Carbon clusters amorphous structures

This picture was found to be consistent with the comparison of Raman spectra and optical gap of a-C H films deposited by RFPECVD, with increasing self-bias [41], It was found that both, the band intensity ratio /d//g and the peak position (DQ increased upon increasing self-bias potential. At the same time, a decrease on the optical gap was observed. Within the cluster model for the electronic structure of amorphous carbon films, a decrease in the optical gap is expected for the increase of the sp -carbon clusters size. From this, one can admit that in a-C H films, the modifications mentioned earlier in the Raman spectra really correspond to an increase in the graphitic clusters size. 

IR, Raman, NMR, ESR, UPS, XPS, AES, EELS, SIMS) [1]. However, some industrial carbon materials such as amorphous carbon films and carbon black cannot be easily characterized from the local-structure point of view by these methods, because these materials usually take amorphous and complex structures. Recently, soft X-ray emission and absorption spectroscopy using highly brilliant synchrotron radiation [2] has been utilized to characterize various carbon materials, because information on both the occupied and unoccupied orbitals, which directly reflect the local structure and chemical states, can be provided from the high-resolution soft X-ray measurements. We have applied the soft X-ray spectroscopy to elucidate the local structure and chemical states of various carbon materials [3]. Additionally, we have successfully used the discrete variational (DV)-Xa method [4] for the soft X-ray spectroscopic analysis of the carbon materials, because the DV-Xa method can easily treat complex carbon cluster models, which should be considered for the structural analysis of amorphous carbon materials. 

Recently, we have shown the possibility of growing a pure carbon amorphous solid containing a significant amount of carbynoid structures by supersonic carbon cluster beam deposition (SCBD) at room temperature and in an ultra-high vacuum (UHV) [22]. 

Charlier et al. [48] used the tight-binding model to study distorted stacking of graphene layers, termed pregraphitic or turbostratic carbon. The turbostratic structure was obtained by generating an amorphous cluster of graphene plates that... 

The model has be, n shown to have good transferability when applied to a variety of crystal structures. This can be seen from Fig. 3 and Tables II and III, where the energies, vibrational and elastic properties for different coordinated crystalline structures obtained from this model are compared with first-principles LDA calculations and experiments. Applications in molecular-dynamics study of the liquid and amorphous phases of carbon as well as the structures of carbon clusters indicate that the potential does a good job of describing carbon systems over a wide range of bonding environments. These applications are reviewed in Section IV. 

The transferability of the tight-binding model is a key issue. Among the tight-binding models proposed so far, the model for carbon developed by Xu et al. [23] is the most successful. The model has been applied to various carbon systems, ranging from clusters to crystalline structures and to the liquid and amorphous structures of carbon. The results from TBMD simulations not only agree well with available ab initio calcula-... 

In the case of calcined kaoUn, individual kaolin plates are thermally fused into a clustered shape. With scalenohedral and aragonitic PCC, either a rosette or repetitive needle structure is formed through carefully controlled reactions of milk of lime and carbon dioxide. Amorphous silica products are chemically aggregated into repeating silica rings. 

The tetrahedral Al incorporated in mesoporous silica reduces considerably the quantity of amorphous carbon, increasing the MWCNTs selectivity, due to the formation of strong Bronsted acidic sites, which allow a better dispersion of iron metallic clusters. The Fe/Al-MCM41 (10) showed the best results in CNT purity and yield. This indicates that the aluminum content and its tetrahedral structural incorporation play an important role in the CNT syntheses. 

A schematic illustration of the model is shown in Figure 10.2.12, together with that of polyhedral nanoparticles which grow as byproducts of MWNTs (see Fig. 10.2.3). An initial seed of an MWNT is the same as that of a polyhedral nanoparticle. Carbon neutrals [C, C2 (19)] and ions (C+) deposit and coagulate with each other to form atomic clusters and fine particles on a surface of the cathode. Structures of the particles at this stage may be amorphous with high fluidity (liquidlike) because of the high temperature ( 3500 K) of the electrode surface and ion bombardment. 

As discussed earlier, it is now possible to make and study deposits of monosized, highly dispersed, transition metal clusters.(S) In this section we summarize results from the first measurements of the valence and core level photoemission spectra of mass selected, monodispersed platinum clusters. The samples are prepared by depositing single size clusters either on amorphous carbon or upon the natural silica layer of a silicon wafer. We allow the deposition to proceed until about 10 per cent of the surface in a 0.25 cm2 area is covered. For samples consisting of the platinum atom through the six atom duster, we have measured the evolution of the individual valence band electronic structure and the Pt 4f... 

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