Synthesis, Optical Spectroscopy and Ultrafast Electron Dynamics of PbS Nanoparticles with Different Surface Capping

Abstract

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.

Role of surface states and defects in the ultrafast nonlinear optical properties of CuS quantum dots

“…The synthesis of the CuS-PVP nanocrystals is similar to the method reported previously for metal sulfide nanoparticles such as PbS, 19 ZnS, 20 and CuS. 21 UV-VIS-NIR absorption spectra were measured using a Varian Cary 5000 spectrophotometer.…”

Efficient Infrared‐Emitting PbS Quantum Dots Grown on DNA and Stable in Aqueous Solution and Blood Plasma

“…[11] The only previous report of DNA-templated growth on PbS has yielded materials with no detectable luminescence in the infrared. [12] We worked instead at a synthesis temperature at which chemical interaction was possible between the metal cations used in PbS growth and at least two classes of sites on DNA: the phosphate backbone, and also DNA's purine and pyrimidine bases. The bases provide an additional opportunity for control over the growth of nanoparticles and the passivation of their surface states.…”

“…For this purpose, various organic thin films and organic thin layers are widely used for producing microdevices and nanodevices under severe space constraints. [12] For the deposition of organic thin layers onto solid substrates, representative organic ultrathin films are self-assembled monolayers (SAMs …”

Optical and Dynamic Properties of Undoped and Doped Semiconductor Nanostructures

“…Other nanoparticles have generally been found to be weakly luminescent or non-luminescent at room temperature, e.g. PbS [91], PbI 2 [92], CuS [93], Ag 2 S [94]. The low luminescence can be due to either the indirect nature of the semiconductor or a high density of internal and/or surface trap states that quench the luminescence.…”