Isolated Nanocrystals

The fluorescent properties of semiconductor nanocrystals have drawn wide attention because of their potential use as labels in fluorescence bio-assays [25, 26, 84, 168-170]. When compared to dyes currently in use, the emission from fluorescent nanocrystals is brighter and sharper. Further, the emission can be brought about by excitation over a broad range of wavelengths. It is therefore possible to excite nanocrystals of several different sizes simultaneously with a single source and obtain well resolved emission at different colours. In order that the nano- 

Several polymer/polyelectrolyte-nanocrystal hybrid devices have been fabricated seeking to exploit the electro and photoluminescent properties of such material [179-188]. Device fabrication in all these cases is by low-cost self-assembly based techniques. These devices utilize thin films of these hybrids obtained either by multilayer deposition or drop/spin casting methods. Thus, solar cells have been made from poly(2-hexylthiophene)-CdSe nanorod multilayers, lasers from drop cast films of CdSe-titania composites and an infrared emitter from multilayers 

8 nm respectively. The excitation was from a single source (mercury lamp with emission wavelength of 365 nm) (adapted from [187]). 

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The definition above is a particularly restrictive description of a nanocrystal, and necessarily limits die focus of diis brief review to studies of nanocrystals which are of relevance to chemical physics. Many nanoparticles, particularly oxides, prepared dirough die sol-gel niediod are not included in diis discussion as dieir internal stmcture is amorjihous and hydrated. Neverdieless, diey are important nanoniaterials several textbooks deal widi dieir syndiesis and properties [4, 5]. The material science community has also contributed to die general area of nanocrystals however, for most of dieir applications it is not necessary to prepare fully isolated nanocrystals widi well defined surface chemistry. A good discussion of die goals and progress can be found in references [6, 7, 8 and 9]. Finally, diere is a rich history in gas-phase chemical physics of die study of clusters and size-dependent evaluations of dieir behaviour. This topic is not addressed here, but covered instead in chapter C1.1, Clusters and nanoscale stmctures, in diis same volume. 

This is a good point to bring up briefly a property of very small crystals (often obtained in CD), which is dealt with in more detail in Chapter 9. The crystal size is, in most cases, much smaller than the size of any space charge layer that would be formed. This means that in an isolated nanocrystal, unless the doping level is very high (usually it is not, as attested to by the high resistivities more of-... 

The ability to synthesize lattices of nanociystals have led to explorations of their collective physical properties. Thus, it is observed in the case, of Co nanociystals (5.8 nm) that, accompanying lattice formation, the blocking temperature increases. 421 FePt alloy nanocrystals yield ferromagnetic assemblies for which the coerrivity is tunable by controlling the parameters such as Fe Pt ratio and the particle size.1431 The evolution of collective electronic states in CdSe nanocrystals have been followed by optical spectroscopic methods. Compared with isolated nanocrystals, those in the lattice exhibited... 

Several applications have been envisaged for nanocrystals, ranging from simple dyes to magnetic-resonance-imaging contrast agents [160], components of electronic circuitry [33, 161] and magnetic media [115], ingredients in catalyst and sensors, and so on. All the above applications seek to exploit the tunability provided by the size-dependent properties of the nanocrystals [4]. Before we dwell upon collective properties in mesostructures, the case of isolated nanocrystals is briefly discussed. 

This study led to the observation of a reversible Mott-Hubbard metal-insulator transition in the nanocrystal ensemble wherein the coulomb gap closes at a critical distance between the particles. Tunnelling spectroscopic measurements on Aims of 2.6 nm Ag nanocrystals capped with decanethiol reveal a coulomb blockade behavior attributable to isolated nanocrystals [203]. On the other hand, nanocrystals capped with hexane and pentane thiol exhibit characteristics of strong interparticle quantum mechanical exchange (see Figure 4.28). Similar behavior was observed... 

An idealized microstructure of such a composite is shown in Fig. 4. In reality, one might also evoke an interwoven bi-phase system as observed by Milligan et al. for ncAu/aSi (see Fig. 6 in [60]) or a bicontinuous two-phase system. However, as will be discussed below, we have not observed any such microstructure in our detailed high resolution transmission electron microscopic study but only isolated nanocrystals imbedded within the amorphous matrix [61,62]. In the interwoven bi-phase system ncAu/aSi, Milligan et al. have observed a signiflcant improvement of the toughness of the composite as compared to a pure ncAu [60]. 

Fig. 1.3 Acid hydrolysis breaks down disordered (amorphous) regions and isolates nanocrystals [34]...

Chemical methods have emerged to be indispensable for synthesizing nanocrystals of various t q)es of materials. These methods are generally carried out under mild conditions and are relatively straightforward. Nanodimensional materials in the form of embedded solids, liquids, and foams have also been prepared by chemical means and such materials have been in use for some time. In the presentation that follows, we focus on the means of producing isolated nanocrystals dispersible in solvents (sols). There are several reviews in the literature focusing on the synthesis of nanocrystals [141-143]. 

The absorption cross section for an isolated nanocrystal is obtained from the oscillator strengths as... 

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