Chinese researchers at East China Normal University (ECNU) have made a groundbreaking observation in the field of two-dimensional (2D) materials science by capturing the atomistic details of phase transitions in α-In₂Se₃, a promising ferroelectric semiconductor. This van der Waals (vdW) layered material retains its ferroelectric properties even at the monolayer level, positioning it as a frontrunner for next-generation non-volatile memory and in-memory computing devices. The study reveals how thermal stimuli or electron beam irradiation trigger intricate atomic rearrangements, offering vital insights into managing phase stability in practical applications.
Understanding α-In₂Se₃: A Ferroelectric Powerhouse
Indium selenide (In₂Se₃), particularly its α-phase, belongs to a family of vdW materials known for their layered structure, where weak interlayer forces allow easy exfoliation into ultrathin flakes. The α-phase, often denoted as 2H-α In₂Se₃ due to its hexagonal stacking, exhibits stable out-of-plane (OOP) and in-plane (IP) ferroelectric polarization. This dual polarization capability is rare among 2D materials and stems from the asymmetric arrangement of In and Se atoms within its In-Se octahedral frameworks.
Ferroelectricity in semiconductors like α-In₂Se₃ enables spontaneous electric polarization that can be reversed by an external field, mimicking biological synapses and enabling multi-state memory logic. Traditional ferroelectrics like lead zirconate titanate (PZT) suffer from scalability issues below 10 nm, but 2D variants like α-In₂Se₃ promise atomic-scale precision. Its narrow bandgap (~1.2-1.4 eV) also supports optoelectronic functions, such as photodetection and light-emitting diodes (LEDs).
The Phase Transition Challenge in Device Fabrication
Phase transitions in In₂Se₃ pose significant hurdles for commercialization. The material exhibits polymorphism—α, β, β′, and γ phases—with low energy barriers (~0.1-0.5 eV) between them. During device processing, elevated temperatures (e.g., >200°C annealing) or operational stresses can induce unwanted shifts from ferroelectric α to paraelectric β phases, degrading performance. Understanding these dynamics at the atomic scale is essential to engineer stable phases.
Prior studies relied on bulk techniques like X-ray diffraction (XRD) or Raman spectroscopy, averaging over ensembles and missing local heterogeneities. Real-time atomic visualization was needed to track ion migrations and vacancy movements.
ECNU's Innovative Approach: In-Situ STEM Observations
Led by Prof. Junhao Chu and collaborators at ECNU's Key Laboratory of Polar Materials and Devices, the team employed aberration-corrected scanning transmission electron microscopy (STEM) for in-situ imaging. Samples of few-layer 2H-α In₂Se₃ were prepared via mechanical exfoliation on TEM grids. Under controlled electron beam (E-B) doses (simulating thermal effects via beam heating) or external heating stages, they monitored atomic positions with sub-angstrom resolution.
The setup captured frames at 0.5-2 frames/sec, revealing directional migrations. Funding from the National Natural Science Foundation of China (Nos. 62274061, 12134003) supported this high-resolution work.
Key Findings: Atomic Dance of Indium Ions and Vacancies
The phase transition initiates at In-Se octahedral sites. Indium ions (In³⁺) in octahedrons migrate laterally within layers, while vacancies in vdW gaps diffuse interlayer. This dual mechanism—intralayer In shuffling and interlayer vacancy hopping—drives the transformation.
- Step 1: Octahedral distortion under stimulus, In atoms shift ~0.5 Å in-plane.
- Step 2: One octahedron splits into two tetrahedral In-Se units, reducing coordination from 6 to 4.
- Step 3: Tetrahedra coalesce across layers, forming a novel 6H-type non-layered phase akin to SiC polytypes.
In the final 6H phase, cation sublattices host 2/3 In and 1/3 vacancies, stabilizing via reduced symmetry. Videos from supplementary materials show this ~10-30 sec process.
Read the full study for detailed atomic trajectories: Atomistic phase transition dynamics of In₂Se₃ semiconductor.
Photo by Jorick Jing on Unsplash
Implications for Next-Gen Electronics
This revelation guides phase engineering. By doping or straining to raise barriers, α-phase stability can extend to 300°C cycles, vital for CMOS integration. Applications include:
- FeRAM (ferroelectric RAM) with >10¹² cycles endurance.
- Neuromorphic synapses for AI accelerators.
- Photodetectors with polarization-tuned response.
In China, where semiconductor self-reliance is prioritized, such 2D ferroelectrics complement Si-based tech amid US export curbs.
ECNU's Leadership in 2D Materials Research
East China Normal University, a top-tier institution in Shanghai, ranks highly in materials science (QS 2026: top 100 globally). Its polar materials lab, under MOE, pioneers vdW ferroelectrics. Recent ECNU works include flexoelectric gating in α-In₂Se₃ and heterostructures with MoS₂ for enhanced photocarrier separation. This study builds on their expertise, contributing to China's 14th Five-Year Plan for advanced chips.
ECNU's interdisciplinary approach—merging electronics, physics, and brain-inspired devices—positions it as a hub for Shanghai's tech ecosystem. For aspiring researchers, ECNU offers robust PhD programs in condensed matter physics; explore opportunities via research positions.
China's Surge in 2D Ferroelectric Innovations
China leads 2D ferroelectrics R&D, with over 40% global papers (Nature Index 2026). Tsinghua and Peking University report bilayer sliding ferroelectrics; CAS advances CuInP₂S₆ devices. In₂Se₃ fits China's semiconductor roadmap, targeting 2nm nodes by 2030. Stats: China's 2D material patents rose 25% YoY (2025 CNIPA data), fueling firms like Huawei's HiSilicon.
Challenges remain: scalability via CVD growth and interface engineering. Collaborative efforts, like National Key R&D Programs, accelerate translation.
Experimental Validation and Theoretical Corroboration
STEM data matched DFT simulations, confirming energy minima for tetrahedral intermediates (~0.2 eV barrier). Temperature-dependent transitions align with prior Raman peaks at 150-250°C. No reverse transition observed, suggesting kinetic trapping in 6H phase—key for metastable engineering.
| Phase | Structure | Bandgap (eV) | Ferroelectric? |
|---|---|---|---|
| 2H-α | Layered octahedral | 1.25 | Yes |
| 6H-new | Non-layered tetrahedral | ~1.5 (est.) | No |
| β | Paraelectric | 1.1 | No |
Future Directions and Global Impact
Next steps: Strain-doping to suppress transitions; heterostructures for hybrid phases. ECNU plans device prototypes. Globally, this advances Moore's Law via ferroelectric gates, reducing power 50x vs. FinFETs. For Chinese higher ed, it underscores university-industry ties, with ECNU partnering CAS institutes.
Prospective students: China's materials PhDs see 20% salary premium (QS 2026). Check China university jobs for openings.
For deeper reading on 2D ferroelectrics: Recent advances review.
Photo by Moughit Fawzi on Unsplash
Stakeholder Perspectives and Real-World Cases
Prof. Junhao Chu notes: "Real-time atomic imaging unlocks phase control, pivotal for reliable nanoelectronics." Industry echoes: SMIC eyes 2D integration for 5G chips. Case: Similar CuInP₂S₆ FeRAM prototypes hit 10⁶ cycles (Tsinghua, 2025).