Luminescent semiconductor nanocrystals

A dielectric oxide layer such as silica is useful as shell material because of the stability it lends to the core and its optical transparency. The thickness and porosity of the shell are readily controlled. A dense shell also permits encapsulation of toxic luminescent semiconductor nanoparticles. The classic methods of Stober and Her for solution deposition of silica are adaptable for coating of nanocrystals with silica shells [864,865]. These methods rely on the pH and the concentration of the solution to control the rate of deposition. The natural affinity of silica to oxidic layers has been exploited to obtain silica coating on a family of iron oxide nanoparticles including hematite and magnetite [866-870]. The procedures are mostly adaptations of the Stober process. Oxide particles such as boehmite can also be coated with silica [871]. Such a deposition process is not readily extendable to grow shell layers on metals. The most successful method for silica encapsulation of metal nanoparticles is that due to Mulvaney and coworkers [872—875]. In this method, the smface of the nanoparticles is functionalized with aminopropyltrimethylsilane, a bifunctional molecule with a pendant silane group which is available for condensation of silica. The next step involves the slow deposition of silica in water followed by the fast deposition of silica in ethanol. Changes in the optical properties of metal nanoparticles with silica shells of different thicknesses were studied systematically [873 75]. This procedure was also extended to coat CdS and other luminescent semiconductor nanocrystals [542,876-879]. 

Maenosono, S., Dushkin, C. D., Saita, S. and Yamaguchi, Y. (2000) Optical memory media based on excitation-time dependent luminescence from a thin film of semiconductor nanocrystals. Jpn. J. Appl. Phys., 39, 4006- 12. 

Colloidal CdS particles 2-7 nm in diameter exhibit a blue shift in their absorption and luminescence characteristics due to quantum confinement effects [45,46]. It is known that particle size has a pronounced effect on semiconductor spectral properties when their size becomes comparable with that of an exciton. This so called quantum size effect occurs when R < as (R = particle radius, ub = Bohr radius see Chapter 4, coinciding with a gradual change in the energy bands of a semiconductor into a set of discrete electronic levels. The observation of a discrete excitonic transition in the absorption and luminescence spectra of such particles, so called Q-particles, requires samples of very narrow size distribution and well-defined crystal structure [47,48]. Semiconductor nanocrystals, or... 

Several examples have been reported recently of solution-processed multilayer electroluminescence devices incorporating semiconductor nanocrystals as the active recombination centers (16-18, 164). Recently, attention has also turned to hybrid electroluminescent devices involving transition metal-doped nanocrystals (104, 165-167). Although many challenges remain, including more specific exploitation of the dopants in many cases, the devices demonstrated to date represent a new direction in application of doped semiconductor nanocrystals made possible by the compatibility of these luminescent nanocrystals with solution processing methodologies. 

In the past decade, lanthanide ions doped in nanocrystalline semiconductors have been the subject of numerous investigations. Although quantum size effects are not expected on lanthanide energy level structures, influence of quantum confinement in semiconductor on the luminescence properties of the lanthanides is expected. One of the advantages of lanthanide-doped semiconductor nanocrystals is that the lanthanide luminescence can be efficiently sen-... 

Generally, quantum size effects are not expected in lanthanide-doped nanoinsulators such as oxides since the Bohr radius of the exciton in insulating oxides, like Y2O3 and Gd2C>3, is very small. By contrast, the exciton Bohr radius of semiconductors is larger (e.g., 2.5 nm for CdS) resulting in pronounced quantum confinement effects for nanoparticles of about 2.5 nm or smaller (Bol et al., 2002). Therefore, a possible influence of quantum size effects on the luminescence properties of lanthanide ions is expected in semiconductor nanocrystals. 

Although Bhargava s mistakes on the shortening of TM lifetime or lanthanide-doped ZnS have been pointed out by other researchers, many scientists still expect that lanthanide-doped II-VI semiconductor nanocrystals may form a new class of luminescent materials. Numerous papers on the luminescence of II-VI semiconductor nanocrystals doped with TM or lanthanide ions have appeared in an effort to achieve high efficient luminescence via ET from II-VI host to lanthanide ions. 

In a representative work, Chen and Rosenzweig34 were able to alter the selectivity of CdS nanoparticles to respond either to Zn2+ or Cu2+ simply by changing the capping layer. They showed that, while polyphosphate-capped CdS QDs responded to almost all mono- and divalent metal cations (thus showing no ion selectivity), luminescence emission from thioglycerol-capped CdS QDs was quenched only by Cu2+ and Fe3+, but was not affected by other ions at similar concentrations. On the other hand, the luminescence emission of L-cysteine-capped CdS quantum dots was enhanced in the presence of zinc ions, but was not affected by cations like Cu2+, Ca2+, and Mg2+. Using this set of QD probes, the authors described the selective detection of zinc and copper in physiological buffered samples, with detection limits of 0.8 pM and 0.1 p,M for Zn2+ and Cu2+, respectively. This was claimed to be the first use of semiconductor nanocrystals as ion probes in aqueous samples. 

Nirmal M, Brus LE. Luminescence photophysics in semiconductor nanocrystals. Acc Chem Res 1999 32 404-14. 

LUMINESCENCE PROPERTIES OF DOUBLE BAND-EMITTING SEMICONDUCTOR NANOCRYSTALS CAPPED WITH DIFFERENT THIOLS... 

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