Synthesis and characterization of Cu x S nanoparticles.
Nature of the infrared band and charge-carrier dynamics
M. C. Brelle, C. L. Torres-Martinez, J. C. McNulty,
R. K. Mehra, and J. Z. Zhang*
*Department of Chemistry, University
of California, Santa Cruz, California 95064
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.
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