Ultrafast electronic relaxation dynamics in PbI2 semiconductor colloidal nanoparticles:: A femtosecond transient absorption study

We report the first direct measurements of ultrafast electronic relaxation dynamics in PbI2 colloidal nanoparticles using femtosecond transient absorption spectroscopy. The PbI2 nanoparticles were prepared using colloidal chemistry methods in different solvents, including ethanol, 2-propanol, 1-butanol, water, and acetonitrile, as well as in poly(vinyl alcohol) (PVA) matrix. The particle sizes were determined using low- and high-resolution transmission electron microscopy and atomic force microscopy, which provided direct evidence of photodegradation of the nanoparticles. The ground state:electronic absorption spectra of aged PbI2 nanoparticles in acetonitrile and alcohol solvents showed two major peaks near 360 and 292 nm, which slightly blue shift with decreasing size. In aqueous solution containing PVA a new sharp excitonic peak appeared at 414 nm, indicative of nanoparticle formation. With excitation at 390 nm and probing in the visible to near-infrared region, the electronic relaxation dynamics in PbI2 nanoparticles-were directly monitored. The electronic relaxation is found to be sensitive to solvent and insensitive to particle size. In acetonitrile the relaxation was dominated by a 75 ps decay. In alcohol solvents, in addition to a 75 ps decay, a fast 6 ps decay was observed. The relaxation in aqueous PVA solution featured a double exponential decay with time constants of 1 and 40 ps, There appeared to be oscillations at early times with, a period changing with solvent but not with particle size. The dynamics observed were somewhat dependent on the probe wavelength and independent of the excitation intensity. The results suggest that the surface plays a major role in the electronic relaxation process of PbI2 nanoparticles. The influence of particle size is relatively minor in the size range studied (3-100 nm), probably because the relaxation is dominated by surface characteristics that do not vary significantly with size and/or the size is much larger than the exciton Bohr radius(1.9 nm)and thereby spatial confinement is not significant in affecting the relaxation process.