Compound Monolayers

Historically, EC-ALE has been developed by analogy with atomic layer epitaxy (ALE) [76-82], ALE is a methodology used initially to improve epitaxy in the growth of thin-films by MBE and VPE. The principle of ALE is to use surface limited reactions to form each atomic layer of a deposit. If no more than an atomic layer is ever deposited, the growth will be 2-D, layer by layer, epitaxial. Surface limited reactions are developed for the deposition of each component element, and a cycle is formed with them. With each cycle, a compound monolayer is formed, and the deposit thickness is controlled by the number of cycles. 

The simplest model for EC-ALE is that a set of conditions is chosen for a cycle, and each cycle produces one compound monolayer. Ideally, the same potentials and solutions are used for each cycle of a deposition. Recently, it has become clear that this is not the case for EC-ALE growth of many compounds on Au, the initial conditions do not appear optimal from start to finish. 

Studies of UPD are important for a number of reasons, most importantly, because they are the formation of the first atomic layer in an electrodeposit. In the present text, they are important because they illuminate the structures of electro-deposited atomic layers, the reactants in EC-ALE. However, such studies must be kept in context, given that the structures of the first UPD layers on a substrate generally have little to do with the structures of subsequently formed compound mono-layers. It has been found that the structures of compound monolayers are determined, for the most part, by the structure of the compound that is forming, perturbed by the lattice mismatch between the deposit and the substrate. The structure of the first atomic layer on the substrate does not appear to be a significant factor in determining... 

The early surface studies described above indicated that compounds were probably being formed with the first cycle and that the deposits had some degree of order. FEED, however, is an averaging technique. A surface can have significant amounts of disorder, relatively small domains, and still give a reasonable FEED pattern. STM was also used in those studies of compound monolayers, however the images collected were generally of very small areas of the deposits—5-15 nm on a side. Those STM studies... 

The mechanism of Cd(II) transfer in methanol across saturated organic compounds monolayers adsorbed on the mercury electrode was also studied [71]. 

Frey, W.,Ringsdorf, H., Sackmann, E., Schneider, J. 1987. Preparation, Microstructure and thermodynamic Properties of Homogeneous and Heterogeneous Compound Monolayers of Polymerized and Monomeric Surfactants on the Air/Water Interlace and on Solid Substrates. Macromolecules 20,1312... 

The electrochemical form of ALE makes use of underpotential deposition (UPD). the electrochemical phenomena where an atomic layer of one element frequently deposits on a second element at a potential prior to (under) that needed to deposit the element on itself fhe driving force for UPD can be thought of as resulting from the free energy of formation of a surface compound. These surface limited reactions are then used in a deposition cycle, where atomic layers of each element are deposited in turn, in order to form a monolayer of the deposit. The number of cycles performed detennines the number of compound monolayers and the thickness of the deposit. One of the main advantages of this methodology is that the electrochemical formation of a compound is broken down into a series of individually addressable steps. Each step in the cycle becomes a point of control over the deposition process. 

UPD can also result in more than an ML in some cases, where small underpotentials are used [5]. If the amount of an element deposited in a cycle is in excess of that needed to deposit a ML of the compound, a second compound layer could be initiated, possibly nucleating asperities and promoting roughing. To prevent roughening, it is advisable to choose underpotentials which will only result in the amount needed to form a compound monolayer or less. It is much better to deposit too little each cycle then too much. 

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