Surface reactions, binary compounds

Eight variants of the DD reaction mechanism, described by Eqs. (21-25) have been simulated. The simplest approach is to neglect B2 desorption in Eq. (22) and the reaction between AB species (Eq. (25)). For this case, an IPT is observed at the critical point Tib, = 2/3. Thus this variant of the model has a zero-width reaction window and the trivial critical point is given by the stoichiometry of the reaction. For Tb2 < T1B2 the surface becomes poisoned by a binary compound of (A -I- AB) species and the lattice cannot be completely covered because of the dimer adsorption requirement of a... 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

The analysis of the surface reaction zone has been applied to laboratory-scale PVD of binary and ternary group II-VI compound semiconductors, such as CdS, ZnS, (Cd ZnJS, CdTe, HgTe, and (Cd HgjTe, and the ternary group I-III-VI chalcopyrite CuInSe2 (17). For example, Figure 14 shows the comparison between the predicted and measured compositions of ternary (Zn Cd jS alloys. The predicted composition is within 3 atom % of the measured composition across the range of composition from 10 to 90 atom %. 

The quaternary solid solution GaxIni tNy Si , can be considered as a regular mixture of four binary compounds GaN, GaAs, InN and InAs in respective proportions [4], Chemical reactions governing the growth of GaJni tNy. S j, layer and the evaporation of main elements from its surface are the following ... 

Reaction rates may be determined by the ease of intracrystalline transport to the surface, or by the chemical change on the surface. These surface reactions often resemble behaviour described in modelling heterogeneous catalytic processes and are usually reversible so that decomposition rates are sensitive to any gases present. Behaviour of reactants of the present group is similar to that of the oxides (Chapter 9), which are in the same class. Little information is available for other binary compounds, fluorides, chlorides, etc., which usually melt rather than decompose. 

Layered nanostructures can be deposited from the electrochemical environment by applying a time dependent voltage program to the working electrode (5) or by using a sequential deposition scheme such as electrochemical atomic layer epitaxy (EC-ALE) (6-10). In EC-ALE, a surface-limited electrochemical reaction, such as underpotential deposition (upd), is used to synthesize a binary compound by successive deposition of each element from its respective solution precursor. EC-ALE is an attractive electrosynthetic alternative to conventional deposition methods that is inexpensive, operates at ambient temperature and pressure and provides precise film thickness control. This technique promises to overcome many problems associated with other electrosynthetic approaches, such as the formation of highly polycrystalline deposits and interfacial interdiffusion. For example, we have recently used EC-ALE to fabricate stable semiconductor heterojunctions with extremely abrupt interfaces (11). 

Studies have also revealed that the implementation of nanostmctured electrode materials can result in the initiation of new lithium storage mechanisms. These effects typically manifest either via a pseudocapacitive storage mechanism that accommodates lithium ions on the surface/interface of the particles below a critical particle size or through a conversion mechanism that involves the formation and decomposition of at least two separate phases [28-31]. The pseudocapacitive mechanism is more pronounced because of the more prominent role of surfaces and grain interfaces in nanomaterials. Reversible conversion reactions based on the reduction and oxidation of metal nanoparticles can ensue between binary compounds comprised of some second or third period element, a transition metal oxide, and metaUic lithium [32-37]. Nanoparticles are extremely effective toward this means because of their large specific surface area that is very active toward the decomposition of the lithium binary compound. Furthermore, reduction of some micrometer sized materials to the nanoscale has been shown to activate or enable reversible electrode reactions that would otherwise not take place, typically materials with low Li-ion diflEiision coefficients. 

During the time dt, the thickness of the ArBs layer increases by dyA3 at interface 3 as a result of diffusion of the A atoms from interface 2 to interface 3 and their subsequent partial chemical reaction (4.2) with the surface B atoms. In the ApBq-B reaction couple the ApBq phase acts as a source of diffusing A atoms. It must be clear, however, that the content of component A in this phase cannot be less than the lower limit of its homogeneity range. Hence, as reaction (4.2) proceeds, the ApBq compound becomes unstable and therefore should undergo a partial transformation into another compound of the A-B multiphase binary system. To reveal the essence of this transformation, let us consider one of the simplest cases, in... 

With time, the Ni3Zn22 phase must be consumed in the course of the latter reaction. However, if the experiment is interrupted before its full consumption, then the layers of all the intermatallic compounds of the Ni-Zn binary system, stable at a given temperature, will be present between nickel and zinc. Moreover, metallographic examination of the cross-section surface after repeated anneals in the as-received condition may well show a greater number of distinquishable layers in the Ni-Zn transition zone than the number of those compounds because some will have duplex structures. 

In spite of their seeming variety, theoretical approaches of different authors to the consideration of solid-state heterogeneous kinetics can be divided into two distinct groups. The first group takes account of both the step of diffusional transport of reacting particles (atoms, ions or, in exceptional cases if at all, radicals) across the bulk of a growing layer to the reaction site (a phase interface) and the step of subsequent chemical transformations with the participation of these diffusing particles and the surface atoms (ions) of the other component (or molecules of the other chemical compound of a binary multiphase system). This is the physicochemical approach, the main concepts and consequences of which were presented in the most consistent form in the works by V.I. Arkharov.1,46,47... 

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