Density-Dependent Carrier Recombination in MoS2 Quantum Dots and Its Implications for Luminescence Sensing of Ammonium Hydroxide

Zero-dimensional molybdenum disulfide (MoS2) quantum dots (QDs) have attracted remarkable interest due to their peculiar properties such as the quantum-confinement effect and high surface area. Exploring recombination dynamics in MoS2 QDs is not only expected to gain a deeper insight into their fundamental physics, it is also important for potential applications in optoelectronics and energy-conversion technology. This study synthesized p-type MoS2 QDs doped with diethylenetriamine (DETA) using pulsed laser ablation method. A hole concentration as high as 2.08 x 10(12) cm(-2) has been demonstrated by gate-dependent conductance measurements. A 110-fold enhancement of photoluminescence in the p- type MoS2 QDs has been found after the introduction of DETA, and the dependence of the radiative and nonradiative recombination of MoS2 QDs on carrier densities were studied. As the carrier density was increased, a decrease of the radiative lifetime was found, which is similar to the behavior of the radiative lifetime in monolayer MoS2. The Shockley-Read- Hall (SRH) and Auger recombination dominates the nonradiative recombination at low and high carrier densities, respectively. The SRH lifetime of MoS2 QDs increases with the increased carrier density, suggesting that the recombination mechanism at the low carrier density is dominated by the SRH recombination. This study found that as the carrier densities exceeded 0.53 x 10(12) cm(-2), the Auger recombination was responsible for the reduction of PL. Furthermore, MoS2 QDs was used as a fluorescent sensor for the detection of ammonium hydroxide (NH4OH). The PL intensity of MoS2 QDs demonstrates a gradual decrease with increasing NH4OH concentration. By investigating the time-resolved PL (TRPL), the mechanism that leads to the decrease of PL in MoS2 QDs is addressed. This investigation is expected to demonstrate a promising development of an effective and low-cost MoS2 QDs-based fluorescent sensor with superior sensitivity for the rapid detection of ammonia in aqueous media.