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 reactio n of NaI (or KI) with Pb(NO3)(2) in H2O, D2O, CH3OH, and C3H7OH solven ts. The observation of absorption features between the 550 and 350 nm region obtained with an integrating sphere strongly suggests PbI2 quan tum 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 nn in the lateral dimension. Atomic force microscopy (AFM) measurements of PbI2 solutions deposite d on mica and highly oriented pyrolytic graphite surfaces indicate tha t the clusters are single layered. The measured height is 1.0 +/- 0.1 nm, which is similar to 0.3 nn larger than the layer thickness observe d for the bulk materials. The swollen layer thickness can be attribute d to the intralayer contraction from the strong lateral interaction am ong PbI2 molecules, which is supported by ab initio calculations. Rama n scattering measurements of the LO and TO modes of PbI2 in bulk and i n the confined state were also conducted in 50-150 cm(-1) region. Thre e bands observed at 74, 96, and 116 cm(-1) for the confined materials are assigned to the TO2, LO(2), and LO(1) modes, respectively. The rel atively small red shift in the LO modes for PbI2 in the porous hosts m ay be caused by the surface phonon of PbI2 nanoparticles confined in t he porous silica.