Self-assembly, stability, and photoresponse of PbS quantum dot films capped with mixed halide perovskite ligands

The type of passivating ligands and the ligand exchange method influence the quality of lead sulfide quantum dot films. This imparts on the efficiency of optoelectronic devices. To get a compact arrangement of the nanocrystals in a thin film (similar to 100 nm) via self-assembling, we used organic-inorganic perovskites with mixed halides for the solid-state exchange of oleic acid ligands on PbS QDs (similar to 4 nm). Formamidinium lead halides FAPbI(x)Br(3-x) (x= 3,2,1,0) were used. X-ray spectroscopy shows that successful replacement of oleic acid with FA happens by short immersion of the films (2 min) in the solution. Transmission electron microscopy shows that nano-scale cracks, short-range ordering, and fusion of the nanocrystals happen during the exchange with FAPbI(3). Gradual substitution of I- with Br- ions improves the film homogeneity with fewer defects (nano-scale holes and cracks). Uniform cubic assembly of the nanocrystals without defects and high coverage density is obtained with FAPbBr(3). The findings are applied to the fabrication of field effect phototransistors on Si/SiO2/graphene wafers. Photoresponse of the devices indicates that the stability of the films in humidified air (26 degrees C and 51% humidity) is improved from less than 14 days for iodine to more than 60 days for bromine. The I-LIGHT/I-DARK ratio, responsivity, and specific detectivity increase by 64%, 55%, and 60%, respectively, with FAPbBr(3). Prevention of lead sulfide nanocrystal aggregation and the formation of large-scale self-assemblies with perovskite ligands is of technical importance for next-generation optoelectronic applications.

Automated summary

The type of passivating ligands and the ligand exchange method significantly affect the quality and stability of lead sulfide quantum dot films, and thereby impacting the efficiency of optoelectronic devices. Using organic-inorganic perovskites as exchange agents, the replacement of oleic acid ligands with perovskite ligands enhances the film homogeneity and reduces defects. The application of this method improves the stability and performance of field effect phototransistors, demonstrating its importance for next-generation optoelectronic applications.