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Two-dimensional transition metal dichalcogenides (TMD), such as molybdenum disulfide (MoS2), have aroused substantial research interest in recent years, motivating the quest for new synthetic strategies. Recently, halide salts have been reported to promote the chemical vapor deposition (CVD) growth of a wide range of TMD. Nevertheless, the underlying promoting mechanisms and reactions are largely unknown. Here, we employ first-principles calculations and ab initio molecular dynamics (AIMD) simulations in order to investigate the detailed molecular mechanisms during the salt-assisted CVD growth of MoS2monolayers. The sulfurization of molybdenum oxyhalides MoO2X2(X = F, Cl, Br, and I)─the form of Mo-feedstock dominating in salt-assisted synthesis─has been explored and displays much lower activation barriers than that of molybdenum oxide present during conventional “saltless” growth of MoS2. Furthermore, the rate-limiting barriers appear to depend linearly on the electronegativity of the halogen element, with oxyiodide having the lowest barrier. Our study reveals the promoting mechanisms of halides and allows growth parameter optimization to achieve even faster growth of MoS2monolayers in the CVD synthesis.

Metal corrosion from exposure to supercritical fluids is a long-standing challenge lacking atomistic understanding for effective corrosion prevention in diverse important applications. Here, reactions relevant to corrosion in supercritical CO2(sCO2), with H2O and NO2as impurities, are revealed using ab initiomolecular dynamics and enhanced sampling methods. While little reactivity is observed in bulk sCO2, the iron surface is found to play a crucial almost-catalytic role in its own corrosion, activating the inert CO2and hence facilitating ∗HCO3−and ∗CO32−formation at the interface. Moreover, the hydrogen bond network of ∗H2O assists the H shuttling to the activated CO2and impurities, forming reactive species observed in experiments. Such atomistic insight elucidates the reaction mechanisms of metal corrosion in aqueous and supercritical fluids and can point to possible remedies.

Pillared 3D carbon architectures, with the graphene layers and carbon nanotubes connected by topological junctions, have been produced and observed, as reported recently. However, the atomistic details of such junctions are hard to discern in microscopy and remain presently unclear. The simplest junction contains six heptagons in the transition region between the nanotube and graphene. Although these junctions make the pillared architectures possible, they are susceptible to failure when the whole structure undergoes mechanical or thermal stress. In this work we consider “nanochimneys”, a variety of special junctions with cones in between the nanotube and graphene parts. We explore the structures of the nanochimneys (NCs) and determine their underlying topological requirements. We also study the thermal conductance of these pillared architectures and show that NCs conduct heat better than regular simple junctions.

Monovalent potassium substitution at the divalent barium sites of Ba2NF, a Ba2N electride analogue, led to fluoride-ion conduction in the two-dimensional (2D) interlayer space. Potassium substitution made the system chemically unstable for x≥ 0.25 in Ba2–xKxNF1–x, and the nominal fluorine stoichiometry was not attained as the composition approached this region. Fluoride ions, fluoride vacancies, and interstitial anionic electrons (IAEs) coexisted in the 2D interlayer space of Ba1.8K0.2NF0.7·0.1e–because the missing charge of the absent fluoride ions was compensated for by IAEs. First-principles calculations demonstrated that the coexistence of these three species hindered smooth fluoride-ion diffusion on account of the IAEs acting as a scattering barrier. Chemical fluorination to replace these residual IAEs with fluoride ions via an exchange reaction led to the coexistence of only fluoride ions and their vacancies, resulting in fluoride-ion conduction. The obtained results demonstrate the first emergence of fluoride-ion conduction in an electride analogue, which is expected to motivate further exploration of this new family of fluoride-ion conductors. Our skeleton-retained anion exchange approach suggests that such 2D electrides may represent a fresh chemical space for realizing monovalent anionic conduction.

Transition metal dichalcogenide (TMD) alloys form a broad class of two-dimensional (2D) layered materials with tunable bandgaps leading to interesting optoelectronic applications. In the bottom-up approach of building these atomically thin materials, atomic doping plays a crucial role. Here we demonstrate a single step CVD (chemical vapor deposition) growth procedure for obtaining binary alloys and heterostructures by tuning atomic composition. We show that a minute doping of tin during the growth phase of the Mo1–xWxS2alloy system leads to formation of lateral and vertical heterostructure growth. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging and density functional theory (DFT) calculations also support the modified stacking and growth mechanism due to the nonisomorphous Sn substitution. Our experiments demonstrate the possibility of growing heterostructures of TMD alloys whose spectral responses can be desirably tuned for various optoelectronic applications.