1H/1T′ phase transition in CVD-grown monolayer MoTe₂ observed by TEM

March 21, 2025 – A team from the Center for Nanotechnology Innovation @NEST (Italy), Ulm University (Germany), the MAX IV Laboratory (Sweden), the University of Trento (Italy), and Sorbonne Université (France) is using graphene encapsulation and controlled thermal annealing in ultra-high vacuum to achieve a switch between the semiconducting 1H phase and the semimetallic 1T′ phase in MoTe₂. The research opens new pathways for phase-change electronics and quantum devices based on 2D materials.

Molybdenum ditelluride (MoTe₂) has attracted considerable attention among transition metal dichalcogenides due to its ability to exist in two stable configurations with only a small energy difference between the semiconducting 1H and the semimetallic 1T′ phases¹. The 1H phase is known for its 1.1 eV band gap² and strong spin–orbit coupling, while the monoclinic 1T′ phase exhibits superconductivity in the monolayer limit³ and has been predicted to be a quantum spin Hall insulator⁴. However, controlling the transition between these phases has proven challenging due to the low stability of monolayer MoTe₂⁵. Decomposition products, including Te-rich or Mo-rich phases, can further hinder phase transitions^5,8. Researchers have reported heterophase junctions and mixed 2H/1T′ phases, achieved by tuning chemical vapor deposition (CVD) growth conditions^6–8 as well as by employing molecular beam epitaxy⁹. Previous studies¹⁰ showed that hetero-contacts formed during CVD can trigger a 1H→1T′ phase transition in monolayer MoTe₂, while local 1T′→1H conversion can be induced by electron beam irradiation under particular conditions^11,12. Beam-induced tellurium vacancy lines, which generate 1H–1T′ interfaces, have also been found to stabilize the 1T′ phase and restrict reversion to the 1H configuration¹². As a result, scientists emphasize that preventing defect formation—such as through encapsulation—is crucial for achieving a reversible 1H/1T′ phase transition.

A major breakthrough in reversible 2H↔1T′ polymorphism in few-layer MoTe₂ was recently achieved by combining laser irradiation and ultra-high vacuum (UHV) annealing under protective encapsulation¹³. Building on this, the authors demonstrated that monolayer MoTe₂ can be encapsulated with CVD-grown graphene and then subjected to thermal annealing in UHV to induce controlled phase transitions while avoiding oxidation. The findings reveal that heating the material to approximately 1090 °C results in a transformation from the 1H to the 1T′ phase, while subsequent annealing at about 900 °C triggers the reverse 1T′→1H transition, resulting in a reversible phase change. The extreme phase transition temperatures observed here—which are significantly higher than those previously reported for few-layer MoTe₂^13,14—highlight the stabilizing effect of graphene encapsulation. To provide further insight into the critical temperatures and phase stability, the researchers also conducted density functional theory simulations.

Through a combination of Raman spectroscopy and transmission electron microscopy (TEM), the research team uncovered detailed structural and chemical changes in MoTe₂ during annealing. After synthesis and encapsulation, the material initially displayed the characteristic Raman modes of the 1H phase (E_2g at 235 cm⁻¹, A_1g at 170 cm⁻¹)¹⁵. As temperature increased, the 1T′ phase began to form at the edges of the flake, but only became fully stabilized above a critical threshold (T_c2) near 1075–1090 °C, marked by the disappearance of the 1H Raman signature¹⁴. Lowering the temperature (to T_c1 ≈ 880–900 °C) led to a reversion from 1T′ back to 1H, enabling a clear, two-way phase transition cycle. High-resolution TEM images revealed that the 1T′ phase can be obtained without introducing significant defects, while the reverted 1H regions contain line defects, such as mirror twin boundaries^12,18. The graphene encapsulation appeared to suppress the formation of tellurium vacancy line defects that otherwise would trap the material in the 1T′ state. (Figure 1)

To further validate phase purity and domain structure, the authors performed µLEEM/LEED and μXPS analysis. The resulting diffraction patterns and XPS spectra highlight the polycrystalline nature of the 1T′ phase and corresponding chemical changes (Figure 2).

By analyzing spatially resolved Raman maps, the scientists quantified the kinetics of the 1T′→1H transformation, observing that the process is remarkably slow at the elevated temperatures required. The measured conversion rate is on the order of 10⁻⁴ s per unit cell (at ~800 °C), corresponding to phase boundary motion of only a few micrometers over 15 minutes. Arrhenius analysis of the temperature-dependent transformation rates revealed an activation energy of approximately 2.0 eV for the 1T′→1H transition. This value exceeds several theoretical predictions (~1.66 eV)^16,21 but aligns with those reported for transitions in bulk-layer materials⁸. The high activation barrier helps explain why such high temperatures are necessary to achieve reversion in these experiments (Figure 3).

The research team also optimized phase quality by adjusting graphene encapsulation and annealing protocols. Optical imaging and spatially resolved Raman mapping demonstrated how the type and thickness of graphene encapsulation influence phase distribution, purity, and defect density within the MoTe₂ layer (Figure 4).

Theoretical modeling played a crucial role in reconciling these experimental observations. Calculations using a fixed-lattice (constant cell) model explained the unusually high 1H→1T′ transition temperatures observed (~1300–1350 K) by incorporating the effects of thermal expansion, leading to approximately 1.1% biaxial strain at 1363 K²⁴. Within this scenario, the 1T′ phase becomes thermodynamically favorable at high temperature, consistent with experimental results. A simple fixed-cell model, however, also predicts that the 1T′ phase remains stable during cooling, contradicting the observed reversion. In contrast, variable-cell (fully relaxed) models underestimate the transition temperature¹⁷ and cannot alone account for the reversibility. The scientists propose a hybrid mechanism: strong interaction between 1H-MoTe₂ and the graphene substrate enforces a fixed lattice during heating, raising the 1H→1T′ transition temperature, while a weaker interaction for 1T′-MoTe₂ permits lattice relaxation and enables the reversion. This view is supported by the rare observation of intact 1T′ domains after sample transfer to TEM grids, suggesting that the 1T′ phase is only metastable when not clamped by the substrate^10,25. Calculated phase diagrams and theoretical results are provided in (Figure 5).

The study demonstrates a reversible semimetal–semiconductor phase transition in single-layer MoTe₂ enabled by thermal cycling under graphene encapsulation. By leveraging CVD-grown MoTe₂ and graphene, the approach ensures large-area uniformity and scalability—traits essential for future device applications¹¹. The graphene/MoTe₂ heterostructure offers a robust platform for exploring phase-change physics and may open new avenues for electronics, spintronics, and quantum devices^26–28. While theoretical work has long predicted promising applications for such phase-engineered two-dimensional systems, experimental efforts are now focused on fabricating devices that can harness the reversible 1H/1T′ transition and evaluating their performance and stability under real-world operating conditions. Prior studies have already shown that few-layer MoTe₂ holds promise in near-infrared photodetectors and spintronic applications^29,30. These latest findings extend such prospects to the monolayer regime and highlight the power of encapsulation and precise annealing to reversibly tailor material properties at the atomic scale.

Resource: Khaustov, V. O. et al. (2025). Reversible semimetal–semiconductor phase transition in CVD-grown monolayer MoTe₂. 2D Materials, 12(2), 025025.