Growth and properties of semiconductor core/shell nanocrystals with InAs cores

Core/shell semiconductor nanocrystals with InAs cores were synthesized and characterized m-V semiconductor shells (InP and GaAs), and II-VI semiconduc tor shells (CdSe, ZnSe, and ZnS) were overgrown on InAs cores with various radii using a two step synthesis. Tn the first step cores were prepared, an d in the second step the shells were grown using high-temperature pyrolysis of organometallic precursors in a coordinating solvent. Core/shell growth was monitored by absorption and photoluminescence spectroscopy. The band ga p shifts to the red upon growth of InP or CdSe shells, while for ZnSe and Z nS shells that have larger band offsets with respect to InAs, the band gap energy is maintained. This behavior is reproduced by band gap energy calcul ations using a particle within a spherical box model. The photoluminescence quantum yield is quenched in InAs/InP core/shells but increases substantia lly up to 20% for InAs/CdSe and InAs/ZnSe core/shells. For InAs/ZnS core/sh ells the enhancement of the photoluminescence quantum yields is smaller, up to 8%. The core/shell nanocrystals were characterized using transmission e lectron microscopy, X-ray photoelectron spectroscopy, and powder X-ray diff raction. X-ray photoelectron spectroscopy provides evidence for shell growt h. The X-ray diffraction peaks shift and narrow upon shell growth, providin g evidence for an epitaxial growth mode. Simulations of the X-ray diffracti on patterns reproduce both effects, and show that there is one stacking fau lt present for very four to five layers in the core and core/shell nanocrys tals. The stability of InAs/CdSe and InAs/ZnSe core/shells against oxidatio n is substantially improved compared with the cores, and the photostability is significantly better compared with a typical near-TR laser dye IR140. C ore/shell nanocrystals with InAs cores are suggested:as a novel type of flu orophores covering the near-IR spectral range, with high emission quantum y ields and improved stability compared with traditional near-TR laser dyes.