Jinju Zheng

Shell-dependent electroluminescence from colloidal CdSe quantum dots in multilayer light-emitting diodes

We report electroluminescence (EL) of colloidal CdSe/CdS, CdSe/ZnS, and CdSe/CdS/CdZnS/ZnS core/shell quantum dots (QDs) in multilayer light-emitting diodes (LEDs) fabricated by spin coating a near monolayer of the core/shell QDs on cross-linkable hole transporting layers. It is found that CdSe/CdS QD-LEDs exhibit a faster decrease in EL quantum efficiency (∼2% at a brightness of 100 cd/m2) with increasing current density and lower maximum brightness than those of CdSe/ZnS QD-LEDs. A more significant redshift and spectral broadening of the EL observed in CdSe core/shell QDs with a CdS or CdS/CdZnS/ZnS shell than with a ZnS shell indicate that the electron wave function can penetrate into the shell under electric field. The difference in device performance and EL spectra results from conduction band offsets between the CdSe cores and CdS or ZnS shells, suggesting the existence of the exciton ionization in the QD-LEDs.

Water soluble CdS nanoparticles with controllable size prepared via femtosecond laser ablation

Small water soluble CdS nanocrystals (NCs) with excellent colloidal stability were prepared by femtosecond laser ablation of a bulk CdS target without using any surfactant. CdS NCs of different sizes ranging from 2.0 to 8.6 nm can be produced by simply adjusting the laser fluence at low values, which are just above the ablation threshold. The obtained CdS NCs show a relatively narrow size distribution, which gradually narrows as the laser fluence decreases, as can be seen in the transmission electronic microscopic images. UV-vis optical absorption, photoluminescence, and fluorescence lifetime were also used to characterize the NCs. These as-prepared small crystals show excellent water solubility and have good colloidal stability. Their size is easily controlled as well. Together, the above-mentioned properties make these NCs of interest for luminescence labeling and sensing applications.