Pure Appl. Chem., Vol. 72, Nos. 1–2, pp. 101–117, 2000. © 2000 IUPAC 101 *Pure Appl. Chem. 72, 1–331 (2000). An issue of reviews and research papers based on lectures presented at the 1 st IUPAC Workshop on Advanced Materials (WAM1), Hong Kong, July 1999, on the theme of nanostructured systems. †Corresponding authors Synthesis and characterization of Cu x S nanoparticles. Nature of the infrared band and charge-carrier dynamics* M. C. Brelle 1 , C. L. Torres-Martinez 2 , J. C. McNulty 1 , R. K. Mehra 2† , and J. Z. Zhang 1† 1 Department of Chemistry, University of California, Santa Cruz, California 95064, 2 Department of Neuroscience and Environmental Toxicology Graduate Program, University of California, Riverside, California 92521 USA Abstract: Cu x S (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 Cu x S nanoparticles show three distinct types of Cu x S formed: a green type assigned as crystalline CuS, and two brown types assigned as crystalline Cu 2 S and amorphous Cu 2 S. 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 Cu 2 S 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 Cu x S samples studied showed interesting power-dependent behavior. At low excitation intensities, the green Cu x S 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. INTRODUCTION Nanoparticles are of great interest due to their extremely small size and large surface-to-volume ratio, which lead to both chemical and physical differences in their properties compared to bulk of the same chemical composition [1–15]. They offer an excellent opportunity to study the effects of spatial confinement on charge carrier behavior and problems related to surfaces or interfaces. The use of time-resolved techniques allows for direct measurements of charge-carrier dynamics in nanoparticles and helps to better understand effects of size and surface on fundamental properties of charge carriers [15–47].