The discovery of inorganic plastic/ductile semiconductors has renewed the traditional understanding of inorganic semiconductors with inherently brittle mechanical properties, offering new pathways for material processing and manufacturing. Indium Selenide (InSe), as the first reported van der Waals crystal with plasticity, not only possesses the excellent physical properties of traditional inorganic semiconductors, but also can be plastically deformed and mechanically processed like metals, holding broad application promise in flexible and deformable thermoelectric energy conversion, photoelectric sensing, and other fields. However, there are still several scientific issues to be resolved, mainly including the deformation mechanisms, and the correlation between plastic deformation and physical properties.
Recently, Professor Tian-Ran Wei from the School of Materials Science and Engineering at Shanghai Jiao Tong University, Professor Xun Shi from the Shanghai Institute of Ceramics, Chinese Academy of Sciences, Professor Ben Xu from the China Academy of Engineering Physics, Professor David Rodney from the University of Lyon, Professor Jie Ma from the School of Physics and Astronomy at Shanghai Jiao Tong University, and Professor Jiong Yang from the Institute of Materials Genome Engineering at Shanghai University, have made a series of advancements in the study of the plastic deformation mechanisms and mechanical-thermal coupling of InSe. These findings have been published in Science Advances and Nature Communications.
(1) Martensitic Phase Transformation in InSe Crystals
Previous studies have proposed several plastic deformation mechanisms for InSe crystals, including multi-center diffuse chemical bonds, interlayer sliding and cross-layer slip, microcracks, and polymorphic transformations. In this work, the research team has unveiled a martensitic transformation in 2D vdW crystal InSe upon compression through a synergy of simulation, theoretical analysis, and experimental validation.
A machine-learned deep potential is developed, which accurately and efficiently simulates the deformation and transformation processes of InSe crystals. Atomistic simulations capture the deformation features of hexagonal InSe upon out-of-plane compression. Unexpectedly, a martensitic transformation, that is, the layered hexagonal structure is converted to a tetragonal lattice with specific orientation relationship, is found, which goes beyond the conventional dislocation slip (Figure 1B). This observation is corroborated by high-resolution experimental observations and theory. Experimentally, the corresponding two phases and their interfaces are observed in the compressed samples, with the orientation relationship being [120]H//[110]T (Figures 1D-F).
To illustrate the impact of martensitic transformation on the plasticity of InSe, various states are selected during deformation marked by stars in Figure 2A, providing a clear illustration of the nucleation and growth process of the new tetragonal phase, as demonstrated in Figure 2B. As the elastic stage concludes, brittle materials, characterized by high bond rigidity, undergo structural damage while simultaneously releasing elastic strain. Nevertheless, plastic vdW materials predominantly convert most of the elastic strain into irreversible plastic deformation through nucleation and growth of new phase upon reaching the critical stress point. The relevant schematic diagram is shown in Figure 2C. The newly formed tetragonal phase not only bridges several vdW layers, augmenting interlayer interaction strength, but also effectively hinders the propagation of cracks along vdW layers. The synergistic effect of these two factors suppresses material cleavage, resulting in unprecedented ductility (Figure 2D).
This discovery deepens the understanding of the plastic deformation mechanisms of vdW crystals and further enriches the scientific connotations of martensitic phase transformations. The related findings are published in Science Advances with the title "Van der Waals semiconductor InSe plastifies by martensitic transformation". Professors Tian-Ran Wei, Ben Xu and David Rodney are the corresponding authors; the postdoctoral fellow Yandong Sun and PhD student Yupeng Ma are the co-first authors.
Figure 1 Phase transformation during InSe compression: numerical and experimental insights. (A) nucleation of multiple tetragonal regions during the compression of hexagonal inSe (the parent phase was identified and removed because of its distinct coordination number). (B) Magnified view of a selected two-phase interface. (C) Single-cell structure of the tetragonal phase along with the atomic arrangement in the (100), (010), (001), and (110) planes. (D) Regions of the two-phase interface found in an experimentally deformed sample, along with the enlarged views of layered hexagonal and tetragonal structures. (E) Orientation relationship (OR) between the product and parent phases in the experiment and compares them with the simulations. (F) electron diffraction pattern of (D) featuring two patterns that correspond to the hexagonal layered phase with [120] zone axis and the tetragonal phase with [110] zone axis.
Figure 2 Martensitic transformation enhances the plasticity of InSe. (A) Strain-stress curves during loading and unloading, (B) illustration of the nucleation and growth process of the new tetragonal phase at the selected states in (A), (C) Schematic diagram of bond-state change during deformation for both brittle semiconductor and plastic vdW semiconductor, (D) visualization of the bridging effect on vdW layers and inhibition of crack propagation along the vdW layers induced by the new phase. The bottom two figures are snapshots of the deformed structure in simulations, with the hexagonal phases in gray and the tetragonal phases in color.
