Particle-surface interaction

From a conceptual point of view, the interaction of an atom or molecule with a solid surface involves the same forces that are known from the theory of chemical bonding. However, there is an important difference with respect to the gas-phase scenario, namely the dimensionality of one of the partners. The surface is a macroscopic medium with an infinite number of electrons that interact with an individual atom or molecule. In spite of this difference, some basic similarities remain, and many of the concepts present in the theory of chemical bonding can be transferred to the molecule-surface interaction. For example, the strength of the interaction is based on the types of force involved. In the case of molecule-metal interactions, two broad categories can he distinguished, namely (i) weak interaction, leading to physisorption, and (ii) strong interaction, which is responsible for chemisorption. 

Physisorption is a process in which the electronic structure of the molecule or atom is practically unaltered upon adsorption. The equivalent mechanism in (gas-phase) molecular physics is van der Waals bonding, in which the attractive force is due to dispersion forces. 

Chemisorption is an adsorption process that resembles the formation of an ionic or covalent bond in molecular physics. Here, the electronic structure of the adsorbate is significantly altered and sometimes, as happens in ionic bonding, charge transfer from one partner to the other takes place (e.g. from substrate to adsorbate). The change of the electronic structure of the adsorbate can even produce new molecules, e.g. as found in dissociative chemisorption. 

Although conceptually one can notice some similarities between adsorption and molecular bonding, as just shown, there are other basic features which are totally different An example is the range of the interaction potential. Consequently, different models to describe bonding in molecular physics and adsorption are required. In order to elucidate this, let us contem- 

CH26 LASER STUDIES OF SURFACE REACTIONS AN INTRODUCTION 

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Energetic particles interacting can also modify the structure and/or stimulate chemical processes on a surface. Absorbed particles excite electronic and/or vibrational (phonon) states in the near-surface region. Some surface scientists investigate the fiindamental details of particle-surface interactions, while others are concerned about monitormg the changes to the surface induced by such interactions. Because of the importance of these interactions, the physics involved in both surface analysis and surface modification are discussed in this section. 

Below are brief descriptions of some of the particle-surface interactions important in surface science. The descriptions are intended to provide a basic understanding of how surfaces are probed, as most of the infonuation that we have about surfaces was obtained tluough the use of techniques that are based on such interactions. The section is divided into some general categories, and the important physics of the interactions used for analysis are emphasized. All of these teclmiques are described in greater detail in subsequent sections of the encyclopaedia. Also, note that there are many more teclmiques than just those discussed here. These particular teclmiques were chosen not to be comprehensive, but instead to illustrate the kind of infonuation that can be obtained from surfaces and interfaces. 

To measure an individual particle surface interaction and its material removal effects. Because of the complexity of the polishing system, it is highly desirable to characterize the physical and chemical behavior of individual interactions while other components are fixed. AFM technology can be provided to explore slurry particle interactions with different surfaces in different liquid ambient. 

H. Oechsner. Secondary Neutral Mass Spectrometry (SNMS) Recent Methodical Progress and Applications to Fundamental Studies in Particle/Surface Interaction. Int. J. Mass Spectrom. Ion Proc., 143(1995) 271-282. 

In summary, the DLVO theory seems to break down at very close separation where interfacial phenomena such as particle-particle interaction (coagulation) and particle-surface interaction (deposition) are important. 

General Observations About x. its Relationship to the Overall Partitioning Coefficient and to the Concept of Surface-Site Heterogeneity. One approach to metal/particle surface interactions which has been developed, historically, in a variety of forms, is a conceptual model that assumes only two conditions for surface sites occupied by an adsorbate or unoccupied. In applying this approach to the solid/aqueous solution interface, the adsorption... 

John, W., Particle-Surface Interactions Charge Transfer, Energy Loss, Resuspension, and Deagglomeration, Aerosol Sci. Technol., 23, 2-24 (1995). 

