Predicting Dislocations and Grain Boundaries in Two-Dimensional Metal-Disulfides from the First Principles

Guided by the principles of dislocation theory, we use the first-principles calculations to determine the structure and properties of dislocations and grain boundaries (GB) in single-layer transition metal disulfides MS₂ (M = Mo or W). In sharp contrast to other two-dimensional materials (truly planar graphene and <i>h</i>-BN), here the edge dislocations extend in third dimension, forming concave dreidel-shaped polyhedra. They include different number of homoelemental bonds and, by reacting with vacancies, interstitials, and atom substitutions, yield families of the derivative cores for each Burgers vector. The overall structures of GB are controlled by both local-chemical and far-field mechanical energies and display different combinations of dislocation cores. Further, we find two distinct electronic behaviors of GB. Typically, their localized deep-level states act as sinks for carriers but at large 60°-tilt the GB become metallic. The analysis shows how the versatile GB in MS₂ (if carefully engineered) should enable new developments for electronic and opto-electronic applications

Intrinsic Magnetism of Grain Boundaries in Two-Dimensional Metal Dichalcogenides

Grain boundaries (GBs) are structural imperfections that typically degrade the performance of materials. Here we show that dislocations and GBs in two-dimensional (2D) metal dichalcogenides MX₂ (M = Mo, W; X = S, Se) can actually <i>improve</i> the material by giving it a qualitatively new physical property: magnetism. The dislocations studied all display a substantial magnetic moment of ∼1 Bohr magneton. In contrast, dislocations in other well-studied 2D materials are typically nonmagnetic. GBs composed of pentagon–heptagon pairs interact ferromagnetically and transition from semiconductor to half-metal or metal as a function of tilt angle and/or doping level. When the tilt angle exceeds 47°, the structural energetics favor square–octagon pairs and the GB becomes an antiferromagnetic semiconductor. These exceptional magnetic properties arise from interplay of dislocation-induced localized states, doping, and locally unbalanced stoichiometry. Purposeful engineering of topological GBs may be able to convert MX₂ into a promising 2D magnetic semiconductor

Half-Metallicity in Co-Doped WSe₂ Nanoribbons

The recent development of two-dimensional transition-metal dichalcogenides in electronics and optoelelectronics has triggered the exploration in spintronics, with high demand in search for half-metallicity in these systems. Here, through density functional theory (DFT) calculations, we predict robust half-metallic behaviors in Co-edge-doped WSe₂ nanoribbons (NRs). With electrons partially occupying the antibonding state consisting of Co 3d_yz and Se 4p_z orbitals, the system becomes spin-polarized due to the defect-state-induced Stoner effect and the strong exchange splitting eventually gives rise to the half-metallicity. The half-metal gap reaches 0.15 eV on the DFT generalized gradient approximation level and increases significantly to 0.67 eV using hybrid functional. Furthermore, we find that the half-metallicity sustains even under large external strain and relatively low edge doping concentration, which promises the potential of such Co-edge-doped WSe₂ NRs in spintronics applications

Environment-Controlled Dislocation Migration and Superplasticity in Monolayer MoS₂

The two-dimensional (2D) transition metal dichalcogenides (TMDC, of generic formula MX₂) monolayer displays the “triple-decker” structure with the chemical bond organization much more complex than in well-studied monatomic layers of graphene or boron nitride. Accordingly, the makeup of the dislocations in TMDC permits chemical variability, depending sensitively on the equilibrium with the environment. In particular, first-principles calculations show that dislocations state can be switched to highly mobile, profoundly changing the lattice relaxation and leading to superplastic behavior. With 2D MoS₂ as an example, we construct full map for dislocation dynamics, at different chemical potentials, for both the M- and X-oriented dislocations. Depending on the structure of the migrating dislocation, two different dynamic mechanisms are revealed: either the direct rebonding (RB) mechanism where only a single metal atom shifts slightly, or generalized Stone–Wales (SW^g) rotation in which several atoms undergo significant displacements. The migration barriers for RB mechanism can be 2–4 times lower than for the SW^g. Our analyses show that within a range of chemical potentials, highly mobile dislocations could at the same time be thermodynamically favored, that is statistically dominating the overall material property. This demonstrates remarkable possibility of changing material basic property such as plasticity by changing elemental chemical potentials of the environment

Tunable Magnetism in Transition-Metal-Decorated Phosphorene

We present a density functional theory study of 3d transition-metal (TM) atoms (Sc–Zn) adsorbed on a phosphorene sheet. We show that due to the existence of lone pair electrons on P atoms in phosphorene, all the TM atoms, except the closed-shell Zn atom, can bond strongly to the phosphorene with sizable binding energies. Moreover, the TM@phosphorene systems for TM from Sc to Co exhibit interesting magnetic properties, which arise from the exchange splitting of the TM 3d orbitals. We also find that strain is an effective way to control the magnetism of TM@phosphorene systems by tuning the interaction of the TM with phosphorene and, thus, the relative positions of in-gap TM 3d orbitals. In particular, a small biaxial strain could induce a magnetic transition from a low-spin to a high-spin state in phosphorene decorated by Sc, V, or Mn. These results clearly establish the potential for phosphorene utilization in innovative spintronic devices

Two-Dimensional MoS₂ Confined Co(OH)₂ Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes

