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(methylmethacrylate) (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, 3.43, and 2.10 Å. 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.

CuxS (x = 1,2) nanoparticles have been synthesized utilizing different capping molecules including polyethyleneglycol (PEG), polyvinylpyrrolidone (PVP), casein hydrolysate-enzymatic (CAS), and bovine serum albumin (BSA). The ground-state electronic absorption spectra of the CuxS nanoparticles show three distinct types of CuxS formed: a green type assigned as crystalline CuS, and two brown types assigned as crystalline Cu2S and amorphous Cu2S. The brown types exhibit a steady increase in absorption toward shorter wavelengths starting at around 650 nm, while the green type shows the same steady increase in absorption, but with an additional absorption band in the infrared (IR). The IR band is attributed to an electron-acceptor state lying within the bandgap. ESR measurements of free Cu(II) ions in solution for all samples show the presence of Cu(II) in the brown amorphous samples, but not in the green or brown crystalline samples. Ultrafast dynamics of photoinduced electrons have been measured for all samples using femtosecond-transient absorption/bleach spectroscopy. In all brown Cu2S samples studied, the early time-transient profiles feature a pulse-width-limited (<150 fs) rise followed by a fast decay (1.1 ps) and a slow decay (>80 ps). These decay dynamics were found to be independent of pump power and stabilizing agent. The fast 1.1 ps decay is attributed to charge carrier trapping, while the long decay may be due to either recombination or deep trapping of the charge carriers. The green CuxS samples studied showed interesting power-dependent behavior. At low excitation intensities, the green CuxS samples showed a transient bleach signal, while at high intensities, a transient absorption signal has been observed. The increased transient absorption over bleach at high intensities is attributed to trap-state saturation. A kinetic model has been developed to account for the main features of the electronic relaxation dynamics.

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