Substitutionally doped 2D transition metal dichalcogenides are primed for next-generation device applications such as field effect transistors (FET), sensors, and optoelectronic circuits. In this work, we demonstrate substitutional rhenium (Re) doping of MoS 2 monolayers with controllable concentrations down to 500 ppm by metal− organic chemical vapor deposition (MOCVD). Surprisingly, we discover that even trace amounts of Re lead to a reduction in sulfur site defect density by 5−10×. Ab initio models indicate the origin of the reduction is an increase in the free-energy of sulfur-vacancy formation at the MoS 2 growth-front when Re is introduced. Defect photoluminescence (PL) commonly seen in undoped MOCVD MoS 2 is suppressed by 6× at 0.05 atomic percent (at. %) Re and completely quenched with 1 at. % Re. Furthermore, we find that Re-MoS 2 transistors exhibit a 2× increase in drain current and carrier mobility compared to undoped MoS 2 , indicating that sulfur vacancy reduction improves carrier transport in the Re-MoS 2 . This work provides important insights on how dopants affect 2D semiconductor growth dynamics, which can lead to improved crystal quality and device performance.

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Magnetic-field-induced phase transitions are investigated in the frustrated gapped quantum paramagnet Rb2Cu2Mo3O12 through dielectric and calorimetric measurements on single crystal samples. It is clarified that the previously reported dielectric anomaly at 8 K in powder samples is not due to a chiral spin liquid state as has been suggested, but rather to a tiny amount of a ferroelectric impurity phase. Two field-induced quantum phase transitions between paraelectric/paramagnetic and ferroelectric/magnetically ordered states are clearly observed. It is shown that the electric polarization is a secondary order parameter at the lower-field (gap closure) quantum critical point, but a primary one at the saturation transition. Having clearly identified the magnetic BEC nature of the latter, we use the dielectric channel to directly measure the critical divergence of BEC susceptibility. The observed power-law behavior is in very good agreement with theoretical expectations for three dimensional BEC. Finally, dielectric data reveal new magnetic presaturation phases in this compound that may feature exotic order with unconventional broken symmetries.