Spectroscopic correlation of chalcogen defects in atomically thin MoS_2(1−x)Se_2x alloys

Engineering of atomically thin transition metal dichalcogenides (TMDs) is highly sought after for novel optoelectronic and spintronic devices. With the limited number of naturally existing TMDs, chalcogen based alloying has become a viable solution for developing TMDs for optical modulators and photovoltaics. Here, we report on detailed optical and microscopic studies of ternary TMD alloys of molybdenum, sulfur, and selenium grown via a single step method. The developed material has tunable band gaps in a broad range 1.5–1.9 eV with the variation in sulfur compositions. Further, the existence of trions, bi-excitons, and defect bound excitons are shown using temperature dependent (4 K−300 K) photoluminescence spectroscopy. A detailed analysis on MoS_1.34Se_0.66 alloy system shows the evidence of new types of defect bound excitons originating at low temperatures along with the presence of bi-excitons having a binding energy of ∼41 meV. The prospects of defect induced quasiparticles are observed in scanning transmission electron microscope assisted analyses and verified using density functional theory calculations. The thermal conductivity values, calculated using micro-Raman studies, of MoS₂, MoSe₂, and MoS_1.34Se_0.66 are found to be 69(±2) W m⁻¹ K⁻¹, 33(±2) W m⁻¹ K⁻¹ and 17(±2) W m⁻¹ K⁻¹ respectively, in agreement with the theoretical predictions. Tunable optical properties of these ternary atomic layers along with moderate thermal conductivity reveal the potential of these layers in modern opto-electronic devices and sensors.