Semiconductor nanoparticles clusters

However, the variety of composite materials to be elaborated by the method is still scarcely explored. For example bi- and multi-metal or semiconductor nanoparticles, included in different matrices (polymeric membranes, porous supports,. ..) have promising applications. New methods of cluster characterization at this extremely low size scale are developed and will improve their study. 

This approach of using a combination of RSe-TMS and Se(TMS)2 for cluster synthesis should allow for the tailored functionalization of semiconductor nanoparticles. Recently, the ability to functionalize the surface of Cu2Se clusters has been demonstrated. The synthesis of l,l -bis(trimethyl-silyl)ferrocene, and its consequent reaction with CuOAc and excess phosphine has produced the polynuclear copper selenide cluster [Cug Fe(77 -C5H4Se)2 4(PPh2Et)4] (51) with surface redox-active ferrocene units that are intimately coupled to the cluster core (Figure 34). ... 

Techniques for the preparation of metal cluster/nanoparticles can be classified into three primary categories condensed phase, gas phase, and vacuum methods. In condensed phase synthesis, metal and semiconductor nanoparticles are prepared by means of chemical synthesis, which is also known as wet chemical preparation. In gas phase synthesis, metal is vaporized, and the vaporized atoms are condensed in the presence or absence of an inert gas. In vacuum methods, the metal of interest is vaporized with high-energy Ar, Kr ions, or laser beams in a vacuum, and thus generated metal vapor is deposited on a support. 

Recent density-functional-based studies on the structural, electronic, and optical properties of CdS, CdSe, and CdS/CdSe semiconductor nanoparticles over a wide range, from clusters containing just a few atoms to nanoparticles with up to about... 

Reversible peaks at about h-0.09 and —0.7 V were attributed to the transfer of electrons from the HOMO and the LUMO to the electrode. The separation between these two peaks matches very well with the band gap obtained by spectroscopic methods. The reversibility of these peaks suggests that elemental semiconductor nanoparticles such as Si do not decompose on charge transfer. The peaks observed in the range -0.7 to 2.3 V are attributed to quantum double layer charging (140, 141), which is commonly observed in the case of monolayered protected clusters (MFCs). 

Density-based methods Wave function-based methods Some technical aspects Excitations in various systems Excitations in metal clusters Excitations in semiconductor nanoparticles Excitations in organic and biological systems Identification of structure Dynamics in excited states Conclusions... 

In this chapter we will review the recent developments in calculating optical excitations. Thereby, we will revise the key methods that are used to calculate excitation spectra in computational physics putting special emphasis on time-dependent density-functional theory. Moreover, we will discuss several recent applications of these methods to various systems, such as metal clusters, semiconductor nanoparticles, organic and biological molecules. Finally, it will be discussed, how calculated excitation spectra can help in revealing the structure of a specific system. 

This chapter is basically divided into a theoretical part and three sections on applications. We will therefore first review in section 2 several methods which are currently used for calculating the optical excitations and excitation spectra of various systems. Thereby, we will put our focus on time-dependent density-functional theory. Sections 3,4 and 5 review recent applications. Section 3 deals with the excitations in various systems, e.g. metal clusters, semiconductor nanoparticles, and organic or biological systems. Finally, we will discuss the latest findings in two more specific areas section 4 will show, how the calculation of excitation spectra can be used to identify a system s structure, especially applied to clusters and nanoparticles and in section 5 we will briefly introduce a newly proposed scheme for calculating dynamics of excited systems. Finally, in section 6 we conclude. 

The above described procedure is suitable for, e.g., metal clusters. Larger, and more stable systems, like wet-chemically produced semiconductor nanoparticles, can be investigated as well. Here, X-ray techniques are usually used in order to determine the particle s structure. 

Most clusters and nanoparticles studied with time-dependent methods are still of quite simple nature, e.g. metal clusters or semiconductor nanoparticles. These systems consist of only a few elements. Several metal clusters exhibit collective plasmon excitations, and semiconductor nanoparticles are known to be able to generate long-lived excitons. To show and explain these is the task of time-dependent methods that are employed to calculate their photo absorption spectra. 

In this presentation, different theoretical methods for circumventing these problems shall be discussed. They shall be illustrated through applications on various types of clusters. These include isolated metal clusters with one or two types of atoms, metal clusters deposited on a surface, nanostructured HAIO, semiconductor nanoparticles, and metaUocarbohedrenes. Special emphasis is put on the construction of descriptors that can be used in identifying general trends. 

Semiconductors are interesting materials first of aU because of their electronic properties. By modifying the material it is partly possible to vary these properties in a controllable way, and further possibilities are obtained by considering semiconductor nanoparticles. When the spatial extension of the clusters becomes comparable with that of an exdton, the electronic and optical properties of the semiconductor will depend markedly on the cluster size. 

Also semiconductor nanoparticles can be prepared in the presence of their polymeric matrix. Polymers with functional groups such as hydroxyl, carboxyl, or amines can form complexes with different metal salts. Subsequent treatment with, for example, H2S gas results in sulfide nanoparticles. Only recently Shen et al. used cadmium acrylate ionomers to form ion cluster, copolymerized with methyl methacrylate, and obtained highly transparent CdS/PMMA nanocomposites... 

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