title: Crystal Phase Transitions in the Shell of PbS/CdS Core/Shell Nanocrystals Influences Photoluminescence Intensity.
abstract: We reveal the existence of two different crystalline phases, i.e., the metastablerock saltand the equilibriumzinc blendephase within the CdS-shell of PbS/CdS core/shell nanocrystals formed by cationic exchange. The chemical composition profile of the core/shell nanocrystals with different dimensions is determined by means of anomalous small-angle X-ray scattering with subnanometer resolution and is compared to X-ray diffraction analysis. We demonstrate that the photoluminescence emission of PbS nanocrystals can be drastically enhanced by the formation of a CdS shell. Especially, the ratio of the two crystalline phases in the shell significantly influences the photoluminescence enhancement. The highest emission was achieved for chemically pure CdS shells below 1 nm thickness with a dominant metastablerock saltphase fraction matching the crystal structure of the PbS core. The metastable phase fraction decreases with increasing shell thickness and increasing exchange times. The photoluminescence intensity depicts a constant decrease with decreasing metastablerock saltphase fraction but shows an abrupt drop for shells above 1.3 nm thickness. We relate this effect to two different transition mechanisms for changing from the metastablerock saltphase to the equilibriumzinc blendephase depending on the shell thickness.
authors: Institute of Physics, Montanuniversitaet Leoben , 8700 Leoben, Austria.
authors: Rainer T
authors: RT
authors: Lechner
authors: Institute of Physics, Montanuniversitaet Leoben , 8700 Leoben, Austria.
authors: Gerhard
authors: G
authors: Fritz-Popovski
authors: Institute of Semiconductor and Solid State Physics and Institute of Experimental Physics, Johannes Kepler University Linz , 4040 Linz, Austria.
authors: Maksym
authors: M
authors: Yarema
authors: Institute of Semiconductor and Solid State Physics and Institute of Experimental Physics, Johannes Kepler University Linz , 4040 Linz, Austria.
authors: Wolfgang
authors: W
authors: Heiss
authors: Helmholtz Zentrum Berlin für Materialien und Energie , 12489 Berlin, Germany.
authors: Armin
authors: A
authors: Hoell
authors: European Synchrotron Radiation Facility , BP 220, 38043 Grenoble Cedex, France.
authors: Tobias U
authors: TU
authors: Schülli
authors: Institute of Semiconductor and Solid State Physics and Institute of Experimental Physics, Johannes Kepler University Linz , 4040 Linz, Austria.
authors: Daniel
authors: D
authors: Primetzhofer
authors: Institute of Semiconductor and Solid State Physics and Institute of Experimental Physics, Johannes Kepler University Linz , 4040 Linz, Austria.
authors: Martin
authors: M
authors: Eibelhuber
authors: Institute of Physics, Montanuniversitaet Leoben , 8700 Leoben, Austria.
authors: Oskar
authors: O
authors: Paris