Jason Heine

Evidence of photo- and electrodarkening of (CdSe)ZnS quantum dot composites

We present a study of the kinetics of photoluminescence (PL) and cathodoluminescence (CL) degradation of semiconductor quantum dot composites, formed by highly luminescent (CdSe)ZnS core-shell nanocrystals embedded in a ZnS matrix. The photoluminescence and cathodoluminescence spectra indicate that both emissions originate from the same near band-edge state of the nanocrystals. We observe a strong decrease in the PL and CL intensities with time. Photoluminescence experiments carried out at high laser fluences (0.5–10 mJ/cm2 per pulse) show that the PL intensity decay with time depends on the size of the nanocrystals and the nature of the surrounding matrix. For instance, close-packed films showed a much slower decay than composite films. The cathodoluminescence intensity degradation is enhanced at lower temperatures. Partial recoveries of the CL signal have been achieved after thermal annealing at temperatures around 120 °C, which indicates that activation of trapped carriers can be induced by thermal sti...

Synthesis of CdSe quantum dot–ZnS matrix thin films via electrospray organometallic chemical vapor deposition

Abstract A modified organometallic chemical vapor deposition technique is used to incorporate luminescing II–VI nanocrystals (NCs) into ZnS thin films. The NCs are synthesized in organic solution and consist of a CdSe core and ZnS shell. The ZnS matrix is deposited by OMCVD from diethyl zinc and hydrogen sulfide while the NCs are delivered to the film surface via electrospray. Varying the size of the CdSe NC core enables tunable emission from the blue to the red. The ZnS shell provides electronic and chemical passivation of the CdSe core improving its luminescence and thermal stability. The thin films exhibit room temperature photoluminescence (PL) and cathodoluminescence (CL) dominated by emission from the NCs. PL quantum efficiencies greater than 10% have been achieved. The dependence of thin film luminescence and microstructure on deposition temperature is studied using optical photoluminescence and X-ray diffraction. Particular emphasis is placed on the relationship between the host ZnS structure and the optical properties of the NC–ZnS composite thin film.

Cathodoluminescence of CdSe/ZnS Quantum Dot Composites

We report cathodoluminescence and photoluminescence spectra originating from ZnS overcoated CdSe nanocrystals of 33 and 42 A diameter imbedded in a ZnS matrix. The thin film quantum dot composites were synthesized by electrospray organometallic chemical vapor deposition. The cathodoluminescence intensity depends on the crystallinity of the ZnS matrix and on the voltage and current density applied. Electron beam irradiation caused a decrease of the luminescence which may be explained by electron ionization of the quantum dots. Quenching of the cathodoluminescence at low temperatures is attributed to a more efficient trapping of the ionized electrons in deep traps inside the ZnS matrix.