Interface particle-substrate

Fulda and Tieke [77] studied the effect of a bidisperse-size distribution of latex particles on the structure of the resulting LB monolayer. For this purpose, a mixed colloidal solution of particles la and lb was spread at the air-water interface. Particles la had a diameter of 434 nm, particles lb of 214 nm. The monolayer was compressed, transferred onto a solid substrate, and viewed in a scanning electron microscope (SEM). In Figure 10, SEM pictures of LB layers obtained from various bidisperse mixtures are shown. 

An interesting example of application of the Wulff rule is given by Ovesen et al. [61]. They have analyzed the kinetics of methanol synthesis on nm Cu particles supported by ZnO. The generalized surface tension for the particle-substrate interface was assumed to be dependent on the reduction potential of the gas phase. The latter resulted in the dependence of the areas of the (111), (100), and (110) facets on the gas-phase concentrations (such changes were observed by using EXAFS). The total reaction rate, represented as a sum of the reaction rates on different facets, was found to be affected by the changes in particle morphology. 

In this case, the Wulff shape is truncated at the interface by an amount Ah, which is proportional to the adhesion energy [3. The latter represents the work to separate the supported crystal from the substrate at an infinite distance, hg and 7s are the central distance and the surface energy of the facet parallel to the interface, respectively. In particular, this theorem shows that the stronger the particle-substrate interaction (given by / ) is the flatter is the supported particle. Equation (3.7) offers a simple way for determining the adhesion energy of a supported crystal from TEM pictures of supported particles observed in a profile view [47]. 

For particles smaller than 6 nm the shape is an half octahedron limited by four (111) faces and truncated by a (001) face at the top, without re-entrant angles at the interface. Particles smaller than 2 nm are dilated by 8% and perfectly accommodated by the substrate. The 5 nm particles are accommodated only at the level of the interface in the first two or three layers of deposited Pd. 

GB Basim, I Vakarelski, PK Singh, BM Moudgil. Role of particle-particle and particle-substrate interactions in CMP. J Colloid Interface Sci, in press, (2002). U Mahajan, M Bielman, RK Singh. Electrochem Solid State Lett 2 46, 1999. J Klein, E Kumacheva, D Mahalu, D Perahia, LJ Fetters. Nature 370 634, 1994. JJ Adler, BM Moudgil. J Colloid Interface Sci, in press, (2002). 

The particle-beam interface is used to remove solvent from a liquid stream without, at the same time, removing the solute (or substrate). 

Fig. 5. A 90° polished cross section of a production white titania enamel, with the microstructure showing the interface between steel and direct-on enamel as observed by reflected light micrography at 3500 x magnification using Nomarski Interface Contrast (oil immersion). A is a steel substrate B, complex interface phases including an iron—nickel alloy C, iron titanate crystals D, glassy matrix E, anatase, Ti02, crystals and F, quart2 particle.

The situation is illustrated in Fig. 3.47. The upper part shows a thin film of Ni deposited on a Si substrate. Only particles scattered from the front surface of the Ni film have an energy given by the kinematic equation, Eq. (3.28), Fi = fCNi o- As particles traverse the solid, they lose energy along the incident path. Particles scattered from a Ni atom at the Si-Ni interface therefore have an energy smaller than On the... 

Once it is recognized that particles adhere to a substrate so strongly that cohesive fracture often results upon application of a detachment force and that the contact region is better describable as an interphase [ 18J rather than a sharp demarcation or interface, the concept of treating a particle as an entity that is totally distinct from the substrate vanishes. Rather, one begins to see the substrate-particle structure somewhat as a composite material. To paraphrase this concept, one could, in many instances, treat surface roughness (a.k.a. asperities) as particles appended to the surface of a substrate. These asperities control the adhesion between two macroscopic bodies. 

Physisorption at interfaces, for example, electrostatic adsorption of charged particles at oppositely charged substrates [14,15,20,23-25,85-117]... 

In 1997, a Chinese research group [78] used the colloidal solution of 70-nm-sized carboxylated latex particles as a subphase and spread mixtures of cationic and other surfactants at the air-solution interface. If the pH was sufficiently low (1.5-3.0), the electrostatic interaction between the polar headgroups of the monolayer and the surface groups of the latex particles was strong enough to attract the latex to the surface. A fairly densely packed array of particles could be obtained if a 2 1 mixture of octadecylamine and stearic acid was spread at the interface. The particle films could be transferred onto solid substrates using the LB technique. The structure was studied using transmission electron microscopy. 

---