Synthesis, optical spectroscopy and ultrafast electron dynamics of PbS nanoparticles with different surface capping

Lead sulfide, PbS, nanoparticles have been synthesized using a number of surface capping agents including poly(vinyl-alcohol) (PVA), poly(vinyl-pyrrolidone) PVP, gelatin, DNA, polystyrene (PS), and poly(methyl-methacrylate) (PMMA). The electronic absorption spectra and particle shapes have been found to depend on the capping molecules used. An excitonic feature at 580 nm was observed for capping with PVA and DNA, while no such excitonic feature was observed for PVP, PS or PMMA. A weak excitonic feature was observed for gelatin. The particle shape varied from cubic, needle to spherical as controlled by the capping agents. For the DNA-capped PbS nanocrystals, HRTEM demonstrated the presence of oval crystals with a diameter of 3-8 nm. Powder X-ray diffraction of the PbS-DNA nanocrystals showed the characteristic peaks for PbS at 2.97, 3.33, and 2.10 Angstrom. The XRD suggested the size of the nanoparticles to be approximately 4 nm. The dynamics of photoinduced electrons in PbS nanoparticles have been determined using femtosecond laser spectroscopy. For all the samples studied the electronic relaxation has been found to be very similar and follow a double exponential decay with time constants of 1.2 and 45 ps. The fast decay can be attributed to trapping from the conduction band to shallow traps or from shallow traps to deep traps while the slower decay is most likely due to electron-hole recombination mediated by a high density of surface trap states that Lie within the band gap. The decay profiles are independent of particle size, shape, surface capping, probe wavelength, and excitation intensity. The results seem to indicate a high density of surface states, consistent with no detectable fluorescence signal at room temperature.