Enhancement of hydrogen sulfide gas sensing of PbS colloidal quantum dots by remote doping through ligand exchange

Colloidal quantum dots (CQDs) are solution-synthesized semiconductor nanocrystals with size typically below 10 nm. Their large surface-to-volume ratio and abundant active surface sites, combined with the grain size effect and solution-processability make CQDs promising building blocks for low-cost and high-performance gas sensors. Here we employed the ligand exchange strategy to develop low-power and highly sensitive H2S gas sensors based on PbS CQDs. Following the layer-by-layer spin-coating of PbS CQDs capped with long-chain oleic acid ligands from synthesis, a surface treatment using different inorganic salts was conducted for ligand exchange in air ambient at room temperature. Upon exposure to 50 ppm of H2S at 135 °C, the resistance of all those sensors decreased shown as the response and the Pb(NO3)2 treatment yielded highest response (4218 at 135 °C) with shortest response/recovery time. We tentatively proposed a H2S-induced and temperature-promoted p-to-n transition of PbS CQDs as the sensing mechanism and the role of Pb(NO3)2 treatment was attributed to an n-type remote doping effect realized by the ligand exchange, which was further supported by the energy dispersive spectrometry (EDS) and ultraviolet photoelectron spectroscopy (UPS) analysis.