Self-surface passivation of CdX (X = Se, Te) quantum dots.

A small portion of a reaction mixture including unpurified CdX (X = Se or Te) quantum dots (QDs), in which unreacted Cd and Se ions were left together with coordinating solvents, was dropped into an organic solvent. The CdX QDs in this organic solution showed enhancement of photoluminescence (PL) efficiency, growth of particles, and focusing of size distribution for more than 10 h at room temperature (RT, -23 degrees C). These effects were attributed to passivation of QDs' surface by Cd and X ions present in the solution. No external energy source was used for these achievements; therefore, the process is termed as self-surface passivation. The self-surface passivation was reproduced using purified CdX QDs with additional Cd and X ions in an organic solvent. The self-surface passivation method was applied to RT-synthesized CdSe QDs, which is characterized by a broad PL spectrum (fwhm - 150 nm) for monodispersed QDs, to modify their emission characteristics. On self-surface passivation, the broad PL spectrum was narrowed (fwhm - 35 nm) and the QDs were grown. The X-ray diffraction measurements of RT-synthesized CdSe QDs and that subsequently aged in 1-butanol showed that crystallinity of the samples was improved on aging.

Role of free cadmium and selenium ions in the potential mechanism for the enhancement of photoluminescence of CdSe quantum dots under ultraviolet irradiation.

The present study describes an enhancement of the photoluminescence of CdSe quantum dots under long-term ultraviolet irradiation in organic solvents. The photoenhancement effect followed multiexponential kinetics and was found to depend on several factors: intensity of ultraviolet light, polarity of the solvent, presence of capping agents on the nanocrystal surface, and presence of free Cd and Se ions in the solution. High intensity ultraviolet irradiation provoked a rapid enhancement of the photoluminescence of CdSe nanocrystals, reaching the maximum with subsequent photoluminescence decay. Low-intensity ultraviolet irradiation provoked a comparatively slow enhancement of the photoluminescence of CdSe nanocrystals, reaching saturation after 5-6 hours of irradiation in organic solvents (butanol and chloroform). The photoenhancement effect was reversible or irreversible depending on the additional ingredients. The role of free Cd and Se in these processes was clarified. The results are discussed in the context of ultraviolet induced liberation of free Cd and Se ions from the nanocrystal surface and their hypothetical reversible deposition with trapping of the surface holes and influencing the efficiency of radiative versus nonradiative exciton decay during the enhancement of photoluminescence.

Fabrication of quantum dot-lectin conjugates as novel fluorescent probes for microscopic and flow cytometric identification of leukemia cells from normal lymphocytes.

The present study describes a synthesis of QD-lectin conjugates and their application for identification of leukaemia cells from normal lymphocytes using fluorescent confocal microscopy and flow cytometry. The results are compared with commercially available FITC-lectin.

Subsecond luminescence intensity fluctuations of single CdSe quantum dots.

Photoluminescence (PL) intermittency characteristics are examined for single quantum dots (QDs) in a CdSe QD sample synthesized at a slow rate at 75 degrees C. Although the PL quantum efficiency was relatively low ( approximately 0.25), we noticed that the PL intensity of single CdSe QDs fluctuated on a subsecond time scale with short-lived "on" and "off" states. The subsecond PL intensity fluctuations of CdSe QDs are different from "on" and "off" PL blinking generally observed for QDs fluctuating on a millisecond to minute time scale. We characterized single QDs by identifying polarized excitations, topographic imaging using atomic force microscopy (AFM), and transmission electron microscopy (TEM). From analysis of the PL intensity trajectories from >100 single CdSe QDs, the average intermittency time was 213 ms. From the PL quantum efficiency, slow growth of QDs, intensity trajectory analyses, and previous reports relating surface trap states and PL properties of QDs, we attribute the subsecond PL intensity fluctuations of single CdSe QDs and short-lived "on" and "off" states to a high-density distribution of homogeneous surface trap states.

Enhancement of the photoluminescence of CdSe quantum dots during long-term UV-irradiation: privilege or fault in life science research?

The present study describes an impressive enhancement of the photoluminescence (PL) intensity of low-temperature synthesized CdSe nanocrystals (75 degrees C) during long-term UV-irradiation. The integrated PL-intensity of CdSe core and CdSe/ZnS core/shell nanocrystals, dispersed in chloroform, enhanced about 3 and 6 times, respectively, during 9 h exposure to UV-light, without any significant changes in the characteristic absorbance spectra and shifting of PL-spectra. After termination of the irradiation a comparatively slow photobleaching was detected with tau(1/2) = 6 h for CdSe core and tau(1/2) = 14 h for CdSe/ZnS core/shell nanocrystals. The most impressive was the effect of UV-irradiation on the photoluminescence of water-soluble CdSe nanocrystals. The integrated PL-intensity enhanced about 10 times during 11 h exposure to UV-light and the improved PL-intensity was preserved during 3 days after termination of the irradiation without any significant photobleaching. The results are discussed in the context of application of CdSe nanocrystals as novel fluorophores in life science experiments.