First-principles study on structural, thermal, mechanical and dynamic stability of T'-MoS2

Interlayer Coupling Dependent Discrete H → T' Phase Transition in Lithium Intercalated Bilayer Molybdenum Disulfide

In this work, the interlayer coupling dependent lithium intercalation induced phase transition in bilayer MoS₂ (BL-MoS₂) was investigated using an atomic-resolution annual dark-field scanning transmission electron microscope (ADF-STEM). It was revealed that the lithiation induced H → T' phase transition in BL-MoS₂ strongly depended on the interlayer twist angle; i.e., the H → T' phase transition occurred in well-stacked H phase BL-MoS₂ (with a twist angle of θ_t = 0°) but not for θ_t ≠ 0° BL-MoS₂. The lithiated BL-MoS₂ appeared in homophase stacking, either T'/T' or H/H (locally, no phase transformation) stacking, without any heterophase stacking such as H/T' or T'/H observed. This finding indicated the H → T' phase transition occurred via a domain-by-domain mode rather than layer-by-layer. Up to 15 types of stacking orders were experimentally identified locally in lithiated bilayer T'-MoS₂, and the formation mechanism was attributed to the discrete interlayer translation with a unit step of (m/6a, n/6b) (m, n = 0, 1, 2, 3), where a and b were the primitive lattice vectors of T'-MoS₂. Our experimental results were further corroborated by ab initio density functional theory (DFT) calculations, where the occurrence of different stacking orders can be quantitatively correlated with the variation of intercalated lithium contents into the BL-MoS₂. The present study aids in the understanding of the phase transition mechanisms in atomically thin 2D transition metal dichalcogenides (TMDCs) and will also shed light on the precisely controlled phase engineering of 2D materials for memory applications.

Two-dimensional MoS2 2H, 1T, and 1T' crystalline phases with incorporated adatoms: theoretical investigation of electronic and optical properties

Although there has been progress in studying the electronic and optical properties of monolayer and near-monolayer (two-dimensional, 2D) MoS₂ upon adatom adsorption and intercalation, understanding the underlying atomic-level behavior is lacking, particularly as related to the optical response. Alkali atom intercalation in 2D transition metal dichalcogenides (TMDs) is relevant to chemical exfoliation methods that are expected to enable large scale production. In this work, focusing on prototypical 2D MoS₂, the adsorption and intercalation of Li, Na, K, and Ca adatoms were investigated for the 2H, 1T, and 1T^' phases of the TMD by the first principles density functional theory in comparison to experimental characterization of 2H and 1T 2D MoS₂ films. Our electronic structure calculations demonstrate significant charge transfer, influencing work function reductions of 1-1.5 eV. Furthermore, electrical conductivity calculations confirm the semiconducting versus metallic behavior. Calculations of the optical spectra, including excitonic effects using a many-body theoretical approach, indicate enhancement of the optical transmission upon phase change. Encouragingly, this is corroborated, in part, by the experimental measurements for the 2H and 1T phases having semiconducting and metallic behavior, respectively, thus motivating further experimental exploration. Overall, our calculations emphasize the potential impact of synthesis-relevant adatom incorporation in 2D MoS₂ on the electronic and optical responses that comprise important considerations toward the development of devices such as photodetectors or the miniaturization of electroabsorption modulator components.

Ultrathin 1T-MoS2 Nanoplates Induced by Quaternary Ammonium-Type Ionic Liquids on Polypyrrole/Graphene Oxide Nanosheets and Its Irreversible Crystal Phase Transition During Electrocatalytic Nitrogen Reduction

Ultrathin nanoplates of metastable 1T-MoS₂ have been successfully stabilized and uniformly distributed on the surface of n-butyl triethyl ammonium bromide functionalized polypyrrole/graphene oxide (BTAB/PPy/GO) by a very simple hydrothermal method. BTAB as a typical kind of quaternary ammonium-type ionic liquids (ILs) played a crucial role in the formation of the obtained 1T-MoS₂/BTAB/PPy/GO. It was covalently linked with PPy/GO and arranged in a highly ordered order at the solid-liquid interface of PPy/GO and H₂O due to Coulombic interactions and other intermolecular interactions, which would induce and stabilize ultrathin 1T-MoS₂ nanoplates by morphosynthesis. The good electrocatalytic activity toward nitrogen reduction reaction (NRR) with strong durability and good stability can be achieved by 1T-MoS₂/BTAB/PPy/GO due to their excellent inorganic/organic hierarchical lamellar micro-/nanostructures. Especially, after the long-term electrocatalysis for NRR at a negative potential, metastable 1T-MoS₂ as the catalytic center undergoes two types of irreversible crystal phase transition, which was converted to 1T'-MoS₂ and Mo₂N, caused by the competitive hydrogen evolution reaction (HER) process and the electrochemical reaction between the electroactive 1T-MoS₂ and N₂, respectively. The new N-Mo bonding prevents Mo atoms from binding to other N atoms in N₂, resulting in the deactivation of the electrocatalysts to NRR after being used for 18 h. Even so, quaternary ammonium-type ILs would induce the crystal structures of transition-metal dichalcogenides (TMDCs), which might provide a new thought for the reasonable design of electrocatalysts based on TMDCs for electrocatalysis.

Two-Dimensional Tellurium: Progress, Challenges, and Prospects

Since the successful fabrication of two-dimensional (2D) tellurium (Te) in 2017, its fascinating properties including a thickness dependence bandgap, environmental stability, piezoelectric effect, high carrier mobility, and photoresponse among others show great potential for various applications. These include photodetectors, field-effect transistors, piezoelectric devices, modulators, and energy harvesting devices. However, as a new member of the 2D material family, much less known is about 2D Te compared to other 2D materials. Motivated by this lack of knowledge, we review the recent progress of research into 2D Te nanoflakes. Firstly, we introduce the background and motivation of this review. Then, the crystal structures and synthesis methods are presented, followed by an introduction to their physical properties and applications. Finally, the challenges and further development directions are summarized. We believe that milestone investigations of 2D Te nanoflakes will emerge soon, which will bring about great industrial revelations in 2D materials-based nanodevice commercialization.

Effect of Structural Phases on Mechanical Properties of Molybdenum Disulfide

Molybdenum disulfide (MoS₂) is a promising layer-structured material for use in many applications due to its tunable structural and electronic properties in terms of its structural phases. Its performance including efficiency and durability is often dependent on its mechanical properties. To understand the effects of the structural phase on its mechanical properties, a comparative study on the mechanical properties of bulk 2H, 3R, 1T, and 1T' MoS₂ was conducted using the first-principles density functional theory. Since considerable applications of MoS₂ are developed through strain engineering, the impact of the external pressure on its mechanical properties was also considered. Our results suggest a strong relationship between the mechanical properties of MoS₂ and the structural symmetry of its crystal. Accordingly, the impacts of the external pressure on the mechanical properties of MoS₂ also greatly vary with respect to the structural phases. Among all of the considered phases, the 2H and 3R MoS₂ have a larger bulk modulus, Young's modulus, shear modulus, and microhardness due to their higher stability. Conversely, 1T and 1T' MoS₂ are less strong. As such, 1T and 1T' MoS₂ can be a better candidate for strain engineering.