Synthesis and characterization of Cu x S nanoparticles. Nature of the infrared band and charge-carrier dynamics

Abstract

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.

Morphology and Composition-Controls of Cu<I><SUB>x</SUB></I>S Nanocrystals Using Alkyl-Amine Ligands

“…These spectra reveal that the Cu-deficients decrease with increase in the number of alkyl-chains because the peak intensity is proportional to the amount of Cudeficients. [12][13][14][15] In the spectrum of copper sulfide NCs prepared with TOCA, no near IR absorption peak was observed, indicating that the brown precipitate is nearly pure Cu 2 S. These results are consistent with the XRD results shown in Fig. 4.…”

Transient absorption of copper selenide nanowires of different stoichiometry

“…The transient absorption spectra and kinetics are in agreement with the energy scheme of copper selenide nanoparticles containing double valence copper as suggested in [17,20,23,24]. This scheme suggests the presence of two intermediate energy levels of different types: deep recombination levels and shallow trapping levels.…”

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.…”