ABSTRACT:We dope CdSe nanocrystals with Ag impurities and investigate their optical and electrical properties. Doping leads not only to dramatic changes but surprising complexity. The addition of just a few Ag atoms per nanocrystal causes a large enhancement in the fluorescence, reaching efficiencies comparable to core−shell nanocrystals. While Ag was expected to be a substitutional acceptor, nonmonotonic trends in the fluorescence and Fermi level suggest that Ag changes from an interstitial (ntype) to a substitutional (p-type) impurity with increased doping.

The structure of CdSe nanocrystals doped with 0.2%–2.5% Ag corresponding to 1.1–13.6 Ag atoms per nanocrystal is studied in detail by a combination of X-ray diffraction (XRD) and X-ray absorption spectroscopy at the Ag–K, Cd–K, and Se–K edges. X-ray absorption near-edge structure (XANES) data are compared with ab initio multiple scattering simulations. Extended X-ray absorption fine structure (EXAFS) spectra are analyzed by reverse Monte Carlo (RMC) simulations. The XANES data provide evidence that Ag is located inside the CdSe nanocrystals, and the EXAFS spectra show that the local structure of Ag can be described by tetrahedral interstitial sites in either wurtzite or zinc blende lattices similar to the coordination of Ag in Ag2Se.

Zinc oxide (ZnO) nanoparticles (NPs) were stabilized in water using the amino acid l-cysteine. A transparent dispersion was obtained with an agglomerate size on the level of the primary particles. The dispersion was characterized by dynamic light scattering (DLS), pH dependent zeta potential measurements, scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, photoluminescence (PL) spectroscopy, and X-ray absorption fine structure (EXAFS, XANES) spectroscopy. Cysteine acts as a source for sulfur to form a ZnS shell around the ZnO core and as a stabilizer for these core-shell NPs. A large effect on the photoluminescent properties is observed: the intensity of the defect luminescence (DL) emission decreases by more than 2 orders of magnitude, the intensity of the near band edge (NBE) emission increases by 20%, and the NBE wavelength decreases with increasing cysteine concentration corresponding to a blue shift of about 35 nm due to the Burstein-Moss effect.

High quality Cr-doped ZnO nanoparticles from the gas phase were prepared and investigated with respect to their structural, optical and magnetic properties. The extended x-ray absorption fine structure and the x-ray absorption near edge structure of the particles verify that after nanoparticle preparation Cr is incorporated as Cr3+ ) at least partially on sites with a 4-fold oxygen configuration, most likely on a Zn site, into the wurtzite lattice. Despite the fact that Cr is known to act as an efficient non-radiative loss centre for near band gap emission (NBE), a pronounced NBE is obtained up to room temperature even for a nominal Cr concentration of 10 at.%. Annealing at 1000 degrees C results in a significant improvement of the photoluminescence efficiency and a reduced PL linewidth down to 2.9 meV at low temperatures while the structural and magnetic data indicate the formation of ZnCr2O4 clusters.

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