A research team led by Prof. CHEN Xingqiu and Prof. SUN Yan at the Materials Artificial Intelligence Division, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS), in collaboration with Prof. NIU Haiyang from Northwestern Polytechnical University, has uncovered a previously unknown one-dimensional "domino-like" phase transformation mechanism in monolayer molybdenum telluride (MoTe₂). The findings, published in Proceedings of the National Academy of Sciences (PNAS), reveal a phase-transformation pathway fundamentally distinct from the conventional martensitic picture and open new avenues for programmable electronic and photonic devices.
Phase transformations are ubiquitous in nature, and understanding their microscopic mechanisms is essential for controlling material properties and designing functional devices. The emergence of two-dimensional materials has reinvigorated phase-transformation research, as reduced dimensionality gives rise to physical behaviors absent in bulk counterparts. In monolayer transition metal dichalcogenides, the phase transformation between the semiconducting 1H phase and the semimetallic 1T' phase has long been understood as a martensitic process, in which atoms undergo concerted shear displacements. However, the high energy barriers predicted by this model appear inconsistent with experimental observations that such transformations can occur under experimentally accessible conditions, leaving the underlying kinetics and microscopic mechanism a subject of longstanding debate.
To resolve this puzzle, the team employed deep learning potential-accelerated molecular dynamics simulations to systematically investigate the 1H-to-1T' phase transformation in monolayer MoTe₂. Contrary to the conventional martensitic picture, they found that the transformation proceeds through a one-dimensional "domino-like" chain reaction: tellurium atoms sequentially hop along a specific crystallographic direction, triggering structural rearrangement accompanied by Peierls distortion and local topological changes. This pathway exhibits a substantially lower energy barrier than the martensitic shear route and gives rise to a free-energy landscape featuring multiple metastable states, distinct from the classical nucleation-and-growth scenario.
The team further elucidated the kinetic origins of single-domain and multi-domain 1T' morphologies observed in simulations with different cell sizes, and proposed controllable phase-transformation strategies based on these kinetic characteristics. Theoretical calculations demonstrate that reversible switching between single-domain and multi-domain configurations enables rapid modulation of electronic states. Moreover, the phase-transformation intermediates accessible through this mechanism exhibit significantly enhanced second-order nonlinear optical responses, with shift current responses in the visible range increasing from approximately 70 μA/V² to about 470 μA/V².
This work not only deepens the understanding of phase-transformation mechanisms in two-dimensional materials but also provides a new paradigm for phase engineering in low-dimensional systems, with promising implications for programmable electronics and optoelectronic devices.
Comparison of conventional martensitic and domino-like phase transformations. (Image by IMR)
Atomic-scale mechanism of the 1H-to-single-domain 1T′ phase transformation in monolayer MoTe₂. (Image by IMR)
Microscopic mechanism for the formation of the multi-domain 1T′ phase during the 1H-to-1T′ transformation in monolayer MoTe₂. (Image by IMR)
Design of materials with large second-order nonlinear optical responses guided by phase-transformation mechanisms. (Image by IMR)
