Van der Waals semiconductor InSe plastifies by martensitic transformation

Abstract

Inorganic semiconductor materials are crucial for modern technologies, but their brittleness and limited processability hinder the development of flexible, wearable, and miniaturized electronics. The recent discovery of room-temperature plasticity in some inorganic semiconductors offers a promising solution, but the deformation mechanisms remain controversial. Here, we investigate the deformation of indium selenide, a two-dimensional van der Waals semiconductor with substantial plasticity. By developing a machine-learned deep potential, we perform atomistic simulations that capture the deformation features of hexagonal indium selenide upon out-of-plane compression. Unexpectedly, we find that indium selenide plastifies through a martensitic transformation; that is, the layered hexagonal structure is converted to a tetragonal lattice with specific orientation relationship. This observation is corroborated by high-resolution experimental observations and theory. It suggests a change of paradigm, where the design of new plastically deformable inorganic semiconductors can focus on compositions and structures that facilitate phase transformations, going beyond the conventional dislocation slip.

Fig. 1.. Plasticity of β-InSe under out-of-plane compression.

(A) Atomic structure of β-InSe with corresponding atomic orientations. (B) Macroscopic morphological changes in samples before and after compression, highlighting the phase transformation region. (C) Stress-strain curves for both numerical and experimental compressions [experimental data from (15)]. (D) Generalized stacking fault energy (GSFE) curves for slip in the [100] and [120] directions parallel the 2D layers. (E) GSFE curves for slip along the [001] direction in two different planes perpendicular to the 2D layers.

Fig. 2.. Overview of InSe DP potential training process.

(A to D) Illustration of the training dataset components: (A) automated data generation from DP-GEN software, (B) deformation configurations in small-scale models, (C) cleavage energy data, and (D) GSFE data. (E and F) Schematic diagrams of the neural network and potential energy surface, respectively. (G to J) Various defects involved in deformation.

Fig. 3.. 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. The sample was unloaded after compression along the [001] direction. (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] direction and the tetragonal phase with [110] direction.

Fig. 4.. Crystallographic relationships during martensitic transformation from hexagonal to tetragonal phases.

(A to C) relate to [120]_H//[110]_T relationship, and (D to F) relate to [100]_H//[100]_T relationship. [(A) and (D)] Comparison of the predicted habit planes (red lines) from the martensitic theory with the actual phase transformation interface, revealing a precise alignment. [(B) and (E)] Schematic diagrams illustrating the coherency axis and specific OR for each OR. [(C) and (F)] MEP curves of the ORs, obtained with the DP, with insets corresponding to the atomic structures of the states marked by red dots on the MEP curve. f.u., formula unit.

Fig. 5.. 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.