structure softening during shearing, achieving a relatively large strain before cleavage. The inspired engineering strategy on synergistically optimizing bonds and defects opens a pathway for designing multi-scale hierarchical inorganic TE materials with excellent overall performance.

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

“…As discussed in our previous work on Bi 2 Te 3 crystals, , there can spontaneously be alternating slips on different Te1 layers during shearing (Figures and a), leading to a larger amount of energy dissipation compared with that of slippage only on a solo layer (until cleavage). For more energy dissipation with less lattice distortion as we expect, an increase in dissipation via enhanced dislocation motion on every Te1 layer is desired on the analogy of a larger hidden length of the SB in self-healing materials. , This idea can be implemented by promoting continuous slippage (rather than a single one to form stable SF with less vdW deformability), particularly the even multiple where unstable SF can be removed if a second partial dislocation followed the path of the first on the same layer.…”

“…In the latter case, there will be repeated changes of the lattice order during continuous slippage, while little disorder ultimately remains after HBS reforming, leading to both high energy dissipation and stable performance against localized deformation. For instance, in the given conditions such as at 300 K and under 1 × 10 9 s –1 rate, one can find a suitable nanostructure size (e.g., 6 nm for the PSP model) to enable continuous slips without disturbing the synergetic evolution of the SB and defect too much and thereby achieve a relatively large fracture strain . The multilayer alternating deformation of the Bi 2 Te 3 nanocrystal (in the order of V4-V3-V4-V6 layers) can be verified by the zigzag-like stress–strain response and the extracted configurations in the xz cross-section (Figures and a).…”

“…The Cartesian z axis is taken along the [0001] crystallographic direction of the layered HBS, making the basal slip plane (0001) correspond to the xy plane. Similarly, the Cartesian y axis is taken along the [12̅10] crystallographic direction that exhibits the orientation effect for the excellent flexibility in the Bi 2 Te 3 -SWCNT TE composite because of the continuous slippage of partial dislocations (e.g., dislocation reaction of or vice versa) . For the nanocrystal model named PSP (where “P” stands for periodicity and “S” for surface), the free boundary condition is adopted in the y direction, that is, the xz surface, while the other two directions are periodic.…”

“…For the sake of observation and discussion, strain-induced defects, especially stacking fault (SF), are shown in the xz cross-section, while heterogeneous local deformation and the lattice disorder (as discussed below) in the yz cross-section and the adjacent Te1 layers coupled by vdW are labeled as “V1–V6” along the z axis. In addition, parts of setups and a detailed discussion of surface effects on nanocrystals can be found in our previous research. , …”

“…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.…”