Carbon cluster deposition

Fig. 10.11 SER spectra of carbon clusters deposited on a rough gold surface measured in air at room temperature. Each spectrum was accumulated for 1 s. The waiting time between recording consecutive spectra was 2 s. (a) Denotes the first spectrum, (g) the last. All spectra have the same intensity scaling, but are spaced upwards in the figure to enhance the clarity of the presentation. (Reproduced with permission from Ref [82].)...

I. SERS Raman Spectroscopy of Mass-Selected Carbon Clusters Deposited on Silver Surfaces... 

Palladium clusters deposited on amorphous carbon have been studied by XPS and UPS [28] and both techniques show broadening of the d-band peak as cluster size increases. The d-threshold shifts towards Ep as cluster size increases. In UPS studies the d-emission of the single atom has its peak at 3.0 eV below Ep, whereas the d-threshold is 2eV below Ep. Palladium clusters evaporated onto Si02 have been studied by UPS [38]. At large coverages of the Pd metal evaporated (> 10 atoms/cm ), a high emission intensity at Ep excited with photons of 21.2 eV (He(I)) or 40.8 eV (He(II)) as excitation source, is observed. This feature is characteristic in the spectra from bulk Pd samples. At the lowest metal coverage (3 x 10 atoms/cm ),... 

Gold clusters deposited on the activated carbon had the smallest average diameter of 1.7nm, while gold on graphite and decolorizing carbon had average particle sizes of 2.8 and 6.8 nm respectively. 

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. 

Deactivation is due primarily to two mechanisms formation of carbon-containing deposits and sulfur poisoning. Carbon deposition may be minimized by the addition of alkali metals, optimization of metal cluster size, and use of oxygen ion-conducting supports. Sulfur poisoning is usually irreversible and there are few reports of catalysts that are tolerant of sulfur levels typical of commercial fuels. 

Alternative procedures to obtain FePt nanoparticles use direct cluster deposition with a particle gun [37], Under normal conditions, the FePt clusters have the fee structure. In this original development, the particles passing through halogen lamps are annealed before deposition. The nanoparticles are embedded in a carbon matrix with typical size of 6 nm. /i0Hc = 0.3 T is obtained. 

Hwang N.M., Evidence of nanometer-sized charged carbon clusters in the gas phase of the diamond chemical vapor deposition (CVD) process. J. Crystal Growth, 204 (1999) 85-90. 

Me90b G. Meijer and D. S. Bethune, Laser Depositions of Carbon Clusters on Surfaces A We89 New Approach to the Study of Fullerenes, ... 

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]. 

The growth of films via SCBD can be viewed as a random stacking of particles as for ballistic deposition [33,34]. The resulting material is characterized by a low density compared to that of the films assembled atom by atom and it shows different degrees of order depending on the scale of observation. The characteristic length scales are determined by cluster dimensions and by their fate after deposition. Carbon cluster beams are characterized by the presence of a finite mass distribution and by the presence of different isomers with different stabilities and relativities. Due to the low kinetic energy of clusters in the supersonic expansion stable clusters can survive to the deposition, while reactive isomers can coalesce to form a more disordered phase [35]. 

Considering the formation and deposition process of carbon clusters by PMCS, it is reasonable to assume that the carbynoid species are formed in the cluster source prior to deposition. The picture coming out from our observations is characterized by relatively fragile sp chains that are quite surprisingly able to survive deposition at kinetic energies per atom, well above the thermal energy measured to induce the sp rearrangement and sp formation. 

Our results also address another relevant aspect of carbon clusters that is, the shape and the hybridization of the precursor aggregates. As we have shown, we are depositing particles in a mass range where fullerene-like shape and sp hybridization should be predominant [46,47]. However, an accurate analysis of the mass spectra shows that the contribution of odd clusters is not negligible (Figure 2.2). This suggests that non-fullerene type of clusters could be more aboundant than expected even for relatively large clusters. Another possibility, supported by the studies of Jarrold and co-workers [44] is that fullerene like clusters can form complexes with the presence of sp chains. 

SCBD SCP SEM SOJT STM supersonic carbon cluster beam deposition self consistent field scanning electron microscope second-order Jahn-Teller effect scanning tunneling microscopy... 

The thermal decomposition of organic compounds can also be employed to generate small carbon clusters or atoms. The borderline with chemical vapor deposition (CVD) as presented in the next section is not really fix. In both cases, the method is based on the thermal decomposition of organic precursors. Processes both with and without catalyst have been reported. Contrary to the chemical vapor deposition, however, the catalyst (if applied) is not coated onto a substrate, but the substance or a precursor is added directly to the starting material ( floating catalyst ). The resulting mixture is then introduced into the reactor either in solid or in liquid state by a gas stream. From this point of view the HiPCo-process could also be considered a pyrolytic preparation of SWNT, but due to its importance it is usually regarded as autonomous method. 

The first MWNTs have been obtained as early as 1976 by iron-catalyzed pyrolysis of benzene. Apart from that, there is a number of methods to produce MWNT, which all of them differ in the way of generating small carbon clusters or atoms from the respective starting materials. They include arc discharge, laser ablation, chemical vapor deposition with and without plasma enhancement or the catalytic decomposition of various precursor compounds. It turned out that MWNTs from low-temperature syntheses bear more defects and, as a whole, are less ordered than those generated at high temperatures. However, these drawbacks can still be compensated by subsequent recuperation of defective samples at elevated temperatures. 

Like in the preparation of single-walled carbon nanotubes, the chemical vapor deposition of MWNT consists in the generation of small carbon clusters or atoms from precursor compounds. The products precipitate in the shape of different carbon materials with the reaction conditions determining the specific stracture... 

Besides small carbon clusters generated in the reaction zone anyway, the presence of hydrogen further gives rise to light hydrocarbons that contribute to the deposition of DWNT as well. Hence, in principle, this is a floating catalyst CVD performed in situ. It has indeed been applied in a multitude of experiments for the deliberate production of double-walled nanotubes. Normally, acetylene is employed as carbon source because apparently it suits best to surround an existing nanotube with a second layer of amorphous carbon (refer to Section 3.3.6). 

The mechanism of nanotube formation in chemical vapor deposition features characteristics rather distinct from those found for the synthesis by arc discharge or laser ablation. Contrary to the latter, a solution of small carbon clusters in and subsequent diffusion through catalyst particles play a minor role in the deposition from the gas phase. The employed hydrocarbons decompose directly on the surface of the catalytic particle. The carbon, therefore, becomes immediately available for nanotube growth. 

Meijer G and Bethune D S 1990 Laser deposition of carbon clusters on surfaces-a new approach to the study of fullerenes J. Chem. Rhys. 93 7800-2... 

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