Until fairly recently, the theories described in Secs. II and III for particle-surface interactions could not be verified by direct measurement, although plate-plate interactions could be studied by using the surface forces apparatus (SFA) [61,62]. However, in the past decade two techniques have been developed that specifically allow one to examine particles near surfaces, those being total internal reflection microscopy (TIRM) and an adapted version of atomic force microscopy (AFM). These two methods are, in a sense, complementary. In TIRM, one measures the position of a force-and torque-free, colloidal particle approximately 7-15 fim in dimension as it interacts with a nearby surface. In the AFM method, a small (3.5-10 jam) sphere is attached to the cantilever tip of an atomic force microscope, and when the tip is placed near a surface, the force measured is exactly the particle-surface interaction force. Hence, in TIRM one measures the position of a force-free particle, while in AFM one measures the force on a particle held at a fixed position. 

In all of the AFM studies mentioned above, the system contained monovalent counterions, and hence the Poisson-Boltzmann theory could be expected to be accurate. Kekicheff et al. [62] studied interactions between mica surfaces and between silicon nitride and mica in Ca(N03) solutions by using both the SFA and AFM methods. As discussed above, the presence of a divalent counterion complicates particle-surface interactions significantly. Both experimental methods showed that there is a strong, attractive force at very small surface separations, a result that could not be explained by the Poisson-Boltzmann equation. The authors interpreted their results by using the AHNC described above, with the primitive model... 

Recent experimental innovations that allow direct measurement of particle-surface interactions include TIRM and AFM. Both methods seem to yield results that are largely consistent with the Poisson-Boltzmann theory. However, the AFM results in particular point to a need for a better understanding of particle-surface interactions at small separations (say, less than 3 nm) and when divalent or other complicated counterions are involved. 

Hence, with porous particles, surface interaction will predominantly occur when the polypeptide and protein adsorbates reach the internal surface of the particles, thus enabling the mass balance, rate-limiting steps, and the mass transfer coefficients to be quantitatively and independently described. If it is assumed that the pores of the porous HPLC particles are initially filled with buffer liquid before the adsorption process starts, then the overall mass balance for a polypeptide or protein in a finite bath is given by... 

Next the difficulties in obtaining a good description of the particle electrode interaction are noticed. For non-electrochemical systems several particle surface interaction models exist of which the perfect sink , that is all particles arriving within a critical distance of the electrode are captured, is the simplest one. However, the perfect sink condition can not be used, because it predicts a continuous increase in particle codeposition with increasing current density, which contradicts experimental observations. Therefore, an interaction model based on the assumption that the reduction of adsorbed ions is the determining factor for particle deposition is proposed. This electrode-ion-particle electron transfer (EIPET) model leads to a Butler-Volmer like expression for the particle deposition rate ... 

The presence of the interface restricts the diffusive motion to a half space. More important is the presence of particle-surface interactions, which can modify the transport properties. 

Figure 13.23a shows the first mechanism, which is mainly chemical in nature. The silica removal rate is accelerated by the ceria-silica interactions, which results in the improved dissolution of the silica substrate during polishing. Figure 13.23b shows the second mechanism, which is based on physicochemical particle-surface interactions in which the ceria-silica bonding does not result in direct modification of the silica substrate but enhances the... 

This is the closest we can come to measuring the particle-surface Interaction during CMP In a direct experiment... 

A complete model for particle-surface interaction would include both (a) Huid mechanical effects as Lhe particle approaches the surface and surface forces. The Hu id mechanical calculations would take into account free molecule effects as the particle comes to within one mean free path of the surface. The presence of thin films of liquids and surface irregularities further complicate the situation. In practice, the design of cascade impactors (Chapter 6) and other devices in which rebound may be important is carried out empirically, by experimenting with various particle.s, coatings, and collecting surfaces. 

Bowling [1988], Visser [1988] and Oliveira [1992], have reviewed particle surface interaction and their work is used extensively in the development of the discussion in this chapter. 

Moaddeb and Koros (1997) described the deposition of silica on polymeric MF membranes as non-uniform. This means that cake characterisation is difficult as a cracks could vary the results. Meagher et al (1996) stated that attractive interaction between membranes and particles would cause a flux decline, even if the particles were aggregated. Aggregation reduced the flux decline if there was no attraction between the membranes and colloids. The authors outlined the restrictions of the gel polarisation model, as the porosity of the deposit is not accounted for in the model. It was also suggested that the resistance of the gel layer is more important than the particle-surface interaction (what is often referred to as adsorption). 

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