Unusual Negative Formation Enthalpies and Atomic Ordering in Isovalent Alloys of Transition Metal Dichalcogenide Monolayers

Isovalent alloys of monolayer transition metal dichalcogenides (TMDs) MX2 (M = Mo, W; X = S, Se) have attracted much attention recently with great promise of expanding TMDs for more applications. In contrast to common substitutional isovalent semiconductor alloys which usually form disordered metastable phases with positive formation enthalpies (ΔH), TMD alloys unusually always have negative ΔH, indicating atomic ordering according to Hume–Rothery rules. As atomic ordering often plays important roles in determining alloy properties, understanding the negative ΔH and ordering effects in TMD alloys is thus of great importance and quite necessary. Using first-principles calculations, we reveal that the negative ΔH of cation-mixed TMD alloys results from energy gain due to charge transfer from weak Mo–X to nearest strong W–X bonds. For anion-mixed cases with stronger M–S bonds, the negative ΔH comes from energy gain due to charge transfer from Se to nearest S atoms. Such charge-transfer-induced energy gain always exists no matter whether TMD alloys are ordered or disordered, but it can be maximized by local Mo–X–W (S–M–Se) ordering in cation-mixed (anion-mixed) alloys. Consequently, the coexistence of partial local ordering and long-range disordering at finite temperatures in TMD alloys could be possible, in agreement with a recent experimental work. The ordering effects on macroscopic properties of TMD alloys such as band gaps and optical absorptions are negligible, but microscopic properties such as defects can be strongly affected. Finally, quaternary TMD alloys by mixing both cations and anions are studied to have a wide range of band gaps for optoelectronic applications.