Influence of the Substrate Surface

Epitaxial layer can be easily obtained on single crystal silicon substrate. (Ill) and (100) oriented substrates are commonly used. Due to the high affinity of 

Metal solvent Melting point C Atomic solubility of silicon 800° C(%) 1,000°C(%)  

De-greasing of the substrate, using solvent like acetone 

Formation of chemical silicon oxide with H2SO4-H2O2 (1 1) 

Another possibility, which has to be used when temperature growth is lower than 900° C, is the addition of reducing agent in the melt like A1 or Mg, which can lead to an oxide-free surface [8]. However, because A1 is a p-type dopant for silicon, one has to reduce its amount or to use a two-melts process, one to remove native oxide, and one for the growth of the active layer [9]. 

---

It is well known that the ensemble of sp -carbon chains is unstable and used to form cross-bonds between the neighboring chains. This results in the formation of sp and sp chemical bonds between carbon atoms. However, on the substrate surface the situation is radically changed due to the interaction of carbon chains with the surface. In this case the growth of well-oriented carbon chains was observed in [9]. The film orientation is found to be strongly dependent both on the film thickness and deposition conditions. For thicker films the influence of the substrate surface becomes negligible and the films are not purely sp. ... 

The influence of the substrate surface tension is further revealed through a consideration of the range in maxima for... 

The presence of a confined interfacial layer, with specific rheological behavior, is proposed to explain this complex behavior. The low stiffness of PDMS allows a competition between the (low) cohesion and the confined chain layer at the PDMS surface and the adhesion level (interfacial interactions between PDMS and substrates). At low speeds, interfacial interactions have a significant effect and partly govern the friction, and at high speeds the influence of the substrate surface becomes negligible and friction is then governed by the polymer s intrinsic viscoelastic behavior. Experimental results underline the subtle competition between interfacial interactions and polymer rheological properties, especially for PDMS samples. Comparison... 

Other forces can arise as a result of elastic strain on the growing film, which can be due to a surface-induced ordering in the first few layers that reverts to the bulk liquid structure at larger distances. This elastic energy is stored in intermolecular distances and orientations that are stretched or compressed from the bulk values by the influence of the substrate at short distances [7]. Similar phenomena are well known to occur in the growth of epitaxial layers in metals and semiconductors. 

In classical kinetic theory the activity of a catalyst is explained by the reduction in the energy barrier of the intermediate, formed on the surface of the catalyst. The rate constant of the formation of that complex is written as k = k0 cxp(-AG/RT). Photocatalysts can also be used in order to selectively promote one of many possible parallel reactions. One example of photocatalysis is the photochemical synthesis in which a semiconductor surface mediates the photoinduced electron transfer. The surface of the semiconductor is restored to the initial state, provided it resists decomposition. Nanoparticles have been successfully used as photocatalysts, and the selectivity of these reactions can be further influenced by the applied electrical potential. Absorption chemistry and the current flow play an important role as well. The kinetics of photocatalysis are dominated by the Langmuir-Hinshelwood adsorption curve [4], where the surface coverage PHY = KC/( 1 + PC) (K is the adsorption coefficient and C the initial reactant concentration). Diffusion and mass transfer to and from the photocatalyst are important and are influenced by the substrate surface preparation. 

In the case of individual Ceo molecules adsorbed on clean Si(lll)-(7 x 7) surfaces the experimental intramolecular structure compares well to ab initio calculations based on density functional theory (DFT) in spite of the covalent character of the substrate (Pascual et al, 2000). In this case a certain degree of uniaxial strain has to be considered to simulate the electronic influence of the substrate. 

To all these intrinsic reasons, one would have to add the expected modifications in the electronic structure of the growing film as it thickens, due to the decreasing influence of the substrate. This can be better judged for a system that is not pseudomorphic, such as Ag/Cu(lll). The large (12%) mismatch between Ag and Cu would provoke such a tremendous compressive stress for a pseudomorphic layer that the Ag layers keep their own lattice parameter from the first monolayer on. For 1 ML of Ag/Cu(lll), the surface state has been found to be 120 meV lower in energy than for bulk Ag(lll) [79], and shifts with increasing Ag coverage to the bulk value. 

