Hossein Taghinejad1 Ali A. Eftekhar1 P. M. Ajayan3 Evan Reed2 Ali Adibi1

1, Georgia Institute of Technology, Atlanta, Georgia, United States
3, Rice University, Huston, Texas, United States
2, Stanford, Palo Alto, California, United States

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) are currently under widespread scrutiny for a diverse set of optoelectronic applications. At the monolayer limit, TMD crystals offer a direct optical bandgap, enabling the light emission within a large spectral range covering a large portion of the visible spectrum and the near infra-red regime. In this line, the synthesis of ternary alloys has served as a powerful technique for changing the optical/electrical bandgap of monolayer TMD crystals beyond what a binary compound (i.e., MX2, M: transition metal and X: chalcogen) may offer. We have previously shown that “ternary” alloys can be produced via doping a “binary” MX2 crystal by an isoelectronic element (e.g., X’) to yield MoX’2xX2(1-x) monolayer alloys [1]. This approach has shown a great promise for the synthesis of lateral heterostructures with arbitrary shapes and dimensions [1, 2]. In this approach, the quality of the as-synthesized alloys depends on the fine details of the starting binary MX2 crystal.
Here, we study the influence of native defects in starting MX2 monolayers on the obtained properties of the MX’2xX2(1-x) alloys. For the demonstration purpose, we use monolayer MoSe2 films as starting crystals in which replacing Se atoms by S atoms yields MoS2xSe2(1-x) ternary compounds. Our results show that native point defects (primarily chalcogen vacancies) in the starting MoSe2 lattice serve as atomic sites from which S atoms incorporate into the lattice of MoSe2 monolayer and form the MoS2xSe2(1-x) alloy. Thus, the abundance of the chalcogen vacancies in the starting MoSe2 films promotes the alloying process and reduces the required temperature needed for this process. We believe that our findings shed light on the fundamental details of the alloying mechanism in 2D TMD crystals as well as opening a new avenue for synthesis of complex alloys via defect engineering.

[1] H. Taghinejad et. al, “Strain relaxation via formation of cracks in compositionally modulated two-dimensional semiconductor alloys” npj 2D Materials and Applications volume 2, article number: 10 (2018).
[2] Mahjouri-Samani, M. et al. Patterned arrays of lateral heterojunctions within monolayer two-dimensional semiconductors. Nat. Commun. 6, 7749 (2015).