RESUMO
Gallium sulfide (GaS) stands out as a versatile nonlinear optical material for green-blue optoelectronic and photocatalytic nano-devices. In addition, the in-plane breaking strain and mechanical strength of layered GaS make it a promising candidate for next-generation flexible nanodevices. The fast and reliable assessment of the number of layers, without sample loss, is key for these applications. Here we unveil the influence of dimensionality in the structural, mechanical, and vibrational properties of GaS by applying density-functional theory-based quantum-simulations and group-theory analysis. We find its intralayer structure and interlayer distances are essentially independent of the number of layers, in agreement with the van der Waals forces as dominant interlayer interactions. The translational symmetry breaking along the stacking direction results in different structural symmetries for monolayers, N-odd layers, N-even layers, and bulk geometries. Its force constants against rigid-layer shear, KLSM = 1.35 × 1019 N m-3, and breathing, KLBM = 5.00 × 1019 N m-3, displacements remain the same from bulk to bilayer structures. The related stiffness coefficients in bulk are C44 = 10.2 GPa and C33 = 37.7 GPa, respectively. This insight into GaS interlayer interactions and elastic coefficients reveals it as a promising lubricant for nano-mechanic applications and it is easy to cleave for thickness engineering, even in comparison with layered graphite, MoS2 and other transition metal dichalcogenides and group-IIIA metal monochalcogenides. We present the GaS Raman and infrared spectra dependence on the layer number as strategies for sample thickness characterization and derive formulas for distinguishing the number of layers in both high and low-frequency regimes. In addition, our analysis of their optical-activity selection rules and polarization dependencies is applicable to isostructural group-IIIA metal monochalcogenides with 2H-layer stacking, such as gallium/indium sulphide/selenide. These results contribute to rapid and non-destructive characterization of the material's structure, which is of paramount importance for the manufacturing of devices and the utilization of its diverse properties.
RESUMO
The layered mineral tilkerodeite (Pd2HgSe3), the palladium analogue of jacutingaite (Pt2HgSe3), is a promising quantum spin hall insulator for low-power nanospintronics. In this context, a fast and reliable assessment of its structure is key for exploring fundamental properties and architecture of new Pd2HgSe3-based devices. Here, we investigate the first-order Raman spectrum in high-quality, single-crystal bulk tilkerodeite, and analyze the wavenumber relation to its isostructural jacutingaite analogue. By using polarized Raman spectroscopy, symmetry analysis, and first-principles calculations, we assigned all the Raman-active phonons in tilkerodeite, unveiling their wavenumbers, atomic displacement patterns, and symmetries. Our calculations used several exchange-correlation functionals within the density functional perturbation theory framework, reproducing both structure and Raman-active phonon wavenumbers in excellent agreement with experiments. Also, it was found that the influence of the spin-orbit coupling can be neglected in the study of these properties. Finally, we compared the wavenumber and atomic displacement patterns of corresponding Raman-active modes in tilkerodeite and jacutingaite, and found that the effect of the Pd and Pt masses can be neglected on reasoning their wavenumber differences. From this analysis, tilkerodeite is found to be mechanically weaker than jacutingaite against the atomic displacement patterns of these modes. Our findings advance the understanding of the structural properties of a recently discovered layered topological insulator, fundamental to further exploring its electronic, optical, thermal, and mechanical properties, and for device fabrication.
RESUMO
Beyond-graphene two-dimensional (2D) materials are envisioned as the future technology for optoelectronics, and the study of group IIIA metal monochalcogenides (GIIIAMMs) in 2D form is an emerging research field. Bulk gallium selenide (GaSe) is a layered material of this family which is widely used in nonlinear optics and is promising as a lubricant. The interlayer coupling in few-layer GaSe is currently unknown, and the stability of different polytypes is unclear. Here we use symmetry arguments and first-principles calculations to investigate the phase stability, interlayer coupling, and the Raman and infrared activity of the low-frequency shear and breathing modes expected in few-layer GaSe. Strategies to distinguish the number of layers and the ß and ε polytypes are discussed. These symmetry results are valid for other isostructural few-layer GIIIAMM materials. Most importantly, by using a linear chain model, we show that the shear and breathing force constants reveal an ultra-weak interlayer coupling at the nanoscale in GaSe. These results suggest that ß and ε few-layer GaSe show similar lubricant properties to those observed for few-layer graphite. Our analysis opens new perspectives about the study of interlayer interactions and their role in the mechanical and electrical properties of these new 2D materials.