Stable Monolayer Transition Metal Dichalcogenide Ordered Alloys with Tunable Electronic Properties

From first-principles calculations, we discover highly stable monolayer transition metal dichalcogenides (TMDs) ternary alloys consisting of group 5 and 7 transition metal elements. We show for Nb_1–<i>x</i>Re_<i>x</i>S₂, Ta_1–<i>x</i>Re_<i>x</i>S₂ and their selenide counterparts that the 1H ordered alloy structures for <i>x</i> ≤ 0.5 are thermodynamically stable, with formation energies an order of magnitude lower than currently known TMD alloys such as Mo_<i>x</i>W_1–<i>x</i>S₂, so that they could potentially be synthesizable using chemical vapor deposition or exfoliation techniques. This class of TMD alloys offer a wide tunable bandgap range of ∼1 eV, displaying metallic to semiconducting behavior versus alloy composition. Importantly, at <i>x</i> = 0.5, the alloys are valence isoelectronic with MoS₂. These stoichiometric compounds, Nb_0.5Re_0.5S₂, Ta_0.5Re_0.5S₂, and their selenide counterparts, exhibit band features similar to MoS₂, but possess significantly smaller bandgaps (∼1 to 1.2 eV). As a result, compared to MoS₂ and WS₂, this class of alloy TMDs display enhanced absorbance in the visible range of the solar spectrum where the solar spectral intensity is the strongest. These ordered monolayer TMD alloys could open doors for designing ultrathin solar absorbers with improved performance

A Comprehensive Search for Stable Pt–Pd Nanoalloy Configurations and Their Use as Tunable Catalysts

Using density-functional theory, we predict stable alloy configurations (ground states) for a 1 nm Pt–Pd cuboctahedral nanoparticle across the entire composition range and demonstrate their use as tunable alloy catalysts via hydrogen-adsorption studies. Unlike previous works, we use simulated annealing with a cluster expansion Hamiltonian to perform a rapid and comprehensive search that encompasses both high and low-symmetry configurations. The ground states show Pt­(core)–Pd­(shell) type configurations across all compositions but with specific Pd patterns. For catalysis studies at room temperatures, the ground states are more realistic structural models than the commonly assumed random alloy configurations. Using the ground states, we reveal that the hydrogen adsorption energy increases (decreases) monotonically with at. % Pt for the {111} hollow ({100} bridge) adsorption site. Such trends are useful for designing tunable Pd–Pt nanocatalysts for the hydrogen evolution reaction