Nanopartides semiconductor

One of the most interesting developments of the past decade has been the merging of the colloidal preparation of nanostructures with synthetic approaches that stem from advanced inorganic chemical routes (see, e.g.. Section 3.2.3), both of which lead to very similar nanoparticles. Both routes yield species that crystallize into superstructures with sizes that may reach several hundreds of microns. When singlecrystal X-ray diffraction (XRD) is performed on these macroscopic crystals, the data obtained yield the exact positions of every single atom in the superstructures. 

Based on the studies of Strickler [1] and Dance and coworkers [2-5] in 1993, Herron et al. reported on the crystallization of Cd32Si4(SPh)36]-4DMF (Ph = C6H5 DMF = dimethyl formamide) as pale yellow cubes [6]. The structure unraveled by single-crystal XRD studies consisted of an 82-atom CdS core that was a piece of the 

Together with the Banin group, the authors of Refs [10-12] carried out optical spectroscopy investigations on some of the cluster molecules obtained ] 14-16]. These materials were treated as the molecular limit of the bulk semiconductor CdSe, and issues such as oscillator strength, steady-state and time-resolved photoluminescence and photoluminescence excitation were addressed. In addition, emission-mediating vibrational modes were detected, and photobleaching effects observed. 

In an investigation of partide-particle interaction in semiconductor nanociystal assemblies, Dollefeld et al. [17] examined (among other structures) the crystalline superstructure of [Cdi7S4(SCH2CH20H)26] (as described in Ref [7]). In the UV-visible absorption spectrum of this compound in solution, the first electronic transition was shifted to higher energies by about 150 meV as compared to the reflection spectrum of the crystalline material. In addition, the transition was broadened from a full-width at half maximum of approximately 390 meV to about 520 meV. Most likely, a complete description of the interaction of semiconductor nanocrystals in crystalline superstructures would include both electronic and dipole-dipole interactions. The electronic coupling may be introduced by covalent 

Yet another development of remarkable nanostmctured materials yields superlattices of nanosized objects. As there is no dear distinction between molecular crystals and superlattices formed from nanopartides, at this point reference will be made to structures composed of very similar (but most likely not exactly identical) nanopartides, namely colloidal partides in the size range 2 to 10 nm. Two excellent reviews by leading experts in the field were produced in 1998 and 2000 [19, 20], the titles of which contained the terms nanocrystal superlattices and close-packed nanociystal assemblies. These are in line with the above-outlined delimitation, although Collier et al. have also reported on molecular crystals (as above). The two reviews comprised approximately 100 pages with some 300 references, and summarized the state of the art at that time in exemplary fashion. The topics induded preparative aspects of the formation of monodisperse nanopartides of various compositions including metals, the superlattice formation itself with some theoretical background, covalent linking of nanocrystals (see below), and an appropriate description of the physical properties and characterization of the nanocrystal superlattices. 

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Molecular self-organization in solution depends critically on molecular structural features and on concentration. Molecular self-organization or aggregation in solution occurs at the critical saturation concentration when the solvency of the medium is reduced. This can be achieved by solvent evaporation, reduced temperature, addition of a nonsolvent, or a combination of all these factors. Solvato-chromism and thermochromism of conjugated polymers such as regioregular polythiophenes are two illustrative examples, respectively, of solubility and temperature effects [43-45]. It should therefore be possible to use these solution phenomena to pre-establish desirable molecular organization in the semiconductor materials before deposition. Our studies of the molecular self-assembly behavior of PQT-12, which leads to the preparation of structurally ordered semiconductor nanopartides [46], will be described. These PQT-12 nanopartides have consistently provided excellent FETcharacteristics for solution-processed OTFTs, irrespective of deposition methods. 

Perhaps the biggest advantage of a water-based synthesis procedure is that bioconjugation of the gold and other metal/semiconductor nanopartides with DNA [32], enzymes [62], antibodies [63] etc. may be easily accomplished. 

Ionic interactions limit the concentration of metal/semiconductor nanopartides in the aqueous phase to very dilute levels, a big drawback in biological labeling of the nanopartides. 

Given the importance of oxide materials, it is only natural to ask whether recent understanding regarding the synthesis, properties and applications of capped metal and semiconductor nanopartides can be extended to oxides. 

The photochemical properties of various nanoassemblies discussed in this chapter highlight the ways in which the metal and semiconductor nanopartides interact with light. Furthermore, one can fine tune these responses by subjecting nano-stmctures to an externally applied electrochemical bias. The ability to functionalize these nanopartides with photoactive molecules has opened new avenues to utilize these nanoassemblies in light energy conversion and catalytic applications. By suitably modulating the fluorescence of the surface bound fluorophore these... 

It should be mentioned at this point that not all aspects of the world of nanoparticles can be considered in a single volume. For instance, the rapidly developing field of nanorods and nanowires has again not been considered, as these species are indeed worthy of their own monographs. The terminus Nanoparticles, as in the First Edition, is restricted to metal and semiconductor species. Numerous other materials exist as nanopartides, while nonmetallic and oxidic nanopartides exist and exhibit interesting properties, especially with respect to their applications. Nevertheless, from a scientific point of view, metal and semiconductor nanopartides play perhaps the most interesting role, at least from the point of view of the Editor. 

During the past two decades, the synthesis or preparation of II-VI semiconductor nanopartides has experienced an enormous development, to the point where the pubhshed material related to the topic has become virtually unmanageable. Nevertheless, the aim of this chapter is to provide a chronological overview of some of the major historical lines in this area, starting with the earliest studies with CdS nanocrystals prepared in aqueous solution. At several points in the story - mostly when successful preparations are first described - the chapter branches into evolving sub-fields, leading to Sections 3.2.1.2 and 3.2.I.3. The remainder of the review then relates to matters distinct from these preparational approaches. More complicated nanoheterostructures, in which two compounds are involved in the build-up of spherically layered particles, are detailed in Sections 3.2.1.4 and 3.2.1.5. 

The interplay between nanopartides and biological systems is of spedal relevance for semiconductor nanopartides, known simply also as quantum dots (QDs). In recent years, these have emerged as ideal systems for molecular sensors and biosensors, based largely on their sizewide variety of chemical functionalities with which QDs can be equipped that makes them ideal partners for different biosystems. In contrast to former passive optical labds, specifically functionalized QDs can operate as optical labds so as to observe the dynamics of biocatalytic transformations and conformational transitions of proteins. This development will surely open a wide variety of doors in modem nanobiotechnology. 

Semiconductor nanopartides are of particular importance due to their optical properties. They act as quantum dots and can be applied as components in optically active materials, as optically active labels for biomolecules and cells and as components in miniaturized optoelectronic devices [16]. Compound semiconductor nanopartides can be prepared by liquid-phase processes. Several groups have investigated the preparation of CdSe partides. The fluorescence emission of these particles is strongly dependent on the particle size. The realization of well[Pg.783]

The specific character of properties, demonstrated by nanoccmiposites is determined by the small size (units of nanometers) of filler partides, comparable with the wavelength of electron, which leads to the so called quantum size effects and the essential ratio of surface to volume in such systems, which increases the role of particle surface and interfaces between particle and polymer media (e.g., in a SO A CdS partide, about 15% of the atoms are on the surface). The latter fact is the reason for the higher chemical activity of nanoparticles and the increase in the role of such surface excitations as surface plasmons in small metal particles and spedfic surface phonon modes both as the increadng role surface states, espedally surface traps in semiconductor nanopartides. 

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