The development of abundant and cheap electrocatalysts for the hydrogen evolution reaction (HER) has attracted increasing attention over recent years. However, to achieve low-cost HER electrocatalysis, especially in alkaline media, is still a big challenge due to the sluggish water dissociation kinetics as well as the poor long-term stability of catalysts. In this paper we report the design and synthesis of a two-dimensional (2D) MoS₂ confined Co­(OH)₂ nanoparticle electrocatalyst, which accelerates water dissociation and exhibits good durability in alkaline solutions, leading to significant improvement in HER performance. A two-step method was used to synthesize the electrocatalyst, starting with the lithium intercalation of exfoliated MoS₂ nanosheets followed by Co²⁺ exchange in alkaline media to form MoS₂ intercalated with Co­(OH)₂ nanoparticles (denoted Co-Ex-MoS₂), which was fully characterized by spectroscopic studies. Electrochemical tests indicated that the electrocatalyst exhibits superior HER activity and excellent stability, with an onset overpotential and Tafel slope as low as 15 mV and 53 mV dec^–1, respectively, which are among the best values reported so far for the Pt-free HER in alkaline media. Furthermore, density functional theory calculations show that the cojoint roles of Co­(OH)₂ nanoparticles and MoS₂ nanosheets result in the excellent activity of the Co-Ex-MoS₂ electrocatalyst, and the good stability is attributed to the confinement of the Co­(OH)₂ nanoparticles. This work provides an imporant strategy for designing HER electrocatalysts in alkaline solutions, and can, in principle, be expanded to other materials besides the Co­(OH)₂ and MoS₂ used here

Universal Descriptor for Large-Scale Screening of High-Performance MXene-Based Materials for Energy Storage and Conversion

Density functional theory calculations are employed to systematically investigate the trend of hydrogen evolution reaction (HER) performance of oxygen-terminated MXenes. By studying 30 transition-metal carbides and 30 transition-metal nitrides, M_<i>n</i>+1C_<i>n</i>O₂ and M_<i>n</i>+1N_<i>n</i>O₂ (M = Sc, Cr, Hf, Mo, Nb, Ta, Ti, V, W, Zr; <i>n</i> = 1, 2, 3), the tendency of oxygen desorption after hydrogen adsorption is elucidated to play a key role in HER performance of oxygen-terminated MXenes. On the basis of these observations, we propose a suitable HER descriptor, oxygen vacancy formation energy (<i>E</i>_f), which scales linearly with the adsorption free energy of hydrogen, Δ<i>G</i>_H. In addition, this new descriptor is linearly correlated with the lithium binding strength on oxygen-terminated MXenes. Therefore, <i>E</i>_f is a universal descriptor for identifying the trend of adsorption processes where adsorbed species donate electrons to oxygen-terminated MXenes. This work provides a general guideline for large-scale screening of promising MXene-based materials for energy storage and conversion

Grain Boundary Structures and Electronic Properties of Hexagonal Boron Nitride on Cu(111)

Grain boundaries (GBs) of hexagonal boron nitride (h-BN) grown on Cu(111) were investigated by scanning tunneling microscopy/spectroscopy (STM/STS). The first experimental evidence of the GBs composed of square-octagon pairs (4|8 GBs) was given, together with those containing pentagon-heptagon pairs (5|7 GBs). Two types of GBs were found to exhibit significantly different electronic properties, where the band gap of the 5|7 GB was dramatically decreased as compared with that of the 4|8 GB, consistent with our obtained result from density functional theory (DFT) calculations. Moreover, the present work may provide a possibility of tuning the inert electronic property of h-BN via grain boundary engineering

Theoretical Investigation of the Intercalation Chemistry of Lithium/Sodium Ions in Transition Metal Dichalcogenides

Among various two-dimensional compounds, transition metal dichalcogenides (TMDs or MX₂) are a group of materials attracting growing research interest for potential applications as battery electrodes. Here we systematically investigate the electrochemical performance of a series of MX₂ (M = Mo, W, Nb, Ta; X = S, Se) upon Li/Na intercalation through first-principles calculations. MoX₂ and WX₂ were found to have lower voltages while those of NbX₂ and TaX₂ were higher than 1.5 V. By applying the rigid-band model, we found that the energy gained for electrons to transfer from Li/Na to MX₂ could serve as a descriptor for characterizing voltages of MX₂.The linear relation between the descriptor and voltages is useful for screening candidates for electrodes with desired voltage. Migration barriers for Li/Na ions were approximately 0.3 eV in MoX₂/WX₂ and 0.5 eV in NbX₂/TaX₂. The low barriers suggest a reasonable rate performance when these TMDs are used as electrodes. By stacking different MX₂ with distinct properties, TMDs heterostructures could be adopted to provide tunable electrochemical properties, including voltage, capacity and electronic conductivity while keeping barriers for Li/Na ions little changed. Thus, this strategy offers another degree of freedom for rational design of layered electrode materials

Intrinsic Structural Defects in Monolayer Molybdenum Disulfide

Monolayer molybdenum disulfide (MoS₂) is a two-dimensional direct band gap semiconductor with unique mechanical, electronic, optical, and chemical properties that can be utilized for novel nanoelectronics and optoelectronics devices. The performance of these devices strongly depends on the quality and defect morphology of the MoS₂ layers. Here we provide a systematic study of intrinsic structural defects in chemical vapor phase grown monolayer MoS₂, including point defects, dislocations, grain boundaries, and edges, via direct atomic resolution imaging, and explore their energy landscape and electronic properties using first-principles calculations. A rich variety of point defects and dislocation cores, distinct from those present in graphene, were observed in MoS₂. We discover that one-dimensional metallic wires can be created via two different types of 60° grain boundaries consisting of distinct 4-fold ring chains. A new type of edge reconstruction, representing a transition state during growth, was also identified, providing insights into the material growth mechanism. The atomic scale study of structural defects presented here brings new opportunities to tailor the properties of MoS₂ via controlled synthesis and defect engineering