Lateral Versus Vertical Growth of Two-Dimensional Layered Transition-Metal Dichalcogenides: Thermodynamic Insight into MoS₂

Unprecedented interest has been spurred recently in two-dimensional (2D) layered transition metal dichalcogenides (TMDs) that possess tunable electronic and optical properties. However, synthesis of a wafer-scale TMD thin film with controlled layers and homogeneity remains highly challenging due mainly to the lack of thermodynamic and diffusion knowledge, which can be used to understand and design process conditions, but falls far behind the rapidly growing TMD field. Here, an integrated density functional theory (DFT) and calculation of phase diagram (CALPHAD) modeling approach is employed to provide thermodynamic insight into lateral versus vertical growth of the prototypical 2D material MoS₂. Various DFT energies are predicted from the layer-dependent MoS₂, 2D flake-size related mono- and bilayer MoS₂, to Mo and S migrations with and without graphene and sapphire substrates, thus shedding light on the factors that control lateral versus vertical growth of 2D islands. For example, the monolayer MoS₂ flake in a small 2D lateral size is thermodynamically favorable with respect to the bilayer counterpart, indicating the monolayer preference during the initial stage of nucleation; while the bilayer MoS₂ flake becomes stable with increasing 2D lateral size. The critical 2D flake-size of phase stability between mono- and bilayer MoS₂ is adjustable via the choice of substrate. In terms of DFT energies and CALPHAD modeling, the size dependent pressure–temperature–composition (<i>P</i>-<i>T</i>-<i>x</i>) growth windows are predicted for MoS₂, indicating that the formation of MoS₂ flake with reduced size appears in the middle but close to the lower <i>T</i> and higher <i>P</i> “Gas + MoS₂” phase region. It further suggests that Mo diffusion is a controlling factor for MoS₂ growth owing to its extremely low diffusivity compared to that of sulfur. Calculated MoS₂ energies, Mo and S diffusivities, and size-dependent <i>P</i>-<i>T</i>-<i>x</i> growth windows are in good accord with available experiments, and the present data provide quantitative insight into the controlled growth of 2D layered MoS₂

Effect of the Ligand Structure on Chemical Vapor Deposition of WN_<i>x</i>C_<i>y</i> Thin Films from Tungsten Nitrido Complexes of the Type WN(NR₂)₃

Tungsten nitrido complexes of the type WN­(NR₂)₃ [NR₂ = combinations of NMe₂, NEt₂, N^<i>i</i>Pr₂, N^<i>n</i>Pr₂, N^<i>i</i>Bu₂, piperidine, and azepane] were synthesized as precursors for aerosol-assisted chemical vapor deposition of WN_<i>x</i>C_<i>y</i> thin films. The effects of the amido substituents on precursor volatility and decomposition were evaluated experimentally and computationally. Films deposited using WN­(NMe₂)­(N^<i>i</i>Pr₂)₂ as a single-source precursor were assessed as diffusion barrier materials for Cu metallized integrated circuits in terms of growth rate, surface roughness, composition, and density. In diffusion barrier tests, Cu (∼100 nm)/WN_<i>x</i>C_<i>y</i> (∼5 nm)/Si samples prepared from WN­(NMe₂)­(N^<i>i</i>Pr₂)₂ were annealed for 30 min at 500 °C and successfully blocked Cu penetration according to four-point probe, X-ray diffraction, scanning electron microscopy etch-pit test, and high-resolution transmission electron microscopy measurements

Plot of the model fits (red dots) to observed data (grey violin plots, with a black bar representing the interquartile range) aggregated by country.

<p>Number of dots per site indicates the number of contributing distributions. The position of the dot on the y-axis represents the geometric mean of the distribution and the size of the dot represents the relative contribution of that subgroup to the full distribution.</p

Summary of the data stratified by country.

<p>Summary of the data stratified by country.</p

Demographic characteristics on admission of patients enrolled in antimalarial drug efficacy trials who received MAS₃, between 1995 and 2007.

<p>Demographic characteristics on admission of patients enrolled in antimalarial drug efficacy trials who received MAS₃, between 1995 and 2007.</p

Model predictions for the probability that an infection with a given clearance half-life is resistant.

<p>This relationship is predicted to be dependent on the underlying proportion of ‘resistant’ infections in the study population. The relationships for underlying proportions ‘resistant’ of 0.1 (green), 0.5 (blue), and 0.9 (purple) are shown. The shaded areas represent the 50%, 80%, 90%, and 95% prediction intervals (from dark to light shading, respectively).</p

Summary of the data from the Thai-Myanmar border stratified by year.

<p>Summary of the data from the Thai-Myanmar border stratified by year.</p

Drug sensitivity of <i>P. falciparum</i> isolates for artesunate (Figure 4a) and mefloquine (Figure 4b).

<p>Isolates from primary infections were collected at SMRU clinics between 1995 and 2007 and assayed for sensitivity to artesunate and mefloquine, IC₅₀ geometric means are given as nM/l with [95% CI].</p

PCR-confirmed novel and recrudescent infections and time to recrudescence.

<p>PCR-confirmed novel and recrudescent infections and time to recrudescence.</p

Percentage of patients who had cleared parasitaemia at Day-2 and Day-3 1995–2007.

<p>Percentage of patients who had cleared parasitaemia at Day-2 and Day-3 1995–2007.</p