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Talc is an insulating layered material that is stable at ambient conditions and has high-quality basal cleavage, which is a major advantage for its use in van der Waals heterostructures. Here, we use near-field synchrotron infrared nanospectroscopy, Raman spectroscopy, and first-principles calculations to investigate the structural and vibrational properties of talc crystals, ranging from monolayer to bulk, in the 300–750 and <60 cm–1 spectral windows. We observe a symmetry crossover from mono to bilayer talc samples, attributed to the stacking of adjacent layers. The in-plane lattice parameters and frequencies of intralayer modes of talc display weak dependence on the number of layers, consistent with a weak interlayer interaction. On the other hand, the low-frequency (<60 cm–1) rigid-layer (interlayer) modes of talc are suitable to identify the number of layers in ultrathin talc samples, besides revealing strong in-plane and out-of-plane anisotropy in the interlayer force constants and related elastic stiffnesses of single crystals. The shear and breathing force constants of talc are found to be 66 and 28%, respectively, lower than those of graphite, making talc an excellent lubricant that can be easily exfoliated. Our results broaden the understanding of the structural and vibrational properties of talc at the nanoscale regime and serve as a guide for future ultrathin heterostructures applications.

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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 β 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 β 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.

We investigate the electronic structure and lattice stability of pristine and functionalized (with either hydrogen or oxygen) α-graphyne systems. We identify lattice instabilities due to soft-phonon modes, and describe two mechanisms leading to gap opening in the Dirac-fermion electronic spectrum of these systems: symmetry breaking, connected with the lattice instabilities, and partial incorporation of an sp 3 -hybrid character in the covalent-bonding network of a buckled hydrogenated α-graphyne lattice that retains the symmetries of the parent pristine α-graphyne. In the case of an oxygen-functionalized α-graphyne structure, each O atom binds asymmetrically to two twofoldcoordinated C atoms, breaking inversion and mirror symmetries, and leading to the opening of a sizeable gap of 0.22 eV at the Dirac point. Generally, mirror symmetries are found to suffice for the occurrence of gapless Dirac cones in these α-graphyne systems, even in the absence of inversion symmetry centers. Moreover, we analyze the gapless and gapped Dirac cones of pristine and functionalized α-graphynes from the perspective of the dispersion relations for massless and massive free Dirac fermions. We find that mirror-symmetry breaking mimics a Dirac-fermion mass-generation mechanism in the oxygen-functionalized α-graphyne, leading to gap opening and to isotropic electronic dispersions with a rather small electron-hole asymmetry. In the hydrogen-functionalized case, we find that carriers show a remarkable anisotropy, behaving as massless fermions along the M-K line in the Brillouin zone and as massive fermions along the Γ-K line.PACS numbers:

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Natural oxidation is a common degradation mechanism of both mechanical and electronic properties for most of the new two-dimensional materials. From another perspective, controlled oxidation is an option to tune material properties, expanding possibilities for real-world applications. Understanding the electronic structure modifications induced by oxidation is highly desirable for new materials like monolayer GeSe, which is a new candidate for near-infrared photodetectors. By means of first-principles calculations, we study the influence of oxygen defects on the structure and electronic properties of the single layer GeSe. Our calculations show that the oxidation is an exothermic process, and it is nucleated in the germanium sites. The oxidation can cause severe local deformations on the monolayer GeSe structure and introduces a deep state in the bandgap or a shallow state near the conduction band edge. Furthermore, the oxidation increases the bandgap by up to 23%, and may induce direct to indirect bandgap transitions. These results suggest that the natural or intentionally induced monolayer GeSe oxidation can be a source of new optoelectronic properties, adding another important building block to the two-dimensional layered materials.

Jacutingaite (Pt2HgSe3) is a recently discovered layered platinum‐group mineral. Recent experimental studies have shown that it displays the properties of a quantum spin Hall insulator (QSHI), and theoretical studies indicate that its two‐dimensional monolayer is a QSHI with a robust topological gap of ∼0.5 eV. Jacutingaite is thus promising for potential applications to nanoelectronics and spintronics. The Raman spectrum of three‐dimensional bulk jacutingaite and the symmetries of its vibrational modes, fundamental for understanding structural modifications of this material, are still unexplored. Here, we address the zone‐center Raman optical phonons of bulk jacutingaite by experiments, symmetry, and first‐principles calculations. The improved synthesis used here provided crystals of higher purity and of micrometer size, allowing the study of single crystals. Polarized Raman spectroscopy was used to assign the symmetries of nine out of the 11 Raman‐active modes expected by group theory and their respective selection rules. The calculated wavenumbers of the Raman‐active modes, in addition to their atomic displacements, are in very good agreement with experiments. In addition, we discuss the use of different exchange correlation functionals within density functional theory, as local functionals and nonlocal functionals that best describe van der Waals interactions. The influence of the inclusion of spin–orbit coupling on calculated vibrational phonon wavenumbers and lattice parameters is commented, and it was found that the local density approximation provides a good description. Our results are of paramount importance to further exploitation of the effects of jacutingaite's structural modifications to tune its properties, as well as for its structural, optical, electronic, mechanical, and thermal applications.