Electronic structure, size-induced

Instead, we believe the electronic structure changes are a collective effect of several distinct processes. For example, at surfaces the loss of the bulk symmetry will induce electronic states with different DOS compared to bulk. As the particle sizes are decreased, the contribution of these surface related states becomes more prominent. On the other hand, the decrease of the coordination number is expected to diminish the d-d and s-d hybridization and the crystal field splitting, therefore leading to narrowing of the valence d-band. At the same time, bond length contraction (i.e. a kind of reconstruction ), which was observed in small particles [89-92], should increase the overlap of the d-orbitals of the neighboring atoms, partially restoring the width of the d-band. 

Beside, the above-mentioned asymmetry of size-depended shifts of electron and hole levels for light-produced Wannier excitons can lead to the photo-induced mutual charging of SC nanoparticles with different sizes in composite film with high SC content. Such process for SC nanoparticles of different electron structure has been noticed in work [29]. It has been shown that the photo-induced mutual charging of SC nanoparticles can increase their photocatalytic activity [29]. 

This paper focuses on the influence of the support on the H/D exchange of CP over supported Pt catalysts. It will be shown that kinetics and selectivities are largely affected by the support material. Particle size effects are separated from support effects. The activity shows a compensation effect, and the apparent activation energy and pre-exponential factor show an isokinetic relationship . This can be explained by different adsorption modes of the CP on the metallic Pt surface. The change in adsorption modes is attributed to a change in the electronic structure of the Pt particles, which in turn is induced by changes in the acid/base properties of the support. 

It is quite challenging to rmderstand in what way the zeolite influences the metal compared to other supports. The electronic changes that could be induced by the pore system are quite subtle and metal particle size effects may overrule these changes [200]. hi comparison to metal-support interactions on macroporous oxides, the interaction between metal particles and the supporting zeolite matrix seems more pronounced. This may be because the metal particles interact with the zeolite lattice over a substantial fraction of their surfece. It has also been suggested that in addition to the intrinsic electronic effects due to the small size of the metal particles in the zeolite cage, a modification of the electronic structure of the metal by the acidic zeolite framework has to be considered [201,202]. 

The binding of ammonia to the cluster induces a change in electronic structure relative to that of the bare cluster. This is probed by reacting ammoniated clusters with hydrogen and comparing the reaction rate constants as a function of cluster size with the naked iron clusters. The absolute reaction rate constants toward H 2 for the fully ammoniated clusters are about an order of magnitude smaller than those for the bare clusters. The minima in reactivity observed for bare iron clusters are shifted to smaller cluster size for the ammoniated species for example, Fe,3 is reactive with H2, but upon... 

In this respect, the cluster size, from which on the defect-induced perturbation on the electronic structure disappears, is a key quantity. The calculations showed [178] that effect of the surface defects on the adhesion energy decreases rapidly with cluster size. Thus, for particles of nanometer size, differences in the bonding to the substrate tend to vanish as the larger polarizability of the particle screens the effect of the defect and the relative effect of defect-related bonds become less important due to the larger number of metal-oxide bonds at the interface. However, because point defects are the most likely sites for the initial steps of nucleation, one has to expect that also large metal particles are still located at these sites unless the temperature is sufficiently high to permit diffusion of particles. 

Figure 6. The variation in the measured ionization potential of mercury clusters as a function of cluster size. The work function for bulk Hg (4.49 eV) is indicated. The dashed line is a plot of the ionization potential calculated for the classical (liquid drop) electrostatic model for a metalUc sphere of diameter d. Region III contains clusters which are classified as insulating. Region II denotes the size-induced metal-insulator transition, in which overlap of the 6s and 6p states sets in at around Hgn. The larger clusters, located in Region I, have valence electronic structures that closely resemble the band structures of liquid and crystalline mercury. Adapted from Rademann. i...

An additional complication arises from the fact that the probability of an electron (or hole) being self-trapped due to the electron - phonon interaction increases strongly as the electronic wave function shrinks in size to the order of atomic dimensions (Emin, 1982). A consequence of this is that electrons in disorder-induced localized states are believed to be more susceptible to small polaron formation and self-trapping than are ordinary extended-state electrons (Emin, 1984 Cohen et al, 1983). Thus, not only does the disordered structure of amorphous semiconductors introduce new physical phenomena, namely, the mobility edge, but also the effect of known phenomena, such as the electron - phonon interaction, can be qualitatively different. 

Theoretical calculations of the electronic structure of metal nanocrystals throw light on the size-induced changes in the electronic structure. Rosenblit... 

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