Molecular collisions layered structure

SIMS is a very surface-sensitive technique because the emitted particles originate from the uppermost one or two monolayers. The dimensions of the collision cascade are rather small and the particles are emitted within an area of a few nanometers diameter. Hence, SIMS can be used for microanalysis with very high lateral resolution (50 nm to 1 pm), provided such finely focused primary ion beams can be formed. Furthermore, SIMS is destructive in nature because particles are removed from the surface. This can be used to erode the solid in a controlled manner to obtain information on the in-depth distribution of elements.109 This dynamic SIMS mode is widely applied to analyze thin films, layer structures, and dopant profiles. To receive chemical information on the original undamaged surface, the primary ion dose density must be kept low enough (<1013 cm-2) to prevent a surface area from being hit more than once. This so-called static SIMS mode is used widely for the characterization of molecular surfaces (see Figure 3.10). 

Nucleation is a crucial step in the whole process of carbonaceous particle formation. According to Frenklach and Wang (1990, 1994), nucleation is controlled mainly by the sticking of PAH sheets during their collisions. Physically bound clusters of PAH are then formed and successively evolve toward aerosol, solid particles and crystallites. As shown in Fig. 25, different polycyclic aromatic layers can form more or less regularly ordered graphite structures, all of which have interlayer distances of about 0.35 nm. These two to four-layer structures are assumed as the threshold of the formation of the solid phase particle inception typically takes place at molecular masses of 1,000-2,000 amu. 

The uniformity of the deposited layer (Table 9.1, no. 3) also differs in both deposition technologies. In OVPD the organic molecules are randomly distributed by intermolecular collisions with carrier gas molecules which results in a very uniform and quantitative coverage of the substrate. OVPD thus also has the potential to cover unintended substrate non-uniformities, for example defects or particles. Consequently OVPD can also be applied to complex three-dimensional structured substrates. A single layer of Alq3, deposited by OVPD on a silicon wafer had a thickness uniformity of only 0.6% standard deviation, and surface roughness analysis by AFM confirmed, with an RMS value of 0.6 nm, that the thickness deviation of the Alq3 -layer is already in the molecular dimension [20]. 

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