Semiconductor nanoparticles excitation

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... 

Kinetic studies of photoreactions on semiconductor nanoparticles are important for both science and practice. Of scientific interest are the so-called quantum size effects, which are most pronounced on these particles shifting the edge of adsorption band, participation of hot electrons in the reactions and recombination, dependence of the quantum yield of luminescence and reactions on the excitation wavelength, etc. In one way or another all these phenomena affect the features of photocatalytic reactions. At present photocatalysis on semiconductors is widely used for practical purposes, mainly for the removal of organic contamination from water and air. The most efficient commercial semiconductor photocatalysts (mainly the TiC>2 photocatalysts) have primary particles of size 10-20 nm, i.e., they consist of nanoparticles. Results of studying the photoprocesses on semiconductor particles (even of different nature) are used to explain the regularities of photocatalytic processes. This indicates the practical significance of these processes. 

The basic theoretical framework for describing electron transfer in bulk solid/Uquid interfaces was developed in the 1960s (Marcus, 1965 Gerischer, 1970 Levich, 1970). Fundamentally, photoinduced electron injection from the molecular excited state to a semiconductor nanoparticle can be described as electron transfer from a discrete and localised molecular state to a continuum of delocalised k states in the semiconductor. As shown in Fig. 11.5, the reactant state corresponds to the electron in the molecular excited state and the product states correspond to the oxidised molecule and the transferred electron in the semiconductor conduction band. There is a continuous manifold of product states, corresponding to the injected electron at different electronic levels in the semiconductor. 

Figure 13.16 Schematic illustration of the light-induced charge excitation (creation of an exciton) in a semiconductor nanoparticle and subsequent charge transfer onto an SWNT, followed by electron transport along the SWNT. (Reproduced with permission with L. Hu et al., Adv. Mater. 2008, 20, 939.)...

Semiconductor nanoparticles exhibit size-dependent unique optical and electronic properties that are different from their bulk counterpart due to quantum confinement. Bulk semiconductor crystal is considered as one large molecule, and electronic excitation of semiconductor crystals generates an electron-hole pair. The size of the delocalization area of this electron-hole pair is generally many times larger than the lattice constant. Decrease in the size of a semiconductor crystal down to a size comparable with the delocalization area of the electron-hole pair or to that of the Bohr excitonic radius of those materials modifies the electronic structure of the nanocrystals. When the particle radius decreases below the Bohr excitonic radius, there is widening in the energy band gap, which results in a blue shift in the excitonic absorption band of a semiconductor crystal. For example, in CdS semiconductor material, the blue shift of the excitonic absorption band is observed to begin at a crystal size of 5-6 nm [138-141]. 

Much of the research interest in nanomaterials is attributable to the remarkably different properties displayed by these fascinating materials. For example, the prediction of size-dependent bandgaps for nanocrystalline semiconductors has excited speculation about their exploitation in novel optoelectronic and photoelectrochemical applications. Due to the relative ease with which they may be synthesized, most work to date has focused on metal nanoparticles and nanocrystalline II-VI (12-16) semiconductors. Despite the tremendous potential of unique properties and applications, however, nanocrystalline III-V (13-15) semiconductors remain largely unexplored. To this end, the work reported here details two different routes for the synthesis of nanocrystalline Ill-V materials, and discusses the characterization of these materials. In particular, it will be shown that stable, conductive, nanocrystalline materials with a reasonably narrow size distribution can be prepared which have a markedly different bandgap than commercial wafers of the bulk material. 

When the nanoparticles become smaller than the exciton-Bohr diameter, semiconductor nanoparticles show quantum size effects due to the spatially confined electron-hole pairs that are created by photo or thermal excitation. The quantum size effects appear most frequently as a bandgap broadening. Hence optical properties such as radiative and nonradiative electronic transitions are significantly influenced by quantum confinement effects in nanoparticles. Although most of the rare earth oxides are insulators, quantum size effects are of particular importance to the bandgap engineering of semiconductors such as CeOa and some rare earth sulphides. 

Transfer from tryptophan residue donors on protein to nanoparticle acceptors The mixed metal, CD4 antibody-5X-aminodextran-(Cd Hg = 1 1)S conjugate prepared ° by a procedure similar to one already reported for the same -CdS conjugate showed an emission spectrum, with 283.2 nm excitation into the CD4 antibody absorption band (Fig. 15), showing (Fig. 16) an intense emission band centered at 339 nm from tryptophan residues of the CD4 antibody and a medium intensity emission band at 660 nm from the mixed (Cd,Hg)S semiconductor nanoparticles. The predominant excitation peak (Fig. 16, top) was at 281.6 nm when emission was monitored at 656 nm. 

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. 

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. 

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