Nanoparticulate semiconductors

In nanoparticulate semiconductors other than Ti02, quantum confinement may become important. 

Finally we address the emulsion layers that contain light-sensitive silver halide. Some of these other layers, as discussed above, contain particulates such as matte beads in overcoats, nanoparticulate semiconductors in antistat layers, copolymeric latexes in subbing layers, etc. Such layers are intrinsically composite multiphase layers, and this is also the case for the light-sensitive emulsion layers. In addition to microcrystals or nanocrystals of silver halide, present to capture light and to form developable... 

Pant and Levinger have measured the solvation dynamics of water at the surface of semiconductor nanoparticles [48,49]. In this work, nanoparticulate Zr02 was used as a model for the Ti02 used in dye-sensitized solar photochemical cells. Here, the solvation dynamics for H2O and D2O at the nanoparticle surface are as fast or faster than bulk water motion. This is interpreted as evidence for reduced hydrogen bonding at the particle interface. 

W/o microemulsion method has been successful for preparing QDs and other semiconductor nanoparticulates. The preparation and properties of these particles have been extensively reviewed [114,186]. 

The 20 A diameter monodisperse crystallites isolated Ifom S. pombe and C. glabrata have been calculated to contain 85 CdS pairs in the lattice coated with approximately 30y-EC peptides (Figure 10). The particle size was determined using transmission electron microscopy and powder X-ray analysis. The analysis of the X-ray diffraction patterns was inconclusive in discriminating between a four-coordinate zinc blende and six-coordinate rock salt lattice structure. The presence of a short coherence length (8 A) can be attributed either to internal disorder or to deviation from pure crystallinity. The absorption red edge of the crystallites is located at about 320 inn. The other properties characteristic of semiconductor nanoparticulates such as luminescence with emission near 460 mn, and electron transfer to methyl viologen... 

On the basis of the above discussion, in its most simple description photocatalysis implies a catalysed process preceded by absorption of a photon by a material acting as the catalyst. Where the photocatalyst is a semiconductor nanoparticulate system, e.g. TiOi, which is the material treated in most detail in this chapter, absorption of photons of energy greater than 3.2 eV (for anatase 3.0 eV for the rutile polymorph) leads to formation of conduction-band electrons and valence-band holes, which subsequently diffnse to the particle surface in competition with bnlk recombination. 

Importantly, physical and chemical properties of nanoparticulate films are markedly different from those of the bulk materials. For example, magnetic nanoparticles can be prepared where only one magnetic domain is present, so that the rotation or alignment of the whole particle implies the rotation or alignment of the magnetic moment [53]. Semiconductor nanoparticles possess strong nonlinear optical properties due to increased oscillator strength within excitonic transitions [54-56]. Electrooptical shifts can be induced in metal particles because the surface plasmon band position depends on the free electron concentration electron injection can be used to modulate the peak position [57]. 

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