CdSe quantum well

The room-temperature fluorescence quantum yield decreases from about 10% for pure CdS to 5% for particles with one monolayer of HgS, to 4% and 3% for particles with two and three monolayers of HgS between the CdS segments, respectively. A feature similar to this has been observed by Zajicek et al. [59] for the photoluminescence from ultrathin ZnSe/CdSe quantum wells. By these authors the reduction of the photoluminescence intensity is attributed to the generation of misfit dislocations when the thickness of the CdSe layers exceeds a critical thickness of 4 + 1 monolayers. In the current status of the investigations it is not yet clear whether this... 

Chemists have synthesized a spectacular array of submicron- and nano-particles with well-defined size and atomic structure and very special properties. Examples include CdSe quantum dots and novel spheres and rods. Transport enters the picture via fundamental studies of the physical processes that affect the synthesis, which must be understood for even modest scale-up from the milligram level. Likewise, processes for assembling fascinating face-centered-cubic crystals or ordered multilayers must concentrate on organizing the particles via flow, diffusion, or action of external fields. Near-perfection is possible but requires careful understanding and control of the forces and the rates. 

A modified SILAR system has been used to grow CdSe in CdS/CdSe core shell semiconductor nanocrystals.12 A cadmium precursor solution, with CdO dissolved with oleic acid in octadecane, was injected onto the substrate, and the Se solution (Se powder dissolved with tributylphosphine in octadecane) was similarly injected. The temperature of the reaction solution was 185 °C. A CdS outer layer in the CdS/CdSe/CdS colloidal quantum wells was deposited by alternating injections of cadmium and sulfur both in octadecane solutions at 230-240 °C. These structures showed high PL quantum yields (20-40%), relatively narrow emission bands, and tunable emission colors from about 520 to 650 nm depending on the number of CdSe monolayers. 

Battaglia, D. Li, J. J. Wang, Y. Peng, X. 2003. Colloidal two-dimensional systems CdSe quantum shells and wells. Angew. Chem. Int. Ed. 42 5035-5039. 

It was really only a matter of time until researchers in the field started doping blue phases with quasi-spherical nanoparticles. This area is very much in its infancy, but the few recent reports already show some promising results. Yoshida et al., for example, reported on an expansion of the temperature range of cholesteric blue phases from 0.5 to 5°C by doping blue phases with gold nanoparticles (average diameter of 3.7nm) as well as a decrease in the clearing point of approximately 13°C [427]. A similar effect was also observed by Kutnjak et al. for CdSe quantum dots simultaneously capped with oleyl amine and TOP (diameter of the core 3.5 nm) in CE8 (Merck) and CE6 (BDH). The authors found that particularly blue phase III was stabilized in these mixtures, blue phase II destabilized, and... 

Table. Values of individual components Xi and their relative amplitudes Ai as well as mean life times for the non-exponential decays of CdSe quantum dots for various molar ratios x=Couinonc/CQD.

The relative solubility of inorganic salts can be used to prepare more complex structures by such methods and examples indude CdS/ZnS [24], CdSe/AgS [25] HgS/CdS [26], PbS/CdS [27, 28], CdS/HgS [29], ZnS/CdSe [30] and ZnSe/CdSe [31] particles. The main constraints on the production of such structures involve the relative solubility of the solids and lattice mismatches between the phases. The preparation of quantum dot quantum well systems such as CdS/HgS/CdS [32, 33], has also been reported, in which a HgS quantum well of 1-3 monolayers is capped by 1-5 monolayers of CdS. The synthesis grows less soluble HgS on CdS (5.2 nm) by ion-replacement. The solubility products of CdS and HgS are 5 X 10 and 1.6 x 10 respectively. The authors reported fluorescence measurements in which the band edge emission for CdS/HgS/CdS is shifted to lower energy values with increasing thickness of the HgS well [33]. 

FlG. 13.8. Schematic and optical properties of the hybrid quantum-well/nanocrystal structure, (a). The structure consists of an InGaN/GaN quantum-well heterostructure with a monolayer of TOPO/TOP-capped CdSe/ZnS core/shell nanocrystals on top of it. Electron-hole pairs in the quantum well can experience nonradiative resonant transfer into nanocrystals. The nanocrystals excited by energy transfer produce emission with a wavelength determined by the nanocrystal size. (b). The emission of the quantum well (blue) spectrally overlaps with the absorption of the nanocrystals (green). For CdSe nanocrystals with 1.9 nm radius, the emission wavelength is around 575 nm (red) (from (13)). 

The second system is based on a quantum dot quantum well (QDQW) model [13,14]. Recently it was shown by Battaglia et al. [14] that it is possible to decouple the QD from the QW by introducing a high band gap barrier between the two. By synthesizing CdSe/ZnS/CdSe css systems it was possible to obtain two different emissions from the core and the outer shell [15]. Inspired by these systems, we synthesized yellow-orange emitting CdSe cores coated with three... 

This section will consider in greater detail specific examples of particular types of nanomaterials interacting with different media in the environment. The fate and transport of carbon-based nanomaterials, including carbon nanotubes and fuUerenes, in aqueous environments and the properties of commercial oxide nanoparticles that affect their removal in water will be discussed. Nanomaterial exposure to soils and porous media, focusing on transport and retention, as well as environmental interactions of cadmium selenide (CdSe) quantum dots with biofilms will be presented. These specific examples provide an idea of the types of environmental interactions that must be considered, and illustrate that environmental impacts of nanomaterials cannot be generalized, but rather, are dependent on properties of the material in question and the environment to which it is exposed or transported. 

The diffusion coefficient of Cd in CdSe-ZnSe single quantum well structures... 

Nanocrystalline particulate films, which exhibit pronounced quantum size effects in three dimensions, are of great interest due to applications in solar cell (108-112) and sensor (57, 113-115) applications. They exhibit novel properties due to not only the SQE manifested by individual nanoparticles but also the total surface area. Unlike MBE and MOCVD methods used to prepare quantum well electrodes, these electrodes can be prepared by conventional chemical routes described in Section 9.5.2.2. For example, II-VI semiconductor particulate films were prepared by using low concentrations of precursors and by controlling the temperature of the deposition bath. Nodes demonstrated the SQE for CdSe thin films deposited by an electroless method (98). The blue shift in the spectra of CdSe films has been demonstrated to be a function of bath temperature. As described in Section 9.5.2.1, electrodeposition of semiconductors in non-aqueous solvents leads to the formation of size-quantized semiconductor particles. On a single-crystal substrate, electrodeposition methods result in epitaxial growth (116, 117), and danonstrate quantum well properties. 

Well-defined CdS/CdSe superlattices have been formed by means of ECALE [74]. In these structures the CdS component - and not CdSe - suffered from substantial crystallographic strain as was evidenced by surface-enhanced Raman spectroscopy (SERS) - a valuable tool for characterizing the superlattice phonons in electrochemical or other ambient environments. Torimoto et al. reported quantum confinement in superlattices of ZnS/CdS grown by ECALE [75]. 

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