Interaction of adsorbed atoms

In this section a method for the direct calorimetric determination of heats of adsorption on evaporated metal films is described and results for the heals of adsorption of hydrogen on nickel, iron, and tungsten are reported. In all cases the heats of adsorption decrease with the fraction of surface covered in a mode that can satisfactorily be explained by interaction of adsorbed atoms. A criterion for mobility of the adsorbed atoms is developed... 

As was already mentioned in the introduction, we have to assume that adatoms of hydrogen adsorbed on a metal surface with a high heat of adsorption must be mobile so that they either can find the sites of highest surface energy or can move as far apart from each other as possible if the heterogeneity of surface energy of the sites is due to interaction of adsorbed atoms on neighboring sites. A rapid redistribution of adsorbed atoms by evaporation and readsorption is out of the question... 

Recently, in order to understand processes on the catalyst surface, in particular structural formations, it has become a frequent practice to apply theories accounting for the interaction of adsorbed atoms. An important microscopic model of such a type is the lattice gas model. Its specific peculiarity is that this model accounts for the interaction of the nearer surface molecules (lateral interactions). It is this model that was applied in refs. 86 and 87. They should be specially emphasized as having exerted a great influence on the interpretation of thermodesorption experiments. The lattice gas model is used, e.g. in a series of investigations by Tovbin and Fedyanin [88, 89] devoted to the kinetics of chemisorption and reactions on catalyst surfaces. In terms of this model, one can interpret the complicated reaction rate dependences of surface coverage observed experimentally... 

The dynamic properties of reaction systems ultimately depend on the nature of interactions between molecules. The nonhnearity of macroscopic rate laws is due to the participation of more than one species in an elementary act and the complex cooperative interaction of adsorbed atoms and molecules with each other and with the catalyst surface. The nonlinearity of macroscopic rate laws is also due to phase transitions in the adsorption layer, surface restructuring during the reaction, catalyst surface (energetic) nonuniformity, and the influence of mass, heat and pulse transfer processes on the reaction rate due to the delay in the feedback. 

Blocking of reaction sites The interaction of adsorbed inhibitors with surface metal atoms may prevent these metal atoms from participating in either the anodic or cathodic reactions of corrosion. This simple blocking effect decreases the number of surface metal atoms at which these reactions can occur, and hence the rates of these reactions, in proportion to the extent of adsorption. The mechanisms of the reactions are not affected and the Tafel slopes of the polarisation curves remain unchanged. Behaviour of this type has been observed for iron in sulphuric acid solutions containing 2,6-dimethyl quinoline, /3-naphthoquinoline , or aliphatic sulphides . 

Adsorbed layers, thin films of oxides, or other compounds present on the metal surface aggravate the pattern of deactivation of metastable atoms. The adsorption changes the surface energy structure. Besides, dense layers of adsorbate may hamper the approach of metastable atom sufficiently close to the metal to suppress thus the process of resonance ionization. An example can be work [130], in which a transition from a two- to one-electron mechanism during deactivation of He atoms is exemplified by the Co - Pd system (111). The experimental material on the interaction of metastable atoms with an adsorption-coated surface of... 

This picture of chemisorbed atoms on jellium, although much too simple, illustrates a few important aspects of chemisorption. First, the electron levels of adsorbed atoms broaden due to the interaction with the s-electron band of the metal. This is generally the case in chemisorption. Second, the relative position of the broadened adsorbate levels with respect to the Fermi level of the substrate metal determines whether charge transfer between metal and adatom takes place and in which direction. 

With increasing covering by foreign atoms, decreases at first linearly and then more slowly to a minimum value o, somewhat below that for monatomic covering, 6 = 1. Then the work function increases again to the value found for the pure foreign metal itself, which is reached when about 5 atom layers (55) of the adsorbed metal have been deposited. The increase of shortly before the coverage reaches the value 6 = 1 is usually explained by the mutual interaction of adsorbed dipoles, but it can also be related to the difference of the electronic interaction on different crystal planes of the sublayer surface (see Fig. 3). 

It is also interesting to consider charge-transfer models developed primarily for metal surfaces. There are clear parallels to the metal oxide case in that there is an interaction between discrete molecular orbitals on one side, and electronic bands on the other side of the interface. The Newns-Anderson model [118] qualitatively accounts for the interactions between adsorbed atoms and metal surfaces. The model is based on resonance of adatom levels with a substrate band. In particular, the model considers an energy shift in the adatom level, as well as a broadening of that level. The width of the level is taken as a measure of the interaction strength with the substrate bands [118]. Also femtosecond electron dynamics have been studied at electrode interfaces, see e.g. [119]. It needs to be established, however, to what extent metal surface models are valid also for organic adsorbates on metal oxides in view of the differences between the metal an the metal oxide band structures. The significance of the band gap, as well as of surface states in it, must in any case be considered [102]. 

