Electronic structure, size-induced changes

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

Systematic size induced changes in the electronic structure of QDs have been extensively studied in the last decade. Sustained efforts have established interesting applications based on the blue-shift of the absorption edge due to the quantum confinement in nanometer-sized samples. Absorption spectroscopy is one of the simplest technique that has been employed to study the electronic structure of nanocrystals. Upon irradiation with light of energy greater than the band gap, NCs are known to absorb photons and promote electrons from the valence band (VB) to the conduction band (CB). The onset of the absorption spectrum generally corresponds to the band gap of QDs. The band gap is size tunable and decreases very sensitively with increase in the size of the dot. This relationship is shown in Fig. 2a for a typical system like the ZnO QDs. This size dependence... 

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. 

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... 

Electronic properties of nanocrystals critically depend on size. This aspect is aptly put forth in the quest How many atoms make a metal . It is clear that as the size of metal nanocrystals is reduced, the accompanjung changes in the electronic structure render them insulating. This transition, called the size-induced metal-insulator transition (SIMIT), has evoked much interest from chemists and physicists alike. A SIMIT is manifested in experiments that measure the electronic band structure and atomistic properties such as ionization energy. 

Although the number of computer simulations on small molecules in membranes performed so far has been rather limited, a consistent picture has begun to emerge from these simulations. In particular, we have markedly improved our understanding of such key issues as the relationship between the distribution of a solute and its chemical and electronic structure, the mechanism of permeation and its dependence on both the size of the solute and the type of the membrane, the nature of solute-induced changes in the structure of the membrane and their influence on permeation. This section is devoted to reviewing this recent progress. 

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