Chemical and Size Characterization of Layered Lead Iodide Quantum Dots via Optical Spectroscopy and Atomic Force Microscopy

Lead iodide (PbI₂) clusters were synthesized from the chemical reaction of NaI (or KI) with Pb(NO₃)₂ in H₂O, D₂O, CH₃OH, and C₃H₇OH solvents. The observation of absorption features between the 550 and 350 nm region obtained with an integrating sphere strongly suggests PbI₂ quantum dot formation in solution. Comparison of spectra of PbI₂ clusters in solution with PbI₂ clusters formed by impregnation of PbI₂ in four different pore-sized porous silica substrates indicates that the PbI₂ cluster size in solution is less than 2.5 nm in the lateral dimension. Atomic force microscopy (AFM) measurements of PbI₂ solutions deposited on mica and highly oriented pyrolytic graphite surfaces indicate that the clusters are single layered. The measured height is 1.0 ± 0.1 nm, which is ～0.3 nm larger than the layer thickness observed for the bulk materials. The swollen layer thickness can be attributed to the intralayer contraction from the strong lateral interaction among PbI₂ molecules, which is supported by ab initio calculations. Raman scattering measurements of the LO and TO modes of PbI₂ in bulk and in the confined state were also conducted in 50?150 cm^-1 region. Three bands observed at 74, 96, and 116 cm^-1 for the confined materials are assigned to the TO₂, LO₂, and LO₁ modes, respectively. The relatively small red shift in the LO modes for PbI₂ in the porous hosts may be caused by the surface phonon of PbI₂ nanoparticles confined in the porous silica.