By their very nature, heterogeneous assemblies are difficult to characterize. Problems include the exact nature of the substrate surface and the structure of the modifying layer. In this chapter, typical examples are given of how surface assemblies can be prepared in a well-defined manner. This discussion includes the descriptions of various substrate treatment methods which lead to clean, reproducible surfaces. Typical methods for the preparation of thin films of self-assembled monolayers and of polymer films are considered. Methods available for the investigation of the three-dimensional structures of polymer films are also discussed. Finally, it will be shown that by a careful control of the synthetic procedures, polymer film structures can be obtained which have a significant amount of order. It will be illustrated that these structural parameters strongly influence the electrochemical and conducting behavior of such interfacial assemblies and that this behavior can be manipulated by control of the measurement conditions. 

Modified TiC>2 surfaces have also found application in the design of fast elec-trochromic devices. The influence of the substrate on the behavior of interfacial assemblies is well illustrated in this book. However, it is important to realize that the electrochromic behavior observed for modified TiC>2 surfaces was not expected. The oxidation and reduction of attached electrochromic dyes are not mediated by the semiconductor itself but by an electron-hopping process, not unlike that observed for redox polymers, where the electrochemical reaction is controlled by the underlying indium-tin oxide (ITO) contact. These developments show that devices based on interfacial assemblies are a realistic target and that further work in this area is worthwhile. 

Therefore, HREELS is capable, like SIMS, of disclosing molecular, long-range information on the polymer surface. However, at this stage, it is fair enough to address our ignorance of the influence of the substrate on the vibrational response of a polymer thin film What are the effects of substrate quality - pure metal or with native oxide -, roughness on the atomic scale, and polarizability Authors (8.11) do feel these external parameters must be controlled and taken into account. 

Re-orientation of surface crystallites, and transfer to the counterface, take place quickly under the influence of sliding so that the coefficient of friction and the shear stress decrease. At the same time the compression of the film under the normal component of the applied stress forces the film material into the low spots of the substrate surface, and it is at this stage that embedding and chemical bonding are likely to become more significant. 

The influence of the substrate will be discussed later, but in general substrates for sputtered molybdenum disulphide films are very smooth, with surface textures of the order of 0.1 //m. The gross keying effect which has been found important for adhesion of bonded or burnished films is therefore not normally available. It has been... 

The steel sample was cleaned by (Ar + H2) plasma, followed by plasma deposition. Transferring the sample from the deposition chamber to the XPS chamber can monitor the surface at any depositing state. As indicated in Figure 33.3, at 5 s deposition, the surface state at this stage reflected the interfacial bonding nature of the plasma film onto the steel. It clearly shows that the C peak comprises peaks at 284.6 eV (C-C) and 283.0 eV, indicative of C-Fe or C-Si bonds. The Si 2p peak also clearly shows a combination of Si-Si (100.6 eV) and Si-Fe (99.7 eV) peaks. Further deposition (15 and 45 s) resulted in establishment of TMS film without influence of the substrate steel, as reflected by C Is (284.5 eV) and Si 2p (100.80 eV) single peaks. 

For the influence of the specific surface area of the semiconductor powder on the rate of product formation, two opposite effects are of major importance [81]. One is concerned with the rate of electron-hole recombination which increases linearly with surface area, and accordingly the reaction rate should decrease. The other is a linear increase in the reaction rate of the reactive electron-hole pair with the adsorbed substrates, which should increase product formation. It is therefore expected that, depending on the nature of semiconductor and substrates, the reaction rate, or increasing surface area. This is nicely reflected by the CdS/Pt-catalyzed photoreduction of water by a mixture of sodium sulfide and sulfite. The highest p values are observed with small surface areas and are constant up to 2 m g". From there a linear decrease to almost zero at a specific surface area of 6 m g" takes place. Upon further increase to 100 m g" this low quantum yield stays constant [82]. 

Covering power describes the extent to which an electrodeposition electrolyte can cover the entire surface of a workpiece being plated, with reasonable uniform thickness. Covering power is influenced by the nature of the substrate surface, the electrolyte composition, the temperature and viscosity, and the current density. 

---