Published in Nano Letters, a team of scientists from the Changchun lnstitute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences and Northeast Normal University reported a novel method to identify the crystal phases of layered gallium sulfide (GaS). Their work sheds light on the coexistence of multiple phases in this material, which could impact its applications in optoelectronics and quantum technologies. Layered GaS, a promising III-VI semiconductor, has attracted attention for its potential in photodetection and nonlinear optics. Theoretical studies predicted four stable stacking orders—β, γ, ε, and δ—with nearly identical formation energies. However, experimental confirmation of these phases remained challenging due to limitations in existing techniques. The team addressed this by developing a polarization-dependent angle-resolved second harmonic generation (SHG) method. Unlike traditional approaches, their technique distinguished out-of-plane tensor components without rotating samples, enabling precise phase identification.
The researchers exfoliated GaS flakes from bulk crystals and analyzed them using SHG under normal and oblique light incidence. They discovered that approximately one-sixth of the samples exhibited SHG patterns consistent with γ-GaS, while the majority were β-GaS. Cross-sectional STEM imaging and Raman spectroscopy validated their findings. Theoretical calculations further revealed that β- and γ-GaS undergo topological phase transitions at different strain levels, underscoring the importance of phase-specific characterization for device design.
This study marks the first experimental observation of γ-GaS and demonstrates the SHG method’s versatility for analyzing van der Waals materials. By eliminating the need for sample rotation, the technique simplifies phase detection in polycrystalline systems. The team’s findings provide a foundation for future research on GaS and highlight the role of stacking order in tuning material properties.
中文
