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–xRe_xS₂, Ta_1–xRe_xS₂ and their selenide counterparts that the 1H ordered alloy structures for x ≤ 0.5 are thermodynamically stable, with formation energies an order of magnitude lower than currently known TMD alloys such as Mo_xW_1–xS₂, 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 x = 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.