. 2000. “
Core/shell semiconductor nanocrystals with InAs cores were synthesized and characterized. III−V semiconductor shells (InP and GaAs), and II−VI semiconductor shells (CdSe, ZnSe, and ZnS) were overgrown on InAs cores with various radii using a two step synthesis. In the first step cores were prepared, and 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 gap shifts to the red upon growth of InP or CdSe shells, while for ZnSe and ZnS shells that have larger band offsets with respect to InAs, the band gap energy is maintained. This behavior is reproduced by band gap energy calculations using a particle within a spherical box model. The photoluminescence quantum yield is quenched in InAs/InP core/shells but increases substantially up to 20% for InAs/CdSe and InAs/ZnSe core/shells. For InAs/ZnS core/shells the enhancement of the photoluminescence quantum yields is smaller, up to 8%. The core/shell nanocrystals were characterized using transmission electron microscopy, X-ray photoelectron spectroscopy, and powder X-ray diffraction. X-ray photoelectron spectroscopy provides evidence for shell growth. The X-ray diffraction peaks shift and narrow upon shell growth, providing evidence for an epitaxial growth mode. Simulations of the X-ray diffraction patterns reproduce both effects, and show that there is one stacking fault present for every four to five layers in the core and core/shell nanocrystals. The stability of InAs/CdSe and InAs/ZnSe core/shells against oxidation is substantially improved compared with the cores, and the photostability is significantly better compared with a typical near-IR laser dye IR140. Core/shell nanocrystals with InAs cores are suggested as a novel type of fluorophores covering the near-IR spectral range, with high emission quantum yields and improved stability compared with traditional near-IR laser dyes.
. 2000. “
We investigate the quantum size effects in the pressure-induced direct-to-indirect band gap transition in InP nanocrystals. Hydrostatic pressures of up to 13 GPa are applied to two different sizes of InP nanocrystal samples in a diamond anvil cell. The band gap pressure dependence and the nature of the emitting states are studied by photoluminescence (PL) and fluorescence line narrowing (FLN) techniques at 10 K. Pressure-dependent FLN spectra show that the nature of the emitting states at pressures up to 9 GPa is similar to that at ambient pressure, suggesting that no direct-to-indirect transition happens below 9 GPa. For both sizes, the PL peak energy exhibits a strong blueshift with rising pressure until approximately 9 to 10 GPa. Above this pressure, the PL peak position slightly shifts red. Beyond 12 GPa, the band gap emission intensity becomes extremely weak and trap emission dominates the PL spectra. As the pressure is released, both the luminescence intensity and the peak position recover in a fully reversible manner. The change in the sign of the band gap energy pressure dependence and the disappearance of the band edge luminescence indicate the pressure-induced direct-to-indirect band gap transition. Contrary to theoretical calculations, no substantial reduction of the transition pressure is observed in the nanocrystal cases compared to the bulk transition pressure.