CdSe/CdS quantum dots

Lin Y-W, Hsieh M-M, Liu C-P, and Chang H-T, (2005) Photoassisted Synthesis of CdSe and Core-Shell CdSe/ CdS Quantum Dots. Langmuir 21 728... 

Xu H, Wu J, Chen C-H, Zhang L, Yang K-L (2010) Detecting hydrogen sulfide by using transparent polymer with embedded CdSe/CdS quantum dots. Sens Actuators B 143 535-538 Yu JY, Chung SW, Heath JR (2000) Silicon nanowires preparation, device fabrication, and transport properties. J Phys... 

Chen, H., R. Li, L. Lin, G. Guo, and J. M. Lin. 2010. Determination of L-ascorbic acid in human serum by chemiluminescence based on hydrogen peroxide-sodium hydrogen carbonate-CdSe/CdS quantum dots system. Talanta 81 1688-1696. 

Other approaches already realized include the development of a simple and rapid method for spironolactone determination based on the quenching of the fluorescence of CdSe quantum dots by the analyte52 or the determination of anthracene with a detection limit of 1.6 x 10-8 M based on the quenching effect of this polycyclic aromatic hydrocarbon on water-soluble CdSe/ZnS quantum dots, prepared via a simple sonochemical procedure using (3-cyclodextrin ((3-CD) as surface coating agent.53... 

Figure 21.2 Chemical composition of the most commonly used quantum dots in biological applications, (a) CdSe quantum dots functionalized with mercaptoacetic acid, whose —SH bonds directly to the semiconductor, leaving the carboxylate group ftee to interact with aqueous solution, (b) CdSe quantum dots with a 1 to 2 nm thick layer of ZnS or CdS, functionalized with mercaptoacetic acid, (c) CdSe/ZnS quantum dots coated with polymers and the protein strepta-vidin. The overall nanocrystal size is a function of the surface coating and/or functionalization. (Repoduced with permission from V. H. Grassian, Nanoscience and Nanotechnology Environmental and Health Impacts. Copyright 2008 John Wiley Sons Inc.)...

A more difficult problem to address is that of the thermal quenching of luminescence. Rathei than recombining radiatively across the band gap, free excitons within a quantum dot are readily bounc to lattice phonons at temperatures exceeding lOO C. " An example of this phonon-driven effect car be found in a Sandia National Lab study on the thermal stability of CdS and CdSe quantum dots encapsulated in both epoxy and silicon as is routinely done for plication to a LED device. " In that study, the luminescence from CdS quantum dots decreased by 50% at 75C. Similar thermal quenching... 

Fig. 17 Fluorescent sensors based on displacement of fluorophores. (a) Chiral boronate 56 containing a fluorophore which is quenched upon displacement (b) CdSe/ZnS quantum dot (QD) modified with a spacer and a boronic acid unit bound to a fluorophore-containing P-cyclodextrin 57, which, upon interaction with the analyte, releases the fluorophOTe, thus pievtaiting energy transfer (ET) from QD (c) guest-induced phosphorescence of CD-l-hromonaphthalene complex...

Figure 6.16 Potential energy diagram showing energetic band positions of cadmium selenide (CdSe] and cadmium sulphide (CdS] quantum dots, semi-conducting SWCNTs (s-SWCNTS] " and the work functions (

Core/shell-type nanoparticles ovm ated with higher band inorganic materials exhibit high PL quantum yield compared with uncoated dots d K to elimination of surface non-radiative recombination defects. Such core/shell structures as CdSe/CdS [6] and CdSe ZnS [7] have been prepared from organometaHic precursors. 

Figure 17.2 (A) Absorption and photoluminescence spectra of CdSe quantum dots prepared from CdO, CdC03, and Cd(AcO)2 in the presence of different ligands. (B) Increase in the optical density (at 400 nm) of a CdSe quantum dot reaction mixture with time under reaction at 75 "C. Color pictures in the inset of B represent CdSe (a,b) andCdSe-ZnS (c) quantum...

Research on semiconductor nanoparticle technology by chemists, materials scientists, and physicists has already led to the fabrication of a number of devices. Initially, Alivisatos and co-workers developed an electroluminescence device from a dispersion of CdSe nanoparticles capped with a conducting polymer349 and then improved on this by replacing the polymer with a layer of CdS, producing a device with efficiency and lifetime increased by factors of 8 and 10, respectively. 0 Chemical synthetic methods for the assembly of nanocrystal composites, consisting of II-VI quantum dot polymer composite materials,351 represent one important step towards the fabrication of new functional devices that incorporate quantum dots. 

Huang, B., and Tomalia, D.A. (2005) Dendronization of gold and CdSe/cdS (core-shell) quantum dots with tomalia type, thiol core, functionalized poly(amidoamine) (PAMAM) dendrons. J. Lumin. Ill, 215-223. 

SILAR has been used for the synthesis of CdS/ZnS coatings for CdSe quantum dots. The precursor solutions were prepared by dissolving CdO, ZnO, and S in oleic acid and octadecane. The final coating consisted of three layers of CdS and three additional layers of ZnS. The photonic band structure of the photonic crystal had a modifying influence on the photoluminescence of the embedded quantum dots.90... 

There are two very broad, general conclusions resulting from the above review. The first is that MoS2-type nanoparticles are very different than other types of semiconductor nanoparticles. Nanoparticles of several different types of semiconductors, such as CdSe, CdS, and InP, have been extensively studied. Experimental and theoretical studies have elucidated much of their spectroscopy, photophysics, and dynamics. The results reviewed above are, in many places, in sharp contrast with those obtained on other types of quantum dots. This does not come as a surprise. The properties of the bulk semiconductor are reflected in those of the nanoparticle, and properties of layered semiconductors are vastly different from those of semiconductors having three-dimensional crystal structures. Although the electronic and spectroscopic properties of nanoparticles are strongly influenced by quantum confinement effects, the differences in the semiconductors cause there to be few generalizations about semiconductor quantum dots that can be made. 

Nanoparticles can be separated by molecular exclusion chromatography just as proteins are separated. Figure 26-15 shows the relation between measured size and retention time of CdSe quantum dots. These are particles containing 2 000 CdSe units in a dense, crystalline core capped by alkyl thiol (RS) groups on Cd and trialkylphosphine (R3P) groups on Se. 

Quantum dots used in liquid crystal systems (ranging in size from a few to tens of nanometers) are usually from the II-VI group, and include CdS, CdSe, and CdTe capped with trioctylphosphine/trioctylphosphine oxide (TOP/TOPO) [101-103], CdS as well as CdTe stabilized with thiols [104], and CdSe protected with amines [105]. 

Depending on the kind of synthesis, these quantum dots can be prepared or size separated into batches covering almost the entire visible spectral range from 400 to 750 nm with, in part, high photoluminescence quantum efficiencies (some stable in air [106], others not [107]). Weller et al. reported on a very efficient synthesis for hydrophilic, thiol-capped CdTe quantum dots [108,109], which can be transformed to lipophilic, alkanethiol-stabilized CdTe quantum dots using a place exchange reaction similar to that for metal nanoparticles described above [110]. A related strategy has also been successfully employed to produce hydrophobic or otherwise functionalized CdS [111] or CdSe quantum dots [112] (Fig. 1). 

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