Atomic-Scale Edge Morphology, Stability, and Oxidation of Single-Layer 2H-TaS2

Computational Methods for Charge Density Waves in 2D Materials

Two-dimensional (2D) materials that exhibit charge density waves (CDWs)—spontaneous reorganization of their electrons into a periodic modulation—have generated many research endeavors in the hopes of employing their exotic properties for various quantum-based technologies. Early investigations surrounding CDWs were mostly focused on bulk materials. However, applications for quantum devices require few-layer materials to fully utilize the emergent phenomena. The CDW field has greatly expanded over the decades, warranting a focus on the computational efforts surrounding them specifically in 2D materials. In this review, we cover ground in the following relevant theory-driven subtopics for TaS₂ and TaSe₂: summary of general computational techniques and methods, resulting atomic structures, the effect of electron–phonon interaction of the Raman scattering modes, the effects of confinement and dimensionality on the CDW, and we end with a future outlook. Through understanding how the computational methods have enabled incredible advancements in quantum materials, one may anticipate the ever-expanding directions available for continued pursuit as the field brings us through the 21st century.

Exploring the growth and oxidation of 2D-TaS2on Cu(111)

In this work, the growth and stability towards O₂exposure of two dimensional (2D) TaS₂on a Cu(111) substrate is investigated. Large area (∼1 cm²) crystalline 2D-TaS₂films with a metallic character are prepared on a single crystal Cu(111) substrate via a multistep approach based on physical vapor deposition. Analytical techniques such as Auger electron spectroscopy, low energy electron diffraction, and photoemission spectroscopy are used to characterize the composition, crystallinity, and electronic structure of the surface. At coverages below one monolayer equivalent (ML), misoriented TaS₂domains are formed, which are rotated up to±13orelative to the Cu(111) crystallographic directions. The TaS₂domains misorientation decreases as the film thickness approaches 1 ML, at which the crystallographic directions of TaS₂and Cu(111) are aligned. The TaS₂film is found to grow epitaxially on Cu(111). As revealed by low energy electron diffraction in conjunction with an atomic model simulation, the (3 × 3) unit cells of TaS₂match the (4 × 4) supercell of Cu(111). Furthermore, the exposure of TaS₂to O₂, does not lead to the formation of a robust tantalum oxide film, only minor amounts of stable oxides being detected on the surface. Instead, the exposure of TaS₂films to O₂leads predominantly to a reduction of the film thickness, evidenced by a decrease in the content of both Ta and S atoms of the film. This is attributed to the formation of oxide species that are unstable and mainly desorb from the surface below room temperature. Temperature programmed desorption spectroscopy confirms the formation of SO₂, which desorbs from the surface between 100 and500 K.These results provide new insights into the oxidative degradation of 2D-TaS₂on Cu(111).