Research On The Gas-Sensing Properties And Mechanism Of PbS Colloidal Quantum Dot Films

Semiconductor gas sensors has become increasingly important for environmental monitoring, industrial manufacturing, and medical diagnosis. Reducing the working temperature and producing flexible gas sensors would boost smart gas-detection technology with low cost and portability. In resent years, taking advantage of the special activity of nanomaterials for room temperature-gas detection has become the research hotspot in the field of gas sensors. Colloidal quantum dots (CQD), inorganic semiconductor nanocrystals with stable dispersibility in the solvent, is emerging as new substitution gas sensing materials due to the small particle size, high surface activity, and good compatibility with flexible substrates. Here we synthesized PbS CQD by hot-injection method and constructed a new structure of gas-sensing material—quantum dots/inorganic ligands—by spin-coating as well as inorganic ligand exchange strategy. The PbS CQD film showed gas sensing effect at room temperature and the gas-sensing mechanism was investigated by employing density functional theory (DFT) simulations. Furthermore, we investigated the effect of micromorphology of CQD and ligand exchange on the gas sensing performance of PbS CQD. Thanks to the solution processing of CQD at room temperature, flexible CQD gas sensors fabricated on paper substrates were demonstrated. Its outstanding bending and fatigue properties could meet the needs of portable and even wearable gas sensing applications. The main results are as follows:1. The NaNO2 treatment in the process of spin-coating could effectively remove the oleate ligands capped on the surface of PbS CQD through ligand exchange, consequently enhancing the interdot carrier transport and producing excellent accessibility of the incoming gas molecules to CQD surfaces, thereby enables the PbS CQDs to be showed excellent NO2 sensing performance at room temperature, including rapid response and high sensitivity, good reversibility. The whole fabrication process is simple operation under mild conditions.2. The adsorption model of NO2 molecule on the surface of PbS CQD was built by DFT simulations. It suggested that the increasing surface acceptor state density induced by the competing adsorption of NO2 and O2 on surface of PbS CQD was the origin of the gas-sensing effect. Theoretical study revealed that NO2 bonds to the PbS CQD surface, introducing p-type doping and resistance reduction. The small binding energy facilitates quick desorption of NO2 from the PbS CQD surface, giving rise to the quick signal recovery.3. The micromorphology of PbS CQD film was controlled and the NO2 gas-sensing performance was optimized by adjusting the synthesis conditions and film fabricating processing of CQD. Through controlling the synthesis temperature as well as the way of quick water cooling, the size and crystal plane of PbS CQD were adjusted. Results indicated that PbS CQD with higher proportion of (200) crystal plane exhibited higher sensor response to NO2, which may be attributed to the higher surface defect of CQD. In addition, appropriately reducing the spin-coating speed was beneficial to obtain more porous PbS CQD film and promote the gas adsorption and diffusion. Through the above controlling, the PbS CQD gas sensor showed response of 90 to 50 ppm of NO2 gas at room temperature with the response and recovery time being 1 s and 25 s, respectively.4. A comparative research of different inorganic ligand exchange indicated that the kinds of inorganic ligand and working temperature were very important for the selectivity of PbS CQD gas sensor. The NaNO2-treated PbS CQD sensor was found to be very sensitive to H2S at 135? and exhibited n-type response. Using Pb(NO3)2 for ligand exchange could realize n-type remote doping effect owing to the increase in Pb excess which not only favors oxygen adsorption but also increases the intrinsic n-type doping level, thus increased the sensor response from 2389 to 4218 toward 50 ppm of H2S gas at 135?.5. Flexible gas sensors based on PbS CQD were constructed by using the inexpensive, lightweight and flexible paper substrates replacing ceramic substrates. When using Au as the device electrode, the response and excellent reversibility of the sensor were fully retained at different bending angles (0?70°); the device showed only a slight decrease in response (7.1% of the initial value) when subjected to 5000 bending and unbending cycles. Furthermore, more simple and cost-effective flexible gas sensors were obtained by using hand-drawn pencil graphite electrode replacing expensive Au electrode combine spray-coating method, which has potentials for largescale and low-cost manufacturing of flexible gas sensors.