Here J, JQ and Ja are the statistical sums of activated complex and gas-phase molecules and of adsorbed atom (adatom), respectively, sA and eD the adsorption and desorption activation energies, a the area of adatom localization, h Planck s constant, and f. the parameters of the activated complex-adatom and adatom-adatom interactions (e < 0 for repulsion and e > 0 for attraction), A the contribution to the complete drop of adsorption heat AQ from the electron subsystem (for a two-dimensional free-electron gas model), x = exp (ej — e) — 1, jc, = = 0), / = 1/kT (k is the Boltzmann con-... 

In 1975 Vannice published a comprehensive study of the kinetics of the methanation of carbon monoxide over various metals.45 He analysed his results in terms of a rate-determining step for the reaction involving the interaction of adsorbed CHOH species and adsorbed H atoms ... 

The existence of a solid itself, the solid surfaces, the phenomena of adsorption and absorption of gases are due to the interactions between different components of a system. The nature of the interaction between the particles of a gas-solid system is quite diverse. It depends on the nature of the solid s atoms and the gas-phase molecules. The theory of particle interactions is studied by quantum chemistry [4,5]. To date, one can consider that the prospective trends in the development of this theory for metals and semiconductors [6,7] and alloys [8] have been formulated. They enable one to describe the thermodynamic characteristics of solids, particularly of phase equilibria, the conditions of stability of systems, and the nature of phase transitions [9,10]. Lately, methods of calculating the interactions of adsorbed particles with a surface and between adsorbed particles have been developing intensively [11-13]. But the practical use of quantum-chemical methods for describing physico-chemical processes is hampered by mathematical difficulties. This makes one employ rougher models of particle interaction - model or empirical potentials. Their choice depends on the problems being considered. 

Nucleation of M/SC particles on defects is typical for systems with weak interaction between adsorbed atoms and substrate lattice. At nucleation on defects coefficient C is the probability for adatom to be captured by defect. As a first approximation, one can assume that defects immobilized diffusing adatoms are uniformly distributed on a surface and their surface concentration is equal to /Vm. In this case C is determined by the relationship between XD and the mean interval dm separating defects of such type (dm = iVnll/2) C = Xu/dm at xudm [51]. 

The 2D electron gas of the Shockley-type surface states plays also an important role in the substrate mediated interaction between adsorbed atoms. 

There are countless examples of the interactions of various atoms and molecules with the clean Si(100) surface. In addition these adsorbate-surface interactions can differ with deposition conditions, such as the rate of deposition or temperature of the sample. For example, even the simplest adsorbate, hydrogen, can etch the surface at room temperature and also form a variety of ordered structures at elevated sample temperatures [57]. A number of adsorbates can form ordered structures commensurate with the surface (e.g. Ag [58], Ga [59], Bi [60]), most transition metals react with the surface to form silicides (e.g. Ni [61], Co [62], Er [63]), halogens can etch the surface at room temperature (e.g. F2 [64], CI2 [65], Br2 [66]), some molecules dissociate on the surface (e.g. PH3 [67], B2H6 [68], NH3 [37]) and other molecules can bond to the silicon in different adsorption configurations but remain intact (e.g. Benzene [69], Cu-phthalocyanine [70], C60 [71]). A detailed review of a number of adsorbate-Si(lOO) interactions can be found in [23,72] and a more specific review relating to organic adsorbates can be found in [22]. As an example of an adsorbate-silicon system we shall here consider the adsorption of a molecule that our group has extensive experience with phosphine. 

Abstract. We describe the state-of-the-art in the creation of ordered superlattices of adsorbed atoms, molecules, semiconductor quantum dots, and metallic islands, by means of self-assembly during atomic-beam growth on single crystal surfaces. These surfaces often have long-period reconstructions or strain relief patterns which are used as template for heterogeneous nucleation. However, repulsive adsorbate-adsorbate interactions may also stabilize ordered superlattices, and vertical correlations of growth sequences of buried islands will be discussed in the case of semiconductor quantum dots. We also present new template surfaces considered as particularly promising for the creation of novel island superlattices. 

---