(2) Plastic Deformation and Thermal Transport Properties of InSe Crystals
As plastically deformable inorganic vdW crystals, InSe offers the opportunity to explore various effects arising from the coupling of its peculiar mechanical behaviors and other physical properties. In this work, the research team has revealed the correlations of plastic interlayer slip, lattice anharmonicity, and thermal transport in InSe crystals by employing neutron scattering techniques.
Neutron diffuse scattering signals (Figure 3B) exhibit a distinct spindle-like feature along the c-axis, indicating a breakdown of long-range order in the c-direction. Figures 3C and D show the evolution of neutron diffraction signals with wave vector q. There is an obvious shift of the reflection peaks for the (K−KL) planes with adjacent series of K value. By contrast, there is no shift for the reflections at (HHL) planes. This observation clearly indicates that there is a relative glide (or slip) of the adjacent (001) planes that tends to happen along the [1−10] direction rather than along the [110] direction.
Figures 4A and B display the phonon spectra measured by inelastic neutron scattering (INS) along the [K−K0] and [HH0] directions, with the white curves representing AIMD calculation results. The results show several significant features: (1) The out-of-plane transverse acoustic phonon (ZA) exhibits an almost "disappearing" overdamped phenomenon, possibly related to the disorder caused by out-of-plane sliding; (2) A large number of low-energy optical and acoustic branches cross throughout the entire Brillouin zone, leading to significant "resonant scattering", which hinders thermal transport; (3) The high-energy phonon band (HEB) and low-energy phonon band (LEB) are roughly parallel in the ΓM direction, showing a "nesting" effect, which opens more phonon scattering paths and significantly enhances the strength of phonon-phonon interactions; (4) The interlayer shear mode (TO, Figure 4A, mode 1) in the LEB is diffuse and has a very low excitation energy (~2 meV), whose vibration direction is consistent with the plastic sliding direction, indicating that this anharmonic shear mode will lead to structural instability and may be the dynamic origin of plastic sliding. The aforementioned phonon resonance scattering and nesting effects cause the soft mode phonons to be strongly scattered, exhibiting large phonon linewidths. Strongly anharmonic phonons significantly affect the material's thermal transport properties. Figure 4D shows that the low-temperature specific heat significantly deviates from the quasi-harmonic phonon calculations, exhibiting a Boson peak characteristic similar to glassy/amorphous materials, implying a large structural disorder and strong anharmonic phonon modes. The high conformity between the calculated and experimental values after deducting the ZA phonon contribution again indicates the overdamped characteristics of the ZA mode, and the thermal conductivity significantly deviates from the typical Debye T3 law at low temperatures.
These findings correlate the macroscopic plastic slip and the microscopic lattice dynamics, providing insights into the mechano-thermo coupling and modulation in 2D and 3D materials. The related findings are published with the title "Uncovering the phonon spectra and lattice dynamics of plastically deformable InSe van der Waals crystals" in Nature Communications. Professors Tian-Ran Wei, Jiong Yang, and Jie Ma are the corresponding authors of the paper.
Figure 3 (A) InSe crystal structure, (B) three-dimensional pattern of the diffuse spindle geometry (−1 ≤ E ≤ 1 meV) at 200 K. Inset schematically shows the interlayer slip in real space. The diffuse scattering intensities along the [001] direction at (C) (K-KL) plane and (D) (HHL) plane, respectively.
Figure 4 The phonon dispersions observed in the dynamical susceptibility χ″(q, E) from (A), (B) inelastic neutron scattering (INS) measurements along [K-K0] and [HH0] directions at 200 K in the (-200) Brillouin zone, respectively; the white lines in (A) and (B) are the AIMD-calculated phonon dispersions and the dash-lines are highlighted for the ZA phonon dispersions. (C) The temperature dependence of energy vs. phonon lifetimes (blue solid squares for 200 K, blue hollow squares for 50 K) and the simulation (purple solid circles for 200 K, purple hollow circles for 50 K). The yellow solid line represents the Ioffe Regel limit, while ω is the phonon frequence and the orange one is guidance for the tendency of calculating lifetime. (D) The Cp/T3 of InSe from measurement (hollow black squares), and the calculations according to the total phonon DOS (purple curve) and the partial phonon DOS without the ZA contribution (brown curve). DOS denotes phonon density-of-state, ZA is the out-of-plane transverse acoustic branch, and T is temperature. (E) In-plane (black squares) and out-of-plane (blue circles) thermal conductivity. The inset shows the low temperature thermal conductivity. The red line is the fitting result of the T−1 law above 150 K.
References:
https://www.science.org/doi/10.1126/sciadv.ado9593
https://www.nature.com/articles/s41467-024-50249-5
Drafted by: Yupeng Ma, Tian-Ran Wei