Size-dependent evolution

In recent publications [120, 121, 122,123] it has been shown that both the ionization potentials and the optical properties of bare and uncharged mercury clusters in a molecular beam experiment demonstrate a gradual size dependent evolution of metallic properties, starting at about 13 atoms and already bulklike at about 70 atoms. It has been predicted theoretically [124] that plasmons should begin to develop for such mercury clusters at about Hgi5. We should keep this in mind in the discussion of the electronic properties of AU55. 

The convergence limits arc denoted in Fig. 10 as oo and oo. In contrast, the solid shows less discrete features. The condensed phase features are broader in width and shifted to higher photon energy. Both dominant features are due to spin-orbit splitting of the lowest core excitons, reflecting considerable changes in electronic structure from the gas to the condensed phase. This size dependent evolution of spectral features is shown... 

Figure 3.123 Size-dependent evolution of powder XRD patterns for CoPt3 nanoc stals. The average nanoc stal sizes were calculated using the Debye-Sherrer equation. Reproduced with permission from Ref [55] 2003, American Chemical Society.

The size-dependent evolutions of the structures and physical and chemical properties of nanoscale clusters have been subjects of continuing basic and applied research interests. Structure determination is one of the outstanding challenges of cluster... 

The size-dependent features of the electronic structure have been explored, with special emphasis on the evolution of the valence band DOS of transition and noble metals as the particle size increases. 

An example of the information obtainable from measurements of aerosol particle composition as a function of size is given in Figure 10. In this study of the evolution of aerosol composition in an urban air-shed, a multi-stage impactor was used to separate particles by size for the determination of size-dependent composition. The particle mass concentration increases as air moves inland from Santa Catalina Island to Long Beach, Fullerton, and finally Riverside, as do substances such as nitrate. Differences in particle composition as a function of size at a given site and from site to site are evident. 

Fig. 15.12 Methanol oxidation on Pd-Al O (mean particle size 6 nm) and Pd(lll) [27, 50, 75]. (a) In situ steady-state PM-IRAS and SFG spectra, shown together with the CHjOH conversion as monitored by gas chromatography, (b) Comparison of Pd3d XP spectra obtained before upper) and after (lower) the methanol oxidation reaction indicated a partial oxidation of Pd nanoparticles during the reaction, whereas Pd(lll) remained metalhc adapted in part from [75] with permission. Copyright (2007) The PCCP Owner Societies, (c) PM-IRAS (p-s) surface vibrational spectra measured during CH OH decomposition on Pd(lll) at 300 K, with the various species indicated. The time-dependent evolution of CH O (as observed by PM-IRAS) and of CH (values deduced from XPS) upon methanol decomposition at -lO mbar suggests a correlation between the two species adapted in part from [27] with permission. Copyright (2005) American Chemical Society, (d) Suggested mechanism of CH OH decomposition and oxidation on Pd catalysts reprinted [50] with permission. Copyright (2007) Elsevier...

Celep, G., Cottancin, E., Lerme, J., Pellarin, M., Arnaud, L., Huntzinger. J.R.. Broyer, M., Palpant, B., Boisron, O., Melinon, P. Size evolution of the optical properties of copper clusters embedded in alumina an experimental and theoretical study of size dependence. Phys. Rev. B 70, 165409 (2004)... 

Another important factor in catalysis is the selectivity of a catalytic reaction. So far, however, information on the atom-by-atom evolution of this astonishing catalytic selectivity is still lacking. In this example, we illustrate such a size-dependent selectivity with the polymerization of acetylene on palladium nanocatalysts [46]. This reaction over supported Pd particles reveals a direct correspondence between reactivities observed on model systems and the behavior of industrial catalysts under working conditions [66]. In ultra-high vacuum (UHV) [67] as well as under high pressure, large palladium particles of typically thousands of atoms show an increased selectivity for the formation of benzene with increasing particle size [66]. In contrast, small palladium particles of typically hundreds of atoms are less selective for the cyclotrimerization, and catalyze butadiene and butene as additional products [66]. 

Figure 25.10 Temperature dependent evolution of the crystallite sizes in the respective structural phases of DIP. Above the transition point the previous crystallite size of about 220 nm is almost restored. In the vicinity of the phase transition, strong effects on the carrier transport might be expected from structural inhomogeneities that appear.

While one unit cell (8 molecules) is very small for most applications, we think it is enough to characterize the overall time evolution of the chemistry. In order to test system size dependence of our results, we performed some simulations with larger cells (2x2x2 unit cells, 64 RDX molecules and 1344 atoms). Both reaction time-scales and equilibrium distribution of products are in excellent agreement with the smaller simulations. Using a single unit cell enabled us to explore lower temperatures where several nanoseconds-long simulations were necessary to reach an equilibrium distribution of products. 

An experimental setup detecting the formed target ions Cs+ or Cs after the collision with clusters could provide pure ct cross sections, i.e. without regard to the evolution of the cluster. The calculated integral ct cross sections cluster size n in Fig. 5. Note that the ground state Cs (6s21S ) is expected to be bound [36]. Obviously, the cross sections for the normal ct process with Cs+ in the exit channel are considerably larger (at least a factor of two) than for the exotic Cs- formation, and the size dependences show... 

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