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.

The phase selection process during the crystallization of undercooled metallic melts is studied in situ by combining the electromagnetic levitation technique with energy dispersive x-ray diffraction of synchrotron radiation. The crystallization of metastable bcc phase in binary Ni-V alloys was identified. A metastable phase diagram of Ni-V alloy is constructed, which shows the primarily solidifying phase as a function of composition and undercooling. The analysis within nucleation theory emphasizes the important role of metal oxide as a heterogeneous nucleation site controlling the phase selection.

Self-assembled InGaAs quantum rings, embedded in a GaAs matrix, were investigated using magneto-capacitance-voltage spectroscopy. The magnetic-field dispersion of the charging energies exhibits characteristic features for both the first and second electron, which can be attributed to a ground state transition from l=0 into l=−1, and a ground state transition from l=−1 into l=−2, respectively. Furthermore, using a combination of capacitance-voltage spectroscopy and one-dimensional numerical simulations, the conduction band structure of these InGaAs quantum rings was determined.

The time-resolved photoluminescence (PL) characteristics of single CdSe/ZnS nanoparticles, embedded in a PMMA layer is studied at room temperature. We observe a strong spectral jitter of up to 55 meV, which is correlated with a change in the observed linewidth. We evaluate this correlation effect using a simple model, based on the quantum confined Stark effect induced by a diffusing charge in the vicinity of the nanoparticle. This allows us to derive a mean distance between the center of the particle and the diffusing charge of approximately 3.3 nm on average, as well as a mean charge carrier displacement within the integration time. The distances are larger than the combined radius of particle core and shell of about 3 nm, but smaller than the overall radius of 5 nm including ligands. These results are reproducible, even for particles which exhibit strong blueing, with shifts of up to 150 meV. Both the statistics and its independence of core-shell alterations lead us to conclude that the charge causing the spectral jitter is situated in the ligands.