Island density

A more detailed picture of the temperature dependence of the growth is given in Figure 2.4, where the island density is plotted as a function of temperature. It can be seen that only in the temperature range from 207 to 288 K the growth is perfectly template controlled and the number of islands matches the number of available nucleation sites. This illustrates the importance of kinetic control for the creation of ordered model catalysts by a template-controlled process. Obviously, there has to be a subtle balance between the adatom mobility on the surface and the density of template sites (traps) to allow a template-controlled growth. We will show more examples of this phenomenon below. 

Figure 2.4 Evolution of the island density with growth temperature for Au/N/Cu(l 0 0). (Reproduced with permission from Ref. [24].)...

We begin our discussion with the diffusion of a Si adatom over a flat terrace. This problem has previously been addressed with ab initio calculations for the case of symmetric dimers. The main result is that diffusion is highly anisotropic on the surface, with fast diffusion taking place over the top of the dimers with a saddle point energy of about 0.60 eV. Slow adatom diffusion is predicted to take place across the dimer rows with a barrier of 1.0 eV. Experiments based on a number counting of the island density are in agreement with these results. ... 

Apparently, epitaxial thin-film model catalysts provide a well-defined initial state for a systematic study of microstructural changes and structure-activity correlations. Model catalysts were prepared for various noble metal-oxide combinations, including Pt, Rh, Ir, Pd, Re supported by Al Oj, SiO, TiO, CeO, VO, Ga Oj, etc. The number density of the metal particles (island density particles per cm ) and their size can be controlled via the NaCl(OOl) substrate temperature during evaporation and the amount of metal deposited (as measured by a quartz microbalance), respectively (Pig. 15.4). 

To prepare model catalysts, metal nanoparticles were grown on the thin film oxide by evaporation (step 3 in Fig. 15.7a). Figure 15.7c and d show STM results for Pd nanoparticles grown on Al3O3/NiAl(110) at 300 and 90 K, respectively [11, 39,43,47,48]. Figure 15.8 displays the dependence of the island density (nanoparticles per cm ), the mean particle diameter, and the average number of atoms per particle on the nominal film thickness [39]. Similar nucleation studies were performed for Pt, Rh, and Ir nanoparticles [39, 46]. 

Fig. 15.8 Deposition of Pd on Al203/NiAl(l 10) at 300 and 90 K substrate temperature dependence of the island density, mean particle diameter, and average number of atoms per particle on the nominal film thickness. Adapted from [39] with permission. Copyright 1999 Elsevier...

The nucleation and growth of islands during submonolayer deposition is, of course, a problem that has been studied for decades [121,122]. Analyses based on mean-field rate equations have led to an understanding of the dependence of mean island density, Nav, on deposition conditions. More recently, kinetic Monte Carlo simulation studies have been used to test... 

From scaling analysis of the island size distribution in Figure 4.3.7a, the surface coverage (9) and island density per unit surface (N) for pentacene films with the same nominal thickness of 0.5 nm can be plotted as a function of increasing... 

Functional Form of the Maximum Island Density Dependences on Deposition Rate andTemperature for Various Growth Regimes and Modes... 

The dynamic scaling assumption relates the island density... 

The temperature dependence of the critical island size for pentacene on Si02 was explored by Tejima et al. [44] and the results are displayed in Figure 5.1.11. The same scaled island size distributions are shown in Figure 5.1.11a for four different substrate temperatures at a constant thickness of -0.2 ML. The curves corresponding to critical island sizes of one, two, and three are shown and it is clear that the different substrate temperatures used result in different critical island sizes. The scaled island densities for thick pentacene layers are plotted as a function of... 

To get an expression for d, we need to explore the relationship between the cross-section of the islands in a given layer with film coverage. If 5 represents the area of a two-dimensional cluster and D its fractal (Hausdorff) dimension, then the island cross-section is proportional to sp, where Pi = HD. D = 2 corresponds to a compact island, but for pentacene that grows by diffusion limited aggregation (DLA), D 513. Noting from before that, prior to coalescence the island density, n remains constant and composed of identical islands, then the total cross-section of a layer becomes d = n SP. Since 9 = n S, then Qp. Similarly, if after coalescence the number of holes in the film also remains constant, then the perimeter of each layer can be related to its coverage as shown in Figure 5.1.12b and c by [29] ... 

Tejima, M., Kita, K., Kuno, K., and Toriumi, A., Study on the growth mechanism of pentacene thin films by the analysis of island density and island size distribution, Appl. Phys. Lett., 85, 3746, 2004. 

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