First monolayer

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

Ellipsometry can be sensitive to layers of matter only one atom thick. For example, oxidation of freshly cleaved single-crystal graphite can be monitored from the first monolayer and up. The best thicknesses for the ellipsometric study of thin films are between about 1 nm and 1000 nm. Although the spectra become complicated, films thicker than even 1 pm can be studied. Flat planar materials are optimum, but surface and interfacial roughness can be quantitatively determined if the roughness scale is smaller than about 100 nm. Thus ellipsometry is ideal for the investigation of interfacial surfaces in optical coatings and semiconductor struc-... 

Langmuir-Blodgett films (LB) and self assembled monolayers (SAM) deposited on metal surfaces have been studied by SERS spectroscopy in several investigations. For example, mono- and bilayers of phospholipids and cholesterol deposited on a rutile prism with a silver coating have been analyzed in contact with water. The study showed that in these models of biological membranes the second layer modified the fluidity of the first monolayer, and revealed the conformation of the polar head close to the silver [4.300]. 

Plotting P/[Va(Po - P)] versus P/Pq yields a straight line with slope a= x l)lxVo, crossing the y-axis at = I/ Vq. The volume adsorbed in the first monolayer is found as Vb = l/(ct + rj). The volume Vq can be converted into the number of molecules adsorbed by No = PVq/RT and if we know how big an area each molecule occupies (j4o) then the total area, A= NqAo, can be found. 

The authors [35] emphasize that their result regarding the first HgS monolayer, which involves reversible underpotential adsorption, suggests that nucleation cannot be considered as a universal mechanism for the formation of anodic films. Analogous conclusions have been inferred for cathodic HgSe films electrodeposited on mercury electrode by the reduction of selenous acid [37] the first monolayer appeared to be reversibly adsorbed, while formation of the following two layers was preceded by nucleation. 

Varazo K, Lay MD, Sorenson TA, Stickney JL (2002) Formation of the first monolayers of CdTe on Au(l 11) by electrochemical atomic layer epitaxy (EC-ALE) studied by LEED, Auger, XPS, and in-situ STM. J Electroanal Chem 522 104-114... 

Self-assembled monolayers (SAMs) [8] The layers are formed by heterologous interaction between reactive groups, such as thiols, and noble metals, such as gold or silver. Since the molecules are selectively adsorbed on these metals, film growth stops after the first monolayer is completed. The molecular aggregation is enthalpy driven, and the final structure is in thermodynamic equilibrium. 

The surface phase diagram of vanadium oxides on Rh(l 11) has been investigated in a series of papers of our group [4, 18, 19, 90, 101-103]. It is characterized by pronounced polymorphism and many different oxide structures have been detected as a function of coverage and growth temperature. The vanadium oxide structures for coverages up to the completion of the first monolayer formed on Rh(l 11) under the different preparation conditions may be subdivided into highly oxidized phases... 

Studies of the first atomic layer in the formation of a compound are essentially studies of UPD. As noted, there are a number of excellent reviews [83-85, 87, 88], To really learn about the structure of UPD layers, single crystals and surface sensitive analytical techniques are required. A recent review covers atomic level studies of UPD on important single crystal electrode interfaces, such as Au, Pt, Ag and Cu [88], but does not go deeply into most of the systems of interest for EC-ALE. In this section, UPD of the first atomic layers in the formation of a compound will be discussed, as will atomic level studies of the formation of the first monolayers of compounds, where information is available. 

The most detailed studies of the first monolayer(s) formed using EC-ALE have been performed on CdTe. Initial studies involved UHV-EC (LEED and AES) and ex-situ STM [156, 157]. In those studies, two structures were identified on Au(lll), a (x/7 x v/7)7 19.1°-CdTe and a (3 x 3)-CdTe. At that time, it was concluded that they both involved a Cd/Te ratio of 1, with the (3 x 3) having a very slightly higher coverage, 0.44 vs 0.43 for the (x/7 x. of both elements. Clear LEED patterns... 

The second UHV-EC study of the first monolayers of CdTe on Au(lll) by this group is just being completed on Au(lll) [236]. Figure 67 is a series of graphs of the Cd/Te ratios from Auger electron spectroscopy, corrected for sensitivity factors, so the ratios are an approximation of the atomic ratios present on the surface. The graphs are a function of the potentials used to form the second atomic layer in the formation of CdTe monolayers in a two step process. 

Atomic level studies of the first monolayer of CdS on Au(lll) have been published by Shannon et al. [165]. STM was used to study the first atomic layer of S and the... 

When a metal is in contact with its metal ion in solution, an equilibrium potential is established commonly referred to as Nernst potential (Er). Metal deposition occurs at potentials negative of Er, and dissolution for E > Er. However, when a metal is deposited onto a foreign metal substrate, which will always be the case for the initial stages of deposition, it is frequently observed that the first monolayer on the metal is deposited at potentials which are positive of the respective Nernst potential [37, 38]. This apparent violation for Nernst s law simply arises from the fact that the interaction between deposit metal and substrate is stronger than that between the atoms of the deposit. This effect has been termed underpotential deposition (upd), to contrast deposition processes at overpotentials. (One should keep in mind, however, that despite the symmetrical technical terms the physical origins of both effects are quite different. While the reason for an overpotential is solely due to kinetic hindrance of the deposition process, is that for underpotential deposition found in the energetics of the adatom-substrate interaction.)... 

Here FMON and - mon represent the available vacant sites and surface sites occupied by B, respectively, of the first monolayer on a solid absorbate. The equilibrium constant KB for the reaction is given by the ratio of the rate constant for k.d for adsorption and k for desorption... 

The second stage of modeling is the introduction of solvated ionic species into the model double layer. Coadsorption of HF and water yields adsorbed HgO ions the solvation stoichiometries of ions in the first monolayer and in subsequent layers are determined. The third stage of modeling is establishment of potential control in UHV. Hydrogen coadsorption is used to deflect the effective potential of the water monolayer below the potential of zero charge. The unique ways in which UHV models can contribute to an improved molecular-scale understanding of electrochemical interfaces are discussed. 

A Ru(0001) sample, with vacuum deposited Cu, has been characterized by cyclic voltammetry by transferring to an electrochemical cell (16). Figures 4e-4h shows the anodic stripping curves for four different coverages of Cu. A single stripping peak was observed at +110 mV for 0.6 ML Cu and shifted to +145 mV for 5.2 ML Cu. This peak represents the removal of the first monolayer of Cu or Cu in direct contact with the Ru surface. The curve for 5.2 ML Cu shows an additional peak at -20 mV for the stripping of multilayer Cu. 

The structure of the first monolayer of Cu is pseudomorphic with respect to the Ru(0001) substrate whereas successive Cu layers grow epitaxially with a Cu(lll) structure. 

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