Single Crystal Growth And Structural Evolution Under High Pressure Of Two Dimensional Layered Materials β-GeSe₂ And GeSe

Recently,two-dimensional（2D）layered materials have become a hot research topic because of their excellent performance and application potential in the fields of photoelectric detection,electronic devices,and electrocatalysts.Especially,how to prepare new high-quality 2D single crystal materials by low-cost and high-efficiency methods is a key concern in this field.In addition,breaking through the inherent properties of materials and seeking new effects of materials under external field regulation are the prerequisites for further expanding the application fields of 2D layered materials.The properties of materials are often determined by their structures.Under the external field regulation,the behavior of material structure changes directly determines the change of its properties.Therefore,clarifying how the material structure changes under the external field is the key to explore new properties of 2D layered materials.As representatives of new 2D Ge-based semiconductor materials,β-GeSe₂ and GeSe are widely used in the fields of photoelectric detection,polarized light detection,and infrared waveguides due to their unique anisotropic crystal structures and optoelectronic properties.However,their high pressure structural evolution is still subject to some controversies.Here,we have successfully prepared high-qualityβ-GeSe₂ and GeSe single crystals using chemical vapor deposition and systematically investigated their structural evolution under high pressure,respectively.In the study onβ-GeSe₂,high-precision high-pressure Raman spectroscopy and synchrotron X-ray diffraction experiments show thatβ-GeSe₂ has no structural phase transition in the pressure range from atmospheric pressure to 13.80 GPa;Some of the chemical bonds break near 6.91 GPa and the material shows disordered behavior.Its structure forms an irreversible amorphous state at 13.80 GPa.Furthermore,the bandgap ofβ-GeSe₂ decreases linearly from 2.59 e V to 1.65 e V as the pressure increases from atmospheric pressure to 12.16 GPa,with a tunability of 36.5%,and the color of the sample exhibits piezochromic phenomenon with increasing pressure.The bandgap ofβ-GeSe₂ shows an irreversible decrease after complete unloading,which is attributed to the incomplete recrystallization of the crystal structure of the material during the unloading process.In the study on GeSe,we have revealed the thermal stability and strong interlayer interactions of bulk GeSe single crystals using temperature dependent Raman scattering experiments.We also found phonon anomalous softening behavior in the high pressure Raman spectra of GeSe,which is consistent with the phonon softening behavior predicted in the theoretical calculation article associated with structural phase transition,indicating that the structural phase transition of GeSe at high pressure is driven purely by lattice distortion.